GSI SLV Duisburg - International Welding Engineer-2015 [PDF]

Zitiervorschau

Gesellschaft für Schweißtechnik International mbH

Welding processes and equipment Materials and their behaviour during welding Construction and design Fabrication, applications engineering

The Welding Engineer‘s Current Knowledge

Edition 2015

International Welding Engineer (IWE)

Welding processes and equipment

The Document contains standards reproduced by permission of DIN Deutsches Institut für Normung e.V. The definitive version for the implementation of this standard is the edition bearing the most recent date of issue, obtainable from Beuth Verlag GmbH, Burggrafenstrasse 6, D-10787 Berlin.

© 2015 SLV Duisburg – Branch of GSI mbH Copyright by SLV Duisburg. All rights reserved

Topic overview

Module 1:

SFI / IWE

Welding Processes and Equipment

Chapter

Topic

1.01

General introduction to welding technology

1.02

Oxy-acetylene welding and related processes

1.03

Electrical engineering, an overview

1.04

The arc

1.05

Power supplies for arc welding

1.06

Indroduction to gas shielded welding

1.07

TIG welding

1.08-1 1.08-2

MIG/MAG welding Flux cored wire welding

1.09

Manual metal-arc welding

1.10

Submerged arc welding

1.11

Resistance welding

1.12-1

Other welding processes (Laser, electron-beam and plasma welding)

1-12-2

Other special welding processes II

1.13

Cutting, Drilling and other joint preparation processes

1.14

Coating process

1.15

Fully mechanised processes and robotic welding © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

Welding processes and equipment

Topic overview Chapter

Topic

1.16

Brazing and (soft) soldering

1.17

Joining processes of plastices

1.18

Joining processes für ceramic and composites

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SFI / IWE

Welding processes and equipment

General introduction to Welding Technology

SFI / IWE 1.01 Page 1

Contents 1

General introduction into the welding technology ....................................................................... 3

1.1 What is welding technology? .......................................................................................... 3 1.2 History of Welding Technology ....................................................................................... 3 1.3 Application areas of Welding Technology ....................................................................... 5 1.4 Definitions of metal welding processes acc. to DIN EN 14610 ....................................... 7 1.4.1 Metal Welding ...................................................................................................... 7 1.4.2 Welding with pressure.......................................................................................... 8 1.4.3 Fusion welding ................................................................................................... 10 1.5 Brazing ......................................................................................................................... 12 1.5.1 Soldering............................................................................................................ 12 1.5.2 Brazing............................................................................................................... 12 1.6 Bonding ........................................................................................................................ 17 2

Basic definitions of welding processes ...................................................................................... 18

2.1 Classification according to the type of energy carrier ................................................... 18 2.2 Classification according to the aim of welding .............................................................. 19 2.2.1 Joint Welding ..................................................................................................... 19 2.2.2 Surface Welding ................................................................................................ 20 2.3 Classification according to the physical sequencing of welding. ................................... 20 2.3.1 Fusion Welding .................................................................................................. 21 2.3.2 Welding with pressure........................................................................................ 21 2.4 Classification according to the level of mechanisation .................................................. 22 3 4 5 6 7 8

Vocabulary definitions .................................................................................................................. 23 Designation, reference- and classification numbers .................................................................. 25 Overview of metal welding processes ......................................................................................... 27 Selection of welding processes ................................................................................................... 29 Health and Safety .......................................................................................................................... 33 Knowledge Questions .................................................................................................................. 34

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Welcome in the world of welding technology and to the course of

International Welding Engineer / Technologist (DVS / EWF / IIW ). Welding is a type of joining that cannot be taken out from today’s world. It is being found in almost every aspect of our life and its aim is to join separate elements of a construction or product which simply cannot be manufactured by just one piece. For the users of welded products welding technology is basically of less importance. They assume that the product will safely comply with the defined functions. This way of thinking goes from dental braces to bicycles, balcony fences to porch roofs, as well as from automobiles, ships, air planes up to bridges houses and skyscrapers. Welding technology comes across to us in many ways without directly being noticed.

Figure 1: Welding Technology “wherever you look“ (Bild: DVS - Deutscher Verband für Schweißen und verwandte Verfahren e. V.)

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General introduction to Welding Technology

1 1.1

SFI / IWE 1.01 Page 3

General introduction into the welding technology What is welding technology?

Description: Welding technology Welding Technology is the sum of knowledge and experience regarding the application of weld technical processes which has been acquired and compiled by experts during many years. Besides welding, brazing (chapter 1.16), bonding (chapter 1.18), mechanical joining and cutting plays an important role in industrial application. 1.2

History of Welding Technology

From the Sumerians to the Laser In the beginning there was fire! For many years and this goes back some 6000 years ago, even for welding technology this was only applicable original energy source. At that time connecting two metallic pieces has been accomplished by forge welding. Both parts were heated by the fire and additionally connected by means of an external force.

Source: presentation „Forge Welding“ by Stefan Griwenka

Figure 2: Forge Welding

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An impression of the historical development of welding technology is given by the following, however without being completeness, chronology: ● ● ● ●

● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

around 4000 b. Chr. Sumerians: welded Parts (Gold to Gold) around 2700 b. Chr. Brazing in Egypt and Mesopotamia. around 2700 b. Chr. Egypt: application of forge welding during the construction of a cupper water pipe line and during the manufacturing of jewelry and decorations out of gold. around 700 b. Chr. Delhi: the Kutub pillar, a welded cast iron obelisk of 16m height.

around 79 b. Chr. Pompeji: Forged welded pipe line 1782 - the physicist Christoph Lichtenberg from Göttingen rather accidentally welds two metals parts (clock spring and knife-blade) by means of electricity being generated by friction. 1809 - the english physicist Humphry Davy uses the electric arc as a light source and detects a deflection by the use of magnets. 1867 - Elihu Thomson discovers the electrical resistance welding of steel. 1881 - 1887 – Nikolai Nikolaijewitsch Bernados from Russia is recognised as the inventor of the electrical arc. Together with Karol Stanislaw Olszewski he tries out the first (electric) manual arc welding process. 1890 - the most widely spreaded and accepted technic of arc welding was developped by the russian enigneer Nikolai Gawrilowitsch Slawjanow. 1895 - Hans Goldschmidt develops Thermit welding (Aluminothermical welding). 1948 - 1950 The so called S.I.G.M.A.-process (Shielded Inert Gas Metal Arc), today’s SMAwelding, was applied in the USA for the first time. In 1950 the introduction in Germany took place. 1951 - Plasma welding, the most recent independent shielded arc welding process was developped. However, the definition of thermal plasma was already introduced by physician Irving Langmuir since 1928. 1951 – Development of electrical slag welding. 1956 – Friction welding was developed in the UdSSR and the USA. 1957 – Ultrasonic welding was developed. 1957 – First electron beam equipment in industry. 1961 – Laser welding was developed. 1970-1980 – Pulsed variants of arc welding processes were developed. 1990 – New high performance variants of MAG welding were developed, for example T.I.M.E.- or "Rapid Melt"-processes. The in the early 70s developed "High Deposition Welding" was the predecessor of these technics. 1995 – Patent publication of Friction Stir Welding (FSW) by TWI (The Welding Institute UK) 2003 – Arc welding is also applied in dental technics. ab 2005 – Development of low-energy processes like EB-Non-Vac, Hybride processes, Arc brazing, CMT, Cold-Arc……. © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Nachdruck und unbefugte Weitergabe sind unzulässig und werden gesetzlich verfolgt

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General introduction to Welding Technology 1.3

SFI / IWE 1.01 Page 5

Application areas of Welding Technology

The area’s in which welding technology is being applied is very diverse. The added value of the separate branches of welding technology in Germany reaches 21.109 Euro per year. The number of employees is around 360.000 people.

Figure 3: Added value of welding technology (DVS-survey of 2012)

In the area of automobile construction many variants of welding technology have been widespread and the added value is the biggest due to the quantity of manufactured vehicles. Even a telephone card which is nowadays part of common life, shows impressively the possibility of welding technology Welding Technology applications can be found in almost any industry branch. ● ● ● ● ● ● ● ●

Steel Construction Pressure Vessel construction Automobile construction Railway vehicle construction Offshore Machine building Medicine technic etc.

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Cooling vessels of stainless steel

Brown coal digger in surface mining

SFI / IWE 1.01 Page 6

Berline’s Reichstagdome

Large Millimeter Telescope on top of the extinct volcano Sierra Negra in Puebla, Mexico

Cargo Lifter Halle

Foundry ladles

Ship construction of Aluminium

Viaduct of Millau

Figure 4: Areas of application of Welding Technology

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General introduction to Welding Technology 1.4

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Definitions of metal welding processes acc. to DIN EN 14610

Metal Welding: Operation which unifies metal(s) by means of heat or pressure, or both, in such a way that there is continuity in the nature of the metal(s) which has (have) been joined. A filler metal, the melting temperature of which is of the same order as that of the parent metal(s), may or may not be used and the result of welding is the weld. Briefly: welding is a way of joining resulting in a positive substance, not releasable joint. DIN EN 14610: This document defines metal welding processes, classified according to their physical characteristics and according to the relevant energy carrier. However welding technology is not limited to the joining of metals; plastics and ceramics can also be joined by welding (see chapter 1.17 and 1.18). 1.4.1 Metal Welding Is a process which joins metal(s) using heat and/or pressure resulting in a continuous inner composition of the joined metal(s). Metal welding is divided into two main categories – Fusion welding und Pressure welding:

Metal Welding

Fusion Welding

Common fusion welding

Resistance fusion welding

- Gas welding (3) - Manual metal arc welding (111) - Gas shielded metal arc welding (13) - TIG welding (141) - Submerged arc welding (12) - Laser welding (52) …. etc.

- Enclosed resistance welding - Elektroslag welding (72)

Pressure Welding

Resistance pressure welding

Common pressure welding

- Resistance spot welding (21) - Resistance seam welding (22) - Resistance butt welding (25) - Flash welding (24)

- Ultrasonic welding (41) - Diffusion welding (45) - Forge welding (43) - Oxyfuel gas pressure welding (47) - Arc pressure welding - Cold pressure welding (48) - Friction welding (42) - …..etc.

especially for longitudinal pipe welds: - RoTating transformer welding (RT) - Resistance welding with sliding contacts (RS) - Inductive resistance pressure welding (RI)

Figure 5: Overview of Metal Welding

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Note 1: Filler materials which have, more or less, the same melting temperature as the base metal(s) to be joined, can be applied. The result of welding is the weld seam. Note 2: This definition includes coating processing. 1.4.2 Welding with pressure Welding with pressure is a process which uses sufficient external forces in order to generate a more or less plastic deformation on both ends of the joining faces. Generally without using filler materials (see figures 6 to 8)

Figure 6: Stud welding

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Figure 7: Flash welding

Figure 8: HF resistance welding (Bonden)

Remark: Usually- but not mandatory- the fusion areas of the parts are being heated in order to establish or ease the joining process.

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1.4.3 Fusion welding Fusion welding is welding without external force whereby the fusion area(s) has (have) to be partly surface-fused. Usually – but not mandatory – molten filler material is being added (see figure 9 to 11)

Figure 9: MAG-welding

Figure 10: Laser welding

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Figure 11: TIG-welding

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1.5 Brazing Brazing is closely related to fusion welding and is characterised by a high flexibility of the materials to be brazed and the possible mix of base materials. In contradiction to welding where melting of the base materials (except some pressure welding processes) occurs, the base materials will remain in the solid phase during brazing. The process of joining is completely based on physical diffusion processes. Brazing is a joining process which uses a molten solder having a liquidus temperature which is lower than the base material’s solidus temperature. The molten solder wets the surfaces of the base material(s) and is being sucked (or in case of being pre-placed: holding position) into the narrow existing gap between the joining parts during or at the end of heating. Besides brazing there is soldering: for further information see chapter 1.16 Note 1: Usually these processes are being applied for metals but it can also be applied for non-metallic materials. The solder material always has a different chemical analysis as the components to be joined. Note 2: If the processing occurs without capillary forces they are often designated as braze welding.

Brazing / Soldering

Brazing

Soldering

Abbildung 12: Unterteilung des Lötens

1.5.1 Soldering Soldering is a joining process using solders having a liquidus temperature of 450°C or below. 1.5.2 Brazing Brazing is a joining process using solders having a liquidus temperature above 450°.

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Brazing and soldering can be classified into additional variants or application technics:

Brazing and soldering

Soldering Soldering via solids

- Soldering with soldering iron (943) - Soldering with preheated blocks - Roll soldering

Soldering via fluid

- Dip soldering (955) - Wave soldering (951) - Drag soldering (944) - Ultrasonic soldering (947)

Soldering via gas

- Flame soldering (942) - Hot gas soldering

Infrared soldering (941)

- SMD-Technic

Electr.current soldering

- Induction soldering in air (946) - Resistance soldering (948)

Furnace soldering (953)

- Electronical plates

Brazing Brazing via fluid Flame brazing (912) Arc weld brazing (972)

- Dip-bath brazing (923) - Salt-bath brazing (924) - Flux-bath brazing (925) - Manual brazing - Flame-area soldering equipment - Manual arc weld brazing - TIG brazing (974) - Plasma arc weld brazing (975)

Beam brazing

- Laser beam brazing (913) - Electron beam brazing (914)

Electr. current brazing

- Induction brazing (916) - Induction brazing with inert atmosphere - Inductive resistance brazing (918) - Direct resistance brazing (918) - Furnace brazing with flux (921) - Furnace brazing with active shielding gas - Furnace brazing with inert shielding gas - Vacuum brazing (922)

Figure 13: Different types of brazing / soldering

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In the following some examples of brazing / soldering applications are shown.

Figure 14: Soldering joint of a cable connection

Figure 15: Induction-High temperature brazing in hydrogen atmosphere

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Figure 16: Brazing / soldering joints in a heat exchanger

Figure 17:

Brazing of galvanised parts

Remark: Pre-brazed galvanised parts via arc weld brazing. The actual brazing is executed in the furnace.

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Figure 18:

SFI / IWE 1.01 Page 16

Induction brazing / soldering

Figure 19: Aluminium-soldering

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1.6 Bonding Bonding is the process of making a permanent joint between parts by using a synthetic material (adhesive) being strengthened via physical-chemical hardening processing, resulting in a joining of the parts by means of as well as surface adhesion and inter- / inner molecule forces (cohesion) of the adhesive. Note: in contradiction to welding bonding is a non-thermal processing. Corresponding to the education of International Welding Engineers IWE according the IIW Guidelines, a similar education for becoming a European Adhesive Engineer is available according to DVS/EWF 3309. Figure 20 to 21 show some examples of bonding applications.

Figure 20: construction related bonding of an automobile windscreen

Figure 21: skin-cut healthcare via adhesive bonding

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As the adhesion processing takes place by chemical processing, the adhesive choice is important and the following must be taken into consideration: ●

What is the maximum load being applied on the adhesive? Adhesives have different strain- and tensile strengths.

●

Which materials are being used? Special adhesives with specific chemical characteristics are designed for specific materials.

●

What is the highest environmental temperature? Some adhesives attain their maximum cure level or highest strength only at higher temperatures.

●

Are there any other influences like humidity, extreme solar irradiation or gaseous atmospheres? Under these circumstances the adhesive could lose its bonding properties very fast.

Basically a surface to be bonded must be pre-treated before the actual bonding processing is being applied. This pre-treatment includes the following: ●

Surface cleaning (eventually with solvents) in order to remove dust, grease or any other particles.

●

Surface roughening via grinding or brushing in order to guarantee the adhesion of the bonding agent.

●

Eventually special treatments like flame treatment, etching or pickling.

2

Basic definitions of welding processes

2.1 Classification according to the type of energy carrier Welding processes are classified according to their type of energy carrier in standard DIN EN 14610. Energy carrier Physical phenomena which make it possible to have the required welding energy either to be transmitted towards the workpiece(s) or to be transformed into the workpiece(s). The following energy carriers with their corresponding numbering are being used in the standard: 1

Solid Body

2

Liquid

3

Gas

4

Electrical discharge

5

Radiation

6

Movement of a mass

7

Electric current

8

Unspecified

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Note: During welding with solids (1), fluids (2), gas (3) or electrical discharge (4) the required energy for welding the workpiece is being transmitted from the outside. However, during welding with radiation, movement of mass or electric current the required thermal energy (or mechanical energy via cold pressure welding) will be generated by energy transition inside the workpiece. For solids, fluids and gas the heat contents is the essential property. Electrical gas discharge and electrical continuity are mechanisms which are providing the welding zone with energy of moving chargecarriers. For the situation of electrical gas discharging this is executed by plasma or sparks and in case of electrical current by resistance heating during which the current is being initiated either through conductivity or through induction. Radiation is energy conduction through diffusion of light waves or through energy carriers. For the movement of mass the essential properties are force and displacement per unit time; different types of movements are translation, rotation and oscillation. 2.2 Classification according to the aim of welding Classification is being made here between joint welding and surface welding. 2.2.1

Joint Welding

Fusion joint welding is the generating of a permanent connection between two or more work pieces by welding.

Figure 22: Resistance spot welding as example for joint welding

For joint welding usually the following joint types are being used:

Butt welds

Fillet welds

Other welds e.g. spot weld

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The type of joint is, for example, defined by: -

Type of the weld joint

-

Type and scope of the preparation, e.g. joint shape

-

The base material

-

The welding process

2.2.2

Surface Welding

Surface welding or surfacing is the creation of a metal layer on a workpiece via welding in order to obtain desired properties and dimensions.

Figure 23: SAW - strip (electrode) surfacing

If the base material and the surfacing material are different a distinction is made, for example, between: Sind der Grund- und Auftragswerkstoff artfremd, wird z.B. unterschieden zwischen: -

Surfacing of armoured protection (plating) Surfacing of a preferable higher wear resistant material compared to the base material

-

Surfacing of claddings (cladding) Surfacing of a preferable higher chemical resistant material compared to the base material.

-

Surfacing of buffer layers (buffering) Surfacing of an intermediate layer (‘black – white‘ joint)

-

Surfacing for repair welding (wear)

The aim of surfacing is to create a load-resistant connection between both materials. 2.3

Classification according to the physical sequencing of welding.

Classification is being made between fusion- and pressure welding processes. Filler materials can be applied in both processes.

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Fusion Welding

Fusion welding is welding without applying external forces whereby the fusion area(s) has (have) to be partly surface-fused; usually – but not mandatory – molten filler material is being added Some fusion welding processes are shown in the following able in relation to their energy carriers: Fusion welding processes Laser beam welding Arc welding Gas shielded metal arc welding Resistance welding Firecracker welding Submerged arc welding Light radiation welding Casting welding Gravity arc welding Gas welding Plasma welding Table 1:

2.3.2

Energy carrier Radiation Electrical gas discharge Electrical gas discharge Electrical current Electrical gas discharge Electrical gas discharge Radiation Fluid Electrical gas discharge Gas Electrical gas discharge

Some welding processes and their type of energy carrier

Welding with pressure

Welding with pressure is welding during which sufficient external forces are being applied in order to generate a more or less heavy plastic deformation on both fusion areas. Usually without adding filler material. Note: Usually- but not mandatory- the fusion areas of the parts are being heated in order to establish or ease the joining process. Some pressure welding processes and their energy carriers are shown in the following table: Pressure welding processes Arc stud welding Friction welding Flas welding Projection welding Forge welding Magnetic pulse welding Resistance welding Explosion welding Resistance spot welding Table 2:

Energy carrier Electrical gas discharge Movement of mass Electrical current Electrical current Movement of mass Movement of mass Electrical current Movement of mass Electrical current

some pressure welding processes and their type of energy carriers

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Classification according to the level of mechanisation

Welding processes can also be classified according to their level of mechanisation. For this purpose the welding processes TIG and MIG/MAG are shown in the following table regarding their mechanisation level.

Figure 24: classification of welding processes according to their level of mechanisation

Figure 25: Automatic welding with welding boom

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3

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Vocabulary definitions

In the following some common day-to-day definitions of welding technology are shown as examples and for the benefit of understanding: ● Burn-off / burn-out:

Burn-off is defined as the loss of alloying elements due to welding

● Run-on plate:

Piece of metal (or even made from any other suitable material) which is positioned in such a way that the total weld cross-section is obtained.

● Dissimilar materials:

Materials which are significantly different regarding their composition or weldability.

● Similar materials:

Materials which are insignificantly different regarding their composition or weldability.

● Dilution:

inevitable pick-up of base material, filler material or from base material of earlier weld runs or layers in the zone of welding.

● Surfacing/cladding welding: ● Run-off plate:

● Both-side welding:

Manufactured plating/ lining through welding.

Piece of metal (or even made from any other suitable material) which is positioned in such a way at the end of the weld that the total weld crosssection is supported until the end of welding in order to prevent end craters. After welding it is to be removed neatly. Welding in a way that the weld joint is being manufactured from both sides.

● Torch angle:

Angle between the centre-line of the welding torch and a reference plane on the workpiece being projected on a perpendicular plane in reference to the weld direction.

● Single-run welding:

Welding is such a way that the weld or plating/lining is being manufactured in one single layer. Note: The actual welding may consists of one or more weld runs.

● One-side welding:

Welding in a way that the weld joint is being manufactured from just one side.

● Welding simultaneously on both-sides:

Welding is being performed on both sides of the weld joint simultaneously.

● Base/parent metal:

The base material of the workpiece to be welded without consideration of platings/ linings.

● Tack welding:

Positioning of the workpieces or assemblies to be welded in a defined order by means of weld points or short weld runs.

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● Multi(ple)-layer /multi-run weld: Welding in such a way that the weld or plating/ lining consist two or more layers. Note: The definition could also follow the number of layers (for example: threelayer welding). ● Step-back welding:

Type of welding technique in which short welds are being welding opposite to the main weld direction and where the end of the short weld overlaps the starting point of the earlier weld run.

● All-weld metal:

Solidified filler material after welding. Its elements can also originate from used additives for welding purposes.

● Fusion line:

Border between the, due to welding, molten base material and not-molten base material which remained solid.

● Weld pool / molten pool:

Fluid of molten filler material and base material.

● Backing strip:

An welding aid made of appropriate material in order to prevent the weld pool falling through the root opening during welding. Simultaneously it can be used as back purging of the root pass.

● Weldability:

Weldability is a material characteristic and is being influenced by the way of manufacturing and to a certain extent by the type construction.

● Both-side single-run welding: Welding of a joint on both sides with just one single run each. ● Welding speed / travel speed: Speed of the welding in the direction of the joint to be weld. ● Weld metal:

The solidified material after welding consisting base material or filler material and base material. Its elements can also originate from platings/ claddings and/or from used additives for welding purposes.

● Weld(ing) time:

Time interval during the actual welding processing.

● Weld / seam:

Area of the welding joint which comprises the total joining of the workpiece(s).

● Welding process:

The activity/operation of welding.

● Weld run sequence:

The sequence of the weld runs being applied in the weld joint or in the applied surfacing/ cladding.

● Weld joint:

Is the area through which the parts are being joined. The type of joint is determined by the constructive layout of the parts.

● Weld zone:

Local, limited area in which the material is being melted during welding and in which the actual joining is being established

● Filler metal:

Metal which contains the filler material.

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● Preheat temperature (Tp): The actual temperature of the workpiece in the welding zone just immediately before the start of the weld processing. Note: Usually it is being specified as minimum value and is more or less identical to the interpass. ● Heat-affected zone (HAZ): Area of the non-molten base material suffering microstructural changes due to the thermal energy that has been put in. ● Gas backing / purging gas:

Gaseous welding aid which prevents the root backside from getting oxidised and which helps to reduce the chance of weld pool falling through during welding.

● Pickup /pick-up:

Difference between the (lower) element analysis of the filler material before welding and the (higher) element analysis of the pure weld metal after welding.

● Two-layer welding:

Welding of the weld joint or surfacing / cladding by means of two weld layers.

● Interpass temperature (Ti): The actual temperature of the workpiece in the welding zone just immediately before the start of the weld processing of the next run. Note: Usually specified as a maximum value (see ISO 13916)

4

Designation, reference- and classification numbers

For the purpose of harmonising the European Market and due to the existing diversity of national designations and abbreviations regarding welding and brazing/soldering processes, the ISO/TC 44 Technical Committee established an international valid system of reference numbering for all welding and brazing/ soldering processes. The valid reference numbers of the welding processes are defined in DIN EN ISO 4063 All welding procedures starting with number 1 belong to the arc welding process. Processes starting with number 11 do belong to metal arc welding without gas protection, for example 111: manual metal arc welding. All submerged arc welding do have the number 12, for example: 121 Submerged arc welding with solid wire electrode, 122 Submerged arc welding with strip electrode.

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The welding processes of gas-shield metal arc welding do have the number 13, for example: 131 MIG welding with solid wire electrode 135 MAG welding with solid wire electrode 136 MAG welding with flux cored electrode All resistance welding processes start with number 2 and the resistance spot welding processes start with number 21. In the following table some welding processes are shown with their corresponding USA designation and reference numbers according to DIN EN ISO 4063: Welding process DIN EN 14610 Metal arc welding without gas protection Manual metal arc welding Self-shielded tubular cored arc welding Submerged arc welding Gas-shielded metal arc welding MAG welding with solid wire electrode MIG welding with solid wire electrode Gas-shielded arc welding TIG welding with solid filler material (wire/rod) Plasma arc welding Laser welding Electron beam welding Resistance welding Resistance spot welding Projection welding Resistance stud welding Wire seam welding Flash welding Resistance butt welding Electroslag welding Friction welding Explosion welding Table 3:

Designation (USA)

Reference number acc.to DIN EN ISO 4063

-

11

SMAW

111

FCAW

114

SAW GMAW

12 13

MAG

135

MIG

131

GTAW

14

TIG

141

PAW LBW EBW RW RSW PW RSW FW UW RES FR EXW

15 52 51 2 21 23 26 22 24 25 72 42 441

Examples of fusion and pressure welding processes

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SFI / IWE 1.01 Page 27

Overview of metal welding processes

Below you see a detailed overview of fusion welding processes. Fusion Welding Processes

Flow (liquid) welding

Gas welding

Liquid heat transfer welding

Aluminothermic welding

Electron beam welding

Light radiation welding

Arc welding

Metal arc welding without (shield)gas protection

Gas-shielded arc welding

Manual metal arc welding

Gravity welding

Firecracker welding

Self-shielded tubular cored arc welding

Gas-shielded narrow gap welding

Electrogas welding

Plasma welding

MIG welding

CO2 welding

Resistance fusion welding

Submerged arc welding

Gas-shielded metal arc welding

Electroslag welding *

Gas-shielded arc welding with non-consumable electrode

MAG welding

TIG welding

(Wolfram) Plasma welding

Atomic-hydrogen welding

Mixed gas welding

Manual metal arc welding (111)

Submerged arc welding (12)

Electron beam welding (51)

Laser welding (52)

Gas-shielded metal arc welding (13)

Gas welding (3)

Figure 26: examples of schematic representation of fusion welding processes

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Below you see a detailed overview of pressure welding processes. Pressure Welding Processes

Cold pressure welding

Friction welding

High (mechanical) energy welding

Ultrasonic welding

Forge welding

Resistaqnce butt welding

Explosion welding

Oxyfuel gas pressure welding

Flash welding

Resistance welding

Resistance spot welding

Arc pressure welding

Diffusion welding

Electroslag welding *

Electroslag welding *

Projection welding

Resistance seam welding

Foil butt-seam welding

Explosion welding (441)

Resistance seam welding (22)

Indirect spot welding (211)

Friction welding (42)

Indirect projection welding (231)

Direct spot welding (212)

Flash welding (24)

Resistance butt welding (25)

Figure 27: examples of schematic representation of pressure welding processes

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SFI / IWE 1.01 Page 29

Selection of welding processes

For selecting the appropriate welding process the following aspects should be considered:

Accessibility

Component‘s Geometry Fixtures / equipment

Requirements Choice of welding process

Economics

Work safety Base material

Welding position Weld location

● Quantity:

In particular the quantity and the level of weld process mechanisation determines the selection.

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● Base material: Specific base material combinations can only be welded through certain welding processes.

● Geometry of the component: In particular the size of the component is a very important criteria for the selection of the welding process

● Economic efficiency:

Costs of investments, production, filler- and auxiliary materials.

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● Facility layout: considerations whether or to what extent a welding process can be executed by appropriate facilities /layouts.

● Requirements:

considerations of operating- and environmental requirements.

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● Accessibility:

In case of limited accessibility the selected welding process must fit to the required weld quality conditions.

● Weld position:

Not all welding processes can be applied in any welding position.

A restriction of the application areas of the welding processes can be applied according the following considerations/ influencing factors:   

   

acc. to the base materials to be welded acc. to the shape of the component (Geometry, sizes) Plate- or weld size areas acc. to the Economic efficiency Efficiency in weld length/ time Deposition efficiency Manufacturing costs acc. to the quantity / time (required quantity) acc. to manufacturing considerations or to the available technic level of the equipment single-production, series-production, mass-production acc. to technological considerations (Quality requirements, scope of testing, reliability) acc. to structural considerations, acc. to the type of loading (predominantly static, dynamic, service-life, reliability)

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DVS Leaflett 2938 „TECHNISCHE UND WIRTSCHAFTLICHE KRITERIEN ZUR AUSWAHL DER FUEGEVERFAHREN IM FEINBLECHBEREICH BIS 3 MM EINZELBLECHDICKE“ shows which technical and economic considerations are to be taken into account for thin sheet manufacturing.

7

Health and Safety

As in many other working areas, many risks jeopardize the welder’s health and physical condition. The acute risks of injury by heat, electric current or crushing hazards are to be seen in a similar way as the long term health risks by fumes, gases and dust but also the possible negative influences of radiation and electromagnetic emissions to the welder’s health. See also chapter 4.05.

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8 (1)

In which standard is „metall welding“ being defined?



The Solidus temperature of the base material will be exceeded The Solidus temperature of the base material will not be exceeded One of the applications of brazing/soldering is the surfacing of a base material Brazing/ soldering cannot be used for the surfacing of a base material Brazing/ soldering is a thermal processing for the positive substance joining and surfacing

Which statements regarding DIN EN 14610 are correct?

     (5)

A filler material is always used during welding During pressure welding a filler material is never being used During pressure welding a filler material is always being used Welding is the joining of materials with the frequent application of heat and/ or force with or without filler material The use of welding additives can ease or establish the welding process

Which of the following statements regarding brazing/soldering are correct?

     (4)

DIN EN 14610 DIN 1910, Teil 2 DIN EN 4711-1 DIN EN 12345 DIN 1901, Teil 1

Which of the following statements regarding welding are correct?

   

(3)

Page 34

Knowledge Questions

     (2)

SFI / IWE 1.01

In this the definitions regarding work processing of welding are being determined In this the processes of metal welding are being listed In this only the pressure metal welding processes are being listed In this only the fusion metal welding processes are being listed In this the processes for fusion- and pressure metal welding are being separated

In which way are the movement- and working sequences being defined for TIG welding and MIG/MAG welding?

    

Mechanical torch positioning Manual torch positioning Manual filler material feeding Mechanical filler material feeding Manual work piece positioning

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TIG welding Resistance seam welding Resistance spot welding Gas welding Metal arc welding

Typical pressure welding process representatives are:

     (11)

Gas welding Friction welding manual metal arc welding Submerged arc welding Flash welding

The arc is being used as energy carrier for the following welding processes?

     (10)

R FW WS RW RP

Which of the following welding processes are fusion welding processes?

     (9)

All arc welding processes start with reference number 3 All arc welding processes start with reference number 1 All gas-shielded metal arc welding start with reference number 13 All gas-shielded metal arc welding start with reference number 32 All submerged arc welding processes start with reference number 12

Which designations letters are being used for the classifications of resistance welding?

     (8)

Page 35

Which of the following statements regarding the reference numbers of welding processes are correct?

     (7)

SFI / IWE 1.01

Resistance spot welding Resistance projection welding Gas-shielded metal arc welding Forge welding Oxyacetylene welding

The applied welding process can be selected according to which criteria?

    

Dimensions of the manufacturing location Geometry of the component Qualification of the welding coordinator Economic efficiency of the welding process Accessibility of the weld

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SFI / IWE 1.02

Oxy-gas welding and related processes

Chapter 1.02:

Page 1

Oxy-gas welding and related processes

Contents 1

Welding gases ..................................................................................................................... 3 1.1 Acetylene (C2H2) ............................................................................................................. 3 1.2 Propane (C3H8) ............................................................................................................... 5 1.3 Methane (CH4) ................................................................................................................ 5 1.4 Hydrogen (H2) ................................................................................................................. 5 1.5 Ethylene (C2H4) .............................................................................................................. 5 1.6 Propylene (C3H6) ........................................................................................................... 5 1.7 Oxygen (O2) .................................................................................................................... 6 1.8 Handling of pressurised-gas cylinders ............................................................................ 7

2

Oxy-acetylene flame ............................................................................................................ 8 2.1 Setting the flame ............................................................................................................. 8

3

Pressure regulator (DIN EN ISO 2503) ............................................................................... 9

4

Hoses and hose connections for gases .......................................................................... 10

5

Welding torch (injector or injector pipe) DIN EN ISO 5172 ............................................ 10 5.1 Design and operating principle ..................................................................................... 10

6

Safety Devices ................................................................................................................... 13 Outlet safety devices for cylinder battery systems ............................................................... 13 Protection of single cylinders ............................................................................................... 14

7

Oxy-acetylene welding ...................................................................................................... 15 7.1 Cost-effective area of application.................................................................................. 15 7.2 Techniques (LW/RW welding) ...................................................................................... 15 7.2.1 Leftward welding ................................................................................................ 15 7.2.2 Rightward welding.............................................................................................. 15 7.3 Welding rods for gas welding (DIN EN 12536) ............................................................. 16 7.4 Designation of gas welding rods ................................................................................... 16 7.5 Marking ......................................................................................................................... 16 7.6 Joint type with gas welding ........................................................................................... 17

8

Related processes ............................................................................................................. 18 8.1 Flame straightening ...................................................................................................... 18 8.1.1 Working rules for flame straightening................................................................. 19

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8.2 Soldering and brazing ................................................................................................... 22 8.3 Oxy-fuel flame cutting ................................................................................................... 23 8.4 Flame heating ............................................................................................................... 23 8.5 Flame cleaning ............................................................................................................. 24 8.6 Cutting with the oxygen lance ....................................................................................... 25 9

Knowledge questions ....................................................................................................... 26

10 Bibliography ...................................................................................................................... 28

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The oxy-fuel gas technology includes all working procedures, in which materials are exposed to the reaction of an oxy-fuel gas flame or an air-fuel gas flame. Welding gases are all fuel gases and pure oxygen used in oxy-fuel technology.

1 1.1

Welding gases Acetylene (C2H2)

Acetylene can be used as fuel gas for all operations in oxy-fuel technology. For gas welding only acetylene is used. The reasons for this are the high flame temperature and the high flame efficiency of the gas. Further advantages are the concentrated heat input and the reducing (carburising) effect of the flame. Acetylene is a chemical compound of carbon and hydrogen. It occurs when calcium carbide is brought together with water. The by-product is lime sludge. Acetylene is increasingly being made from mineral oil. Properties and hazards during handling of acetylene Acetylene is a colourless, non-toxic, but slightly narcotic gas. In its pure state, it is odourless. Commercial acetylene contains traces of impurities which give the gas its garlic-like odour. Under conditions of increased temperature and pressure, it tends to decay into its components carbon and hydrogen. The maximum overpressure in supply lines is therefore limited to Maximum 1.5 bar over pressure Acetylene is explosive at a concentration of 2.4% to 80% in air. It is lighter than air and rises. It reacts with copper and copper alloys with over 70% copper content as well as with silver and silver alloys. Storage of Acetylene Acetylene is broken down at higher pressure into its components carbon and hydrogen, and must therefore be stored differently to any other fuel gases. To prevent the decay, acetylene must be stored in several small chambers. This is achieved by a porous mass that is installed in the steel cylinder. To increase the storage ability further, acetone is added to this porous mass in which acetylene dissolves.

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Acetylene Cylinder Protective cover Cylinder valve Outlet connection

Table 1: Delivery types (high-pressure gas cylinders, solvent acetone) Type

Volume (litres)

Weight with filling (kg)

Filling Pressure (bar)

Acetylene content (kg)

10 20 40 48 50

10 20 40 40 50

23 42 74 76 77

18 18 18 19 19

1.6 3.3 6.3 8.0 10.0

Red ring Identification colour

Highly porous mass

The dissolving power of acetone depends on the pressure and the temperature. 1 litre of acetone dissolves approx. 25 litres of acetylene at 15°C and 1 bar pressure. At a pressure of approx. 20 bar, 1 litre of acetone can dissolve up to 500 litres of acetylene. In addition to pressure, the dissolving power also depends on the temperature. Low temperatures: High dissolving power High temperatures: Low dissolving power Consequence: Change in gas pressure for same content. The gas pressure of e.g. a newly filled acetylene cylinder falls from approx. 19 bar at 20°C to approx.12 bar at temperatures below 0°C. An indication of the actual content via the cylinder pressure is only possible to a limited extent.

Figure 1: Acetylene cylinder

Working rules   

The consumption rate during continuous operation is limited to 500 – 700 l/h. Briefly (up to 20 min.) 1,000 l/h may be consumed. (The indicated values refer to a 40l cylinder). If you exceed the maximum permissible consumption rate, the solvent cannot release acetylene fast enough. The solvent is drawn out of the cylinder and damages pressure regulators, safety devices and the fuel gas hose. If a larger amount of acetylene is required, several single cylinders are to be connected via cylinder connectors. Care should be taken to ensure almost the same contents pressure and the same type of solvent. Alternative: Use cylinder bundles. Sample calculation for extraction: Formula: Average value of the welding torch x 100 E.g. torch size 4-6: (4+6) = 5 x 100 = 500 l/h 2 size 20-30 torch

E.g. torch size 20-30: (20 + 30) = 25 x 100 = 2,500 l/h 2

As in the final example the permissible consumption rate of a single cylinders is exceeded considerably, 4 cylinders are to be connected together in continuous operation.

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1.2 Propane (C3H8) Propane is a colourless, non-toxic, but slightly narcotic gas. Propane is primarily generated from crude oil. It is easy to handle because it can be stored in liquid state at room temperature under its own vapour pressure of only 7 bar. Propane cylinders must not be exposed to high temperatures. At high temperatures the gas expands and fills the entire cylinder volume. There is a risk of the cylinder bursting. Propane has a higher heat value than acetylene, but it has a considerable lower flame efficiency when welding, which is particular important during welding. It is therefore not suitable for welding. The flame temperature is lower than that of the oxyacetylene flame and the oxygen quantity required for combustion is almost four times higher than with acetylene. Propane has a low explosion limit and a high density. Leaking gas collects in low-lying areas. 1.3 Methane (CH4) Natural gas consists mainly of methane. The composition depends on the natural gas deposits, so the combustion properties are also different. Natural gas is mainly used for heating purposes. It can be stored in compressed form in cylinders, but is usually supplied to the buyer directly via pipelines. Methane is a light gas and its lower explosive limit is higher than that of most other gases. The heat value is low and little heat is generated by the primary flame. 1.4 Hydrogen (H2) Hydrogen is a colourless, odourless and non-toxic gas. It is the lightest of all gases. It is a highly inflammable gas and burns with an invisible flame. Hydrogen is manufactured industrially by the electrolysis of water. It is transported as a gas under high pressure in cylinders or in liquid state. 1.5 Ethylene (C2H4) Ethylene is a colourless gas with sweetish, slightly mouldy smell. It is slightly toxic. It can be used for flame cutting and similar processes. The heat value is approximately the same as with acetylene, but less heat is generated by the primary flame. 1.6 Propylene (C3H6) Propylene is a colourless gas with a slightly sweetish smell. It is a non-toxic, but has a slight narcotic effect. It can be used for flame cutting and similar processes. Propylene is supplied as a liquefied gas. The properties are similar to those of propane. Table 2: Physical Properties of fuel gases Fuel gas

Hydrogen Acetylene Propane Natural gas

Heat value MJ/m3 10.8 57.0 93.2 36.0

Combustion velocity m/s 8.9 13.5 3.7 3.3

Flame temperature °C 2,500 3,150 2,750 2,770

Flame power kW/cm 13.98 42.74 10.27 8.51

2

Density kg/m 0.08 1.09 1.88 0.67

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3

Explosion limits in air % 4.0…74.5 2.4…80.0 2.0…9.5 5.0…15.0

Welding processes and equipment

Oxy-gas welding and related processes 1.7

SFI / IWE 1.02 Page 6

Oxygen (O2)

Oxygen production/properties/storage The manufacture of oxygen is carried out almost exclusively by air liquefaction with subsequent decomposition of the air into its components. This occurs due to the different boiling points of the individual components. The oxygen extracted in this way has a purity of 99,999% (5.0). The standard purity of oxygen filled into cylinders is 99.5% (2.5). Properties and hazards when handling oxygen Oxygen is a colourless, odourless and tasteless gas with a density of 1.43 kg/m 3 (thicker than air). Combustion reactions occur faster than in air even with slightly increased oxygen levels. Above an oxygen concentration of 30%, these can be explosive.    

Oxygen under pressure coming into contact with oil or grease may result in spontaneous ignition. The maximum consumption rate depends on the cylinder size and the maximum flow rate of the pressure regulator. Excessive consumption rates lead to the icing up and freezing of the pressure regulator. Open oxygen cylinder valves slowly, otherwise internal ignition in the pressure regulator may be caused as a result of the pressure surge. Never use oxygen to ventilate containers, rooms etc. In comparison to air the following effects of oxygen are to be considered:  Required ignition energies are considerably lower,  The ignition temperature of the materials is lower,  The combustion temperature and combustion velocities are higher.

Storage of oxygen Oxygen is stored under high pressure in gaseous state in steel cylinders. State of the art gas cylinders predominantly have a filling pressure of 200 bar. Newer cylinders have 300 bar. Protective cover Cylinder valve Outlet connection Identification colour

Table 3: Steel cylinders for gaseous oxygen Type 50 40 10

Cylinder volumes (litres) 50 40 10

Cylinder pressure (bar) 200 150 200

Oxygen quantity (litres) 10,000 6,000 2,000

The maximum consumption rate depends on the cylinder size and the maximum flow rate of the pressure regulator. Base ring

Figure 2: Oxygen cylinder

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Storage can also be in liquid form in thermally insulated tanks. In an evaporator liquid oxygen is reconverted into gaseous oxygen. 1 litre of liquid oxygen produces about 850 litres of gaseous oxygen.

Figure 3: Cold gasifier system

1.8         

Handling of pressurised-gas cylinders Pressurised-gas cylinders must not be thrown, struck or rolled when lying. Only transport, store and deliver with a safety cap attached. Protect from falling using chains or clamps. Do not install in corners or near to stairs or narrow passages. Do not open oxygen cylinders abruptly (stored heat). Protect against strong heating. Refilling from large to small cylinders requires specialised knowledge and is therefore not permissible. When transporting gas cylinders, national and international regulations regarding the transport of hazardous goods via road, rail and inland waterways must be observed. Pressurised-gas cylinders must be regularly inspected.

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2

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Oxy-acetylene flame

The flame results from ignition of the gas mixture which flows out of the torch nozzle. The flame is composed of the flame cone and the outer flame. In the flame cone, a partial combustion of acetylene takes place with the oxygen supplied from the cylinder. In the outer flame, the complete combustion of the gas takes place with oxygen from the air. The maximum temperature is approx. 3,200°C at a distance of 2 - 5 mm after the flame cone. The welding flame not only has the task of melting the surfaces to be joined and the filler material but also of protecting the weld pool against negative influences from the air.

Figure 4: Oxy-acetylene flame

2.1    

Setting the flame For the welding of ferrous metals an acetylene/oxygen ratio of 1:1 is set (normal flame). Excess acetylene has a carburising and hardening effect. Excess oxygen leads to the oxidisation (combustion) of the material. The flame can be adjusted to be hard or soft depending on the setting of different gas volumes using the regulating valves.

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Page 9

Pressure regulator (DIN EN ISO 2503)

The pressure regulator has the task of reducing the gas pressure to the working pressure and keeping it constant when consuming gas. Primary (Cylinder) Pressure Manometer

Pressure Manometer for working pressure Regulator valve Nonreturn valve

Blow-off valve Recoil spring Cylinder connection

Housing Dirt filter

Regulator pin

Diaphragm Membrane

Regulator valve spring

Hose connection

Spring cap Relief bore Adjusting screw

Figure 5: Pressure regulator

Design characteristics   

Oxygen pressure regulators must be resistant to internal ignition, all parts must be kept oil and grease free. Acetylene pressure regulators must be designed and manufactured such that the maximum back pressure of 1.5 bar cannot be exceeded. A dirt filter must be integrated.

Table 4: Recognition features of pressure regulators Inscription Code letter Cylinder connection Hose connection

Oxygen

Acetylene

Propane

O

A

P

R ¾" Right-hand thread

Clamp connection

W 21.8 x 1/14" left-hand thread

Right-hand thread

Left-hand thread

Left-hand thread

Working method The pressure regulator is a membrane-controlled valve. When the adjusting screw is turned in, the regulator valve is being adjusted via the regulator valve spring, diaphragm and regulator pin lift. The adjusting screw is therefore used to set the working pressure. The flowing gas exerts a back pressure on the membrane. During gas consumption equilibrium occurs at the membrane between the force of the set spring and the gas back pressure and the force of the regulator valve spring. Working rules   

Before connecting the pressure regulator check the cylinder connection for cleanliness (blow out) and check the seal. The adjusting screw must be relieved if the pressure regulator is not in operation. Leak detection spray is to be used to check the seal. Soap solution is not permitted with oxygen due to the possible grease content.

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4

Page 10

Hoses and hose connections for gases

Table 5: Identification colour codes and nominal parameters of gas hoses Features Internal diameter in mm Identification colour

Oxygen 4.0;

5.0; 6.3;

Acetylene, hydrogen 8.0; 10.0;

blue

12.5; red

16.0;

Propane, natural gas

Shielding gases

20.0; orange

black

Examples

Right-hand thread

Left-hand thread with Left-hand thread with surface notch surface notch

Right-hand thread

Connections

Working rules      

5

Burned and porous gas hoses must be replaced. Tying wire must not be used for fastening. Gas hoses must not be hung over the cylinders. For the joining of gas hoses double hose coupling nipples are to be used. With acetylene no copper or copper-bearing materials with more than 70% copper may be used. The minimum length of the gas hoses is three metres.

Welding torch (injector or injector pipe) DIN EN ISO 5172

In the welding torch acetylene and oxygen are mixed. The mixture ratio is kept constant. The flow rate of the gas mixture is adapted to the ignition speed and the flame cone is formed. 5.1

Design and operating principle

The welding torch is made of the main parts handle (with hose connections and torch valves) and the welding attachment (with injector, mixer, mixing tube and welding nozzle). The injector consists of the pressure and the suction nozzle. The oxygen flows through the pressure nozzle at an operating pressure of 2.5 bar. Oxygen causes a suction effect in the area of the suction nozzle due to its high flow rate when escaping from the pressure nozzle. Acetylene flows with a pressure of 0.2 to 0.7 bar into the suction nozzle and is drawn into the mixing hose by the oxygen stream, mixed with oxygen in the mixing tube and ignited at the outlet of the welding nozzle.

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Figure 6: Welding Torch Circular channel

Oxygen Fuel gas

Welding attachment

Suction nozzle Union Nut

Figure 7: Injector area of an injector-type blowpipe (detail A)

Inscription

 A i 1 S 2.5 bar

= = = = =

Handle Pressure Nozzle

Manufacturer's mark Gas type (Acetylene) Injector-type blowpipe (suction blowpipe) Size 1, workpiece thickness of 0.5 - 1 mm which is welded with this welding attachment Working pressure to be set for oxygen

Malfunctioning of the injector-type blowpipe Popping Characteristic: Cause: Remedy:

Banging noise, explosive spraying of the weld pool. Discharge speed lower than ignition speed blowpipe nozzle expanded. Replace blowpipe nozzle, set flame larger.

(flame

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set

too

small);

Welding processes and equipment

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SFI / IWE 1.02 Page 12

Backfiring Characteristic: Cause: Remedy:

Sudden extinguishing of the flame, clear, shrill whistling, strong heating of the welding attachment. Too strong heating of the welding attachment, several blow outs as a result, Loose welding nozzle (torch nozzle). Immediately close both valves on the handle and the let the torch cool.

Flashback Characteristic: Cause: Remedy:

Loud explosive bang, bursting of the acetylene hose in several locations, strong soot generation, peculiar smell. Serious reduction in the flow rate in particular with large torches, continuing backfiring, leaking connection between handle and welding attachment, e.g. loose union nut. Close both cylinder valves immediately. Remove pressure regulator from the acetylene cylinder and check the cylinder.

Testing the torch (suction test) The function of the torch can be tested by means of a suction test. Procedure: 1. Make the oxygen ready for operation, i.e. set the pressure. 2. Shut off acetylene supply (close cylinder). 3. Unscrew acetylene hose from the handle. 4. Open the acetylene and oxygen valves on the handle. If the welding torch is functioning correctly, a clear suction effect can be felt at the acetylene connection of the handle. Working rules    

Firmly tighten the union nut on the handle. Set pressures correctly (oxygen in accordance with marking, acetylene approx. 0.4 bar.) Sequence when igniting the flame: First open the oxygen valve at the handle, then open the acetylene valve, ignite the flame and set it. Sequence when extinguishing the flame: First close the acetylene valve, then close the oxygen valve.

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Page 13

Safety Devices

Incorrect operation or defective servicing of welding or cutting equipment can cause flashbacks from the torch into the fuel gas cylinder. It can result in internal ignition in pressure regulators and pressure equipment which causes bursting of the hose, destroying of the welding torch or the cylinder to explode. The reasons for the flashbacks are for example dirty torch nozzles, loose connections between welding or cutting torch head and handle or defective gaskets in the area of the torch. These faults cannot always be ruled out, so appropriate safety devices are required. The accident prevention regulations stipulate that each outlet for consuming equipment (e.g. welding torch) in which acetylene is burned with oxygen or compressed air is to be equipped with a safety device. This means that there are different requirements for acetylene cylinder battery and individual cylinder systems. Outlet safety devices for cylinder battery systems At each outlet of an acetylene cylinder or an acetylene cylinder battery system safety devices must be installed. They must prevent:  Flashbacks from the torch into the supply system  Return of oxygen into the supply system for fuel gas  Continuing flow of fuel gas following a flashback The safety devices are equipped with a flame arrester (Sinter Metal), a gas back-flow nozzle and a cut-off valve. Temperature controlled

Pressure trolled

con-

Dirt filter Warning lever Flash-back arrester (Pressure controlled) Back Flow Nozzle Flame arrester (Sinter metal)

Flash-back arrester

(temperature controlled)

Pressure-relief valve

Figure 8: Outlet safety devices

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Protection of single cylinders If only one individual consuming device is connected to an acetylene cylinder, a single cylinder safety device can be used in place of the outlet safety device. It protects against back-flow and flash, but has no flashback arrester. Back Flow Nozzle

Figure 9: Single cylinder safety device

Flame arrester (Sinter metal)

Installation types Single cylinder safety devices can be integrated   

immediately after the pressure regulator in the hose directly on the handle

Figure 10: Single cylinder fuse on the handle

Single cylinder safety device

Supply of several consumers from a single cylinder Taking into account the permissible consumption rate of an acetylene cylinder (max. 700l/h under continuous operation) it is possible to connect for example two welding torches to one acetylene cylinder. Protection against flashbacks is provided with two outlet safety devices. Single cylinder safety devices are not sufficient here.

Figure 11: Supply of several consumers from one acetylene cylinder

Periodic testing of safety devices According to current standards, safety devices against flash-back must be tested at least once a year for gas flow-back, tightness and flow rate.

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Oxy-acetylene welding

Description of the welding process The heat source for welding is a flame which is created with the fuel gas acetylene and pure oxygen. The gases flow through the welding torch with a mixing ratio of 1:1. Acetylene is ignited and burned after emerging from the welding torch. The flame temperature is approx. 3,200 °C. To burn acetylene completely, oxygen from the air is required. The filler material required for welding is additionally added. 7.1   

Cost-effective area of application Welding of sheet metals and pipes made from non-alloyed steels up to approx. 5 mm Repair welding Pipeline construction, installation sector

1 = Oxygen Cylinder 2 = Acetylene Cylinder 3 = Outlet safety devices 4 = Oxygen hose 5 = Acetylene hose 6 = Handle 7 = Welding rod 8 = Welding nozzle 9 = Workpiece 10 = Welding flame

Figure 12: Component parts of a single-cylinder system

7.2 Techniques (LW/RW welding) There are two techniques for producing welded joints: 7.2.1 Leftward welding Guide welding torch in a straight line, move the welding rod in a dabbing action (the welding torch follows the welding rod). Advantages: Disadvantages:

Smooth or only slightly scaled weld surface; favourable use up to 3 mm workpiece thickness. Easily moulded weld pool, complete fusion difficult to control.

7.2.2 Rightward welding Guide welding torch in a straight line, move the rod in a circular motion (the welding rod follows the welding torch). Advantages: Disadvantages:

Targeted heat input, ensured complete fusion, lower cooling speed, better protective effect of flame. Difficult to use below 3 mm workpiece thickness. High requirement for the manual skill of the welder.

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7.3 Welding rods for gas welding (DIN EN 12536) For the joint welding of general structural steels and heat-resistant steels the welding rods are divided into six rod classes. Information on which steel grades are to be coordinated with the respective rod classes is given in the following table.

16Mo3

X

X

X

X

10CrMo9-10

P 295

X

13CrMo4-5

P 235 P 265

P 235

L 235 L 245 L 290 L 360

S 355

16Mo3

X

P 265

Suitable welding rod class

I

S 235 S 275

S 185

1)

Steel type

Sheet metal and strips made of heat-resistant steels according to DIN EN 10028

Steel type

Pipes according to DIN EN 10216

General structural steels according to DIN EN 10025

Steel tubes for pipelines according to DIN EN 10208

Table 6: Allocation of gas welding rods to different base materials

X

II

X

X

III

X

X

X

X

X

IV

X

X

X

X

X

X

V VI

X

2)

X

2)

1)

Weldability of steel S185 is limited Multi-pass welding. Note: Gas welding of heat-resistant steels, e.g. 13CrMo4-5 etc., no longer corresponds to the state of the art. The allocation of the relevant rods is therefore rather theoretical in nature. 2)

7.4

Designation of gas welding rods

The designation comprises of the name, the DIN EN number, the designation for gas welding and the welding rod class. Example: 7.5

Welding rod DIN EN 12536 - O III

Marking

The welding rods must be provided with a permanent, clearly recognisable class designation (labelled with Roman numerals). An additional colour marking at the rod ends is possible

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Page 17

Joint type with gas welding

The joint type to be selected depends on     

Material Material thickness Welding process Welding position Accessibility

DIN EN ISO 9692 contains the types of joint preparation for welding processes. Table 7: Joint forms for butt welds, welded on one side

Measure in mm

Workpiece thickness

Designation

Weld joint

Joint type Dimensions:

Symbol

Representation

Angle

Gap

Cross-section

Root Flank face height height c h



b

t2

Butt weld

-

-

-

-

t4

Square butt weld

-

b=t

-

-

t>4

Single-V butt weld

30°    60°

b4

c2

-

Table 8: Types of groove weld for fillet welds, welded on one side

Designation

t1 > 2 t2 > 2

Fillet weld, T-joint

t1 > 2 t2 > 2

Fillet weld, Lap joint

t1 > 2 t2 > 2

Fillet weld, Corner joint

Symbol

Usually without filler material -

Measure in mm

Weld seam Workpiece Thickness

Remarks

Joint type Representation

Cross-section

Dimensions: Angle Gap  b

70 °    100 °



60 °    120 °

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b2 (target is b = 0)

b2 (target is b = 0)

b2

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Oxy-gas welding and related processes 8

Page 18

Related processes

8.1 Flame straightening Formation of shrinkages and stresses in the workpiece If a metal is heated uniformly, it expands and becomes larger. The subsequent cooling shrinks it again. When the initial temperature is reached, it has its initial dimensions again. It does not shrink beyond the initial level. The workpiece behaves differently if expansion is prevented or the component is only partly heated. During heating the softest point – the point of heating – upsets (deforms). When cooling the workpiece, shrinking occurs around the deformed area. Bending (angular distortion) or tension results if the material is clamped.

L = Longitudinal shrinkage Q = Transverse shrinkage D = Thickness shrinkage W = Angular shrinkage

Figure 13: Shrinkage types

Straightening process With flame straightening, the component is quickly, specifically and locally heated into the plastic range. The temperature at which plastic deformation occurs is approximately 550°C for steel and, for aluminium and its alloys around 350°C–400°C. Upsetting occurs due to obstructed thermal expansion (an important requirement for flame straightening). In order to achieve upsetting, auxiliary materials are required, which prevent expansion. During cooling, the workpiece shortens around the deformed area which leads to the desired length or shape change. Contrary to mechanical straightening in which the “short side” is stretched, with flame straightening there is a shortening of the “long side”. The final result of straightening only becomes visible on reaching the ambient temperature. Four factors cause flame straightening: Heating  Prevented expansion length during cooling



Upsetting (deformation)



Reduction

Figure 14: Heating and prevented expansion

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in

Oxy-gas welding and related processes

8.1.1

Working rules for flame straightening

1.

Measuring

SFI / IWE 1.02 Page 19

Shape and size of dimensional deviation are determined by measuring the workpiece. With flame straightening workpieces can be only shortened. Welds contract after welding and are shorter than their surroundings. For this reason, never heat on the weld seam. Determine flame straightening and its position and where necessary mark it. 2.

Prevention of thermal expansion

During the heating process the workpiece expands. In order to achieve good straightening results, expansion must be prevented. This can take place through the component's own weight and shape or by additional measures. 3.

Selection of fuel gas and torch

Acetylene is recommended as the fuel gas. Other fuel gases, such as propane or natural gas have a too low flame efficiency and flame temperature to achieve quick and concentrated heating. Areas next to the straightening point are also heated. Bulges can occur as a result. The torch size depends on the size of the structure, the material and the material thickness. 4.

Create locally limited heat accumulation

Successful straightening depends on local and targeted heat accumulation. The areas are to be kept small. Several small patterns figures work better than one large pattern figure. Heat wedges must be strictly limited. At the flame straightening point the material must be plasticised. In the plastic range, the yield point is very low, and the material in the heated area is upsetted. During cooling the material shrinks and achieves the desired deformation. 5.

Shrinkage

The material shrinks as long as it has not yet reached its ambient temperature. Clamping means which are used to prevent expansion are gradually released. The straightening process can be accelerated, but not improved, by cooling with compressed air or water. This straightening must be checked by measuring. Only after this, any necessary new straightening points are determined.

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Prevention of thermal expansion The upsetting (deformation) of the straightening point is a precondition for straightening. If the component is not rigid enough, additional measures must be put in place to prevent thermal expansion during the heating procedure. An additional prevention of thermal expansion from the outside is particularly important in less rigid components.

Figure 15:

Prevention by own weight

Figure 16:

Prevention by the inherent rigidity

Figure 17:

Prevention by additional restraint

Heat patterns with flame straightening In order to achieve optimal straightening results, different heating patterns are used depending on the component and the deformation. Heat can be applied as a heat point, heat line, heat wedge, heat oval or as a combination of several heat patterns. Heating point The heat point is preferably used for flame straightening thin plates, for the removal of buckling. It must be small. The workpiece is heated through in order to achieve two-dimensional shortening of the component. Many small points are more effective than one large one. The workpiece is heated from the outer area to the centre. The straightening of components can be performed with the help of perforated plates. The prevention of expansion takes place by clamping the component between a perforated plate and a counterplate. The bores in the perforated plate determine the distance between the individual heating points.

Figure 18: Heating points

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Heat oval Pipes can be straightened simply and effectively with the flame. The main use is the elimination of deformations, which result from one-sided connecting of pipe branches. This deformation is repaired by applying oval-shaped heat spots on the other side of the pipe connection. The tube wall is heated through. The basic rule is: the long side of the oval is always in the pipe longitudinal direction.

Figure 19: Heat oval

Heat lines for the elimination of angular distortion Angular distortion is the most frequent and the most distinctively visible deformation type. It can be removed in many cases by one or more parallel heat lines drawn on the opposite side. It is particularly effective if only 1/3 of the workpiece thickness is heated to the flame straightening temperature. Sheet thicknesses above 4 mm are straightened with 3 parallel lines. Five-line heat flows are used for sheet thicknesses above 8 mm.

Heat lines

Figure 20: Heat lines

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Wedge-shape heating The heat wedge is the heat pattern with the greatest straightening effect. It is mainly used on profiles and vertical ribs to achieve major deformations. The component is always exposed uniformly from the wedge tip outgoing up to the baseline. It is necessary to ensure that the form and the size of the wedge is set to size of the component (1). The heat wedge contour must be strictly defined, sharply pointed and long. The height of the wedge is to be chosen in such a way that the wedge tip just crosses the bending line of the profile (3). With this procedure the rigidity of non-heated material areas is used to prevent expansion.

Baseline of the heat wedge Bending line

Figure 21: Heat wedge

It is recommended to mark the shape of the heat wedge on both sides of the component in order to ensure an as exact as possible opposite heating. Heating is performed from the wedge tip to the baseline of the wedge (best upsetting). 8.2

Soldering and brazing

Soldering and brazing are thermal processes for joining and surfacing materials with the help of a molten filler material - the solder and if necessary flux. The working temperature of the solder is below the melting temperature of the materials to be joined. The solder diffuses into the grain boundaries. Adhesion and a type of alloy formation between the base material and the solder occurs. The strength of the solder joint mainly depends on the type of the joint, the properties of the solder and the base material. In soldering technology, we distinguish between soldering, brazing and high-temperature soldering in accordance with the working temperature of the solder. For further information see chapter 1.16.

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Page 23

Oxy-fuel flame cutting

Oxy-fuel flame cutting is a thermal cutting process, in which the main part of the energy required for the process is obtained from the heat released by the combustion of the material. The material to be cut is locally heated by the oxy-fuel gas flame to ignition temperature at the workpiece surface and is then burned by the oxygen stream. The heat resulting from the combustion of the material allows a continuous combustion into the depth and into feed direction. Oxy-fuel flame cutting has the largest application in terms of workpiece thickness. Standard torches are generally suitable for the range of 3 300 mm, special torches up to 1,000 mm and more. For further information see chapter 1.13. 8.4

Flame heating

The term “flame heating” means all applications in which the flame induces heat in a workpiece without melting it. In flame heating, the workpiece is heated to change its characteristics, for example, to reduce deformation resistance. It is also used for preheating when welding, cutting etc. Flame heating is also used during hot-forming, e.g. for bending and flaring of pipes, etc. Here the area to be deformed is locally heated to the correct temperature. The hot-forming temperature is approx. 900°C. Simple welding torches and special torches are used. When heating very large parts the torches are often water-cooled and the ignition and extinguishing processes are effected automatically. Flame heating can be carried out both by manually and mechanised. The measurement of the temperature is performed using temperature indicating crayons, spring- or contact thermometers. When selecting the fuel gases for the different processes certain factors must be considered: 

Is a fast and concentrated heating process of importance?



Is heating through of the workpiece of importance?



Does the water vapour content of the flame play a role?

Table 9: Flame temperatures and water vapour content Type of flame Acetylene/oxygen

Flame temperature °C 3,150

Acetylene/compressed air

2,300

Propane/oxygen

2,800

Propane/compressed air

1,925

Methane/oxygen

2,770

Water content of flame % approx. 3.5 approx. 30 approx. 40

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8.5

SFI / IWE 1.02 Page 24

Flame cleaning

With flame cleaning undesired layers are removed using fuel gas/oxygen torches, e.g. rust scale, paint etc. Following setting, the flame cleaning hand torch is set down on the surface to be worked. The torch head slides on the steel or concrete. with the mechanised torch the nozzles are at a distance of about 1.2 to 2 cm from the surface. The flame cleaning torch must have an inclination angle of approx. 45 degrees to the surface and the tips of the flame cones must touch the surface.

Figure 22: Flame cleaning

Working method The flame cleaning torch may not be applied at an angle. The flame must have an even effect across the entire width of the torch. With steel, inclination of the torch is necessary in the feed direction, but not with concrete. The torch feed rate for steel is 3.0 to 5.0 m/min, for concrete 1.0 to 3.0 m/min. Training Technical personnel for flame cleaning can be trained according to guideline DVS 1147 – flame cleaning –.

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8.6

SFI / IWE 1.02 Page 25

Cutting with the oxygen lance

Flame boring with the oxygen lance is a thermal cutting process. It is used for the flame boring of mineral or metallic materials. For flame boring, oxygen core lances or oxygen-powder lances are used, with core lances primarily being used. Equipment and accessories

Figure 23: Oxygen lance

Working method After ignition (ignition temperature approx. 1,200°C) the oxygen core lance is pressed against the material (concrete, stone metal) with the help of a welding or a cutting torch. Through the constant combustion of the iron by the oxygen flow sufficient heat is available to melt the material locally. The emerging iron oxide forms (e.g. molten stone) a fluid slag with the material that is carried away by the oxygen flow. In this way a bore is created which can be integrated to any depth.

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Knowledge questions Acetylene decomposes down at higher pressure. Which value must the working pressure therefore not exceed?

    (2)

Close the acetylene and oxygen cylinders Allow the torch to whistle Build in outlet safety devices Immediately close both valves on the handle and the let the torch cool

What is the purpose of the outlet safety device?

    (6)

The cylinder valve and pressure regulators can freeze The acetone is carried away The cylinder is heated strongly There are no negative consequences

What should you do in the case of backfiring in the welding torch?

    (5)

Because of its good ignitability Because of the low ignition temperature Because of the high flame intensity Because of the low density

Which consequences should be expected if large amounts of oxygen are taken from the cylinder?

    (4)

1.5 bar (overpressure) 2.5 bar (overpressure) 15.0 bar (overpressure) 19.0 bar (overpressure)

Why is acetylene used as a fuel gas for welding?

    (3)

Page 26

Ensure an even gas flow Allow a normal flame setting Prevent flame back-fire and gas backflow Extinguish a hose fire

How can acetylene decomposition be initiated?

   

Too low gas consumption rate By too strong external heating of the cylinder By abrupt opening of the cylinder valve By too large gas consumption rate

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SFI / IWE 1.02 Page 27

Which of the following straightening patterns is particularly good for flame straightening of profiles?

   

Heat point Heat lines Heat wedge Heat oval

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10 Bibliography /1/ DIN EN ISO 2503: Pressure regulating valves for gas cylinders for welding, cutting and related processes. /2/ DIN EN ISO 5172: Gas welding equipment; torches for welding, heating and cutting. /3/ DIN EN 12536: Rods for gas welding of non-alloyed and heat-resistant steels. /4/ DVS leaflet 0201: Technical gases for welding, cutting and related processes: Oxygen. /5/ DVS leaflet 0202: Technical gases for welding, cutting and related processes: Acetylene. /6/ Flame straightening: Werksbilder Linde AG (basics of flame straightening.)

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Electrotechnics, a review

Chapter 1.03:

SFI / IWE 1.03-1 & 1.03-2 Page 1

Electrotechnics, a review

Contents 1 2 3 4 5 6 7 8 9 10

Introduction ......................................................................................................................... 2 Ohm’s law, circuit resistance, series connection ............................................................. 2 Parallel circuit, electrical power ......................................................................................... 4 Electrical Work (energy), heating (joule) effect of current ............................................... 5 Capacity, capacitor.............................................................................................................. 6 Inductivity, Coil .................................................................................................................... 7 Electromagnetism ............................................................................................................... 8 Transformer ......................................................................................................................... 9 Force-effect on Current-Carrying Conductors in Magnetic Fields (Motor Principle) .. 10 Induction of Movement (Generator Principle) ................................................................. 10 10.1

Generation of a Sinusoidal Alternating Voltage .................................................................... 11

11 Characteristic Values of Alternating Current and Voltage ............................................. 12 12 Effective Value of Current (r.m.s.) .................................................................................... 13 13 Outputs (power) of alternating current circuits, cos .................................................... 15 14 15 16 17

Three-Phase Alternating Current ..................................................................................... 19 Diode and rectifier ............................................................................................................. 21 Thyristor and Transistor ................................................................................................... 23 Hazards to persons via contact with electric voltage and current, occupational safety .................................................................................................................................. 25 18 Literature ............................................................................................................................ 27 19 Knowledge questions ....................................................................................................... 28

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Electrotechnics, a review

1

SFI / IWE 1.03-1 & 1.03-2 Page 2

Introduction

The process heat required for welding is largely achieved by means of electrical energy. Consequently, knowledge of the electrical principles involved is of great importance in order to understand the behaviours in the arc, the welding circuit and the power sources. By means of simple examples the welding coordinator is given some “handholds” for the understanding regarding the contents of the chapters “Welding processes and Equipment” and for the area of measuring and testing as part of “Manufacturing and Application Technic”.

2

Ohm’s law, circuit resistance, series connection

The explanation of the mentioned subjects via some simple calculation example. A grass field should cut by means of a lawn-mower. The lawn-mower is connected to the power supply of U0230V via a cable drum of 100m. Is it possible that the fall of voltage over the extension cable is so big that the voltage that “reaches” the engine is too low? Sketch

Figure 1:

Series connection

For common extension cables the single conductor’s cross-section is 1,5 mm2 . The line is made from copper with a conductivity of cu = 56 [m/Ω*mm2] The Schuko plug socket contains a mains voltage of Utot = 230 V

1,00 A

230 V

RL1 Rm

Utot

Rm=engine resistance RL1=resistance of the way-to line RL2=resistance of the way-back line

RL2 Figure 2:

Series connection with three resistances

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Electrotechnics, a review

SFI / IWE 1.03-1 & 1.03-2 Page 3

The addition of the separate voltage-drops over the resistances equals the “driving” voltage.

Utot  U RL1  U Rm  U RL 2 The current in a series connection equals the sum of the values through each branch resistance.  ges   RL1   Rm   RL 2

The total resistance equals the sum of the separate resistances

Rtot   RL1   Rm   RL 2 In this way the voltage drop over the way-to line can be calculated according the ohm’s law.

I

U  U fall  I tot  ( RL1  RL 2 ) R

( RL1  RL 2 )  total resistance of the extension cable

The cable resistance can be calculated according:

resistance of line 

total length of line conductivity  cross-section

Length=  m  Cross-section (area)=  mm 2 

RL=

 m  Conductivity  (kappa)=  2    mm 

l A

The conductivity  is the value of the specific resistance. The specific resistance specifies the magnitude of the resistance in a line of 1m length having a cross-section of 1mm2 at an ambient temperature of 20°C. The problem of unallowable large voltage drops in a series connection could lead to a decrease of arcpower within the welding circuit. Examples of such appearances are (too) high resistances at the current transmission in the contact tip, wrongly laid-out secondary lines as well as charred or not correctly fitting main supplies.

Cable Cross-section=70mm2

Figure 3:

voltage losses over the welding lines depending to the line length under several current intensities.

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Parallel circuit, electrical power

Three electric users should be all connected to one main supply secured by a wire-fuse of I=13A. The identification plates show the following: Cement mixer P1=1000W Radiant Heater P2=1500W Spotlight P3=500W

+ -

Figure 4:

Parallel circuit consisting three resistances

Parallel connection of three resistances Question: is the wire-fuse of 13 A sufficient for the total current I tot ? The electrical power Pel is calculated via the multiplication of the actual voltage and the current draw.

Pel  U  

 W

In a parallel circuit: equal voltages over all resistances U tot  U R1  U R 2  U R13 The total current equals the sum of all single (branch) currents

1 1 1 1    Rtot R1 R2 R3 The value of the total resistance is less than the lowest value of one of the separate resistances. Is the total current more than 13A?

I tot 

U tot  Rtot

 .... A

respectively I tot 

P1  P2  P3  U tot

 .... A

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Electrical Work (energy), heating (joule) effect of current Energy is the capability to execute work

If one elevates or moves a body, mechanical work is being executed. This work Wmech is related to the applied force F and the covered distance s . Similar considerations are leading to the definition of electrical work. If charge carriers with an electric charge Q (Q / Time  current) are being moved by the force of the electrical voltage U , work is being executed; electrical work Wel A vessel with a weight force of F=4186 N is to be lifted over 1m (s=1m)

Figure 5:

Electrical work, Mechanical work, heating effect of current.

For achieving this the generator has to deliver a voltage of U=230V and current of I=1A during a period of t=18,2 sec.

Electrical work

Wel = U    t

 Ws

With an equal energy amount of Wel  4186 Ws a water mass of 1kg can be heated for 1K. The required amount of heat equals 1kcal or 4186 Joule.

Amount of (current )heat 1Ws ˆ 0,001 kWs ˆ

Q  I2  Rt

J 

0,001 kWs  0,278  10 6 kWh 3600 s

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Capacity, capacitor

A capacitor is formed by two differently charged geometrical bodies which are located in a defined distance from each other. Usually the “bodies” are two parallel plates. The capacitor has the ability to store electrical energy by means of an electrical field for a limited period of time. Its capacity and the amount of the applied voltage determine the level of energy to be stored. The capacity itself depends on the size of the plates, the distance between both plates and on the type of material in between. If a voltage is being applied no electrical current is running through the capacitor except a charging current. In terms of time, current and voltage are running opposite to each other during processing. The current is leading in phase! After charging the capacitor has the same voltage as the power source.

Figure 6:

Capacitor charging curve

Term:

Capacity

Abbreviation:

C

Unit:

Farad, F[As/V]

Symbol:

┤├

Figure 7:

Capacitor

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Inductivity, Coil

When applying DC to a coil an electric current is flowing through the coil which produces a magnetic field. The coil has the ability to temporarily store electrical energy by means of an electrical field. Switching off the feeding current results in a collapse of the magnetic field. The energy being stored in the coil will be set free again. In terms of time, current and voltage are running opposite to each other, but reverse to the situation in a capacitor. The current is lagging (following in phase)!

Figure 8:

Inductivity in DC circuit

Term:

Inductivity

Abbreviation:



Unit:

Henry, H[Vs/A]

Symbol:

Figure 9:

Coil

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Electromagnetism

In every conductor through which a current is running (=moving electrical charges) a magnetic field is deployed which surrounds the conductor in a circular shape (ring)

H

Figure 10: Distribution of magnetic field lines around a current-carrying conductor If the conductor is shaped like a coil a magnetic field is generated similar to the one of a bar-magnet.

Figure 11: Distribution of magnetic field lines in a coil Two parallel current-carrying conductors will produce a force-effect. CURRENT  MAGNETIC FIELD  FORCE Between conductors with the same current flow direction, attraction forces become effective In the case of conductors with opposite current flow directions, repulsion forces become effective.

inverse current flow direction

Figure 32:

same current flow direction

Force effect between parallel current-carrying conductors

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Page 9

Transformer

Magnetic flux Every current-carrying conductor produces a magnetic field. The intensity of the magnetic field, the magnetic flux (density)  is calculated as follows:



Z Φ I1 I2 U1 U2 N1 N2

Apparent impedance [Ω] Magnetic Flux [Vs] Primary current [A] Secondary current [A] Primary voltage [V] Secondary voltage [V] Number of primary windings Number of secondary windings

Rm

Magnetic resistance

A  Vs   

f

Frequency

[Hz]

4,44  2  

U2  N2 Figure 13:

 t

Transformer (principle)

VOLTAGE  CURRENT  MAGNETIC FLUX  VOLTAGE

U1

U   1  1  Z

  N   1 1  Rm 

    U2  N 2 or U 2  4, 44     N 2  f t  

Induced Voltage A voltage is being induced inside a coil when the magnetic flux  is being changed.

U 2 induced Voltage

 V

 change of magnetic flux  Vs t

Time

s

N 2 Number of turns on the secondary side General transformer relation: U1 N   1  2 U 2 N 2 1

Figure 14: Display of the electromagnetic values of a transformer

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Page 10

Force-effect on Current-Carrying Conductors in Magnetic Fields (Motor Principle)

Current-Carrying conductors will be deflected in a magnetic field.

F  Bl 

F = Deflecting force

[N]

B = Magnetic flux density

V  s   2  m 

l=

Effective length of conductor

[m]

=

Current

[A]

Figure 15:Deflection of a current-carrying conductor in a magnetic field

10 Induction of Movement (Generator Principle) If a conductor is being moved inside and perpendicular to the magnetic field direction, a voltage is being generated (induced) during this movement. Voltmeter

U = induced voltage

Voltmeter

B = magnetic flux density U  Bl v

v = speed l=

[V] V  s  m2    m  s  

effective [m] conductor length

Weight in motion

Wire loop stationary

Figure 46:

Wire loop in motion

Voltage generated through movement of a conductor inside the magnetic field

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10.1 Generation of a Sinusoidal Alternating Voltage The most important way to generate a sinusoidal voltage is given by the principle of motion induced voltage in which mechanical work is converted into electrical energy. To achieve this, a conductor has to be rotated inside a magnetic field. Example angle of rotation 60°: The conductor loop moves almost vertically to the field lines; the current is approx. 87% of the maximum voltage.

u = Momentary voltage

u  uˆ  sin 

uˆ = Peak voltage (read: “U-roof”)

[V]

[V]

sin  = sine of the angle of rotation

Figure 57:

Generating a sinusoidal alternating current (AC)

If a conductor loop (= 1 winding) is rotated continuously in a homogeneous magnetic field, the induced voltage and the height of the induction current is changing equally to the sinus of the rotation angle.

Figure 68:

Induced momentary voltage in relation to the angle of rotation of the conductor loop

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11 Characteristic Values of Alternating Current and Voltage Amplitude, Momentary Value u, i u, i u, i t

u, i

Figure 79:

Amplitude and momentary values during sinusoidal alternating voltage

The peak value (maximum value) defines the highest value of a sinusoid. It is usually marked as “ û ”. Momentary values (instantaneous) value is the exact value measured at the given moment. Momentary values are marked with “ u ”. Similar considerations are valid for the related currents. Period, frequency, phase

Figure 20:

Period and duration of period of a sinusoidal alternating voltage

Period, Duration of Period One complete course of a sinusoid, consisting of both positive and negative half-waves, is called ”1 cycle” (you may also find: 1 period) The time used to complete one cycle is called period ”T”. It is measured in seconds.

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Frequency The frequency indicates the number of cycles that are completed within one second Its unit is ”hertz” (abbreviated Hz). The following applies: 1 Hz is one swing in one second f=Frequency [Hz] T=Duration of period [s] 1 Hz =

1 s

f=

1 T

12 Effective Value of Current (r.m.s.) The root mean square value of an alternating current is the value which produces the same heat as an equal constant current would produce. The amount of heat produced inside a resistance equals: Q = 2  R  t and can be illustrated graphically. For calculation reasons a resistance value of R=1Ω and certain time interval of t=time of period T will be used.

Figure 21:

Determination of direct current / alternating current- amount of produced heat

The area of the rectangular  2  t symbolises the amount of heat energy being generated by the direct current I generated inside the resistance R during a time period of t=T

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Alternating Current – amount of heat energy

Figure 1: representation of the produced amount of heat by an alternating current

Figure 2 & 24:

Repositioning of the alternating current area into a rectangular having the same surface content for the area (=amount of heat) of a direct current the the Umlegen der Fläche der WechselstromWärmearbeit in einem Rechteck mit dem gleichen Flächeninhalt wie bei der Gleichstrom Wärmearbeit

For sinusoidal alternating current:  eff 

iˆ 2

Generally:

 0,707  iˆ

 eff 

1 T

T

i

2

 dt

o

Another way of determining the mean value of a current’s path is by the arithmetical mean value: 



1T  i  dt T0

Alternating Current (AC) values are determined by r.m.s. (effective value), Direct Current (DC) values in an arithmetical way.

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13 Outputs (power) of alternating current circuits, cos An ideal transformer with a thermal efficiency of 100% (no thermal losses), a ratio of n=100 and cos  0.84 is connected to a power supply of U=100V

Figure 25:

Installation circuit set-up for determination of the cos value

The electrical output which is converted into heat via the resistance in the secondary circuit, can be calculated as follows:

P   2  R  10 A  0,1   10 W 2

P  Active Power W 

or P    U  10 A  1 V  10 W The transformer’s power consumption (primary) is:

S  U    100 V  0,119 A  11,9 VA

S  Apparent Power VA

Since the transformer does not have heat losses there has to be another reason for the difference between consumption (11,9VA, primary) and output (secondary).

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Power at active load (current flow by ohmic resistance)

p = u .i

Active power

u i

i u p

Current and voltage curves are phase balanced, the zero cross-overs occur simultaneously. If the corresponding instantaneous values of current and voltage are multiplied, instantaneous values of efficiency are received. To receive the actual power the arithmetic average has to be calculated.

P  U  R

t Figure 86:

Current, voltage and power curve with active load

Power at ideal coil (inductive load) Current and voltage are out of phase, zero-crossover takes place at different times. Current is “lagging behind” voltage by 90°. The power curve shows positive and negative power/time areas.

negative power corresponds to:  the power utilized in the circuit / returned to the mains.

u

iL

iL u p

As the positive power is:  equal to electrical power supplied to the circuit / drawn from the mains,

p = u. i L

Figure 97:

Current, voltage and power curve with inductive load

The power taken from the mains for a short time (“borrowed” from the mains), serves for the creation of a magnetic field of the coil. The power which is later “given” back is being created through the depletion of the magnetic field. The energy swings therefore back and forth between the generator and the consumer. This power which cannot be used effectively (such as to produce heat or light, etc.) is called inductive reactive power. The arithmetic mean of the power curve (p(t)) is 0 which means that the real power consumption is 0. Inductive reactive power QL QL = inductive reactive power QL  U  L IL = inductive current

[var] var=volt-ampere-reactive [A]

The inductive current being used for the calculation of the inductive reactive power is lagging the primary voltage by 90°. If capacitors are being applied capacitive current will occur. Multiplication of this capacitive current  C with the voltage U gives the reactive power Q C Capacitive Reactive power QC QC = capacitive reactive power QC  U  C IC = capacitive current

[var] [A]

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Power during loading with active and inductive resistance Using the example of a transformer fed with 100 V the mains are loaded by active- (resistance R) and reactive power. The resulting power consumption is called apparent power.

p = u.i

i u p

S  U 

i

Even in this case current and voltage are not in phase and, the zero crossings are not simultaneous. The current is lagging the voltage by a phase shift of 0° <  < 90°.

u

Abbildung 3: Current, voltage and power

curve of a lossy coil

The power curve shows positive and negative areas with different sizes and proceeds mainly in the positive area of the diagram. Therefore, active and reactive power can be found. Calculating the arithmetic mean of the power curve the part of active power is obtained. The apparent power can be calculated as follows:

S  P2  Q2

S Apparent power P Active power Q Reactive power

[VA] [W] [VAr]

All kinds of power can be displayed graphically. In this triangle the angle C , to be more precise, the cos, has a special meaning. This value is called the power-factor of the circuit.

Figure 10: Power triangle

As the welding power can, more or less, be equated with the active power and the apparent power with the connected load of the power source, a power-factor cos  approaching 1 is a good utilisation of the mains supply.

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Active power

S1 Welding current circuit

Q1

In the example of the transformer fed with 100V the cos  cos  

P 10,0 W   0,84 S 11,9 VA

This value results in a phase shift between current and voltage of:

1,8 ms 20 ms

cos   0,84 ˆ   32

Figure 30: Phase shift between current and voltage

20ms

360

1,8ms

32

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14 Three-Phase Alternating Current The three-phase supply network is being used between the energy producer (Power Plant Generator) and the energy user (for example: welding power source). At the very beginning there is the three-phase alternating current generator.

fixed part of the generator

moving part of the generator

Figure 31: Typical basic circuit of alternating current generator When rotating a magnetic field, in three coils displaced by 120°, three sinusoidal AC-voltages displaced 120° to each other are generated. In a pure 3-phase alternating current circuit originally 6 wires are to be used. This is called an ”unchained” or ”open” 3-phase-circuit.By concentrating (interlinking) the wires a star-connection is formed that requires only 3 or 4 wires.

Figure 11: unchained 3-phase-circuit

Figure 123: producer (generator) and user (load) in starconnection circuit

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Voltage

Note:  All 3 phases have the same frequency.  All 3 phases have the same peak value.  All 3 phases have a phase-shift of 120° to each other.

Figure 34:Diagram of three-phase Alternating Voltage

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15 Diode and rectifier Diode Diodes are comparable to non-return valves in a water circuit. They are semiconductor components which allow the current to flow into one direction only while blocking the current flowing into the reverse direction.

Figure 13:

Diode - symbol

With the simplest rectifier circuit with only one diode, only the positive half-wave is delivered to the load resistance R, while no current flows through the load during the negative half-wave. The ripple w is largest in this circuit. This type of circuit is not suitable for arc welding.

w > 100% Figure 14:

Half-wave rectifier circuit

AC Bridge Rectifier With the three-phase current bridge rectifier the negative half wave of the AC is ”bend upwards” by the DC rectifier and turns therefore into a positive current time area by the load (user).

w ~ 48%

Figure 15:

Bidirectional rectifier circuit, bridge rectifier (Graetz bridge)

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Three-Phase Current Bridge Rectifier For the arc a current is used containing only a small portion of the alternating current. The ”remaining ripple” should stay low. The “remaining ripple” is defined as the relation between the alternating current share  AC to the direct current share  DC : W 

 AC %  DC

The smaller the numerical value of the remaining ripple, the better the rectifier works which is around a numerical value of 48 % for an AC-bridge rectifier circuit. In arc welding power sources three-phase current bridge rectifier circuits are being used. The remaining ripple is only just 4.2 %.

Figure 16:Three-phase bridge rectifier, rotary current bridge rectifier

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16 Thyristor and Transistor Thyristor Like diodes thyristors can be compared to non-return valves, but have the ability to control the current flow. Therefore, they are semiconductors with an “On – Off” switch function. If there is a control voltage at the thyristor AC can flow at the positive half-wave of the AC. The time when the control voltage is applied can be chosen (ignition time). Therefore the electric power can be controlled quickly, continuously varying and nearly without losses. The negative half-wave is always blocked

Cathode

Control

Figure 4: Thyristor circuit

Figure 5: Supply voltage UAC, Arc-ignition control currentIG , load current IL

Transistor Transistors can be compared to extremely fast adjustable hydraulic valves. They are controllable semiconductors which are able to switch up to 300 A in a period of a few microseconds (per transistor). They also can be used as variable resistances. In switched power sources transistors are used as fast On/Off-switches.

Collector

Control Figure 6: Transistor circuit

Figure 7: Supply voltage UDc, control current IB, load current IL

The "basic" version of the transistors is the so-called bipolar types that distinguish themselves by their easy way to be controlled.

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The field effect transistors (FET) belong to the unipolar transistors. Their input resistance is that high that their output can be controlled. There is practically no flow of control current, only a gate voltage is applied at the input. In the modern current sources IGBT`S (insulated gate bipolar transistors) are used. They are a mixture of the bipolar transistors with a special field effect transistor (MOS-FET). Their main field of application is higher voltages (starting from several 100 V), high power (switched currents max. up to 4 kA), operating frequencies up to 200 kHz.

Figure 82: picture of a IGBT ABB 5SNA 2400E120100 (Voltage VCE = 1200 V, Current  c = 2400 A)

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17 Hazards to persons via contact with electric voltage and current, occupational safety On this subject, there is a separate SFI course unit that covers the hazards and occupational health and safety regarding to welding.

Danger due to electrical Voltage

Danger due to electrical Current

• •

•

Running through the human body

AC more dangerous than DC Risk prevention  Protective Work Clothing!!  creates an increase of the human resistance Rhuman resulting in  a decrease of the current  human through the human body.

Voltage U

For DC-currents almost no risk if no portion of alternating current is availabel (< dI/dt)

•

Alternating current (AC) with high frequency ratios  larger dI/dt for example Current impuls >> 10 kA for example Resistance welding > 60 kA Risk prevention

•

Reading of operating manuals Keep distance to /away from currentcarrying parts!

•

magnetic field force H ~ 1/l², l = distance from conductor. The field force reduces quadratic with the distance. Often it is already sufficient to hold a distance of a few cm for risk prevention.

Rhuman Ihuman

a. Risk of electrical voltage

Transformatorprinzip b. Risk of magnetic fields

Figure 9:Electrical hazards for humans

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Voltage Current Ohmic resistance Specific electrical conductivity Active power Electrical work Heat Capacitance Inductance Apparent impedance Magnetic flux Magnetic resistance Frequency Magnetic flux density Apparent power Reactive power cos phi Period duration

Symbol U I R

Page 26

Units V A





l  * mm 2 W Ws oder kWh Joule oder kJ Farad F [As/V] Henry H [Vs/A]

Pel Wel Q C L Z  Rm F B S Qel cos  T



Vs A/Vs Hz Vs/m2 VA var s

Relations Series circuit

U g  U1  U 2  U3  g  1  1   2 R g  R1  R 2  R 3

Parallel circuit

Ug  U1  U2  U3  g  1   2   3

R Ges 

RG 

1 G Ges



1 1 1 1   R1 R 2 R 3

R1  R 2 R1  R 2

R1  R 2  R R G  0,5  R

Ohm's law Cable resistance

I=

U R

RL 

l I  A A

Active power Electrical work Amount of heat Induced voltage

Pel = U*I Wel = U*I*t Q = I2 * R *t

Induced voltage

U2  2 * *  * f * N2

U2  

 t

[ ] [W] [Ws], [J] [J] [V] [V]

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SFI / IWE 1.03-1 & 1.03-2

Electrotechnics, a review Transformer main equation Frequency

Square mean (effective value)

ü

N1 U 1 I 2 * * N 2 U 2 I1

f 

1 T

[Hz]

 eff 

Arithmetic mean

I

Rectified value

1 T2 0 i  dt T

1 T 0 i  dt T

III 

Power factor cos , 

Page 27

1 T 0 IiI  dt T

cos  

P P  S S

 U  Ι  cos

[A] [A] [A]  for 50 Hz

 for > 50 Hz

Power in the DC circuit (5)

P

Reactive power

Q el  U    sin 

[VAr]

Apparent power

S = U I

[VA]

Apparent power

S  P 2  Q2

[VA]

w

[W]

18 Literature /1/ Heinz Meister: Elektrotechnische Grundlagen; Vogel Buchverlag Würzburg

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SFI / IWE 1.03-1 & 1.03-2 Page 28

19 Knowledge questions (1) What are the most important electrical variables in a basic electrical circuit?

    

Voltage U Ohm's law Current I The welding circuit Resistance R

(2) Electrical resistance is...

    

power. the quotient of current and voltage is reduced in line with the increasing cross-sectional area. The cross-section. proportional to the flow of the gas in the hose package. increases the longer a welding cable is.

(3) A serial circuit of 4 different resistors…

    

is the sum of all resistances. The voltage drop across all resistances is the same. The same current flows through all resistors. is the sum of all conductivity values. the total resistance is less than the lowest single resistance.

(4) The parallel connection of 2 different resistors…

    

at the same resistances the total resistance is R/2. is the sum of all resistances. the total resistance is lower than the lowest single resistance. the voltage is the same over all resistances. at the same resistances the total resistance is 2R.

(5) The magnetic field….

    

surrounds a current-carrying conductor. around an electrical conductor is proportional to the voltage drop via the conductor A force (F = BXJ) acts on a current-carrying conductor in the magnetic field. can be generated by rubbing a rubber rod on fur. around an electrical conductor is proportional to the current that flows into the conductor.

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SFI / IWE 1.03-1 & 1.03-2 Page 29

(6) Which statement on electric work is correct?

    

The “counter” provided by power supply companies indicates the electric power consumed. The electric power can be calculated based on the indication of W el = 40 kWh and the corresponding time indication t = 30 min. The heat produced by the current can be calculated using the following formula: Q = I²•R•t The numerical values of the units Ws, Joule and Nm can only be compared with each other with the aid of conversion factors. The heat produced by the current can be calculated using the following formula: Q = U²•R•t

(7) Which of these statements are incorrect?

    

Direct current always flows in one direction Direct current changes its polarity every 50s Three-phase current is also referred to as rotary current cos φ is the power factor and φ is the phase shift between U and I cos φ indicates the efficiency of a machine

(8) How is the electric power P calculated?

    

P = U•I U = R•I P = I²•R P = U²•G 1 P = I²•R•t

(9) Which of the following are electrical components?

    

transformer welding tongs inductance capacitor cable insulation

(10) Capacitors

    

store the electrical energy proportionally to U². store the electrical energy proportionally to I². are semiconductor components. is a “phase shifter” which delays the the voltage signal in relation to the current signal. is a “phase shifter” which delays the current signal in relation to the voltage signal.

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(11) Inductance

    

stores the electrical energy proportionally to U². stores the electrical energy proportionally to I². is a semiconductor component. is a “phase shifter” which delays the voltage signal in relation to the current signal. is a “phase shifter” which delays the current signal in relation to the voltage signal.

(12) The transformer

    

only transfers AC signals. only transfers DC signals. In the transformer, the output voltage behaves to the input voltage like the input number of windings do to the output number of windings. In the transformer the input voltage behaves to the output voltage like the input number of windings do to the output number of windings. In a transformer, the current transfer ratio is inverse to the voltage ratio.

(13) Which of the following are electronic components?

    

diode choke thyristor transistor transformer

(14) Which of these statements are incorrect?

    

Diodes are used for rectifying electric variables (voltage, current). The half wave rectifier has the lowest ripple. The 3-phase bridge circuit has the lowest ripple. The transistor is a mechanical switch. The transistor can be used as an analogue variable resistor or as switch.

(15) By which of following the human body will be at risk?

    

Via high electrical voltage-carrying conductors within minimal distance. By touching voltage-carrying conductors (for example 150V AC). By direct current carrying isolated conductors. By the presence near AC-carrying conductors with high current amplitudes and large dI/dt . By following the safety requirements and wearing gloves, safety shoes and protective clothing

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The arc

Chapter 1.04:

SFI / IWE 1.04 Page 1

The arc

Contents 1

Introduction ......................................................................................................................... 2

2

Some basic physical definitions ........................................................................................ 2

3

How an arc is generated – ignition mechanisms .............................................................. 5

4

Arc Voltage characteristic-curve........................................................................................ 7

5

The arc in the magnetic field .............................................................................................. 8

6

Arc with non-consumable electrode (TIG, plasma) .......................................................... 9 6.1 TIG process .................................................................................................................... 9 6.2 Plasma process ............................................................................................................ 10

7

Arc with consumable electrode (gas-shielded metal arc welding) ............................... 10 7.1 MIG welding .................................................................................................................. 10 7.2 MAG welding ................................................................................................................ 10

8

AC welding ......................................................................................................................... 13

9

Arc-characteristics of manual metal arc welding ........................................................... 13

10 Arc-characteristics of submerged arc welding ............................................................... 14 11 Arc-characteristics of stud welding ................................................................................. 14 12 Hazards when using the arc during joining (welding) and separating (cutting) metals ................................................................................................................................. 14 13 Questions ........................................................................................................................... 16 14 Bibliography and further information on arc welding .................................................... 18

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1

SFI / IWE 1.04 Page 2

Introduction

An arc is a particular kind of self- sustaining gas discharge at relatively high currents (1-1,500 A) at low burning voltages (15-50 V). The electric current flows via the plasma column, the arc. This plasma is like a hot, electrically conductive gas. The strength of the flowing (welding) current governs the plasma temperature. This thermal energy is used to join or separate metals by melting them locally within a limited area. Today, arcs are used mainly in combination with  non-consumable electrodes, e.g. with TIG and plasma welding  consumable electrodes, e.g. with manual electric, MIG/MAG and submerged arc welding  stud welding  plasma cutting  plasma spraying This chapter explains some of the key fundamentals on how electrical gas discharges and how high-temperature plasma are generated.

2 

Some basic physical definitions Atom

The atom is the smallest particle of a chemical element which still possesses its chemical characteristics. An atom consists mainly of a nucleus, with protons and neutrons, and electron shells with electrons. Protons are positively charged, electrons negatively. Neutrons are electrically neutral. Protons and neutrons determine an atom's weight. Electrons orbit the nucleus of the atom in paths (shells) in the atomic shells. We say an atom is electrically neutral if it has the same number of protons as electrons. Noble gases have all their electron shells completely full of electrons. That means they are chemically inactive (inert).



Ion

We use the term 'ion' if an atom has more or fewer electrons in its outermost shell than positively charged particles (protons) in its nucleus.

Positive ions ⊕  There is at least one electron missing in the outermost shell, the charge of the nucleus predominates. ⇒ The ion is positively charged.  We call such ions cations, because they move towards the cathode.

Negative ions ⊖  The atom has at least one more electron in its outermost shell than protons in its nucleus, the number of electrons predominates. ⇒ The ion is negatively charged.  We call such ions anions, because they move towards the anode. We talk about ionisation when atoms are stimulated into emitting or absorbing electrons, for example by firing electrons or ions at them, so that they become ions themselves. Free ions are also created by processes such as dissociation, when dissolving salts, (e.g. NaCl) for example, in water.

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Page 3

_ _

_ K+

Nucleus P  Proton Positive charge

_

_ P+ N P+

a

N  Neutron

_

_

_

_ b

_ _

_

Neon _

_

_

_

_

K+

Helium

K+

_

_

_

K+

_

_

_

_ c

Lithium

d

Fluorine

K+ the atomic nucleus is positively charged. The number of protons and electrons is the same It is outwardly electrically neutral. Figure 1:



Schematic model of an atom

Molecule A molecule is a chemical compound of two or more atoms. There are two basic types of bond: - Atomic bond  E.g. Hydrogen as molecule H2. - Ionic bond  E.g. NaCl



Comparison Atom : Electron Hydrogen atom Diameter, effective diameter ⇒ approx. 2⋅104: 1 Weight ⇒ approx. 2⋅104: 1

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The arc

Figure 2: 

Page 4

Work function, typical values - Cu 4.3 – 4.50 eV - Al 3.2 – 4.00 eV -W 4.56 – 4.60 eV - Ag 4.05 – 4.50 eV - Ni 5.00 eV -K 2.25 eV - Na 2.28 eV - Cs 1.7 – 2.14 eV - Ba 1.8 – 2.52 eV

Potential well (model)

Work function

The term 'work function' refers to the energy required to bring electrons to the surface, for example from a metal. Figure 2 clearly shows how much of this work is required, using the potential well model. Some specific values relating to work function are given in the adjacent table. What matters above all is the relationship between the different materials, e.g. tungsten W and barium Ba.

Conclusion:

The smaller the work function, the less energy is needed to release free electrons from the metal surface of the cathode.  the more reliable the ignition process is  the more current can be applied to the cathode. With TIG welding, including 'rare earths' in the tungsten electrode improves how a no-contact arc can be generated with a high-voltage pulse (see section 1.06 TIG welding). 

Plasma - is the fourth state of aggregation (Figure 3) - is a fully or partially ionised gas  consisting mainly of electrons and ions, as well as atoms and molecules  and appears electrically neutral outwardly  but behaves like an electric conductor to electrical and magnetic fields. can be divided according to pressure into low-, normal- and high-pressure plasma. is referred to as cold or hot plasma, depending on its temperature.

In the case of arc welding, the plasma (arc) is created by an electrically induced, self-sustaining gas discharge, and thus generally corresponds to a high-temperature plasma at normal (atmospheric) pressure The number of ions + electrons increases with the

44

> 2000 K

3

4 3 2 1

Plasma gaseous liquid solid

2

Temperature >

Temperature

temperature

N[ion+electron]= F (T)

2

Density

Figure 3:

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3

Page 5

How an arc is generated – ignition mechanisms

3

Cathode

_

3 3

e

Voltage

_

E Electric field

V High-voltage Pulse approx. 8 kV

Electron movement

+

Figure 4: Igniting arc by field emission

E

„hot“ electrode

Voltage

e

e

Workpiece Anode

W-electrode Cathode

e

V

e

Electron movement

e

+

Electric field

Workpiece Anode

Figure 5: Thermal emission

Field emission () In electrical engineering, the term 'field emissions' refers to electrons being ejected from a metallic surface (conductor), the cathode, by applying a high electrical field strength. Now we will describe the main requirements for using contact-less ignition of the arc in TIG welding as our example: - TIG welding uses tungsten electrodes as the cathode. They have a very high work function (see table Figure 2). Embedding materials with a low work function, such as 'rare earths', makes it easier for electrons to escape and so ensures that the arc is ignited more reliably. - The pointed design of the electrode enables a high electrical field strength density. - To initiate the ignition process, a high ignition voltage pulse (approx. 8 kV) is applied between the cathode and the anode. - The closer the electrodes are to the workpiece, the faster the arc can ignite. - With TIG welding, mainly argon is used as the shielding gas. The arc ignites faster than when using helium. Pure helium requires higher no-load (open-circuit) voltages. The contactless arc ignition method -

The high-voltage pulse forms an ionised conductive channel between the electrode and the workpiece. Initially, a so-called 'cold' discharge takes place. This 'cold' discharge enables with a corresponding no-load (open-circuit) voltage of the welding power source the ignition of the ('hot') welding arc.

Thermal emission () If the electrode (cathode) is heated strongly (>> 1,000 K), this makes it more likely that the free electrons in metals can escape from the surface of the metal. A kind of space charge occurs directly at the surface. Applying an electrical field accelerates the electrons getting out of the 'hot' surface. Thermal emissions are vitally important to maintain a plasma (welding arc). The hot cathode provides the electrons for the arc.

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Page 6

E Anode

Cathode

_

-

-

+ I

A

I

-

-

+

A

Atom

Movement► Movement►

-

+ I

+ Ion

I

- Ion

-

Elektron

electric field E mechanical impuls

U Figure 6:

Impact ionisation

Impact ionisation () As soon as the field or thermal emissions give rise to free-charged-particles, mainly electrons between the electrodes (cathode/anode), the electrical field accelerates them. They impinge on atoms and/or molecules. This creates ions and further free electrons. This process is referred to as impact ionisation. Because multiple electrons are generally released, the process is highly accelerated, (avalanche effect). This very quickly creates a conductive 'channel' (plasma), the arc.

Current conduction in the arc plasma Current is carried in the plasma via ions and electrons. At the anode and cathode boundaries, a charge is exchanged with the ions (anions and cations), because the cathode “injects” only electrons into the plasma and the anode “absorbs” only electrons from the plasma. In the conductors connected to the welding current source, the charge is also carried (current flow) entirely via electrons.

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SFI / IWE 1.04 Page 7

Arc Voltage characteristic-curve

Figure 7:

Voltage characteristic in the arc (shown in diagram form)

An arc is divided into three areas, the extremely narrow drop areas in front of the cathode and anode and the arc column. The drop areas consist mainly of 'ion clouds'. These have a 'braking' effect on the electron flow and generate in this way the higher voltage drop compared with the longer plasma column (Figure 7). Existing physical models do not describe the drop areas very adequately. Between the two drop areas is the plasma column. With a predominantly gas plasma (TIG, plasma welding, the voltage drop depends mainly on the process gas used (e.g. Ar or He). In the case of arcs with consumable electrodes, the metal vapour also has a decisive influence on the electrical resistance (voltage) of the plasma column. Additive elements, as used in submerged arc welding (powder), manual electric welding (electrode coating) and flux cored arc welding (flux core) or similar admixtures affect the voltage drop in both the drop areas and in the plasma column.

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SFI / IWE 1.04 Page 8

Temperatures

P

A

K

Figure 8:

5

A Anode spot approx. 4,000 K P Plasma column approx. 20,000 K K Cathode spot (hotspots) approx. 3,600 K The anode is generally hotter than the cathode because fast electrons and positive ions generally impact on just a very small area of the surface, while electrons are emerging from different, very rapidly moving, points on the cathode and moreover, the emission of electrons simultaneously generate a cooling effect on the cathode surface. All the temperatures are also determined by the welding process itself and its background conditions, such as process gas, powder, base materials and/or other factors

MIG/MAG arc

The arc in the magnetic field

The electrons and ions in the arc move from the cathode to the anode or vice versa, depending on whether they are negatively or positively charged. Current flowing through a conductor generates a rotationally symmetrical magnetic field, as we saw in chapter SFI 1.03 section 3. The same happens with a current-carrying plasma, the arc. The plasma column is highly mobile, so the arc can easily be deflected by a magnetic field. This is shown by Figure 9 the three finger (right hand) rule in terms of the effects of the three vectors, current (density) j, magnetic flow density B and the resulting force F.

Figure 9:

Arc in magnetic field

Targeted deflection of the arc via a magnetic field, -

also known as magnetic oscillation of the arc, can be used with TIG and MIG/MAG welding to melt the flanks more effectively during narrow gap welding (see chapters 1.07 and 1.08 TIG and MIG/MAG welding). is used with magnetically impelled arcs (MIA welding) for welding pipes etc. Applying a symmetrical magnetic field causes the arc to rotate around the pipe ends to be welded (chapter 1.12-2). is also used with stud welding (magnetic rotating arc welding) to improve the weld quality (chapter 1.12-2).

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SFI / IWE 1.04 Page 9

External magnetic fields can also cause welddefects, however. As, here, the arc is clearly deflected visibly, like blowing on a candle flame, this phenomenon is also known as the arc blow effect. -

6

This can cause lack of fusion when welding pre-magnetised plates, for example, because the arc is magnetically deflected away from the weld joint. The arc may also be deflected due to incorrect routing of the ground return conductor, because the return current in the conductor creates a magnetic field which interferes with the arc.

Arc with non-consumable electrode (TIG, plasma)

6.1

TIG process

The arc is generated between the tungsten electrode as the cathode and the workpiece as anode. The process gases used are mainly inert gases such as argon, helium or mixtures of the two. The geometric shape and temperature (energy) of the arc plasma is primarily determined by the process gas. The proportion of metal vapour and/or metal ions in the plasma is comparatively low compared with the consumable electrode processes. There are two ways of igniting an arc: 1. Contactless  via a high-voltage pulse, as described above.  arc ignites due to field emission 2. via short-circuit ignition:  The tungsten electrode touches the workpiece and then the welding current is switched on. A programmable short circuit current heats the electrode. When the electrode is then lifted, the thermal emissions and rapidly rising welding voltage, assisted by the field emission, ignite the arc. The tip of the electrode is extremely hot, so the required ignition voltage is very low.  The arc is ignited by a combination of thermal and field emissions. Note: Using a high proportion of helium (>90%) in the process gas makes contactless ignition more difficult, because the ignition voltage required is higher than with argon-rich gases.

current for the working point Aso

Figure 10:

I/U curve for TIG welding, shielding gas argon

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SFI / IWE 1.04 Page 10

The current/voltage curve () The current/voltage curve is divided into two areas. The Ayrton area of the curve is unsuitable for welding. From approx. 50 A, the arc voltage increases more or less proportionally with the current, which is why it is also known as the ohm area. The arc voltage increases as the arc gets longer (red curve) and decreases as it gets shorter (blue curve). If helium is used as process gas instead of argon, the arc voltage is significantly higher for the same arc length. The characteristic is comparable, however. 6.2 Plasma process The plasma process differs from the TIG process in that the arc is constricted by means of an additional plasma jet. This plasma arc is ignited in two stages. First a pilot arc is ignited using a high-voltage pulse. This plasma ionises the path between the cathode and anode for the actual plasma arc. In the second stage, the actual plasma arc is ignited merely by applying the no-load (open-circuit) voltage and switching on the welding power source. (chapter 1.12-1).

7

Arc with consumable electrode (gas-shielded metal arc welding)

7.1 MIG welding The arc is formed between the consumable wire electrode as anode and the workpiece as cathode. The process gas (Inert gases) used is mainly argon. The arc consists of a plasma component, which is generated mainly by metal evaporating at the droplet-end of the wire and secondly by the composition of the process gas. 7.2 MAG welding The difference between MAG and MIG welding lies in the process gas used. It contains Active gas components, such as CO2, O2 etc. The arc is always ignited by creating a short circuit between the anode (wire) and cathode (workpiece). Two types are distinguished here: LIFTARC  First, the wire moves towards the workpiece, causing a short circuit between the wire electrode and the workpiece as the counter-electrode. The high current density at the tip of the wire causes the surface of the wire electrode to heat up strongly without melting. After a time determined by one of the process variables, such as the short circuit current, wire material etc., the wire is moved backwards. The short circuit bridge is broken. The combination of thermal and field emissions then ignites the arc. Via short-circuit ignition  Here again, the wire moves towards the workpiece, causing a short circuit between the wire and the workpiece. The short circuit current rises rapidly to a high value. The wire melts under the high current density, usually close to the surface of the workpiece. The short circuit bridge is broken and an arc is generated. Once again, the arc is ignited by a combination of thermal and field emissions. Arc curve (MIG/MAG welding) The MIG/MAG welding arc curve is comparable to that for TIG welding. The additional metal vapour in the arc modifies the voltage drops in anode and cathode, as well as in the arc column. The influencing factors here are as follows: -

current density (wire diameter) type of process gas  inert gases, argon-rich gas mixture, carbon dioxide material to be welded

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SFI / IWE 1.04 Page 11

Forces in the arc

P P

a

Schematic diagram.

Figure 11:

b HS image of a pulse arc P Pinch force = FL Lorenz force

Forces in arc (MIG/MAG welding)

Figure 11 a shows the forces involved during MIG/MAG welding. One major factor which affects how drops are detached is the magnetic Lorenz force (FL), also known as pinch force (P), at the transition point between the wire end and weld-drop: i.e., the greater the welding current, the greater the Lorenz force. Through appropriate programming of the current pulses (pulse width/amplitude etc.), detachment of the drops can be influenced. A stable arc also depends very much on using suitable process gases (chapter 1.06). The arc is not only affected by the base material to be welded in how they vaporise or what they are made of but, for example, also by the viscosity and surface tension which counteract or support the Lorenz forces controlling the droplet detachment. The longer the arc, the greater the suction forces, i.e. the oxygen in the air may impair the process. The welding speed above all causes lateral forces in the arc which may also affect process stability at high speeds. Process dynamics

Weld pool

Figure 12:

Time constants in MIG/MAG arc

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SFI / IWE 1.04 Page 12

The process dynamics in MIG/MAG welding are characterised by three characteristic areas: 1. By the material transition, the time constant for the drop detachment is in the range τ ~ 1 ms - 10 ms. This is determined: - by the wire diameter, - material composition and - process gas. 2. By the plasma, the arc, the dynamic time constant τ lies between a few μs to approx. 1 ms. 3. By the weld pool in the base material, this time constant τ can have greatly fluctuating values from ~1 ms to ~ 1 s. It is determined by: - the thickness of the base material, - the composition of the material - by the heat dissipation (e.g. fasteners/clamping tools) - etc. In ranges 1 and 2, modern welding equipment can control the material transition very well. Keyword  energy-reducing processes (chapter 1.08). Arc types for MIG/MAG welding There are basically five arc transition types for the MIG/MAG welding process. They are largely determined by the wire feed speed (welding current). These arc types can be modified by process controlled welding technology in order to extend the range of available applications. 1.

Short arc

 the material transition (drop transition) occurs during the short circuit phase. The short circuit phase and ignition phase alternate on a regular basis.

2.

Transition arc

 the material transition occurs sporadically with and without short circuiting. The drop transition is extremely irregular. This arc type is not used in practice, being superseded by other types, such as the pulse arc.

3.

Spray arc

 the material transition takes the form of small drops without short circuiting due to the very high current density at the wire electrode.

4.

Pulsed arc

 the material transition is very regular. Detachment of the drops always occurs without short circuiting, due to the supporting influence of the current pulses on the pinch force. Ideally, each pulse sends one drop into the weld puddle.

Rotating arc

 the material transition occurs in the form of a rotating chain of drops virtually without short circuiting, upon further increase of the current density.

5. 6.

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SFI / IWE 1.04 Page 13

AC welding

TIG welding AC TIG welding is primarily used for making welded joints in predominantly aluminium and magnesiumbased alloys. -

If the aluminium base material is used as the cathode, the oxide skin is broken up through the electrons escaping from the surface of the metal. This process is referred to as the cleaning effect on the surface of aluminium materials. Using the tungsten electrode as the positive pole means that the tungsten tip becomes much hotter than the negative pole, so the tungsten electrode needs to have a greater diameter to assist with heat dissipation. If an aluminium base material is used as the anode, the extremely strong focussing around the anode creates deeper weld penetration. The negative tungsten electrode remains colder. During zero cross-over of the welding current when the polarity changes, the arc extinguishes but reignites immediately upon a steep voltage zero cross-over. Only in the event of 'flat' zero crossovers (sinusoidal) are high-voltage pulses necessary to assist reigniting.

AC MIG/MAG welding Based upon the different operating mechanisms at the cathode with hotspots (emerging electrons) spread over a 'large' area, or at the anode with electron entry concentrated in a 'small' area, the polarity change can also be used to additionally control the heat input in the workpiece and/or wire electrode. Figure 13 shows that when the wire electrode is the positive pole (anode) with respect to the workpiece, the electrons enter the wire at an extremely limited area. This causes the wire end to become hotter than when the polarity is reversed. The pinch forces can be supported much more effectively. If the wire electrode is acting as the cathode, on the other hand, the cathode emission points move back and forth across the surface of the wire. The wire is heated more evenly over a larger area. The drop becomes larger and is also colder. The pinch forces are Figure 13: AC MIG/MAG arc considerably lower. If the polarity is now changed again, even a lower pulse energy is required to release this large drop. Lower energy input is therefore required in the base material during welding.

9

Arc-characteristics of manual metal arc welding

The arc is always ignited via a short circuit. The high current density heats the contact point rapidly and by pulling the electrode away quickly the short circuit interrupts and the arc ignites. The arc is stabilised by elements in the electrode coating. The coating also determines which polarity can be used during welding. Various types are also AC-compatible. (chapter 1.09).

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The arc

SFI / IWE 1.04 Page 14

10 Arc-characteristics of submerged arc welding With UP welding, the arc is also always ignited by a short circuit. The high current melts the short circuit bridge. The short circuit bridge is broken and an arc is created. Covering the arc with powder forms a cavern in which the arc can burn protected from the outside air. The solidifying melt in the weld seam is covered with slag. The SAW powder also contains elements which stabilise the arc (chapter 1.10).

11 Arc-characteristics of stud welding Stud welding involves using an arc to weld (threaded) bolts or pins to a base material (chapter 1.12-2). Arc ignition  Short circuit ignition The ignition process is similar to MIG/MAG welding, but the short circuit current is considerably higher  Lift ignition Lift ignition corresponds to the LIFTARC ignition of MIG/MAG welding with different process parameters Welding arc  The welding arc corresponds to the short and spray arc with MIG/MAG welding, but with a much shorter arc length and generally greater welding currents.  With 'magnetic rotating arc' welding, a magnetic coil is wound around a sleeve (small tube). The arc is being propelled into a rotary motion which enables a very even heating of the joint surface.

12 Hazards when using the arc during joining (welding) and separating (cutting) metals Temperature High temperatures occur in the arc >> 1,000 K, so strictly observe fire safety rules. - Heat is transmitted without contact via the heat radiation of the arc. - The arc heats other sheet sections indirectly. - During manual metal arc welding, ensure that the welding electrode is set down safely. - Welding can cause 'hot' spatter. Radiation During TIG, plasma, MIG/MAG and manual metal arc welding, radiation is generated which is: -

 risk of dazzle eyes and impair vision Highly intense in the invisible range o UV (ultraviolet) range  extremely harmful to eyes and skin o IR (infra-red) radiation  strong heating of the skin and eye-damaging Highly intense in the visible range

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The arc

a

TIG welding CrNi of (welding current: 180 A)

Page 15

b

MIG/MAG welding of CrNi (welding current: 180 A)

Figure 14: Radiation spectrum during arc welding Figure 14 shows the radiation spectra for a TIG and a MIG/MAG arc. In particular for the MIG/MAG process, the very high intensities in the UV radiation range can be seen, while a high level of infrared radiation occurs during TIG welding.

Table 1:

Different types of radiation and the risks they involve

Different types of radiation and the involved potential hazards are shown in Table 1. For welders and anyone else working in the welding area continuously or for longer periods, must wear protective clothing and goggles (DIN EN 166/169) to protect their eyes and skin. Generation of harmful substances The immense heat of the arc generates: - fine dust, ultra-fine particles in the micron range - reaction products of gases, metals etc. The type and amount of harmful substances generated depends on numerous factors: - the welding/cutting process used - the process gases used - the working materials involved

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SFI / IWE 1.04 Page 16

13 Questions (1)

Which of these statements are incorrect?  Atoms consist of electrons, protons and neutrons  Ions are either anions or cations  Atoms are charged particles  Molecules consist of ions  Electrons are heavier than protons

(2)

What do we mean by plasma?  The fourth state of aggregation  An inert gas  A plastic material  An ionised gas  A supercooled liquid

(3)

Which of these statements are incorrect?  Plasma is electrically conductive, like an electrical conductor  The plasma is deflected by a magnetic field  The blow effect of the arc is caused by the hot melt (thermal effect)  PINCH force refers to the ejection of drops (spatter)  PINCH force means the magnetic effect of the weld current due to Lorenz forces during detachment of the drop from the wire

(4)

What mechanisms are required for the generation of charge carriers?  Field emission  Thermal emission  Magnetic emission  Evaporating of water  A high ignition voltage

(5)

What are the charge carriers in the arc?  Fast-moving atoms  Ions  Molecules  Electrons  Cations

(6)

The arc is ignited  By deliberately inducing a short circuit  By the gas pre-flow  By a contactless high-voltage pulse  By a long arc  By a pilot arc

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The arc (7)

Which of the following are components of a welding arc?  Pulsed arc  Arc plasma  Short arc  Anode and cathode drop areas  Ions and electrons

(8)

Which welding processes do not use process gas?  TIG welding  Plasma welding  MIG/MAG welding  Manual electric welding,  Submerged arc welding

(9)

Which arc is not an MIG/MAG arc?  Short arc  Plasma arc  Spray arc  Pilot arc  Pulsed arc

SFI / IWE 1.04 Page 17

(10) What harmful effects does an arc have on people?  It emits UV radiation  The voltage drop in the arc  Heat radiation in the infra-red range  It produces fine dust  The ions radiating from the arc (11) What personal protection equipment do welders need?  Gloves  Apron to protect them against radioactive radiation from the arc  Goggles against high levels of UV radiation from the arc  Waterproof clothing  Flame-retardant clothing

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SFI / IWE 1.04 Page 18

14 Bibliography and further information on arc welding [1] Schellhase, Martin: Arc welding - a technological tool, DVS-Verlag, Düsseldorf 1985 [2] H. Cramer; Pommer, S.: Overview of modern arc processes and their material transitions in MIG/MAG welding DVS Congress, Hamburg 26.-29.09.2011, for more information contact GSI NL SLV Munich, Schachenmeierstraße 37, D-80636 Munich, www.slv-muenchen.de [3] M. Schnick: Visualisation of shielding gas covering in arc welding, Dresdner Fügetechnisches Kolloquium 2012, Dresden 29. and 30.032012, additional information also under http://micron.mw.tu-dresden.de/fue/fuetec.htm [4] D. Uhrlandt: Emission spectroscopy in MIG/MAG welding arcs, Dresdner Fügetechnisches Kolloquium 2012, Dresden 29. und 30.032012, additional information: INP Greifswald e.V., Felix-Hausdorf-Straße 2, D-17489 Greifswald, www.inp-greifwald.de

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Power sources for arc welding I

Section 1.05-1:

SFI / IWE 1.05-1 Page 1

Power sources for arc welding I

Contents 1 2 3

4

Introduction ..................................................................................................................................... 2 The welding power circuit .............................................................................................................. 2 Operating principles of welding power supplies .......................................................................... 3 3.1 Welding transformer................................................................................................................... 3 3.1.1 Principle ......................................................................................................................... 3 3.1.2 Setting principles ............................................................................................................ 4 3.2 Welding rectifier ......................................................................................................................... 5 3.3 Electronic power supplies .......................................................................................................... 7 3.3.1 Thyristor welding rectifier ............................................................................................... 7 3.3.2 Transistor welding power supply .................................................................................... 7 3.4 Welding converter .................................................................................................................... 13 Technical data (rating plate) ........................................................................................................ 13

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Power sources for arc welding I 1

Page 2

Introduction

Starting with the basic electrical circuit (script SFI 1.03 Figure 1) this section 1.05 I/II looks at the requirements for arc welding power supplies in more detail.

Figure 1:

Arc welding power supply

What requirements must an arc welding power supply meet? Mains connection Mains voltage Mains current Connected power e.g.: 4(5) conductor system) UN 400 V, IN 3 x 32 A

e.g.:2(3) conductor system) UN 230 V, IN 16 A Table 1:

2

Welding Power Supply

Power supply

Welding arc

Input mains galvanically isolated from welding Welding cable circuit connections Working safeguards

Process-adapted welding characteristics

Operability, setting aids (software) interfaces, e.g. robots, recording/ backing up data, calibrating

No load (R ∞) Ignition aid (UZ ~8 kV) Short circuit (R  ~0) Dynamic, static characteristic Process stability e.g. spatter prevention

Power sockets standardised plug connections

Requirements for a power source for arc welding

The welding power circuit

Figure 2: Basic power circuit for welding In welding, appropriate static characteristics are used for each welding process. A characteristic shows the relationship between current and voltage as the load resistance changes (arc). We say the characteristic has a constant voltage (CV) characteristic if the voltage remains more or less constant as the current increases or reduces by just a few volts per 100 A. This is shown in red in Figure 3. However, if the voltage decreases steadily as the load increases (R  0 Ω), we call this a falling characteristic (constant current, CC). The current reaches a stable limit (blue characteristic, Figure 3).

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Power sources for arc welding I

Page 3

Constant Voltage (CV) (flat) characteristic: Constant Voltage (flat) characteristic

MIG/MAG welding Submerged arc welding

Constant Current (CC) (falling) characteristic:

Constant Current (falling) characteristic

Manual electric welding, Submerged arc welding TIG welding plasma welding a)

Power supply characteristic

Figure 3:

b)

Welding process

Allocation of power supply characteristic to welding process

This power supply characteristic, also known as a static equipment characteristic, can be generated in a number of different ways: 1 via the design structure 2 via an electromagnetic control 3 via electronic control

3

Operating principles of welding power supplies

3.1 3.1.1

Welding transformer Principle

Law of induction U2 ~ -dΦ/dt U1 / I1 U2 / I2 N1 N2

Figure 4:

Primary voltage/current Secondary voltage/current Number of primary windings Number of secondary windings

Transformer - principle

The transformer does not transfer DC ! The arrangement of the primary and secondary windings in a transformer as welding power supply, can be used to generate a flat (CV) or falling (CC) characteristic. A welding transformer with a falling (CC) characteristic is used for AC-TIG, submerged arc and manual electric welding. With AC TIG welding, a capacitor must also be connected in series, because the differing behaviour of the positive and negative current component at the tungsten electrode can generate a DC component which could destroy the transformer.

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Power sources for arc welding I

Page 4

α Characteristic angle of inclination function of connection between primary and secondary windings and resistance losses in coils.

a) falling characteristic Primary and secondary windings are far removed from one another  'soft' transformer Figure 5:

b) constant voltage characteristic Primary and secondary coils are closely interlaced with one another  'hard' transformer, the larger α, the 'softer' the transformer

Transformer - characteristic

The mains (primary) voltage between two wires is normally 230 V in single-phase networks (L1/N/PE) 230 V and 400 V in three-phase networks (L1/L2/L3/N/PE) (e.g. L1/L2). Different voltages can easily be generated by adjusting the winding taps accordingly (in the chemical industry, for example). In the simplest case, there are four conditions a welding transformer must meet: 1. It must convert the mains voltage to the welding voltage (US)  US> IN 2. It must ensure optimum welding characteristics  ignites well, low spatter 3. It must protect workers  contact voltage (idling (U0))  U0 ≤ 48 V AC e.g. 113V DC 4. The mains power must be isolated galvanically from the welding circuit The mains fuses in the standard three-phase sockets, e.g. 16, 32, 63 A etc. prevent the equipment drawing too much power from the mains supply. This may also mean limiting how long the power source is switched on for in exceptional cases if the thermal effects of the current exceed the fuse characteristic. In principle, however: The (mains) fuse serves to protect the mains supply  Fire safety! 3.1.2

Setting principles

a

Step switch (multiple contact)

b

Variable resistance (choke/ inductance coil)

c

Magnetic shunt (scatter core)

d

Transductor

Figure 6: Transformer setting principles

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SFI / IWE 1.05-1 Page 5

a Step switch (multiple contact) The step switch changes the transformer's conversion ratio. The welding voltage can be set in discrete voltage steps. Advantages: - a robust and highly cost-effective solution for simple applications - is still used frequently for MIG/MAG welding rectifiers (see section 3.2 Welding rectifier) Disadvantages: - is limited in use by limited number of steps, voltage steps too great - the smaller the voltage steps, the more winding taps are required, increasing the production costs - the no-load voltage is determined by the preselected step U0 ≠ constant

b Variable resistance (choke/ inductance coil) Including a variable resistance in the welding circuit can be used to set the working point (welding voltage and current) during welding. Using a choke in an AC circuit is particularly effective. This reduces the resistive component considerably. The choke therefore heats up less than a purely ohmic (DC) resistor.

c Magnetic shunt (scatter core)

The secondary voltage U2 is a function of the magnetic flux φ2. The position of the scatter core (moving plate assembly) in the transformer magnet circuit governs how much of the primary generated component of the magnetic flux φ1 is derived. What this means is that, when the scatter core is completely 'immersed', the derived flux is governed by the ratio of the magnetic crosssectional area of the secondary scatter core and the secondary winding core ( φ1 –φS =φ2). φ2 min equals minimum weld current. If the scatter core is lifted out of the magnetic circuit completely, the total magnetic flux φ1 ~ 2 (φS = 0) will pass through the secondary coil. The welding current now reaches its maximum value I2 max. φ1 = φ2 max.

d Transductor The transductor works on the principle that a strong magnetic inundation will saturate the iron core, so it behaves as if it were air (RmagnFe ~ RmagnAir). If we replace the mechanical adjustment of the magnetic shunt (section c.) with an electrically variable magnetic circuit, we get a transductor. A transducer has an auxiliary winding through which a DC control current flows, controlling the magnetic flux φS in the same way. This controls the magnetic resistance of the magnetic shunt. A transductor is also known as a magnetic amplifier, as a small output can be used to control a large one. This adjustment principle makes it possible to use an electrical control without any additional mechanical intervention. This means it can be controlled remotely, which is a first step towards electronic power supplies. 3.2

Welding rectifier

A welding rectifier consists a transformer and a bridge rectifier in sequence. The rectifier converts ACcurrent to DC-current. The residual ripple with a three-phase full bridge rectifier is very low (4%). Using a welding circuit choke, the current ripple can even further be reduced. The size of the choke has quite a significant effect on the welding characteristics: igniting an MIG/MAG arc requires the lowest possible inductance, for example, whereas a higher inductance is used during the welding process itself. That’s why so-called electronic chokes are often used. These can be used to adjust the inductance and hence the dynamics of the different process conditions during welding.

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Power sources for arc welding I

Mains voltage

Secondary voltage

DC 4% ripple

Page 6

Welding Voltage

DC −

AC ~

DC +

Transformer

Rectifier

Choke

Welding arc

Figure 7: Principle of a welding rectifier

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SFI / IWE 1.05-1 Page 7

3.3 Electronic power supplies Electronic power supplies use electronic components (thyristors, transistors) to set/control the welding voltage/current. There are two characteristic types of component which govern the control principle here: 1. Thyristor welding rectifiers (network-synchronous regulation (control)) 2. Transistor power supplies (control independently of frequency and phase of input network)

Diode

Thyristor

Transistor

Figure 8: Electronic components 3.3.1 Thyristor welding rectifier Thyristor power supplies are similar in design to welding rectifiers (see Figure 3.2). Half or all the diodes in the rectifier block can be replaced by thyristors (semi-/fully-controlled three-phase bridge). A thyristor works like a diode in the first instance, but only if it is controlled via the gate (control electrode G) during the positive half-wave. The electronic control can be used to control the welding voltage (welding current) synchronously with the mains frequency. Additional control circuits can be used to generate different characteristics (falling  current-controlled, flat  or voltage-controlled). The problem with this control principle is mainly that it causes more ripple than uncontrolled rectifiers, that is, the welding circuit choke must normally be larger than with an uncontrolled welding rectifier (diodes). They are still used for simply applications, manual electric, TIG, MIG/MAG welding even today. They can still be used costeffectively today, especially with submerged arc welding in the larger welding current ranges. 3.3.2 Transistor welding power supply Transistor welding power supplies use transistors to control the electrical output for the welding arc. “Transistor” here is a general term (see part SFI/IWE 1.03 Electrical principles) for a controllable electronic semiconductor element. Today, the output voltage and current and dynamic characteristics are controlled almost exclusively digitally by one or more processor systems. This creates more complex ways of influencing the welding process. Three different switching principles are used, as follows: 1. Starting from the welding rectifier, a transistor is added as an analogue actuator downstream. The transistor acts as a variable resistor. The secondary choke can be eliminated or replaced by a filter with a surge protection device. Analogue transistor welding power supply 2. The setup is the same as in 1 in principle, except now the transistor works as a 'switch'  Secondary switched-mode transistor welding power supply 3. The primary switched-mode transistor welding power supply uses a different principle All laptops use a similar power supply today.  Primary switched-mode transistor welding power supply

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Power sources for arc welding I

Page 8

Analogue

Secondary switched-mode

Inverter - primary switched-mode Figure 9: Electronic welding power supply - transistor welding power supply 1. Analogue transistor welding power supply, 2. Secondary switched-mode transistor welding power supply, 3. Primary switched-mode transistor welding power supply (inverter) Combining different control principles can be used to create other versions. Two of these will be explained in more detail here: 4. the transistor welding power supply in hybrid form, 5. the combined primary and secondary switched-mode power supply. 3.3.2.1 Analogue transistor welding power supply DC −

US IS

Regler

US

IS

DC +

Intermediate circuit filter Figure 10: Analogue transistor welding power supply (principle) Transformer

Rectifier

Transistor controller

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Welding arc

Welding processes and equipment

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Power sources for arc welding I

Page 9

An analogue power supply uses a transistor on the secondary side of a transformer followed by a rectifier. The first part can be compared to that of a welding rectifier. This eliminates the step switch and the actual inductance coil. The intermediate circuit filter (choke and capacitor) is used to smooth the intermediate circuit voltage. The transistor downstream is wired in series with the welding arc. The way it functions may be regarded in simplified form as a variable resistor. The way it actually works is considerably more complex, however, so the dynamic transition method e.g. short  arc can be influenced. The functioning principle can then be compared with an electronic choke. The control electronics can vary the welding current and voltage level and behaviour via the output component (transistor) within a few microseconds. This can be used to eliminate interference-induced current or voltage fluctuations. The response time is 40 µs – 50 μs. This creates completely new control strategies, especially with MIG/MAG welding. This makes the process virtually spatter-free, while at the same time the faster control can make the process much more stable while inducing less heat. Advantages: - very fast control, response times 400 V AC)

Figure 15: Comparison: conventional (50 Hz) – transformer inverter (50 kHz) -transformer 3.3.2.4 Hybrid power supplies Hybrid power supplies are welding power supplies which combine a conventional welding rectifier with an electronic control, such as a transistor. The transistor can work as a switch or analogue as electronic control resistor. The main advantage is that the power transistor does not have to output as much power. With MIG/MAG short arc welding, for example, the short-circuit current is provided by the conventional power supply, while the pulse is generated via the electronic control. The analogue component alone (conventional welding rectifier) operates, or the electronic component (transistor) is switched in, depending on the present position of the working point. 3.3.2.5 Combined power supplies

Figure 16: Secondary switching device

primary

and

secondary

switched-mode

For AC TIG aluminum welding, for example, an AC voltage/current is required. This is included downstream with the electronic welding power supplies, such as a primary or secondary switched-mode power supply, for example. Figure 16 shows the structure of this inverter. Transistors (switches) S1/S3 and/or S2/S4 are always switched ON or OFF at the same time. If S1/S3 are on, S2/S4 break and vice versa. This can be used to reverse the polarity without swapping the welding electrodes. AC TIG welding uses a switching frequency of approx. 25 – 200 Hz. This means the AC voltage/current can be set from a purely sinusoidal or trapezoidal form to a steep rectangular form. The rectangular form assists fast reigniting when the voltage passes through zero. The acoustic pressure is extremely high, however. The sinus form involves less acoustic pressure but does not reignite as efficiently.

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3 f 1 ~

1 ~ f 1 f 2

SFI / IWE 1.05-1 Page 13

Generating the AC voltage electronically means the positive and negative half-waves can be matched to the requirements of the welding process separately. The symbols are shown in Figure 17.

Figure 17: Symbol for a primary/secondary supply

3.4

Welding converter

The welding converter is like a generator in the way it is used to generate power. When used in welding, this generator has a modified structure with specific current/voltage characteristics, such as a constant voltage characteristic for MIG/MAG welding or a falling one for manual electric welding. Aggregates are generators driven by a combustion engine (diesel). The term motor-generator set means a generator which is permanently coupled to an electric motor. The size depends on the welding power required and the generator speed. The higher the speed, the smaller the size. DC welding generators are the oldest power sources for arc welding. Today, aggregates are still used mainly where no electrical mains supply is available, such as on sites on remote terrain. There are two kinds of welding generators: a brush welding generators, which generate pure DC with very little ripple. b brushless welding generators, which generate AC which is rectified by a rectifier block and smoothed by a welding circuit choke. There is little difference between the two types of generators when it comes to welding. Brushless welding generators ignite better. With no collectors or carbon brushes, they are virtually maintenance-free. Advantages: - the manual electric welding process is particularly suitable for many welding jobs in rough site operation - aggregates can be used independently of the mains - the mechanical rotation mechanism is slow to respond, which means transient interruptions to the mains supply can be bridged easily (electric motor operation) - suitable for welding processes with falling and constant voltage characteristics Disadvantages: - high weight, large volume, high noise level - not easy to control  more spatter than with electronic power supplies with MIG/MAG welding - no MIG/MAG pulse welding - not widely used today  Generators are required on sites for many items of equipment, such as angle grinders, lights etc.; electronic power supplies can also work very stably on mobile generators.

4

Technical data (rating plate)

The 'technical data' for the user is documented in the equipment specifications (operating instructions) for the power supply in accordance with the relevant standards based on EC marks. Each power supply has a rating plate required by law with the main technical data in short form.

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SFI / IWE 1.05-1 Page 14

Figure 18: Rating plate for welding power supplies. Electrical characteristics for selecting and comparing welding power supplies. The plate layout standards can be found in EN 60974-1 (= IEC 60974-1 and/or VDE 0544-1). Each welding power supply must have a rating plate attached to or printed on it, which is divided into three sections: 1. Top section (general data):

gives the name of the manufacturer, distributor or importer and welding equipment data.

2. Centre section (welding data):

gives details of the welding power circuit.

3. Lower section (mains supply):

details of mains supply, including protection class, cooling type etc.

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SFI / IWE 1.05-1 Page 15

Table 1: Overview of welding power supply, types Figure 18 shows some typical examples of rating plates used in practice. The information is not always standardised and may be very minimal on portable welding inverters, due to their small design. Manufacturer Trading company etc.

Figure 19: Typical rating plate for welding power supplies

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SFI / IWE 1.05-1 Page 16

Welding processes and equipment

Power sources for arc welding II

Chapter 1.05-2:

SFI / IWE 1.05-2 Page 1

Power sources for arc welding II

Contents 1

Welding characteristics of arc welding power supplies ................................................................. 2 1.1 Static characteristic curve .............................................................................................................. 2 1.1.1 Flat (horizontal) curve ......................................................................................................... 2 1.1.2 Falling curve ........................................................................................................................ 3 1.2 Dynamic properties ........................................................................................................................ 4 1.3 Synergy curve................................................................................................................................. 5 2 Duty cycle ............................................................................................................................................. 6 3 Standardised working voltage / arc characteristic .......................................................................... 8 4 No-load voltage .................................................................................................................................... 9 5 Power supply mains connection ..................................................................................................... 11 6 Type of cooling .................................................................................................................................. 11 7 Protection class ................................................................................................................................. 12 8 Testing welding power supplies ...................................................................................................... 12 9 Bibliography ....................................................................................................................................... 12 10 Knowledge questions ....................................................................................................................... 13

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SFI / IWE 1.05-2 Page 2

Welding characteristics of arc welding power supplies

Welding characteristics of arc welding power supplies are characterised by two definitions: 1. The static current/voltage characteristic curve which describes how the voltage (U) and current (I) of a power supply responds if we vary the arcing load (resistance). It can be obtained by measuring current and voltage for an adjustable ohmic load resistance 2. how current and voltage respond dynamically (over time) to faults during welding, such as a short circuit. 3. the on each other adjusted setting of welding parameters and synergy curves 1.1 Static characteristic curve The static characteristic curve of a welding power supply is determined, firstly, by the structural design and/or secondly by the electrical control circuit. The falling characteristic of a welding transformer, for example, is generated by the type of connection (simple) between the primary and secondary windings. Transistor power supplies, on the other hand, always have a control circuit. This 'measures' the welding current and voltage, and the output curve then follows a selected form, falling or flat, current (I) or voltage (U) being controlled. 1.1.1 Flat (horizontal) curve With a flat or horizontal curve (Figure 1), the voltage remains largely constant as the load increases (R1 < R0). It is the voltage which determines the welding process. With conventional power supplies, the slope α of the curve is determined mainly by internal resistances and how the transformer is connected. The short circuit current, where it intersects the x-axis, is usually well outside the working range, and depends on the design and how slow-blowing the mains fuse is.

α Curve slope

Figure 1:

Flat curve

MIG/MAG welding – control procedure when varying arc length Δ I control 3

2

1

4

AP1 Working point when starting welding. Wire feed speed and welding voltage are set.

5

AP2 The jump in arc length (U2 > U1) causes AP 1 to move towards AP 2  IS becomes smaller. AP3 The welding current falls, so the consumption rate falls also  The arc becomes shorter. This takes us to AP 3, which is not the same as AP 1, because the free wire length is shorter.

U UL

AP4 The jump in arc length (U4 > U3) causes AP 3 to move towards AP4  IS becomes smaller.

0 l IL

AP5 Due to the higher welding current, deposition speed is larger  arc length gets shorter. This takes us to AP 5, which is the same as AP1.

0

U

51

U2 U1,3,5

2

U4

Figure 2:

Δ I control is also known as internal control, because the control process depends on the power supply and its curve alone.

4 3

ΔI Δ I control

I

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With curves which can be generated via a regulator (such as transistor power supplies), the slope α can also be selected as a parameter (e.g. 1V/100A). The max. short-circuit current is controlled electronically. It is determined, firstly, by the maximum load rating of the power supply components and secondly by the technological requirements involved in Figure 3: the welding.

1.1.2

Flat curve Symbol

Falling curve

Figure 4:

Falling curve

Figure 5:

Falling curve Symbol

A falling curve indicates a power source at a 'constant' current: i.e. as the load increases, the voltage at the arc reduces, while the current remains largely constant. Maximum constancy is achieved with electronically controlled power supplies, as shown in Figure 4. The blue line shows a transformer with scattering core control, for example, With both manual electric and TIG welding, the arc length is determined mainly by how the electrode is guided manually (distance from the workpiece). This method is known as external or ΔU control. With automatic welding, there has to be an additional external control circuit to keep the arc voltage and hence the arc length constant. Additional sensors (distance or arc measurement) can also be used to keep the distance from the torch constant  ΔU ≈ 0. With submerged arc welding too, a falling curve is preferred when welding, especially for large wire or strip cross-sections. The feed speed of the consumable electrode is controlled as a function of the arc voltage.

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Power sources for arc welding II

Page 4

Figure 6: ΔU control, falling curve Figure 7: ΔU control, basic diagram If the arc length changes lLB0 at the working point set AP0, the steeply falling load curve of the welding power supply generates a change in the welding voltage above all ΔU1/2. If the arc gets longer (lLB1 > lLB0) this causes the working point AP0 to shift AP1, increasing the voltage by +ΔU1. To reduce this difference again, the controller is used to increase the wire feed speed v D0  vD1. ΔU1 is reduced back to ~0, restoring the arc to its original length lLB0. 1.2 Dynamic properties The dynamic characteristics of a welding power supply are determined decisively by the control principle and/or structural design. With a welding rectifier, for example, the inductance coil (choke) is decisive for the dynamic characteristics such as ignition and spatter with MIG/MAG short arcs. With an inverter, on the other hand, the choke has virtually no effect on the welding characteristics. The dynamics are governed almost exclusively by the control dynamics of the electronic power supply, and can therefore be optimised to suit the welding process. MIG/MAG welding    

Short-circuit arc ignition MIG/MAG short arc  reigniting after the short circuit  spatter prevention (Figure 8a) MIG/MAG pulse arc  creating current or voltage pulses (Figure 8b) MIG/MAG AC welding  rapid reigniting at zero voltage

Additional inductances, such as coiled welding cable, should be avoided. They alter the welding characteristics.

a

Short arc

Figure 8:

b

Pulsed arc

Setting options for electronic power supplies with MIG/MAG welding

IP Ig tr tP tf

Pulsed current Basic current Pulse up-slope time Pulse width (pulse time) Pulse down-slope time

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Power sources for arc welding II Figure 9:

SFI / IWE 1.05-2 Page 5

Pulse characteristics with MIG/MAG pulse arcs

TIG welding/plasma welding 



Arc ignition o electronic power supplies can be used to generate a very fast current rise, making even short ignition possible without necessarily creating tungsten inclusions o via high-voltage pulse  contactless arcing o ignition even with AC TIG welding with DC  higher open circuit voltage  better ignition characteristics AC welding o high voltage and current rise rate  reliable arc reigniting at zero voltage without additional high-voltage pulses (HF) o fast switching from AC to the safe reliable DC operation if the power is interrupted (open circuit)

Manual metal arc welding As with MIG/MAG welding, the welding process is determined by the dynamics of the power supply. The inductance of conventional welding power supplies is of decisive importance here. With a single-phase mains supply (230 VAC) in particular, the welding characteristics can be improved quite decisively through using inverters and higher welding and short-circuit current.

Figure 10: Examples of synergy curves

1.3 Synergy curve The synergy curve is a database with working points saved for MIG/MAG welding, as shown in Figure 10. They are generally obtained by conducting appropriate welding tests. The intermediate values are calculated using an algorithm specified by the equipment manufacturer.

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Synergy curves:  helps selecting the right parameters for different welding tasks  recall of tried and tested records from a data memory  enable a simple procedure for the welding task e.g. fillet weld with details: material, thickness, wire material, diameter etc..; the welding parameters are then selected automatically and can be adjusted to a minor extent via correction menus  but can also be adjusted to suit specific characteristics of the user  this requires being able to access the program and having suitably trained specialists

2

Duty cycle

When current flows through an ohmic resistance heat is generated. If a given output is required of a power supply, its components, e.g. transformer, diodes, power components etc.. are heated until a thermal balance is established between heat losses generated and heat dissipated. The maximum welding current shown on the Figure 11: Rating plate on time rating plate (see setting range line) cannot be demanded indefinitely. If too much power is drawn from the source for too long, its electronic components will overheat. That means they will exceed their maximum temperature, which is indicated by their insulation class (e.g. for a transformer F = 155°C). There are certain duty cycles which must not be exceeded to avoid overheating, depending on the welding current level. Definition of duty cycle

Run time = 10 min Run time = total of all weld and break times up to 10 min. The 10 min run time applies to the arc welding process. With resistance pressure welding, a run time of 1 min. is assumed. The permissible current height for a given duty cycle can be calculated as follows:

Is  ID 

100 % ED %

IS = welding current ID = constant current

Power supplies for manual metal arc welding: the current/voltage data of 35%, 60% and 100% duty cycle are stated on the rating plate.

Figure 12: Example of welding current and temperature for a duty cycle of 100%

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I s  150 A 

SFI / IWE 1.05-2 Page 7

100 %  254 A  250 A 35 %

Power supplies which are also suitable for mechanised and automatic welding (e.g. MIG/MAG process) are generally used for longer welding times. Typical values on the rating plate are 60% and 100% duty cycle.

Figure 13: Example of welding current and temperature at for a duty cycle of 35% and 60%

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Page 8

Standardised working voltage / arc characteristic

US [V] 40 c

30

a

20

b

10 0 100

0

200

300

400

500

600

IS [A]

Figure 14: Standardised voltage-current operating lines for different processes acc. to EN 60974-1 a

Manual electric welding,c

U 2  20 V  0.04 b

up to 44 V > 600 A constant U2 = 44 V

V  I2 A

up to 34 V > 600 A  constant U2 = 34 V

TIG welding

U 2  14 V  0.05 c

V  I2 A

MIG/MAG welding

U 2  14 V  0.05

V  I2 A

Submerged arc welding

up to 44 V > 600 A  constant U2 = 44 V with a falling curve like a with a flat/horizontal curve like c

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Figure 15: Example of determining welding current design values for a manual electric welding power supply

4

No-load voltage

The no-load voltage is the voltage between the two welding sockets on the welding power supply for the torch and workpiece clamp, or at the ends of the welding lines to the welding point (torch, workpiece) if the welding current circuit is 'open', i.e. no welding current is flowing. With manual metal arc welding particularly, the welder is in danger when replacing the welding electrodes, due to the no-load voltage. To avoid putting the welder at risk, under industrial insurance board rule BGR 500/Part 2, section 2.26: “Welding, cutting and related procedures”, the adjustable no-load voltage must, considering conditions of use and voltage type, not exceed the maximum values specified in the following table.

Table 1: Requirements for no-load voltage for arc welding power supplies

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If a welding task is in an area of increased electrical danger (conductive environment, confined space, enforced posture, high ambient temperature or relative humidity), only power supplies may be used who’s no-load voltage does not exceed 113 V DC or 48 V AC RMS. These limits must be observed even in the case of a fault. Power supplies which comply with these conditions receive the following marking: According to BGR 500, welding power supplies may exceed the maximum no-load voltages if they are equipped with self-actuating self-monitoring no-load voltage reduction devices. These are also known as risk reduction devices, because during welding in AC mode, there is always a DC voltage as a no-load voltage (cf. Table 1). The AC process only starts when the welding current starts flowing, and reverses immediately (DC) if the power flow is interrupted. The function of the risk reduction device must be capable of being checked without using any tools. It’s recommended to have the precautions against hazardous currents flowing through the body being checked on a quarterly basis. These relatively short monitoring periods are appropriate: e.g. during carelessness handling of the electrode holder, a current flow via the earth wire to the source may arise leading to a possible burning off. In the case of a fault, the safety earth protection class is no longer effective, and the welder is at extreme risk.

With the power supply switched on, the electrode holder is lying on the housing.

Photo: burned-out earth wire.

Due to the high current flow (may exceed 50 A), the earth wire burns out.

If there is a housing short, current may flow through the body.

Figure 16: Earth wire destroyed by leakage currents (potential diversion)

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5

SFI / IWE 1.05-2 Page 11

Power supply mains connection Power factor cos φ The power factor cos  indicates what percentage of the apparent power consumed (S1 in kVA) is converted to actual power (P in kW). Typical values for cos :

Figure 17: Rating plate - power factor cos φ

Welding transformers: Welding rectifiers: Transistorised welding power supplies:

0.40 - 0.80 0.80 - 0.95 0.90 - 0.99

What this means in practical terms is that a welding transformer operating on a 32 A mains supply can be replaced with an inverter of the same welding output which only needs a 16 A supply. The inverter also has a higher electrical efficiency and a lower weight. Connection and fuse protection of the power supply

Figure 18: Connection and fuse protection of the power supply The field with the plug symbol indicates whether a welding power supply operates on AC, two-wire mains supply (L1, N; L1 or L2) equivalent to single-phase operation, or on 3-phase supply (L1, L2, L3, (N)). The works electrician must connect the network plug as specified to the power supply.

6

Type of cooling

The AF code letters (formerly just F) mean that, when drawing maximum permitted power from the supply, components such as the transformer and electrical output controls (diodes, transistors) must be externally cooled by fans. The internal area (ventilation ducts) must be cleaned regularly, depending on the ambient conditions at the location where the supply is used. No forced ventilation options: S stands for self-cooling, no fan required

Cooling type S

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Power sources for arc welding II 7

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Protection class

Figure 19: Protection class The protection class codes indicate how the power supply housing provides contact protection against live and/or dangerous moving parts. They also indicate how the source is protected against liquids penetrating equipment. The first digit indicates the protection class for contact and foreign body protection and the second the level of protection against water penetration. According to DIN EN 60974-1, power supplies in enclosed spaces must have a protection class of at least IP 21. A power supply to be used for arc welding outdoors must have a protection class of at least IP 23. The first code 2X means: “protection against penetration by solid foreign bodies more than 12 mm in diameter preventing penetration by fingers or similar objects”. The second code X1: “protection against penetration by dripping water (water drops) falling vertically. It must not have any harmful effect.” The second code X3 means: “protection against penetrating water falling at any given angle up to 60° to the vertical (water spray). It must not have any harmful effect.” e.g. could reduce dielectric strength by forming a conductive film of water.  Destruction of the power supply due to spark-over.

8

Testing welding power supplies

BGR 500 requires arc welding power supplies to be inspected and tested on a regular basis. The main electrical tests are as follows:  Earth wire test  Insulation test  Leakage current  Checking the function of the risk reduction device Dirt deposits must be blown out regularly, especially in environments which are more prone to be contaminated by dust or similar. The exact conditions are given in [2].

9

Bibliography

[1]

Rosenfeld, W.; Baum, L.: “[New TIG and MIG/MAG welding power supplies”, basic info, 38th special convention, “Welding in container and plant construction", Munich, 02-05 March 2010

[2]

DIN EN 60974-4:2011 (VDE 0544-4) Arc welding equipment - Part 4: Periodic inspection and testing

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10 Knowledge questions (1) What do we mean by an arc welding power supply?     

The electrical energy source for arc welding. Power supplies which are only suitable for DC welding. Equipment specifically made for welding to control and regulate the welding process. The 'black box' as the link between the mains power supply and arc. The power supply is always a transformer.

(2) What is the purpose of the power supply?     

Separate the mains power supply galvanically from the welding power output. Limit the no-load voltage. The fan is used to improve the air indoors. Control and regulate the welding current and voltage according to the process conditions. Transmit mains problems to the welding process.

(3) What control principles for current and voltage are used in electronic welding power supplies?     

Transformer with scattering core, The inverter as primary switched-mode transistor power supply with pulse width control, The welding rectifier with step switch, Thyristor welding rectifier with phase section control, The converter with a brush setting.

(4) Which of the statements about the rating plate below are correct?     

It gives the technical data in short form. It merely states the welding power in watts. Welding processes are indicated by code symbols. Mains voltage, mains current and welding voltage and current are stated according to duty cycle values. The manufacturer's or dealer's specifications are not mandatory.

(5) What does the term 'curve' mean in a welding current curve?     

The static current/voltage curve (flat, falling) for the power supply, It means a welding process, Assigns mains voltage to welding current, The dynamics of the welding current and voltage, A synergy curve is a static and dynamic assignment of process data for a welding process for a defined performance range (one-button operation).

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(6) Which statements are correct?     

The flat curve is used for MIG/MAG welding. The flat curve is used for manual electric welding. The flat curve is used for plasma welding. The falling curve is used for MIG/MAG welding. The falling curve is used for TIG welding.

(7) Pulse welding ...     

Imposes high demands on welding power supplies in terms of fast current and voltage control. Also works without process gas. Works with argon-rich process gases. Only works with electronic welding power supplies. Is a special kind of MIG/MAG welding.

(8) What controls are used in welding?     

ΔI control for MIG/MAG welding ΔI control for submerged arc welding ΔI control for manual electric welding ΔU control for MIG/MAG welding ΔU control for manual electric welding

(9) Which statements are incorrect?     

Duty cycle is a term used for fan operation. Duty cycle is equivalent to the welding time divided by 10 min run time, stated as a percentage. Protection class IP23 is suitable only for operating power supplies in enclosed spaces. AF means cooling without fans. S indicates equipment without fans (self-cooling).

(10) Which of the statements below regarding the no-load voltage in a welding power supply are correct?     

The no-load voltage is also known as the mains input voltage. The no-load voltage must be stated on the rating plate. The no-load voltage limits the use of the power supply. The no-load voltage is irrelevant for the use of the power supply in practice. No-load voltage is only important with TIG welding.

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(11) What do we mean by these terms?     

[S] marking  the power supply has an no-load voltage of < 113 V DC and an RMS value of 48 V AC. S stands for self-cooling. [S] means  a risk reducing device, e.g. switching from DC operation in no-load mode to AC operation in welding mode and vice versa. Potential diversion  is a metrology problem. Potential diversion  always occurs if the welding return line is not directly connected to the welding point, but is only connected via the earth wire (PE line): this always occurs for example if the welding electrode holder with a live electrode is laid on a power supply with a metal housing.

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Introduction to Gas Shielded Arc Welding

Chapter 1.06:

SFI / IWE 1.06 Page 1

Introduction to Gas Shielded Arc Welding

Contents 1

2

3

4

5 6

Classification of gas-shielded welding processes ................................................................ 3 1.1 Gas-shielded metal arc welding (MIG/MAG) ........................................................................ 3 1.2 Tungsten Inert Gas Welding (TIG) ........................................................................................ 4 1.3 Plasma gas-shielded metal arc welding (Plasma MIG/MAG welding) ............................... 5 1.4 Tungsten Plasma welding ......................................................................................................... 5 Shielding gases ............................................................................................................................ 6 2.1 Requirements for shielding gases ......................................................................................... 6 2.2 Properties of shielding gases ................................................................................................. 7 2.3 Heat conductivity of shielding gases and gas constituents ................................................. 8 2.4 Classification and designation of shielding gases to DIN EN ISO 14175 .......................... 8 2.5 Purities and dew point .......................................................................................................... 10 Examples of classification and designation of shielding gases to DIN EN ISO 14175 ....................................................................................................................... 10 3.1 Examples of classification .................................................................................................... 10 3.2 Examples of designations .................................................................................................... 11 Choice of Shielding Gases ....................................................................................................... 11 4.1 Active shielding gases for MAG- welding of steel .............................................................. 11 4.1.1 Argon-/CO2-mixtures ................................................................................................. 12 4.1.2 Argon-/O2-mixtures .................................................................................................... 12 4.1.3 Argon-/CO2 -/O2 –mixtures ....................................................................................... 12 4.1.4 Carbon dioxide (CO2) ................................................................................................ 12 4.2 Inert shielding gases for TIG welding .................................................................................. 13 4.2.1 Argon .......................................................................................................................... 13 4.2.2 Helium, Argon/helium- mixtures ............................................................................... 13 4.2.3 Argon/hydrogen- mixtures ........................................................................................ 13 4.2.4 Argon of high purity (4.8 minimum).......................................................................... 13 4.3 Inert shielding gases for MIG-welding ................................................................................. 13 4.3.1 Argon .......................................................................................................................... 13 4.3.2 Helium......................................................................................................................... 14 4.3.3 Argon/Helium-mixtures ............................................................................................. 14 Shielding gases and gas-mixtures in dependence of the welding process and base material ......................................................................................................................................... 14 Shielding gas consumption and measuring ......................................................................... 15 6.1 Gas flow consumption setting ................................................................................................. 15 6.1.1 Flow rate measuring/setting via a flow-orifice ......................................................... 15 6.1.2 Flow rate setting via a floating device ..................................................................... 15 © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorized disclosure are prohibited and will be prosecuted in accordance with the law

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7

Fabrication, transport and storage of shielding gases ...................................................... 16 7.1 Storage of shielding gases in gas cylinders ....................................................................... 17 7.2 Liquefied gases ..................................................................................................................... 17 8 Factory supply of shielding gases.......................................................................................... 17 9 Safety measurements for handling, transport and application of shielding gases ...... 18 9.1 Handling of (high) gas cylinders .......................................................................................... 18 9.2 Colour identification of gas cylinders (DIN EN 1089) ................................................................ 18 9.3 Hazardous goods sticker ...................................................................................................... 19 10 Bibliography ................................................................................................................................ 20 11 Questions ..................................................................................................................................... 21

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Introduction to Gas Shielded Arc Welding

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Page 3

Classification of gas-shielded welding processes

The term 'gas shielded welding' covers all arc welding processes in which the consumable filler material or non-consumable electrode and weld pool are protected from the degrading influences of the ambient air by an added shielding gas. These gas shielded welding processes have been classified into two main groups according to the melting behaviour of the electrode.  

Gas-shielded Metal Arc Welding (MIG/ MAG welding), having a consumable electrode Gas-shielded Tungsten Arc Welding (TIG welding), having a non-consumable electrode

A more detailed classification is shown in figure 1. Gas-shielded Arc Welding Gas-shielded Metal Arc Welding

Metal Inert Gas Welding with flux cored wire

Electric Gas Welding

MIG Metal Inert Gas Welding

Plasma metal gas-shielded welding

MAG Metal Active Gas Welding

MAG-C CO2-welding

MAG-M Gas mixture welding

Gas-shielded Tungsten Arc Welding

Metal Active Gas Welding with flux cored wire

Plasma Beam welding

TIG Tungsten Inert Gas Welding

Plasma Arc Welding

Plasma Welding

Plasma JetArc Welding

Powder Plasma Welding

Figure 1: Classification of gas shielded welding processes /2/

1.1

Gas-shielded metal arc welding (MIG/MAG)

For gas-shielded metal arc welding the following process variables have been differentiated for Metal Inert Gas Welding (MIG) and Metal Active Gas Welding (MAG). Process numbers according to DIN EN ISO 4063 Metal-active gas welding with solid wire Metal-active gas welding with flux cored wire Metal-active gas welding with metal cored wire Metal-inert gas welding with solid wire

135 136 138 131

The heat-source for welding is an ignited arc between the base metal and the continuous filler metal electrode. In order to protect the melt pool from the degrading influences of the surrounding air a shielding gas is required. For MIG-welding of non-ferrous materials argon, helium or mixtures of both are being used. For MAGwelding active shielding gas is being used for metallurgical or technical reasons. For welding of unalloyed, low-alloyed and high-alloyed steels active shielding gases are used like CO2 (MAG-C), ArgonCO2-mixtures, Argon-Oxygen-mixtures and more component-mixture- gases (MAG-M).

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Wire electrode Wire feeder guiding rolls

Contact tube

Shielding gas Shielding gas nozzle

Power supply Weld Arc Workpiece

Figure 2: Principle of metal gas shielded welding works

1.2

Tungsten Inert Gas Welding (TIG)

The heat-source for welding is an ignited arc between the base metal and a non-melting tungsten electrode. The filler material is added manually into the arc where it is melted. The heated tungsten electrode, the melt pool and the surrounding area are protected by the shielding gas from the surrounding air. Shielding gases like Argon, Helium or mixtures from both and for specific base materials also Argon-Hydrogen-mixtures are being used. The thermal capacity of the Tungsten electrode is limited. This sets restrictions to the deposition rates and the welding speed but on the other hand this enables a precise weld-layout even with an imprecise weld preparation. Process numbers according to DIN EN ISO 4063 Tungsten-inert-gas welding with solid wire or rod Tungsten-inert-gas welding without filler wire or rod Tungsten-inert-gas welding with flux cored wire or rod Tungsten-inert-gas welding with solid wire or rod with deoxidizing gas (partly)

141 142 143 145

Tungsten electrode Contact tube Shielding gas Shielding gas nozzle

Power supply

Filler material

Workpiece

Arc

Figure 3: Tungsten inert gas welding (TIG)

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Introduction to Gas Shielded Arc Welding 1.3

SFI / IWE 1.06 Page 5

Plasma gas-shielded metal arc welding (Plasma MIG/MAG welding)

This process is a combination of gas-shielded metal arc welding (MIG/MAG) and plasma welding. At the beginning of the weld the annular plasma-arc is heating the base material before the continuous filler material is being added. Two independent power sources are used; one with the constant current characteristic curve (Plasma power source) and the other one with a constant voltage characteristic curve (MIG/MAG power source)

Plasma (focussing) gas

Plasma gas Cooling water

Impuls generator

Shielding gas

Plasma Power supply

Constant Voltage Power supply

Figure 4: Plasma MIG/MAG welding

1.4

Tungsten Plasma welding

The arc is located between the tungsten electrode and either a water-cooled copper-nozzle (nontransferred-mode) or a base material (transferred-mode) located in a gaseous atmosphere. This coppernozzle (orifice-nozzle) is constricting the arc and increases the density-capacity. Tungsten electrode Cooling water Shielding gas Plasma gas Orifice nozzle

Plasma nozzle

Figure 5: Tungsten Plasma welding.

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Introduction to Gas Shielded Arc Welding 2

SFI / IWE 1.06 Page 6

Shielding gases

During gas-shielded arc welding it is essential to protect the weld pool from the degrading influences of the air. Without this protection the weld pool would be mainly contaminated with nitrogen- and oxygen inclusions and embrittlement of the weld metal would become evident. Shielding gases protect the weld pool, the melting filler material and the non-consumable tungsten electrode, principally against the effects of the air. 2.1

Requirements for shielding gases

Active shielding gases are gases which cause the arc to interact chemically and physically with the filler and base material. Inert shielding gases are gases which do not enter into chemical reactions during welding. Shielding gases for welding and cutting differ in terms of: 1. 2. 3. 4. 5.

how they react during welding (inert, oxidising, slow-reacting, reducing) density (heavier/lighter than air) thermal conductivity and capacity ionisation/dissociation energy and hence in the arc voltage, in relation to a given arc length (arc curve) boiling point

Summary of requirements for shielding gases General requirements: – – – – – – – –

Suitability for all types of arcs Sufficient shielding effect depending on place of use and type of weld Favourable melt flow characteristics Not sensitive to impurities in the weld seam area Can be used for all wire diameters Slag formation and/or distribution over the weld surface Resistance to pore formation Prevention of weld spatter

Physical requirements: – – – – – –

Ignition behaviour at start of welding Arc stability, i.e. constant arc Arc shorting out and re-igniting Plasma formation/electrical conductivity Dissolution characteristics depending on the material Degassing characteristics

Thermal requirements: – – –

Heat conductivity Heat capacity Heat transfer capacity, i.e. heat transfer coefficient

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Metallurgical requirements: – – – –

Element pick-up Element burn-off Preservation of mechanical-topological characteristics Preservation of corrosion-resistance characteristics

2.2

Properties of shielding gases

Which gases or gas mixtures are suitable for shielding gases depends mainly on the type of base material. Many of the base material groups can be welded with different types of shielding gases so that many relationships have to be considered before making a choice of shielded gas. Shielding gases are standardized in DIN EN ISO 14175. In this standard the most common, for practical use, shielding gases are listed according to their chemical behaviour and composition. Table 1: Gas characteristics to DIN EN ISO 14175

Gas

Chem. symbol (s)

Argon Helium Carbon dioxide Oxygen Nitrogen Hydrogen 1) 2)

Specific characteristics at 0 °C and 1.013 bar (0.101 MPa)

Reaction behaviour during welding

Density (air = 1.293) [kg/m³]

Relative density to air

Boiling point [°C]

1.784 0.178 1.977 1.429 1.251 0.090

1.380 0.138 1.529 1.105 0.968 0.070

- 185.9 - 268.9 1) - 75.5 - 183.0 - 195.8 - 252.8

Ar He CO2 O2 N2 H2

Inert Inert Oxidising Oxidising 2) Slow-reacting Reducing

Sublimation temperature Nitrogen behaves differently with different materials, and the effects may be adverse.

The huge difference in the density of Argon and Helium is being reflected to the required flow quantity. In case helium- or helium holding shielding gases are used the actual gas flow quantity must be increased by 2 or 3 times when using an argon reducer. Based upon these density differences between Argon and Helium correction factors have to be considered for making settings of argon calibrated pressure reducers. Table 2: Density and correction factor depending on the helium part in argon based shielding gases Gas, / gas mixtures

100% Argon 75% Ar + 25% He 50% Ar + 50% He 25% Ar + 75% He 100% He

Density At 15°C, 1 bar 3 [kg/m ] 1,78 1,29 0,92 0,54 0,17

Correction factor flow-reading multiplied with 1,00 1,14 1,35 1,75 3,16

Example: Helium- Argon mixture Gas 25% Ar + 75% He

Flow-reading

Correction factor

Actual gas flow

12 l/min

1,75

21 l/min

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Heat conductivity of shielding gases and gas constituents

Figure 6: Heat conductivity of shielded gas constituents (according to Linde)

The heat conductivity of the shielding gas affects how the weld seam is formed, the weld pool temperature, weld pool degassing and welding speed. During MIG and TIG welding of aluminium materials, the welding speed and weld penetration behaviour with can be increased considerably by adding helium or, during TIG welding austenitic of austenitic steels, steels by adding hydrogen. Chemical characteristics influence the metallurgical behaviour and the weld seam surface. For example, oxygen and carbon dioxide for example, lead to the burning off of alloy elements and low viscosity of the weld pool; both gases are oxidants. Hydrogen is a reducing gas; argon and helium do not react with metals, as they are inert. 2.4

Classification and designation of shielding gases to DIN EN ISO 14175

Gases and gas mixtures are classified by the number in accordance with the above international standard, followed by the symbol for the gas as in Table 3 NOTE: This classification is based on the reaction behaviour of the gas or gas mixture. Main group: The letters and numbers used for the main group are as follows: — — — — — — —

I: M1, M2 and M3: C: R: N: O: Z:

inert gases and gas mixtures; oxidising gas mixtures with oxygen and/or carbon dioxide; highly oxidising gases and gas mixtures; reducing gas mixtures; slow-reacting gas or gas mixture with nitrogen; oxygen; gas mixtures with constituents which are not included in Table 3 or gas mixtures whose compositions are outside the ranges stated in Table 3.

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Sub-group The main groups apart from Z are divided into sub-groups, depending on the presence and content of certain constituents which affect the reaction (see Table ). The values stated in Table 3 are nominal values. Table 3: Classification of process gases for welding joints and related processes Symbol

Constituents in percent by volume (nominal)

Main group

Subgroup

I

1

oxidising CO2

inert O2

Ar

2

Rest 0.5 ≤ CO2 ≤ 5

Resta

0.5 ≤ CO2 ≤ 5

a

3 M2

R N

0.5 ≤ O2 ≤ 3

Resta

0.5 ≤ O2 ≤ 3

Resta

0.5 ≤ CO2 ≤ 5

0

5 < CO2 ≤ 15

Resta

1

15 < CO2 ≤ 25

Resta 3 < O2 ≤ 10

Resta

3

0.5 ≤ CO2 ≤ 5

3 < O2 ≤ 10

Resta

4

5 < CO2 ≤ 15

0.5 ≤ O2 ≤ 3

Resta

5

5 < CO2 ≤ 15

3 < O2 ≤ 10

Resta

6

15 < CO2 ≤ 25

0.5 ≤ O2 ≤ 3

Resta

7

15 < CO2 ≤ 25

3 < O2 ≤ 10

Resta

1

25 < CO2 ≤ 50

2

C

Rest

4

2

M3

10 < O2 ≤ 15

Resta

3

25 < CO2 ≤ 50

2 < O2 ≤ 10

Resta

4

5 < CO2 ≤ 25

10 < O2 ≤ 15

Resta

5

25 < CO2 ≤ 50

10 < O2 ≤ 15

Resta

1

100

2

Rest

a b

0.5 ≤ H2 ≤ 5

0.5 ≤ O2 ≤ 30

1

Resta

0.5 ≤ H2 ≤ 15

2

Rest

a

15 < H2 ≤ 50

Rest

a

3

Rest

a

4

Resta

1

100

5

Z

0.5 ≤ He ≤ 95

Resta

2

O

N2

100

3 1

Low reactivity

H2

100

2 M1

Reducing He

1

0.5 ≤ N2 ≤ 5 5 < N2 ≤ 50 0.5 ≤ H2 ≤ 10

0.5 ≤ N2 ≤ 5

0.5 ≤ H2 ≤ 50

Rest

100

Gas mixtures with constituents which are not included in the table or gas mixtures whose compositions are outside the ranges indicated.b

For the purpose of this classification, argon may be substituted partially or completely with helium. Two gas mixtures with the same Z classification must not be interchanged.

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Purities and dew point

The purity and dew point of the gas constituents and gas mixtures must meet the requirements specified in Table 2. Moisture content can be expressed either as a concentration in ppm (parts per million) or as the dew point at 0.101 MPa in °C. Purities and moisture contents of specific gas mixtures are not covered in the standard DIN EN ISO 14175. Table 2: Minimum requirements for purities and moisture contents of gases and gas mixtures Dew point at 0.101 MPa

minimum

°C

Inert

99.99

−50

40

Main group/gas

I

Moisture content (volumes) ppm maximum

Purity Vol.%

M1

a

Gas mixture

99.9

−50

40

M2

a

Gas Mixture

99.9

−44

80

M3

a

Gas Mixture

99.9

−40

120

Carbon dioxide

99.8

−40

120

R

Reducing

99.95

−50

40

N

Nitrogen

99.9

−50

40

O

Oxygen

99.5

−50

40

C

a

IMPORTANT, please note: a higher purity and/or lower dew point may be advisable for certain applications to avoid potential oxidisation and contamination. a

Nitrogen: max. 1,000 ppm

Note: on the market the purity of gases is generally indicated by number codes. For example, Argon with a purity of 99,996% is indicated by Argon 4.6.

3 3.1

Examples of classification and designation of shielding gases to DIN EN ISO 14175 Examples of classification

Number of this international standard, followed by the symbol for the gas or gas mixture (main group and sub-group) EXAMPLE 1

Gas mixture with 6% carbon dioxide, 4% oxygen, the rest argon Classification: ISO 14175 – M25

EXAMPLE 2

Gas mixture with 30% helium and the rest argon: Classification: ISO 14175 – I3

EXAMPLE 3

Gas mixture with 5% hydrogen and the rest argon: Classification: ISO 14175 – R1

EXAMPLE 4

Gas mixture with 0.05% oxygen and the rest argon: Classification: ISO 14175 – Z

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Examples of designations

The base gas symbol must be followed by the symbols for the other constituents in decreasing order of percentage content followed by the values for the nominal composition in per cent by volume, separated by a dash. EXAMPLE 1

Gas mixture with 6% carbon dioxide, 4% oxygen and rest argon Classification: ISO 14175 – M25 Designation: ISO 14175 – M25 – ArCO – 6/4

EXAMPLE 2

Gas mixture with 30% helium and rest argon: Classification: ISO 14175 – I3 Designation: ISO 14175 – I3 – ArHe – 30

EXAMPLE 3

Gas mixture with 5% hydrogen and rest argon: Classification: ISO 14175 – R1 Designation: ISO 14175 – R1 – ArH – 5

EXAMPLE 4

Gas mixture with 7.5% argon, 2.5% carbon dioxide and the rest helium: Classification: ISO 14175 – M12 Designation: ISO 14175 – M12 – HeArC – 7.5/2.5

For gas mixtures with constituents listed in Table 3 but whose contents are outside the ranges stated, the base gas symbol must be preceded by the letter Z. This is followed by the symbols for the components as stated above, followed by the values for the nominal composition in percentage by volume, separated by a forward slash. EXAMPLE 5

Gas mixture with 0.05% oxygen and rest argon: Classification: ISO 14175 – Z Denomination: ISO 14175 – Z – ArO – 0.05

For gas mixtures with constituents which are not listed in Table 3, the base gas symbol must be preceded by the letter Z. This is followed by the symbols for the constituents as stated above, but with a plus sign before the constituents not listed, followed by the values for the nominal composition in per cent by volume, separated by a dash. EXAMPLE 6

4 4.1

Gas mixture with 0.05% xenon, chemical symbol Xe, rest argon: Classification: ISO 14175 – Z Designation: ISO 14175 – Z – Ar+Xe – 0.05

Choice of Shielding Gases Active shielding gases for MAG- welding of steel

Shielding gases which are reacting with the melting filler wire and the weld pool are designated as active shielding gases (Metal Active-Gas welding). CO2 or O2 and/or CO2 portions in Active shielding gases consists CO2 or for mixture gases with high argon content, components of O2 and/or CO2. The chemical behaviour of oxygen-emitting shielding gases is called “oxidising” which are mainly used for welding of steel. Compared to welding with pure argon, welding of steel with oxidising, argon-rich shielding gases and pure CO2 creates fewer pores, less undercut and a more stable arc.

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Argon-/CO2-mixtures

For un-alloyed and low-alloyed steels Ar/CO2-mixtures are used with CO2-portions of 4 – 25% (common used shielding gas: 82% Ar + 18% CO2). This shielding gas type is suitable for welding solid wires and most of the flux cored wires. Compared to pure Argon with an increasing CO2 percentage the side penetration becomes more, less pores-intensity and a bigger slag quantity is created. For welding in the short-arc mode Ar/CO2 mixtures are suitable for welding thin sheet metals and gap bridging. For welding in spray-arc mode only a restricted percentage of CO2 is possible. With percentages of 20% of more even at high arc-intensities short-circuit situations are occurring during the drop transfer. Even during pulsed-welding the drop-transfer is becoming more difficult with increasing CO2 percentages and therefore the CO2-percentage is limited to 20% For welding of high-alloyed, austenitic CrNi-steels the C O2 percentage is limited to 5% in order to prevent Cr-depletation at the grain borders and the corresponding intergranular corrosion. In this situation an argon-rich mixture gas with 2.5% C O2 is common. 4.1.2

Argon-/O2-mixtures

Oxygen percentages of 1 – 12% are being used for welding of steels. Compared to pure argon a higher side penetration and a more stable arc are achieved. With increasing oxygen percentages the surface tension of the weld pool is lowered which enables, especially in the flat welding position, more smoother and flat weld seams. For welding in the short-circuit-arc mode Ar/Oxygen-mixtures are very suitable and in the spray- and pulsed mode welding it creates a very stable arc. The spray arc range already starts at lower arc-intensities. 4.1.3

Argon-/CO2 -/O2 –mixtures

Common mixtures are Argon with 3-8% O2 and 5-15% C O2. These mixtures are suitable for welding unalloyed and low-alloyed steels. In the short-circuit-arc mode it is most convenient for welding thin sheet metal and gap-bridging. In the spray-arc mode the drop transfer volume is small and less spatter. 4.1.4

Carbon dioxide (CO2)

CO2 is mainly used for welding un-alloyed steel. The CO2 arc is, next to metal-vapours, mainly influenced by the heat conductivity of the gas. CO2 conducts/ transfers the heat very well so that under similar current-intensity conditions the voltage-setting is about 4 volts higher. The penetration profile under similar arc conditions is significantly wider compared to argon-rich mixtures. The out-of-position welding is improved, for example welding down of thick components. The drop transfer is even at higher arc-intensities not free of short-circuit situations. The forces of the arc and the short-circuit conditions are making the drop transfer more difficult which creates a more increased spatter and the weld pool is placed into a swinging motion. This results in a coarser weld seam appearance. When welding with CO2 mixtures the setting of the arc-voltage and the current intensity has to be done more accurate compared to mixture gases. The welder requires a certain experience for setting the working point. The use of an adjustable inductance is to be preferred to limit the short-circuit-current-peaks during the drop transfer. Welding of thin sheet metals ( 4.8

Argon with purity > 4.8

Duplex- und Super-Duplex-Steels

Argon Ar + N2 ≤ 5%

Argon

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Table 6: Shielding gases for MIG/MAG welding in dependency of the base material Base material MIG MAG

Non-alloyed and low-alloyed steels

Ar 92% + O2 8% Ar 82% + CO2 18% Ar 90% + CO2 5% + O2 5% Ar 82% + CO2 14% + O2 4% CO2 100%

high alloyed corrosion-, acid- and heat resistance steels, creep- and lowtemperature resistance steels

Ar 97% + O2 3% Ar 98 % + CO2 2% Ar 69% + He 30% + O2 1%

SFI / IWE 1.06 Page 15

Root shielding gas

Argon N2 90% + H2 10% Ar 90% + H2 10%

Aluminium, Al-alloys Copper- and Cu-alloys

Argon 100% Ar 50% + He 50%

Argon

Nickel and Ni-alloys

Argon 100%

Argon

Gas-sensitive alloys like Titanium, Tantalum etc.

Argon with purity ≥ 4.8

Argon with purity ≥ 4.8

6

Shielding gas consumption and measuring

The shielding gas consumption (shielding gas flow) is determined by a number of factors, as follows:    

the welding process and weld pool size the electrode size (diameter) with MIG/MAG welding gas type and composition size of welding torch/gas nozzle

As a guidance/setting value the shielding gas flow for MAG welding can be calculated via the formula: wire-diameter-size x 10-12. Example: wire diameter=1.2mm  glas flow rate: 14 l/min. For TIG welding the gas-flow setting can be achieved from the gas nozzle inside diameter: with a gass nozzle inside diameter of 8mm a gas flow rate of 8 l/min is required. 6.1

Gas flow consumption setting

In order to reduce the cylinder pressure, nowadays often set to 300 bar, it is required to use a pressure reducer which sets the gas to the desired pressure and enables a suitable gas-flow rate for the specific weld job.

6.1.1

Flow rate measuring/setting via a flow-orifice

In the shielding gas supply line a calibrated orifice with a defined flow cross-section is integrated which sets the flow rate in dependency of the gas pressure. With the pressure adjustment screw the required gas pressure is set after which the gas flow manometer is showing the flow-rate. 6.1.2

Flow rate setting via a floating device

The pressure is set constantly. By turning the adjustment screw the flow-cross-section, respectively the gas-flow rate will be changed. The flowing gas is lifting the floating device (ball) accordingly to the flowrate. The flow-rate value is to be read from the top of the ball.

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1 Bottle pressure manometer

SFI / IWE 1.06 Page 16

1 Bottle pressure manometer

2 Gas flow manometer

2 Flow tube with floating device

3 Pressure adjustment screw

3 Adjustment screw

4 Shut-off valve

4

5 Calibrated orifice

Type of gas

6 Type of gas

Figure 7: Shielding gas flow setting and measuring device

7

Fabrication, transport and storage of shielding gases

Argon, nitrogen, oxygen and inert gases are produced mainly by liquefying air in modern air separation plants. Helium is an exception which comes from natural gas sources in the USA, and is therefore cheaper to produce there than separating it from the surrounding air having a helium content of 0.00052%. For liquefying the air, temperatures near to minus 200 °C are required. For the following separation of the different components each critical temperature point is successively being used Fabrication The six basic shielding gases are manufactured differently

Argon, nitrogen and oxygen are manufactured by liquefying air, consisting 78%N2, 21%O2 and 1% nobelgases, in large air separation plants. For the separation of liquefied air into it’s components, the different critical temperature points are being used.

Helium is just for a very small part available in the air and is being produced more economically from earth-gas-sources in the USA and East-Europe. The in the market used carbon dioxide (CO2) is mainly being extracted from natural resources in Germany. A part is also extracted out of the Chemical Industry Natural hydrogen (H2) is mainly enclosed in water and can be extracted out if with a huge energy input by electrolysing. The in the market available quantities are mainly produced via the chlorine-alkaline-electrolyse process in which the chlorine and caustic soda are manufactured from a watery saline solution. Schematic representation of a separation unit

Figure 8: Surrounding air separation unit

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Storage of shielding gases in gas cylinders

Oxygen, argon, nitrogen and hydrogen are gases which are in gaseous form at ambient temperature and pressure, and are supplied mainly in steel cylinders. The maximum permitted pressures are 150 or 200 bar, the latest cylinders have 300 bar. The actual pressure can fluctuate from the given pressures and even depends on the surrounding temperature. Carbon dioxide, CO2 liquefies at around 54 bar and + 15 °C. In a cylinder full of liquefied gas, the gas is liquefied completely except for a small gas cushion. Table 7: Storage of gases Argon, helium, gas mixtures: gaseous Cylinder capacity in l 10 20 Filling pressure in bar 200 200 Gas content in m³ 2 4

Cylinder capacity in kg Content in l Gas content in m³

Carbon dioxide: liquid 10 20 13 26 5 11

50/50 200/300 10/15

30 40 16

If larger amounts are removed, icing may occur. For liquefied gases like CO2, the use of a pre-heater in front of the pressure reducer is preferred. 7.2

Liquefied gases

Liquid gases are supplied in insulated tanks at suitably low temperatures. Carbon dioxide is supplied in cylinders in liquid form at ambient temperature. Liquid gases must be converted to their gaseous state before using. In order to produce gas mixtures, the liquid gas constituents must be vaporised before mixing. Argonoxygen mixtures can also be stored pre-mixed in liquid form, without needing a mixer unit to supply them.

8

Factory supply of shielding gases

In general the required shielding gases are supplied in separate gas cylinders. But for higher demands and efficient working the shielding-gas supply comes from cylinder bundles or batteries and a cold gasification system via a central ring-supply to the specific area. An example is given by figure 9.

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Cold gasification system for Argon Cylinder bundle

Cylinder battery

Shielding gas

Forming gas

Ring supply

Single cylinder supply

Fixture for purging of root pass

Cylinder bundle

Cylinder battery

Figure 9: Central shielding-gas supply system

9 9.1

Safety measurements for handling, transport and application of shielding gases Handling of (high) gas cylinders

Gas cylinders have to be transported with care and are not allowed to be thrown or rolled. During transport or storage the safety caps have to be attached properly. Gas cylinders are to be protected against any uncontrolled heat- or impact- influences. Vertical stored cylinders have to be secured for falling down via chains or brackets. In the workshop area only the actual to be used gas cylinders are to be found. A collecting of gas cylinders should be avoided. The refilling from larger cylinders into smaller cylinders requires special knowledge and is prohibited for this reason. The gas cylinder storage area must be ventilated sufficiently. The density of each shielding gas partly differs enormous from the surrounding air density. Shielding gases which are heavier than the surrounding air are replacing the breathing air and are a danger for the employees. Hydrogen containing shielding gases can create dangerous concentrations of detonating hydrogen gas in the surrounding air. The stamp marking of gas cylinders is according to DIN EN ISO 13769. On the gas cylinder shoulder for example the following details are marked: the test date, empty weight, capacity and working pressure. 9.2

Colour identification of gas cylinders (DIN EN 1089)

The colour identification is realised by colouring the cylinder shoulder or the complete cylinder according to the type of gas. The colour identification is only used as additional information. The only mandatory identification is defined by hazardous good stickers.

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until 2006

until 2006

Actual

Nitrogen

Page 19

until 2006

Actual

Hydrogen

Dark green

Black

red

red

Dark green

grey (dark green, black)

red

red

Actual

Oxygen (technical)

Carbondioxide

Forming gas (Mixture nitrogen / hydrogen)

blue

white

grey

grey

red

blue

blue (grey)

grey

grey

red (dark green)

Acetylene

Helium

grey

Mixture Argon/CO2

yellow

maroon

grey

brown

grey

Bright green

yellow (black)

maroon (black, yellow)

grey

grey

grey

grey

Argon

9.3

red

Xenon, Krypton, Neon grey

Dark green

grey

grey (dark green)

grey grey (black)

Pressed air Bright green

grey

Bright green

grey (bright green)

grey

grey

Hazardous goods sticker

The hazardous stickers, as shown in figure 11 and 12 meet the transport regulations (GGVS/GGVE). The new sticker (figure 11) is marked with the warning word “Danger” = “GEFAHR”. Above this the new symbols are given. On the left side the danger- and safety details (P- and H-marks). On the right side, additional information.

Figure 11: hazardous goods sticker (new)

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1 2 3 4

Risks and safety information Hazard labels Gas composition of the gas or gas mixture Manufacturer's product name

5 6 7 8

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EEC no. for individual substances or the word 'gas mixture' Full gas details to GGVS Manufacturers' instructions Manufacturers' name, address and telephone no.

Figure 3: Hazardous goods sticker

10 Bibliography /1/ DIN EN; DIN EN ISO 13769: gas cylinders – stamp marking DIN Deutsches Institut für Normung e.V.; Berlin; Beuth Verlag GmbH /2/ DIN EN 14610 welding and allied processes – definitions of metal welding processes /3/ TRGS 510 Technical regulations for compressed gases /4/ TRBS 3145 Assembly and storage of gas cylinders / vessels

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11 Questions (1)

Which functions does the shielding gas have for Gas-shielded Metal Arc Welding and Gasshielded Tungsten Arc Welding?    

(2)

Which shielding gases are applicable for MAG- welding?    

(3)

Liquid Gaseous Dissolved Solid, by means of dry ice

How are gas cylinders designated permanently    

(7)

The shielding gas has a composition of 82% Argon, rest CO2 The shielding gas has a composition of 82% CO2, rest Argon The shielding gas is suitable for the TIG welding of steel It´s a mixture gas made of main group M2 and subgroup 1

In which manner is CO2 being stored in gas cylinders?    

(6)

Active shielding gases destroy the tungsten electrode Inert shielding gases improve the penetration profile Inert shielding gases are more economical than active shielding gases During the application of active shielding gases the creation of pores is initiated.

What means the designation M21 of a shielding gas?    

(5)

Argon and Helium Nitrogen and hydrogen Carbon dioxide and mixture gases Krypton and Xenon

For TIG welding only inert gases can be used. Why?    

(4)

Cooling of the electrode Protection of the electrode and the weld pool from the degrading influences of the surrounding air Preventing fume initiation Cooling of the welding torch, in order to weld with not-water-cooled torches

By means of specific colouring all around the cylinder By means of the hazardous sticker By stamping of the designation on the cylinder shoulder By means of specific colouring on the cylinder shoulder

Which kind of shielding gas supply is more or less the appropriate choice for small and middle big companies?    

Central shielding gas supply with cold gasifier system Central shielding gas supply with cylinder bundle Separate cylinder shielding gas supply Central shielding gas supply with cylinder battery

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TIG Welding

Chapter 1.07:

SFI / IWE 1.07 Page 1

TIG welding

Contents 1

Principle ............................................................................................................................... 3

2

Welding unit and the welding process .............................................................................. 3

3

Shielding gases used and application of the working process ...................................... 4

4

TIG welding torches (types) ............................................................................................... 4 4.1 Air-cooled TIG welding torches can be used up to an arc amperage of approx. 250 A. . 5 4.2 Water-cooled TIG welding torches are used for high arc power and duty cycle. ............ 5

5

Tungsten electrodes ........................................................................................................... 6 5.1 Electrode tip shapes ....................................................................................................... 8 5.2 Classification of tungsten electrodes in accordance with DIN EN ISO 6848 ................... 9

6

Influence of the shielding gas on the penetration profile .............................................. 10

7

Ignition of TIG arcs ............................................................................................................ 10 7.1 Contactless Ignition ...................................................................................................... 10 7.2 Lift-arc Ignition .............................................................................................................. 11

8

Purging ............................................................................................................................... 11 8.1 Purging gas and work safety ........................................................................................ 12 8.2 Purging fixtures ............................................................................................................. 14

9

TIG welding of aluminium ................................................................................................. 16 9.1 Overview ....................................................................................................................... 16

10

TIG welding aluminium with alternating current...................................................... 16

11

TIG welding aluminium with direct current .............................................................. 18

12

Weld preparation for TIG aluminium welding .......................................................... 20

13

Welding defects .......................................................................................................... 21

14

Process variants of TIG welding ............................................................................... 24 14.1

15

Possible mechanisations levels of TIG welding. ..................................................... 25

Process variants of TIG welding. .............................................................................. 26 15.1

TIG pulsed welding ................................................................................................ 26

15.2

TIG welding with filler wire...................................................................................... 27 15.2.1

TIG welding with cold wire ....................................................................... 27

15.3

TIG welding of rotated components ....................................................................... 28

15.4

Orbital TIG welding ................................................................................................ 29

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15.5

Orbital TIG Narrow gap welding ............................................................................. 32

15.6

TIG hot wire welding .............................................................................................. 34

15.7

TIG multi-cathode welding...................................................................................... 34

15.8

TIG welding with a double gas nozzle .................................................................... 36

15.9

TIG spot welding .................................................................................................... 36

15.10

TIG welding with two torches (simultaneously) ...................................................... 38

15.11

TIG Key-hole welding. ............................................................................................ 38

16 Test questions ................................................................................................................... 41

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Page 3

Principle

A tungsten electrode is clamped inside the welding torch, providing the location where the welding current is introduced. An arc is formed between the tungsten electrode and the workpiece that fuses the base material and melts the added filler metal. Inert shielding gas streams out of the welding torch and screens off the glowing tungsten electrode and the weld pool from air.

2

Welding unit and the welding process Shielding gas supply

Power source with control and arc-ignition

Filler material

Welding Torch

Welding hose with current-, gas- and water supply

1 2 3 4 5 6 7 8 9 10 11 12 13

Work cable Work piece

Mains connection Welding power source Welding current cable (electrode Welding work cable (workpiece) Work clamp Shielding gas cylinder with pressure reducer and gas flow meter Shielding gas hose Welding Torch Welding rod Workpiece Tungsten electrode Collet and current transfer Arc

14 Liquid weld metal 15 Solid weld metal

16

Inert gas shield

Figure 1: Schematic diagram of the TIG welding equipment and the welding process

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Shielding gases used and application of the working process

In the case of tungsten inert gas welding, only inert shielding gas is used as the glowing tungsten electrode must never be subjected to chemical reactions: - argon (Ar) - helium (He) - mixtures of argon (Ar) and helium (He) and hydrogen (H2). The tungsten inert gas welding method enables the welding of steel and non-ferrous metals in all positions. Material thicknesses of 0.5 mm to 5 mm allow economical applications; in the case of thicker workpieces only root penetration passes will be welded with this welding process. Important areas of application are aviation and aerospace technology, precision mechanics, construction of chemical equipment, apparatus and containers/vessels.

4

TIG welding torches (types)

In the case of TIG welding, gas or water-cooled torches are used depending on the required arc powers. The basic design of a torch is as follows:

Torch cap

Tungsten Electrode

Collet

Torch Body Isolation Ring

Push-button switch

Torch Collet Holder with Gaslens

Collet Holder Shielding Gas Nozzle

Figure 2: Basic design of a torch

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SFI / IWE 1.07 Page 5

Air-cooled TIG welding torches can be used up to an arc amperage of approx. 250 A.

Tungsten electrode

Torch handle

Cable connection

Argon hose with cables

Figure 3: Cross section of an air-cooled welding torch

4.2

Water-cooled TIG welding torches are used for high arc power and duty cycle.

Figure 4 shows the cross-section of a water-cooled torch.

Cooling water return Cooling water and welding current supply

(shielding) gas nozzle

Shielding gas connection

Torch handle

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Figure 4: Cross-section of a water-cooled TIG torch

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Tungsten electrodes

The quality of a TIG weld seam basically depends on the kind of tungsten electrodes used and the shape of the electrode tip. We distinguish between pure tungsten electrodes and those with oxide additives. The differences between these kinds of electrodes are based on the electron-emission-energy (Figure ). This shows that, in order to reach a stable ignited arc, a pure tungsten electrode gets approx. 1000 °C hotter than a thoriated tungsten electrode. The required arc current density of the pure tungsten electrode is in the liquid phase of the electrode tips, whereas the necessary emission-energy for the thoriated electrode takes place in the solid state of the electrode material. Table 1 shows the suggested arc current values based on electrode diameter, current type and polarity. Thorium is increasingly replaced by other oxides.

The required current density for stable arc

Figure 5: Electron emission: Density j of the electron-flow as a function of temperature T for pure and thoriated tungsten electrodes: Tth: Temperature of thoriated electrode during welding Tr: Temperature of pure tungsten electrode during welding

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(Extract from DIN EN ISO 6848)

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SFI / IWE 1.07 Page 8

Electrode tip shapes

The tungsten electrode's tip shape has an impact on the shape of the arc and therefore on the shape of the heat flow to the workpiece (see Figure ). The electrode end is mainly defined by the kind of current and the polarity as well as the thermal strain (thermal capacity) which is defined by the level of the amperage. In case of direct current with negative polarity, a cone-shaped tip can be kept under lower current levels. By raising the level of arc current, the electrode tip is liquefied and turns into a hemisphere with a diameter of about the electrode thickness (see Figure ).

Broad, flat penetration

Narrow, deep penetration

Figure 6: Penetration dependency of the electrode tip shape for TIG welding under same welding current

Type of current

Figure 7: Development of the electrode shape at different amperages

The formation of the electrode tips is similar in case of other oxide additives.

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Classification of tungsten electrodes in accordance with DIN EN ISO 6848

According to Table 2, tungsten electrodes must be marked with a coloured ring on one of the electrode’ s tips depending on their chemical composition. The width of each coloured ring must be at least 3 mm. Tungsten electrodes may alternatively have their own symbols, which are etched into the surface of the electrode near to one end. Table 2: Requirements on the chemical composition of tungsten electrodes (extract from DIN EN ISO 6848)

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Influence of the shielding gas on the penetration profile

The shape of the arc is also considerably influenced by the type of shielding gas used. In case of active gases the physical characteristics of the different thermal conductivities play a part on the dissociation. Figure 5 shows the penetration profiles of dummy runs and Figure 6 of fillet welds in the base material X5CrNi18-10 (1.4301) of TIG welding under different shielding gases.

Figure 5: Penetration profiles of TIG welding with different shielding gases in a 5 mm thick plate, current 130 A, arc length 4 mm, welding speed 15 cm/min.

Figure 6: Fillet weld penetration profiles of TIG welding with different shielding gases, base material 1.4301

7

Ignition of TIG arcs

A TIG arc can be ignited by the tungsten electrode either through contacting the workpiece or without contacting (contactless) the workpiece 7.1 Contactless Ignition Contactless ignition has the advantage of preventing contamination of the welding area with tungsten particles and avoiding a specific movement of the welding torch for starting the ignition. In earlier days using the contactless ignition, the ionisation of the arc area (initiation of charge carriers) was carried out by high frequency (HF). Nowadays, this is usually being executed by high voltage pulses. However, the designation HF-ignition is still being used even for the high voltage pulses. Due to the voltage amplitudes, the energy content of the pulses and the short-time high frequency electromagnetic fields of the HF-ignition and High Voltage Pulse ignition, specific risks for humans and sensitive electronic equipment will arise. Electronic equipment which is located near the welding current cable of the torch (some meters) should be interference free (suitable for industrial use) in order to prevent defects or malfunctioning. The touchable areas of the TIG-torch, the welding hose and connectors must be well isolated in order to prevent undesired high voltage flashovers (sparks). Absolutely dry and isolating welding gloves in case of manual torch welding should be self-evident. Some company specific- or association related limitations for using the HF-ignition could be valid. The maximum voltage amplitude (and the allowable energy content) for HF-ignition is being standardised in accordance with the specific equipment and is around 12kV (kilovolt) for manual torch welding and up to 20kV for mechanised torch welding. TIG power sources usually support the HF-ignition through a © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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specific ignition-current which can be adapted manually or automatically by selecting the tungsten electrode diameter. Depending on technical and standardised limitations of HF-ignition in combination with local conditions it may occur that the HF-ignition is not working well. The following information is given: –

The HF-ignition will become worse with increasing helium content in the shielding gas. It could be helpful, but with appropriate effort, to start under pure argon conditions and subsequently switching over to helium. The type and condition (correct grinding) of the tungsten electrode influences the ignition. a conductive ring around the front area of the gas nozzle changes the electrical field and may improve the ignition process. the welding hose should not be located in close surrounding to other electrical conductors (even electrical cables, cable booms, etc.) as subsequently the ignition energy can be transferred locally. the ignition sparks could affect surface areas outside the weld seam by micro craters. Although the ignition sparks are following the shortest, electrical favourable path, they show a preference for sharp peaks, edges and specific surface conditions (partly oxides)

– – – –

A contactless ignition without facing the above mentioned “problems” is possible by using specific torches having an (already ignited) active pilot-arc inside which on its turn can ignite the main arc. This type of contactless ignition is very common for micro plasma welding. 7.2

Lift-arc Ignition

The ignition of the arc will be activated by making a contact between the tungsten electrode and the workpiece which creates a local heating and generates sparks at the time of ending the contact. These sparks ignite the arc. This processing requires a specific movement which could consist a slight touching, scratching or a push-hold-lifting movement with a certain holding time. TIG power sources support the lift-arc-ignition through a specific ignition-current which can be adapted manually or automatically by selecting the tungsten electrode diameter.  Short circuit ignition  Ignition with high-voltage pulse generator (high frequency ignition)  Lift - arc ignition Table 3: Influence of different gases on ignition characteristic, arc stability, joint type and welding speed. Ignition

Arc stability

weld-widening

Penetration

Welding speed

Argon

XXX

XXX

XXX

XX

XX

Argon/hydrogen mixture

XXX

XXX

XX

XX

XXX

X

X

X

XXX

XXX

Helium/argon mixture 25/75

XX

XX

XXX

XX

XXX

Helium/argon mixture 50/50

X

X

XX

XXX

XXX

Influence/ shielding gas

Helium

8

Purging

Purging takes place almost exclusively during welding of high-alloyed steels.

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Purging primarily forms a gas shield at the weld root. Without this shielding gas, oxidisation may occur in the vicinity of the weld root (discoloration that can generally occur without an optimal gas shield in the area surrounding the weld surface, too). Annealing colours are thin oxide layers that are formed due to heat on the surface of Cr-Ni steels, if exposed to air. They must be removed, or better still, their generating should be avoided, so that the chemically resistant passive layer can develop, which is responsible for the corrosion resistance of these materials. The annealing discoloration can be removed by blasting, grinding or pickling (please note: remove pickling residues sufficiently, otherwise there is a corrosion risk). Inside pipes and containers this is usually not possible; thus purging here may require remedial measures. Purging must already occur during tack-welding. To a certain extent the purging gas pressure gives also a certain support preventing the root weld metal from excessive drop through and helping in making a weld root.

8.1

Purging gas and work safety

Argon and nitrogen are nor toxic or flammable. It should be observed however, that during purging gas proceedings of vessels the oxygen is purged so that during work activities in such vessels an air supply is required (air; not pure oxygen!!) in order to avoid the risk of suffocation. Purging gases with hydrogen content (for obtaining an oxide-free weld root) are flammable depending on their hydrogen content. Therefore, EN ISO 14175 specifies that purging gases with over 10% hydrogen content need to be flared. This is mostly to be achieved through the use of a constantly burning pilot flame. Flammable gas mixtures are present if the hydrogen content in the air is between 4 and 75 vol%. When components with inaccessible areas cannot ensure sufficient purging of the trapped air, purging gas with less than 4% hydrogen or only argon or nitrogen are to be used.purging The residual oxygen content is leading for the purging-effectiveness of the area. purging When welding stainless steels, a sufficient dilution is usually achieved if approx. 2.5 - 3 times the geometric volume of the area to be purged is set for the quantity of the purging gas to be used. Example: Pipe internal diameter Flush length Pipe volume Gas flow rate Flush factor Gas volume 2.5 x 14

= = = = = =

132 mm 1,000 mm 14 l 10 l/min 2.5 35 l flushing time 35: 10

=

3.5 min

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In table 4 the information is given which type of purging (=shielding) gas should be used for different purging materials. Table 4 Purging gases

Materials

Argon-hydrogen mixtures

Austenitic Cr-Ni steels Ni and Ni based materials

*) Nitrogen-hydrogen mixtures

Steels, except high-strength fine grain structural steels, austenitic Cr-Ni steels

Argon

Austenitic Cr-Ni steels, austenitic-ferritic steels (duplex), gas-sensitive materials (titanium, zirconium, molybdenum), hydrogen-sensitive materials (high-strength fine grain structural steels, copper and copper alloys, aluminium and aluminium alloys as well as other non-ferrous metals), ferritic Cr steels

*) Nitrogen

austenitic Cr-Ni steels, austenitic-ferritic steels (duplex)

*) For titanium-stabilised stainless steels, titanium-nitride may form on the full penetration root run (yellow discolouration) when using nitrogen or nitrogen-hydrogen mixtures. The question of whether to leave this titanium-nitride in place is to be decided separately in each individual case.

When, for example larger containers are to be purged the relative density of the purging gas used is to be considered.

Figure 7: Relative density of different gases used for purging

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Purging fixtures

Figure 8:

Schematic diagram showing the use of shielding gas to protect the top and bottom when welding sheet metals

Figure 9: Purging fixture for pipe profiles

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Figure 103: Different means of purging

Figure 114: discoloration and early corrosion due to inadequate gas shielding

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Figure 125: “Burned” weld root due to an absolutely inadequate gas shielding

9

TIG welding of aluminium

9.1 Overview The following aluminium materials can be welded using the TIG welding process. A. Pure aluminium (Al 99.9; Al 99.5 etc.) with high corrosion resistance but low strength (80 N/mm²), which can be increased (130 N/mm²) by cold forming (roll forming etc.). The effects of cold work hardening are lost by welding in the weld area. B. Self-hardening aluminium alloys (AlMn; AlMg 3, etc.). higher strength through alloying elements (240 N/mm²). Cold-forming leads to increased strength (320 N/mm²), which is lost by welding in the weld area again. C. Heat-treatable aluminium alloys (AIMgSi1; AIZnMg1 etc.) Adequate strength characteristics (380 N/mm²) are achieved by using thermal treatments (precipitation processes). The loss of strength in the weld area after welding can partially be regained by ageing at higher temperatures (100-250 C°) or by storage at room temperature. The main problem of aluminium welding is the high melting temperature of the oxide layer. In TIG welding the oxide layer is usually not destroyed by flux (as it is the case with oxy-fuel gas welding, which is now rarely used for aluminium) but by physical effects of the electrical current in the area of the arc root. The prerequisite for welding joints without trapped oxides and therefore also without incomplete fusion, is a primarily pre-weld treatment of the workpieces in the weld zone and possibly also of the filler metal shortly before welding by pickling or by mechanical means e.g. by brushing. Brushes with bristles made of highly alloyed material are used; these should never be used to brush ferrous materials.

10 TIG welding aluminium with alternating current Alternating current welding is currently most frequently used in practical manufacturing applications. Cleaning takes place in the positive half wave, while during the negative half wave the tungsten electrode, which is strongly heated up earlier, can now cool down. Consequently, in case of alternating current welding the advantages of the two kinds of direct current polarity are combined. Since the arc

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extinguishes at every current zero crossing (Figure ), work used to be performed with a high frequency overlay (150 kHz at 1,500 to 2,000 V) in order to facilitate the re-ignition of the arc. These devices have now been replaced by impulse generators that no longer have an output of constant high-frequency voltage impulses, but rather impulses with the same rhythm as the supply voltage (Figure ) and therefore have less influence over the radio and TV reception in the close environment.

Figure 16: TIG arc when using alternating current

Figure 17: Impulse generator; voltage impulses

Table 5: Reference values for TIG welding of aluminium materials with alternating current Workpiece thickness mm

Shape of Tungsten groove electrode diameter weld mm

Welding current *)

Filler rod diameter mm

A 1 2 3 4 6 8 10 12

II II II II V V V V

1.6 2.4 2.4 3.2 3.2 4.0 4.8 6.4

50 ... 60 60 ... 90 90 ... 150 150 ... 180 180 ... 240 200 ... 280 260 ... 350 320 ... 400

Argon consumption

Amount of Layers

L/min. 2 2 3 3 4 4 5 5

4 ... 5 5 ... 6 5 ... 6 6 ... 8 8 ... 10 8 ... 10 10 ... 12 12 ... 14

1 1 1 1 2 2 2 ... 3 3

*) Values for butt welds; in the case of fillet welds these should be increased by 10 to 20%.

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11 TIG welding aluminium with direct current During direct current welding, different temperatures occur at the anode (+) and the cathode (-) because of physical characteristics, see Figure and Figure 19.

Penetration ratio

Figure 18: Direct current arc, electrode as cathode (-); penetration ratios

Penetration ratio

Figure 19: Direct current arc, electrode as anode (+); penetration ratios

In case of the arrangement shown in Figure , with the electrode as the cathode, the emitted electrons hit the workpiece poled as the anode and generate a lot of heat by converting kinetic energy on the hitting point and thereby achieving deep penetration. In comparison, the electrode tip is only heating up a little bit because of upcoming gas ions which, in contrast to the electrons, however show a larger mass but smaller amounts and in particular, not as fast as the electrons. The oxide layer is not destroyed by using this polarity, so that processes with this type of polarity seem to be unsuitable first for the welding of aluminium. In case of the arrangement shown in Figure 19, with the electrode as the anode, the emitted electrons hit the electrode and they heat it up substantially. In comparison, the workpiece which is poled as the cathode only heats up a little bit. Therefore only a flat penetration arises. This polarity leads to a "cleaning effect" i.e. the oxide layer is torn up and removed. This effect is explained by the fact that the quite heavy ions meet the oxide skin and destroy it. At this polarity, however, the high thermal load on the tungsten electrode leads to the rapid destruction of the tungsten. By using this kind of polarity several welding procedures are carried out by using disproportionally thick tungsten electrodes for thin plates. However, TIG-welding using this kind of polarity is of little technical significance.

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Concerning TIG welding with a negative polarity of the electrode, a method has recently been developed which, instead of the usual inert gas argon, makes use of helium. This is based on special characteristics of this gas. Due to the higher ionisation energy compared to argon, a greater welding voltage of approx. 75 % occurs at the same current levels (Figure 20) and this also leads to a higher thermal input into the workpiece. The higher thermal conductivity of helium is another advantage compared to argon. Because of its lower electrical conductivity, one of the disadvantages of helium is the turbulent arc and the difficult arc ignition during TIG welding. In a lot of cases, mixtures of argon and helium result in a practical compromise. From an economic point of view, it also has to be considered that helium is more expensive than argon and that, due to its lower specific weight relatively more helium than argon has to be used for gas shield purposes.

Figure 20: Arc voltage vs. welding current when using different gases (according to Schnöbel)

The higher energy input when helium is used results in higher welding speeds (Table ), lower pre-heat temperatures at the same penetration rate (Figure ) and a lower tendency for porosity due to a hotter weld pool with lower viscosity and better degasification possibilities. Table 6: TIG welding of AlMg3, double V weld, 16 mm sheet thickness Gas

Welding current (A)

Argon Argon + 30 % helium Argon + 70 % helium

400

Welding voltage (V) 29 30.5 33

Welding speed (cm/min) 45 50 60

Figure 21: Influence of pre-heating on penetration

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It is expected that TIG welding of aluminium workpieces using helium will be increasingly adopted in the future, especially for automated welding. See Table for reference values for welding with direct current. Table 7: Reference values for TIG welding of aluminium with direct current Workpiece thickness mm

Type of groove weld

1 2 3 4 5 6 8 10

II II II II II II II II

 mm

Degrees

A

Welding speed cm/min

1.6 2.4 2.4 2.4 2.4 3.2 3.2 4.0

90 90 90 90 90 90 60 60

85 110 150 180 200 220 265 320

120 100 80 80 70 70 60 50

Tungsten Electrodes

Welding current

Helium consumption l/min

Amount of Layers

15 15 15 15 20 20 25 25

1 1 1 1 1+1 1+1 1+1 1+1

12 Weld preparation for TIG aluminium welding The preparation of joints is standardised according to DIN EN ISO 9692-3. Table shows some common weld profiles based on the standard. Table 8: Weld profiles for butt welds, one-sided (extract from DIN EN ISO 9692-3)

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13 Welding defects When considering welding defects, a distinction should be made between general welding defects based on wrong torch and rod manipulation, incorrect workpiece preparation, gas shielding and additional defects that occur during aluminium welding. For respective data see Table to Table 11. Table 9: Errors due to defects in the weld joint preparation and the shielding gas

Defects

Reasons

Remedy

Dull surface, weld edges rough, too little flow

Incorrect preparation of the weld area and welding rod (not metallic clean)

Brushing, grinding, pickling, blasting

Pores

Workpiece dirty, oil, grease, paint, moisture

Cleaning, gloves clean?

Surface oxidised, dull, incorrect melting flow

Air in argon, leaking hoses and gas nozzle sucks air in, swirled air, draft, torch distance too large, argon flow too high

Control of argon flow, torch inclination, draft, fan wind, nozzle size, argon l/min

Whitish smoke, electrode tip oxidised

Lack of argon

Bottom has annealing colours, grey oxidisation, rough, burned surface

Too little back purge

Dark sediments, pores, unstable arc

Water leaks into torch, condensed water in torch

Arc flickering, condensate of metallic vapour, lower penetration

Dirty electrode tip

Control torch, water solenoid valve does not close during welding pauses, prepare electrodes again

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Table 10: General faults due to torch and welding rod manipulation Fault

Possible effects

Page 22

Table 11: Typical defects during TIG welding of aluminium materials Oxide Inclusions

Causes Insufficient welding current - excessive gap, lower web edge not broken

Arc is too long

Weld areas not cleaned, hot rod end is taken out of the shielding gas area after dipping and is dipped back into the weld pool after it has reacted with the oxygen in the air

Notch

Cleaning effect of the arc does not penetrate significantly below the pool

Oxide pores Without joint preparation or filler metal

Low penetration

I-shaped weld on excessively thick workpieces Welded on both sides in succession Workpiece distortion

Torch angle too big

Gas absorption

Welded on both sides simultaneously Torch tilted

Bead, single-sided notches

Pores Torch offset

Single-sided root fusion defect

I-shaped weld on excessively thick plates

Hydrogen input, humidity in oxide layers, grease and paint residuals in the welding zone, on the rod-surface, leaking water cooling, condensed water in torch head (if cooling water circulation is not interrupted during pauses) Arc instability during welding, especially at the start of welding and the welding over tack-welds Cooling rate is too high: pores in the weld interface between the weld and the base material are caused by the insufficient degassing of the base material.

Wire end moves out of shielded zone after welding

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A reason for metallurgic pores is always that hydrogen has dissolved in the material and absorbed while welding and is then unable to escape after solidifying. The reason for this is the change in solubility of hydrogen when it changes from a solid state (0.036 ml/100 g Al) to a liquid state of matter (0.7 ml/100 g Al-weld pool) and additionally in the strong increase of the solubility of the weld pool at a rising temperature of about 50 ml/100 g Al shortly before reaching the melting point. This entails a 70-fold increase in the H2 solubility from the melting and boiling point, compared with a 1.6-fold increase for steel, Figure . Especially super-purity and pure aluminium tend to the formation of pores in the weld. The outgassing process can be improved by a higher heat input (pre-heating 100 to 250 °C) and a lower welding speed. To avoid pores, the highest cleanliness is required concerning grease, oil, moisture, etc. in the zone of the weld and at the filler material.

Figure 22: Typical H2 solubility in aluminium depending on temperature

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14 Process variants of TIG welding Negative pole welding

Key-hole welding

Under pure Helium

Pulsed welding

Hot-wire welding

Positive pole welding

Cold-wire welding

Alternating current welding

Narrow Gap welding

Orbital welding

Figure 23: TIG welding variants

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14.1 Possible mechanisations levels of TIG welding.    

manual welding partially mechanised welding fully mechanised welding automatic welding

Table 12: Examples of classification according to degree of mechanization (extract from DIN 1910-100:2008-02) Motion and working processes Designation Short symbol

Tungsten inertgas welding TIG (141)

Torch/workpiece guidance

Filler metal feed

Handling of workpieces

manual welding manual

manual

manual

partially mechanised welding

manual

mechanical

manual

fully mechanised welding

mechanical

mechanical

manual

automatic welding

mechanical

mechanical

mechanical

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15 Process variants of TIG welding 15.1 TIG pulsed welding TIG pulsed arc welding which is a relatively new arc welding process modified only by the type of current, differs from TIG direct-current-welding only by a special power supply that generates e.g. sinusoidal or rectangular direct current or direct current pulses with adjustable impulse parameters (pulse amplitude, pulse frequency, duty cycle). During the high current pulses in the pulse arc process, a lot of heat is brought into the welding area. The weld material is melted. During the pulse phase (base time) with low current only little heat is applied to the workpiece. The weld pool stays comparatively cool. These low currents during the base time only serve to maintain the arc in order to avoid disruptions and ignition difficulties. When welding with a filler wire or rod the filler material is fused with the base material during the high current pulse phase. The pulse frequency is usually between 0.5 Hz and 10 Hz. The weld heat input can be considerably changed by the choice of time-periods and current values. In the extreme case, a weld seam may consist of adjacent or overlapping fusion welding points. TIG pulsed arc welding allows the area of application of the TIG process to be extended to low power levels and low material thicknesses and the weld seam appearance can be further improved, too.

The most important weld parameters are:  Pulse current

Ip

 Background current IG  Pulse current time

tp

 Background current time  Pulse frequency

tG fp = 1/tc (tc = Pulse cycle duration)

Figure 24: Basic weld parameters for TIG pulsed arc welding

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The following advantages and disadvantages apply when comparing TIG welding with even- or pulsed shaped arcs. Advantages of the TIG pulsed arc welding:

Disadvantages:

 possibilities of lower energy inputs

 welding equipment is expensive

 better depth-to-width ratio in the case of higher thickness

 setting up the equipment is more complicated

 more stable arc  more uniform root formation  better out-of position weldability  less workpiece distortion  better modulation of the welding pool  better gap bridging ability

15.2 TIG welding with filler wire Filler material is used whenever a groove needs to be filled during welding e.g. a single-V butt weld or a fillet weld is performed, or sufficient weld cap and root excess is required. 15.2.1 TIG welding with cold wire Welding with cold wire is the most popular TIG application. The filler wire can be fed manually or mechanically. The separation of the arc heat and the filler wire in TIG welding results in the situation that the wire is only fed at the time a sufficiently large weld pool has been formed. This allows a high weld quality to be achieved, securely helping to avoid initial fusion defects and cold locations. Thus, TIG welding is used most frequently in applications where high quality welds are required, as in thin sheet and root welding. A disadvantage of cold wire welding is the limited deposition rate. The feed rate of the cold wire can be adjusted only in a limited range (approx. 0.2-1.0 m/min) without running into problems. If the cold wire feed rate is too low, the wire does not melt evenly. If the rate is too high, the arc cannot melt the wire completely.

Figure 25: TIG welding with cold wire / Photo: Linde

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15.3 TIG welding of rotated components A conventional fully mechanised TIG welding unit with a controlled turntable can often be used with a

rotating pipe and stationary torch. Since the root weld and the subsequent beads from the start to the end of the weld must be free of defects, process control is absolutely essential. Small pipe diameters and pipe material with good heat conductivity properties increase the pre-heating temperature at the welding point continuously. This requires continuous changing of the welding data. We recommend using program control to retrieve the required welding data.

Figure 26: TIG welding with cold wire / Photo: Linde

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15.4 Orbital TIG welding Similar to TIG welding with a rotating pipe, the orbital welding process can be used with a stationary pipe and a rotating torch to join pipe diameters from < 10 mm to > 1,000 mm and pipe wall thicknesses from some tenth of millimeters to 50 mm or more. Also, a diverse selection of materials may be welded using this process. Un-alloyed, low-alloyed, heatresistant, high-strength and corrosion-resistant high alloyed steels can be joined using orbital TIG welding. Furthermore, joints of nickel-based materials, Cu and Al alloys are possible. This usually requires complex welding equipment, in order to meet the requirements for high quality. Thin-walled pipes and pipes with very small dimensions are often welded without filler material (and also root welds on thick pipes). For larger pipes, cold- and hot-wire TIG welding is used.

Figure 27: Stationary apparatus for the preparation and orbital welding of pipes (without filler wire) / Photo: Protem

Figure 28: Orbital TIG welding without filler wire Photo: Protem

Figure 29: Open orbital TIG-system with cold-wire feeder

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In orbital TIG welding, the actual welding position continuously changes (e.g.: from horizontal to welding down, from overhead to welding up). In one cycle only these weld position changes require up to 40 different welding data settings. Equipment that can perform these functions is expensive and requires substantial experience in programming. The welding heads can be simple clamps or complex tools that enable all functions of the welding process to be remotely controlled in the smallest area.

Figure 30: Closed Orbital TIG heads of different diameters (without filler material feeder) Photo: Fronius

Figure 31: Orbital TIG welding head for larger pipe diameters (with filler material feeder) Photo: Fronius

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Figure 32: Orbital TIG welding equipment with filler material feeder. Picture below: in action

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15.5 Orbital TIG Narrow gap welding In the welding industry both the Orbital TIG welding variant with a rotating torch around a fixed component, and the one with the fixed torch and a rotating component become more and more important. Orbital TIG narrow gap welding is only applied from a certain pipe diameter due to the increasing wall thickness. Due to the high weld preparation effort it is only economical to use narrow gap welding for using thicknesses just from and above 25mm. Compared to conventional TIG welding the Orbital TIG narrow gap welding time reductions can go up to 5 to 10 times for wall thicknesses of 60mm and above. Not only is less weld metal needed also the disposition rate can be considerably increased by using the hot-wire variant. In order to apply the orbital TIG narrow gap welding at all, it is absolutely necessary to comply with the mechanical preparation of the joint layout and the positioning of the component.

Figure 33:

Comparison of weld volume savings for TIG/MAG/UP narrow gap welding processes (according to Siemens KWU)

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Figure 34: Orbital TIG narrow gap welding

Figure 35: Macro of cross-section of a 10CrMo9-10 (1.7380)

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15.6 TIG hot wire welding The wire is heated by an additional power source up to the melting point and can be added into the weld pool without extracting significant heat from the arc. The heating of the wire is executed by resistance heating without creating an additional arc. Advantages:  High deposition efficiency  High welding speed  Low risk of lack of fusion  Low dilution of the base material  High deposit quantities with same welding currents (30-50%)  More simple welding in out-of-positioning welding  Smaller Heat Affected Zone

Legend: 1 2 3 4 5 6

Power supply Wire electrode Wire electrode, hot wire electrode (with current) Weld Arc Workpiece

Figure 36: TIG hot wire welding process

15.7 TIG multi-cathode welding This version (one torch with several electrodes) is used for producing thin-walled pipes, for example pipes formed from sheet metal which are longitudinally welded. For an economic production, very high welding speeds are required. This can be achieved by arranging several TIG welding torches in series at small intervals or, with special torches, where several electrodes are arranged to each other in an isolated way. Welding speeds of 10 - 20 m/min are obtained, many times faster than what can be achieved with individual torches. This process variant requires special purpose machines and is little used

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Figure 37: TIG multi-cathode welding

Figure 38: TIG multi-cathode welding

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15.8 TIG welding with a double gas nozzle This process is again used very little in practice. Certain gas mixtures can be achieved using different compositions of gas in the outer and inner gas shield in order to achieve economic and quality advantages. However, changes in general conditions means that stable gas combinations are often not achieved.

Centre/inner gas

Shielding gas

Figure 39: TIG welding with a double gas nozzle

15.9 TIG spot welding TIG spot welding is used to join two overlapping sheets of thin metal by a spot weld (with or without filler material). A joint similar to resistance spot welding can be achieved. However, if there is no sufficiently large hole in the top layer, the smallest disturbances e.g. - gap between the top and bottom sheet metal - contaminations - coatings may affect the welding process so much that the joint between the two sheets of metal is not successfully created. With a hole in the top sheet (5-7 mm) and filler material, sound joints are possible with short welding time (around one second). The advantages, such as single-sided accessibility and free of spatter, are diminished by the high cost and low cost efficiency. The process is little used.

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Figure 40: TIG spot welding

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15.10 TIG welding with two torches (simultaneously) ~ 10°- 20°

Both sides TIG welding in a single weld pool by two welders is preferable to be executed in the PF welding position. Other welding positions are also possible. Welding position PA is difficult. The filler wire does not to be added from both sides. Especially for welding thick aluminium materials the gap should not be too small in order to prevent the loss of the flank cleaning effect (arc bow starting points) and creating the risk of getting oxide inclusions. ~7

0°80

°

4-10

8-20

70° 2-5

3-6

Figure 41: TIG welding with two torches (simultaneously)

70°

Figure 42: Weld preparation for both side TIG welding (simultaneously)

The most important advantages of welding simultaneously with two TIG welding arcs are:      

More simple weld preparation of components up to 10 mm thickness Gap distances up to the material thicknesses can be controlled Pores, lack of fusion or oxide inclusions are to be neglected if correct weld preparation and trained welders are involved. Also for high heat conductivity base materials preheating is mostly not necessary. No distortion (symmetrical welding) and small weld layout. An advantage for repair welding when facing bigger gaps after repair-preparations

When welding with alternating current it is important that: The power sources should be set in a way that in both arcs the phase-period and frequency are equal. For power sources having no fixed frequencies these requirements are hard to accomplish. If the current directions of each arc are different, the open-circuit voltage is above an acceptable value and the arc goes from electrode to electrode which strongly affects the cleaning-effect and arc-working negatively.

15.11 TIG Key-hole welding. The industry has a high demand of efficient joining techniques for reliable and secure welding under retention of base material properties especially in the heat affected zone. TIG key hole welding does have advantages and efficiency-benefits regarding to:  weld preparations from metal thicknesses of d=3-12mm  number of weld runs compared to MIG/MAG- and conventional TIG welding

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Essentials for application:  Conventional TIG power source  No complex weld preparation, only plane-machining or –turning  Less to no filler material  TIG-quality

Figure 43: Principle of TIG key hole welding

Figure 44:TIG key hole welding with filler wire

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12 mm CrNi Sheetmetal Parameter

Conventionel TIG

TIG key-hole

V-Naht 60° 7 1000 g/m 320 A 200 mm/min 35 min pro m

Butt weld 1 50 g/m 640 A 300 mm/min 3 min 20 sec. pro m

Macro-etching

Weld preparation Number of layers Filler quantity Current I Welding speed Welding time

Figure 45: Macro-etching of TIG key-hole welded 6mm CrNi steel (1.4301),weld current 430A, welding speed 85cm/min

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16 Test questions 1)

Why is the temperature at the anode higher than at the cathode?

     2)

A power source with a steep falling characteristic curve is suitable for which welding process?

     3)

in order to achieve a symmetrical mains loading in order to prevent the rectifying effect destroying the high melting oxide layer during the positive phase of the current wave lower thermal stressing of the wolfram electrode than with plus poled electrode lower thermal stressing of the wolfram electrode than with minus poled electrode

What is the reason for using contactless arc ignition during TIG welding?

     5)

MAG TIG SMAW TIG Plasma MIG

Why is alternating current used for TIG welding of aluminium?

     4)

due to the emission of protons by the impact of electrons due to the high voltage drop at the anode due to the emission of electrons out of the anode due to the less heat conductivity

To prevent tungsten contamination in the base material Due to the low open-circuit voltage In order to prevent the contamination of the tungsten electrode Due to the low ionisation voltage of Argon with steep falling static characteristic curves a contactless arc ignition is not possible

Which advantage is given by using helium as shielding gas during TIG welding?

    

improved arc-ignition less shielding gas flow-quantities required Higher heat capacity of helium arc enables higher welding speeds decreasing the pore formation for some base materials lower shielding gas price

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6)

Active gases can be used Better control of the welding pool more easier parameter adjustment/setting better suitability for out-of–position-welding cost savings in machinery and shielding gas expenses

Why are Argon-CO2-mixtures not being used for TIG welding?

     10)

Voltage decreases Voltage increases Voltage remains the same Voltage collapses (short-circuiting)

Which advantages does pulsed arc welding have on TIG welding?

     9)

lower thermal stressing of the electrode To achieve a higher temperature of the tungsten electrode deeper penetration of the base material for ionisation of the arc-bow to prevent contamination of the tungsten electrode

The situation is given that during TIG welding the arc has to be hold at a poor accessible location. What kind of influence does this have on the arc voltage value?

    8)

Page 42

For which reason is the tungsten electrode connected to the minus pole during TIG welding of steel?

     7)

SFI / IWE 1.07

the gas costs for Argon-CO2-mixtures are much higher than those of inert gases the setting of the correct shielding gas flow quantity will become more difficult in order to prevent oxidation of the tungsten electrode these gas mixtures can only be used for welding non-ferro metals these gas mixtures require a specific training of the welder.

Which of the following gases or gas mixtures are suitable as purging gases during TIG welding?

    

Argon Ar+8%O2 Ar90%+H210% N290%+ H210% Ar82%+ 18%CO2 (M21)

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Chapter 1.08-1:

SFI / IWE 1.08-1 Page 1

MIG/MAG welding

Contents 1

Term ................................................................................................................................................. 3

2

Application areas, adoption of the process .................................................................................. 4

3

Construction of GMAW welding equipment .................................................................................. 4 3.1

Power source unit .................................................................................................................. 5

3.2

Control unit ............................................................................................................................ 6

3.3

Hose assembly and welding power cables............................................................................. 7

3.4

Welding torch ......................................................................................................................... 9

4

Selection criteria for the welding equipment .............................................................................. 11

5

Arc length control for gas-shielded metal arc welding .............................................................. 11

6

Filler materials and shielding gases ............................................................................................ 12 6.1

Wire electrodes .................................................................................................................... 12

6.2

Shielding gases ................................................................................................................... 15 6.2.1

The Argon Arc during steel welding ........................................................................ 16

6.2.2

The CO2 Arc ........................................................................................................... 17

6.2.3

Classification and characteristics of different shielding gases for gas-shielded metal arc welding ................................................................................................... 18

7

8

MIG/MAG welding equipment settings ........................................................................................ 24 7.1

Setting parameters for welding with step-switched welding rectifiers ................................... 24

7.2

Influence of the arc voltage .................................................................................................. 25

7.3

Influence of the wire feed rate .............................................................................................. 26

7.4

Influence of contact tube distance ........................................................................................ 27

7.5

Influence of welding speed .................................................................................................. 27

7.6

Influence of the electrode wire positioning on the weld profile and the edge penetration...... 28

7.7

Influence of the root gap when welding square butt welds ................................................... 29

7.8

Influence of the torch angle .................................................................................................. 29

7.9

Influence of the weld position ............................................................................................... 30

Wire electrode polarity, forces in the arc and metal transfer modes (arc types) ...................... 33 8.1

Polarity of the wire electrode ................................................................................................ 33

8.2

Influence of the wire electrode diameter on deposition rate and weld run profile .................. 34

8.3

Forces in the arc .................................................................................................................. 35 8.3.1

Spray arc mode ...................................................................................................... 37

8.3.2

Short spray arc mode ............................................................................................. 39

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8.4 9

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8.3.3

Long spray arc mode ............................................................................................. 42

8.3.4

Globular spray arc mode ........................................................................................ 43

8.3.5

Short-arc mode ...................................................................................................... 44

8.3.6

Pulsed Arc Mode .................................................................................................... 47

GSMA-welding with cyclic changing arc power .................................................................... 50

Potential weld defects during MIG/MAG-welding ....................................................................... 51 9.1

Lack of fusion during weld start, weld overlap and –restart, weld-end crater ........................ 51

9.2

Lack of fusion inside the weld .............................................................................................. 52

9.3

Pores ................................................................................................................................... 55

9.4

Weld run undercuts .............................................................................................................. 57

9.5

Cracks ................................................................................................................................. 58

10 Joint preparation (Overview) ........................................................................................................ 58 11 Weld pool support (weld backing) ............................................................................................... 61 12 Advantages and disadvantages of Gas Shielded Metal Arc Welding ........................................ 62 12.1

Advantages of MIG/ MAG welding ....................................................................................... 62

12.2

Disadvantages of MIG / MAG welding ................................................................................. 62

13 Process variants of MAG welding ................................................................................................ 62 13.1

MAG-Spot welding ............................................................................................................... 63

13.2

Gas shielded metal arc brazing ............................................................................................ 63

13.3

MIG/MAG-High Performance Processing............................................................................. 65 13.3.1

High Performance weld processing with single wire electrode................................ 66

13.3.2

High performance dual-wire weld processing ......................................................... 67

13.4

Plasma Gas Shielded Metal Arc Welding ............................................................................. 70

13.5

Laser- Gas Shielded Metal Arc Welding .............................................................................. 71

14 Electrogas welding ....................................................................................................................... 71 15 Test questions............................................................................................................................... 73 16 Bibliography .................................................................................................................................. 75

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Page 3

Term

Gas-shielded metal arc welding (GMAW) is an arc welding process a continuous endless, depositing wire electrode with a shielding gas blanket, Figure 1. GMAW welding can be applied partially mechanical with a manually fed torch or fully automated. Direct current is used, and the wire electrode is usually the positive pole. For special welding tasks in the low power range, using electronic power sources (with an inverter in the output current circuit), even adapted alternating currents can be applied for reducing the heat input.

Figure 1: Construction of GMAW welding equipment. /SLV Munich/

Table 1: Designations and process types for the MIG/MAG welding process Short symbol

Code according to ISO 4063

GMAW

13

Generic term

Metal-arc inert gas welding with solid wire electrode

MIG

131

Inert shielding gas (argon, helium and argon/helium mixtures)

Metal-arc inert gas welding with flux-cored wire electrode

MIG

132

Inert shielding gas (argon, helium and argon/helium mixtures)

Metal-arc inert gas welding with metal powder-filled wire electrode

MIG

133

Inert shielding gas (argon, helium and argon/helium mixtures)

Metal-arc active gas welding with solid wire electrode

MAG

135

Argon-filled gas mixes or 100% CO2

Welding process Gas-Shielded Metal Arc Welding

Remarks

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Metal-arc active gas welding with flux-cored wire electrode

MAG

136

Argon-filled gas mixes or 100% CO2

Metal-arc active gas welding with metal powder-filled wire electrode

MAG

138

Argon-filled gas mixes or 100% CO2

CO2 welding

MAGC

/

Shielding gas 100% CO2

Mixed gas welding

MAGM

/

Argon-filled gas mixes

MF

114

-

-

Self-shielded flux-cored arc welding MIG/Mag Spot welding

With self-protecting cored-wire electrodes Short-period welding without toch/nozzle movement

Further designations and ISO 4063 codes are listed in Table 1 and 2 for each conventional and special process. Table 2: Special welding process with melting wire electrode (High-performance welding process: see section 13) Welding process Electrogas welding Plasma MIG Welding

2

Short symbol

Code according to ISO 4063

EGW

73 151

Remarks Welding in vertical position Plasma and MIG arc in the torch (plasma arc arranged concentrically) (plasma- and gas metal arc arranged in serie)

Application areas, adoption of the process

Gas-shielded metal arc welding has become widely used in the last 50 years. These processes account for about 70% of the filler materials used. Gas-shielded metal arc welding is used throughout in the metalworking industry, steel construction, shipbuilding, container construction and vehicle construction industries in a wide component wall thickness range. Next to un- and alloyed steels also CrNi steels and aluminium materials are joined. GMAW-brazing is mainly used for the joining of metals containing thin layers of Zinc.

3

Construction of GMAW welding equipment

Components of GMAW welding equipment according to Figure 1:  Power component (welding rectifier)  Wire feed unit (wire spool holder, wire feed motor, wire feed roll)  Hose package  Welding torch  Cooling equipment  Shielding gas equipment  Control and adjusting elements. Figure 2 shows the structure of electronic GMAW welding equipment.

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Figure 2: Structure of electronic GMAW welding equipment. /SLV Munich/

3.1 Power source unit Section 1.04 includes the types of power components that convert the mains power for the different process types. For conventional GMAW welding (short-arc, spray-arc mode) using a rectifier, basically power sources are being used having constant voltage characteristics. The static characteristics of the power supply in the range of the welding data have an almost horizontal course (inclination 1 to 5 V/100 A). Figure 3 displays the positioning of the static characteristic curve and the corresponding arc voltage settings which is being controlled by coarse- and fine coarse switches (e.g. winding tapping of the transformer). Power sources with stepless (continuous) controlled static characteristics properties enable a continuous arc voltage setting (Figure 4).

Figure 3: Setting of the static characteristics using power supplies with step switching. /SLV Munich/

Figure 4: Setting of the static characteristics using electronic power supplies. /SLV Munich/ The slope of the static characteristic is variable and usually assigned to programs

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Normally the arc voltage settings are already adapted to the required wire feed speed within the programs of high performance power sources. With the adjustment wheel normally only small changes are being controlled. Recommendations for the use of welding power supplies in relation to current intensity, wire electrode diameter and type of torch cooling are shown in Table 3. Table 3: Recommendation values for the selection of power supplies for MAG steel welding Setting range of the power source supply at 100% ED 150…180 A 180…250 A 60…350 A 70…450 A 70…600 A

Recommended wire electrode diameter [mm]

Recommended torch cooling

0.8...1.0 0.8...1.0 0.8...1.6 0.8...2.0 1.0...2.0

Gas Gas (water) Water Water Water

Power supplies for gas-shielded metal arc welding with pulsed arc (wire diameter of 1.2 mm and mixed gases with up to 20% CO2 content) should enable sufficient pulsed peaks (480 to 600A) and upslope welding speeds. It is to be noted that for power sources having adjustable static characteristic slopes, every adjustment will lead to different arc voltages and consequently effective arc lengths. 3.2 Control unit The control unit is usually a plug-in unit integrated inside the power supply or is mounted externally for fully mechanised or automated welding. During partial mechanised (manual) welding, the main functions of shielding gas supply, wire feed and welding current feed are controlled by a two- or four-cycle (torch) control. During the two-cycle mode the torch switch button has to be activated continuously during welding. In the four-cycle mode the welder only presses the button once for starting and once for ending the welding processing. The duration of the shielding gas flow before arc-ignition and after arc-extinction can accordingly be extended. Additional control functions like reduced ignition feed speed of the wire electrode just till shortly after the arc ignition and the burn-back time, can be adjusted to the required arc- and deposition rate settings. An adjustable burn-back time prevents the freezing of the wire-end into the weld pool and controls the droplet size at the end of the wire. Welding power sources containing high advanced electronics often do have the possibility of enabling a final puls at the end of a weld run in order to establish a small, needle-pointed wire end. And needle pointed wire electrodes do ease the weld arc ignition processing. Power sources with advanced electronics (mainly secondary or primary clocked transistor power sources) are mainly used for fully automated welding equipment as well as for gas-shielded metal arc welding with pulsed arc. The start- and the ending of a weld run can be optimised through different ways if the machines are equipped accordingly. Figure 5 shows some examples.

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Figure 5: Variable welding parameters to improve the quality of welding at the start-/ the end of a weld run /2/

Wire feeding For gas-shielded metal arc welding, the wire electrode is pulled off from the coil and pushed by a wire feeding unit through a hose assembly towards the contact tube in the torch measuring several metres in length. Figure 6 shows schematically, next to the standard types, also wire feeding systems for higher feeding capacities and for pressure-sensitive wires which are equipped with multiple wire feeding rolls. The wire feed unit usually uses hardened rolls which incorporate a wedge-shaped groove or a groove type which is adjusted (semi-circular or angled) to the wire diameter in case of soft wires

Figure 6: Wire feeding systems /2/

3.3

Hose assembly and welding power cables

The hose assembly and the welding torch belong to the highly stressed wear parts of the gas-shielded metal arc welding process. The hose assembly leads the wire electrode, the welding current, the shielding gas and the cooling water to the torch. Signal wires in the hose assembly allow process controlling from the torch itself. The hose assembly should be as short as possible in order to prevent small changes in wire feeding speeds causing unregularly weld runs and weld spatter. Usual lengths are 2-3 meters. For arc stability reasons hose assemblies lengths up to about 5 meters may still be used for steel wires of 1.2 mm wire electrode diameters and under the restriction of limited hose curvatures during processing. Thinner steel wired or softer wires (e.g. aluminium) may cause wire feeding faults. For this reason, for automatized applications (e.g. welding robots) the use of an additional drive in the torch or close to the torch is highly recommended. Examples of facilities for improvement of the welding feed speed consistency and for the extension of working area are shown in figure 7.

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Figure 7: Wire feed units and ways to extend the working area. /2/ At high current, the power cable in the hose assembly is cooled by water. The copper cable is surrounded by a hose which is cooled by the cooling water flowing back from the torch. This means that the cable diameter and also the weight of the hose assembly can be kept low. However, smaller cable cross-sections do increase the voltage drop and power loss over the hose assembly. In the hose assembly the steel wire electrode is surrounded by a wire filament made of steel and for an aluminium wire electrode by a tube made of plastic. The inside width of the wire filament or the plastic tube is about 0.5-1 mm larger than the wire diameter. If the inside diameter of the filament or tube is too big possible jamming of the wire occurs by piling up. Particles being peeled off from the wire should be blow-out after each wire-spool exchange. When not used in full, long welding power cables should be coiled not only once but twice, or otherwise the increase of the inductive resistance will render the welding current insufficient for certain processes See figure 8. Furthermore it is to be noted that longer hose assemblies do have a higher voltage drop which may lead to larger parameter adjustments and the use of a high(er) performance power source.

Figure 8: Influence of additional inductances when welding with the short and pulsed arc. /SLV Munich/

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Welding torch

A GMAW welding torch consists of a torch body, a contact tube, a shielding gas nozzle and for semimechanised welding a handle with an integrated switch for process controlling.

Figure 9: Sectional drawing of a GMAW welding torch in gasand water-cooled design for semi-mechanised welding

Figure 10: Examples of forced-contact current contact tubes /SLV Munich

The contact tube transfers the welding current into the sliding wire electrode. In practice this current transfer is easily interrupted due to high specific current loads of the contact areas, the often not optimal electrical properties of the welding wire surface and the limited contact-pressure. The bore in the contact tube, which is about 0.2 mm (steel) and about 0.4 mm (aluminium) larger than the wire diameter, expands conically after a certain length of time If any deposits are adding up inside the contact tube through contaminated wire surfaces the inner diameter is being reduced and will slow down the wire feeding. These inner diameter changes do influence the free-end wire length (contact-tube distance) which results in an unregular arc and currenttransfer and spatter may become more. The contact-tube which consists of E-Cu, CuCr or CuCrZr must be replaced when worn out. It is not possible to provide reference values for this replacement, as the service life may be easily influenced by the current load and the pollution of the wire surface through e.g. drawing soap. A modification of the contact tube and change of wire spool may increase service life. The optimal contact tube material for the production shall be found by testing. E-Cu has excellent electrical characteristics, however, it wears faster if under mechanical load. Harder and heat resistant contact tube materials (some are available made of special materials such as WCu or WAg) have lower abrasion, still they are worn by electroerosive processes. The cost of a contact tube is lower compared to the cost of the standstill of a welding robot equipment caused by a defective contact tube. A regular change is usually carried out in mass production before the contact tube wears out. Forced-contact current contact tubes generally improve the introduction of the current signal into the wire electrode, see fig 8. In case of defective wire electrode surfaces the improvements are mostly modest.

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When cleaning the torch, it is important to make sure that the shielding gas flow is free, the water cooling is tight and that no electrical leakage current occurs due to spatter bridges at the workpiece of the gas nozzle. Control and adjustment elements For the adjustment of the arc and the transfer of material for conventional welding equipment only two or three setting parameters are necessary. These are the voltage (mostly two interval switches) and the wire electrode feed rate (welding current.). Commonly used scale values from 1 to 10 that do not allow allocation to the actual values, should belong to the past. The third setting parameter given by high-quality welding equipment, which affects also the transfer of material with short-circuits, is the choke (inductivity in the welding current circuit). Modern welding rectifiers are often equipped with stepless controlled chokes. The actual arc voltage is clearly lower than the set open-circuit voltage. The reasons are the voltage drop, caused by the slope of the static current/voltage characteristics and the ohmic losses in the welding circuit. To some extent, it can be measured precisely only between the contact tube and the workpiece. This is mostly too complicated in practice. If measured at the terminal of the power supply, the voltage at the voltage drop in the hose assembly and in the workpiece cable and also at the contact points is higher than the actual arc voltage. Figure 9 shows the user interface display of a transistorised welding machine displaying the most important values, having adjustment possibilities for saved programming as well as setting and finetuning arrangements (arc- and deposition rates, arc bow length and dynamic parameter control). Different concepts are used for the electronically controlled welding equipment. A small (often too small) screen almost always indicates the setting data. With the suitable control elements different menu levels may be seen on this screen. The processes can be confusing, when the parameters are to be changed and saved. As the capabilities of the different power components affect the welding process, only small differences exist if simple operability and the quality of the available programs are important selection criteria.

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Figure 11: User interface of electronic welding equipment with an adjusting wheel for the arc and for the deposition rate (U + vDr in the picture, synergy characteristics) and an adjustment wheel for the correction of the arc length and the dynamic characteristics (e.g. speed of the current change, pulsed current, …) /2/

All electronic welding equipment manufacturers offer setting data saved in the device. For special conditions programs can be loaded later. This may make the selection of the right setting data easier. But as they can't be taken as optimal from the experienced welders for all the possible welding positions, weld profiles and welding techniques, simple corrections may be needed. Setting and saving your own program shall also be possible.

4

Selection criteria for the welding equipment

When purchasing MIG/MAG welding equipment the welding properties (for example: quality of predefined setting data for common weld taks), the handling and the efficiency are important evaluation criteria along with the technical values. Table 3 and 4 contain some indications regarding these issues. Table 3: Selection criteria for MIG/MAG welding power supplies Power supply, type

Price

Application area

Pulsed operation

Power supply with interval switch

Low

Manual welding in series, preferably for steel

No

Secondary clocked transistorcontrolled power supply

High

Manual welding, fully automated welding, also robots

Primary clocked transistorcontrolled power supply

High

Manual welding, fully automated welding, also robots

5

Weld quality

Compensation Mechanisation of mains capability voltage

Good, if the choke is adjustable

No

No

Yes, for all materials

Very good

Yes

Can be automated by leading voltage or bus

Yes, for all materials

Very good

Yes

Can be automated by leading voltage or bus

Arc length control for gas-shielded metal arc welding

In order to obtain an even weld quality, the arc length must be kept constant by gas-shielded metal arc welding. For this reason the traditional arc length control is too complex and is therefore being achieved through auto-balancing inside power sources showing a minor slope of the static characteristic curve (constant voltage characteristic). Power sources with constant voltage characteristics have the advantage that changes in the arc length and therefore in the arc resistance cause only a small change of voltage but a high change of current. Figure 22 depicts schematically the migrations of arc working points when the arc length is changed. This effect of the self-correction of the arc length by current changes is called “internal regulation” or “I-Regulation”. High-end power sources are as well as using constant voltage characteristic curves for some program types but are also using constant current output if the voltage values do not exceed a certain predefined range. If the actual voltage values are outside this interval (below or above) current intensities will be changed accordingly within small steps for the adjustment of the arc length. Nowadays programs are not only using the conventional short-circuit welding behaviour but are using more and more specific features of current- and voltage control in order to influence process dynamics.

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Figure 12: Correction of arc length when welding over a trapezoidal notch by changing of current (internal regulation, - I-Regulation) /2/.

Page 12

Figure 13: Control options for arc length stabilisation for pulsed arc welding /2/.

By pulsed-arc welding arc length corrections are carried out according to samples in Fig 13. It is regulated mostly by the pulse frequency and the background current quick changes.

6

Filler materials and shielding gases

6.1 Wire electrodes The wire electrodes for the most important materials are standardised. These are listed in the table below. Table 5: Wire electrodes for MIG/MAG-welding (extract from /7/) EN-/prENNumber EN ISO 14341 EN ISO 17632 EN ISO 1071 EN ISO 21952 EN ISO 17634 EN ISO 14343

Title Welding consumables - Wire electrodes and weld deposits for gas shielded metal arc welding of non alloy and fine grain steels - Classification Welding consumables - Tubular cored electrodes for gas shielded and non-gas shielded metal arc welding of non-alloy and fine grain steels - Classification Welding consumables - Covered electrodes, wires, rods and tubular cored electrodes for fusion welding of cast iron Classification Welding consumables - Wire electrodes, wires, rods and deposits for gas-shielded arc welding of creep-resisting steels - Classification Welding consumables - Tubular cored electrodes for gas shielded metal arc welding of creep-resisting steels Classification Welding consumables - Wire electrodes, strip electrodes, wires and rods for arc welding of stainless and heat resisting

Issue

In connection with DIN

*)

2011

ISO

IDT

2013

2014

VGL DIS 14174

2012

DIN EN ISO 21952

IDT

2014

DIN EN ISO 17634

IDT

2009

DIN EN ISO 14343

IDT

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DIS 11837

DIN EN ISO 14343

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EN ISO 17633 EN ISO 16834, EN ISO 18276 EN ISO 18273,

steels - Classification Welding consumables - Tubular cored electrodes and rods for gas shielded and non-gas shielded metal arc welding of stainless and heat-resisting steels - Classification Welding consumables - Wire electrodes, wires, rods and deposits for gas shielded arc welding of high strength steels Classification Welding consumables - Tubular cored electrodes for gasshielded and non-gas-shielded metal arc welding of high strength steels - Classification Welding consumables - Wire electrodes, wires and rods for welding of aluminium and aluminium alloys - Classification

DIN EN Welding consumables - Solid wire electrodes, solid strip ISO 18274 electrodes, solid wires and solid rods for fusion welding of nickel and nickel alloys - Classification DIN EN Welding consumables - Solid wires and rods for fusion ISO 24373 welding of copper and copper alloys - Classification DIN EN prEN ISO

= German standard = European standard = European draft standard = International standard

Page 13

2010

DIN EN ISO 17633

IDT

2012

DIN EN ISO 16834

IDT

DIN EN ISO 16834

2006

DIN EN ISO 18276

IDT

DIN EN ISO 18276

2014

DIN EN ISO 18273 substitute for DIN 1732-1 Substitute for DIN 1736-1 and DIN 1736-2 Substitute for DIN 1733-1

VGL IDT

2010

VGL

IDT = DIN is IDENTICAL to the European Standard/draft VGL = DIN is comparable with the European Standard/draft *) Type of the relationship ISO/DIS = International draft standard

For welding of unalloyed steels and fine-grained steels wire electrodes are used, according to EN ISO 14341 (Table 6). Table 7 shows the applications for wire electrode according to EN ISO 13341 Table 6: Wire electrodes and weld metal for gas-shielded metal arc welding of unalloyed steels and fine-grained steels according to EN ISO 14341 (short version)

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Table 7: Application areas of the wire electrodes according to EN ISO 14341

Type G2Si

For welding of construction steels under weak oxidising argon-filled mixed gases of groups M12 to M14, according to DIN EN ISO 14175.

Type G3Si1-

For welding of construction steels under argon-filled mixed gases of groups M12 to M27, according to DIN EN ISO 14175. For welding with mixed gases from the main group M3 or CO2 the yield point and the weld metal toughness decrease (tables 5-2).

Types G4Si1 and G3Si2

For welding of construction steels under mixed gases of the groups M2, M3 or C1 according to DIN EN ISO 14175.

Type G2Ti

This wire electrode type shows a significantly high content of titanium compared to other types. The element leads to fine grain in the steel. Therefore the application area of the wire electrode is in the welding of fine-grained structural steels.

Types G3Ni1

The large nickel content of the wire electrodes provides an increase in toughness, especially at low temperatures. The application of these types is recommended, if very low-temperature-tough steels are to be welded or particularly high toughness is required in the weld metal.

Types G2Mo and G4Mo

By adding molybdenum, the yield strength is increased. These wire electrodes shall be used when steels like 16Mo5 are to be welded.

The standards EN ISO 21952 (creep resistance steel) and EN ISO 14343 (stainless and heat resisting steel) have a similar structure. Figure 14 and 15 show some examples.

EN ISO 21952-A Number of standard with the classification acc. to chem. composition Symbol for wire electrode for MIG/MAG welding Cr 0,90 – 1,30 % Mo 0,40 – 0,65 % Si 0,50 – 0,80

- G CrMo1Si

ISO 14343-A

- G 19 12 3 L

Number of internation standard with classification acc. to system A Symbol for wire electrode for MIG/ MAG welding Cr 18,0 - 20,0 % Ni 11,0 - 14,0 % Mo 2,5 - 3,0 % Low carbon

Figure 2: Example of a wire electrode being used Figure 3: Example of a wire electrode being used for base material 13CrMo4-5 for base material X2CrNi19-11 Wire electrodes for gas-shielded metal arc welding are mostly wound on bobbins. Depending on the usage, this bobbin can be very small (1 kg) or very large (some weight 100 kg). For large consumers, there are also barrels beside the large coils, from which the wire is drawn out by a device. The common wire electrode diameters are: (0.6); 0.8; (0.9); 1.0; 1.2; (1.4); 1.6 [mm]. The wire diameter of 0.9 mm is usually used in the automotive industry. © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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The wire diameters of 1.6 to 3.2 mm are usually used for flux-cored wires for deposition welding, more rarely for solid wires due to higher arc forces and a more difficult arc ignition. High requirements are placed upon the wire surface for transferring the welding current from the contact tube into the wire. See figure 16. Therefore wires for unalloyed, creep-resistant and high-strength steels are mostly covered with a thin copper layer. Stainless and heat-resistant steels shall not be coppered. The low electrical- and heat conductivity of these materials make the welding current transfer more difficult so that wire feeding faults up to the adhesion of the wire electrode in the contact tube are inevitable for certain wire electrode qualities.

Figure 16: Wire electrodes with poor current transfer characteristics. /2/

6.2

Shielding gases

Shielding gases are required during the metal transfer in the arc in order to protect the weld pool and the back of the weld (weld root) against oxidation and undesired gas absorption from the air. Depending on the material, process variants and requirements of the bead profile, shielding gases of different composition will be used. Shielding gases for welding vary in heat-conductor effectiveness inside the arc at high temperatures. See figure 17. The heat content of polyatomic gases is larger than those of monoatomic gases regarding the energy absorption during the thermal dissociation. See figure 18.

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Figure 17: Heat conductivity of gases at Figure 18: Heat content of monoatomic and different temperatures diatomic gases in relation to the temperature An important temperature is the evaporation point of the base- and filler material (about 2900 °C for steel) because a metallic vapour is being initiated at the surface of the liquid wire electrode at these temperatures. Therefore the shielding gas is blended more strongly with metallic vapour during increasing arc intensities of gas-shielded metal arc welding. The charged particles that are required for the current conduction in the arc originate mainly from the metal vapour because they are easier to be ionised. As the shielding gases transport only few charged particles, they influence the arc mainly by their heat conductivity and heat content. Argon displays a lower heat conductivity at high temperature, compared to other gases. The higher heat conductivity of the polyatomic gases (CO2, H2, O)2) in the temperature range between approx. 2,000 and 4,000°C arises as a result of the energy absorption in case of thermal dissociation (e.g. Q + CO 2 = CO + ½ O2, Q + H2 = 2 H). During the recombination inside the arc shell and closely above the weld pool, the stored energy is being released again and contributes to the welding heat input. The high affinity of the oxygen to most of the elements in the materials to be welded causes a rapid formation of oxides at the given temperatures. The energy gain by the oxidation process is relatively small compared to arc energy. Shielding gases with good heat conductivity properties decrease the arcbow diameter via this cooling effect. This cross-section reduction increases the arc-bow resistance so that with equal current intensities the welding voltage increases which correspondingly increases the arc performances. The good protection characteristics of CO2 are related to its high density and the volume increase through heating and thermal dissociation. The influences of shielding gases for gas-shielded metal arc welding will be explained more deeply in the following chapters by comparing the use of Argon and CO2 during steel (electrode diameter 1,2mm) welding at moderate power settings (Vwire 10m/min)

6.2.1 The Argon Arc during steel welding The wire electrode end offers only a small area for the arc attachment immediately after arc ignition. The temperature at this point will be a little over the evaporation point of steel (2,900°C). At this temperature the heat conductivity of argon is not sufficient for a strong cooling of the arc-bow shell and consequently the constricting of the arc. As a result the arc attachment will be able to rise up along the wire surface © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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which influences significantly the material transfer with argon and argon-rich gas mixtures. See picture 19, left. The wire end will be heated by the arc attachment from outside to inside over a larger area. The pinching force, the forces initiated by the metal vaporising and the surface tension will now become effective: the liquid wire-end will be constricted and small drops will move into the weld pool at an adequate current intensity and arc length. The extremely hot droplet stream causes a finger shaped penetration in the weld run centre. This effect is often wrongly defined as “argon-finger”. After the ignition-phase of the arc the current intensity and pinching force are variable along the arc axis. Subsequently pressure differences arise which create a flow stream inside the arc towards the weld pool. More cooler argon continues to follow and promotes the limitation of the arc’s cross-section.

Figure 28: Arc attachments for argon and argon-rich gas mixtures with small proportions of CO2 (spray arc) and 100% CO2 (long arc) /2/

6.2.2 The CO2 Arc Compared to the argon-arc a larger cooling of the arc-bow shell will become effective using 100% CO2 and argon-rich gas mixtures with more than 25% CO2 in the arc area. The shielding gas CO2 dissociates at high temperatures (starting at 1,600 °C). In particular the heat absorption during dissociation does increase the heat conductivity and heat content of these gases. The cooling effect of CO2 on the arc-bow shell is stronger than argon does and consequently constricts the arc and the arc attachments more and consequently heats the wire end only over a short area. Figure 19, right. The wire end is therefore always hotter than an area just above (e.g. 3mm) the wire end. Consequently this lower temperature is the reason that the pinch force will not become effective for constricting and detaching small drops. In addition the compressed arc and the metal evaporation on the arc attachments areas generate repulsive forces which are obstructing the material transfer and deflect asymmetrically. Figure 19. Only when the accumulated wire dimension is large enough, the material transfer will take place depending on the effective arc length with or without short-circuit. Figure 20 shows a short-circuit-free material transfer at a sufficient arc length. During short-circuit-material-transfers the partly high short-circuit currents must be suppressed with electric or electronic measures for spatter reduction during arc re-ignition.

Figure 20: Large-volume material transfer without arc rupture by welding with CO2 with sufficient arc length (long arc).

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The u-shaped penetration profile for welding with CO2 is created by the good heat conductivity, the association of CO and O (energy release) close to the weld pool surface, the higher arc-voltage compared to argon and by the continuously moving of the arc-attachment. Figure 19 and 20.

6.2.3

Classification and characteristics of different shielding gases for gas-shielded metal arc welding The shielding gases are standardised by EN ISO 14175. Table 8 contains an overview of classifications. All types of shielding gases can be classified with limited symbols by using main- and subgroups. Examples of gas classifications (also specific gases) and their applications are shown in table 9. The standard does not give any information regarding the behaviour and their solubility into the weld metal. Also application recommendations for welding tasks are not included. Table 8: Classification of process gases for joint welding and for related processes, extract from EN ISO 14175

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The given minimum values for purity and humidity of gases as well as the mixture accurateness as given in the DIN EN ISO 14175 are usually adequate in praxis. For steel welding very often, next to CO2, argon-rich gas mixtures (e.g. Ar + CO2, Ar + O2, Ar CO2 +O2, Ar + He + CO2) are applied. The high affinity of the oxygen to most of the elements in the materials to be welded causes, depending on the oxygen level, a rapid formation of oxides at the given temperatures and are mainly deposited on the weld run. The oxidation level of the shielding gas (100% CO2 corresponds to ca. 10% O2) affects the loss of alloying elements and therefore the mechanical-technological properties of the weld metal. With increasing oxidation level usually the yield strength, elongation and toughness will be reduced. For this reason the weld metal properties are classified in the filler material catalogues according to their oxidation level (C and M). For lower oxidation levels (M) the mechanical properties of the weld joint become better. The influences of shielding gas composition on weld bead profile- and surface appearance are shown in figure 21

100% CO2

82% Ar + 18% CO2

92% Ar + 8% O2

Figure 21: The influence of shielding gas composition on the penetration profile and deposits of oxides (slags) on the bead /SLV Munich/

Table 9: Summary of the properties of shielding gases for unalloyed steels and fine-grained structural steels (central area of the deposition efficiency). Shielding gas

Spatter portion

Penetration shape

Melting loss

Pore frequency

Mech. tech. Properties

82 Ar, 18 CO2

low

good

low

moderate

good

90 Ar, 10 CO2

low

finger-shaped penetration in the middle bead

low

moderate

good

70 Ar, 30 CO2

stronger

good (V to u-shaped

stronger

moderate

moderate

92 Ar, 8 O2

low

finger-shaped penetration in the middle bead

stronger

stronger

good

88 Ar, 12 O2

low

finger-shaped penetration in the middle bead

very high

stronger

moderate

strongly

very good (u-shaped)

very high

low

moderate

100% CO2

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For welding CrNi-steels the CO2-content of gas mixtures will be limited to 2-3% in order to prevent carbon pick-up. Ar-O2-gas mixtures (1-3%) will generate higher oxide deposits on the weld run. Special gases replace part of the argon in CrNi steels and nickel-based materials with helium (fewer oxides on the weld run and higher welding speeds possible). Table 10 contains, according to base materials grouping, often applied shielding gases with designation examples for gas-shielded metal arc welding (EN ISO 14175) Table10: Often applied shielding gases with designation examples for gas-shielded metal arc welding. Base material

Non-alloyed and alloyed steels, creep-resistant steels

Composition

Designation acc.to DIN EN ISO 14175 Group class.

Designation acc.to DIN EN ISO 14175

100 % CO2 argon-rich gas mixtures: with: 5 - 15% CO2

C1

C1

M20

with: >15 – 25 % CO2

M21 M21 M22 M22 M23 M23 M24 M25

M20-ArC-10 M20-ArHeC-30/10 M21-ArC-18 M21-ArC-25 M22-ArO-4 M22-ArO-8 M23-ArCO-4/3 M23-ArHeOC-18/3/2 M24-ArCO-10/3 M25-ArCO-13/4

with: >3 -10 % O2 with: >0,5 - 5 % CO2 und 3 - 10 % O2 with: >5 - 15 % CO2 und 0,5 - 3 % O2 with: >5 - 15 % CO2 und >3 - 10 % O2 Argon-rich gas mixtures: with: >0,5 - 5 % CO2

CrNi steels

Aluminium and aluminium alloys Nickel and nickel alloys

with: >0,5 - 3 % O2

Argon Argon with over 60 % Helium Ar-He-O2-mixtures Argon -He -CO2-mixtures Ar-He- H2 -CO2-mixtures

M11 M12 M12 M13 M13 Z I1 I3 Z Z Z Z

Designation examples for practical gas mixtures

M11-ArHeHC10/1,2/0,8 M12-ArC-2,5 M12-ArHeC-5/0,5 M12-ArHeC-20/2 M13-ArO-1 Z-ArHeCO-30/1/0,1 I1 I3-He-30 Z-ArHeO-30/0,3 Z-ArHeC-30/0,05 Z-ArHeHC-30/2/0,05 Z-ArHeHC-30/2/0,12

The shielding gas supply is provided either from pressure cylinders with 200 or 300 bar internal pressure or more frequently, from welding manufacturers by a ring circuit with central supply of the liquid phase. In both cases the pressure must to be reduced and the flow rate must be measured. There are two methods for flow rate measurement:  with a pressure nozzle (capillary), see Figure 31  with floating element, see Figure 32. In the case of the pressure nozzle, the pressure, which is built on the flow of some gases, is measured and read on a Manometer, whose scale is calibrated in l/min.

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When measuring with a floating element, a ball is pressed up by the gas in a conical glass tube with scale. The position of the ball corresponds to the flow speed. An additional review of the shielding gas amount by a measuring tube, which is placed on the shielding gas nozzle is recommended for the practitioner. Although both systems are not calibratable, it will be accurately enough for practical use. The necessary amount of shielding gas depends on the materials, the current power, the welding position and the shielding gas composition, Figure 24.

Figure 22: Shielding gas volume measurement with pressure nozzle. /2/

Figure 23: Shielding gas volume measurement with floating element. /2/

The necessary shielding gas flow rate in the case of deviation of the gas composition of the gas, for which the measuring instrument is calibrated (e.g. argon), is to be determined according to the following formula:

Vx  VArgon 

 Argon x

vX = gas amount to be calculated [l/min] vArgon = gas amount indicated on the rotameter [l/min]. 3 Argon = density of argon (1.748 l/min) [kg/m ] 3 X = density of the gas to be found [kg/m ]

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Figure 24: Shielding gas amount in relation with gas nozzle diameter and current power. /2/

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MIG/MAG welding 7

Page 24

MIG/MAG welding equipment settings

7.1 Setting parameters for welding with step-switched welding rectifiers Initially only the relations for setting and adjustment of the working points inside the area of the arc characteristic curves are shown. The characteristics of material transfers and influences of the shielding gases remain disregarded. Arc length at melting wire electrodes is difficult to be determined. Changes frequently arise as a result of material transfer and from the migration of the arc attachments at the wire end and on the weld pool. Therefore, in the following you can read about the effective arc length. Simple power supplies have 2 buttons for setting the arc, the voltage setting and the wire feed. For more complex devices an adjustable inductivity (choke) is added to flatten out current peaks with short circuits during material transfers. Figure 25 Shows the movement of the working point on the chosen static characteristic curve (machine) by changing wire feed rates which causes current intensity (by changing the arc resistance) and arc length changes. Figure 26 shows the movement of the working points when different static characteristics are used at a constant wire feed rate. By increasing the voltage, the current intensity does not increase or increases only marginally as the arc resistance also increases. Figure 27 shows the movement of the working point on the arc characteristic curve (line) by simultaneous changing the static characteristics and the wire feed rate. Although the arc- and the deposition rates changes by moving the working point along the arc characteristic line, the effective length of the arc stays the same. The illustration also shows that the arc voltage necessarily increases as the wire feed rate increases.

Figure 25: Movement of the working points upon the static characteristics by wire feed rate changes /2/

Figure 26: Movement of the working points when different static characteristic are used for constant wire feed rate /2/

Figure 27: Movement of the working points upon characteristics of the arc by simultaneous straightening of the static characteristics and the wire feed rate. /2/

Figure 28 shows a summary of how the effective arc length and the bead profile change by the movements of the working points. Modifications of the shielding gas composition may affect the arc length at constant wire feed rate and voltage. For example with significantly more CO2 in argon shielding gas mixtures, the voltage must be lifted. Figure 29 shows examples of operating ranges for different shielding gases.

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Figure 28: Change of the working point in the operating zone by adjusting wire supply speed rate and/or the static characteristics of the welding power supply. The consequences on the effective arc length, current power and the bead profile schematised in the lower part of the diagram. /2/

Figure 29: Welding voltage and welding current for different shielding gases (values taken from welding tests) Wire: G3Si1, Ø 1,2 mm.

7.2 Influence of the arc voltage The arc voltage determines the arc length, bead width, amount of metal vapour, magnetic deflectability of the arc and the arc pressure on the weld pool. If the arc voltage is high, the seam will be flat and wide and undercuts may occur. In a very long arc the alloying elements may be more strongly burned-off in the arc and the fume emission increases. A too low voltage results in narrow, highly raised welds and increasing short-circuits during material transfer. Figure 30 shows this on the basis of fillet welds.

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Figure 40: Influence of the arc voltage for fillet welds in the spray arc area. /2/

7.3 Influence of the wire feed rate Wire speed rate is directly proportional to the deposition rate and is almost linear in relation to the welding current. Figures 31 and 32 show how an increasing wire feed rate increases the deposition rate and also the current intensity and the penetration depth. Modifications of the wire feed rate require a voltage change if the effective arc length is to remain the same.

Figure 31: Influence of the current intesity and the wire feed rate during arc voltage changes. /2/

Figure 32: Application areas in relation to arc- and deposition rates. /SLV Munich/

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SFI / IWE 1.08-1 Page 27

Influence of contact tube distance

The contact-tube distance is defined as the distance between the contact tube end and the workpiece surface, Figure 33. In practice, for the lower current ranges lower values (approx. 10-15 mm) and for the upper current ranges larger (15-20 mm) values are being used, in order to relieve the contact tube and the gas nozzle thermally (reference values in Fehler! Verweisquelle konnte nicht gefunden werden.). Figure 33 shows the influence of the contact tube distance on current intensity, and therefore on the penetration depth.

Figure 33: Influences of the contact tube distance /2/

For fillet welds in acute angles, the contact tube can protrude from the gas nozzle so that a sufficient penetration is ensured. 7.5 Influence of welding speed With constant welding data, an increase of welding speed reduces the weld cross-section, see Figure 34. If the same weld geometry is to be achieved with increased welding speed, the welding voltage and wire feed speed must be increased. Welding speed is not arbitrarily selectable. For manual welding, welding speeds of 40-60 cm/min are useful, since with higher values a manual welder may not be able to lead the torch uniformly any more. For mechanised welding, the welding speed can rise up to the process limit. If it is too high, undercuts may occur and the weld will be too high and narrow. Welding speeds of 1-1.5 m/min are used for many welding tasks during batch production. If welding speed is reduced to values below 40 cm/min, an ahead-moving weld pool can reduce the penetration depth severely and thus lack of fusion faults may occur, Figure 35. Generally it is better to weld a seam quickly in three layers than slowly in one layer.

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vS = 20 cm/min vS = 40 cm/min vS = 70 cm/min Figure 34: Influence of welding speed upon the weld profile when making fillet welds. Unchanged parameters: vDr = 10 m/min; IS = 300 A; US = 29 V; Shielding gas: 82% Ar + 18% CO2;

/2/

1 Minimum penetration with ahead-moving bath, 2 Maximum penetration with correct welding speed, 3 Lower penetration with faster welding.

Figure 48: Influence of welding speed on the penetration depth. /SLV Munich/

7.6

Influence of the electrode wire positioning on the weld profile and the edge penetration

The lower the arc intensity and the higher the welding speed, the more precisely the wire electrode must be guided/positioned. Figure 36 shows the influence of the torch positioning accuracy at medium deposition rates.

Wire electrode 3 mm left out of the corner

Wire electrode at the corner

Wire electrode 3 mm above out of the corner

Figure 36: Influence of the precise positioning of the wire electrode and the component position deviations./2/

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SFI / IWE 1.08-1 Page 29

Influence of the root gap when welding square butt welds

Root gaps (in dimensions) improve the depth of fusion and reduce bead height because a part of the weld metal fills up the gap area, see Figure 37. The same tendency applies also for other butt- and joint welds with gaps

Figure 37: Influence of the root gap when welding square butt welds . /2/

7.8

Influence of the torch angle

The torch angle in reference to the welding direction will affect the penetration shape and the outer weld geometry whether a pushing, neutral or pulling position is being applied. A pushing positioning widens the weld but the penetration depth and the weld run height are becoming less. When using a too extreme pushing torch position (>120°) the ahead running weld pool reduces the penetration extremely causing lack of fusion. During welding in pulling position, the weld run becomes smaller, more raised (higher) and the penetration deeper. Figures 38 and 39 show the influences graphically. The torch angle during vertical down welding should be about 90° to slightly pulling in reference to the sheet surface. Aluminium materials should always be welded in a pushing position. Welding in pulling position often generates a dark surface (condensate of vapour out of the arc zone on top of and near the weld run).

Figure 38: Influence of the torch angle on the bead profile. /2/

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Figure 39: Influence of torch angle on the weld profile when making fillet welds.

7.9 Influence of the weld position The welding positions are marked according to EN ISO 6947. When welding in the horizontal weld positions, the highest deposition rates can be achieved, where welding speeds need to be increased accordingly, in order to avoid an excessively large, ahead-moving weld pool. This applies in particular for welding in the vertical down position. Only with optimised parameters and limited weld run thicknesses it will be possible to achieve satisfactory penetration characteristics, figure 40.

Figure 40: Defect-free vertical-down welds for structural steels with the shielding gas CO 2 achieved through compliance with tight parameter ranges. /2/

Figures 41 and 42 are showing the specific influence of the travel speed during welding in vertical down position. If the travel speed is too less the fluid weld pool tries to overtake the arc, figure 41 left. In order to avoid lack of fusion the travel speed has to be higher as the speed of the weld pool which is falling down. The arc is set briefly in the vertical-down position (up to the spatter limit). Short and transition arcs are appropriate for this. An “a dimension” of up to 3.5 mm is achieved with steel in this position in a single pass.

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Figure 41: Influence of welding speed on the edge fusion of vertical-down welds. /2/

Figure 42: Vertical-down welds on steels having lack of fusion caused by ahead-moving weld pool.

Welding aluminium in the vertical down position is even more difficult due to the lower viscosity than welding steel. All weld tilting angles are adjustable on the revolving pipe or with positioning equipment. Figure 43 shows the effects on the weld geometry.

. Figure 43: Influence of the weld position (the weld angle) on the weld geometry on the rotating pipe. /2/

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The vertical-up weld position for heavier welds has good shape abilities using weaving torch movements and reduced arc intensity setting, see Figure 44 and Figure 45. The welding speed and deposition rate are small.

Stringer bead in position PG.

Arc trace along the bevel faces

Arc trace along the workpiece flanks

Figure 44: Wire electrode guide and layer structure when welding vertical-up fillet welds. /2/

Figure 45: Wire electrode guide and layer structure when welding vertical-up butt welds. /SLV Munich/

The controlled cycle switching during stepped adjustment of the power source or with periodically changing arc performance by electronic welding units mean that vertical uphill welds can be formed faster without oscillation and free of lack of fusion faults, Figure 46.

Figure 59: vertical-up weld, fully mechanised, welded at intervals; a = 4.5 mm. /2/

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SFI / IWE 1.08-1 Page 33

Wire electrode polarity, forces in the arc and metal transfer modes (arc types) Polarity of the wire electrode

Figure 47: Overview of the influences of the electrode wire polarity. /2/

In gas-shielded metal arc welding, positive or negative poled wire electrodes can be used for welding. Figure 47 shows the most important influences on electrode wire polarity. Figure 48 shows the cleaning effect during MIG/MAG welding. In MAG-welding, complete oxide eliminations is impossible due to the oxygen and/or CO2 supply in the shielding gas. Figure 48 shows the differences during metal transfer in the spray arc area during MAG welding.

MAG surface welding on steels with a positive poled wire electrode.

MIG surface welding on aluminium material with a positive poled wire electrode.

/SLV Munich/

/SLV Munich/

Figure 48: The cathodic cleaning effect by station preceding the weld pool with spots at which short-term electron emission takes place and particularly oxidic substances are removed (cathodic cleaning effect).

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Short arc applications with negatively poled electrodes allow very low levels of heat-input to be applied, but create narrow and raised beads during welding and soldering on steels. Electric power supplies with an inverter in the output circuit allow a negative polarity of the electrode wire according to phase (short and pulsed arcs.) The advantages of each polarity type can be exploited within certain limits.

+

-

Spray-arc with positive poled wire electrode

Spray-arc with negative poled wire electrode

Figure 49: Influence of the electrode wire polarity during MAG welding with medium arc performance.

8.2 Influence of the wire electrode diameter on deposition rate and weld run profile The electrode diameter (Ø 0,8 mm, 0,9 mm 1,0 mm, 1,2 mm, 1,6 mm) will be chosen depending on the type of weld. The current-carrying capacity increases with the wire electrode diameter, see fig 50. Important influences which have to be taken into account are shown in figure 51. Figure 52 shows an overview of the influences of electrode wire diameter to the weld run profile under similar deposition rates or current intensities.

Figure 5: Deposition rates in relation to current intensity and wire diameters /2/

Figure 6: Influences of wire electrode diameter for shielded-gas metal arc welding. /2/

Figure 7: Influences of wire electrode diameter to weld run profiles under similar deposition rates or current intensities. /2 /

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SFI / IWE 1.08-1 Page 35

Forces in the arc

Different forces are applied to the wire electrode end, the arc attachment on the wire, the weld pool, and in the arc which can influence the metal transfer from the electrode wire into the weld pool and the weld run profile. Figure 53 below shows a schematic view of the most important forces. Additionally, the distribution of the temperature at the end of the wire plays an important role during droplet detachment.

Figure 53: Forces during the metal transfer inside the arc. /2/

Figure 54: Arc attachments for argon and mixed gases with small CO-2 levels (spray arc) and CO2 (long arc) /2/

The pinch force is an electromagnetic force which applies on every current-carrying conductor and grows proportionately according to the square of the welding current and decreases proportionately according to the square of the cross-sectional area. However, this force is not large and can only become effective, if the wire end is semi-solid or liquid. Using low current, the pinching force is not large enough to sufficiently affect the drop formation. Large drops are transferred into the weld pool. For small currents, gravity and surface tension are the main influences for metal transfer.

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The so-called critical current value is exceeded at higher currents: the pinch force can constrict the liquid wire end and detaches small droplets. The prerequisite, however, is that shielding gases with low heat conductivity (Ar, Ar + O2, Ar + CO2 10

D(oppel)Y-Fuge

1b4 40°    60°

a b c

In Sonderfällen auch für kleinere Werkstückdicken und Prozess 3 möglich; Gegenlage ist angegeben

111 141

  60° 2.4

Gegenlage ist angegeben

111 141

  60°

> 10

-

13

-

2

40°    60°

2.3

(nach ISO 4063)

111 141 3

Bemerkungen

52

0

V-Fuge

Darstellung

111 141

  60° 2.2

Empfohlener Schweißprozess c

2c6

h1 = h 2 = t c 2

13

Für Schweißen in Position PC nach ISO 6947 (Querposition) auch größer und/oder unsymmetrisch. Die angegebenen Maße gelten für den gehefteten Zustand. Der Hinweis auf den Schweißprozess bedeutet nicht, dass er für den gesamten Bereich der Werkstückdicken anwendbar ist.

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Table 15: Weld joint preparation for fillet welds, welded single sided (Extract from DIN EN ISO 9692-1) Kennzahl Nr.

Werkstückdicke

Art der Schweißnahtvorbe reitung

Symbol

Schnitt

Maße

(nach ISO 2553)

t mm

Winkel , 

Spalt b mm

Empfohlener Schweißprozess a (nach ISO 4063)

3.1.1

t1 > 2 t2 > 2

Stirnfläche rechtwinklig

70°    100°

2

3 111 13 141

3.1.2

t1 > 2 t2 > 2

Stirnfläche rechtwinklig

-

2

3 111 13 141

3.1.3

t1 > 2 t2 > 2

Stirnfläche rechtwinklig

60°    120°

2

3 111 13 141

Darstellung

a Der Hinweis auf den Schweißprozess bedeutet nicht, dass er für den gesamten Bereich der Werkstückdicken anwendbar ist. b Symbol ist nur für  = 90° anwendbar.

Table 16: Weld joint preparation for MIG welding of aluminium (thin wire) Werkstück dicke t mm

Nahtart

Nahtaufbau (schematisch)

Abstand b mm

Steghöhe c Öffnungswinkel  mm

Bemerkungen

1 ... 4

0 ... 0,25 t -

-

Einspannvorrichtung mit Badsicherung erleichtert das Schweißen erheblich

5 ... 10

< 1,0

-

von beiden Seiten geschweißt

90°

Badsicherungen erleichtern das Herstellen fehlerfreier Wurzellagen erheblich

60°

Wurzellagen in Zwangspositionen können ohne Badsicherung leichter mit dem Prozess WIG oder WPL geschweißt werden

.

5 ... 12

< 1,0

5 ... 20

< 1,0

-

2,5

2,5

Table 17: Weld joint preparation for MIG welding of aluminium (thick wire) /2/ Werkstück

Nahtart

Nahtaufbau

Abstand b

Steghöhe c Öffnungs-

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MIG/MAG welding dicke t mm

(schematisch) mm

mm

Page 61

winkel 

Beilage mindestens 12 mm dick > 18

5 ... 7

5

70°

Drahtelektrodendurchmesser: 2,4 oder 3,2 mm Steghöhe abhängig von der Stromstärke

18 bei 90° etwa 700 A

> 30

Drahtelektrodendurchmesser: 2,4 oder 3,2 mm

11 Weld pool support (weld backing) Weld backing for butt welds (unlikely for fillet welds) is necessary when welding on both sides is not possible or prohibited. Using weld backings is usually combined with additional effort. It has to be checked which types of weld backings are suitable or whether another joint type could be applied. Figure 113 shows some examples of weld backings for MIG MAG welding. For smaller, straight products often clamping devices of copper bars are applied. Normally they have a groove which can be flushed with shielding- or backing gas according to requirements and base material characteristics. For intensive use the copper bars are often water cooled. Copper bars without groove could initiate defects in the root pass. Other types of weld backing are ceramics, glass or use of powder coated adhesive tapes (one-time use only). In case of larger components often also powder cushions are used for weld backing (see also submerged arc welding). If weld backings are being used that remain attached to the component it should be noticed that grooves and gaps remain which are unfavourable for dynamic and corrosive load conditions. Weld backings Bei Schweißbadsicherungen, die angeschweißt werden, ist zu beachten, dass Kerben und Spalten bleiben, die bei dynamischer und korrosiver Beanspruchung problematisch sein können. Schweißbadsicherungen, die bei Drehteilen oder Pressteilen am Bauteil angebracht werden, können zugleich eine Fixierung bilden. Weld backings which are integrated by the component are very effective but expensive. For example an Y-joint can be welded, grinded out from the backside and welded again. A less expensive but nevertheless a good method is the double-sided welding technique which is often applied for submerged arc welding but also for MIG / MAG welding. At first the component is welded from one side over more than half of the component thickness. Then the component will be turned, followed by welding the back side (other side) resulting in an overlap of both root passes. In many cases, for example facing high quality requirements and/ or difficult weld courses/routings it is useful to use the TIG welding process for welding the root pass(es).

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Figure 113: Schweißbadsicherungen für das MIG/MAG-Schweißen /2 /

12 Advantages and disadvantages of Gas Shielded Metal Arc Welding 12.1 Advantages of MIG/ MAG welding Almost all weldable base materials can be welded using the MIG/MAG welding process. Non-alloyed and alloyed steels, as well as CrNi-steels are being welded using the MAG welding process (active, CO2 and/or oxygen containing shielding gases). Other materials like aluminium, magnesium, nickel based materials, copper, titanium etc. will be welded by MIG welding using inert shielding gases. The achievable deposition rates are high combined with sufficient weld quality. The welder’s requirements are relatively low for welding unalloyed and alloyed steels in mass production. However, for welding high quality products made of steel, Cr-Ni-steels, aluminium etc. an adequate training of the welder is absolutely necessary. The processing can easily be mechanised (Robotics). Components of 0.8 thicknesses and above can be butt welded or with a T-joint. Root passes can be welded with (+ adequate joint preparation) or without weld backing /fixtures. The weld processing can be executed for out of position welding. The acquisition costs are relatively low for standardised power sources. 12.2 Disadvantages of MIG / MAG welding Lack of fusion at weld starts and re-starts cannot always be prevented due to the immediate start of filler metal feeding at the very beginning of the arc’s heat input. When welding with (too) low travel speed potential lack of fusion is present due to an ahead-moving weld pool. The shielding gas atmosphere must be maintained by appropriate fixtures in case of outdoor welding or welding inside draughty halls. Mass production MAG welding of steel with reduced quality requirements requires only a limited welder’s training. For higher requirements like thicker components and other materials a more specific welder’s training is absolutely necessary. Even for full mechanised welding this training is essential as although the welder does not guide the torch by hand, he/she must know exactly how to set-up the machine and torch positioning.

13 Process variants of MAG welding In the following some process variants of MAG welding will be described.

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13.1 MAG-Spot welding During the MIG/MAG spot welding the torch will not or only very limited (e.g. Alu base materials) be moved over the weld piece. The torch will be positioned to the work piece by a specific shielding gas ceramic ferrule, see figure 114. This ceramic ferrule contains openings for shielding gas output and weld observation. Usually always overlapping sheets are welded in which the upper sheet contains a drill hole for a substantial improvement of the joint’s safety in particular when there is a gap between the upper and lower sheet. Only in case of using very thin sheets and accurate fitting the drill hole of the upper sheet can be omitted. Figure 115 shows the common used applications in automotive repair.

Figure 114: MAG-Spot-

welding. /2/

Figure 115: Examples of applications for MAG spot welding of thin sheet metals. /2/

13.2 Gas shielded metal arc brazing During GSMA-brazing the work pieces are connected by Cu-base filler materials having a lower melting point than that of the work piece base materials. Like other brazing processes the work pieces are ideally only wetted via a thin diffusion layer. In praxis however it is inevitable to get some fused surfaces of the base materials. Nowadays the application of GSMA-brazing is mainly concentrated to sheet metal welding of non- and low alloyed materials with zinc coatings or to aluminium base materials. Figure 116 shows schematically the brazing of thin zinc coated (7 to 10 m) steel sheet metals. Depending on the type of material transfer and arc length the zinc burn-off next to the weld run is about 1 to 2 mm width, see figure 117. Common used joint types are shown in figure 118. The weld run profile can be influenced by the choice of arc type, see figure 119.

Figure 116: Schematic view of GSMA-brazing of thin coated steel sheet metal

Figure 117: Zinc burn-off next to the weld run

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Figure 118: Common used types of GSMA-brazing joints Kurzlichtbogen

Impulslichtbogen

Figure 119: weld run profile influenced by arc type. /2 /

Advantages of GSMA-brazing of zinc coated steels are:  less heat-input as regular welding  almost no spatter and pores  good gap-bridging capabilities  high brazing speed  no flux required  no weld run undercuts Disadvantages of GSMA brazing of steel:  higher filler material prices than regular filler materials for steel welding  for thin sheet metal brazing flat weld runs are generally only achievable using the pulsed arc mode  In work piece areas having large deformations (high residual stresses) it is possible that liquid brazing weld metal penetrates into the base material via grain boundaries. Common used brazing filler metals are: CuSi3, CuSi2Mn, CuAl8, CuAl8Ni2, CuMn13Al. High alloyed brazing filler materials do have higher strength properties. Argon can be used as shielding gas for all brazing filler materials. Slightly oxidising shielding gases like Ar + 2% CO2, Ar +1% O2 do improve process stability substantially.

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13.3 MIG/MAG-High Performance Processing The standard MIG MAG weld processing can already be considered as high performance processing. Nevertheless, for a long time they have been trying to improve the power efficiency. Power efficiency means a higher deposition rate, a higher welding speed or both. Additionally the economic efficiency can partly be raised through weld cross-section reduction. In exploiting the economic opportunities of gas shielded metal arc weld processing or their combinations with other joining processes it should be noticed that often to more restrictions have to be complied with in order to guarantee sufficient weld quality. The high requirements to the weld processing stability can only be achieved through excellent contact properties of the wire electrodes and optimal torch configuration. Limits are set to manual GSMA-welding via the thermal stressing of the welder (even with adequate protection/ safety clothing) and via the high requirements to the torch positioning accuracy. For partly mechanised MAG welding the maximum deposition rates are around 9kg/h (18m/min with 1.2mm wire diameter) and the maximum speed is around 60cm/min.

Figure 120: Power ranges of standard MAG welding, MAG high Performance welding with one or two wire electrodes as well as for Laser-MAG welding, all under optimised conditions./2/ DVS-Leaflett 0909-1 “Basics of high performance gas shielded metal arc welding with solid wire - definitions and terms” shows an overview of the several processes. Figures 120 and 121 show an overview of the power ranges of conventional (standard) and high performance processes using one and two wire electrodes. In the late 1980s the so called T.I.M.E. high performance weld process with patented shielding gas composition (65% Ar, 26,5% He, 8% CO2, 0,5% O2; Licensee Fronius) became known in Germany. With the relative expensive 4-component shielding gas, good results have been achieved using spray arc mode and rotating spray arc mode. More or less at the same time the multiple wire process application has been developed further by the SLV München in cooperation with a few power source manufacturers. Both directions, either using one or two wire electrodes, have substantially increased the economic efficiency in many applications.

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In DVS leaflett 0909-1 new processing symbols have been specified for high performance MIG / MAG welding. However, these terms although they have been recommend by experts, will not be used in our lectures as they have not yet been adopted into standards and basic rules. DIN 1910-100 “Welding and allied processes – vocabulary” contains, among others, classification and denomination possibilities of weld arc processes from the partially withdrawn DIN ISO 857-1 standard. For the rotating spray arc mode no designation has been introduced lately. 13.3.1 High Performance weld processing with single wire electrode Stimulated by the T.I.M.E.-process (initially only designated for the rotating arc) other authorities also investigated the areas of high electrode wire speeds out of which a series of interesting types of welding processes have been originated. Figure 121 shows the different types of material transfer depending on the wire speed and voltage. Figure 122 and 123 show the schematic views of material transfer for rotating and high performance spray arc welding. In Abbildung 120 sind die unterschiedlichen Arten des Werkstoffübergangs in Abhängigkeit von Drahtgeschwindigkeit und Spannung aufgetragen. Abbildung 121 und Abbildung 122 zeigen die Schemen der Werkstoffübergänge beim Schweißen mit dem rotierenden Sprühlichtbogen und Hochleistungssprühlichtbogen.

Figure 121: Power ranges of single wire MAG welding and the typical weld run profile that can be acieved./2/

13.3.1.1 Rotating arc welding Around 1970 the first experiments of rotating arc weld processing have been carried out using thin wire electrodes with speeds of more than 60m/min. However, for the at that time intended application area, surfacing welding, the processing was not stable enough. With specific 3- and 4-component shielding gases wire speeds of up to 50m/min (1,2mm wire diameter) have been achieved. Nowadays, in praxis mainly wire speeds of 20-25m/min are being used. A wider and almost always sufficient penetration depth is being achieved in the PA- and PB welding position, figure 122.

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MIG/MAG welding Specific features for rotating arc welding:  argon-rich gas mixtures with 2-4 components,

   

often

with

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up

to

30%

added

He

quantities

(3,1 % O2, 25 % He, Rest Ar, 25 % CO2, 25 He, Rest Ar, 2 % O2, 25 % CO2, 26,5 % He, Rest Ar 30 % He, 10 % CO2, Rest Ar, 8% CO2, Rest Ar) longer free wire-end (pre-condition for rotating)) high performance, wider penetration, no depth-penetration in the middle of the weld run within the transition area from spray arc mode to rotating arc mode instability is possible Companies name: T.I.M.E. Process, Rapid-Melt, Linfast-Concept

Figure 122: Schematic view of rotating spray arc mode /2 /

Figure123: Schematic view of high performance spray arc mode. /2 /

13.3.1.2 High performance spray arc welding This type of arc, figure 123, generates a very deep but narrowed penetration in the middle of the weld run. Due to its disruption susceptibility in combination with pore formation this application is rarely used. 13.3.2 High performance dual-wire weld processing At this moment two types of dual-wire weld processes are being used: the MIG/MAG dual-wire process and the MIG / MAG tandem welding process. See figure 124, top. The welding current supply is different for both types of processing. The dual-wire system uses one power source to provide both wires with welding current and the tandem welding process uses a separate power source for each wire. Figure 124, below. Figure 125 shows the torch for MIG /MAG tandem welding. By repositioning of both torches there are additional application opportunities available for practical use. See figure 126. The advantage of this high performance multiple wire welding process is the larger power range in terms of deposition rates and welding speeds. By using tandem welding weld processing can be executed having different types of arc modes. For both types of weld processing the welding speed can be adjusted over a wide range. Compared to single wire welding the welding speed can be doubled. Usually the heat input of dual wire welding is less than for single wire welding and can be adjusted according to the welding task. The MIG / MAG multiple-wire weld processing can weld all weldable base materials. 13.3.2.1 MIG/MAG-Dual-wire welding By using the same contact tube both electrode wire voltages are more or less similar. The welding current follows Ohmic’s law (parallel connection of resistors) and usually the wire current intensity will also be more or less the same for both wires. However there might be disturbances via different contact characteristics between the wires and the contact tube due to surface defects of both wires. There is also the possibility of using different welding speeds, for example to adjust a higher wire speed over the first wire (shorter arc) and a somewhat longer arc over the second wire. By doing this a high(er) penetration combined with a smooth weld surface can be achieved. © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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MIG/MAG-Dual-Wire Welding: argonrich gas mixtures, pulsed- or spray arc mode low to very high power ranges possible very high welding speeds achievable Processing with two wires, one power source, one torch, one contact tube

MIG/MAG-Tandem Welding: argonrich gas mixtures, short-circuit-, pulsed- or spray arc mode low to very high power ranges possible very high welding speeds achievable Processing with two wires, two power sources, one torch, two potential-separated contact tubes improved processing stability and easier process optimising opportunities than MIG/MAG dual wire welding

Figure 124: GSMA-processes with two wire-electrodes. /2/

Figure 125: Torch for GSMA tandem welding. /2/

Figure 126: Process variants of tandem welding by contact tube adjustment. /2/

The magnetic arc blow which moves both arcs towards each other in case of equal poled wire electrodes, is used here in a positive way. The magnetic deflection controlled by the optimised distance between the wires which depends on the current intensity and type of shielding gas, enables a centralised material droplet transfer into the weld pool. Figure 125.

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The clearly enlarged contact area of the arc just before the material transfer enables a sufficient melting of the base metal even at high welding speeds. And after the material transfer the enlarged contact area of the arc enables the formation of wide and flat weld runs without having undercuts. Even with high welding speeds and high deposition rates the multiple wire welding processes do not have crossing arcs which allows a processing of high efficiency even for less accurate pre-assembled components 13.3.2.2 MIG/MAG-Tandem welding Nowadays this type of weld processing is being preferred above the dual-wire welding technique due to its high stability during interruptions and the better adjustment features of both arcs to welding jobs. The achievable deposition rates and welding speeds are more or less the same The magnetic arc blow which is also active during tandem welding can however be widely eliminated by using two pulsed arcs which are 180° phase shifted. In figure 127 and 128 the magnetic arc blow is schematically shown with and without phase shifting. The deposition rate for steel welding using phase shifted pulsed arc mode is limited to around 18kg/h. Without this phase shifting deposition rates of more than 25kg/h are being achieved during spray-arc or pulsed arc mode. For sheet metal applications welding speeds over 3m/min are possible.

Figure 127: Magnetic arc blow and deflection of the arc and the material transfer during synchronised pulses. /2 /

Figure 128: Reduced magnetic arc blow via phase shifting of pulses. /2/

Additional applications are MIG/MAG tandem surface welding and tandem welding with flux cored wires. The investment costs for multiple wire applications are relatively low as only two commercially available pulse power sources and one multi-wire nozzle are required. Both welding processes are only available as fully mechanised systems. Commercially used argon-rich shielding gas mixtures are usually being used for steel welding. Figures 129 and 130 are showing weld cross-sections of tandem welding with high welding speeds.

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Figure 129: MAG-multiple wire welding of a fillet weld (a-size = 4 mm) in steel. Sheet thickness 5mm, deposition rate 14,8 kg/h, welding speed 1,9 m/min.

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Figure 130: MIG-Tandem welding of a overlap joint of aluminium AlMg2,7Mn, sheet thickness 4 mm, deposition rate 6,6 kg/h, welding speed 3,0 m/min.

13.4 Plasma Gas Shielded Metal Arc Welding Figure 130 shows schematically the principle of plasma gas shielded metal arc welding. This welding process is a combination of a plasma torch and a gas shield metal arc torch. At the weld’s beginning the plasma arc will heat up (melts) the base metal before the wire electrode material transfer starts. The danger of pore formation and lack of fusion will therefore be reduced (e.g. for Al base materials, sheet metal area). The torch nozzle does have a relatively large diameter. The plasma- and gas shielded metal arc can be arranged in series. Figures 132 and 133.

Figure 131: schematic view of plasma – GSMA welding with concentric arranged plasma arc. /2/

Figure 132: Schematic view of plasma – GSMA welding with both arcs being arranged in series. /2/

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Figure 133: Schematic view of plasma –GSMA welding with welding torches arranged in series and two weld pools. /2/

13.5 Laser- Gas Shielded Metal Arc Welding During high power laser welding of thicker components (steel and al-base materials) the welds are often not sufficiently filled. However, through combinations with arc bow processes component deviations and orientation deflections of work pieces can be compensated much better even at high welding speeds. An overview of today’s applied process combinations of Laser- GSMA-welding is shown in figure 134.

Figure 134: Schemes of Laser-GSMA-welding. /2 /

14 Electrogas welding Figure 135 shows the principle. Thick, vertical positioned components are being welded with high deposition rates using a square butt weld with gap. A large weld pool is generated which is being contained by sideways positioned water cooled copper plates. The fixture with torch and copper plates will continuously be pulled upwards in accordance with the rising weld pool. Due to the large weld pool the mechanical properties of these joints can be less than welds being welded with a multiple layer technique. By using filler metals containing fine grain formation additives improvements are possible.

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Figure 135: Schematic view of Electrogas Welding.

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15 Test questions (1)

How do MIG and MAG welding processes differ from each other?

     (2)

Which shielding gases are used for gas-shielded metal arc welding?

     (3)

P,S C Si, Mn Al, Cu Ni, Mo

What causes an increase in the distance between the contact tube and the workpiece in case of gas-shielded metal arc welding?

     (5)

CO2 or Ar/CO2-mixes for welding of unalloyed steels Ar and He for welding non-ferrous metals CO2 for welding high alloyed steels CO2 for welding non-ferrous metals Ar/CO2-mixes (with limited CO2 content) for welding of high alloyed steels

The wire electrode materials according to EN 490 for unalloyed steels and fine-grained steels to 500 N/mm2 yield point (G2Si1, G3Si1, G4Si1) differ mainly in the alloy content of:

     (4)

By the slope of the static the power supply By the type of current used By the polarity of the electrode used By the shielding gas used By the deposition rate

Less spatter Lower current Less penetration Fewer pores Worse current transfer to the wire electrode

How can the internal control correct an arc in GMAW welding, when the arc became too long due to an interference?

    

By increasing the wire feed rate By reducing the operating voltage By reducing the current By changing the torch distance By adjusting the welding speed

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Slag amount Welding Voltage Welding current Deposition rate Arc Length

Which are the most important advantages of MAG-welding over manual arc welding?

     (9)

By reducing the contact tube distance By increasing the wire feed rate By reducing the operating voltage By increasing the welding speed By increasing the operating voltage

Which parameters and influencing values change considerably, when the wire feed rate is increased during gas-shielded metal welding?

     (8)

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How can the arc length be reduced permanently during gas-shielded metal arc welding?

     (7)

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It is possible to weld with high alloy steels lower equipment costs higher deposition efficiency higher welding speeds good suitability for welding fine sheets

Which shielding gas produces the highest slag content when MAG welding with higher arc power?

    

82% Ar + 18% CO2 100% CO2 97% Ar + 3% O2 70% Ar + 30% He 92% Ar + 5% CO2 + 3% O2

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16 Bibliography /1/

DIN 1910-100:2008

/2/

Hertz G. and R.Rompe: Introduction to plasma physics and its technical application

/3/

Baum L. and V. Fichter: The shielding gas welder, Part II, MIG/MAG welding. Welding technology practice, Volume 12 (1999), DVS-Verlag, Düsseldorf.

/4/

Ruckdeschel, W.: Material Transfer during MIG/MAG welding. Linde-Bericht 70-F-51 (1970).

/5/

Aichele G. and A. A. Smith: MAG-Welding (1975), DVS-Verlag, Düsseldorf.

/6/

Knoch, R. and W. Welz: Active gas pulsed arc welding with transistorised power supplies. Welding and cutting 38 (1986), Leaflet 2, pp. 67-71.

/7/

Pomaska H. U.: MAG welding “no book with seven seals”. Linde AG.

/8/

Schambach, B.: Current of conditions of welding European standards, extra edition to the 26. Special Conference “Welding in equipment and container construction” (2000), Munich.

/9/

Dilthey, U.: Energy balance of the arc column. DVS reports 30 (1974), pp. 139-156.

/10/

Knoch R.: Welding designations for the MIG/MAG welding process. DVS reports, leaflet 91, DVS-Verlag.

/11/

Schellhase, M.: The welding arc - a technological tool. Leaflet 84, DVS-Verlag, Düsseldorf.

/12/

Welz, W. and R. Knoch: Setting and classifying impulse arcs in metal active gas welding. Welding and cutting 41 (1989), leaflet 12, S.658-660.

/13/

Welz, W. and R. Knoch: Examination of pulsed MAG welding. Welding and cutting 41 (1989), leaflet 10, S. 542-547.

/14/

Knoch, R.: Measured variables and measuring instruments in gas-shielded metal arc welding. Welding and cutting 38 (1986), leaflet 7, S.330-334.

/15/

Knoch R.: Increasing the ignition and start-up phase with metal electrodes active gas welding. Welding and cutting 35 (1983), leaflet 8, S. 370-376.

/16/

Knoch, R.: Examination of the ignition process and the initial joint faults in metal active gas welding. Welding and cutting 35 (1983), Leaflet 9, pp. 432-435.

/17/

DVS Guideline 0912, Part 1 - Avoidance of lacks of fusion. Part 2 - Avoidance of pores.

/18/

Knoch, R. and A. W. E. Nentwig: Quick MAG welding with multiple wire electrodes. DVS reports no. 162 (1994), page 77-81.

/19/

Baum, L. and R. Knoch: Higher economy by the high power MAG process. DVS reports no. 183 (1997), page 50-55.

/20/

Marfels, W.: The arc welder, welding technology practice DVS-Verlag, Düsseldorf.

/21/

DVS leaflet 0909-1: Process principles of MSG heavy-duty welding with solid wire electrodes, definitions and terms.

/22/

DVS leaflet 0926: Requirements of power sources for gas-shielded metal arc welding.

/23/

DVS leaflet 0926-3: Process parameters and equipment technology for pulsed arc welding

/24/

DVS leaflet 0932: MAG setting practice – engineering and component-dependent influences on the weld geometry.

/25/

DVS leaflet 0932: MAG setting practice – engineering and component-dependent influences on the weld geometry.

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After frequent torsional, bending and tensile loads, broken wires and ruptured hoses are to be expected after a certain time, Figure 13. For the purposes of the warranty, stable welding parameters must be monitored at all contact points in the welding current. Elastic workpiece clamps with relatively small contact area are unsuitable for larger welding currents, Figure 14.

Figure 8 shows the structure of the wire feed. In order to achieve a good welding result, the wire electrode must exit the contact tube at a steady speed. Slowing the wire will extend the arc and in extreme cases it may cause backfiring in the contact tube. An uneven wire feed rate (stutter) leads to short circuits during welding and thus to an intensified spattering. Figure 9:. One or more rolls may be used, Figure 12. The thin wire electrodes, particularly those made from aluminium, tend to buckle if not fed appropriately

minimizes the size as the droplet size on the wire end will be minimized (the wire “creeps” with lower feed up to ignition) , Hot start (higher initial current pulse for better ignition), and arcing time (a selectable arcing time prevents the wire ends from "freezing" in the end crater and) are required.

The four-cycle torch control, when the welder only uses the switch to ignite and terminate the welding process, secures the gas shielding at the start of the seam and at the end of welding. When using power supplies with adjustable slope of the static characteristics it is important to make sure that the operating voltage and therefore the effective arc length change with each adjustment of the slope.

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Control unit Figure 7: When using electronic welding equipment, variable welding parameters are available for the start and the end of bead.

Figure 6: Two-cycle control /SLV Munich/

Four-cycle control

Figure 8: The structure of the wire feed. /SLV Munich/

Figures 10 and 11 show the frequently observed faults in relation to the wire feed.

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Figure 9: Wire Feed Rolls: Wedge grooves for hard wires and round grooves for soft wires. /SLV Munich/

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Figure 10: Deformation and increased friction caused by faults in the area of the wire guide. /SLV Munich/

Figure 11: Faults caused by excessively large or small grooves in the wire guide and by too intensive electrode pressure. /SLV Munich/

Figure 12: Wire feeding systems. /SLV Munich/

Figure 13: Cable break in the hose assembly.

Figure 14: Damaged workpiece clamps.

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/SLV Munich/

. Figure 15 shows examples of short and pulsed arcs. Figure 16 shows wire feeding systems and auxiliary materials for extending work space.

Figure 16: Wire feed units and ways to extend the work space. /SLV Munich/

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The suitable power supply shall be chosen according to Table 3 depending on the application.

Therefore the values for setting the voltage (open-circuit voltage) should be calibrated in Volts and the wire electrode feed rate should be calibrated meters per minute, for example The arc voltage, at a 3 V/100 A voltage drop and other losses from approx. 1 V/100 A about 4 V/100 A welding current is lower than the set open-circuit voltage. This significant difference requires the specific provision of what has been measured and where in the welding data sheets.

and a mounting tube for machine torches. An arc ignites between the wire exiting the contact tube and the workpiece, Figure 1. The arc and the highly heated weld pool are protected by a shielding gas flowing from the shielding gas nozzle. The water-cooled torch conveys the radiation heat of the arc from the contact tube and from the gas nozzle as well as the resistance heating through the welding current in the whole torch via the cooling water. Non-cooled (gas or air-cooled) torches in the upper power range may reach a temperature up to 700 °C even after short welding time in the contact tube and in the shielding gas nozzle /6/. As the copper of the contact tube softens, wear increases and spatter adheres more easily to the gas nozzle because of the higher temperature, and makes torch cleaning difficult. Therefore water-cooled torches shall always be used at higher welding current. The welding torches are organised by current values and shall be monitored, as too largely dimensioned torches are heavier and load the welder more. It is important to pay attention that the argon-filled shielding gas mixes load the torches thermally more than CO2. The adjustment of the shielding gases may require a stronger torch. Figure 17 shows a GMAW welding torch. The sectional drawing shows the structure of the torch in case of a gas- and water-cooled torch. For larger arc powers torches with directly cooled gas nozzles are also available for larger arc powers.

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. With a high specific current load this is possible only with a wear on the contact tube. The bore in the contact tube, which is about 0.2 mm (steel) and about 0.4 mm (aluminium) larger than the wire diameter, expands conically after a certain length of time. The exposed wires change and if the deposits in the contact tube get contaminated wire surfaces, the arc becomes irregular and may spray more heavily. The quality of the wire electrodes has a significant influence on the contact tube service life. Figure 18 shows frequent causes for the distortion of the contact tube. Melting caused by wire surface errors, very high pulsed current during pulsed arc welding or arc ignition. Spatters at the bore edge “lengthen” the contact tube (with poor electrical conductivity) and cause considerable slow-down. Spatter from the arc zone or burst resulted from the melting of the wire electrode in contact points with very high local current density. Deposits of abrasive particles and draw-aids caused by insufficiently cleaned wire surfaces or of abrasive particles caused by errors of the wire feed units. A too big contact tube bore, particularly with wires of low bending and straight torches frequently cause changing contact points and free wire electrode lengths. Wires with larger bending improve the contact quality and also increase the friction in the feed hose and in the contact tube. Figure18: Frequent faults in contact tubes /SLV Munich/

Figure 19: Forced-contact current contact tubes generally improve the introduction of the current signal into the wire electrode.

Figure 19: Examples of forced-contact current contact tubes /SLV Munich

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Figure 20:.

Figure 20: Leakage currents in the torch reduce the welding current at gas nozzle contact with the workpiece and damage the torch body /SLV Munich/

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The rating plate of a MIG/MAG power supply, according to EN 60974-1 may make it easier to select the correct welding equipment, as important characteristics, such as the power range and the network load are provided. (see section 1-3) Table 3 provides reference values for the design of MIG/MAG welding equipment depending on the wire diameter. The bold-printed values represent the average values for the spray arc when welding is done with argon-filled mixed gases. The voltage values below are appropriate for the short arc with low current. Depending on the application, the arrangement of the power supply, the wire feed unit, the hose assembly and the torch may vary significantly. The standard equipment for welding in a cabin or with components with limited dimensions is power supplies with integrated or externally added wire feed unit. The hose assemblies are mostly not longer than 3 meters. If larger components are being welded, the power supply and the wire feed unit may be separated. A lighter wire feed unit can be placed close to the welding point by suitable equipment. Figure 16: Other types use additional driving motors in or close to the torch. It must be considered that longer hose assemblies have higher voltage decrease and therefore a power supply with higher power may be required. Table 6 summarises the selection criteria. Table 6. Criteria for selecting and evaluating MSG welding equipment a) Technical parameters - The principal of electrical circuit in the power supply unit - Setting range and load capacity (permissible duty cycle for large loads) - Workpiece cable diameter and workpiece connection type - Constant or stepped setting of voltage - Step intervals for voltage setting - Adjustable additional inductivity (choke) or electronic influence of the short-circuit current peaks (MSGk, MSGü, MAGl) - Setting and control options for pulsed arc welding - Single Button setup and program preparation - Suitability for several arc welding processes and types of currents(GMAW, WIG, E) - Timers for spot or interval welding - Adjustable shielding gas pre- and the post-flow time - Stabilization of welding parameters by change of mains voltage - Contact protection (protection against solid bodies and water, min. demand for the workshop IP 21) - Wire feed system and wire feed length - Quality of feeding rolls (groove profile, groove surfaces and even pitch diameter) - Regulation of the wire feed rate - Wire feed unit in the welding equipment or free to move - Torch design - Cooling equipment for electrical power components and the torch - Noise and draughts - Measuring instruments (U, .I, wire feed rate) - Monitoring devices b) Welding characteristics - Arcing (current rise rate, adjustable free arcing time and wire starting speed, current pulse with the arc interruption, reversible wire movement at ignition) - Arc stability, arc length control (characteristics, inertia of the “internal regulation”, controller settings, hose assembly length, additional wire feed aids) - Appearance of spattering (choke setting, program quality, program number, …) - Weld appearance (profile, weld ripples) © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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- Pulsed arc current (maximum peak current, current rise and fall rate, background current stability, shape of the curve transitions) c) Handling - Control elements (arrangement, operability with gloves, protection against damages) - Adjustment precision and clarity of the control elements or screens - Adjustment of the current pulse smoothing (choke) in steps, continuously or not adjustable - Change of the arc power with a single button - Number, quality and adjustment options of saved welding data sets (working points and/or jobs) - Storage spaces for your own welding data sets - Program upgrades for different arc powers at the start and the end of the welding - Program upgrades for periodically changing arc powers - Remote adjustment possibilities or interface type - Warning indicators - Permission to work under special electric risks - Pole reversal possibility (AC applications, flux-cored wires) - Transportation possibilities - Stability - Connections to different mains voltages - Connections to welding cables and torches - Space requirement and format - Risks of injury by housings with sharp edges and poorly secured lid - Too narrow chambers for wire spool and wire feed unit

d) Economic viability -

Purchase price Reliability Power factor (cos ) Efficiency Stand-by losses Maintenance costs and diagnostic system Spare parts (costs, delivery time)

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The surface tension (interface stress) has a strong influence on droplet detachment on the heated wire end. Metallic bare wire surfaces, as they occur with inert shielding gases, have a high surface tension. Therefore, there is a tendency for large droplets to be formed. If some oxygen is added to the shielding gas in the form of O2- or CO2- additions, oxide islands will be formed on the melted material. These reduce the surface tension (just like detergents with water) and promote the formation of small droplets when melting the filling material. The direction of gravitational pull depends on the weld position. The composition of the shielding gas also has a major influence on droplet detachment. Welding shielding gases vary in quality as thermal conductors at the high temperatures, as they occur in the arc starting point and in the arc (see Figure ). An important temperature is the evaporation point of the material (about 3,000 °C for steel), because this is the approximate temperature that will establish itself on the melting wire electrode. The surface tension (interface stress) is another strong influence on the drop detachment on the heated wire end. The surface tension can be reduced by temperature increases and chemical changes of the material surface. Bare metal wire surfaces, as they occur in inert shielding gases (without oxides), have a high surface tension (also droplets with very thick oxide layers behave similarly to droplets with high surface tension.) There is a tendency to form bigger droplets. Shielding gas composition also has a major impact on droplet detachment. Welding shielding gases are differently good heat conductors at high temperatures, as they occur in the arc starting point and in the arc. Argon is a poor thermal conductor at high temperature, compared to other gases. Compared to CO 2 (good thermal conductor), the arc attachments on the wire end and the arc cross-sectional area remain bigger, because the jacket of the arc is cooled down by the argon. The arc increases with sufficient current at the jacket surface of the wire end. This rise has a decisive influence over the transfer of metal for argon and argon-filled mixed gases, Figure 58. The wire end is heated by the arc starting point (anode fall area) not just from the front side but also over a larger area from the outside to the inside. A longer, highly heated zone forms on the wire end. The pinch force can take effect and constrict the liquid wire end, so that depending on the temperature or the current, small drops are transferred into the weld pool. It is completely different with 100% CO2 and argon-filled mixed gases with more than 25% CO2. The shielding gas CO2 dissociates at temperatures around 2,000 to 3,000 °C. Thermal conductivity is very good (see maximum with CO2 in figure 24). The higher cooling of arc jacket by CO 2 guides in comparison to argon to an arc attachment with a smaller surface and a smaller arc diameter. The pinch force cannot become effective, because the arc is only attached to the wire end and so no sufficient wire length is heated. The higher current density of the CO2 arc causes recoil forces that prevent smallvolume metal transfer. Mixed gases therefore cannot contain an arbitrary amount of CO 2, Figure 59. The weld pool can be repressed by the sum of the forces in the arc, Figure 60. For higher arc powers and limited arc lengths, the arc can work in the hollow in the weld pool, if large penetration depth is achieved and spatter ejection is minimised.

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Der tropfenförmige Übergang (Abbildung 57) kommt in der Praxis selten zur Anwendung, weil das große Tropenvolumen und die kleine Tropfenfrequenz nur sehr kleine Schweißgeschwindigkeiten in der Pos. PA zulassen würde. Im oberen Feinblechbereich können jedoch größere Spalte mit flachen Raupen überbrückt werden.

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Chapter 1.08-2:

SFI / IWE 1.08-2 Page 1

Flux-cored wire welding

Contents 1

Gas-protected flux-cored wire electrodes ......................................................................... 2 1.1 Definition of flux-cored wire electrodes ........................................................................... 2 1.2 Cross-section shapes and manufacturing of the flux-cored wire electrodes ................... 3 1.3 Filling types and properties of the flux-cored wires ......................................................... 5 1.4 Shielding gases for flux-cored wire electrodes ............................................................. 11 1.5 Sample applications...................................................................................................... 11 1.5.1 Welding of pipe half shells on container outer skins for water cooling ............... 12 1.5.2 Forks of forklift truck as welded constructions ................................................... 12 1.5.3 Mobile crane booms........................................................................................... 13 1.5.4 MAG orbital welding with flux-cored wires ......................................................... 14 1.5.5 Use of flux-cored wire in shipbuilding................................................................. 14 1.5.6 Use of flux-cored wire in the offshore field ......................................................... 16

2

Test questions ................................................................................................................... 17

3

Literature ............................................................................................................................ 18

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SFI / IWE 1.08-2 Page 2

Gas-protected flux-cored wire electrodes Definition of flux-cored wire electrodes

Flux-cored wire electrodes can be called inverted stick electrodes, if they form a slag. This definition is accurate to a certain extent because the flux-cored wires involved in this chapter do not contain any gas forming agents in the core, and therefore have to be processed under shielding gas as usual for the MAG process. Whereas the external coating of the stick electrode contains all necessary raw material components, in the flux-cored wire electrode these are located inside (the flux), which is surrounded by a circular jacket, see Fig 1.

Slag-forming constituent Arc stabilizers Alloy elements Metal powder Micro-alloys (Gas- forming agent)

Figure 1: Comparison of stick electrodes and flux-cored wire electrodes The filling flux usually consists of several raw materials and, among other things, includes arc stabilizers for achieving high process stability, alloying elements for the use of metallurgy and partly micro-alloy elements to improve the quality of the metal weld through nucleation. The latter provides the weld metal a fine grain microstructure. Here it should be stated that the gas forming agents and reinforced deoxidizers appear only in case of flux-cored wires without shielding gas, which are not discussed here. For guidance, Figure 2 shows some raw materials. Usually, weld fillings are present as dry mixtures or agglomerates. Each flux-cored wire type has a formula specific to alloy type and diameter, which describes the composition of the raw materials and the manufacturing conditions. The filling materials have, in connection with the shielding gas, influence on arc stability, spatter formation, bead profile, bead surface, removability of the slag, seam lay-out in out-of-position welding and mechanical-technological weld metal properties. Gas-shielded flux-cored wires require a shielding gas blanket of the same quality as solid wire electrodes.

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Figure 2: Some raw materials used in the production of the filling flux 1.2

Cross-section shapes and manufacturing of the flux-cored wire electrodes

According the construction of the wire cross-sections, seamless and enclosed flux-cored wire electrodes exist, which also differ in the way they are manufactured (Figure 3).

Seamless flux-core wire electrode

Figure 3: Usual cross-section shapes for flux-cored wire electrodes /2 and 3/

As a pre-material for enclosed types a narrow strip, with a slightly more than 10 mm width and less than 1mm thick, is used and brought first into a U-shape by bending rolls. After this, a continuous filling of the mostly dry mixed flux takes place. Subsequently, the cross-section is closed and is usually reduced and compressed by rolling. In the second step, the completed semi-finished product is drawn and /or rolled to the final dimensions, see Figure 4.

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Figure 4: Manufacturing process for enclosed flux-cored wires The classic production of the seamless flux-cored wires uses a strip of approx. 50 mm width and about 2 mm thickness, from which continuous tubes are manufactured using high-frequency welding. After the recrystallisation annealing process, the tube is drawn to the filling diameter. The filling flux, in this case agglomerated, is inserted into the pipe by vibration and pre-compressed.

Manufacturing of the tube rom the solid strip by HF welding

Recrystallisation annealing and calibrating to filling diameter Manufacturing of the agglomerated filling flux and insertion of the filling flux by vibration

Drawing to annealing diameter and annealing of the filled tube

Drawing to the final dimension Copper-plating the surface

Winding to deliverable form

Figure 5: Manufacturing process for seamless flux-cored wires /3/

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Next, the semi-finished product is drawn to the annealing diameter, see Figure 5. During this heat treatment, the strain strengthened casing is soft-annealed, reducing the hydrogen originating from the filling flux to far below 5 ml/100 g. Next, the wire is drawn to its final dimensions by several steps, repeatedly wet-cleaned and finally copper-plated and polished. The flux-cored wire electrode manufactured by this method is absolutely protected against moisture absorption and can be stored without limitation. Re-drying is not necessary. As the coating does not contain any stiffness changes, the wires do not twist and ensure no disruptions during feeding. The coppered wire surface improves current transmission and reduces nozzle wear. A further developed process variant works with direct filling before the tube welding station, whereby the filled tube is continuously annealed after compression. The wire is finally wound into coils in preparation for delivery.

1.3

Filling types and properties of the flux-cored wires

Currently, flux-cored wire electrodes are available with and without slag (Figure 6). The slag-forming types contain rutile or basic components and are to be processed with a slightly trailing torch position due to the risk of slag inclusions as with stick electrodes (Figure 7). Titanium-oxide is the main component here and appears as dark grey slag on the surface of the weld. Because of the fluoride components, the basic flux-cored wires produce a very thin liquid slag that is yellow ochre to light-green in colour.

Figure 6: Filling types of flux-cored wire electrodes /3/ The slag-forming components, especially basic components, influence the mechanical data of weld metal positively. They reduce the risks of incomplete fusion and pore formation to a minimum. Accordingly, the slag-forming flux-cored wire electrodes represent an alternative to stick electrodes in welding technology terms, regarding practical application cases. A major advantage of the rutile types with fast-solidifying slag is the fact that they can be used very economically due to their excellent weld pool plasticity for out-of-position welding applications especially in the rising position. The slagless metal powder flux-cored wire electrodes are to be regarded as alternatives to solid wire electrodes. Despite having no slag, these types show the flux-cored wire-specific advantages in terms of weld quality. An exception is the tendency to porosity if the distance from the contact tip is too great.

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Figure 7: Welding technique when welding rutile flux-cored wires In terms of the absolutely certain ability to reignite and the almost spatter-free process behaviour, these wires were originally developed for robotic applications. A further advantage to the slag-forming flux-cored wire electrodes can be found in multi-layer welding, as the intermediate weld cleaning process is omitted here.

Figure 8: Welding techniques with metal powder flux-cored wires The flux-cored wires for the most important material groups are standardised. These are listed in Table 1.

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Table 1: Standards for flux-cored wire electrodes for MIG/MAG welding EN-/prENNumber

Title

Issue

EN ISO 17632

Welding consumables - Tubular cored electrodes for gas shielded and non-gas shielded metal arc welding of non-alloy and fine grain steels - Classification

2008

EN ISO 17634

Welding consumables - Tubular cored electrodes for gas shielded metal arc welding of creep-resisting steels Classification Welding consumables - Tubular cored electrodes and rods for gas shielded and non-gas shielded metal arc welding of stainless and heat-resisting steels - Classification Welding consumables - Tubular cored electrodes for gasshielded and non-gas-shielded metal arc welding of high strength steels - Classification Welding consumables - Welding consumables for hard-facing

EN ISO 17633 EN ISO 18276 EN 14700 DIN EN prEN ISO

= = = =

German standard European standard European draft standard International standard

In connection with DIN

*)

2006

DIN EN ISO 17632 replaces EN 758 DIN EN ISO 17634

IDT

2011

DIN EN ISO 17633

IDT

2006

DIN EN ISO 18276

IDT

ISO

DIN EN ISO 18276

2005

IDT = DIN is IDENTICAL to the European Standard/draft VGL = DIN is comparable with the European Standard/draft *) Type of the relationship 1) ISO/DIS = International draft standard

Flux-cored wire electrodes for GMAW welding are mostly wound on spools. Depending on the usage, this spool can be very small (1 kg) or very large (several hundred kilos). For large consumers, along with large spools there are also containers from which the wire is drawn mechanically. The common wire electrode diameters are: 1.0; 1.2; (1.4); 1.6 [mm]. For special tasks, dimensions from 2.0 to 3.2 mm are also available for gas-shielded arc welding. The metal powder flux-cored wire electrodes for which mainly mixed gases are used, have three arc ranges, i.e. short-, mixed /globular- and spray arcs (Figure 9). The surprisingly low spatter formation in the short arc range allows universal application possibilities for the wire also with manual use. This applies specifically to the good gap bridging capabilities (root welding) and out-of-position welding. The torch position is similar to that when welding with solid wire electrodes, i.e. either slightly pushed or neutral. (Figure 8). The mixed/globular-arc with spatters around 200A should not be used. When pulsed power sources are used, even in this range the metal powders also weld almost entirely without spatter.

Shielding gas: M21 Diameter: 1.2 mm

350 A 300 A 250 A 200 A

Spray-arc

150 A

Mixed / Globular-arc

70 A

Short-arc

Figure 9: Arc ranges of the metal powder flux-cored wires Another development is the basic cored wire with out-of-position suitability. Such wires have less slag and are manufactured with higher fill factors. The fill factor provides the weight ratio of the coating and © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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the weld filling. The main demand for the development of this basic generation originates from the shipbuilding industry. Here the rutile types with fast-solidifying slag are predominantly used for singleside welds mostly on ceramics in a perpendicular position. The shipyards need to process an increasing number of thick cross-sections for both the shipbuilding industry and offshore technology components, for which the rutile type is not suitable because its resistance to cracking is not sufficient under extremely adverse conditions. The welding characteristics of the above basic flux-cored wire electrodes with out-ofposition suitability are as follows: - Fine to medium droplet transfer - Slag with medium support, easy loosening - Minimising of the back formation - Possibility of welding over rolled and primer coatings Such flux-cored wires are used mostly on the - pole because of the better welding properties, which may be a disadvantage depending on the application and the machine availability. In general, the power supply with constant current characteristic is still to be regarded today as the standard power source for all gas-shielded flux-cored wire electrodes. The use of pulse technology offers additional advantages in terms of the absence of spatter. With reference to the optimising of welding properties, it should be noted that the pulse frequency should be between 50 and 100 Hz. Flux-cored wire electrodes are more expensive than solid wire electrodes. Therefore they are used only where good welding properties and/or high weld qualities provide an advantage. Table 2 shows the description of the most important properties of different weld fillings. Table 2: Properties and applications information for flux-cored wire electrodes for MAG welding of unalloyed steels and fine grained steels Rutile type -

-

Basic type

stable arc and little spatter good side wall penetration lower penetration in the weld centre compared to solid wire smooth weld surface and good removability of slag wires with rapidly solidifying slag advantageous for welding in out-of-positions on thicker workpieces mechanical and technological properties of the weld metal, especially at low temperatures, strongly depending on the alloy type and the degree of purity of the raw materials.

-

-

larger droplets and bead less smooth than for rutile weld filling during welding with CO2 more spatter as with Ar-CO2mixtures power supplies with good dynamic characteristics required in out-of-position welding, a pulsed arc is favourable good mechanical and technical properties of the weld metal reduced cracking susceptibility for conditionally weldable steels (e.g. steels with increased Ccontent, heat-resistant steels, low-temperature resistant steels, fine-grain steels with higher yield points, thick steel castings) torch polarity according to manufacturer's specifications (frequently negative pole)

Metal powder type -

-

in the short arc range, fewer spatters than with solid wire well suitable for root welding the spray arc range starts earlier than for solid wire better reignition and less smoke than with slag-forming cored wires, therefore suitable for robotic welding more resistance heating in the free wire end than for solid wires higher deposition rat than for solid wires with the same welding parameters and other conditions

The properties and application information above provide the following reasons for using flux-cored wire electrodes:

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- More secure sidewall fusion, tolerant of incomplete fusion - Good wetting, notch-free transitions, smooth surface - High crack resistance - Low spatter droplet transfer - High process stability - X-ray-safe welds - Applicability of micro-alloy elements - Good suitability for out-of-position welding - Economic manufacturing As a result of these properties, the manufacturing cost rather than the wire price determines the selection of the flux-cored wire application which can be much more favourable in case of the right application. Therefore, the approval to use flux-cored wire technology needs to be verified separately for each individual case. Chapter 1.5 shows some sample applications. Table 3 is an abstract of EN ISO 17632 for flux-cored wire electrodes of unalloyed steels and finegrained structural steels. The standards for flux-cored wire electrodes EN ISO 17634 (heat-resistant steels), EN ISO 17633 (stainless and heat-resistant steels) and EN ISO 18276 (high-strength steels) have a similar structure./3/

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Table 3: Abstract of EN ISO 17632 for flux-cored wire electrodes of unalloyed steels and fine-grained steels with and without shielding gas

ISO 17632-A - T 46 3 1Ni B M 4 H5 Symbols for yield point, strength and strain Code letter

Min. yield strength MPa

Tensile strength MPa

Minimum strain %

35 38 42 46 50

355 380 420 460 500

440 - 570 470 - 600 500 - 640 530 - 680 560 - 720

22 20 20 20 18

Designation for diffusible hydrogen content Symbol H5 H10 H15

Code letter for recommended welding position Code letter 1 2 3 4 5

Symbols for impact properties Symbol Z A or Y 0 2 3 4 5 6

Minimum impact energy 47J °C no requirement +20 0 -20 -30 -40 -50 -60

Designation of the alloy type

Welding position PA, PB, PC, PD, PE, PF & PG PA, PB, PC, PD, PE & PF PA & PB PA PA, PB & PG

Symbol for shielding gas Symbols M and C refer to the shielding gas specified in ISO 14175:1997. The symbol C is to be used, if classified with the shielding gas ISO 14175-C1, carbon dioxide. Self-protecting flux-cored wire electrodes are designated with the symbol N.

Designation of welding filler

Designation of the chemical composition of the pure weld metal Chemical composition %

Hydrogen content in ml/100 g of weld metal 5 10 15

Symbol Characteristics

a,b)

Mn Mo Ni Without symbols 2 0,2 0,5 Mo 1,4 0,3 - 0,6 0,5 MnMo >1.4 - 2.0 0,3 - 0,6 0,5 1Ni 1,4 0,2 0,6 - 1,2 2Ni 1,4 0,2 1,8 - 2,6 3Ni 1,4 0,2 2,6 - 3,8 Mn1Ni 1,4 - 2,0 0,2 0,6 - 1,2 1NiMo 1,4 0,3 - 0,6 0,6 - 1,2 Any other agreed chemical Z composition a Single values in chart are maximum values. b The results have to be rounded to the same position as fixed values by using ISO 31-0: 1992, Appendix B, Rule A.

R P B M V W Y Z

Rutile, slowsolidifying slag Rutile, fast solidifying slag Basic Metal powder Rutile or basic / fluoridic Basic/fluoridic, slow-solidifying slag Basic/fluoridic, fast solidifying slag Other types

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Single run (S) Multi-run (M) welding

Shielding gas

S&M

Required

S&M

Required

S&M S&M

Required Required not required not required not required

S S&M S&M

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1.4

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Shielding gases for flux-cored wire electrodes

Rutile types, especially with fast solidifying slag, can be successfully processed with CO2 and mixed gases of CO2 + Ar. When the application takes place under on-site conditions, pure CO2 is to be chosen to ensure porosity free welds. Otherwise the Ar content can be raised to 90%. Depending on the manufacturer's specifications, 3-component mixtures of Ar+CO2+O2 can also be used. However it must be considered that slag will be more fluid because of O2-containing gases and the welding characteristics are negatively affected, particularly in out-of-position welding. The position is similar to basic cored wires, whereby the spatter amount increases as the CO2 content rises. Metal powder flux-cored wires are predominantly welded under mixed gases from group M21. The gas mixtures of the groups M20, M23 and M24 are also suitable, although overheating phenomena can be expected in this case. Theoretically, metal cored wire can also be processed under CO2.

1.5

Sample applications

The selected applications are intended to provide an overview of the range of applications. This includes bulk structural steels, heat-resistant and cast steel grades up to high-strength, quenched and tempered fine-grain structural steels with Rp0.2 1,100 Mpa. Suitable flux-cored wire types are now available even for high-alloy steel grades as well as for black-and-white compounds (Figure 10). There are also self-protecting flux-cored wires (Open-arc types) containing gas-forming agents in their filling which can be welded without requiring additional shielding gas. These are not considered here.

Figure 10: Application areas and processes of flux-cored wire electrodes

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Welding of pipe half shells on container outer skins for water cooling

When welding pipe half shells onto container/vessel or base outer walls, 100% leak tightness is required. Figure 11 shows an example with the joint preparation commonly used in such cases. The goal is to avoid crevice corrosion and requires a suitable root formation. In this case, a rutile type with Ø 2.0 mm was used in a full-mechanised process. Rutile cored wire with Ø 2.0 mm at G+Pol under CO2 with: 500A, 33V and 105 cm/min

S= 10 mm

Figure 11: Welding of pipe half shells to outer container walls As can be seen in the microsection on the right of figure 11, a very secure penetration shape is achieved due to the widely ignited arc-bow. Although different wall thicknesses are welded here, because of its wetting ability, the flux-cored arc ensures spatter-free, x-ray-proof welds without fusion errors right from the first layer. 1.5.2

Forks of forklift truck as welded constructions

Usually, the forks of a forklift truck are bent to an L-shape from different materials at austenitic temperatures. Due to hot-forming, the strength properties are lost so far that the forkes of heavy-duty forklift trucks do not meet the requirements for nominal capacity any more. Therefore, these components are manufactured from high-strength steels with the help of MAG welding robots using high-strength metal powder flux-cored wire electrodes.

Figure 12: Forks of a forklift truck, incl. weld preparation

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High flexibility is achieved by this technique, because the delivery times are shortened by around 80% even in case of low quantities and a broad selection spectrum. The weld and the component preparation are shown on Figure 12. The welding of the double-bevel butt weld takes place in the flat position in two steps. When generating the root with a root gap of approx. 4 mm, ceramic rods of circular cross-section are used as weld pool support. After welding the first side, the component is turned around and the other side is welded. The pre-heating temperature is about 150 °C. In order not to exceed the maximum working temperature of 260 °C, the work is performed at different locations. Although welding is performed here in highly stressed zones, contrary to the principles of welding technology, the welds are absolutely free of defects despite steep side-walls and very large sheet thicknesses. 1.5.3

Mobile crane booms

The primary concern in crane construction is to reduce the intrinsic weight in order to increase the payload capacity. This fact explains the use of high-strength, quenched and tempered fine-grain structural steels. The booms of a heavyweight mobile crane shown in Figure 13 made of XABO 90 steel with a yield point of Rp 0.2 = 890 N/mm2 are welded with basic flux-cored wires. The wires are alloyed with Mn-CR-Ni-Mo. Such flux-cored wires have a basic slag characteristic and offer sufficient protection against hydrogen-induced crack formation. The pre-heat and interpass temperatures are within the range of 120 °C to 150 °C.

Figure 13: Heavyweight mobile crane

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MAG orbital welding with flux-cored wires

Orbital welding involves making a butt weld between two axially symmetrical components (e.g. pipes) that cannot be rotated. Typical areas of application are pipelines for crude oil and gas, pipelines for distance heating and water, pipe structures in equipment and container/vessel construction. For this task, rutile wires with fast-solidifying slag have proven successful, since due to the supporting feature of the slag, the weld pool is prevented from falling down when welding out of position. Figure 14 shows suitable equipment which is moved along a guiding rail being fixed immediately next to the weld. Fillingand cap runs are usually produced beginning at PE (6 o'clock) and rising to 12 o'clock. In most cases, the welding involves weaving. According to the power source type, the root can be welded down-hill starting at PA (12 o'clock) to PE, with metal flux-cored wires preferred. For this method, the opening gap is 3-4 mm.

Figure 14: MAG orbital welding equipment

1.5.5

Use of flux-cored wire in shipbuilding

Rutile flux-cored wires have found their way into shipbuilding due to the universal application range, in all positions with high deposition rates without parameters changes. From the 1980s onwards, stick electrodes were almost completely replaced by flux-cored wire technology. Beside the fillet welds for inner base constructions, base bulkheads, frame side rails and external coating, the flux cored wire proved to be effective for butt joints. The sheet segments are tack-welded with a root gap of 4 - 6 mm. Since this is about curved construction parts, having low buckling strength during the assembly phase, the turning of the component shall be suspended during production. In order to be able to weld regardless of this, “single-side welding” shall be used on ceramic support. For this purpose, profiled ceramic rails are attached to the bottom side of the external base material, as shown in Figures 15 and 16.

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Ceramic rail

Page 15

Self-adhesive film aluminium foil

Figure 15: MAG single-side welding with flux-cored wires on ceramic backing supports. The ceramic backing intended only for a single use can be immediately removed after welding the root run. Typical MAG vertical welding equipment has a very low total weight of approx 15 kg. The air-cooled torch system enables currents of up to 260 A in connection with  1.2 mm flux-cored wire electrodes. All welding parameters are entered into the control interface for each run. The ceramic backing rail guarantees an excellent root formation, eliminating the need for any post-work.

S= 20 mm

Figure 16: Mechanised MAG flux-cored welding of the section walls in shipbuilding

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1.5.6

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Use of flux-cored wire in the offshore field

The increasing scarcity of oil and natural gas from onshore sources necessitates increased efforts to search for natural raw materials on the world's seas. Because of this, offshore activities are becoming increasingly lucrative, thus currently numerous offshore drilling rigs are being constructed. This sample application shows semi-submersible floating rigs that can be deployed on a mobile basis. These are produced as pontoons, they float and have their own power units, enabling them to be relocated to new positions, on the one hand, and to fix the platform over the drilling site, on the other (Figure 17).

Figure 17: Semi-submersible drilling rig (Drilling Rig TDS 2,000 P) Figure 18 shows a pontoon consisting of three blocks with a length of 105 m.

Figure 18: Prefabrication of the pontoons of a drilling rig Since the components cannot be turned - similar to shipbuilding - and work must be carried out outdoors during the manufacturing of the pontoons, the rutile flux-cored wire under CO2 is used for all weld types in all positions. For indoor prefabrication, mixed gases from group M21 are preferred.

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Test questions

(1)

What is a flux-cored wire?

     (2)

a coiled type of solid wire electrode an endless, inverted stick electrode a bended strip electrode a filler material filled with gas a tube which forwards powder fort he filling of the joint during welding Which types of flux-cored wires exists according to their structure?

     (3)

water annealed flux-cored wires normalised flux-cored wires seamless flux-cored wires (shape) enclosed flux-cored wires basic flux-cored wires Which types of flux-cored wires exist according to their weld filling?

     (4)

flux-cored wires with basic components filling rutile flux-cored wires gasfilled flux-cored wires flux-cored wires without slag metal powder flux-cored wires For which welding procedures are flux-cored wires available?

     (5)

for SAW welding for friction welding for flash welding for ES welding for EG welding

Which reasons determine the use of flux cored wires in the MAG process?

    

less sensitive for lack of fusion lower deposition rate but therefore better weld pool control low porosity sensitivity good flexibility of the wire welding in water is no problem

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Flux-cored wire welding

(6)

What is the meaning of micro-alloying technic in flux-cored welding?

     (7)

addition of Mn into the joint for achieving a more fine microstructure addition of Si into the joint for achieving a more fine microstructure addition of B into the joint for achieving a more fine microstructure addition of Ti into the joint for achieving a more fine microstructure addition of N into the joint for achieving a more fine microstructure Which standards for flux-cored wires are known in Europe?

     (8)

flux-cored wires for welding creep resistant steel flux-cored wires for laser welding flux-cored wires for welding high strength steel flux-cored wires for welding cast iron materials flux-cored wires for welding standardised profiles In which application areas are flux-cored wires mainly used?

    

3

Page 18

Automotive Off-shore industry Heating installations Shipbuilding Chemical industry

Literature

/1/

DIN paperback 191, Welding technology 4, Beuth-Verlag

/2/

DVS leaflet 0941-1 Flux-cored wire electrodes for joint and surface welding

/3/

EN ISO 17632 Welding consumables - Tubular cored electrodes for gas shielded and non-gas shielded metal arc welding of non-alloy and fine grain steels - Classification EN ISO 17634 Welding consumables - Tubular cored electrodes for gas shielded metal arc welding of creep-resisting steels - Classification EN ISO 17633 Welding consumables - Tubular cored electrodes and rods for gas shielded and non-gas shielded metal arc welding of stainless and heat-resisting steels Classification EN ISO 18276 Welding consumables - Tubular cored electrodes for gas-shielded and non-gasshielded metal arc welding of high strength steels - Classification

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Chapter 1.09:

SFI / IWE 1.09 Page 1

Manual Metal Arc Welding I

Contents 1. 2.

Introduction, historical background .................................................................................. 4 Technique ............................................................................................................................ 5 2.1 2.2

3.

Welding equipment technology ......................................................................................... 7 3.1

3.2

4. 5.

Storage (extract from DVS 0957) .............................................................................................................. 18 Moisture absorption of coated electrodes............................................................................................... 18 Re-drying .................................................................................................................................................. 19 Negative consequences of raised moisture absorption of electrodes ..................................................... 20

Specific hazards during manual metal arc welding ....................................................... 20 7.1

7.2 7.3 7.4 7.5

8.

Production ................................................................................................................................................ 13 Core rods for electrodes ........................................................................................................................... 14 Composition of the coating, coating materials ........................................................................................ 14 Standard analysis of the most important coating types .......................................................................... 14 Functions of the stick electrode coating .................................................................................................. 15 Classification of the stick electrodes ........................................................................................................ 16 Dimensions and permitted deviations of stick electrodes (extract from DIN EN ISO 544) ...................... 16 Identification according to DIN EN ISO 544 .............................................................................................. 17 5.8.1 Identification on the component .............................................................................................. 17 5.8.2 Identification on each packaging unit ....................................................................................... 17

Recommendation for storage and re-drying of coated electrodes ............................... 18 6.1 6.2 6.3 6.4

7.

Welding power sources .............................................................................................................................. 7 3.1.1 Characteristic curve of manual metal arc welding...................................................................... 8 3.1.2 Technical parameters .................................................................................................................. 8 Welding accessories ................................................................................................................................. 11

Standards and fields of application of stick electrodes ................................................ 12 Stick electrodes for manual metal arc welding .............................................................. 13 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

6.

Application areas ........................................................................................................................................ 6 Advantages and disadvantages .................................................................................................................. 6

Electrical hazard ....................................................................................................................................... 20 7.1.1 Danger zone of welding equipment and mains connection ..................................................... 20 7.1.2 Danger zone for welding cables, workpiece connection .......................................................... 20 Danger zone of the arc ............................................................................................................................. 20 Danger zone of the weld pool and slag .................................................................................................... 21 Welding fumes and dust........................................................................................................................... 21 Personal safety equipment....................................................................................................................... 21

Labelling of stick electrodes according to international standards ............................. 22 8.1 8.2

Classification is based on eight characteristics according to DIN EN ISO 2560-A: ................................... 22 Sample identifier ...................................................................................................................................... 23

9. Criteria for selecting stick electrodes for manual arc welding ..................................... 24 10. Covered electrodes of non-alloy and fine-grain steels, DIN EN ISO 2560-A ................ 25 10.1 Application area DIN EN ISO 2560-A ........................................................................................................ 25

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SFI / IWE 1.09 Page 2

Covered electrodes with acid coating: A .................................................................................................. 25 Covered electrodes with cellulosic coating: C ......................................................................................... 26 Basic-coated stick electrodes: B ............................................................................................................... 27 Stick electrodes with rutile coating: R, RR, RC, RA, RB ............................................................................. 28 High-deposition electrodes “high-efficiency electrodes” ........................................................................ 29 Sample applications for stick electrodes according to DIN EN ISO 2560-A.............................................. 30

11. Stick electrodes for weatherproof, high-strength and low-temperature toughness steels, DIN EN ISO 18275-A .............................................................................................. 32 12. Stick electrodes for creep-resistant and high-temperature steels, DIN EN ISO 3580-A ............................................................................................................ 34 13. Stick electrodes for stainless and heat-resistant steels, DIN EN ISO 3581-A .............. 36 13.1 Stick electrodes for black-and-white joints, DIN EN ISO 3581 – A ........................................................... 38

14. Filler materials for hard-facings, DIN EN 14700 .............................................................. 40 15. Stick electrodes for repair welding of cast iron, DIN EN ISO 1071 ............................... 44 15.1 Identification of the electrodes according to DIN EN ISO 1071 ............................................................... 44 15.2 Hot welding of grey cast iron with similar filler material ......................................................................... 45 15.3 Cold welding of grey cast iron with a dissimilar filler material ................................................................ 45

16. Special electrodes ............................................................................................................. 48 16.1 16.2 16.3 16.4

Cutting/chip-out electrodes ..................................................................................................................... 48 Cutting electrodes - hollow stick electrodes ............................................................................................ 48 Carbon arc electrodes (usually copper plated with arc-air process) ........................................................ 48 Preheating electrodes .............................................................................................................................. 48

17. Welding procedures .......................................................................................................... 49 17.1 Joint preparation ...................................................................................................................................... 49 17.1.1 Butt welds ................................................................................................................................. 49 17.1.2 Fillet welds ................................................................................................................................ 49 17.2 Stick electrode handling and weld layout ................................................................................................ 50 17.3 Magnetic Arc blow.................................................................................................................................... 53 17.4 Efficiency of stick electrodes .................................................................................................................... 54 17.5 Selecting a favourable welding position................................................................................................... 55

18. Economics ......................................................................................................................... 56 19. Vertical down welding....................................................................................................... 58 19.1 Coated stick electrodes for pipe-line construction .................................................................................. 58 19.2 Welding power sources ............................................................................................................................ 59 19.3 Working methods for vertical down welding with cellulose-coated stick electrodes ............................. 60 19.3.1 Joint preparation ....................................................................................................................... 60 19.3.2 Working techniques, handling stick electrodes ........................................................................ 61 19.3.3 Weld run layout......................................................................................................................... 62 19.4 Summary................................................................................................................................................... 63

20. Weld imperfections and their possible causes .............................................................. 64 20.1 20.2 20.3 20.4 20.5 20.6 20.7

Slag inclusions........................................................................................................................................... 64 Gas inclusions (pores) ............................................................................................................................... 64 End crater ................................................................................................................................................. 64 Cracks in weld transition .......................................................................................................................... 64 Root fault .................................................................................................................................................. 64 Undercuts ................................................................................................................................................. 65 Hydrogen-induced cracks ......................................................................................................................... 65

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21. Bibliography/sources........................................................................................................ 67 22. Question............................................................................................................................. 67

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1. Introduction, historical background Manual Metal arc welding (MMAW), also known as Shielded Metal Arc Welding (SMAW / USA), is one of the oldest fusion welding processes. Already in 1885, an arc was created between a carbon electrode and the workpiece in order to melt the metal. The currentless feeding of the filler material was done in a similar way as it is still being done in today´s Oxyfuel Gas welding or TIG welding. In 1890 the patent of the Russian engineer Slavianoff combined both the filler wire and the electrode by using the filler material as an arc carrier. However these electrodes were not yet coated, making them very difficult to weld (e.g. missing ionisation) and the surrounding air had a considerable negative influence on the weld pool (pores, oxidation etc.). In 1908 The Swedish patent “Electrode and process for electric soldering” was filed by the Swedish engineer Oscar Kjellberg which paved the way for coated stick electrodes. At this time, coatings were produced by repeatedly dipping the rod into a paste of coating materials. Around 1935, this very complex process was replaced by the press technology which is still common use today. So it became possible to produce stick electrodes with a thick coating of homogeneous composition and with exact calibration. By 1938, approx. 50% of all stick electrodes were produced using pressing technology. Today´s production methods differ hardly in any way from earlier times, however, due to further development of coating materials and manufacturing optimisation (e.g. extrusion presses), a substantial increase in output and quality improvement has been achieved. Manual metal arc welding has also substantially been influenced by constant improvements of equipment manufacturing.

Figure 1: Manual metal arc welding

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2. Technique Manual Metal Arc Welding (process number 111) is an arc welding process and is described as: “Manual Metal Arc Welding process with a coated stick electrode”. This welding process is very versatile, can be used in all welding positions, without complex protection precautions, especially outdoors, and is the only process that can be used under water. With manual metal arc welding, all weldable ferrous metals, nickel and nickel alloys can be welded using coated stick electrodes. Welding of copper and aluminium materials is no longer being discussed in the new standards for manual metal arc welding and is hardly any more used in practice. For manual metal arc welding, the arc ignites between a coated melting stick electrode and the workpiece. The arc and the weld pool on the base material side are protected from the air by the surrounding shielding gas and a slag blanket.

Figure 2: Process Principle

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Mains connection Welding power source Welding current conductor (electrode) Welding current conductor (workpiece) Stick electrode holder Stick electrode Work clamp Workpiece Arc Stick electrode core rod Coating of the electrode stick Drop transfer Shielding gases from stick electrode coating Liquid slag Solidified slag Liquid weld metal Solidified weld metal

Figure 3: Details of the arc

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Application areas

Manual metal arc welding makes it possible to weld both thin sheets  1.5 mm in a single layer run and thicker sheets (usually up to 20 mm) with multi-layer runs. According to the overall filler material consumption and the fact that in recent years the use of manual arc welding has continuously been reduced in favour of MIG/MAG welding, the ratio of manual metal arc welding is still about 7.5%. Of this is used: 30 %

in handcraft, in small and medium-sized enterprises,

30 %

in shipbuilding,

20 %

in chemical industry, automotive and in surface welding

10 %

in pipeline construction,

5%

in steel construction, usually on building sites,

5%

in the boiler and pressure vessel construction,

Figure 4: Manual Metal Arc Welding, vertical down welding in open air

2.2

Advantages and disadvantages

The advantages of manual metal arc welding are:  Relatively low equipment purchase price  High production safety without complex precautions (e.g. outdoors)  Excellent quality values with lower error probability (lack of fusion, pores)  Versatility (wide selection of electrodes)  Welding under special conditions (e.g. underwater welding and cutting) Disadvantages are:  Relatively low deposit rate: 0.5 to 5.5 kg/h, average approx. 1.5 kg/h  Thin sheets of ≤ 1.5 mm cannot be welded continously without faults.  Weld quality mainly depending on the welder´s skill.

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3. Welding equipment technology As it is seen from the process principle, the  welding power sources,  coated stick electrodes and  accessories (tools such as slag hammers, wire brushes and safety clothing) belong to the basic equipment of manual metal arc welding. 3.1

Welding power sources

Welding power sources convert the high mains voltage into a low welding voltage respectively low mains current into the required high welding current. For this particular reason the most simple power sources are welding transformers which only provide alternating current. However not all types of electrodes are appropriate for welding with alternating current. The welding current can be adjusted with a moving shunt- or reactor-core or in case of even older equipment by changing the coupling of coils. This multiple-step switch does not allow fine adjustment, so that these welding transformers are no longer suitable for today's requirements. Welding rectifiers, transformers with a subsequent diode or thyristor rectifier supply direct current, but the low open circuit voltage sometimes makes it impossible to ignite certain electrode types like e.g. cellulose-coated electrodes. For pipeline construction, where preferentially cellulose electrodes are used, welding converters, in the form of a welding generator are implemented. It comprises a drive motor and the generator, to generate the welding current. The advantages of the welding converter are the very good welding properties and for the arrangement as a welding set, no mains connection is required. The disadvantage is its lower efficiency compared to other power sources. The low weight, the portable design and the good efficiency lead to an increase switching towards welding inverters having additional technical advantages like:      



 

Adjustable static power characteristic curves, steep slope, with constant current characteristics. Type of current: Direct current (and/or alternating current for multi-processing units: TIG/SMAW). Pole inversion +/Compensation of mains voltage. “Hot-Start” function: increase of welding current during the ignition phase. “Anti-stick” function: shortly before drop transfer in the short-circuit, the maximum welding current is achieved, therefore adhesion is prevented; when the stick electrode freezes, the controller turns off the welding current so that the stick electrode is not tempered or damaged and the welding equipment is not overloaded. “ArcForce” function: for too low welding voltage (< 8V), the welding current increases automatically, the arc “burns itself freely” and remains almost constant. Highly recommended for large droplet electrodes which must be welded with very short arc (e.g. basic electrodes). Required for all cellulose coated vertical down welding electrodes. Pulse function (Enables stringer bead technic also for vertical up welding of high-strength and CrNi materials using lower heat-input. Measuring instruments, programming box and auxiliary components. A new generation of welding inverters allow frequently used welding parameters to be stored under quick selection buttons. Wireless and wired remote controllers allow the current to be fine-tuned when welding.

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1 Arc too long 2 Arc correct Welding Current I  A 

Figure 5: Static power characteristic-curves

3.1.2 3.1.2.1

Page 8

Characteristic curve of manual metal arc welding

Welding Voltage U  V

3.1.1

SFI / IWE 1.09

3 Arc too short

Technical parameters Electrode-arc ignition and welding current

With all stick electrodes, the arc is ignited through making contact with the work piece. Creating a shortcircuit situation, having a very high current-density in a very small contact area, leads to a fast ionisation and formation of the arc. Igniting aids on electrode ends and the Hot-Start function facilitate this process. To prevent sticking, the practice of igniting the electrode like a match on the work piece has been adopted. In any case, it must be assured that stray-arcs have to be over-welded and do not lie outside the welding area. In manual metal arc welding, rutile and acid-coated electrodes are predominantly used with straight polarity (direct-current electrode negative, DCEN). The ionisation for these types is good, depending on the composition of the coating. Therefore the arc burns more quietly and with better focussing on the work piece. The advantage is the lower electrode temperature. The current load of the electrodes decreases, it tends less to overload. Welding with alternating current is generally also possible. Basic- and high-alloy covered electrodes are mainly used on reverse polarity (direct-current electrode positive). The physical properties of the arc´s basic-coating components provide a more stable arc, a deeper penetration and a lower burn-off of alloying elements when welding on reverse polarity.

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Typical welding data: Welding current (Is): 40 A to approx. 360 A Current per mm core rod diameter: 20 A to 60 A (depending on the welding position and electrode type) Direct current (+/- pole) or alternating current Rule of thumb as a mean value: Is in [A]  40x core rod diameter in [mm]

Table 1: Current levels depending on the core rod diameter Core rod diameter dE in mm Current IS in amperes [A] Rule of thumb for current in amperes [A]

2

2.5

30 – 80

3.2

4

5

50 – 100 90 – 150 120 – 200 180 – 270

20 to 40 x dE

30 to 50 x dE

6 220 – 360 30 to 60 x dE

3.1.2.2 Arc length In manual metal arc welding only the current is adjusted. The voltage-setting results from the arc length as seen in Figure 5: Static power characteristic and a mean value can be found via the following equation: 20 V  0,05  lS In principle, however, the arc should always be kept very short. Rule of thumb: All electrodes: Arc length = max. 1.0 x core rod diameter Exception: Basic electrodes and highly alloyed electrodes: Arc length = max. 0.5 x core rod diameter

3.1.2.3

Welding current according to the thickness and type of electrode coating

A reduction or an increase of the welding current is required by the following technical conditions; the values (current range) specified by the electrode manufacturer should be taken into account.   

thin-coated electrodes = less current medium thick coated stick electrodes = normal current thick and very thick coated stick electrodes = increased current

3.1.2.4   

Welding current according to the metal sheet thickness/tube wall thickness t in [mm]

low thickness t ~ 1.5… 3 < 8 mm medium thickness t ~ 8… 10 mm higher thickness t  10 mm

  less current   normal current   increased current

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3.1.2.5

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Welding current according to the welding position, weld type and weld composition PA (flat position)

PB (horizontal position)

Fillet weld

  increased current for root pass ,

Butt weld

  less to normal current for root,

Fillet weld

  increased current for root pass,

filling layers and cover layers. increased current for filling layers and cover layers normal current for filling layers and cover layers

PC (lateral position)

Fillet weld

  increased current for root pass,

normal current for filling layers and cover layers Butt weld

  less to normal current for root pass,

increased current for filling layers and cover layers PD (half overhead position)

Fillet weld

PE (overhead position)

Fillet weld

  increased current for root pass,

normal current for filling layers and cover layers

  increased current for root pass,

normal current for filling layers and cover layers Butt weld

  less current for root, normal current

for filling layers and cover layers PF (increasing)

PG (downward)

Butt and filled seams

  normal current for root pass,

Butt and filled seams

  normal to increased current for root

filling layers and cover layers pass, filling layers and cover layers

Figure 6: Welding current according to the welding position, welding method and weld composition

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Page 11

Welding accessories

These include: -

the welding cables, an all-insulated electrode holder, the workpiece clamp1, a welding table with fume extractor and protective (wall) screens

Fully insulated stick electrode holder

Threaded work piece clamp

Work Clamp

Earth clamp

Figure 7: Welding accessories  Fully insulated stick electrode holder  Workpiece clamps

In addition tools like: -

a slag hammer, wire brush, file or grinder, a weld gauge, temperature measuring instrument and a metal bucket for collecting the stub ends.

1

Although it can be practical to use a solenoid as earth connection, this is not advised. Splinters, abrasive dust and clinker residues adhere already after a short use. Here, due to the resulting poor contact, a “jumpover” may take place with a short arc. The consequences from this are stray arcs or a burning of the ground connection.

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4. Standards and fields of application of stick electrodes Many different materials can be welded using stick electrodes. As the quality of materials increases, not only the manual skill requirements of the welders are rising but also his technical expertise in order to avoid errors during manufacturing. In the following you will find the current standards for stick electrodes according to the areas of application: DIN EN ISO 2560-A

-

DIN EN ISO 18275-A

-

DIN EN ISO 3580-A

-

DIN EN ISO 3581-A

-

DIN EN ISO 14172

-

DIN EN 14700

-

DIN EN ISO 1071

-

Stick electrodes for manual metal arc welding of non-alloy and fine grain steels. Up to a minimum yield point of 500 MPa Stick electrodes for manual metal arc welding of high-strength steels. yield point > 500MPa Stick electrodes for manual metal arc welding of creep-resisting steels Stick electrodes for manual metal arc welding of stainless and heat-resisting steels Stick electrodes for manual metal arc welding of nickel and nickel alloys Welding consumables for hard-facing Stick electrodes, wires, rods and tubular cored electrodes for fusion welding of cast iron

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5. Stick electrodes for manual metal arc welding 5.1

Production

Stick electrodes consist of the core wire and a mainly mineral coating, usually bound with a coating binder (usually water glass). While unalloyed, soft cores are generally used for non-alloyed and low alloyed covered electrodes, so that additional alloying for increasing strength and toughness is achieved via the coating, for high-alloy covered electrodes a distinction is made between (rod) core- and coating alloyed electrodes. Nowadays stick electrodes are produced as extruded stick electrodes (see also Figure 8). The composed mixture, according to the respective standards (different mineral and metallic raw materials and coating binder water glass), is pressed onto the (rod) core with a pressure of approx. 350 to 500 t. The still moistened electrodes are brushed (holding- and striking end), marked (according to DIN/EN/ISO and/or the company name) and depending on the type of coating and after being collected, will be “dried” (burned) at a temperature of approx. 180°C (cellulose) to 480 °C (basic H5) in a furnace. Followed by packaging into cardboard boxes (+ PE-foil), depending on the quality and type, into air-tight tins or vacuum packages.

Figure 8: Manufacturing of stick electrodes

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Core rods for electrodes

Core rods for unalloyed electrodes include relatively similar compositions; the carbon or manganese content can slightly fluctuate depending on the yield point and tensile strength: C 0.06 to max. 0.12% Mn 0.3 to max. 2.0% P, S  0.030 % Si 0.06 % High-alloy core rods contain the alloy composition required for the base material. 5.3

Composition of the coating, coating materials

The materials used for electrode coatings are very numerous. These are mainly ores and minerals in the form of oxides, hydroxides, carbonates, carbonyls, silicates, chlorides, fluorides and other structures of metals and non-metals as well as ferrous alloys, organic substances and compositions. Both the welding characteristics of a stick electrode and the mechanical properties of the weld metal are influenced accordingly by the coating. This homogeneous mixture generally includes the following main components: slag-forming substances (quartz, fluorspar), – deoxidising materials (ferromanganese, ferro-silicon, aluminium), – gas shield forming substances (cellulose, calcite), – arc-stabilisation substances (potash-feldspar, rutile), – binding material (potash and sodium water glass) and, if required, – alloying elements (ferrochromium, Nb, Si, nickel flux, etc.). Iron powder can also be added to increase the deposition rate. –

5.4

Standard analysis of the most important coating types

Table 2: Standard analysis of main coating types (data in %)

Cellulose type “C” Cellulose Rutile TiO2 Quarz SiO2 FeMn Waterglass

40 20 25 15

Droplet transfer: medium sized drops Toughness values: Good

Acid-type “A” Magnetite Fe3O4 Quarz SiO2 Lime stone CaCO3 FeMn Waterglass

2

Rutile-type “R” 50 20 10 20

Rutile TiO2 Magnetite Fe3O4 Quarz SiO2 Lime stone CaCO3 FeMn Waterglass

45 10 20 10 15

Basic type “B” Calcium fluoride CaF2 45 Calcite CaCO3 40 Quarz SiO2 10 FeMn 5 Waterglass

Droplet transfer: Droplet transfer: Droplet transfer: fine medium sized drops up medium sized drops up droplet and spray type to fine sized drops to large sized drops Toughness values: Toughness values: Toughness values: Normal Good Very good

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Functions of the stick electrode coating

The functions of the stick electrode coating are as follows:     

Increasing the ignition properties, ionisation of the arc-area and stabilisation of the arc. Forming shielding gas for the protection of the damaging effect of the air (N and O). Slag formation for protecting the generated weld (weld surface and root), for a cooling delay as well as for shaping of the weld. Metallurgical influence of the weld metal, i.e. oxidation and de-oxidation as well as for setting of harmful accompanying elements (S, P) or additional alloying. Increasing the deposition rate by so-called “high-efficiencyelectrodes” with additives like e.g. Iron powder.

Non-covered elektrode

Coating thickness

Covered elektrode

Material transfer

Steady / stable arc

unstable arc

Gap bridging ability

Weld appearance

Slag

Shielding gas surrounding

Penetration depth

Figure 9: Functions of the stick electrode coating

Figure 10: Influence of the coating thickness

The coating thickness and the composition of the coating of the stick electrodes have a significant influence upon:    

the strength and toughness properties of the weld metal (mechanical data of weld metal) hot-cracking and/or cold-cracking behaviour (hydrogen content in the weld metal) the weld- and ignition characteristics of the stick electrode and the metal transfer the gap bridging abilities, the weld seam appearance and the penetration depth

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Classification of the stick electrodes

The stick electrodes can be classified based upon of the following criteria: According to the purpose of use

Surface welding, Joint welding, Cutting, Underwater cutting and welding

According to the chemical composition of the weld metal

Non-alloyed and low alloyed, high-alloyed electrodes for high-strength, creep-resistant, heat-resistant, stainless steels or non-ferrous metals as well as cast iron.

According to the technological properties According to the mechanical-technological quality values of the weld metal, the current type, polarisation, deposition rate, welding positions, weld shapes and hydrogen content of the weld metal. according to the method of production according to the coating type

5.7

Press casing, double press casing, (dipping electrodes) Acidic A Basic B coated Rutile R Cellulose C

Dimensions and permitted deviations of stick electrodes (extract from DIN EN ISO 544)

Table 3: Dimensions and permitted deviations Stick electrode (rod) Core diameter

dimensions in mm

Diameter permitted deviation

Length

Length permitted deviation

± 0.06

200 to 350

±6

± 0.10

275 ≤l< c 450

±6

1.6 2.0

ab

2.5 3.2 4.0 5.0 6.0 8.0 a

b c

Other dimensions may be agreed. For intermediate dimensions, permitted deviations according to this table are to be applied. Dimensions for the core wire. For special cases (e.g. gravity arc welding) length l< 1,000mm.

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SFI / IWE 1.09 Page 17

Identification according to DIN EN ISO 544

5.8.1

Identification on the component

Stick electrodes must be durably marked with a traceable supplier´s trade name or with the electrode classification on the coating close to clamping end or on the clamping end itself.

Figure 11: Identification on the stick electrode

5.8.2

Identification on each packaging unit

The packaging, even on the smallest unit, must contain the following information:          

Name of the manufacturer and supplier Trade name Designation according to the respective international standard Dimensions (diameters and lengths) Charge-/ batch- or manufacturing number Type of current, recommended current ranges, polarity Nominal net weight or quantity Re-dry process regulations or information to appropriate sources of information (e.g. with basic electrodes) Occupational health protection and accident warning information as well as: Approvals 1) (if applicable) Here: American Bureau of Shipping (ABS), Bureau Veritas (BV), Det Norske Veritas (DNV) Germanischer Lloyd (GL), and Lloyd´s Register (LR) 1) In accordance with regional building regulations, construction products for metal construction require according to the building rules list B Part 1 an analysis in form of an accordance certificate (CE marking, “Conformité Européenne”) of a test-, monitoring- and certification-authority. This authority list according to the regional building regulations is issued by the DIBt. Figure 12: Designation on the smallest packaging unit (source: ESAB GmbH

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6. Recommendation for storage and re-drying of coated electrodes 6.1 Storage (extract from DVS 0957) Basically the coated electrodes should be stored in their original package until they will be used. The withdrawal of the electrode packages should be in accordance with their sequence of storage entrance. In order to protect the coated electrodes from damages due to moisture, they have to be stored in dry areas. The minimum requirements for suitable electrode storage are:  The storage room must be weatherproof, ventilated and if necessary be heatable.  Ceiling, floor and walls must be dry  No open water surfaces allowed in the room  The room is equipped with shelving units or pallets A direct contact of the electrode packages with the floor or walls should be avoided as well as extreme storage temperature conditions below 0°C and above 30°C. The electrode packages should be stored on wooden pallets or in shelves having a distance of 30cm of the wall. Under these conditions, as mentioned above, conventional packed coated electrodes could be stored for a longer period although the maximum storage period of 2 years should not be exceeded. For special packed packages no specific restrictions are valid. DO NOT throw packages during storage and retrieval or repositioning. The storage height of the covering boxes and cases should be limited to 6 or max. 8 units and for highefficiency electrodes max. 4 units. 6.2 Moisture absorption of coated electrodes. Figure 13: moisture absorption of basic coated electrodes at room temperature under different humidity. Moisture content ofderthe coating [%] % Feuchtigkeitsgehalt Umhüllung

10 9

rrelative e l a t i vLuftfeuchtigkeit e a ir h u m id ity

8

95%

7 6 5 4 3 75%

2 1 0

60% 0

5

10

15

20

25

30

Lagerungsdauer an feuchter Luft [Tage] Storage duration under moist air conditions  Days (nach Böhler AG)

High air humidity leads to higher moisture absorption The coating of the electrode rod is – depending on the type of coating – more or less hygroscopic and contains water. Figure 14 shows the sources of hydrogen which could enter the weld metal. Even during storage and during processing the coating can pick-up water from the air.

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1

Coating: Outer surfaces and capillary water, absorbed water, crystallisation water and colloidal absorbed water, constituents water

2

Core rod surface: drawing grease

3

Core rod: Atomic, molecular, ionised and bound hydrogen

4

Arc bow surface: Surrounding air, air humidity

5

Base material: Surface contamination like rust, oil, grease, paint Atomic, molecular, ionised and bounded hydrogen

In principle coated electrodes should only be welded in their dry state condition. Cellulose coated electrodes however are an exception to this: they require a certain controlled rest humidity. That’s the reason why today’s packaging is specifically adjusted to the required needs (plastics, tin can, vacuum etc.) 6.3 Re-drying For re-drying of coated electrodes the following reference values are valid:    

low alloyed, basic coated electrodes Medium alloyed basic coated electrodes for fine-grain steels and creep-resistant steels High alloyed coated electrodes basic- and rutile coated Cast iron electrodes

250° - 350°C / ca 2 hours 300° - 350°C / ca 2 - 3 hours 300° - 25°C / ca 2,5 hours 120° - 200°C / ca 1 hour

Excepted from this are special packed packages like vacuum packed electrodes which are ready for use during 10-12 hours after opening. The number of re-drying processing of each electrode is limited to about 3 times or the maximum redrying time is set to 10 hours. However, mandatory are always the manufacturer’s re-drying recommendations. In order to prevent new moisture absorption the electrodes should be, if not otherwise stated by the manufacturer, stored (intermediate) after the re-drying until processing as follows:  100° - 150°C up to 8 hours in mobile quiver  120° - 200°C up to 4 weeks in a stationary drying oven

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Manual Metal Arc Welding 6.4

Page 20

Negative consequences of raised moisture absorption of electrodes

Table 4: negative consequences of raised moisture absorption of coated electrodes. Changes to the electrode  Blisters on the coating  Swelling of the coating  Core rod corroding (for un-alloyed electrodes)

Changes to welding characteristics    

Uneven melting of the coating Instable arc Raised spatter formation Worsening of slag removal

Weld metal property      

Undercut formation Change of weld appearance Change of weld metal composition Pore formation Decrease of fracture elongation due to hydrogen embrittlement Cold cracking

7. Specific hazards during manual metal arc welding 7.1 Electrical hazard Electric current is an energy carrier (heat source) for all processes of arc welding technology. It means a risk to the welder, since in the welding circuit not all live parts are protected from direct contact. An electric current flowing through the human body can cause life-threatening injuries depending on the type of current, the amperage, the exposure duration and the electrical route. Therefore attention should be paid laying down the electrode-holder in an isolated manner without holding any stick electrode. 7.1.1

Danger zone of welding equipment and mains connection

Welding equipment being used in areas of high potential electrical risk requires the marking S. Older equipment marked with K or 42V are also allowed. For equipment being used in the open field the marking should show at least protection degree IP 23 /4/ 7.1.2

Danger zone for welding cables, workpiece connection

The welding current should form a visible unit between the welding power source, the welding power cables and workpiece. Welding cables must be insulated and protected against damage. The workpiece cable must be connected using large surface contacts, and directly with the welding point on component to be welded. Otherwise, eddy currents may occur that may lead to the malfunction of the ground conductors, scorching in the area of gears, shaft bearings, suspensions etc. and may affect the preset welding parameters. 7.2

Danger zone of the arc

The arc emits UV radiation, from which the welder has to protect itself with safety clothing and eye protection. Within 30 seconds, critical values are exceeded for the unprotected skin. Suitable protective filters (9 – 15) protect the eyes. Stray radiation from reflecting areas shall be considered, too (e.g. slabs, polished metal surfaces, etc.). The welding area has to be “isolated” in order to protect other parties.

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Danger zone of the weld pool and slag

Protective clothing adapted to the working conditions, closed and fire-retardant, protects from the metal and slag spatters occurring when welding. Slag may be hammered off only using protection shielding, safety screen or safety glasses, otherwise eye injuries may be caused by hot slag slivers. 7.4 Welding fumes and dust During welding, gas and particulate pollutants are formed with a particle size usually under 1 µm. Welding fumes can thus be inhaled. They must be extracted from the point of origin inside workshops. Paint and coatings in the weld area that have not been removed may emit further toxic gases in the arc during vaporisation, e.g. zinc oxide. This could lead to poisoning. Sufficient care is often forgotten when performing manual metal arc welding. Carcinogenic welding fumes from CrNi basic electrodes are a particular concern in this context. The proportions of chromium VI compounds and nickel-oxides are very high. Rutile CrNi electrodes poses a much lesser risk, however effective extraction is required in both cases (see also BGI 616 /9/).

Hazardous substances that can be inhaled during welding

Gases

Fumes and dust

Toxic

Inert

Nitric oxides CO CO2 Ozone Carbonyl chloride

7.5

Fine dust Sodium oxide Aluminium oxide Chromium III oxide Magnesium oxide

Toxic Calcium oxide Fluoride Manganese oxide Zinc oxide Lead oxide Copper oxide

Carcinogenic Chromium VI oxide Nickel oxide Cadmium oxide Cobalt oxide Beryllium oxide

Personal safety equipment

Personal safety equipment includes:       

A welder helmet or a mask with suitable level of eye protection, A closed, dry protective suit, Dry leather gloves, Safety glasses Ear protection (is recommended to wear these also during welding e.g. under construction site conditions, since metal spattering can cause injury in the auditory canal during vertical down welding, among others. Mandatory above 85 dB (A), which is exceeded when grinding off the welds. In confined spaces, when welding gas and remote heating pipes, an approved fire-resistant welder protective suit is to be worn; leather safety clothing is recommended here. High closed safety shoes with gaiters prevent severe burns in the foot region due to any metal and slag drop.

Never wear any synthetic work clothing, installation gloves with material insets, etc.

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8. Labelling of stick electrodes according to international standards In order to simplify the designation of filler materials, a uniform, material-dependent labelling system was introduced. The standards contain either a classification for designation based on strength, elongation after fracture and impact energy of pure weld metal or the chemical composition. e.g for high-alloyed filler materials. The ratio of yield point to tensile strength of weld metal is generally higher than that for the base material. Users should note therefore that a weld metal that reaches the minimum yield point of the base material, does not necessarily reach its minimum tensile strength. If, during use, a specific minimum tensile strength is required, the tensile strength must be considered accordingly for the choice of the filler material.

8.1

Classification is based on eight characteristics according to DIN EN ISO 2560-A: 1) 2) 3) 4) 5) 6) 7) 8)

Identifier of the product/the welding process; Identifier for the strength properties and the elongation after fracture of weld metal; Identifier for the impact energy of weld metal; Identifier of the chemical composition of the pure weld metal; Identifier for the coating type; Identifier for the efficiency and the type of current; Identifier for the welding position; Identifier for the hydrogen content of the weld metal.

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Sample identifier

Mandatory identifying characteristics of the classification Number of ISO standard. The letter A indicates a classification according yield point and impact energy value of 47 J. Indicates a stick electrode Identifier for yield strength. For electrodes, which are suitable for multi-pass welding, the code designation “35, 38, 42, 46 or 50” indicates the minimum yield point of 355 MPa, 380 MPa, 420 MPa, 460 MPa or 500 MPa. Identifier for the Charpy V impact value. It indicates the temperatures, at or above which the pure weld metal reaches or exceeds 47J. Identifier for the chemical composition of pure weld metal. Abbreviation for the coating type

DIN EN ISO 2560 - A

E

46

6

Mn1Ni

B

3

2

H5

Non-mandatory identifier characteristics Identifier for the effeiciency and type of current Identifier for the welding position Identifier for diffusible hydrogen content in ml/100g. H5, H10, H15 gives the maximum content of diffusible hydrogen of 5ml/100g, 10ml/100g, 15ml/100g deposited weld metal.

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9. Criteria for selecting stick electrodes for manual arc welding Stick electrodes are selected according to material and welding criteria. An initial comparison is made between the mechanical quality data for the filler material and the quality values of the base material, whereby the minimum requirements of the base material shall be achieved in the pure weld metal, too. The selection takes place according to the following criteria: 1. the base material to be welded – for unalloyed steels and fine-grained steels – for high-strength steels – for creep-resistant steels – for stainless and heat-resistant steels – for nickel and nickel alloys – for cast iron materials further subdivided into: – the chemical composition – the metallurgical and physical properties 2. the load capacity of the component classified according to: – structural design of the component – static or dynamic load – load-carrying state (amount of load) 3. the welding task classified according to: – welding conditions, – working position, – welding power source, – type of coating 4. cost effectiveness classified according to: – deposition rate, – efficiency, – heat-input Coated stick electrodes are usually selected according to the catalogues for filler materials from the manufacturers (also called “welding guides”). It should be noted that the mechanical properties of pure weld metal used for the classification of the covered electrodes can deviate from those that can be achieved in production welding. This is due to variations when performing the weld, for example covered electrode diameter, oscillation, welding position and the material composition. Making a production work sample is therefore recommended for special situations.

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10. Covered electrodes of non-alloy and fine-grain steels, DIN EN ISO 2560-A 10.1 Application area DIN EN ISO 2560-A This international standard sets the requirements for the classification of stick electrodes, the welded weld metal, after a post weld heat treatment, of non-alloy and fine grain steels with minimum yield strength up to 500MPa or minimum tensile strength up to 570MPa. According to DIN EN ISO 2560-A, a wide range of stick electrodes is available with very different coating compositions. A distinction is made between base- and mixed-types. Table 5 provides an overview of the different coating types: Table 5: Coating types according to DIN EN ISO 2560 - A

Type A C R RA RB RC RR B

Coating Acid Cellulose Rutile Rutile-acid Rutile-basic Rutile cellulosic Thick rutile Basic

The same variety is not available for medium and high-alloyed electrodes. So there are stick electrodes for high-strength steels according to DIN EN ISO 18275 with just a basic coating, creep-resistant electrodes according to DIN EN ISO 3580 and stainless/heat-resistant electrodes according to DIN EN ISO 3581 with just rutile or basic coating. 10.2 Covered electrodes with acid coating:

A

Composition The coating of this electrode type is characterised by high content of heavy metal oxides (Fe3O2; Fe2O3, SiO2) and– as a result of the high oxygen-potential – de-oxidising materials (ferromanganese). Characteristics The generated combustion heat makes these electrodes the “hot” stick electrode type. They are much more sensitive to solidification cracks than covered electrodes with other coating types. The mechanical data of weld metal is low. Stick electrodes with acid coating are of only limited use for out-of-position welding. As a result of the high oxygen content of the coating components (Magnetite Fe3O4) a high burn-off in alloying elements (especially manganese) will occur during welding. Applications: Nowadays pure acid covered electrodes are not being used any more. One of the benefits is the smooth weld drawing and the ease with which slag can be removed.

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C

Composition Covered electrodes of this kind have a high amount of combustible organic substances in the coating, especially cellulose. A defined residual moisture is required for an intensive arc. Characteristics Because of the high amount of organic content, and therefore a low slag content and the strong arc, these electrodes are especially suitable for vertical down welding. A high welding speed is achieved at a high deposition rate. Although the fumes are substantial, this is not a problem when working outdoors. Xray-safe pipe circumferential welds are achieved with good to excellent quality values of weld metal. The welds have a coarse-flaked weld appearance. Applications: Underground pipe construction and pipeline construction are the main areas of application of cellulosic electrodes. For pipes ≥ DN 100 up to 60% time may be saved compared to vertical welds. Already for root welding, larger electrode diameters (Ø 3.2 mm to 8 mm wall thickness/Ø 4.0 mm from 8 mm wall thickness) and higher currents can be used. This enables a higher welding speed and therefore also a higher economic efficiency is obtained. Cellulosic electrodes are weldable with direct current: Root at the – pole. Hotpass, filling- and cap-layers are welded at the + pole. A type selection: DIN EN ISO 2560-A

E 42 2 C 25 E 46 3 C 25 E 50 3 1Ni C 25

Note: Vertical welds must be welded in a multi-layer technology “in heat”. The hotpass must be welded no more than 10 minutes after completion of the root welding, to avoid having cracks in the weld. The interpass temperature should not be lower than 80°C and for higher tensile strength pipes, 150°C. Further special techniques and welding equipment with special properties for vertical down welding are described in paragraph 19.

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Page 27

B

Composition The thick coating of this stick electrode is characterised by the high content of about 80% of calcium carbonate (CaCO3) and calcium fluoride (CaF2). To improve the welding characteristics, especially for welding with alternating current, larger amounts of non-basic components (e.g. rutile and/or quartz) might be necessary. This older basic type is the so-called “lime-type” type (Kb-electrodes). In newer basic electrodes, part of the carbonates has been replaced by cryolite (Na3AIF6 - aluminium trisodium hexafluoride). These electrodes have a steady arc, a more uniform weld quality and can still be safely mastered with lower currents. Less slag is detached than with the “Kb” electrode. There is little oxygen present in the arc. The loss of alloying elements is therefore low. Characteristics Basic-coated stick electrodes have two special characteristics:  high impact energy of the welded material, especially at low temperatures;  high crack resistance, better than for other types (the high metallurgic degree of purity of weld metal reduces the risk of hot cracking, and a lower hydrogen content reduces susceptibility to cold cracking). This requires dry stick electrodes. The hydrogen content should not exceed the upper limit H = 15 ml/100g weld metal. Basic electrodes are hygroscopic; regulations for a dry environment are required to be observed. Basic type stick electrodes are appropriate for all welding positions – except the vertical down position. Applications: The very ductile, low-temperature toughness weld metal is suitable for the following:  at low yield point for shrink-limited components,  for high-strength steels as soft weld metal for rigid structures.  for larger weld cross-sections,  for large component thicknesses (>20mm)  for welding steels (FU) contaminated with P, S and N2; these steel accompanying elements are released and transferred into the slag  and with C-contents above 0.22% (including cast steel and rail welds) Drying process: Baking for about 2 hours at 250 to 350 °C, then in the drying oven at 150 °C, then keep warm in the oven or portable drying unit at 100°C - 150 °C until welding takes place. However, in each case it is the manufacturer's specification that is relevant. Disadvantages  Hard to remove slag, rough weld appearance  Difficult handling like poor ignition- and re-ignition properties  Moisture absorption (hygroscopic) A type selection: DIN EN ISO 2560-A E 38 2 B 22 H 10 E 46 8 3Ni B 73 H5

E 42 5 B 32 H5 E 50 4 2Ni B 42 H5

E 46 4 B 42 H 10

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Manual Metal Arc Welding 10.5 Stick electrodes with rutile coating:

SFI / IWE 1.09 Page 28

R, RR, RC, RA, RB

Composition The main component of the coating is rutile (TiO2); this has a less oxidising effect in the arc, the arc atmosphere is more neutral, the alloy burn-off is lower. The mechanical properties of weld metal of rutile electrodes must be given particular consideration for steels with higher Mn-content (S355). When choosing these stick electrodes, the specification sheets for the filler materials (from the electrode manufacturer's inspection body) should be used. Characteristics Rutile types are the most used electrodes in practice because of the huge amount of possible application areas. Advantages of rutile-coated stick electrodes  Easy handling  Good ignition and re-ignition characteristics  Finely rippled weld appearance  Easy removability of slag  Direct and alternating current welding Disadvantages  Not applicable for high-carbon steels with C > 0.2%  Higher hydrogen content (approx. 20 ml/100g weld metal)  Risk of cracks in component thicknesses over 25 mm  Lower impact strength at lower temperatures (compared to basic covered electrode) Rutile covered electrodes are mainly welded with direct current, -pole or with alternating current. Stick electrodes with rutile coating:

R

Stick electrodes of this kind create a more coarse droplet transfer than the thick rutile type. They have good gap bridging abilities and are suitable for welding thin sheet metal. Their disadvantage is the strong crater formation. Suitable for all welding positions except for vertical down position. Example: DIN EN ISO 2560-A E 42 0 R 12 Thick stick electrodes with rutile coating:

RR

With stick electrodes of this type, the ratio between coating and core rod diameter is  1,6. The high rutile amount of the coating, the good ignition capability, a fine-droplet metal transfer and the finely rippled, even welds are all characteristic. Example: DIN EN ISO 2560-A E 42 0 RR 12 Stick electrodes with rutile cellulose coating:

RC

The composition of this stick electrode's coating is similar to that of the rutile type electrodes. However, it contains higher amounts of cellulose. Weld metal is viscous with a lower slag amount, therefore stick electrodes of this type may also be used for welding in vertical down position. All-round electrode in areas of common, practical use. Not approved for root welding with butt and fillet welds in the metal and pipeline construction. Example: DIN EN ISO 2560-A E 38 0 RC 11 © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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RA

The welding characteristics of the stick electrodes of this mixed type are comparable to the ones of the acid type. However large amounts of iron oxide replaced by rutile in the coating for these stick electrodes. This is the reason why most thick coated stick electrodes can be used for welding in all positions – except the vertical down position. These stick electrodes have high deposition rates, current load capacity and easily removable porous slag. Rutile-acid electrodes are rarely used nowadays. They are particularly suited as seal welds on acute angles. They form concave smooth welds, the root point is determined safely for fillet welds. Because of the low Si-content, they are very well suited to subsequent galvanising, enamelling and rubberising. The loss of alloying elements is, similar to the acid types, relatively high. Stick electrodes with rutile-basic coating: RB This coating type is characterised by a high content of rutile in combination with increased basic content, medium-droplet metal transfer and a thin fluid slag. These mostly thick coated stick electrodes have – along the good mechanical characteristics of the welds – good welding characteristics in all welding positions with the exception of the vertical down position. They are very frequently used as vertical up welding electrodes in pipeline and steel construction, for out-of-position welding and for root welding. Example: DIN EN ISO 2560-A E 38 2 RB 12

10.6 High-deposition electrodes “high-efficiency electrodes” Composition These stick electrodes contain additional iron powder in their coating which easily doubles the amount that is put in by the core rod. Characteristics (see also 17.4) Welding is carried out for RR- and RA-types with direct current at the – the pole or alternating current RB-types, preferred direct current + pole, possible also direct current – pole or alternating current B-types direct current at + pole The current settings are significant higher compared to the electrodes with lower efficiency (105%). The arc-time is shortened and the run-out length increases only insignificantly. High-efficiency electrodes with an efficiency of 160% can be used for the PA- en PB- weld position. Electrodes having an efficiency of 160% to 220% can only be used in the flat position (PA- welding position) due to the very low viscosity of the weld metal. Achievable a-size: core rod diameter + 0,5mm Application A preferential application is longer continuous fillet welds (e.g. excavator construction, shipbuilding). Selection of type: DIN EN ISO 2560-A

E 42 2 RA 73 E 42 0 RR 73 E 42 2 RB 53 E 42 4 B 73 H 5

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10.7 Sample applications for stick electrodes according to DIN EN ISO 2560-A Butt and fillet welds on thin profiles Material S235JR DIN EN ISO 2560-A DIN EN ISO 2560-A DIN EN ISO 2560-A

E 42 0 RR 12 (more suitable for fillet welds) E 38 2 R 12 or E 38 2 RB 12 (more suitable for butt welds, gap bridging abilities)

Fillet welds on double-T beams Material S235J0 Welding position: PB DIN EN ISO 2560-A DIN EN ISO 2560-A

E 42 0 RR 73 E 42 2 RB 53

Double single-V butt welds on tension rods with large workpiece thickness Material S355J3 Welding position: PA DIN EN ISO 2560-A DIN EN ISO 2560-A

E 38 2 B 12 H10 E 42 4 B 32 H10

Fillet welds on consoles (t = 10 mm) Material S235JR Welding position: PB, PF DIN EN ISO 2560-A

E 38 2 RB 12

Butt welds in pipes (L235J2) Weld 1 welding position PJ DIN EN ISO 2560-A E 42 2 C 25 Weld 2 welding position PH DIN EN ISO 2560-A E 38 2 RB 12 Figure 15: Suitable stick electrodes

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MMA Welding II

Page 31

Table 6: Stick electrodes for non-alloy and fine-grain steels, DIN EN ISO 2560-A

E

46

6

Mn1Ni

B

4

2

H5

E:Identifier for shielded metal arc welding Minimum yield point 1)

Code number

Tensile strength [MPa]

[MPa]

1)

2)

Minimum elongation after fracture [%]

35 38 42 46

355 380 420 460

440 to 570 470 to 600 500 to 640 530 to 680

22 20 20 20

50

500

560 to 720

18

Hydrogen content in ml/100 g of weld

Symbol

metal max.

H5

5

H 10

10

H 15

15 Welding positions in accordance with

Code number

The lower yield point (ReL) is valid. In case of not clearly defined tensile yield point, the 0.2% yield strength (Rp0.2) is valid.

DIN ISO 6947

1

In all positions

2

All positions, except the vertical down position

3

Butt weld in flat position, fillet weld in flat and horizontal position

4

Butt weld in flat position, fillet weld in flat position

Measurement length is identical to the fivefold test diameter Identifier/

Minimum impact work

code number

47 J at °C

Z A 0 2 3 4 5

no requirements + 20 0 - 20 - 30 - 40 - 50

6

- 60

Vertical-down position and positions Like

5

for code letter 3.

Code number

Yield %

Type of current

1

≤ 105

Direct and alternating current

2

≤ 105

Direct current

Chemical Composition

3

> 105

≤ 125

Mn

Mo

Ni

4

> 105

≤ 125

Direct current

No designation Mo MnMo 1Ni Mn1Ni 2Ni Mn2Ni 3Ni

2.0 1.4 1.4 to 2.0 1.4 1.4 to 2.0 1.4 1.4 to 2.0 1.4

0.3 to 0.6 0.3 to 0.6 -

0.6 to 1.2 0.6 to 1.2 1.8 to 2.6 1.2 to 2.6 2.6 to 3.8

5

> 125

≤ 160

Direct and alternating current

6

> 125

≤ 160

Direct current

Short symbol

Type of coating

1NiMo

1.4

0.3 to 0.6

0.6 to 1.2

A

Acidic type coating

any other agreed chemical composition

C

Cellulosic type coating

R

Rutile type coating

B

Basic type coating

% (mass percentage)

Alloy symbol

Z

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7

> 160

Direct and alternating current

8

> 160

Direct current

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11. Stick electrodes for weatherproof, high-strength and low-temperature toughness steels, DIN EN ISO 18275-A For manual metal arc welding of high-strength fine-grain structural steels (>500 MPa) in steel construction, it is almost exclusively basic (B) coated stick electrodes that are used. Parts of DIN EN ISO 2560-A were used to ensure uniformity in the identifiers. Composition The composition of coating differs from the basic electrodes according to DIN EN ISO 2560-A due to a high content of alloying elements like e.g. Mn, Ni, Cr and Mo. Characteristics Basic-coated high-strength stick electrodes have the following special characteristics:  high impact energy of weld metal, without brittle fracture at low temperatures (- 80 °C)  high crack resistance,  outstanding strength properties also for high temperatures (to approx. 440° C), however the yield strength decreases significantly in this case. Applications  statically and dynamically higher loaded welded structures. e.g in steel construction, equipment and vehicle construction.  butt weld joints for concrete reinforcement steel welding,  for high-strength steels as soft weld metal (under-matching) for rigid structures.  large weld cross-sections and component thicknesses  for welding of steels (FU) contaminated with P, S and N2, these steel accompanying elements are released and transferred into the slag  and with C content above 0.22% (including cast steel and rail welds) Characteristics Indications of the strength of weld metal properties refer to the welding condition. If there is a T added to the identifier, the values refer to the stress (-relieved) annealed state Sample identifier: DIN EN ISO 18275-A E 55 3 MnMo B T 4 2 H10 (650 °C, holding time 15 hours). A Z for alloying elements indicates that the chemical composition is outside the defined limits. Note: Similar electrodes with z values are not comparable to each other. Sample identifier: DIN EN ISO 18275-A E 55 5 Z 2Ni B 45 (basic vertical down welding electrode) Work rules Basic electrodes for welding of fine-grain steels must be re-baked, at higher temperatures than Belectrodes according to DIN EN ISO 2560-A: 2 hours at 300 - 350 °C, but maximum 10 hours. The manufacturer's specifications are relevant. Electrodes are usually equipped with ignition aids (additional shielding gas cloud during first ignition, avoiding of starting-pores). The hydrogen content of weld metal must not exceed 15 ml/100 g weld metal. -

Use targeted heat-input Monitoring of heat-input Stringer beads instead of oscillating weld beads No excessively thick layers, particularly in position “PF” Working temperature not to exceed 200 °C. Heat-input value for wall thickness < 15 mm: Wall thickness = energy per unit length kJ/cm. Grinding of all start-stop points (pores after extinguishing of arc because of lack of shielding gas)

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Explanation of the identifiers of the DIN EN ISO 18275-A E

62 7 Mn1Ni B 3 4 H5

E:Identifier for shielded metal arc welding Minimum yield point 1)

Code number

Tensile strength [MPa]

[MPa]

Minimum elongation after fracture

Symbol

[%]

H5

5

H 10

10

H 15

15

55 62 69 79

550 620 690 790

610 to 780 690 to 890 760 to 960 880 to 1,080

18 18 17 16

89

890

980 to 1,180

15

1)

The lower yield point (ReL) is valid. In case of not clearly defined tensile yield point, the 0.2% yield strength (Rp0.2) is valid.

2)

Measurement length is identical to the fivefold test diameter Identifier/

metal max.

Welding positions in accordance with

Code number

DIN ISO 6947

1

In all positions

2

All positions, except the vertical down position

3

Butt weld in flat position, fillet weld in flat and horizontal position

4

Butt weld in flat position, fillet weld in flat position

Minimum impact work

code number

47 J at °C

Z A 0 2 3 4 5 6 7

no requirements + 20 0 - 20 - 30 - 40 - 50 - 60 - 70

8

-80

Alloy symbol

for code letter 3.

Code number

Yield %

Type of current

1

≤ 105

Direct and alternating current

2

≤ 105

Direct current

3

> 105

≤ 125 Direct and alternating current

Chemical Composition

4

> 105

≤ 125

Direct current

% (mass percentage)

5

> 125

≤ 160

Direct and alternating current

6

> 125

≤ 160

Direct current

Ni

Cr

Mo

MnMo Mn1Ni 1NiMo 1.5 NiMo 2NiMo Mn1NiMo Mn2NiMo Mn2NiCrMo

1.4 to 2.0 1.4 to 2.0 1.4 1.4 1.4 1.4 to 2.0 1.4 to 2.0 1.4 to 2.0

0.6 to 1.2 0.6 to 1.2 1.2 to 1.8 1.8 to 2.6 0.6 to 1.2 1.8 to 2.6 1.8 to 2.6

0.3 to 0.6

0.3 to 0.6 0.3 to 0.6 0.3 to 0.6 0.3 to 0.6 0.3 to 0.6 0.3 to 0.6 0.3 to 0.6

Mn2Ni1CrMo

1.4 to 2.0

1.8 to 2.6

0.3 to 0.6

0.3 to 0.6

any other agreed chemical composition. It is possible that two covered electrodes in same classification in Z are not interchangeable

Vertical-down position and positions Like

5

Mn

Z

Hydrogen content in ml/100 g of weld

7

> 160

Direct and alternating current

8

> 160

Direct current

Short symbol

Type of coating

B

Basic type coating

Most covered electrodes of this type are basic coated and have a B designation. For cellulose coated and other coating types see DIN EN ISO 2560 The symbol T indicates that strength, elongation and impact properties are obtained in the classification of the depicted weld metal for the state after a postweld annealing process at temperatures between 560 °C and 600 °C for 1 hour. The specimen must be left for cooling to 300 °C in the furnace.

Table 7: Stick electrodes for weatherproof, high-strength and low-temperature toughness steels, DIN EN ISO 18275-A

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12. Stick electrodes for creep-resistant and high-temperature steels, DIN EN ISO 3580-A This standard includes a classification for the identification of coated stick electrodes with the chemical composition of the pure weld metal. Composition Stick electrodes for creep-resistant steels are available as rutile and basic-coated types, while hightemperature electrodes are only available with basic coating. In order to increase creep resistance, low amounts of Cr, Mo and V are used for alloying (CrMo1) at max. working temperatures of up to 550 °C. Over 550 °C to about 600 °C, additionally a resistant to scaling is required that in turn requires higher alloying with Cr, Mo and V (CrMo2). Special electrodes are additionally alloyed with W, Nb, Ni for working temperatures of up to 650 °C. Stick electrodes of this type are usually alloyed-core types. Characteristics Rutile CrMo electrodes are weldable without pre-treatment like rutile covered electrodes. Basic CrMo electrodes must be baked at approx. 300 °C to 350 °C for two hours. High creep rupture strength and toughness behaviour also in the long-time range up to 650 °C. Sample applications Steam turbines, boiler and pipeline construction Allocation of suitable stick electrodes to certain heat-resistant steels: Base metals: 16Mo3 Filler materials: E Mo R 1 2 13CrMo4-5 E CrMo1 R 1 2 (E CrMo1 B 2 2 H5) 10CrMo9-10 E CrMo2 B 2 2 H5

Characteristics For thicker materials in butt joints (single-U butt weld preparation, so-called U-butt weld), the root is TIG welded, the filler and top beads are frequently welded with a stick electrode for economic reasons. Cracks in welded joints can be caused or influenced considerably by hydrogen. The risk of hydrogeninduced cracking increases with raised alloy content and the level of stresses. Cold cracking susceptibility is also even lower, if the hydrogen content of weld metal is lower. Hydrogen in weld metal is generated e.g. from basic stick electrodes not being appropriately re-baked. Depending on the base material, pre-heat and interpass temperatures of approx. 250 °C to 350 °C as well as post-weld heat treatment, annealing 660 ° - 750 °C, holding 1/2 – 2 hours (in the furnace), are necessary.

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Explanation for the identifiers of DIN EN ISO 3580 - A Table 8: Extract from DIN EN ISO 3580 Identifier for the chemical composition for the classification according to Nominal composition Alloy type (ISO 3580-A) (ISO 3580-B) Mo (1M3)

Chemical composition, % mass fraction

C

Si

Mn

P

S

Cr

Mo

V

Other elementsd

0.10

0.80

0.40 to 1.50

0.030

0.025

0,2

0.40 to 0.70

0.03

—

(Mo)

1M3

0.12

0.80

1.00

0.030

0.030

—

0.40 to 0.65

—

—

MoV

—

0.03 to 0.12

0.80

0.40 to 1.50

0.030

0.025

0.30 to 0.60

0.80 to 1.20

0.25 to 0.60

—

CrMo0.5

(CM)

0.05 to 0.12

0.80

0.40 to 1.50

0.030

0.025

0.40 to 0.65

0.40 to 0.65

—

—

(CrMo0.5)

CM

0.05 to 0.12

0.80

0.90

0.030

0.030

0.40 to 0.65

0.40 to 0.65

—

—

—

C1M

0.07 to 0.15

0.30 to 0.60

0.40 to 0.70

0.030

0.030

0.40 to 0.60

1.00 to 1.25

0,05

—

CrMo1

(1CM)

0.05 to 0.12

0.80

0.40 to 1.50

0.030

0.025

0.90 to 1.40

0.45 to 0.70

—

—

(CrMo1)

1CM

0.05 to 0.12

0.80

0.90

0.030

0.030

1.00 to 1.50

0.40 to 0.65

—

—

CrMo1L

(1CML)

0.05

0.80

0.40 to 1.50

0.030

0.025

0.90 to 1.40

0.45 to 0.70

—

—

(CrMo1L)

1CML

0.05

1.00

0.90

0.030

0.030

1.00 to 1.50

0.40 to 0.65

—

—

CrMoV1

—

0.05 to 0.15

0.80

0.70 to 1.50

0.030

0.025

0.90 to 1.30

0.90 to 1.30

0.10 to 0.35

—

CrMo2

(2C1M)

0.05 to 0.12

0.80

0.40 to 1.30

0.030

0.025

2.0 to 2.6

0.90 to 1.30

—

—

(CrMo2)

2C1M

0.05 to 0.12

1.00

0.90

0.030

0.030

2.00 to 2.50

0.90 to 1.20

—

—

CrMo2L

(2C1ML)

0.05

0.80

0.40 to 1.30

0.030

0.025

2.0 to 2.6

0.90 to 1.30

—

—

(CrMo2L)

2C1ML

0.05

1.00

0.90

0.030

0.030

2.00 to 2.50

0.90 to 1.20

—

—

—

2CML

0.05

1.00

0.90

0.030

0.030

1.75 to 2.25

0.40 to 0.65

—

—

—

2C1MV

0.05 to 0.15

0.60

0.40 to 1.50

0.030

0.030

2.00 to 2.60

0.90 to 1.20

0.20 to 0.40

Nb 0.010 to 0.050

—

3C1MV

0.05 to 0.15

0.60

0.40 to 1.50

0.030

0.030

2.60 to 3.40

0.90 to 1.20

0.20 to 0.40

Nb 0.010 to 0.050

CrMo5

(5CM)

0.03 to 0.12

0.80

0.40 to 1.50

0.025

0.025

4.0 to 6.0

0.40 to 0.70

—

—

DIN EN ISO 3580-A: E

E:Identifier for shielded metal arc welding

CrMo1 B 4 4 H5

Symbol

Hydrogen content in ml/100 g of weld metal max.

H5 H 10

5 10

Code

Type of cover

Code number

Yield %

Type of current

H 15

R B

Rutile type coating Basic type coating

1

≤ 105

Direct and alternating current

≤ 105

Direct current

Code number 1 2 3

2 3

> 105 ≤ 125 Direct and alternating current

4

> 105 ≤ 125

Direct current

5

> 125 ≤ 160

Direct and alternating current

6

> 125 ≤ 160

Direct current

7

> 160

Direct and alternating current

8

> 160

Direct current

4 5

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15 Welding positions in accordance with DIN ISO 6947 In all positions All positions, except the vertical down position Butt weld in flat position, fillet weld in flat and horizontal position Butt weld in flat position, fillet weld in flat position Vertical-down position and positions Like for code letter 3.

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13. Stick electrodes for stainless and heat-resistant steels, DIN EN ISO 3581-A This standard includes a classification for the designation of coated stick electrodes with the help of the chemical composition of the pure weld metal. Composition Stick electrodes can be rutile- or basic-coated; in both cases the stick electrodes are welded at the DC + pole (rutile-coated partly also possible with AC). Both types have alloy rod cores. Alloying elements are indicated as whole percentages without symbols in the order Cr, Ni, Mo. Characteristics The characteristic feature of these stick electrodes is high corrosion resistance, low-temperature toughness to about -200 °C and scaling resistance to about 900 °C Applications Welding of stainless steels for applications e.g. in equipment construction, pipeline construction, steam power plant construction, chemical industry, food industry, etc. Work rules High-alloy stick electrodes must be re-baked according to manufacturer's specifications (approx. 2 hours at 250 °C – 350 °C). Non-observance of this measure will lead to porosity, particularly at the start of the weld. Alloyed-core chromium nickel electrodes are to be welded with low heat-input. The interpass temperature should not exceed 150 °C. Select small electrode diameters and low amperages. Avoid large welding pools to keep the residual welding stresses as low as possible, but perform root welding with sufficiently large cross-section, otherwise stress cracking might occur in longitudinal direction. All weld start- and stops must be carefully removed. Stray arcs, e.g. due to wrongly applied workpiece clamps, are to be avoided because very fine cracks may occur, which can then be the cause of intercrystalline corrosion. High-alloy weld metal can be made considerably more resistant or insensitive to hot cracks by adding manganese and/or molybdenum. On the other hand, niobium, particularly with nitrogen, can favour hot cracking together with contaminations like phosphorus, sulphur and boron. Full-austenitic steels without Mn- and/or Mo-alloy content are especially at risk here. The correct choice of the filler material is very important in this case. Special attention must be paid therefore to the weld preparation and the cleanliness of the weld faces and weld environment. Oil, grease, paint or coatings, etc. must be completely removed. As for the corrosion resistance, the filler material should be as similar as possible or lightly over-alloyed. Excessive alloy differences have a very negative influence, as the lower alloy medium may become under attack. To maintain corrosion resistance, provided by the “passive layer”, it is absolutely necessary to remove all impurities (e.g. oxide layers, scale, tarnish, slag residues and spatters) mechanically by brushing, grinding, spraying or chemically by pickling after welding.

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Safety guidelines When welding basic-coated CrNi stick electrodes, the welding fumes contain chromium VI compounds. Here, welding must be performed using local fume extraction – or with filter protective masks (P3). Highalloy rutile-coated electrodes have significantly lower chromium VI content. Allocation of suitable stick electrodes to certain high-alloyed materials: To ensure the most suitable filler material/alloy type for special applications, it is highly recommended using the Schaeffler-diagram for help. Example: Ferritic chromium steels  base metal: x suitable electrode: x suitable electrode:

X6Cr13 material no. 1.4000 DIN EN ISO 3581-A - E 13 B 42 DIN EN ISO 3581-A - E 18 8 Mn B 1 2

Risks and countermeasures Tendency to coarse-grain-growth and hardening because of martensite formation. In order to avoid cracks in the heat-affected zone (HAZ) and to minimise welding residual stresses, a pre-heat and an interpass temperature of 200 °C to max. 300 °C is to be selected. Weld with low heat-input, prefer small diameters with low currents, favour stringer beads, avoid oscillating weld beads. Annealing after welding (700° - max. 800 °C) improves toughness in the HAZ, reduces residual welding stresses and re-establishes resistance against intercrystalline corrosion. Martensitic chromium steels  basic metal: x suitable electrode: x suitable electrode:

X20Cr13 material no. 1.4021 DIN EN ISO 3581-A - E 13 B 22 DIN EN ISO 3581-A - E 19 9 Nb B 2 2

Risks and countermeasures The risk of cold-cracking is even higher here than with ferritic chromium steels. Martensitic chromium steels have a very poor weldability due to the higher C-content. Up to a carbon content of 0.2%, welding can be performed with austenitic filler materials, above 0.2% carbon content filler materials from DIN EN 14172 shall be used. For welded constructions, these chromium steels above C>0.2% are actually not suitable.  base metal: X30Cr13 material no. 1.4028 DIN EN ISO 14172 - E Ni 6082 (NiCr20Mn3Nb) x suitable electrode: Austenitic steels  base metal: x suitable stick electrodes:  x  x

base metal: suitable electrode: base metal: suitable electrode:

X5CrNi18-10 material no. 1.4301 DIN EN ISO 3581-A - E 19 9 R 1 2 DIN EN ISO 3581-A - E 19 9 Nb R 1 2 X6CrNiMoTi17-12-2 material no. 1.4571 DIN EN ISO 3581-A - E 19 12 3 Nb R 1 2 X2CrNi18-9 material no. 1.4307 DIN EN ISO 3581-A - E 19 9 L R 1 2

Risks and countermeasures Risk of hot cracking. For countermeasures, see working rules.

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13.1 Stick electrodes for black-and-white joints, DIN EN ISO 3581 – A For joining high-alloyed base materials with non- or low-alloy base materials, a weld is generated which consists partially of the two deposited materials and the filler material. Depending on the extent of mixing and the requirements for mechanical-technological properties of the joint, a more or less over-alloyed filler material is required here, in order to avoid any excessive martensitic structure contents. The Schaeffler diagram is considered again here as a valuable help. If non-alloyed filler materials according to DIN EN ISO 2560-A are used in black-and-white joints, hardening will occur in the weld, even if “only” high-alloyed 18-10 filler materials according to DIN EN ISO 3581 - A are being used. For the welding of unalloyed steels (S235) with high-alloyed steels (X6CrNiTi18-10), different types of “over-alloyed” filler materials can be used. Proven alloy types for welding different steel types are: DIN EN ISO 3581 – A E 18 8 Mn6 R 1 2 DIN EN ISO 3581 – A E 23 12 L R 12 DIN EN ISO 3581 – A E 23 12 2 LR 1 2 Example:

(Material no. 1.4370 corresponds to DIN EN 14700 E FE10 also suitable for manganese steels) (Material no. 1.4332) (Material no. 1.4459)

Welding of supports (S235) on container walls from austenitic chromium steel 18-10.

Figure 16: Simple design

Figure 17: Higher-value design

① Un-alloyed S 235 ② DIN EN ISO 3581–A E 18 8 Mn6 R 1 2 ③ High-alloyed X6CrNiTi18-10 ④ DIN EN ISO 3581–A E 19 9 Nb R 1 2 For fillet welds on black-and-white joints, a “coating-alloyed” covered electrode (non-alloyed rod core) is preferred. Identifier MP = metal flux: (Higher sustainable current-load than alloyed-core electrodes). Example: DIN EN ISO 3581 – A E 23 12 MP R 1 2 DIN EN ISO 3581 – A E 18 8 Mn6 MP R 7 3 Welding position Efficiency > 160% © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised transmission are prohibited and shall be legally pursued

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MMA Welding II Explanation for the identifiers of DIN EN ISO 3581 - A Identifier for the classification according to Nominal compositionb,c,d Alloy typed,e (ISO 3581-A) (ISO 3581-B)

C

Si

Mn

P

S

Cr

Ni

Mo

Cu

Nb + Ta

N

0,08 0,08 0,08 0,04 to 0,08 0,04 0,04

1,2 1,2 1,00 1,00 1,00 1,2

2,0 2,0 0,5 to 2,5 0,5 to 2,5 0,5 to 2,5 2,0

0,030 0,030 0,04 0,04 0,04 0,030

0,025 0,025 0,03 0,03 0,03 0,025

18,0 to 21,0 17,0 to 20,0 17,0 to 20,0 17,0 to 20,0 17,0 to 20,0 17,0 to 20,0

9,0 to 11,0 10,0 to 13,0 11,0 to 14,0 11,0 to 14,0 11,0 to 14,0 10,0 to 13,0

0,75 2,0 to 3,0 2,0 to 3,0 2,0 to 3,0 2,0 to 3,0 2,5 to 3,0

0,75 0,75 0,75 0,75 0,75 0,75

8 × C to 1,1 — — — — —

— — — — — —

— (307)

0,20 0,04 to 0,14

1,2 1,2

4,5 to 7,5 3,0 to 5,0

0,035 0,035

0,025 0,025

17,0 to 20,0 18,0 to 21,5

7,0 to 10,0 9,0 to 11,0

0,75 0,5 to 1,5

0,75 0,75

— —

— —

(19 12 3 Nb) 19 13 4 N L 22 9 3 N L (22 9 3 N L) 23 7 N L

318 — (2209) 2209 —

0,08 0,04 0,04 0,04 0,04

1,00 1,2 1,2 1,00 1,0

0,5 to 2,5 1,0 to 5,0 2,5 0,5 to 2,0 0,4 to 1,5

0,04 0,030 0,030 0,04 0,030

0,03 0,025 0,025 0,03 0,020

17,0 to 20,0 17,0 to 20,0 21,0 to 24,0 21,5 to 23,5 22,5 to 25,5

11,0 to 14,0 12,0 to 15,0 7,5 to 10,5 7,5 to 10,5 6,5 to 10,0

2,0 to 3,0 3,0 to 4,5 2,5 to 4,0 2,5 to 3,5 0,8

0,75 0,75 0,75 0,75 0,5

6 × C to 1,00 — — — —

— 0,20 0,08 to 0,20 0,08 to 0,20 0,10 to 0,20

23 12 2 L

(309LMo)

0,04

1,2

2,5

0,030

0,025

22,0 to 25,0

11,0 to 14,0

2,0 to 3,0

0,75

—

—

18 8 Mnc 18 9 Mn Moc

b c d e

Chemical Compositiona % (mass fraction)

(347) (316) 316 316H 316L (316L)

19 9 Nb 19 12 2 (19 12 2) (19 12 2) (19 12 3 L) 19 12 3 L

a

Page 39

Single values are maximum values. Stick electrodes, for which no chemical composition is indicated, are to be coded similarly and marked by symbol Z in front. The ranges for chemical composition are not defined. There is the possibility that two electrodes with the same Z-classification are not interchangeable for each other. The sum of P and S must not exceed a mass fraction of 0.050%, except for 25 7 2 N L; 18 16 5 N L; 20 16 3 Mn N L; 18 8 Mn; 18 9 Mn Mo and 29 9. The designation in parenthesis [e. g. (308L) or (19 9 L)] indicates that the stick electrode is almost, but not completely, in compliance with the other designation system. The right designation for a given composition range is the designation without the parentheses. If a component is present that has a limited chemical composition, which corresponds to both designation systems, it may be equipped with both designations. The stick electrode must be analysed for all specific elements for which the values are indicated. If the presence of other elements is indicated during the course of the analysis, the amount of these elements must be determined, in order to ensure that their total sum (except iron) does not exceed a mass fraction of 0.50%.

DIN EN ISO 3581-A:

E

23 12 2 L R 3 2

E:Identifier for shielded metal arc welding Code

Type of cover

Code number

Yield %

Type of current

R B

Rutile type coating Basic type coating

1

≤ 105

Direct and alternating current

2

≤ 105

Direct current

L: Low Carbon

Code number 1 2 3

3

> 105 ≤ 125 Direct and alternating current

4

4

> 105 ≤ 125

Direct current

5

5

> 125 ≤ 160

Direct and alternating current

6

> 125 ≤ 160

Direct current

7

> 160

Direct and alternating current

8

> 160

Direct current

Welding positions in accordance with DIN ISO 6947 In all positions All positions, except the vertical down position Butt weld in flat position, fillet weld in flat and horizontal position Butt weld in flat position, fillet weld in flat position Vertical-down position and positions Like for code letter 3.

Table 9: Extract from DIN EN ISO 3581-A

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14. Filler materials for hard-facings, DIN EN 14700 Stick electrodes for surface welding are classified and selected according to DIN EN 14700: 1. 2. 3.

alloy identifier according to DIN EN 14700 the hardness of pure weld metal and the weld metal properties according to Table

Only those weld metal properties are listed that are particularly characteristic in addition to the hardness values. Sample identifier of stick electrodes for wear-resistant hard-facings according to DIN EN 14700: Identifier characteristics of the classification Number of EN standard. The first characteristic indicates the product shape, here a coated stick electrode, see table Table 10. Table 10: Identifier for the product form

DIN EN 14700

E

Fe10

Short symbol E S T R B C P

Product form (consumable) Stick electrode Solid wire and solid rod Cored wire and cored rod Cast rod Solid strip Sintered rod, cored strip and sintered strip Metal powder

the second characteristic gives the chemical composition via an alloy identifier, here Fe10 see table 11. Full-austenitic Mn-Cr alloyed stick electrode for high-wear-resistant hard-facings, that are subject to extreme compressive- and impact loads (e.g. Fox BMC from Böhler).

Figure 18: (Work photo by Messer – Lincoln)

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Table 11: Extract from DIN EN 14700 – Alloy symbols and chemical composition Alloy a Suitability symbol

Chemical composition in % (m/m) C  0.4 0.4 to 1.2 0.2 to 0.5 0.2 to 1.5

Cr  3.5 7 1 to 8 2 to 6

 0.5  2.5  0.2 0.2 to 2 0.3 to 1.2

 0.1  10 4 to 30 5 to 18

Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 Fe7 Fe8 Fe9 Fe10 Fe11 Fe12 Fe13 Fe14 Fe15 Fe16

p ps st s t (p) cpstw gps cpt gpt k (n) p c k (n) p z cnz c (n) z g g (c) g gz

Fe20

cgtz

Ni1

cpt

 0.25  0.3  0.08  1.5 1.5 to 4.5 4.5 to 5.5 4.5 to 7.5 Hard b materials 1

Ni2

ckptz

Ni3

Ni – 1 5 4 17 to 22 –

Mn 0.5 to 3 0.5 to 3 3 3 1 3 3 0.3 to 3 11 to 18 3 to 8

Mo 1 1  4.5  10 3 to 5

W 1 1  10  19 – – –

V 1 1  1.5 4 – –

Nb – – – – –

– – Co, Si Co, Ti Co, Al Ti Si Si, Ti Ti Si Cu – B, Ti – B B, Co

Rest Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe Fe

3

3 2  4.5 2  1.5 4 4 4 4 2 9

–

–

–

–

–

–

–

Fe

15 to 30

Residue

0.3 to 1

6

2

1

–

Ni

 0.1

15 to 30

Residue

 1.5

 28

8

1

4

cpt

1

1 to 15

Residue

0.3 to 1

6

2

1

–

Ni4

ckptz

 0.1

1 to 15

Residue

 1.5

 28

8

1

4

Si, Fe, B Co, Si, Ti Si, Fe, B Co, Si, Ti

Ni20

cgtz

–

–

–

–

–

–

–

–

NI

Co1 Co2 Co3

cktz t z (c s) t z (c s)

Hard b materials  0.6 0.6 to 3 1 to 3

20 to 35 20 to 35 20 to 35

 10 4 4

0.1 to 2 0.1 to 2

 10 –

2

1

–  15 4 to 10 – 6 to 14 –

1 – –

Co Co Co

Cu1

c (n)

–

–

6

 15

–

–

–

–

Al1

cn

–

–

10 to 35

 0.5

–

–

–

Cr

gn

1 to 5

Rest

–

1

–

–

– 15 to 30

Fe Fe Fe Al, Fe, Sn Cu, Si Fe, B, Si, Zr

c: g: k: () a b

6 –

 19 17 to 22 18 to 31 17 to 26

3 7 to 11 8 to 20 9 to 26

 6.5 25 to 40 20 to 40 10 to 40

4 4 4 –

–

3 0.5 to 3 0.5 to 3 0.5 to 3 0.5 to 3

 10 1  10 –

Other

2 – – – – – – –

1 2 1 – – – – – –

8

 10

 10  10

stainless n: cannot be magnetized resistant to abrasion p: impact resistant cold hardened s: edge retention may not apply to all alloys of this classification

t: z: w:

 1.5  1.5  1.5 – –

–

Ni Ni Ni

Cu Al Cr

heat-resistant scaling resistant precipitation hardened

Alloys which are not listed in this table are to be coded similarly but the symbol Z is to be put in front. Tungsten fused carbide or tungsten carbide broken or spherical.

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Table 12: Extract from DIN EN 14700 – Suitability of the alloys for different loads Requirement Alloy symbol

Mechanical Friction

Impact

Hardness range

Thermal high temperature

Thermal shock

corrosive

crack resistant

workability

Alloy/microstructure

[HB]

[HRC]

Fe1

3 and 4

2 and 3

4

4

4

1

1

ferritic/martensitic

150 to 450

-

Fe2

3 and 4

2

4

4

4

2

3

martensitic

-

30 to 58

Fe3

3

2

2

2

3

2

2

martensitic + carbide

-

40 to 55

Fe4

2

2 and 3

1 and 2

1 and 2

3

2 and 3

3 and 4

martensitic + carbide

-

55 to 65

Fe5

2

1

1

1

2

1

1

martensitic

-

30 to 40

Fe6

1

1

2 and 3

2 and 3

4

2 and 3

3 and 4

martensitic + carbide

-

48 to 55

Fe7

2

2

1 and 2

1 and 2

1 and 2

1

1 and 2

ferritic/martensitic

250 to 450

-

Fe8

1 and 2

1 and 2

4

4

3

2 and 3

3 and 4

martensitic + carbide

-

50 to 65

Fe9

4

1

4

4

2 and 3

1 and 2

3

austenitic

200 to 250

40 to 50

b

Fe10

4

1

1 and 2

1

2

1

2

austenitic

180 to 200

38 to 42

b

Fe11

4

3

1

4

1

1

1

austenitic

-

-

Fe12

4

3

1

4

1

1

1

austenitic

150 to 250

-

Fe13

1

4

2

4

4

4

4

martensitic/austenitic + FeB

-

55 to 65

Fe14

1

3 and 4

3

4

2

4

4

martensitic/austenitic + carbides

-

40 to 60

Fe15

1

4

2

4

3

4

4

martensitic/austenitic + carbides

-

55 to 65

Fe16

1

4

1

4

3

4

4

martensitic/austenitic + carbides

-

60 to 70

Fe20

1

3

3

4

3

4

4

Hard materials in a Fematrix

1,500 HV to 50 to 60 2,800 HV (hard (matrix) materials)

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Sample applications 1. Digger tooth made from Mn high-strength steel, X120Mn12

Weld Mn high-strength steels as 'coldly' as possible (water bath) - low current - small electrode diameter - short arc - only stringer beads etc. 2. Digger tooth made from low alloy cast steel GE360 (1.0597) ≙ S355J2C (+N) Preheat to 200 - 300 °C

A distinction is drawn between friction- and impact wear. The cracking risk increases with increasing application thickness and surface area. Often a lattice or spot hard-facing application is sufficient (heavy equipment). Pure weld metal is only to be found in the third layer. In case of fewer layers, choose higher values (e.g. hardness) for electrode 1 and 2.

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15. Stick electrodes for repair welding of cast iron, DIN EN ISO 1071 Cast iron materials are weldable and can be controlled with suitable process technology. This is true for both cast iron with flake- or spheroidal (nodular) graphite and for malleable cast iron. The parameters that influence the weld suitability of cast iron types are as follows:      

high C-content embrittlement and low elongation after fracture high residual stresses, and therefore risk of cracking low melting point low viscosity of melt pool high content of P and S.

DIN EN 1011-8 contains valuable recommendations for the welding of cast iron. 15.1 Identification of the electrodes according to DIN EN ISO 1071 Stick electrodes are classified according to their chemical composition as table 15 similar and table 16Table different. Classification entails four characteristics: a) b) c) d)

the first characteristic consists of the identifier for the product; the second characteristic describes the material to be welded (C for cast iron); the third characteristic includes a designation for the chemical composition of the stick electrode the fourth characteristic consists of the code number for the efficiency and the type of current. Designation characteristics of the classification Number of ISO standard Identifier for the product form, here a coated stick electrode, see table 13 Table 13: Identifiers for the product type

DIN EN ISO 1071

E

C

NiFe-1

3

Code E S T R B C P

Product form (consumable) Stick electrode Solid wire and solid rod Cored wire and cored rod Cast rod Solid strip Sintered rod, cored strip and sintered strip Metal powder

Code number for the efficiency and type of current Identifier for the alloy composition, here NiFe1 see table 16. Identifier for the material to be welded, C stands for cast = cast iron

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15.2 Hot welding of grey cast iron with similar filler material The whole component is heated slowly to approx. 600 °C, is being welded, and cooled down slowly in the oven or in hot sand or ashes. Chemical composition of the electrodes and pure weld metal according to DIN EN ISO 1071; Examples as per table 14 and table 15Table: E C FeC-3

Weld metal: Flake graphite; basic-graphitic coating; welding of cast iron with flake graphite

E C FeC-GF (Basic ferrite structures) and E C FeC-GP2 (pearlite basic structure) Weld metal: Spheroidal graphite; basic-graphitic coating; Welding of cast iron with spheroidal graphite and neutral annealed (black) malleable cast iron Table 14: Filler materials for weld metal, similar to the basic metal

Symbol b FeC-1 c FeC-2 FeC-3 FeC-4 FeC-GF FeC-GP1 FeC-GP2 a

Microstructure Flake graphite Flake graphite Flake graphite Flake graphite Basic structure ferritic, spheroidal graphite Basic structure pearlitic, spheroidal graphite Basic structure pearlitic, spheroidal graphite

Form of product E, R E, T E, T R E, T R E, T

a

For identifiers see Table For coated electrodes, the core rod consists of cast iron. For coated electrodes, the core rod consists of unalloyed steel

b c

Table 15: Extract from DIN EN ISO 1071: Chemical composition, similar. Symbol FeC-1

E, R

FeC-2

E, T

FeC-3

E, T

FeC-4

R

FeC-5

R

FeC-GF

E, T

FeC-GP1 R FeC-GP2 E, T Z a b c

d e

Chemical composition %

Form of product

E, R, T

C 3.0 3.6 3.0 3.6 2.5 5.0 3.2 3.5 3.2 3.5 3.0 4.0 3.2 4.0 2.5 3.5

Si to 2.0 3.5 to 2.0 3.5 to 2.5 9.5 to 2.7 3.0 to 2.0 2.5 to 2.0 3.7 to 3.2 3.8 to 1.5 3.0

Mn to to to

0.8 0.8

1.0

to 0.60 0.75 to 0.50 0.70 to 0.6 to 0.10 0.40 to 1.0

P

S

Fe

Ni

d

a, b, c

Cu

e

Other

Resi– – Al: 3.0 dual Resi0.5 0.1 – – Al: 3.0 dual Resi0.20 0.04 – – – dual to 0.50 to Resi0.10 – – – 0.75 dual to 0.20 to Resi- 1.2 to 0.10 – Mo: 0.25 to 0.45 0.40 dual 1.6 ResiMg: 0.02 to 0.10 0.05 0.015 1.5 – dual Ce: 0.20 to ResiMg: 0.04 to 0.10 0.05 0.015 0.50 – dual Ce: 0.20 ResiMg: 0.02 to 0.10 0.05 0.015 2.5 1.0 dual Ce: 0.20 Any other agreed chemical composition 0.5

0.1

Sum otherwise. elements 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Single values are maximum values. The results have to be rounded on the same position as fixed values by using Appendix B, Rule A from ISO 31-0:1992. For weld metal and the electrodes according to this table, the elements indicated in the table are to be applied. If it is shown that other elements are included then their contents shall be determined; this ensures that their total value does not exceed the maximum limit value for “the sum of other elements” in the final column of the table. The value for nickel may contain the accompanying element cobalt. The value for copper may contain the accompanying element silver.

15.3 Cold welding of grey cast iron with a dissimilar filler material

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The advantage of cold welding of cast iron is that in the case of a repair in all positions (except for PG), the welding can be performed without complex pre-heating. In addition, it is often impossible to dismantle or remove the component. The thermal load for the welder is reduced, contrary to hot welding. A disadvantage is the difference in colour of the weld metal. Workpiece preparation    

Use a suitable test method (e.g. liquid penetration test) to locate the cracks precisely. Drill crack-ends to avoid the notch effect and therefore prevent the propagation of the crack. Depending on the wall thickness, bore diameters between 5 mm and 10 mm are to be selected. Remove the crack, preferably thermally rather than mechanically, e.g. with chip-out electrodes. Gouging with compressed air or grinding (with oil or fat-polluted components) are not appropriate, as otherwise when welding gasification and a porous weld metal is to be expected. Completely remove cast skin and all residues (e.g. from chip out, oil, grease, etc.). in the weld areas.

Performing cold welding Predominantly nickel, nickel-iron or nickel-copper filler materials are used according to DIN EN ISO 1071.       

Follow the baking instructions provided by the electrode manufacturers. Select small electrode diameters (2.5 mm or 3.2 mm); start with the smaller one Keep arc-time as less as possible and amperage as low as possible In the pilgrim-step-process, weld short weld sections (rule of thumb: length of weld = core rod diameter x 10; approx. 20 mm – 30 mm length; width max. one electrode diameter) Hammer down each weld bead immediately in the “red-hot” state with the hammer. Grind off pores immediately. The welded component may be only hand-warm max. 60 °C to avoid heat stresses. In single cases (large workpieces) it can be required that the workpiece is being pre-heated to approx. 150 °C and hold this temperature until the completion of the welding operations

For this, the following stick electrodes may be used according to Table: DIN EN ISO 1071 E C ST Unalloyed weld metal for repairing small holes and cracks. Because of the carbon absorption from the cast iron, the weld metal will largely be martensitic and can only be worked on by grinding. E C Ni-Cl-A 1 Basic-graphitic nickel iron electrode with high nickel content. Because of the high phosphorous content in cast iron, the weld metal is more sensitive to hot cracks. Contains more aluminium than the filler material E C Ni-Cl to improve welding characteristics. The alloyed aluminium is dissolved in the weld metal and may reduce toughness. E C NiFe-2 3

Basic-graphitic electrode. Multi-pass welding of cast iron with spheroidal graphite and black malleable cast iron. Mixed joints between cast iron and steel.

E C NiCu 1

Basic nickel copper electrode. Well suited for fill layers in multi-pass welding for bigger weld cross-sections (flake and spheroidal graphite and black malleable cast iron). Good bonding on aged cast iron. Advantage: colour similarity.

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Page 47

Extract from DIN EN ISO 1071: Chemical composition of dissimilar sticks and wire electrodes as well as of weld metal of coated stick electrodes and cored-wire electrodes.

Chemical composition %

a, b, c, d

Sum of other elements

Symbol

Product type

Fe-1

E, S, T

2.0

1.5

0.5 to 1.5

0.04

0.04

Residual

-

-

-

1.0

St Fe-2

Ni-Cl-A NiFe-1

E, S, T E, T E S E E, S, T

0.15 0.2 2.0 1.0 2.0 2.0

1.0 1.5 4.0 0.75 4.0 4.0

0.80 0.3 to 1.5 2.5 2.5 2.5 2.5

0.04 0.04 0.03

0.04 0.04 0.03 0.03 0.03 0.03

Residual Residual 8.0 4.0 8.0 Residual

min. 85 min. 90 min. 85 45 to 75

0.35 2.5 4.0 2.5 4.0

Nb + V: 5.0 to 10.0 Al: 1.0 Al: 1.0 to 3.0 Al: 1.0

0.35 1.0 1.0 1.0 1.0 1.0

NiFe-2

E, S, T

2.0

4.0

1.0 to 5.0

0.03

0.03

Residual

45 to 60

2.5

NiFe Cl NiFeT3-Cl

E T

2.0 2.0

4.0 1.0

2.5 3.0 to 5.0

-

0.04 0.03

Residual Residual

40 to 60 45 to 60

2.5 2.5

Al: 1.0 Carbide forming: 3.0 Al: 1.0 Al: 1.0

NiFe-Cl-A

E

2.0

4.0

2.5

-

0.03

Residual

45 to 60

2.5

Al: 1.0 to 3.0

1.0

E

2.0

1.0

10 to 14

-

0.03

Residual

35 to 45

2.5

Al: 1.0

1.0

NiCu NiCu-A

S E, S E, S

0.50 1.7 0.35 to 0.55

1.0 1.0 0.75

10 to 14 2.5 2.3

-

0.03 0.04 0.025

Residual 5.0 3.0 to 6.0

35 to 45 50 to 75 50 to 60

2.5 Rest 35 to 45

Al: 1.0 -

1.0 1.0 1.0

NiCu-B

E, S

0.35 to 0.55

0.75

2.3

-

0.025

3.0 to 6.0

60 to 70

25 to 35

-

1.0

Z

E, S, T

any other agreed chemical composition

Ni-Cl

NiFeMn-Cl

a b c

d e f

C

Si

Mn

P

S

e

Fe

Ni

Cu

f

Other

1.0 1.0 1.0

Single values are maximum values, if not stated otherwise. The results have to be rounded on the same position as fixed values by using Appendix B, Rule A from ISO 31-0:1992. For weld metal and the rods according to this table, the elements indicated in the table are to be applied. If it is shown that other elements are included then their contents shall be determined; this assures that their total value does not exceed the maximum limit value for “the sum of other elements” in the final column of the table. Some copper-tin filler materials are not included in the table; they may successfully be used for braze welding of cast iron. Weld metal differs from cast iron in its colour. The value for nickel may contain the accompanying element cobalt. The value for copper may contain the accompanying element silver.

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16. Special electrodes 16.1 Cutting/chip-out electrodes Stick electrode with special coating for chipping out, gouging, piercing and cutting (scrap cut) without oxygen. The metal is melted by the arc and through the strong gassing from the special coating, it is blown out. Application  Easy joint preparations  Gouging of root welds  Removal of surplus weld metal  Gouging of cracks for repair welding (see 15.315.3.) etc. Keep electrode vertical, until the arc ignites. Afterwards tilt by an angle of 15 - 20°. Push forward with sawing movement, so that the molten material is blown up forward. Repeat for deep grooves. The chip out speed is about 100 - 150 cm/min. For high-alloyed steels, the carbonised layer edge area must be removed in the cut zone. An extractor is required for indoor applications because of the strong fumes produced. 16.2 Cutting electrodes - hollow stick electrodes Special electrode holder with current- and and compressed air supply (5 bar) required. Also for coarse cutting of parts which are lying upon each other with overlap. Underwater cutting is possible with water-repellent protective layers on the electrode coating. Welding equipment: DC + pole on the electrode, per mm electrode diameter approx. 50 A. Strong sparks and fumes produced. 16.3 Carbon arc electrodes (usually copper plated with arc-air process) Special electrode holder with current and compressed air supply (min. 5 bar) necessary. Application  To use for non-, low and high-alloyed steels  Gouging for back run welding  Gouging of weld errors  Cutting of welded parts  DC + pole on the electrode  Per mm electrode diameter ca. 50 A  Strong fume production  Loud 16.4 Preheating electrodes For pre-heating work in common practical use; there is no metallic weld metal; strong fume production.

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Welding procedures

17.1 Joint preparation 17.1.1 Butt welds Table 17 lists the most important types of joint preparation for butt welds. DIN EN ISO 9692-1 gives additional recommendations for the joint preparation. Chamfering the joint edges is usually achieved by flame cutting (steel) or plasma cutting (CrNi steels); other mechanical and thermal processes like turning, milling, gouging torches, etc. are also possible. It is also recommended that the oxide layers resulting from flame cutting should be removed before manual arc welding. Coatings, paints etc. should be removed from the weld area, too. For smaller pipe diameters in pipeline construction, the joint preparation mostly takes place through grinding with angle grinders. Clean joint preparation facilitates welding operations and influences welding speed positively. A single-U butt weld preparation is often used for thick-walled pipes, in hydraulic equipment and power plant construction, for example. This preparation is more economical due to less weld volumes. Table 17: Weld joint preparation, manual metal arc welding of steel

Workpiece Type of joint thickness Weld layout preparation s / t mm up to 3

Distance b mm

Root face thickness c mm

Included angle, α, β

Remark

≈t

3

≤4

≤2

60°

> 10

1≤b≤3

≤2

60°

> 12

1≤b≤3

h≈4

60° ≤ α ≤ 90° 8° ≤ β ≤ 12°

6≤R≤9

17.1.2 Fillet welds The most common welds used in steel constructions are fillet welds. Generally, they are applied as isosceles weld shapes. Fillet welds usually require no special joint preparation, but the distance b between the two components should be kept as short as possible. In the unfavourable case, slag can run via the gap to the weld pool and thus lead to weld defects.

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Table 18: Fillet weld preparation

Preparation

Type of joint preparation

Weld structure

Gap b mm

Angle α

Remark

Fusion face rectangular

≤2

70°≤ α ≤100°

to aim for b=0

Fusion face rectangular

≤2

60°

to aim for b=0

Fusion face rectangular

≤2

60°≤ α ≤120°

17.2 Stick electrode handling and weld layout Stick electrode handling and weld layout are based upon:  Base material and weld thickness,  Type of joint preparation and weld layer,  Weld position,  Magnetic Arc-blow,  Heat effect of the welding arc,  Coating type and the thickness of coating of the stick electrodes.

1. Forward movement 2. Possible weaving movement 3. Feeding movement

Figure 19: Stick electrode control

The stick electrode is tilted to approx. 10° in the direction of welding. The best results are achieved by positioning the stick electrode perpendicular to the workpiece. The magnetic arc blow (see 17.3) may require the angle to be changed.

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Stringer and weave weld beads When the stick electrode is being moved quickly with minimal oscillation, we refer to stringer beads. When the stick electrode is moved in an oscillating action, we refer to weave weld beads. Figure 20: Stringer and weave weld beads

The following recommended working methods are intended as a first point of reference. This is valid for stick electrodes. Fillet welds in flat position (PA) Root pass Fill run (-layer with stringer beads) Cap run (-layer with stringer beads) Figure 21: Weave beads

Figure 22: Stringer beads

Fillet weld horizontal (PB) Root pass Fill run (-layer with stringer beads) Cap run (-layer with stringer beads)

Figure 23: Examples of the layer structure

Fillet welds in vertical up position (PF) Max. 2x electrode Ø

Open

Closed

Triangular weaving Root fillet weld

Weaving pattern Final pass

Figure 25: Stick electrode handling fillet weld PF-position

Fillet weld PF-position Root pass Cap run (layer) Figure 24: Layer sequence

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Butt welds in welding upward position (PF)

Electrode handling for the butt weld root pass and fill layers

Weaving pattern Final pass

Root pass Cap run (layer) Figure 27: Weld layout butt weld

Fillet welds in overhead position (PE)

Root pass 1st . fill layer 2nd. fill layer Cap layer

Figure 28: Stringer beads PE V-butt weld in horizontal position (PC)

Root pass 1st . fill layer 2nd. fill layer 3rd. fill layer Cap layer

Figure 29: Stringer beads PC

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17.3 Magnetic Arc blow Like every electrical conductor, the arc is also surrounded by a magnetic field. If the magnetic field is prevented from expanding evenly, the arc is deflected: this is the so-called (magnetic) arc blow effect. Frequent reasons for arc blow are: 

Welding at the edge of the workpiece



Welding next to large workpiece masses



Welding close to a work-clamp connection

a) Welding at the edge of the workpiece

b) Welding next to large workpiece masses

c) Welding close to the work-clamp connection Figure 30 a, b, c: Causes of arc blow

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The following measures for preventing magnetic arc blow: – – – – – –

Keep arc length short Change the angle of the stick electrode Attach an earth clamp on both sides or shift the earth clamp Make numerous tack welds Select the correct welding sequence If possible use AC rather than DC current

Figure 31: Altered angle

17.4 Efficiency of stick electrodes

Efficiency in % 

Weight of weld metal  100% Weight of melted core rod

The efficiency can be increased over 100% by adding more iron powder into the coating. Example: Efficiency 105% Ø 4 mm  450 mm

Efficiency 160% Ø 4 mm  450 mm

Weight of the core rod about 40 g coating without iron powder

Weight of weld metal equals the weight of the molten core rod.

Weight of the core rod about 40 g coating contains iron powder (approx. 25 g)

Weight of weld metal is about 60% greater than the weight of the deposited core rod (core rod + iron powder).

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a-size 105% a-size 160%

Figure 32: Change of a-size with high-efficiency stick electrodes with hardly modified run-out length High-efficiency stick electrodes can be welded in horizontal position, partly only in flat position (PA). In addition, the current is to be increased compared to stick electrodes with a normal efficiency. Iron- or metal powder in the stick electrode coating increases the metallic efficiency and deposition rate. 17.5 Selecting a favourable welding position All fusion welding processes achieve the highest deposition rates, a good weld structure and a deep penetration in the flat position (PA). Table 19 shows how the weld position influences production time according to Malisius. Figure 33 "Comparison of manual metal arc welding times based on the flat position PA” shows the compares of approximate working times for shielded metal arc welding related to the flat position (PA) with the same welding cross-section (according to Aichele). Figure 34 shows the deposition rates of different fusion welding processes.

PA

PB

PC

PF

PD, PE

100%

130%

180%

220%

220 to 250%

Figure 33: Comparison of manual metal arc welding times based on the flat position PA

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Table 19: Influence of the welding position on the production time Welding position

Number of beads

Stick electrode diameter mm

Pure welding time min/m

Production time min/m

Comparison %

Fillet welds, a-size 6 mm horizontal

(PB)

2

5

8.5

15.7

100

flat position

(PA)

1

5

8.1

14.2

90

vertical up (uphill)

(PF)

2

4

12.4

24.2

154

vertical down (downhill)

(PG)

3

4

12.6

24.7

158

overhead

(PE)

5

4

13.0

34.0

217

Butt welds, 8 mm Single-V edge preparation without back-welding of the root horizontal, flat

(PA)

2

4

13.7

25.0

100

perpendicular to vertical wall

(PC)

5

3.25/4

16.2

31.2

125

vertical up (uphill)

(PF)

2

3.25/4

16.3

31.3

126

vertical down (downhill)

(PG)

4

3.25/4

16.5

31.7

127

overhead

(PE)

5

3.25/4

20.0

54.0

216

18.

Economics

Today welding with stick electrodes is mainly used in applications where “heavy-duty welding processes” are either out of the question, or are not cost efficient. This is particularly the case when welding outdoors on building sites. Simple equipment or good weld metal quality are often an advantage. Figure 34 compares deposition rates. Under comparable conditions between MAG welding and welding with stick electrodes, MAG welding will come off best in a pure comparison of deposition rates. Nevertheless, there are many areas of welding technology where stick electrodes are the answer, e.g. repairs, installation welding, welding in the open, vertical down welding in pipeline construction and also many tasks in container and plant construction, as the examples show.

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Covered electrodes

SFI / IWE 1.09-1 Page 57

Shielded Metal Arc Welding

Figure 34: Deposition rates of different fusion welding processes (Aichele).

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Vertical down welding

Manual metal arc welding with stick electrodes using the vertical down welding technique has become the most economic manual welding method in underground pipeline construction. For regulations see also /4/ /5/ /6/ and /7/ (regulations of the DVGW, API and TÜV) Main influencing factors are: 

pipe base material and the pipe dimensions,



the stick electrodes used, the welding power sources and the equipment,



the personnel conditions: trained and experienced welders and an experienced welding supervisor,



technical welding expertise experience during production planning, the welding process, the weld post-processing and the inspection technology as well as



the local conditions and the weather conditions.

Figure 35: Construction site - installing a natural gas pipeline in Germany

19.1 Coated stick electrodes for pipe-line construction In underground pipeline construction, approx. 85% cellulose coated vertical-down weld electrodes are used in Germany. However, basic coated vertical down weld electrodes are also used, mostly for high-strength pipe steels. This section discusses vertical down welding with cellulose coated stick electrodes. The cellulose-coated stick electrodes (for properties see also 10.3) create a sharp and penetrating arc due to the organic elements in the coating in combination with a defined residual moisture (should not be redried) and little slag which allows welding in downward position. The shielding gas atmosphere

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consists of carbon and hydrogen. The existing hydrogen favours vertical down welding, however, it leads to increased hydrogen content in the weld metal. For a faster hydrogen emission (effusion) after welding, it is necessary to preheat the pipes between 40°C and 150°C (depending on the wall thickness, 5mm – 25mm) before welding; this also reduces the risk of underbead cracking. The cellulosic stick electrodes have thin to medium thick coatings. The following stick electrode types are mainly used: DIN EN ISO 2560-A E 42 2 C 25 (e.g. Thyssen Cel 70) For all pipe welds/runs in downward position. Particularly suitable for root runs (also uphill). DIN EN ISO 2560-A E 46 3 C 25 (e.g. Thyssen Cel 80) For all pipe welds/runs in downward position. For root runs, Hotpass, fill and cover passes. DIN EN ISO 2560-A E 50 3 1Ni C 25(e.g. Thyssen Cel 90) For all pipe welds/runs in downward position. Particularly suitable for Hotpass, fill and cap layers.

19.2 Welding power sources It is also important to choose the right “vertical down weld safe” welding power source. Cellulose-coated stick electrodes asking special requirements for the welding power sources. These are:



a steep-falling power characteristic curve with the highest possible open circuit voltage (< 80 V to 90 V), pure direct current, with low harmonic wave content, an adjustable current increase near short-circuit-situations, ArcForce (see figure 36),



a remote setting option, allowing a welding current to be set as a function of the welding position and



pole reversibility: - pole for root welding + pole for Hotpass, fill and cap layers.

 

These conditions are often satisfied by mobile welding aggregates (diesel/petrol engine plus DC generator). There are also “vertical down weld-safe” welding inverters available in the selection of power sources.

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Figure 3613: Power characteristic of a welding power source with short circuit current increase for vertical down welding (Source: EWM Hightec Welding GmbH)

19.3 Working methods for vertical down welding with cellulose-coated stick electrodes Vertical down welding with cellulose-coated stick electrodes requires some specialities in the working technique. The following requirements are discussed below: 1. 2. 3. 4.

Joint preparation Working techniques, handling stick electrodes Welding the root run (rootpass) Welding the filler beads and the top run)

19.3.1 Joint preparation For pipes with wall thicknesses up to approx. 20 mm, a single-V butt weld preparation is installed with root face and an opening angle of 60° according to figure 37 at the factory location. (For pipes with a nominal diameter of up to 80 mm, it is often sufficient to have a bevel angle less than 60°). Pipe pieces or segments must be prepared accordingly by hand. Clean joint preparation is essential for fast, economical welding. Tolerances for the joint preparation must be respected and the in this way prepared weld must not be damaged during transport or on site. Welding is conducted with the use of internal or external centring equipment without tack welds. The centring equipment can be removed if, for external centring at least 60% of the root run, for internal centring the root run and the Hotpass need to be completed. Details to the type and the use of the centring equipment are found in the welding procedure specification.

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If no centring equipment can be used, tack welding shall be carried out with the welding process intended for root run. At least three tacks must be distributed uniformly along the pipe's perimeter. The maximum distance should not exceed 400mm or 25 x T. The tack welds should be at least 25 mm for pipes ≤ DN 400 and at least 50 mm for pipes > DN 400. Cracked tack welds may not be over-welded but are to be ground and re-welded /3/. 60°

1,5 + 0,8 1,5 + 0,5 Figure 37: Single-V butt weld preparation for Figure 38: vertical down pipe welding

Internal centring equipment for vertical down welding of underground pipes

19.3.2 Working techniques, handling stick electrodes Vertical down welding starts from the 12 o'clock position and moves downwards on both sides. The cellulosic coated stick electrodes are used almost vertically (about 10 angle), as shown in figure 39. Holding the stick electrode like this, a round welding hole is formed and the arc burns more on the inside of the pipe during root welding.

Cap layer

Figure 39:

Relationship between stick electrode angle and Figure 40: welding position

Vertical down welding with cellulose coated stick electrodes

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19.3.3 Weld run layout

Cap layer

Fill- /medium layers

Figure 41: Weld layout

1 2 3–8 9

Root run Hotpass (2nd run or hot run) Fill- /medium layers Cap layer

minimum number of layers 

t 3

Table 20: Technical parameters for vertical down welding based on the weld layout of figure 41.

Wall thickness El.-Ø [mm] [mm]

Root run Is [A]

Hotpass

Curren t type, El.-Ø polarit [mm] y

Fill layer

Cap layer

Is [A]

Current type, polarity

El.-Ø [mm]

Is [A]

Current type, polarity

El.-Ø [mm]

Is [A]

Current type, polarity

3–4

2.5

50– 80

=/-

3.25

120– 140

=/+

3.25

100120

=/+

3.25

80100

=/+

4–6

3.25

80110

=/-

4.0

180190

=/+

5.0

160180

=/+

5.0

140160

=/+

6 – 10

4.0

120140

=/-

4.0

170190

=/+

5.0

180200

=/+

5.0

160180

=/+

10 – 15

4.0

140160

=/-

4.0

170190

=/+

5.0

180220

=/+

5.0

170200

=/+

15

4.0

140160

=/-

5.0

190210

=/+

5.5

220250

=/+

5.0

170200

=/+

19.3.3.1 Welding the root run (rootpass) The stick electrode, set to the – pole, has to be forwarded without weaving with such a speed that the weld metal can develop a closed weld bead above the round welding hole. The arc “blows” through the gap and also melts the root's side. The two edges have to be captured by the root run. After the root welding, lateral slag is ground off, and the weld metal elevation in the middle of the weld is ground flat. For pipe diameters > 400mm, it is usual to have two welders working simultaneously on opposite sides to avoid distortion and to keep the groove width constant.

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19.3.3.2 Welding the filler beads and the top run The first filling run, the Hotpass is welded with the maximum current specified by the manufacturer for this stick electrode diameter on the + pole. The root is partly re-melted again, is being “through-annealed” and slag residues are being removed by both the specific stick electrode positioning (6 o'clock and 12 o'clock position approx. 80° - 90°, 3 o'clock and 9 o'clock position approx. 45°) and the high current. The Hotpass must be performed immediately after the welding of the root, i.e. be welded in “hotsituation”. No more than 10 minutes can be allowed to elapse between welding of the root and the hotpass for high-strength tubular steels. Also, until the hotpass is finished, the pipe must not be moved, otherwise there is a high risk of underbead cracking. For the further fill layers, the current is slightly reduced, and the whole width of the seam is welded with slightly weaving or stepping movements. The weaving width must not exceed 3 times the stick electrode core rod diameter. Compensating runs may be required in the pure vertical down positions (2 – 4 o'clock and 10 – 8 o'clock), in order to keep the weld thickness uniform. (Higher welding speed and lower material input). The cap layer is also welded on the + pole, but the current intensity is reduced by 20 to 30 A (see also Table 20). The weld upper faces are over-welded up to 1.5 mm. If performed correctly, the reinforcement of the weld is about 1 to 2 mm. Pores occur here if the weld metal is overheated or the weaving width is too large. After completing the welding tasks, the weld should be covered for another approx. 30 minutes at 150 C to accelerate the effusion of the hydrogen.

19.4 Summary Vertical down welding in pipeline construction: Special cellulose vertical down weld electrodes, “vertical down weld-safe” welding power sources and weld-positioning equipment are required. The vertical down welding requires a separate welder certificate, with special requirements for the DVGW range; these are regulated in the DVGW worksheet GW 350, for example. Preparation of the welds is slightly more complex and requires greater care. Welds must be finished by welding “in heat” to increase the effusion of the hydrogen. The interpass temperature should not fall below 80°C, or 150°C for high-strength pipes. With the right technique, X-ray-safe welds are achieved with good to excellent weld quality values of the weld metal, with a high welding speed and a high deposition rate. The heat-input is much lower than for vertical up welding. The welds have a slightly coarse-flaked appearance.

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Weld imperfections and their possible causes

20.1 Slag inclusions Possible causes:   

current intensity inadequate (too low), welding speed too high slag residues have been welded over when welding several runs

20.2 Gas inclusions (pores) Possible causes:   

dirty workpiece surface (rust, grease, coating materials), arc too long, basic-coated stick electrodes not dry enough

20.3 End crater Possible causes:  

stick electrode removed too quickly from the melt, especially in case of large welding currents, risk of shrinkage cracks

20.4 Cracks in weld transition Possible causes:  

material is unsuitable, cooling down (quenching, insufficient protection against surrounding conditions) too soon after welding

20.5 Root fault Possible causes:  

slag has penetrated root area due to excessive distance from end face current intensity too low, slag-leading in front of melt pool

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20.6 Undercuts Possible causes:    

excessive welding current, stick electrode held too obliquely, arc too long, moving too fast on weld edges

20.7 Hydrogen-induced cracks The risk of hydrogen-induced cracking arises from a hydrogen embrittlement, i.e. the ductility change of the metal arises from:  Hydrogen absorption and settlement into the metal lattice.  In combination of (tensile-) stresses and critical micro-structures (imperfections) Hydrogen-induced cold cracking is one of the most dangerous welding defects. It is not easy to detect immediately after welding and often occurs not until 3 to 20 hours later. Possible sources of hydrogen during manual metal arc welding are:  Work piece  Ambient air  Covered stick electrode Wrongly prepared work pieces in relation to remaining primer-, painting- or coating- residues in the near surrounding of the weld area, are potential hydrogen sources. Also inadequate or wrongly drying or preheating of the weld area can lead to increased hydrogen contents. Acetylene (C2H2) is much more suitable as a fuel gas for the drying process or pre-heating process than propane, for example (C3H8). The absorption of hydrogen from ambient air should not be forgotten, either. The high arc energy can cause the moisture from the ambient air in the arc to be partially separated into atomic hydrogen and absorbed in the weld pool. Thus, particularly when using basic stick electrodes, short arcs should be used in welding to reduce the arc surface and therefore also the absorption surface for the hydrogen. Example: In the case of basic stick electrodes with hydrogen content of max 5 ml/100 g of weld metal (H5), the diffusible hydrogen content is in the weld metal, at an arc length of 0.5 x core rod diameter, at about 4.5 ml. An extension of the arc to 1.5 times the rod core diameter increases the hydrogen content to 6 ml/100 g. As the atmospheric moisture rises, the absorption of hydrogen may increase further with a long arc depending on the air temperature. Often, however, the stick electrode is the main moisture source. Selecting the wrong coating type, if a lower hydrogen absorption is required, can be the primary reason for subsequent defects.

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Where the hydrogen content of basic coated stick electrodes (5ml- to 15ml/ 100 gr weld metal) is still unproblematic, hydrogen contents of 20 – 40 ml for comparable RB-electrodes or 40 – 60 ml/100gr weld metal for C-electrodes are highly critical. Failure to (adequately) re-dry basic stick electrodes is, however, often the main cause of hydrogen induced cracks. Some welders still tend to believe that moisture will “evaporate” in the arc, like water droplets on a hot plate. As a result, stick electrodes that have not been (adequately) dried are used for welding, or the stick electrodes are incorrectly stored before welding, causing the hydrogen content to increase, which can lead to hydrogen-induced cold cracking. This hydrogen is embedded at the ends of notches and gaps and leads to an increase of the stress condition. This again lets the crack grow and allows still more H2 to diffuse. The combination of hydrogen, less deformable microstructures in the HAZ and a multi-axial stress condition, e.g. at surface notches can lead to a delayed fracture in case of high-strength steels. Counter measures: Cracking can be prevented:     

A perfect weld preparation Preheating Application of hydrogen reduced, correct dried and stored filler materials Compliance to the work procedures / welding procedures, e.g. multi-layer welding with “Cel” (see also 19.3) Work piece annealing for one or more hours at about 250 °C immediately starting out of the welding heat (soaking).

Figure 42: Fish eyes

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21. Bibliography/sources /1/ /2/ /3/ /4/ /5/ /6/ /7/ /8/ /9/

Killing. Handbuch der Schweißverfahren, Teil 1. s.l. : DVS-Verlag Düsseldorf. DVS. Merkblatt 0957: Umgang mit umhüllten Stabelektroden – Transport, Lagerung und Rücktrocknung umhüllter Stabelektroden. VDE 0470-1, Schutzarten durch Gehäuse (IP Code). 2000. American Petroleum Institute. API Standard 1104. DVGW GW 301. DVGW GW 350. VdTÜV - Merkblatt 1052 Aichele, calculation and economics BGI 616 “Beurteilung der Gefährdung durch Schweißrauche“

22. Question (1)

How should basic electrodes be used in welding?

    (2)

Which characteristic defines the yield point for E 38 2 RB 12 stick electrodes?

    (3)

C content Cellulosic type coating Rutile cellulosic type coating Rutile basic type coating

Which electrode type is best suited for welding low alloyed steel cast GE360 (1.0597)?

    (5)

E 38 2 RB

What does the code letter C mean in case of a stick electrode E 42 2 C 25?

    (4)

With alternating current Predominantly downward With long arc After re-drying (e.g. 2 h. with 300° - 350 °C)

DIN EN ISO 3581 – E 23 12 2 LR 12 DIN EN ISO 2560 – E 38 4 B 22 DIN EN ISO 2560 – E 38 2 RB 12 DIN EN ISO 1071 – E C NiFe-1 3

Which method is recommended for cold-welding of a grey cast iron component?

   

Slowly pre-heat component to 600° C, slowly cool after welding. Weld component in water pool Weld component with the pilgrim step process Weld component “under heat”.

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(6)

What is the meaning of the letter T when identifying stick electrodes DIN EN ISO 18275-A E 55 3 MnMo B T 4 2 H10 ?

    (7)

Protection of the core wire Reduction of the blast effect Generation of a shielding gas Stabilisation of the arc

What do the coating thickness and composition influence?

    (12)

Core wire diameter x 40 in amperes Core wire diameter x 40 in volts High voltage, low current Depending on the coating approx. 100 A

What is the role of the coating on stick electrodes?

    (11)

0.5 x core wire diameter 1.0 x core wire diameter 1.5 x core wire diameter The arc length is irrelevant

What is the formula for current with stick electrodes?

    (10)

Voltage Current Resistance Arc length

How long should the arc be maintained when welding with rutile electrodes?

    (9)

Special chemical composition of coating Filler material stabilised with titanium The filler material is a core stick The strength properties are only achieved after stress relieving.

Which welding parameters do you set for manual metal arc welding?

    (8)

Page 68

The droplet size and the penetration The gap bridging ability The appearance of the weld No influence

Which marking must a welding power source carry, if it is to be used in areas of increased electric danger

 

CE mark GS mark

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PA PB PF PD

They are redried at approx. 150 °C for 1 hour. They are redried at 100 °C for approx. 2 hours. No further treatment. They are redried according to manufacturer specifications (approx. 2 hours at 300°C).

Name the causes of undercuts?

    (18)

Weld with a long arc Change the angle of the stick electrode Keep arc length short Weld with Direct Current

What happens to basic stick electrodes prior to welding…?

    (17)

Dry the stick electrodes Weld at a “single temperature” Hammer off the weld Do not produce the seam in a single action.

Which welding position is recommended for a stick electrode with 200% efficiency?

    (16)

S, K or 48 V Ü – mark

Which measures are available against arc blow?

    (15)

Page 69

Name a practical solution for expelling the hydrogen that enters the coating in case of vertical down welding?

    (14)

SFI / IWE 1.09-1

Current too low Arc too short Excessive welding current Stick electrode removed too quickly from the end of joint

Name the causes of slag inclusions?

   

Moistened stick electrodes Current too high Current too low Root gap too large on fillet welds.

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Submerged arc welding

Chapter 1.10:

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Submerged arc welding

Contents 1 2

3 4

5

6

7

General Introduction to Submerged Arc Welding......................................................................... 3 1.1 Principle of submerged arc welding ........................................................................................... 3 Construction of a SAW welding equipment .................................................................................. 6 2.1 Welding equipment .................................................................................................................... 6 2.1.1 Current (welding) contact tip .......................................................................................... 7 2.1.2 Wire feed device ............................................................................................................ 7 2.1.3 Wire coils ...................................................................................................................... 7 2.1.4 Powder supply and extraction device ............................................................................. 7 2.1.5 Control ........................................................................................................................... 7 2.2 Travel speed device (Relative movement between the element and the welding head) ............. 9 2.3 Power Supply .......................................................................................................................... 10 2.3.1 Regulation of the Arc Length ........................................................................................ 10 Weld preparation ........................................................................................................................... 11 Filler- and auxiliary materials ....................................................................................................... 12 4.1 Electrodes................................................................................................................................ 12 4.1.1 Wire Electrodes ............................................................................................................ 12 4.1.2 Strip electrodes ............................................................................................................ 15 4.2 Welding Flux ............................................................................................................................ 15 4.2.1 Tasks of the welding powder ........................................................................................ 15 4.2.2 Classification of Welding Flux (Powders) ..................................................................... 16 4.2.3 Storage and re-drying of flux ........................................................................................ 20 4.2.4 Identification and designation of welding powders ........................................................ 20 4.3 Wire / flux combination............................................................................................................. 22 4.3.1 Metallurgical behaviour ................................................................................................ 22 4.3.2 Designation of a wire/flux combination ......................................................................... 22 Procedure parameters, weld pool backing.................................................................................. 23 5.1 Deposition rate......................................................................................................................... 23 5.2 Influence of the electrode position on penetration depth and weld geometry ........................... 23 5.2.1 Flux granularity ............................................................................................................ 23 5.3 Weld pool backing system ....................................................................................................... 23 Options to increase performance in submerged arc welding .................................................... 24 6.1 Increase in output in single wire SAW ...................................................................................... 25 6.2 Performance increase for multiple-wire SAW ........................................................................... 26 6.2.1 Double wire welding with shared feed system .............................................................. 27 6.2.2 Multi-wire welding using separate feed systems ........................................................... 28 6.2.3 Multi-wire welding with additional cold/hot wire ............................................................ 29 6.2.4 Multi-wire welding with cold wire filler and/or metal powder .......................................... 30 Process variants ........................................................................................................................... 30

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7.1 Submerged arc welding with strip electrode ............................................................................. 30 7.1.1 SAW Joint welding ....................................................................................................... 30 7.1.2 SAW surface layer welding .......................................................................................... 30 7.2 Narrow Gap Submerged Arc Welding ...................................................................................... 31 7.3 Horizontal welding with Submerged Arc Welding ..................................................................... 33 7.4 Shape welding ......................................................................................................................... 34 8 Imperfections in submerged arc welding and corrective measures ......................................... 35 9 Applications of submerged arc welding ...................................................................................... 38 9.1 Shipbuilding ............................................................................................................................. 38 9.2 Vessel construction .................................................................................................................. 39 9.3 Pipe welding ............................................................................................................................ 41 10 Application of submerged arc deposition welding ..................................................................... 43 11 Test questions............................................................................................................................... 45 12 Bibliography .................................................................................................................................. 47

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1

SFI / IWE 1.10 Page 3

General Introduction to Submerged Arc Welding

Definition of submerged arc welding according to DIN EN 14610, DIN ISO 857-1 and DIN1910-100 According to DIN 1910 Part 2 Submerged arc welding is part of the “covered/submerged” arc welding processes. The ignited arc is protected from the atmosphere by a flux which is located between the electrode and the workpiece (in special cases also between two electrodes) in a welding cavity filled with gases (CO, CO2, CH4 and H2) and vapours; these are generated partially from the melting of the surrounding flux and the evaporation of flux components. At the same time, the electrode (filler material) is also melted and transferred into drops to the molten base material. Here the filler material and the base material are melted by the arc and forming a weld pool, that solidifies during the course of the welding into a weld bead. The weld is created by the relative movement between welding head and workpiece. Slag is deposited on the weld and supports weld forming. The non-melted welding powder is sucked off and can be supplied into the flux circuit again.

1.1

Principle of submerged arc welding Section enlargement

Figure 1: Principle of submerged arc welding

The cross-section in Figure 1 shows that the weld pool is covered by the powder which results in a very good thermal efficiency grade, which leads to a highly efficient melting performance. During the submerged arc welding process there are metallurgical processes active in the drip stadium, in the weld pool reaction and in the dilution area with the base material. The reactions in the drop stage and in the weld pool are determined by the welding flux and the filler material. The effects of the welding filler and the base material can be found in the dilution area which makes the chemical composition of the weld metal and its mechanic-technological characteristics largely dependent on the used wire/powdercombination, which consequently always has to be adjusted to the base material and the requirements of the weld. In Figure 2, deposition rates of conventional welding processes are compared to submerged arc welding.

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Figure 2:

Page 4

Comparison of the deposition rates of different welding methods

The usual deposition rates for submerged arc welding with a wire electrode diameter of 4 mm is 7 to 8 kg/h with a duty cycle of 100%. Submerged arc welding is a fully mechanized high performance method, which is normally used for material thicknesses of 5 mm and above. Table 1 Some typical areas of application for submerged arc welding Table 1: Application examples for submerged arc welding Industry

Type of components

Shipbuilding

Kind of Seam

Panel production Section construction

Butt- and fillet welds Butt- and fillet welds

Pipe production

Pipes Structural pipes Coiled pipes

Longitudinal and circumferential welds Longitudinal and circumferential welds circumferential welds

Tank/vessel construction

Chemical reactors Columns Pressure vessels

Longitudinal and circumferential welds Longitudinal and circumferential welds Longitudinal and circumferential welds

Fitting construction

Valve housing

Longitudinal and circumferential seams

Crane and bridge construction

Arc conductor Floor slab Profiles

Butt and fillet welds Butt and fillet welds Butt and fillet welds

General steel construction

Thick-walled components with long seams (e.g. Driven piles)

Butt and filled seams

Beside joint welding (limited to the flat position PA, fillet weld PB, and horizontal butt weld), where usually wire electrodes are used, cladding of components, aimed for protection against corrosion and/or wearing, is a very important application area of submerged arc welding. For cladding tasks usually the

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double wire submerged arc welding, or the submerged-arc strip welding processes are used, since a generally required low dilution of the base material can be achieved using both processes. Corresponding to the numerous application areas, with vessel construction, shipbuilding and pipe construction being the most prominent ones among them, there is also a wide range of materials used. Table 2: Materials processed with submerged arc welding The material groups which are processed nowadays using submerged arc welding. Table 2: Materials processed with submerged arc welding Materials

Example

Standardisation

Minimal Plate Thickness

Unalloyed and low alloyed materials

S235JRG2 (Rst37-2) S355J2G3 (St52-3)

DIN EN 10025-1, -2 (DIN 17100)

8 mm

High alloyed materials  Cr-Ni steels

X 2 CrNiMo 17122

DIN EN 10088-3 (DIN 17440)

6 mm

Heat resistant materials

16 Mo 3 (15 Mo 3) 13CrMo4-5(13CrMo44)

DIN EN 10028-2 (DIN 17155)

8 mm

Case hardening steel (with limitations)

16 MnCr 5

DIN EN 10084

8 mm

Fine grain construction steel

P 355 N, P 460 N

DIN EN 10025-4 DIN EN 10028-3 DIN EN 10025-4

8 mm

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Construction of a SAW welding equipment

A complete SAW welding machine consists of a welding head, a welding power supply, mechanical units for positioning and moving the welding head and the workpiece. Figure 3 shows the schematic design of a submerged arc welding system. Figure 4 shows the possible structure of a single wire SAW machine.

tot Copper base

Figure 3: Schematic view of a SAW machine

Figure 4: real SAW machine

2.1

Welding equipment

The welding equipment consists of the following components: - Current (welding) contact tip - Wire feeder - Powder supply and extraction device - Control and regulation devices The essential parts are described in the following: © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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Page 7

Current (welding) contact tip

In continuous operation the current contact tip must provide uninterrupted current transmission and maintain a permanently precise wire guiding. Due to the required high thermal and mechanical resistance, it is recommended that the current contact tip is made of a copper alloy (CuCrZr). Figure 5 shows different implementations of current contact nozzles.

Figure 5: Current contact tips

2.1.2

Wire feed device

A high-capacity and adjustable motor with gear and reliable mechanics for force transfer has the function to pull the wire electrode from the coil (reel) and to push it through the current contact tip towards the welding joint. Wire straightening fixtures allow an accurate aligned wire output from the lower end of the contact tip. 2.1.3

Wire coils

Suitable attachment fixtures are to be mounted for installing wire electrode coils which are available in different sizes and weights in accordance with DIN EN 756. 2.1.4

Powder supply and extraction device

The flux supply can be provided – depending on the company's operating conditions or requirements – manually before welding, or during the welding with the help of a powder feeder device. The non-melted flux is extracted and can eventually be supplied to the flux circuit again. 2.1.5

Control

With the control, the main parameters “amperage”, “voltage” and “welding speed” are set. These parameters cannot be set independently of each other but must be adjusted together because the value of each parameter influences the value setting of the others. The influences of the welding parameters are shown schematically in Figures 6-8.

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2.1.5.1 Current strength The current strength basically influences the penetration depth.

Figure 6: Penetration as function of current strength As a reference value 100 A can be seen to be equivalent with a penetration depth of approx. 1 mm in the square butt joint. Usual amperage values for a wire electrode diameter of 4 mm are at I = 600 A to approx. 800 A. These values result in a current load which usually is about 150 to 200 A/mm wire diameter. Accordingly, the current density is approximately 48 to 64 A/mm2. It has to be considered that the current density influences the extent of the penetration more than the amperage does.

2.1.5.2 Voltage The influence of voltage on weld width

Figure 7: Weld width as function of voltage The usual voltage values are set to approx. 30 to 40 V.

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2.1.5.3 Welding speed Welding speed influences both the weld width and the weld depth

Figure 8: Weld width and depth as a function of the welding speed The usual welding speeds for submerged single wire welding are approx. 55 cm/min. If the chosen welding speeds are too low, the ignited arc is located on top of the ahead running molten weld pool material, which causes low penetration depth and fusion errors. Too high welding speeds cause significant irregular constrictions of the weld. From the points 2.1.5.1 to 2.5.1.3 follows a practically relevant welding parameter setting, for example:

I

=

600

A

U

=

30

V

v

=

55

cm/min

Wire diameter

=

4

mm

A heat-input of approx. 20 kJ/cm is generated out of these values.

2.2

Travel speed device (Relative movement between the element and the welding head)

The travel speed device generates the relative movement between the component and the welding head with wire electrode. Depending on the component geometry and the joints to be welded, various systems are used.    

Beam carriage (longitudinal welds) Welding masts (longitudinal welds) Roller blocks, wheels (circumferential welds) Roller tables (longitudinal welds)

   

Movement of the welding head Movement of the welding head or the component Movement of the component Movement of the component

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2.3

SFI / IWE 1.10 Page 10

Power Supply

Using direct current with the  pole on the wire electrode is normally used for welding with one wire. For multi-wire welding it is favourable to use direct current and  pole for the first electrode, and to use AC for the following electrodes (to avoid arc-blow). The power supplies are usually rectifiers, and one has to consider that due to the high degree of automation, high amperages have to be available for a 100% duty cycle. A power supply with a constant current characteristic (external regulation) is used for wire electrodes exceeding 3 mm, while for smaller wire electrode diameters a constant voltage characteristic is used (internal regulation). 2.3.1

Regulation of the Arc Length

For uniform welding results (weld width, -height, and -depth) the arc length and consequently the welding voltage and current must be kept constant. Depending on the characteristic curves of the power supply this is ensured with ∆U or ∆I regulation. 2.3.1.1 ∆U-Regulation The ∆U-regulation (“external regulation”) is normally used with constant current characteristic curve and when using bigger electrodes (diameter exceeding approx. 3 mm). The wire feed speed is controlled and adjusted according to the arc length. One possibility of the regulation involves the connecting of the arc voltage directly or as a proportional voltage value to the armature voltage of the electrode drive motor. If the arc length increases for any reason, the voltage of the arc and consequently the supply voltage of the conveyor motor increases. The rotating is now going faster whereby the arc length becomes shorter. In case of inadvertent shortening, the wire feeder is slowed down: the arc length increases now back to the original length. 2.3.1.2 ∆I-Regulation The I-∆ regulation (“separate internal regulation”, actually not a regulation, but a process using autobalance compensation) is used for power supplies with constant voltage characteristics); wire feed speed is constant here. Keeping the arc length constant is accomplished by the strongly differing high deposition rates of the arc length changes and the ensuing movements of the working point. If for any reason the arc is smaller, the amperage, based on the characteristic curve, increases strongly and consequently the melting speed as well: with continuing constant wire feed, the preselected arc length is again being achieved fast. During unintentional arc extension, the current value decreases strongly, electrode melts slower, and the pre-set arc length is established again, see also chapter 1.04 “The arc.” A reliable regulation is assured only in case of thin electrodes (diameter smaller than approx. 3.0 mm). It is also used in submerged arc strip welding.

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Weld preparation

Due to the deep penetration a special weld preparation and the use of backing is necessary. The weld weld preparation is standardised according to DIN EN ISO 9692-2, Table 3 and 4. Table 3:

Joint preparations for butt welds, welded on one side (dimensions in millimetres)

Table 4:

Joint preparations for butt welds, welded on both sides (extract from DIN EN ISO 9692-2)

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Filler- and auxiliary materials

In submerged arc welding normally electrodes of the same type, or electrodes which are adapted to the base material as far as possible, are used along with granular, mineral powders. 4.1

Electrodes

Depending on the intended application, wire (joint welding) or strip electrodes (surface welding) are used; if required also flux-cored electrodes and filled strips can be used. 4.1.1

Wire Electrodes

Wire electrodes (or solid, and/or in special cases, tubular wire electrodes) are used as filler materials in joint welding. The solid wire electrodes are made from wire rod and are extended to the required size by cold draw. The surface is slightly coppered in order to minimise friction resistance and to improve current transfer; to some extent this layer is also useful for corrosion protection. To prevent welding defects resulting from contact problems in the contact tip, the surface of the wire electrode must be a smooth surface, free from crevices, grease and rust scars, and appropriately calibrated. Normally, wire electrodes with a diameter of 3 and 4 mm are used. For high amperages, wire electrodes of 5 mm diameter are also used. If the welding area is hard to access, or if there is a danger that the root may fall through, 3 mm wire electrodes are used. The choice of the electrodes is made under the criterion that the mechanical-technological properties of the weld metal and the base material match as far as possible; It must be considered here though, that the welding powder exerts a more or less strong metallurgical influence on the chemical composition of the weld metal, and therefore on its key properties. For an error-free welding sequence the wire electrode must be perfectly coiled up. Deliverable ring sizes are listed in DIN EN ISO 544. To prevent confusion among them, each ring must be labelled on the exterior side, using the following indicators: - Trade name - DIN EN designation (e.g. Wire electrode DIN EN ISO 14171-A-S2Mo) - Production and batch number - Wire diameter - Net weight - Manufacturer or supplier. Table 5 shows the filler materials for non- and micro-alloyed materials.

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Any agreed chemical composition

) Ti: 0,10 % up to 0,20% , B 0,005 % up to 0,020%

d

) The chemical composition of the finished product, Cu including copper coating ≤ 0.30%, Al ≤ 0.030%. b ) Single values in table are maximum values c ) Wire electrodes not listed in the table must be named similarly, starting with the letter SZ. The ranges of the chemical analysis are not fixed. The possibility is given that two electrodes with the same Z-classification cannot be exchanged.

a

Chemical composition in % (percentage)a b c d

Table 5:

Short symbol

Submerged arc welding SFI / IWE 1.10 Page 13

Filler materials for non- and micro-alloyed materials (Excerpt from DIN EN ISO 14171)

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Page 14

By processing CrNi-steels, usually 3 mm wire electrodes are used to lower the heat input. Filler materials for high-alloyed materials are to be found in DIN EN ISO 14343. Table 6 shows an extract from this standard. Table 6:

Symbols for the chemical composition of wire electrodes, wires and rods for the welding of high alloyed materials Chemical composition in % (m/m)1) 2) 3) 4)

Alloy symbol

Martensitic/ ferritic 13 13 L 13 4 17 Austenitic 19 9 L 6) 19 9 Nb 6) 19 12 3 L 6) 19 12 3 Nb 6) Ferriticaustenitic high corrosionresistant 22 9 3 NL 8) 25 7 2 L 25 9 3 Cu NL 8) 25 9 4 NL 8) Full-austenitic high corrosionresistant 18 15 3 L 9) 18 16 5 NL 9) 9) 19 13 4 L 20 25 5 Cu L 9) 20 16 3 Mn L 9) 25 22 2 NL 9) 27 31 4 Cu L 9) Special types 18 8 Mn 9) 20 10 3 23 12 L 6) 23 12 Nb 23 12 2 L 29 9 Heat resistant steels Types 16 8 2 19 9 H 19 12 3 H 22 12 H 25 4 25 20 9) 25 20 Mn 25 20 H 9) 18 36 H 9) 1) 2) 3) 4) 5) 6) 7) 8) 9)

C

Si

Mn

P 5)

S 5)

Cr

Ni

Mo

Other elements

0.15 0.05 0.05 0.12

1.0 1.0 1.0 1.0

1.0 1.0 1.0 1.0

0.03 0.03 0.03 0.03

0.02 0.02 0.02 0.02

12.0 to 15.0 12.0 to 15.0 11.0 to 14.0 16.0 to 19.0

— — 3.0 to 5.0 —

— — 0.4 to 1.0 —

— — — —

0.03 0.08 0.03 0.08

0.65 0.65 0.65 0.65

1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5

0.03 0.03 0.03 0.03

0.02 0.02 0.02 0.02

19.0 to 21.0 19.0 to 21.0 18.0 to 20.0 18.0 to 20.0

9.0 to 11.0 9.0 to 11.0 11.0 to 14.0 11.0 to 14.0

— — 2.5 to 3.0 2.5 to 3.0

— Nb 6) — Nb 6)

0.03 0.03 0.03

1.0 1.0 1.0

2.5 2.5 2.5

0.03 0.03 0.03

0.02 0.02 0.02

21.0 to 24.0 24.0 to 27.0 24.0 to 27.0

7.0 to 10.0 6.0 to 8.0 8.0 to 11.0

2.5 to 4.0 1.5 to 2.5 2.5 to 4.0

0.03

1.0

2.5

0.03

0.02

24.0 to 27.0

8.0 to 10.5

2.5 to 4.5

N 0.10 to 0.20 — Cu 1.5 to 2.5; N 0.10 to 0.20 N 0.20 to 0.30; Cu 1.5; W 1.0

0.03 0.03 0.03 0.03 0.03 0.03 0.03

1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.0 to 4.0 1.0 to 4.0 1.0 to 5.0 1.0 to 5.0 5.0 to 9.0 3.5 to 6.5 1.0 to 3.0

0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.02 0.02 0.02 0.02 0.02 0.02 0.02

17.0 to 20.0 17.0 to 20.0 17.0 to 20.0 19.0 to 22.0 19.0 to 22.0 24.0 to 27.0 26.0 to 29.0

13.0 to 16.0 16.0 to 19.0 12.0 to 15.0 24.0 to 27.0 15.0 to 18.0 21.0 to 24.0 30.0 to 33.0

2.5 to 4.0 3.5 to 5.0 3.0 to 4.5 4.0 to 6.0 2.5 to 4.5 1.5 to 3.0 3.0 to 4.5

— N 0.10 to 0.20 — Cu 1.0 to 2.0 — N 0.10 to 0.20 Cu 0.7 to 1.5

0.20 0.12 0.03 0.08 0.03 0.15

1.2 1.0 0.65 1.0 1.0 1.0

5.0 to 8.0 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5

0.03 0.03 0.03 0.03 0.03 0.03

0.03 0.02 0.02 0.02 0.02 0.02

17.0 to 20.0 18.0 to 21.0 22.0 to 25.0 22.0 to 25.0 21.0 to 25.0 28.0 to 32.0

7.0 to 10.0 8.0 to 12.0 11.0 to 14.0 11.0 to 14.0 11.0 to 15.5 8.0 to 12.0

— 1.5 to 3.5 — — 2.0 to 3.5 —

— — — Nb 7) — —

0.10 0.04 to 0.08 0.04 to 0.08 0.04 to 0.15 0.15 0.08 to 0.15 0.08 to 0.15 0.35 to 0.45 0.18 to 0.25

1.0 1.0 1.0 2.0 2.0 2.0 2.0 2.0 0.4 to 2.0

1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 1.0 to 2.5 2.5 to 5.0 1.0 to 2.5 1.0 to 2.5

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

14.5 to 16.5 18.0 to 21.0 18.0 to 20.0 21.0 to 24.0 24.0 to 27.0 24.0 to 27.0 24.0 to 27.0 24.0 to 27.0 15.0 to 19.0

7.5 to 9.5 9.0 to 11.0 11.0 to 14.0 11.0 to 14.0 4.0 to 6.0 18.0 to 22.0 18.0 to 22.0 18.0 to 22.0 33.0 to 37.0

1.0 to 2.5 — 2.0 to 3.0 — — — — — —

— — — — — — — — —

If not defined Mo < 0.3%; Cu < 0.3% and Ni < 0.3%. Single values in chart are maximum values. Wire electrodes not listed in the table must be named similarly, starting with the letter Z. The results have to be rounded on the same position as fixed values by using Appendix B, Rule A from ISO 31-0:1992. The sum of P and S should not exceed the value 0.050%, with the exception of 25 7 2 L, 18 16 NL, 20 16 3 Mn, 18 8 Mn and 2 99. Si is to be added to the alloy short symbol if Si > 0.65 to 1.2%. Nb min. 10 × % C, max. 1.0%; up to 20% of the Nb- content can be replaced by Ta. Wire electrodes with these symbols are usually chosen for specific properties and are not directly interchangeable. Pure weld metal is in most cases full-austenitic and therefore can be prone to microcracks or hot cracks. The forming of cracks is reduced by increasing the manganese content in the weld metal. Under consideration of this fact the manganese ratio was extended for some types.

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SFI / IWE 1.10 Page 15

Strip electrodes

Generally, joint welding using strip electrodes is barely used. For submerged surface welding with strip, strip electrodes of 60 mm width and 0.5 mm thickness, with tendency towards considerable more width (100 mm), are used. The advantage of this type of electrode – as a result of the low penetration coupled with high melting deposition efficiency, lies in the small dilution of non-/ low-alloyed base material with the higher alloyed material of the strip electrode. The thickness of the layer is at about 3.5 to 5 mm.

4.2 4.2.1

Welding Flux Tasks of the welding powder

Welding powders are granular, meltable, mineral substances, which fulfil similar tasks in submerged arc welding as the coating of the rod electrode, Figure 9: The Welding Flux Serves for: a. increasing the conductivity of the arc gap therefore: better ignition more stable arc b.

creation of slag which:

can be expanded to a stable cavern. protects the transferring drop protects the molten welding metal forms the bead with low heat input! (stringer beads) prevents the too fast cooling off of the weld. influences the drop size.

c.

creation of a protective gas area (agglomerated flux) out of:

e.

carbonates (e.g.: CaCO3)

de-oxidisation and to alloy reinforcement (agglomerated flux) by: adding Mn, Si, Cr, Ni, Mo etc.

Figure 9: Tasks of the welding powder

Welding fluxes are standardised according to DIN EN ISO 14171.

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Classification of Welding Flux (Powders)

4.2.2.1 Classification according to type of manufacturing We distinguish between fused (molten) and agglomerated (bonded) welding fluxes. Sintered welding fluxes are not very important in practice. F (fused) : Molten flux A (agglomerated / bonded) : Agglomerated flux M (mixed) : Mixed flux

Manufacturing of fused flux Fused flux becomes a glasslike product by re-melting the raw material components (Figure. 9). Subsequently the desired grain size is achieved by crushing and filtering to the right size. The bulk weight and consumption are higher than with the agglomerated flux. Due to their glasslike surface these fluxes are less sensitive to moisture. Fused flux is produced at a relatively low cost, but the high production temperatures have the unfavourable effect that they lead to some chemical reactions, which cannot be utilised any more during welding. The use of fused flux is of minor significance. Manufacturing of agglomerated (bonded) flux Agglomerated flux consists of grains which are joined together with the help of a bonding agent (water glass), the grains coming from a mixture of finely granulated single components, which are heated after granulation at a temperature of 600 to 800 °C (Figure 10) Since the temperatures needed for flux production are below the reactivity of the raw materials, metallic deoxidising and alloying constituents may be added to the mix, which lead to the desired metallurgical reactions (deoxidation, alloying with e.g. Mn and Si) during the droplet- and bath reaction process. So they can be utilized for the mechanicaltechnological properties of the weld, and also for the welding characteristics. The manufacturing of these fluxes is expensive, and the consumption is low due to the low bulk weight. Due to their grain structure and their manufacturing type, these fluxes are hygroscopic and must be verified with utmost care regarding their moisture content, before using them. These fluxes are often and widely used.

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Figure 9: Producing fused flux (Source: Messer Griesheim)

Figure 10:

Page 17

Production of agglomerated (bonded) flux (Source: Messer Griesheim)

4.2.2.2 Classification of Welding Fluxes according to mineralogical structure The welding process and the mechanical-technological properties of the weld are strongly influenced by the powder type and the characteristics of the resulting slag. Table 7a shows the classification of fluxes according to their mineralogical structure, Table 7b according to the characteristic properties of the different flux types. Figure 7a: Classification of Welding Fluxes according to mineralogical Structure Manganese silicate type Main constituent parts

Manganese oxide MnO Quartz SiO2

Other Bauxite possible fluorite components Sum of the main constituent parts (min.)

Calcium silicate type

Al2O3 CaF2

50%

Aluminate Rutile type

Quarz Calcium oxide Magnesium oxide

SiO2 CaO MgO

Bauxite Fluorite manganese oxide

Al2O3 Quarz CaF2 Manganese MnO oxide Zirconium oxide

60%

Bauxite Rutile

Aluminate base type

Fluoride base type

Al2O3 Bauxite Al2O3 (min. 20%) Calcium oxide TiO2 Calcium oxide CaO Magnesium oxide Magnesium oxide MgO Manganese oxide Fluorite SiO2

Manganese oxide Quartz

MnO ZrO2

45%

CaO MgO MnO CaF2

MnO Quarz SiO2 (max. 20%) SiO2 Bauxite Al2O3

45%

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50%

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Table 7b: Classification of Welding Fluxes according to Characteristic Properties

Out of Table 7b, the following conclusion can be made regarding the particular flux types: - Manganese silicate type Most fluxes of this type cause pore-safe welds even on rusty and polluted base metals Comparable with acid coated electrodes - Calcium silicate type Is a “universal” flux for one- and multi-layer technology comparable to acid base coated electrodes - Aluminate rutile type Their use is especially recommended in case of high welding speeds. The slag can be removed easily, and usually sets itself apart. Comparable with rutile coated rod electrodes - Aluminate basic type In this group we find the typical fluxes that are welded with alternating current, which cover the area of slightly alkaline to alkaline. Comparable with rutile basic coated rod electrodes - Fluoride basic type The basic and highly basic fluxes are grouped together beneath; they are suitable for alternating current welding only with reservations, but produce outstanding mechanical properties (toughness!) Comparable with basic coated rod electrodes

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4.2.2.3 Classification according to the degree of basicity Depending on their mineralogical construction and chemical composition, welding fluxes greatly influence the mechanic-technological characteristics of the weld. As characteristic parameter the degree of basicity is defined according to Boniczewski. It is defined as

According to the degree of basicity, the fluxes are distinguished: B1 B=1 B1 B3

“acid” “neutral” “basic” “highly basic”

Powder Powder Powder Powder

Figure 11 illustrates the influence of the flux composition on the impact energy of the weld material.

Acid

Figure 11: Influence of flux composition on the impact energy of the pure weld metal

Results of welding tests are shown, in which multi-position welds with at least 8 layers were welded for generating “pure weld metal” (see the defined welding conditions in DVS 0907 and DIN 700), always with electrode S1 but with a flux of different basicity in each test. For determining the impact energy, the test pieces have been removed out of the cap layer. This procedure guarantees that the results are independent of the used base material.

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The results show that the ductility properties of the weld metal become better and better when the basicity is increased. The reason behind this is the higher degree of purity, which is manifested in the lower oxygen content in the weld metal, which is realized by using basic flux. 4.2.3

Storage and re-drying of flux

Due to their grain structure and different ways of manufacturing, the welding fluxes are more or less sensitive to moisture, they must therefore be stored in a dry (or preferably air-conditioned) place. To prevent the forming of pores and hydrogen-induced cracks, which happens in particular by welding highstrength steels, the powders must be dried before use according to manufacturer's specifications. In various welding procedures, a general re-drying is mandatory. Usual re-drying temperatures are: Fused flux: Agglomerated flux:

250 °C, at least, 2h 300 to 400 °C, at least. 2 h

The manufacturer's specifications are mandatory; useful information can also be found in the DVSGuideline 0914 (“Processing and storage of welding powders”). When re-drying, in each case keep in mind not to damage the flux by a too high temperature or a too long dry time. 4.2.4

Identification and designation of welding powders

Similar to electrodes, the fluxes must be marked uniquely to avoid being mix-up, and the essential properties are as follows: - Commercial name - Name according to DIN EN ISO 14174 - Production and batch number - Net weight - Manufacturer or supplier - Grain size. According to the manifold influence possibilities of the welding flux, the designation is relatively extensive which however enables a fairly good prediction of the suitable usage. The following statements are used in the designation. Index for the application class, powder class (table 8) Powder class 1: Fusion/surface welding of unalloyed and low-alloyed steels as well as general construction steels, high-strength and heat-resistant steels Powder class 2: Fusion/surface welding of stainless and heat resistant Cr and CrNi steels and/or nickel and nickel alloys Powder class 3: Flux for surface welding, and pickup of C, Cr or Mo Powder class 4: Fluxes other than classes 1-3, for example, flux for copper alloys Index of the metallurgical behaviour Indices 1 to 9 They designate the pick-up or burn-off of alloy elements With flux class 1, the pick-up or burn-off of the elements Si and Mn in this sequence, is given by numbers.

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Table 8: Meaning of indices for the metallurgical behaviour

Metallurgical behaviour

Code letter

Burn-off

1 2 3 4 5 6 7 8 9

Pick-up and /or burn-off Pick-up

Quantity by powder in pure weld metal % higher 0,7 over 0,5 to 0,7 over 0,3 to 0,5 over 0,1 to 0,3 0 to 0,1 over 0,1 to 0,3 over 0,3 to 0,5 over 0,5 to 0,7 Higher 0,7

Symbol for welding current type AC for alternating current DC for direct current Suitability for AC generally implies suitability for DC as well Symbol for hydrogen content: H5, H10 or H15 The hydrogen content in the applied weld material is indicated; H5 means maximum 5 ml hydrogen/100 g pure weld metal. The precondition is that dry welding flux has been used Current carrying capacity rating, grain size range The current carrying capacity rating of the flux depends on different welding conditions. Therefore the flux designation does not propose marking it. The information coming from the flux manufacturer must be used. The granularity marks the lowest and the highest grain size of the welding powder (e.g. Granularity 2 to 16 means grain sizes from 0.2 to 1.6 mm)

An example of flux designation Welding Flux ISO 14174 – S F C S 1 6 7 AC H10 The meanings here are: DIN EN ISO S F CS 1 67 AC H10

Valid standard Flux/Submerged arc welding Fused flux Powder type calcium silicate Application, powder class 1 Metallurgical behaviour: 6: Pickup of Si of 0.1 to 0.3% 7: Pickup of Mn of 0.3 to 0.5% Suitable for alternating current Hydrogen content of 10 ml/100 g (pure) weld metal

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Wire / flux combination

The choice of wire and flux is made under consideration of welding and metallurgical rules and laws. From the welding perspective, AC suitability, current carrying capacity and high-speed welding properties are of importance here. Due to its cooperation with the electrode, the metallurgical behaviour of the welding powder influences the chemical composition of the welds, which in turn affects the mechanical-technological properties of the weld, so it must be included among the choice criteria.

4.3.1

Metallurgical behaviour

During the submerged arc welding of unalloyed and low-alloyed steel, pickup or burn-off of alloy elements occurs due to the material slag - and the metal-gas-reaction (droplet reaction in gas-filled welding cavern). The elements C, Si and Mn are affected here. According to their percentage in the metal (electrode and weld pool on the work piece) and in the slag, a pickup or burn-off of the elements C, Si and Mn takes place, depending on the reaction affinity. Therefore the chemical composition of the weld fusion differs more or less significantly from the one calculated based on the dilution (experienced values: approx. 1/3 electrode material and 2/3 base material give, in the case of submerged arc welding with wire electrode, the weld metal of the seam). The metallurgical behaviour is determined by the powder manufacturer, based on the guidelines described in DVS 0907. For this purpose, multi-pass welds are (at least. 8 layers) welded under controlled welding conditions with electrodes that differ from each other in their Si and Mn contents. The comparison of the detected chemical composition of the “pure weld metal” with the alloy content of each used electrode, shows the pickup or the burn-off of the examined wire/flux combination. This enables a statement about the metallurgical behaviour of the powder which is an individual property of the flux, and consequently it must be considered when choosing the right powder. 4.3.2

Designation of a wire/flux combination

In DIN EN ISO 14171 there are also symbols which provide information about the mechanical property values of the weld metal of a wire/flux combination

Example of a designation Wire/flux combination ISO 14171- A-S-46 3 AB S2 ISO 14171-A: Number of the international standard with classification according to yield strength and impact energy value of 47 J. S Flux/Submerged arc welding 46 Value for the mechanical properties (min.yield strength) : 460N/mm2 3 Value for the impact energy: 47 Joule at -30 °C AB Flux typ: Aluminate Basic type S2 Wire electrode S2 (1%Mn)

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Procedure parameters, weld pool backing

5.1

Deposition rate

The deposition rate depends considerably on the current intensity, and for a given current intensity on the current density which can be influenced via the wire diameter. The current density in the electrode is set to 30 ÷ 90 A/mm2 (thumb rule for current level: diameter multiplied by 100 to 200) but can occasionally be set to a higher value in order to boost the performance. 5.2

Influence of the electrode position on penetration depth and weld geometry

With electrode positioning, the weld seam geometry can be influenced within certain limits: With a “dragging” positioning of the electrode, the weld will be deeper and narrower, while with a “pushing” positioning some widening of the weld is possible, even at higher welding speeds, see Figure 12.

Figure 12: Weld geometry of external circumferential welds and its dependence on electrode position

5.2.1

Flux granularity

Welding fluxes for SAW are delivered in various grain sizes. Fine-grained fluxes enable faster welding, and have higher current carrying capacity. They provide a denser filling, and therefore more powder consumption. Rough fluxes lead -because of the loose accumulation -to better gas release and therefore higher pore safety.

5.3

Weld pool backing system

Because of the partly quite large welding pools, it must be assured by suitable measures that the weld pool does not fall through. If the root face is not high enough and/or in case of a too large root opening, weld pool backings must be used (Figure. 13). When using copper bars, you must ensure by intensive cooling and/or a constructive design that the copper bar is not melted off at the interface of weld and copper. The consequence might be brazing/solder copper fracture.

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Manual run

Figure 13: Common weld backing types for submerged arc welding

6

Options to increase performance in submerged arc welding

Single wire SAW has a wide usage range, and even thick plates can economically be joined by several runs. Figure 14 shows the cross section of a submerged multi-layer welding; with backing layer.

Figure 14: Multi-layer submerged arc welding on a 30 mm thick plate made of low-alloy steel (SLV Duisburg)

Increased quality requirements and the constantly growing labour costs and additional expenses make it necessary to further increase the efficiency of the already high performing wire electrode SAW. Due to the increase o As the intensity of the melting and the deposition rate increases with the increase in amperage, a certain increase of productivity can be made by using a higher welding speed and /or by welding thick plates with fewer layers.

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The range of the practically usable current carrying capacity is limited by the current capacity rating of the flux. Additionally, with high amperage or energy concentration, the resultant unfavourable weld geometry can lead to hot cracks, pores or undercuts on the edges.

6.1

Increase in output in single wire SAW

The modification of the conventional single-wire-SAW technology provides the possibility to achieve higher welding output and maintaining the usual joint quality (Figure. 15). So, a longer free wire ending (Figure 16) or a smaller wire diameter leads to higher deposition rates as a result of the stronger resistance heating. This can be used – especially in case of surface welding – to reach a noticeable increase in efficiency. By using metallic fillers (same material or specific composition) a higher welding speed can be reached, too. It should be noted that the metal powder, which is applied concentrically around the electrode and adheres there firmly due to the magnetic field of the electrode, extracts heat as well as from the slag while passing through, but also from the entire welding zone. As the amperage stays unchanged this method has its limits in respect to weld imperfection prevention. (Attainable increase of deposition rate is approx. 20 – 30%). Finally the negative polarisation of the electrode causes an increase of the deposition rate by approx. 20%, depending on the welding flux and amperage, in contrast to the positive polarisation, at the same energy amount.

Figure 15: Options to increase performance in single-wire SAW

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Figure 16: Normal and extended free wire electrode ending in SAW

6.2

Performance increase for multiple-wire SAW

The most effective method for performance increasing consists in the simultaneous melt-off of several electrodes and the supply of filler wire and/or metal powder (Figure 17). These process variants allow to largely eliminate the disadvantages of single-wire welding like the unfavourable penetration geometry and energy concentration, and also to improve the seam weld by controlling the penetration and the heat input, and finally to considerably reduce the production times. Today, the pure multi-wire systems are the most economic and the furthest developed processes; the remaining process variants are reserved to special applications.

Figure 17: Options to increase performance in multiple-wire SAW

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Double wire welding with shared feed system

At the beginning of the development of the multi-wire SAW in the years 1950 to 1960, first two electrodes were used in transverse or tandem position. The two electrodes are moved by one wire feeder unit and are connected to one energy source (double wire welding Figure 18). The effort regarding device technology is low here, because only a modified contact tip and a modified wire feed roll are necessary. In this process, a better gap bridging ability can be reached in comparison to the single-wire technology, because the energy concentration in the workpiece can be controlled, at least within certain limits. Arranged in an offset pattern, the arcs can be pointed for example towards the two fusion faces, whereby bigger tolerances in the joint preparation are permissible, and the weld pool backing can eventually be omitted. Deposition rate is approx. 50% higher, due to the increased current density resulting from the smaller wire diameters, at the same energy input. Shorter production times are reachable Two-wire welding Characteristics: two wire electrodes one power supply one control unit Advantages:

high deposition rate good gap bridging abilities high welding speed

Typical welding data: Electrode Diameter: 2.5 mm Current: approx. 800 A Voltage: approx. 32 V Welding speed: to approx. 120 cm/min

Figure 18: Double wire welding with shared feeder system

The fact however, that in this process technology – one energy source and one common wire feeder unit – the penetration ratios and deposition rates are closely connected with each other, led to the development of further multi-wire types in which many of these disadvantages are largely eliminated.

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Multi-wire welding using separate feed systems

With the currently common submerged multi-wire welding process, two (tandem welding), three or more electrodes, each of which is connected to its own power supply, are fed to the welding point by separately controlled feeder systems; all electrodes melt down into a shared chamber. Consequently, different power and voltage values can be set up on the individual electrodes, and weld geometry can be influenced in a targeted manner. Parallel to the mentioned advantages, the number of parameters, which must be optimised for achieving good results, also increases by a multiple Regarding the energy input the following must be considered: Current type and polarity of the electrodes Electric circuit (parallel, series) Phase sequence and displacement Properties of the energy source Welding parameters

In general one can say that using pure DC systems is to be avoided because of the too strong mutual influence of the arcs. The supply of the first electrode with direct current and positive polarization brings advantages regarding process stability. The circuit in tandem welding is shown in Figure 19.

Figure 19: Multi-wire welding (tandem) using separate feeder systems

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A “tractor” for the welding of a tandem weld (here fillet weld) as well a tandem weld are shown in Figure 20.

Figure 20: SAW fillet weld with tandem process and a tandem weld (Plant pictures: ESAB)

The submerged multi-wire method is used with great success in large pipe production, in vessel/machine/bridge construction and shipbuilding.

6.2.3

Multi-wire welding with additional cold/hot wire

Whereas the supply of the currentless wire (“cold wire”) is not justified because of the relatively smaller increase in deposition rate speed, a further increase in efficiency can be achieved if another currentcarrying wire (“hot wire”, not arc!) (Figure 21) is added. Even in case of a marginally increased total amperage (filler wire: 150 to 200 A, 12 to 14 V) up to 20% shorter production times are achievable. A further advantage of this technique is the overall lower temperature of the weld pool and the lower burn-off in alloying elements of the filler wire.

Hot wire

Figure 21: Multi-wire SAW with additional cold/hot wire

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Page 30

Multi-wire welding with cold wire filler and/or metal powder

As opposed to the hot wire addition, this procedure is characterised by the fact that no additional energy is brought in. At identical electrical energy supply, the heat surplus created in the overheated slag- and weld pool by the arc, is used to melt off additional metallic materials, for example wire or metal powder. As a result of the ensuing higher deposition rates (approx. 20%) the welding speed also increases in the same magnitude, with identical energy input. The amount of metal powder is limited by the occurrence of fusion errors, slag inclusions, and unfavourable bead geometry due to a too high cooling effect.

7

Process variants

In the following, process variants are described which are also applied with great success. 7.1 Submerged arc welding with strip electrode By using strip shaped electrodes, both joint and surface welding can be performed. 7.1.1

SAW Joint welding

Joining by welding, with the strip electrode placed along the welding direction, brings similar advantages as double-wire welding. With the strip in transversal or diagonal position, a lower penetration and a better gap bridging ability can be provided. The application - if used at all - is limited to special cases. 7.1.2

SAW surface layer welding

The main application of submerged arc welding with strip electrode is surface-layer (deposition) welding (Figure 22). It is used for remanufacture to apply layers that are very different from the base material, and also for maintenance/repair of layers similar to the base material which are worn away or abraded. Above all, corrosion-resistant or hard/wear-resistant layers are applied.

Figure 22: Submerged arc strip cladding, schematic representation and image section of a welding facility (ESAB)

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With submerged arc surface layer welding, one must consider that - By the dilution of base and cladding material some undesired structure formations may arise with unfavourable characteristics. When using strip shaped electrodes in surface welding, the usually targeted low dilution (approx. 20%) is attained by the arc's running back and forth constantly at the melting strip edge, preventing a deep fusion. Welding is done using direct current and positively polarised electrode, whereby power supplies with constant voltage characteristic are preferred. You can see the definition of dilution in Figure 23.

Figure 23: Dilution shown on a strip weld cladding (acc. to Schofer, ESAB)

7.2

Narrow Gap Submerged Arc Welding

The use of this process variant takes place with thick workpieces in the reactor, boiler and pressure vessel construction, as well as in mechanical engineering e.g. turbine waves, rotors and crank shafts. Non-alloy, low and high-alloy materials are welded. The aim is to keep the weld cross section small with the help of a special joint form type in order to save filler material. More favourable stress ratios in the joint, and a considerably shorter production time are also related to this. Submerged narrow gap welding can be carried out– depending on the properties of the component parts – as SAW single-wire, tandem or double wire processes. Weld joint search systems (tactile or non-contact e.g. using laser beam) are implemented, in order to prevent imperfections which can very be hard to eliminate especially on thick components. Special attention must be paid to the current contact unit in the narrow gap, in order to prevent short-circuits that inevitably lead to errors, imperfections. Figure 24 shows the corresponding indications.

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Advantages:  Reduction of seam volume  Reducing the welding time  Improvement of the internal stress condition Disadvantages:  High investment costs  Very high requirements regarding process safety  High repair costs

Figure 24: Narrow Gap Submerged Arc Welding Table 9:

[mm2] Saved volume

Table 9 shows data concerning the saved weld volume in relation to the thickness of welded sheet metals, and the weld cross-section compared with a single-V butt weld preparation. Figure 25, left, shows a complete, functional narrow gap welding machine, that was exhibited as a demonstration machine (ESAB) on the Essen fair “Welding & Cutting 2009”. Figure 25, right, shows a view into the welding groove during welding. The cross-section in Figure 26 shows the multi-layered structure of a narrow gap welding.

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Figure 25: The submerged narrow gap tandem welding facility

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View into the welding gap

Figure 26: Cross-section of the upper part of narrow gap SAW (AREVA)

7.3

Horizontal welding with Submerged Arc Welding

With fixtures suitable for attaching the welding heads and supporting the welding flux, horizontal butt welds can be performed (Figure. 27, 28). For joining prefabricated container shots, welding equipment is available that – after attaching them to the components with the corresponding assembly fixtures – perform a girth (circumferential) weld on two container shots in a fully automated way.

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Characteristics: - one wire electrode for every welding head - one power supply - one control Advantages:

- welding on-site of big construction elements are possible

Execution: - One-sided or simultaneous two-sided welding is possible

Figure 27: Horizontal submerged arc welding

Horizontal submerged arc welding with flux fed belt

Figure 28: Structure of horizontal welding with flux fed belt

7.4

Shape welding

Shape welding was developed as an alternative for the production of medium and thick forgings. With this process semi-finished products or welded-on pieces are manufactured out of weld metal only. For doing this multi-layer welding is used where both wire or strip shaped filler materials are involved. This specific technic, as well as the metallurgical influencing of the weld metal which is possible via the coordination of the wire- and strip-/flux combination, makes the component generated in such a way superior to the forged material. This almost isotropic behaviour with outstanding mechanicaltechnological properties, the generally lower C-contents of “shaped weld metal” are noteworthy, as well as the unnecessary forming, hardening and tempering. Despite the mentioned advantages this process variant did not/does not find large-scale industrial application; its use is rather restricted to special cases.

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Imperfections in submerged arc welding and corrective measures

The submerged arc welding is basically a welding process barely susceptible to faults. The nevertheless occurring defects can be grouped to “internal defects” and “external defects” as shown in Figure 29.

Figure 29: Internal and external defects in submerged arc welding

Some particularly important imperfections and remedies are described in the following. 1. Cold cracks by hydrogen The hydrogen brought into the weld metal by wet flux, polluted plate surfaces etc. dissociates in hydrogen atoms in the arc. These are concentrated e.g. in tension fields, or at discontinuities (e.g. dislocations, microcracks in the HAZ, and similar). In these areas, hydrogen induced cracks can occur, if the hydrogen cannot leave the weld metal via effusion. These diffusion processes and therefore the appearance of cracks can take days and weeks to complete, so this crack formation may possibly manifest just after some time has passed. Actions to improve matters:  Re-dry the welding flux for approximately 2 hours at 250 – 300 °C  Pre-heating, and with it - Decreasing of hardness values - Reducing residual stress - Improvement of [H] diffusion from critical zones

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2. Lamellar tearing Due to long stretched sulphide inclusions stresses in the thickness direction of plates can only be absorbed in a restricted extent. Too high stresses lead to cracking. Due to the high weld volume submerged arc welding seams are especially prone to lamellar tearing. Action to improve matters:    

Amended construction Change of welding order Improved base materials (z-values) Buttering of base material

3. Hot cracking The weld solidifies in a temperature interval. Sulphur and phosphorus form with other steel accompanying components (e.g. Mn, Ni) low-melting phases. These low-melting phases move ahead of the solidification front towards the weld centre. The shrinkage stress created by the cooling off, rip the still liquid areas apart. Action to improve matters:      

Steels with lower amounts of sulphur and phosphorus Depth and width ratio of the seam W/D > 1, Figure 30 Lowering of welding speed to achieve a more favourable structural constitution in the weld centre Use of acid flux to increase the amount of oxygen in the weld pool Reduction of weld metal volume Reducing the heat input

W/D < 1 Figure 30: Solidification at different D/W ratios

D/W ~ 1

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4. Pores 4.1 Mechanical pores Mechanical pores are created e.g. via holes and crevices. Caused by the heating up of the air in these cavities the gas volume expands and gets into the weld metal. Pores will also be created by strong arcblow. 4.2 Metallurgic pores Metallurgic pores always origin from the weld metal. They develop by pool reactions (e.g. CO) or by solubility decline of an element at decreasing temperature (H2, N2). Pores can also initiate via the blowing effect of the arc when there is an insufficient degasification of the welding pool. Action to improve matters:  Deflect arc-blowing effect into more favourable directions by changing earth clamping  Dry the flux  Clean the plates 5. Slag inclusion Slag inclusion can result from unfavourable wire-flux combinations, unfavourable weld shape, inappropriate welding parameters, or an unfavourable welding sequence (Figure 31). Slag inclusions also take place after an insufficient fusion by the arc. Action to improve matters:  Change welding sequences  Selection of flux with good slag release  Change welding parameters (e.g. changing the voltage to avoid “ears”) correct

wrong

Figure 31: Avoiding slag inclusions by changed welding sequence

With a root pass-backing weld situation, the penetration depth can be too low to fuse the root face or to achieve a sufficient overlapping of the weld runs. Action to improve matters:  Increase of amperage  Better alignment of construction pieces.

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Defects on the weld surfaces and in the weld root area Failure mode

Corrective measure

Reinforced weld Weld height too low

Increase welding voltage Increase current Reduce voltage Finer flux Bigger wire diameter Correcting the forward travel speed

Weld surface is too rough Top bead depression (e.g. for circumferential welds on containers) End crater Undercuts (butt welds) Undercuts (fillet welds) Fall through of the root/fall back of the root

9 9.1

Run-on and run-off plate Choose different flux (Conductivity of slag) Optimize voltage Optimize welding voltage Optimize the wire position Optimise welding parameter

Applications of submerged arc welding Shipbuilding

Submerged arc welding is used extensively in ship building, especially for long, straight welds. Figure 32 shows a few weld seams that typically have to be produced in the ship building.

Typical SAW welded joints

Figure 32: Submerged arc welding in shipbuilding (source: Lincoln Smitweld)

Tandem welding is often applied in the pre-assembly stage, while three-wire and multi-wire welding is used occasionally (Figure 33), when the necessary sheet thicknesses and component dimensions are given and allow to do so. Depending on the production line, welding is carried out on both sides as well as on one side (with backing).

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Figure 33: SAW multi-wire welding set (image detail)

9.2

Vessel construction

In vessel construction, round and longitudinal welds are created by submerged arc processes if the material thicknesses are over approx. 5 mm. The weld preparation and the filler material/flux powder is chosen according to the material being used. The manufacturing of vessels and components being non-corrosive is particularly interesting in this field. To this end, CrNi steels are processed in accordance with DIN EN 10088-3. This process demonstrates a number of specific characteristics with regard to submerged arc welding: Heat input must be controlled and restricted. Consequently, a reduced heat input of approx. 15 kJ/cm is used. In practice, wire electrodes with a diameter of 3 mm are being welded with reduced welding parameters (I ~ 450 A). In addition, several layers are welded. During welding, a temperature of approx. 150°C must be maintained between the layers. Due to modified thermal conductivity and expansion properties of these steels, in contrast to ferritic materials, considerable warpage occurs. When using flux powders with acid powder properties considerable chromium burn-off occurs which must be compensated for by choosing an appropriate flux powder respectively powder with chromium support. Figure 34 shows an example of the weld preparation and the layer structure of a submerged arc weld in a CrNi vessel having a wall thickness of approx. 13 mm. The weld was prepared as a Y-seam. The increased root opening size, as compared to the ferritic materials, and the complex layer structure are important here.

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Figure 34: Weld preparation and layer structure when performing submerged arc welding on CrNi steels

A submerged welding set from the area of vessel construction is shown in Figure 35.

Figure 35: Submerged arc welding equipment in vessel construction (ESAB)

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SFI / IWE 1.10 Page 41

Pipe welding

During pipe production by means of submerged arc welding the welds can be created as helical or as longitudinal welds. An interesting variant is the spiral pipe welding here. Spiral pipes have the advantage that:  They can be produced “endlessly”, even on the construction site  The weld seam is not oriented in the direction of the main load  The production process can largely be automated It must be considered however that  that although the pipe is produced without ending, it must be cut to a length of approx. 12 to 18 m for subsequent transport  based on the manufacturing procedure, the spiral pipes have a higher flow resistance compared to the longitudinal welded pipes, which cannot be neglected looking at the partly large transport routes of the medium. Longitudinally welded pipes are produced from sheets which are shaped into the split tube with a 3-roller bending machine, or in a press line in different press operations (initial bending, U and O forming). These pipes are used in the areas of offshore technology and pipeline construction, etc. Due to the large piece numbers and pipe lengths, high welding speed rates are aimed at and reached with the nowadays available welding fluxes and welding equipment. Nowadays, longitudinally welded pipes are normally produced with lengths of up to 12 m. Figure 36 shows an accordingly typical edge preparation (wall thickness approx. 16 mm, schematically) for the two-pass method (i.e. first run followed by backing run) in longitudinal welding.

Figure 36: Weld preparation of “longitudinal welded large pipes” for welding in the two-pass method (run/backing run)

The special welding sequence is carried out with the following welding procedures and welding parameters: 1. Welding the tack weld with e.g. MAG, a wire electrode (3 mm) (Current strength approx. 1,300 A, welding speed approx. 8 m/min) 2. SAW three or four-wire welding of the weld (inside) (Total current value approx. 3,000 to 3,500 A, voltage between 34 and 38 V, welding speed up to approx. 2.20 m/min) 3. SAW three or four-wire welding of the weld (cap layer) (Total current value approx. 3,000 to 3,500 A, voltage between 34 and 42 V, welding speed up to approx. 2.20 m/min)

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The longitudinal welding of pipes is shown in Figure 37.

Figure 37: Longitudinal welding of cylindrical pipe sections (ESAB)

Fine grain structural steels are frequently welded with the SAW process, too. Maintaining the correct heat-input and the choice of suitable filler materials and flux powders are to be observed. Regarding the welding flux special attention must be paid to re-drying. Table 10 provides an overview of the distinctive features associated with the processing of fine-grained structural steels by submerged arc welding. Table 10: Characteristics of processing fine-grained structural steels by submerged arc welding Designation

C

Si

Mn

Cr

Ni

Mo

S2

0.12

 0.15

1.0

-

-

-

S2 Mo

0.12

0.15

1.0

-

-

0.5

S2 NiMo 1

0.10

0.15

1.0

-

1.0

0.5

S2 NiCrMo 1

0.10

0.15

1.0

0.45

1.0

0.5

S1 NiCrMo 2.5

0.08

0.15

0.5

0.70

2.5

0.6

typical materials P 255 N P 355 N P 420 N S 690 Q S 690 Q

Standard analysis of wire electrodes for submerged arc welding of fine-grained structural steels in accordance with DVS 0918

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Flux powder type in accordance with DIN EN 760

Production type in accordance with DIN EN 760

Drying temperature (minimum duration 2 hours)

Page 43

Maximum recommended Temporary storage re-drying time temperature °C

MS CS

B

approx. 300°C

10 hours

150

MS CS

F

approx. 150°C

30 days

150

AR AB

B

approx. 300°C

10 hours

150

AR AB

F

approx. 150°C

30 days

150

FB

B

approx. 350°C

10 hours

150

FB

F

approx. 250°C

20 hours

150

Standard values for the re-drying of flux powders in accordance with DVS 0914 Diffusible hydrogen content in the weld metal cm³/100g HD value according to DIN EN ISO 3690

Preferred application

Preferred flux powder type

15 General structural steels Vessel/boiler plates

MS CS AR AB

Fine-grained structural steels with a minimum yield strength < 390 N/mm²

CS AR AB FB

10

7 390 N/mm² to < 690 N/mm² 5

 690 N/mm²

(CS) (AB) FB FB

General recommendation for maximum hydrogen content in submerged arc weld metal in accordance with DVS 0914

10 Application of submerged arc deposition welding An important submerged arc welding method is submerged arc strip cladding. Technically a cladding, suitable for the load application, is attached to a non-alloyed base material. This cladding can protect the component against  corrosion and  wear. Restrictions for the cladding of components (pipes) are given in the material thickness of the base material which should be > 15 mm, and the external diameter must not be less than 300 mm. Surface coatings of approx. 3 - 5 mm width / layer are reachable using strip cladding. The dilution with the base material is approx. 20%. This frequently allows the required chemical compositions of the component surfaces to be achieved within just 1 or 2 layers. Strips with a dimension of 60 x 0.5 mm are used as standard for corrosion protection. In wear protection, strips of 30 x 0.5 mm are used when working with components with small diameters. When working with larger diameters, 60 x 0.5 mm strips are used. As deposition welds for wear protection require high hardness values, filler strips or sintered strips are used in order to achieve the corresponding alloy compositions.

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Sample applications:  Corrosion protection * Cladding of tube plates in tube-bundle heat exchangers * Cladding of connecting pieces etc. in large-dimension vessel construction  Wear protection * Cladding of continuous cladding rolls for steel production * Cladding of components, pipes etc. in the sand, gravel and cement industries In Table 41 a few solid-strip electrodes for cladding are presented. Table 41: Range of solid-strip electrodes for anti-corrosion and anti-wear protection C

Si

Chem. Composition in % Mn Cr Ni

Mo

Other

X 30 Cr Mo W 6

0.3

0.5

1.5

6

0.2

1.5

1.6 W

Wear protection

X 2 Cr 13

0.02

0.2

0.4

12

-

-

-

Wear protection

X 6 Cr 17

0.05

0.4

0.4

17

-

-

-

Wear protection

X 2 Cr Ni 2412

0.02

0.5

1.7

24

12.5

-

-

Corrosion protection

X 5 Cr Ni Nb 199

0.015

0.3

1.5

20

10

-

0.8 Nb

Corrosion protection

X 12 Cr Ni 2520

0.15

0.2

4.0

25.5

20.4

-

-

Corrosion protection

Ni Cr 21 Mo 9 Nb

0.02

0.25

0.1

21.5

Rest

8.5

3.0 Nb

Corrosion protection

Strip electrode

Application

A machine for the submerged arc welding of pipes of small outside diameter is shown in Figure 37.

Figure 37: Surface welding with strip electrode on a pipe (ESAB)

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11 Test questions (1)

When using the SAW hotwire process, in comparison to conventional submerged arc welding,

    (2)

Among others, the ∆U regulation is used in SAW in order to keep the arc length constant. This regulation

     (3)

  

(5)

(6)

Is made to keep the welding speed constant Is made to keep the weld geometry constant Needs a constant voltage characteristic Needs a constant current characteristic Keeps the wire feed rate constant

Agglomerated fluxes have the following properties:



(4)

The energy per unit length significantly increases Increased deposition rate Reduced welding speed Welding speed possibly slightly increased

High water content due to the manufacturing process (low annealing temperature) They are hygroscopic; therefore they must be carefully stored and eventually according to the manufacturer's specifications are re-dried Because of the manufacturing type, undesired metallurgical reactions appear only during welding Because of the manufacturing type, desired metallurgical reactions only occur during welding

The copper coating of wire electrodes in submerged arc welding  Should be thick in order to ensure good gliding in the contact tip  Protects the wire electrode from corrosion  Improves somewhat the current input  Must be thin and strongly adhesive, in order to avoid soldering fracture by peeling copper particles Please mark the correct statements concerning submerged arc welding It is a fully mechanised welding process. Its use usually just makes sense starting from a material thickness of 30 mm. Its use generally makes sense only with a material thickness of 6 mm and above. It is used for cladding work since during the process a strong dilution of the base material is caused.  It is used for cladding work since during the process a slight dilution of the base material is caused.

   

Please mark the correct statements concerning tandem submerged-arc welding.  Two wire electrodes are used in welding  Only one power source is needed.  Two power sources are needed.  The arc-blowing effect can be influenced by the polarity of the power source.  It is welded with two strip electrodes.

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(7)

Which tasks does the welding flux have, in submerged arc welding?  The welding flux serves to increase the conductivity of the arc gap/width.  To avoid the extreme creation of slag.  It is supposed to cool down the weld faster.  It serves for an increase of the arc resistance.  It serves for the shielding gas flow formation.

(8)

Which statements concerning submerged arc welding are correct?  Usually welding is done with the negative pole of the wire electrode by using direct current.  If the wire electrode diameter exceeds 3 mm a constant current characteristic is used (external regulation).  In the case of a smaller wire electrode diameter it is worked with a constant voltage characteristic (internal regulation).  Usually the power sources are clocked.  Usually the power sources are pulsed.

(9)

Which rules of thumb concerning submerged arc welding are correct?  For the selection of current value (amperage) the following applies: I = 100 – 200 * wire diameter  For the welding speed: v = amperage I / 10 * plate thickness t  For the selection of the wire electrode diameter applies I = 200 – 300 * wire diameter.  For the penetration depth with the square butt joint: t approx. 1 mm per 100 A.  For the penetration depth with the Y-butt joint: t approx. 0.7 mm per 100 A

(10)

What applies for the welding fillers of submerged arc welding?  The carbon content of the filler material S1 is at 0.08 to 0.09 %.  With high amperage wire electrodes with a diameter of 5 mm are used.  The filler materials for high-alloyed steels are to be found in DIN 8557.  When fabricating Cr-Ni-steels wire electrodes of a 3 mm diameter are used.  By risk of root fall through, 3 mm wire electrodes are used.

(11)

Which of the following defects in submerged-arc welding belong to “internal” defects?  Metallurgic pores  Undercuts  Slag inclusion  Root fall through/Root fall back  Lamellar tearing

(12)

Which of the following defects in submerged-arc welding belong to the “external” defects?  Excess weld metal  Undercuts  Slag inclusion  Root fall through/Root fall back  Pores

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12 Bibliography DIN EN ISO 14171 - Welding consumables - Solid wire electrodes, tubular cored electrodes and electrode/flux combinations for submerged arc welding of non-alloy and fine grain steels - Classification (2011-01) DIN EN ISO 14174 - Welding consumables - Fluxes for submerged arc welding and electroslag welding Classification (2012-05) M 0948 Submerged arc welding and its process variants (12/1995), DVS guideline, arc welding M 0907-3 Determination of pick-up and burn-off in submerged-arc welding fluxes - Usage of flux powder diagrams (9/2006), DVS regulations, submerged arc welding M 0914 Processing and storage of flux in submerged arc welding (9/2006), DVS guideline, arc welding M 0917 Submerged arc welding of austenitic steels (10/2006), DVS guideline, arc welding 0918 Submerged arc welding of fine grained structural steels (9/2005), DVS guideline, arc welding M 0928 Submerged arc welding of the austenite-ferrite compounds (10/2006), DVS guideline, arc welding M 0936 Submerged arc welding (narrow gap) (12/1988), DVS guideline, arc welding M 0940 Submerged arc welding - surface welding with tape electrode (3/1991), DVS guideline, arc welding

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Resistance welding I

Chapter 1.11-1:

SFI / IWE 1.11-1 Page 1

Resistance welding I

Contents 1 Overview of the resistance welding processes ................................................................ 2 2 Principle of resistance welding .......................................................................................... 2 3 Resistance spot welding .................................................................................................... 3 3.1 3.2 3.3 3.4 3.5 3.6

4

Examples of resistance welding machines ................................................................................. 3 Resistances in the welding circuit .............................................................................................. 5 Influences on the resistances..................................................................................................... 6 Thermal balance ........................................................................................................................ 7 Application of resistance spot welding ....................................................................................... 7 Welding parameters ................................................................................................................... 8

Types of spot welding......................................................................................................... 9 4.1 Dual-sided welding (direct spot welding) .................................................................................... 9 4.2 Single-sided welding (indirect spot welding) ............................................................................... 9

5

Typical faults ..................................................................................................................... 10 5.1 Shunting effect ......................................................................................................................... 10 5.2 Inductive losses ....................................................................................................................... 11 5.3 Electrode wear ......................................................................................................................... 12

6 7 8

Safety information ............................................................................................................. 12 Bibliography ...................................................................................................................... 12 Question............................................................................................................................. 13

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1 Overview of the resistance welding processes Other than many other welding processes most of the resistance welding processes require an electrode force for processing and therefore a pressing force is applied onto the workpiece. Therefore this type of welding is often called “resistance pressure welding process”. In the references and in the text below often only the term “resistance welding” is used, even though this is not completely correct. These processes should not be confused with the resistance fusion welding processes that can be performed without a pressing force. The main application of the resistance pressure welding processes is in the field of thin sheet metal construction of approx. 0.5 to 3 mm individual sheet thickness. Mainly overlap seams are produced here. In individual cases of course considerably thinner but also thicker sheets can be welded with suitable special machines. The butt welding types enable the joining of weld cross-sections of up to approx. 100,000 mm², depending on the capacity of the machine. Resistance welding equipment can also be used basically for other processes using the benefits of resistance heating, e.g. soldering or hot riveting. Metal welding processes

Fusion welding

Conventional fusion welding processes

Pressure welding

Resistance fusion welding

Resistance pressure welding

Resistance pressure welding

Manual metal arc welding

Chamber welding

Resistance spot welding

Arc pressure welding

Shielded gas metal arc welding

Electroslag welding

Resistance seam welding

Cold pressure welding

Projection welding

Friction welding

Submerged arc welding Laser beam welding

Pressure butt welding

…..

…..

Flash butt welding

Figure 1: Classification of the welding processes according to DIN 1910-100 (extract)

2

Principle of resistance welding

In order to create a welding spot, the temperature of the materials to be welded has to be increased, in general, up to the melting temperature. The achieved temperature increase follows (ideally) the following formula:

T = Q/(m/* c)

T Q m c

= = = =

temperature difference [K] amount of heat [J] weight of the heated materials [kg] spec. heat [j (kg of K)]

For resistance welding, heat is created in the material, as the electric welding current flows through the material (resistance). The created heat arises according to Joule's law: 2

Q=I *R*t

Q I R t

= = = =

heat current resistance time

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Resistance spot welding

Abbreviation according to ISO 4063: process 21 I

Process features:

   

F

workpieces overlapped pencile-shaped electrodes (current density) weld nugget in general one joint for each welding sequence

Field of application (examples):

 

For steel:

0.5…3.0 mm (0.05…30 mm)

For Al:

0.5…2.0 mm (0.1…8 mm)

 For simple resistance spot welding at least the following parameters have to be adjusted: 

Squeeze time

tV

[cyc]



Welding time

tS

[cyc]



Hold time

tN

[cyc]



Welding current

IS

[kA]



Electrode force

FE

[N]

Figure 2: Typical sequence of a resistance spot welding process

3.1

Examples of resistance welding machines

Depending on the requirements of the production, different types of welding machines are used. This can be stationary equipment, so-called pedestal welding machines, or also mobile equipment, so-called weld guns. In addition to the widespread standard equipment that can be bought practically out of the catalogue, there is also the possibility of building custom made machines for very special applications.

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The fundamental construction of the machines is comparable and is shown using the example of a pedestal spot welding machine.

Figure 3: Pneumatically operated spot welding machine (schematic)

Manufacturer: DALEX

Manufacturer: DALEX

Manufacturer: NIMAK

Figure 4: Pedestal spot welding machine (pneumatic) and spot welding guns Above: manual gun; below: robot gun in a special customised design

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A relatively high force has to be transferred to the workpieces. This means that the welding machine has to produce this force, has to be constructed properly stable in order to avoid a too strong deformation. The force is generated generally, by a compressed air system (pneumatic). In exceptional cases hydraulic or purely mechanical (e.g. foot lever) systems are also used. Since a few years an increased number of servo-electric drives have also been used. For generating high amperage usually the transformer is integrated into the welding machine. In most cases alternating current (50 Hz) is used. During the welding current flow, the generated heat losses have to be discharged. This mostly requires water cooling. For standard machines the initial start for welding is executed by a foot-operated switch or finger switchbutton (clamp); for special-purpose machines or machines integrated into in automated manufacturing lines this is controlled via a switching contact in the superordinate sequence control. 3.2

Resistances in the welding circuit

Looking at Joule’s law there is only one resistance that seems to be responsible for the heat production. In reality several resistances have to be taken into account. With respect to the closer range of welding two groups of resistances are distinguished, material (bulk) resistances and contact resistances. 

resistances in the upper and lower electrode (material resistance)



resistances in the upper and lower sheet (material resistance)



contact between the upper and lower electrode for the sheet (contact resistance)



contact between the sheet metals (contact resistance)

REM

RM

RMM RM REM Material resistance Figure 5: Resistances on the welding spot

Further resistances are active, e.g. in the cables, electrode arms and at their contact points, however they do not have to be considered here.

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3.3

SFI / IWE 1.11-1 Page 6

Influences on the resistances

The surface resistances are influenced by the electrode force and the state of the material surface. The material resistances are dependent on the leading cross-section, on the temperature and basically on the conductivity of the material which becomes clearly noticeable during welding. The following illustrations describe the relationships.

Surface resistance RS

Electrode force FE Figure 6: Influence of the electrode force on contact resistances (schematic)

It should be noticed that the resistances change over the course of the welding time. Where the contact resistances (RS) are dominating at the beginning, they will recede into the background compared to the material resistances (RM) during the subsequent course. The material resistances increase with the temperature. Resistance

RM

RS [cyc, ms]

RS

RM

Figure 7: Dynamic resistance curves (non-alloyed steel sheet)

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Page 7

Thermal balance

Qtot   2  R  t QVS

QVS

QVE

QVL

QVL

Q eff .

QVE

QVS

QVS

Qtot  Qeff .  QV QV  QVE  QVL  QVS



Qtot  100 [%] Qeff .

Figure 8: Thermal balance of spot welding

3.5

Application of resistance spot welding

Resistance spot welding is preferably used in the mass production of thin metal sheet processing. Particularly the appliance industry (“white goods”) and the car industry are mentioned here. On a steel body of a medium-sized vehicle for example approx. 5,000 spot welds are used. Therefore in this field resistance spot welding is the dominant joining technique. Detail

not welded

Detail

welded

Figure 9: Examples of resistance spot welded parts

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Figure 10: Micro-section of a resistance spot weld (DC04; t=2.0 mm) 3.6

Welding parameters

A great number of different recommendations related to the setting of the welding parameters are available for special tasks. However, it has always to be considered that the required parameters depend on many influencing variables. These include the materials that are to be welded and their thickness(es), surface refinement, and also those parameters which at first appear insignificant, e.g. electrode geometry, cooling, machine behaviour, etc. Therefore the listed values in the table below should only serve as a rough indication for the dimension of the parameters that have to be set. Mild steel (uncoated)

St, zinc coated (elo...hot dip)

Stainless steel

Aluminium

[cyc]

t=thickness of the thinner sheet

Table 1:

Reference values for resistance spot welding

In addition to the materials listed above, most metals can also be resistance welded, however depending on their physical features more or less easily. For surface refined materials sufficient electric conductivity is a basic requirement as a rule. Plastic-coated or painted sheet metals can therefore mostly be joined by resistance welding only with significant limitations. In case of an insulating one-sided coating often only one-sided welding from the rear (uncoated side) is possible. Further reference values for the welding parameters are found in the data sheets of the machine manufacturers or the DVS leaflets [1].

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4 4.1

Page 9

Types of spot welding Dual-sided welding (direct spot welding)

In case of common spot welding two welding electrodes are positioned on the opposite sides of the workpieces. In this way the welding current can flow directly through the area to be welded. Usually one weld nugget is created for each welding sequence. Most standard machines are produced for this type.

Two-sheet welding (single-phase)

Three-sheet welding (dual-phase)

Figure 11: Direct spot welding of two or more sheet metals

If several spots are welded, this can be achieved with a special type of dual-sided welding, also with only one power source.

tot

Balance (equilibrium of forces)

1

2

Figure 12: Parallel spot welding (several spots at the same time)

4.2

Single-sided welding (indirect spot welding)

In many situations it is impossible to position the electrodes on the opposite sides of the workpiece. Examples are large car floor assemblies where machines with large throat depths would be required for dualsided welding. In such cases the special technology of “indirect welding” can be applied.

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Copper base

Copper base

Figure 13: Types of single-sided welding

In the case of indirect welding generally at least two spot welds are generated for each sequence.

5

Typical faults

5.1

Shunting effect

A frequent problem is the occurrence of shunting during resistance welding. Shunting is the effect that part of the current flows not directly through the weld nugget but finds its way via different routes. In some cases the shunting effect can be negligibly small. In many cases however the size of the weld nugget is considerably affected. Some causes of the shunting effect are: 

distance between the spot welds too small with respect to material and thickness



intense contact of the parts that are to be welded close to the welding



contact between the workpieces and current-carrying parts (electrode/-holder/-arms)



indirect welding, especially in the case of disregarding the positioning of the sheet thickness



contact between the workpieces and electrical conductors or inserting fixtures

Itotal

Ishunt

Iweld

I

weld

I

total

I

shunt

Figure 14: Parallel connection in the case of dual-sided welding

The level of the shunting current is generally dependent on the distances between spot welds, sheet thickness(es), conductivity of the material and surface refinements.

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The single-sided spot welding is a special case of shunting effect. FAVOURABLE

tot

UNFAVOURABLE

tot

e

e

shunt

shunt

Copper base

Copper base

Figure 15: Shunt effect for single-sided welding, different sheet thicknesses

Here, compared to dual-sided welding, the current flows not exclusively through the spots to be welded but a part is lost by shunting current. Therefore in the case of single-sided welding, generally a lower quality is to be expected. 5.2

Inductive losses

In the secondary circuit losses arise from the ohmic resistance as well as from the inductive resistance of the machine. The inductive resistance depends on the size of the secondary window “A” (vertical spacing * throat depth), on the frequency of the secondary current as well as on the material mass and geometry of magnetisable components, fixtures or workpieces in the secondary circuit. The secondary achievable current is reduced by the increase of inductivity.

Figure 16: Increased inductivity depends on the workpiece (magnetisable material mass)

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SFI / IWE 1.11-1 Page 12

Electrode wear

High electrode force, current and heat deform the electrode contact surfaces with increasing number of spot welds. If the weld current is kept the same during the production, the current density will decrease and the welding quality will worsen. In addition to geometric wear the alloying pick-up between the electrode and the material/coating of the assembly part, should be considered. Due to this usually poorer conducting and deformed contact areas are developped. The achievable lifetime is strongly dependant on the material to be welded, type and material of the electrodes, cooling conditions, welding parameters, cycle time etc. Generally some 1,000 spots can be achieved (in extreme cases however significantly less).

6

Safety information

During resistance welding high electrode forces are applied. Therefore the machines have to be operated with considerable caution and crushing hazards have to be reduced by safety measures. For this the gap between the electrodes in open position is to be set as small as it is required for the welding of the workpiece.

Expulsions /spatters should be avoided by correct adjustment of parameters, nevertheless they occur occasionally. To avoid injuries in the eyes, hands or bodies, transparent safety glasses, gloves as well as appropriate work clothing have to be worn.

The magnetic fields arising from the high currents on the secondary circuit could lead to malfunctioning of electronic devices. Therefore persons with active implants, e.g. cardiac pacemakers, may not be employed in the vicinity of resistance welding machines.

The electric voltage on the secondary circuit is not dangerous, because the most frequently used maximum no-load voltage is about 15 V.

7 [1] [2]

Bibliography DIN-DVS Taschenbuch 312/1 Schweißtechnik 9 [DIN-DVS Code of Practice 312/1 Welding technology 9] DVS 2902 (parts 1-4) Spot welding of steels up to 3 mm

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Resistance welding I

Page 13

8 Question (1)

Which parameters have to be set for the resistance spot welding?     

(2)

Which of the following statements about the shunting effect are correct?     

(3)

Room temperature Electrode shape and material Type of base material Surface refinements Squeeze time

Which fundamental statements for adjusting the welding parameters are correct?    

(5)

Shunting currents are reducing the welding quality In case of materials with lower conductivity there is no shunting effect The larger the distance between the spot welds, the lower the shunting effect current Positioning and holding devices can cause shunting currents and should, if necessary, be insulated from the workpieces An increase of the sheet thickness reduces the shunting currents

Which factors influence the resistance welding strongly?     

(4)

Welding speed Preheating current Welding current Welding time Electrode force

Thicker sheets require higher current Aluminium is welded with lower current than steel Thicker sheets require higher electrode forces A coating of materials basically does not influence the required current

Which hazards exist during the resistance welding?     

Crushing Welder's arc eyes Dusts, gases, and fumes Noise Expulsions/ splatter

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Chapter 1.11-2:

SFI / IWE 1.11-2 Page 1

Resistance welding II

Contents 1 Electrodes for resistance welding ..................................................................................... 2 1.1 Types of electrodes (special spot-welding) ................................................................................ 2 1.2 Electrode materials .................................................................................................................... 3

2

Seam welding ...................................................................................................................... 5 2.1 2.2 2.3 2.4

3

Projection welding .............................................................................................................. 8 3.1 3.2 3.3 3.4

4 5

Technique .................................................................................................................................. 5 Weld types ................................................................................................................................. 5 Weld Shapes ............................................................................................................................. 6 Types of resistance seam welding machines ............................................................................. 7 Principle ..................................................................................................................................... 8 Projection geometries ................................................................................................................ 8 Reference values and weldable materials ................................................................................ 10 Welding machines for projection welding ................................................................................. 11 3.4.1 Properties of projection welding machines ................................................................. 12

Bibliography ...................................................................................................................... 12 Question............................................................................................................................. 13

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1

SFI / IWE 1.11-2 Page 2

Electrodes for resistance welding

Electrodes for resistance welding are not filler materials, unlike e.g. with manual metal arc welding. The tasks of the electrodes is to transmit the electrode force and the welding current to the material to be welded. After the welding current flow, they (mostly) need to transmit heat away from the workpiece, in order to cool the melted weld nugget. 1.1

Types of electrodes (special spot-welding)

The electrode composition could be as follows: 

Solid electrode (single piece)



Electrode holder + electrode cap



Cap with internal cone



Cap with external cone



Threaded spot welding electrode (for higher electrode forces)

Electrode shank (holder)

a) one-piece

b) two-piece with electrode holder cap

c) two-piece with screw-in tip holder

Figure 1: Basic types of electrodes

In the production usually standard electrodes or caps are used. The dividing into electrode holders and caps serves the purpose that you only need to replace small, inexpensive parts when replacements are needed. Standard electrode caps are available in 13, 16 and 20 mm (d1) nominal diameters.

Figure 2: Different electrode caps according to DIN EN ISO 5821 (selection)

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For a long life-time of the electrodes, sufficient cooling is essential. For uncoated steel sheets and favourable conditions, 1000 or more welds are possible but with surface-refined (e.g. Zn) – depending on the type and thickness of the layer – it can drop to 10% of this value. If aluminium is welded then due to the so-called “alloying pick-up” of Al to Cu, the fast changing surface of electrodes may require re-work already after 10-30 welds. The basic design of electrode cooling as well as some important distance settings are shown in the following picture.

Figure 3: Electrode cooling

1.2

Electrode materials

Resistance welding electrodes are usually made of copper alloys. According to the requirements the corresponding material must be chosen on a task-specific basis. The main criteria for this are presented in the following diagrams. Finally the choice is always a compromise between hardness properties and electrical conductivity.

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Page 4

Usual electrode materials: RP: CuCrZr (with aluminium also CuAg) RR: CuCrZr RB: CuCoBe or CuCrZr

T Electrical

Figure 4: Important properties of electrode materials: hardness at elevated temperatures, good tempering properties and electrical conductivity

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Page 5

Seam welding

Abbreviation according to ISO 4063: Process 22 I

Process features:

F



workpieces overlapped (if required, butt is also possible)



cylindrical, mostly driven electrodes (current density)

 

weld nuggets continuous welds or roll-spot welds are possible

Field of application (examples):

 

for steel:

0.5…2.0 mm (0.05…6 mm)

for Al:

0.5…1.5 mm (0.1…3 mm)

 2.1

Technique

If several weld nuggets or welds strokes (lines) are to be welded after each other in a line or if a continuous weld is to be manufactured, seam welding can be used. Instead of pencile-shaped electrodes, specially formed wheels are used, in order to transfer and concentrate force and current to the workpiece at the welding point. At first sight the shunting effect looks like to be a significant problem here. Due to high temperatures in the welded seam just immediately before, this influence isn't as grave.

Figure 5: Principle of seam welding

2.2

Weld types

A

B

C

D

A

Overlapped weld

B

Mask weld

C

Butt weld with foil

D

Overlapped weld with foil

E

Overlapped weld with electrode wire

E

Figure 6: Weld types for seam welding

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SFI / IWE 1.11-2 Page 6

Weld Shapes

Depending on whether rows of spots or weld lines or continuous welds have to be welded, and depending on the material and plate thickness, different current programs will be used.

Roll spot welds

Continuous welds

All: Pulse – pause current programm Pulse – pause current programm

cool

Continuous current

Figure 7: Current programs and respective weld joints

The distances between the welded sections can be regulated by adjusting the current or the cooling time (pause) as well as by changing welding speed.

Radiator

Heat exchanger (stainless steel)

Cans

Figure 8: Examples of resistance seam welded parts [Pictures SLV Duisburg]

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SFI / IWE 1.11-2 Page 7

Types of resistance seam welding machines

Depending on the components to be welded, different types of welding machines are required. In particular travelling wheel seam welding machines are usually custom-built models. Longitudinal weld machines are, for example, used for producing tubular parts like cans.

Travelling wheel seam welding

Figure 9: Seam welding machines for different tasks

A standard machine for horizontal seam welding is shown in the following figure.

[Fa. NIMAK]

Figure 10: Pedestal resistance seam welding machine

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Page 8

Projection welding

Abbreviation according to ISO 4063: Process 23 Process features:

    

workpieces overlapped (T-joint is also possible) workpieces responsible for current concentration weld nuggets large diameter electrodes multiple joints at same time

Field of application (example):

 3.1

 

for steel:

0.8…3.0 mm (0.5…8 mm)

for Al:

1.0…2.0 mm (0.5…3 mm)

Principle

Projection welding is characterised in particular by the fact that the current density required for welding is generated not through the shape of the electrodes, but by the component.

Figure 11: Concentration of the current by use of an embossed projection

The simultaneous welding of several projections shortens the production times and is therefore often more economical than spot-welding. The number of projections that can be welded at the same time depends strongly on the capacity of the welding unit used. 3.2

Projection geometries

In practice both standardised and not standardised projections are used. Both “embossed” round projections according to DIN EN 28167 and linear- and annular projections according to DIN 8519 are standardised projections. However, not standardised embossed projections are also used. The general rule is that the stiffness of a projection should be sustained as long as possible in order to have to current sufficiently concentrated during the welding process. Sheet thickness

Projection diameter d1 [mm]

t [mm]

Group A

Group B

Group C

t  0.5

1.6

2.0

2.5

0.5 < t  0.63

2.0

2.5

3.2

0.63 < t  1.0

2.5

3.2

4.0

1.0 < t  1.6

3.2

4.0

5.0

1.6 < t  2.5

4.0

5.0

6.3

2.5 < t  3.0

5.0

6.3

8.0

Table 1: Selecting the projection size [according to DIN EN 28167]

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dimensions (mm)

d1

1)

+ 0.1 0

1.6 2 2.5 3.2 4 5 6.3 8 10 1) 2)

a

2)

0.4 0.5 0.63 0.8 1.0 1.25 1.6 2.0 2.5

d2

0.5 0.63 0.8 1.0 1.25 1.6 2.0 2.5 3.2

The diameter of stamp d3 has to be  d1 Permissible difference of projection heights when welding multiple projections at the same time: 5%

Table 2: Dimensions of standard projections [DIN EN 28167]

Figure 12: Component with approx. 20 projection welds (circular projection)

A further characteristic feature of projection welding appears when using “natural” projections. The best known examples are probably reinforcement mats, which are moulded into concrete ceilings, as well as wire products like shopping trolleys. Here are some typical projection welds with “natural” projections (crosswire welds), see figure 13:

Figure 13: Examples of not standardised shapes of projections

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Figure 14: Examples for projection welding: Nuts, threaded bolts and other welding parts

Figure 15: Examples for projection welding: wire components

3.3

Reference values and weldable materials

Similar to resistance spot welding, with projection welding, iron and non-ferrous metals can be welded. 

Steel



Steel, galvanised



Aluminium



Brass



Copper

The projection welding of soft materials or alloys is, due to the required fast follow-up behaviour of the machine, problematic and can only be used safely with custom made welding machines.

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Weld time

Diameter of projection Sheet thickness t [mm]

Table 3: Reference values for projection welding steel sheets [according to DVS 2905]

The indicated reference values refer to the welding of one single projection. Usually multiple projections are welded at the same time. For that reason the reference value of the welding current and the electrode force is to be multiplied by the number of projections to be welded. Welding time remains the same for all projection numbers. 3.4

Welding machines for projection welding

The general assembly of a projection welding machine corresponds to that of a spot welding machine. However, due to the often significantly higher electrode forces, the machine frame is designed much more rigidly (deformation). The high necessary welding currents require also a larger dimensioning of the secondary circuit (e.g. Transformer). A further difference between projection and spot welding machines are the clamping plates with T-slots which are used for the fixation of the workpiece holders and guarantee short set-up times for the changing of welding tasks.

[Fa. DALEX]

[Fa. NIMAK]

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3.4.1 Properties of projection welding machines Due to deformation effects, uneven force distributions may arise with large throat depths. Material resistances within the machine as well as the current displacement effects due to alternating current, can lead to an irregular current flow distribution in the weld metal.

Unequal force distribution under load conditions is caused by the deformation of a C-frame- machine

Current distribution due to ohmic resistances The current distribution decreases due to the longer paths and hence higher resistances to the outside

Current distribution with alternating current Current density is following the skin effect.

Figure 17: Current- and machine-caused effects during projection-welding [DVS 2905]

The above mentioned effects are to be avoided by appropriate measures as far as possible or are to be compensated otherwise the homogeneity of welding would be impaired.

4 [1] [2] [3]

Bibliography DVS 2903 Electrodes for resistance welding DVS 2906 (parts 1-4) Resistance roller seam welding DVS 2905 Projection welding of steels

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5 Question (1)

Which statements about the electrodes are accurate?     

(2)

Which of the following statements about resistance seam welding are correct?    

(3)

Here, hold time strongly influences the melting, unlike in the case of spot welding Cool time is important for weld distances Welding time affects the weld width considerably Welding speed is an additional setting parameter The welding current affects the weld width considerably

Projection welding machines    

(5)

Only continuous welds can be produced this way As is the case for spot-welding, only lap joints can be produced The current density occurs as during spot-welding through the shape of the electrodes Tubular parts can also be welded

Which parameters are important for resistance roller seam welding?     

(4)

In order to make the most of the optimal conductivity, if possible pure copper materials are used (E-Cu) The electrodes should be water-cooled In mass production preferably electrode caps are used The electrodes should be made as small as possible Electrode hardness at elevated temperatures is negligible

Smaller projection welding tasks can be carried out on spot welding machines Projection welding machines must have a good follow-up behaviour Due to the generally stable construction of projection welding machines deformation is not a problem The current distribution across a workpiece can be uneven in the machine

Which statements about projection welding are correct?    

Shunt problems should be given special attention during projection-welding Projection welding additionally allows overlap joints and T-joints The current concentration for weld nugget formation takes place through the shape of components to be welded and can be spot or line shaped Projection welds, unlike spot welds, can be produced only with direct current

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Chapter 1.11-3:

SFI / 1.11-3 Page 1

Resistance Welding III

Contents 1 Butt welding ......................................................................................................................... 2 1.1 1.2 1.3 1.4 1.5

2

Overview of the process variants ............................................................................................... 2 Pressure Butt Welding ............................................................................................................... 2 Flash butt welding ...................................................................................................................... 3 Machines for butt welding with pressure and flash butt welding ................................................. 5 Sample applications ................................................................................................................... 6

Electrical assembly (components) of resistance welding machines.............................. 9 2.1 Types of Current ...................................................................................................................... 10 2.2 Controlling the welding parameters .......................................................................................... 11 2.2.1 Welding control panel ................................................................................................................... 11 2.2.2 Operating modes ........................................................................................................................... 13 2.2.3 Examples of possible current programs ........................................................................................ 13

3

Quality assurance ............................................................................................................. 14 3.1 Testing ..................................................................................................................................... 14 3.1.1 Destructive testing ........................................................................................................................ 14 3.1.2 Non-destructive testing ................................................................................................................. 15 3.2 Measuring, monitoring and controlling ..................................................................................... 15 3.3 Training.................................................................................................................................... 16 3.3.1 Practitioner for resistance welding ............................................................................................... 16 3.3.2 Welding Specialist for resistance welding ..................................................................................... 16 3.3.3 Other courses ................................................................................................................................ 16

4 5

Bibliography ...................................................................................................................... 17 Test questions ................................................................................................................... 18

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Resistance Welding III

1 1.1

Page 2

Butt welding Overview of the process variants

Butt welding using resistance heating is divided into: Pressure butt welding 25 Resistance butt welding

No preheating 242 Flash butt welding 24 With preheating 241

Figure 1: Classification of butt welding processes using resistance heating [ISO 4063]

The processes differ in terms of the maximum weldable cross-sections, the appearances of the weld and the achievable weld qualities. 1.2

Pressure Butt Welding

ISO 4063-25 Process features:

   

Butt joint Clamping electrodes Preparation: smooth workpiece ends Upset metal

Application area (example):

 

For steel:

 0.5 to 30 mm, max. 1,000 mm²

For Al:

only small wires (oxides!)

 The process flow for butt welding with pressure can be described as follows: 

Press the joining parts together



Switch on the current



Wait until welding heat is achieved



Upsetting (if required, with strongly increased force)



Switch off the current



Remove workpiece after cooling time

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Page 3

Figure 2: Sample process sequence for butt welding with pressure, with upsetting [DVS 2931]

Guiding values for the welding parameters when butt welding steel with pressure: 

Welding force:

approx. 40 N/mm²



Welding current:

approx. 60 to 120 A/mm²



Upset force:

approx. 150 N/mm²



Jaw clamping force: approx. 2 x upset force

Maximum weldable cross-section: approx. 1,000 mm². The prerequisite for this welding process is that the joint faces must be coplanar and clean. Only in this way an uniform heating across the entire cross-section is possible. Any contaminations on the joint faces can be included in the joint plane which has a quality reducing influence on the weld. The loss of length during welding must be taken into consideration at the design stage. 1.3

Flash butt welding

ISO 4063-24 abbreviation (also: RA) Process features:

­­ ­

     

­

­

­ ­ ­

Butt joint Clamping electrodes Preparation: Roughly cut joint faces Flashing and upsetting cycle If required, pre- and post-heating Burr

Application area (examples):





For steel:

 1.0 to 300 mm, max. 100,000 mm²



For Al:

max. approx. 10,000 mm²

Flash butt welding features a significantly more complex process sequence. Here we differentiate in principle between the following two main variants: a) Flash butt welding without pre-heating = “Cold flash butt welding” (for large surface cross-sections, e.g. sheet metal parts) b) Flash butt welding with pre-heating (for compact cross-sections, e.g. rails and shafts)

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The cycle can be described as follows:        

Switch on the voltage Move the platen until the joining parts touch and the current flows Pull back the platen Repeat multiple times Slowly move the platen forward with constant sparking Joining parts melt and flash off through resistance heating Move the platen forward abruptly Switch off voltage

Preheat (only for b)

Flashing

Upsetting

If required, flat flashing and post heat treatment can also be performed in the machine beforehand.

Figure 3: Flash butt welding phases [DVS 2901-1] E1, 2:

Free clamping length per workpiece end

E1 + E2:

Distance between current-carrying clamping jaws

V1, 2:

Pre-heating length loss per workpiece end (if applicable, includes the flat flashing loss)

A1,2:

Flashing length loss per workpiece end

U1, 2:

Upsetting length loss per workpiece end

L1, 2:

Length allowance per workpiece end

L1 + L2:

Total length loss

Figure 4: Clamping length and length allowance [DVS2901-1]

The length loss that occurs during individual phases of flash butt welding must be taken into consideration at the design stage.

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Page 5

Guide values for the welding parameters when flash butt welding steel: 

Preheating current:

approx. 3 to 12 A/mm²



Flashing current:

approx. 2 to 7 A/mm²



Flashing speed:

approx. 1 to 5 mm/s



Upset force:

approx. 150 N/mm²



Upset speed:

approx. 25 to 200 mm/s



Jaw clamping force:

approx. 2 x upset force

Maximum weldable cross-section: approx. 100,000 mm². In case of closed cross-sections, higher values have to be used for currents and forces. 1.4

Machines for butt welding with pressure and flash butt welding

The essential components of pressure- and flash butt welding machines are: 

Machine frame



Clamping towers with clamping jaws



Platen



Current circuit with transformer



Welding control panel



Power system Clamping towers Spanntürme

FFSp Sp

Spannbacken Clamping jaws Joining part Fügeteil

FFUSt

Clamping jaws Spannbacken Joining Fügeteilpart

Schlitten Platen

MaschinenMachine gestell frame

Transformer Transformator

Figure 5: Basic structure of a pressure- and flash butt welding machine.

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Clamping area of the wire welding machine [IDEAL]

Pressure butt welding machine with cutting device [IDEAL]

Figure 6: Typical machines for pressure butt welding

Wire welding machine [IDEAL]

Flash but welding machine for truck wheel rims [IDEAL]

Figure 7: Typical flash butt welding machines

1.5

Sample applications

The following images show sample applications of parts that have been welded using either flash butt welding or with pressure butt welding.

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Page 7

Motorcycle wheel rims, aluminium [DVS 2901]

Wheel rim, steel - welded ring and finished product [DVS 2901]

Housing welding, steel [DVS 2901]

Rail welding, steel [DVS 2901]

Motorcycle wheel rim, aluminium [IDEAL]

Mitre welding for staircase construction, steel [SLV Duisburg]

Truck axles, steel [BPW]

Chain links, steel [DVS 2901-3]

Figure 8: Sample applications – Pressure- and flash butt welds

Application areas for resistance butt welding include: 

Automotive industry



Household appliances



Wire, chain and cable industry

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Mechanical and electrical engineering



Construction industry



Iron and steel industry, mining and railway construction



Chemical apparatus construction

SFI / 1.11-3 Page 8

The weldable cross-section shapes are manifold.

Figure 9: Examples of possible cross-section shapes for resistance butt welding and recommended clamping jaws [DVS 2901-1]

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2

Page 9

Electrical assembly (components) of resistance welding machines

The essential assembly of resistance welding machines for spot, projection and seam welding is largely similar. Key differences arise for the drive of the wheels in resistance seam welding, the shape of the electrodes for projection welding and of course the performance of the force and current supply. 1 1 2

3 4

17

18 9

8

15

16

14

6 7

10 13

11

5

12

1

Pneumatic electrode force system

2

Upper arm

3

Machine frame

4

Welding transformer

5

Foot-operated start switch

6

Lower arm (base)

7

Current plate for lower arm fastening

8

Secondary coil of the welding transformer

9

Primary coil of the transformer

10

Step switch

11

Compressed-air connection

12

Machine terminal block

13

Weld control

14

Lower arm (extension)

15

Electrode holder

16

Electrodes

17

Current cable

18

Current bar

Figure 10: Schematic view of components of a resistance welding machine

The electrical system generally comprises the transformer, with rectifier, if required, and the welding control cabinet comprising the control unit and power switch. A step switch can be included in the transformer to allow the general pre-selection of the welding current. Welding control cabinet Schweißsteuerung

Steuerteil Control unit

Actuators Stellglieder I1 I1

Is Is

Leistungsteil Power switch

U10

Elektroden Electrodes

Start Start

U10

U U20 20 Step switch Stufenschalter

Schweißtransformator Welding transformer Figure 11: Scheme of the electrical components of a resistance welding machine

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Resistance Welding III 2.1

Page 10

Types of Current

In principle, both AC and DC welding machines are used. The most widely used, due to their simple and therefore reasonably priced structure, are the AC machines. Circuit diagram L1 L2 L3

Current Waveform

Alternating current “AC” 50/60 Hz

I2

Still the most frequently used current type due to the investment costs. Unfavourable asymmetrical mains load! L1 L2 L3

Direct current, conventional “DC”

I2

For high-performance projection welding machines as well as for spot welding of aluminium. Capacity utilisation of all 3 network phases!

Direct current, inverter “MF-DC”

L1 L2 L3

I2

An “easy” solution, if DC is required at transportable spot welding guns. In the last few years, also increasingly used for high-performance projection welding machines. Normally with 1,000 Hz systems, but also with 10 or 25 kHz for small parts or particularly lightweight systems. Capacity utilisation of all 3 network phases!

Capacitor discharge “CD”

L1 L2 L3

I2

Use in projection welding. Welding times are very short, in order to bring as little heat into the component as possible (distortion minimisation, marking on the rear for materials with surface finishing). Minimal mains load!

Figure 12: Overview of common types of current in resistance welding

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Controlling the welding parameters

Special weld control panels are used to set the welding parameters. In principle, the squeeze time, welding time and hold time as well as the level of the welding current must be set at least. The electrode force is set in most machines not via the control panel but via a pressure reducer which must be adjusted manually. Depending on the manufacturer and type, the control panels can however feature numerous additional functions of which some are illustrated in the following images. 2.2.1 Welding control panel Nowadays, digital control panels with synchronous control are used almost exclusively in the industrial sector. They are either designed for just one welding program or can store several programs, which then can be selected for the relevant welding task.

Single program control panel

Multi program control panel

Figure 13: Typical control panels for individual welding systems

Power semiconductors are used to set both current and IGBTs for MF-DC equipment. Thyristors are used for AC control panels and conventional DC welding systems.

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Page 12

Control unit Power control

Figure 14: Antiparallel connection of the thyristors in AC machines

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Page 13

2.2.2 Operating modes The controls generally feature various operating modes which can be selected depending on the application: 

Single spot mode



Series spot mode



Seam and continuous operation

2.2.3 Examples of possible current programs The use of various current programs can be practical depending on the requirements of the weld connection and/or the materials to be welded. The following image depicts a selection of the usual variants.

tV tV

tS tS

tS tS tP tP IMP=3

tN tN Single-pulse welding

tup

Multi-pulse welding

tdown

ts

Upslope and downslope

tVWtVW

tK tK

tRKtRK

tNWtNW

Current program with pre- and postheat

Force and current program

Figure 15: Current programs, examples

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3

Page 14

Quality assurance

A particular difficulty for quality assurance of resistance-welded parts is that the welding process which generally is very short cannot be observed, as welding takes place hidden between the two parts to be joined. Consequently, various options for quality assurance are defined: the testing of parts after welding (destructive, and - with restrictions - also non-destructive), measurement and monitoring during welding and finally the use of process controllers. 3.1

Testing

3.1.1 Destructive testing In principle, we differentiate in testing between simple methods that can be performed in the workshop and laboratory testing methods that require testing machines. Process acc.to ISO 4063

Test methods

(21)

(22)

(23)

(24) (25)

Peel testing (DIN EN ISO 10447 DIN EN ISO 14270)

X

X

X



Chisel testing (DIN EN ISO 10447)

X

(X)

X



Shear testing (DIN EN ISO 14273)

X

X

X



Cross tension testing (DIN EN ISO 14272)

X

(X)

X



Torsion testing (DIN EN ISO 17653)

X



X



Pressure testing (DIN EN ISO 17654)



X





Tensile test



(X)



X

Erichsen cupping test



(X)



(X)

(X) not usual/practical;  not possible

Table 1: Overview of destructive test methods (selection)

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Table 2: Example of destroyed spot welds (chisel test)

Of course, metallographic examinations are also common. 3.1.2 Non-destructive testing In most cases, non-destructive test methods provide only insufficient statements regarding the quality of the joint. They must under all circumstances be backed up with destructive methods. Process acc.to ISO 4063

Test methods

(21)

(22)

(23)

(24) (25)

RT - Radiographic testing

(X)

(X)

(X)

X

UT - Ultrasonic testing

X

(X)

(X)

X

PT / (MT) - Surface crack testing

X

X

X

X

VT - Visual testing

(X)

(X)

(X)

(X)

(X) no connection with strength or not usual

Table 3: Overview of the non-destructive test methods

3.2

Measuring, monitoring and controlling

In principle, the welding process can be verified by measuring mechanical and electrical parameters. Monitoring and controlling are also possible. Behaviour when the following are changed: Measured variable

Welding current Welding Voltage Resistance curve Electrode movement, thermal expansion ++ Always suitable

Sensor

Rogowski coil Electrode contact Rogowski coil + Electrode contact Displacement sensor or acceleration sensor + Generally suitable

Mains voltage

Electrode force

Electrode shape

Workpiece surface

Shunting

Expulsion

Weld nugget diameter



++ ++





+







+



+

+

+



+

++



+

+



+

+

++



+

+



+

+

° Partially suitable

Table 4: Measured variables for monitoring and control (selection)

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Control systems

Reference value

Application

Time control

Voltage

Change of mains voltage, electrode force, sheet thickness, parallel connection Change of mains voltage, electrode force, parallel connection Change of mains voltage, sheet thickness, parallel connection

Electrode movement Resistance curve Phase-shift control

Current Voltage Power Electrode movement

Combined control

No findings No findings Change of mains voltage, electrode shape, surface condition, electrode force, parallel connection Change of mains voltage, electrode shape, surface condition, electrode force, parallel connection

Resistance (Preheating phase) Electrode movement

Disturbance variables, e.g. parallel connection, network voltage, sheet thickness

Monitoring

Measurement of thermal expansion, measurement of current, voltage or power integral

Automatic readjustment

Stepper control for the welding that follows measurement

Process controls

Here the current time or level is readjusted during welding, e.g. after ½ cyc.

unsuitable

Measurement of the surface temperature

Table 5: Overview of the usual control variants

3.3

Training

One of the most important ways of ensuring the quality of production is, however, the appropriate qualification of the employees involved. While the standards and regulations do not as yet require proof of qualification, the demand for such proof, e.g. from automotive suppliers, has increased considerably in the last number of years. The DVS® and EWF have developed various training courses, which require participants to sit a final examination. The following are recommended: 3.3.1 Practitioner for resistance welding This one-week course [7] comprising theoretical and practical modules is suitable for all people dealing with resistance welding. Operating personnel on the machines, welding coordination personnel, planners and also employees from quality assurance and design are all part of the target audience. 3.3.2 Welding Specialist for resistance welding As resistance welding is only insufficiently covered during normal training for welding coordination personnel, welding specialists and welding engineers, a special three-week course [8] with theoretical and practical modules was developed. 3.3.3 Other courses Corresponding DVS® courses also exist, and indeed were specifically developed, for designers and testers in the area of thin sheet metal structures. Furthermore, certain SLVs (centres for welding-related instruction and experimentation) offer advanced seminars which handle special topics in greater depth.

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SFI / 1.11-3 Page 17

Bibliography

[1]

DIN-DVS Taschenbuch 312/1 Schweißtechnik 9 [DIN-DVS Code of Practice 312/1 Welding technology 9] [2] DIN-DVS Taschenbuch 312/2 Schweißtechnik 11 [DIN-DVS Code of Practice 312/2 Welding technology 11] [3] DVS 2901 (Teile 1-3) Abbrennstumpfschweißen [DVS 2901 (Parts 1-3) Flash butt welding] [4] DVS 2916 (Teile 1-6) Prüfen von Widerstandspressschweißverbindungen [DVS 2916 (Parts 1-6) Testing of resistance pressure welded joints] [5] DVS 2931 Pressstumpfschweißen [DVS 2931 Butt welding with pressure] [6] DVS 2916 (Teile 1-6) Prüfen von Widerstandspressschweißverbindungen [DVS 2916 (Parts 1-6) Testing of resistance pressure welded joints] [7] DVS-EWF 2940 Europäischer Einrichter für das Widerstandsschweißen EWP-RW (EWF 621) - Ausbildung, Prüfung und Qualifizierung - [European resistance welder/fitter EWP-RW - Training, testing and qualification] [8] DVS-EWF 2941 Europäischer Fachmann für das Widerstandsschweißen (EWS-RW) (EWF 525) - Ausbildung, Prüfung und Qualifizierung - [European specialist for resistance welding (EWS-RW) - Training, testing and qualification] [9] DVS 2945 Prüffachkraft für Dünnblechverbindungen [DVS 2945 - Inspection Expert for Thin Sheet Joints] [10] DVS 2946 Prüffachmann für Dünnblechverbindungen [DVS 2946 - Inspection Expert for Thin Sheet Joints] [11] DVS 2948 Schweißkonstrukteur für das Widerstandspressschweißen [DIN 2948 - Welding Constructor for Resistance Pressure Welding]

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5 Test questions (1)

With which of the welding processes listed below can one create butt joints?     

(2)

  

Switching times Evaluating welding quality Setting the welding current

What current types are used for resistance welding?     

(5)

In addition to round cross-sections, sheet profiles, for example, can also be welded together Components are heated via arc heating A length loss occurs during welding Flash butt welding of steel is only suitable for small welding cross-sections (max. 1000 mm2)

What tasks are performed by a normal welding control panel?   

(4)

(21) (23) (22) (25) (24)

Which statements about flash butt welding are correct? 

(3)

Spot welding Projection welding Resistant seam welding Butt welding with pressure Flash butt welding

Mostly alternating current Rarely alternating current Direct current Pulsed currents over 100 kHz Mostly capacitor discharge

What destructive test methods are used for resistance spot welding?     

Peel test Chisel test Charpy test Deflection measurement Dye penetrant test

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Chapter 1.12-1:

SFI / IWE 1.12-1 Page 1

Special welding processes I

Contents 1

Plasma process ................................................................................................................... 3 1.1 Plasma welding............................................................................................................... 5 1.1.1 Principle of plasma welding ................................................................................. 5 1.2 Technical equipment for plasma welding ........................................................................ 8 1.2.1 Plasma welding systems...................................................................................... 8 1.2.2 Power sources for plasma welding ...................................................................... 9 1.2.3 Plasma torch ...................................................................................................... 10 1.2.4 Influencing factors of plasma arc welding. ......................................................... 11 1.2.5 Weld pool backing/gas backing ......................................................................... 11 1.3 Wearing materials and consumables ............................................................................ 12 1.4 The welding process ..................................................................................................... 12 1.4.1 Striking the arc ................................................................................................... 12 1.4.2 The welding sequence ....................................................................................... 13 1.5 Welds, materials and applications of plasma welding using images ............................. 17 1.6 Process variants ........................................................................................................... 21 1.6.1 Micro-plasma welding ........................................................................................ 21 1.6.2 Plasma-powder joining and arc surfacing .......................................................... 22 1.6.3 Plasma MIG welding .......................................................................................... 23 1.6.4 Plasma-hot-wire surfacing ................................................................................. 25 1.6.5 Plasma soldering ............................................................................................... 26 1.6.6 Plasma spot welding .......................................................................................... 26 1.7 Bibliography .................................................................................................................. 28 1.8 Test questions .............................................................................................................. 29

2

Electron beam material processing ................................................................................. 30 2.1 Basics ........................................................................................................................... 30 2.2 System technology ....................................................................................................... 32 2.3 Process variants in addition to welding technology....................................................... 37 2.3.1 EB drilling........................................................................................................... 37 2.3.2 Surface treatment by electron beam .................................................................. 38 2.4 Electron beam welding ................................................................................................. 40

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2.4.1 Process flow of electron-beam welding.............................................................. 40 2.4.2 Preparation and design of welded joints ............................................................ 41 2.4.3 Sample applications ........................................................................................... 42 2.4.4 Troubleshooting in electron-beam welding ........................................................ 46 2.4.5 Process-specific advantages and disadvantages .............................................. 46 2.5 Test questions .............................................................................................................. 47 2.6 Bibliography .................................................................................................................. 48 2.7 Overview of standards: Electron beam technology ....................................................... 48 3

Laser welding .................................................................................................................... 49 3.1 Summary description of the procedure ......................................................................... 49 3.1.1 Description of the general principle .................................................................... 49 3.1.2 Beam generation ............................................................................................... 49 3.1.3 Laser types ........................................................................................................ 51 3.1.4 Effect of the laser beam ..................................................................................... 51 3.1.5 Component geometry and processing materials ................................................ 52 3.1.6 Process-specific advantages and disadvantages .............................................. 52 3.1.7 Areas of application ........................................................................................... 53 3.1.8 Beam focussing ................................................................................................. 54 3.2 Laser types – detailed description ................................................................................ 55 3.2.1 CO2 laser ........................................................................................................... 55 3.2.2 Solid-state laser ................................................................................................. 57 3.2.3 Slab laser ........................................................................................................... 59 3.2.4 Fibre laser .......................................................................................................... 60 3.2.5 Diode laser......................................................................................................... 61 3.2.6 Summary ........................................................................................................... 62 3.3 Procedure for laser material processing ....................................................................... 62 3.3.1 Laser cutting ...................................................................................................... 62 3.3.2 Welding .............................................................................................................. 62 3.3.3 Variants of laser welding .................................................................................... 66 3.3.4 Surface treatment .............................................................................................. 67 3.4 Test questions .............................................................................................................. 71 3.5 Bibliography .................................................................................................................. 73

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Plasma process

In 1955, UCC (Union Carbide Corporation) sold a plasma torch for the plasma cutting of aluminium under the name “Heliarc-Cutting”. One year later, this torch was taken over by the Linde company and renamed the “Presslichtbrenner”. The development of the plasma process was largely due to the increased use of nickel-chromium steels and aluminium alloys. Neither group of materials could be cut using autogenous flame cutting. The plasma procedure differs from other arc welding processes in that the arc does not operates freely, but is instead constricted through a water-cooled copper nozzle. This increases its power density and consequently yields a wealth of particular features compared to the free-igniting arcs. The arc generally is ignited between a non-consumable tungsten electrode and the workpiece. The plasma nozzle is de-energised, depending on the type of circuit. An inert gas (argon, called plasma or also centre gas), which is heated to a very high temperature in the arc, flows from the borehole of the nozzle, is partly ionized and becomes electrically conducting (plasma effect). Through the shape of the plasma nozzle and the quantity of plasma gas, the arc can be adjusted within a very wide range to the welding task in hand. The plasma welding is generally mechanized; this is preferable for long welds. Butt joints without weld preparation can be welded on sheets with wall thicknesses ranging from 0.1 (microplasma) up to approx. 10 mm. Filler material is generally only used to compensate for a weld backslide, if the gap width, which should not exceed max. 10% of the sheet wall thickness, is too large. Argon, with the customary purity of 4.6, is used as the “plasma gas”. With reactive materials such as e.g. titanium or zirconium it can be necessary to use a higher purity (4.8). Building on the plasma torch discussed above, a wealth of procedures have been developed for joining, surfacing and cutting numerous materials. Table 1: Breakdown of plasma processes, applications and parameters

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In DIN EN 14610 and DIN 1910-100 (Welding and allied processes - Vocabulary), plasma cutting is categorised as follows: Plasma welding (15), arc welding, whereby the plasma (electrically conducting, ionized gas) of a constricted arc is used, which in turn is divided into:  Plasma arc welding (transferred arc), whereby the electrical energy source is connected between the electrode and workpiece.  Plasma arc welding (non-transferred arc), whereby the electrical energy source is connected between the electrode and the constricting nozzle.  Plasma arc welding with semi-transferred arc, whereby a non-transferred arc and a transferred arc burn.  Powder plasma welding, welding with the addition of metal powder. The respective circuit types are shown in Figure 1.

Figure 1: Arc types in plasma welding (according to Baum)

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Plasma welding

The higher power density of the constricted plasma arc allows higher welding speeds to be achieved compared to TIG welding and also allows thicker workpieces to be welded with a butt joint, without weld preparation. Otherwise, plasma welding is similar to TIG welding on many fronts. The filler materials are comparable, although plasma welding is rarely used for filling layers. In principle, all electrically conducting and fusion-weldable materials can be processed. 1.1.1

Principle of plasma welding

In physics, a plasma is defined as an electrically conducting gas (fourth state of matter). It comprises charge carriers (electrons, multiply charged ions) and neutral particles. The charge carriers emit the energy they absorb through dissociation and ionization into the environment and, in particular, into the workpiece, in the form of recombination heat. In contrast to the free-igniting and strongly divergent TIG arc, the edges of the plasma arc are practically parallel and, consequently, the distance between the torch and the workpiece is less critical than it is for TIG welding, Figure 2.

Figure 2: Free-igniting and strongly divergent TIG arc (left) and constricted and practically parallel plasma arc (right) Due to the practically parallel edges of the plasma arc, changes in the torch distance do not impact as much on the penetration as they would with a TIG arc. The direction of the constricted arc is also considerably more stable, compared to the free-igniting arc, and is barely diverted by e.g. an edge offset of the workpiece to be welded, Figure 3.

Figure 3: Consequence of the rigidity of the arc

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(Laser, electron-beam and plasma welding)

Figure 4 depicts a comparison between the basic principles of the TIG and plasma welding processes.

Figure 4: Comparison of TIG and plasma welding (procedural principle), according to Baum A TIG arc burns freely between a non-consumable tungsten electrode and the workpiece. Depending on which shielding gas is used, the penetration shapes and or the welding speed can change quite considerably at times under otherwise constant conditions. A key reason for this is the heat conductivity of the gases. Figure 5 shows that e.g. He has 5 - 10 times greater conductivity than Ar in the temperature range from 1,000 - 10,000 K. From the energy realized in the arc, a good heat-conducting gas (helium) gives off more heat into the environment and therefore also into the workpiece.

Figure 5: Thermal conductivity of gasses at high temperatures

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(Laser, electron-beam and plasma welding)

A current can only be transferred via a gas if this gas is ionised. A very hot arc core is formed. The temperature decreases radially towards the outside. This temperature drop (temperature gradient) differs in the steepness and width of the drop depending on the gas used. In the case of a good heat-conducting gas (e.g. He), the temperature drop is flatter and in a poor heat-conducting gas (e.g. Ar) it is steeper, Figure 6.

Figure 6: Temperature profile of TIG (different shielding gases) and plasma arcs If one considers the temperature drop and compares it to the respective typical penetration shape, one notices that the progression is the same, Figure 6, above. In the case of the plasma arc, the temperature decrease from the arc core towards the outside is strongly increased by the water-cooled copper nozzle (thermal pinch effect). With a plasma nozzle borehole of e.g. 3 mm, the temperature in the arc centre decreased very steeply over a distance of less than 1.5 mm radially towards the outside from about 20,000 K, Figure 7.

CATHODE OPEN ARC 40 CFH ARGON 200 AMPS 15 VOLTS

CONSTRICTED ARC (3/16 IN DIA. ORIFICE) 40 CFH ARGON 200 AMPS 30 VOLTS

Temperatures (Cº)

ANODE

and higher

Figure 7: Temperature distribution for a TIG and a plasma arc

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Page 8

Advantages of plasma arc welding over TIG welding       

Focussed arc, not sensitive to changes in arc length Arc with directional stability even at very low current strengths Flashlight effect to adjust the torch Higher welding speeds particularly with keyhole effect Low heat input, low distortion, narrow heat-affected zone Good weld shape (weld width to depth = 1:1 to 1:2) Secure penetration and low level of excess weld and root sag

Disadvantages of the plasma process  

Special training of the operating personnel Equipment is more expensive

Figure 8 shows a comparison of power densities of different welding processes.

MANUAL METAL ARC WELDING MAG WELDING PLASMA WELDING LASER WELDING EB WELDING Power Density [W/cm2] Figure 8: Power densities of different welding processes 1.2

Technical equipment for plasma welding

The technical equipment is comparable to TIG welding; the key difference is in the torches. 1.2.1

Plasma welding systems

Figure 9 presents a sketch of the possible structure of a complete plasma welding system. Control and regulation device Choke coil

High voltage pulse generator

Energy sources for the nontransferred arc (indirect plasma generation)

Energy sources for the transferred arc (direct plasma generation)

Figure 9: Fundamental structure of a plasma welding system (here with two power sources) [1]

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Figure 10 presents the possible installation of a real plasma welding system.

Figure 10: Possible installation of a real welding system (photo courtesy of Air Liquide) 1.2.2

Power sources for plasma welding

Constant-current sources (direct current or alternating current) are used, i.e. a small change in the arc length brings about a large change in the arc voltage and no or only a minor change in the welding current; as is also the case with TIG welding. As with TIG power sources, the welding current is controlled via moving core, transductor, thyristors or transistors. The open-circuit voltage of these energy sources is generally increased up to the maximum permissible limit (VDE), in order to guarantee good ignition properties. Depending on the intended purpose, the power sources are equipped with additional devices, e.g. increase and fall of plasma gas and current, pulsed welding current etc.

Figure 11: Example of a power source for plasma welding (photo courtesy of SBI)

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SFI / IWE 1.12-1 Page 10

Plasma torch

Plasma torches can be obtained in both manual and mechanical formats. They are a little larger than TIG torches. Torches with negatively polarised electrodes achieve very high service life; the wearing part here is the copper nozzle. Torches that also permit welding with positively polarized electrodes (welding of Al and Mg materials, electrode loading up to approx. 8 times higher) are offered only by a handful of companies. The tungsten electrodes must be centred very accurately, so that the radial distance between the nozzle and the electrodes is perfectly equal across the scope of the electrodes. The axial distance between electrodes and nozzle is determined by means of setting gauges and should be kept to  0.1 mm. If the electrode is overloaded during TIG welding (too much current), it will melt off. The damage is relatively little. If on the other hand, a plasma torch is overloaded, generally a higher level of damage is given - nozzle melted, electrode damaged, possibly even damage to the torch body. For this reason, the specified reference values for the max. current strength must be maintained precisely. For each torch, there are electrodes and nozzles with different bore holes. Nozzles for positively polarized electrodes are designed differently than for negatively polarized electrodes. In general: Negatively polarized electrode - Electrode, sharp - Interior of nozzle, conical borehole Positively polarized electrode - Electrode, round-head - Interior of nozzle, semi-circular, drilled out. Regrinding of tungsten electrodes should be done precisely according to the template (of a new electrode) on a grinding machine. Regrinding by hand is problematic as even a slight eccentricity would permit the plasma jet to burn on one side. The most important settings for plasma arc welding are: 1. Welding current intensity to suit the welding task 2. Plasma gas quantity 3. Diameter of plasma nozzle borehole Figure 12 shows a machine plasma torch. Furthermore, a TIG torch with strike aid, a TIG torch with drag nozzle for welding reactive materials such as titanium and vanadium and a TIG torch with the option for magnetic arc deflection through a coil are shown.

Figure 12: Machine plasma torch with various TIG torches (photo courtesy of Air Liquide)

Figure 13 depicts other plasma torches.

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Machine torch

Quick Connector torch, Robotics

SFI / IWE 1.12-1 Page 11

Manual torch

Figure 13: Modern plasma torches (photo courtesy of MIG-O-Mat) There are also plasma torches on the market that feature “bypass boreholes”. The aim of these nozzles which have a slightly different design, is to constrict the plasma jet a little more outside the nozzle, by using cold plasma gas guided along the side from two boreholes in order to increase the achievable weld speed (power density) a little more.

1.2.4 Influencing factors of plasma arc welding. The most important influencing factors of Plasma welding are      

Current intensity: Adjusted to the welding task; with increasing sheet thickness, the current intensity and plasma gas quantity are to be increased Weld type: Higher current strength for fillet welds than for butt welds Technology: Higher plasma gas quantity required for keyhole technique Plasma nozzle ø: Adjusted to the welding task Plasma gas: Always argon (purity 4.6) Shielding gas: Structural steels - Ar, Ar+CO2, Ar+O2 CO2 CrNi steels - Ar or Ar-H2 mixtures (e.g. 6.5% H2) Ti, Zr - Ar (possibly higher purity 4.8 required) Al, Mg - Ar, Ar-He mixtures

The use of helium as a shielding gas for Al alloys introduces a higher quantity of heat into the welding point, making faster welding possible; the frequency of pores can be reduced by adding helium. 1.2.5

Weld pool backing/gas backing

In order to obtain a well-formed and oxide-free weld root, gas backing is often used particularly when welding high-alloy and stainless steels, Figure 14. As with TIG welding, if a smooth and oxide-free weld root formation is required, a backing gas (argon) or a purging gas (nitrogen with a small quantity of hydrogen, 5 to max. 15%) is supplied to the weld pool backside. © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised transmission are prohibited and shall be legally pursued

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Weld pool backing

Figure 14: Weld pool backing

1.3

Wearing materials and consumables

Tungsten electrodes with larger diameters than those used in TIG welding are used; either from pure tungsten or with small quantities of additives (e.g. cerium oxide, lanthanum oxide or similar). The electrodes have a substantially longer service life compared to those used in TIG welding. The water-cooled nozzles made from copper materials have service lives of, at times, several shifts. When the nozzle borehole is visibly worn out, it must be replaced. The nozzles have a simple structure and are not costly. Only inert gasses are used as the plasma gas. In the shielding gas, active gasses such as hydrogen, helium and carbon dioxide can also be added in small quantities, depending on the application. The filler materials correspond to those used in TIG welding. Filling layers are generally not created with plasma welding. If filler materials are added (desired weld reinforcement for butt joint, compensation of weld with lack of incomplete penetration where the gap between the metal sheets is too large), additives of the same type are used, as, like in TIG welding, there is no burn off of alloy elements.

1.4

The welding process

Plasma welding is generally mechanized. The arc is ignited inside the torch, so that there can be no contact between the workpiece and the electrode. 1.4.1

Striking the arc

Electrode

Negative pole

English designations: DCSP (Direct Current Straight Polarity) For most applications the electrode is negatively polarized (cathode). The workpiece is positively polarized (anode). A WIG arc is struck without contact using high-voltage pulses. As the electrodes in the plasma torch are in the nozzle, a pilot arc is required to strike the main arc from the electrode to the

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workpiece. The pilot arc is struck with high-voltage pulses between the electrode and the nozzle (current strength approx. 10-20 A). This pilot arc ionises the plasma gas issuing from the nozzle. After switching on the welding current circuit, the arc jumps from the electrode, through the nozzle, to the workpiece (Figure 15).

Figure 15: Principle of ignition in plasma welding with negatively polarised electrodes (acc. to Baum) 1.4.2

The welding sequence

With plasma welding, welds can be created with thicknesses ranging from just a few tenths of a millimetre through to approx. 10 mm in the butt weld, without weld preparation. Sheet thickness ranges generally used in practice are listed in Table 2. Different working methods are applied, from microplasma welding of the thinnest components through light sheet metal welding (also called the push-through technique or (soft) plasma welding) through to welding using the keyhole technique (also called keyhole welding or plasma keyhole welding). In the thinner sheet thickness range, the heat of the plasma jet is applied to the sheet surface and the welding speed results from the electrical energy supplied in relation to the heat conductivity and thickness of the workpiece. Relationships between the workpiece thicknesses to be welded, current intensities and the welding procedure designation are summarised in Table 2.

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Joining by welding Current intensity [A] 0.1 ... 10 1 ... 20 5 ... 40 40 ... 100 100 ... 200 100 ... 350

Microplasma welding

Plasma and plasma keyhole welding

Foil Thin sheet Thin sheet Thin sheet ---

---Plasma welding Plasma welding Plasma keyhole welding

0.05 ... 0.2 mm 0.2 ... 0.5 mm 0.5 ... 1 mm 1 ... 2 mm

0.5 ... 1.5 mm 1.5 ... 3 mm 3 ... 10 mm

Table 2: Sheet thickness ranges for plasma welding 1.4.2.1 Key-hole welding. If the plasma gas quantity is increased when plasma welding with currents > 100 A, the exiting plasma jet is capable of pushing the molten weld metal to the side and to pierce through the sheet. If, after piercing the sheet, the torch is moved forward, the weld metal that was pushed to the side flows back together again behind the keyhole, Figure 16 and 17.

. Figure 16: Techniques for plasma welding (Baum)

Plasma welding torch

Weld

Figure 17: Formation of the keyhole

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This penetration is visibly identifiable on the backside of the sheet metal and consequently it can also be checked. In keyhole welding, the plasma jet pierces the workpiece because of the arc pressure (influenced by the quantity of the plasma gas). The arc attachment point is not focussed on the workpiece surface but instead moves inside the keyhole. The heat energy of the arc attachment point and the emitting heat of the gas being cooled down and simultaneously being recombined at the workpiece surface, are giving off heat across the entire sheet thickness to the component. The degree of efficiency of heat input is thereby considerably greater than with a heat flow from the sheet surface to the workpiece. The result is a high welding speed with comparatively large sheet thickness. Advantages of the keyhole technology      

Reliable penetration Uniform weld root Butt joints for CrNi steel approx. 3 – 9 mm (without) or with small quantities of filler material Butt joints for structural steel approx. 4 – 8 mm (without) or with small quantities of filler material Butt joints for aluminium alloy approx. 5 – 7 mm (without) or with small quantities of filler material Butt joints for titanium approx. 3 – 10 mm (without) or with small quantities of filler material

Note:

   

The filler material can be necessary if the gap between the sheets, (this should not be more than approx. 10% of the sheet thickness) is in the upper range and if without filler material the top of the weld would backslide a little. For thick sheets, the weld root with plasma keyhole and filling layers can be welded with another procedure. With aluminium alloys, a filler wire can be helpful during keyhole welding due to the stronger root formation.

Low heat input (small heat-affected zone) Low distortion Comparatively high welding speed High weld quality

As with TIG welding, plasma welding does not give rise to fusion faults. The comparatively high welding speed is shown in Figure 18 in the form of a comparison with TIG welding on the same material.

Figure 18: Comparison of welding speed between TIG and plasma welding

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As of a sheet thickness of approx. 3 mm, the keyhole effect can be created – exclusively by increasing the quantity of plasma gas and the welding speed increased. With smaller sheet thicknesses, the quantity of molten metal surrounding the keyhole can be too little, thereby no longer forming a coherent weld seam (holes) behind the keyhole after hardening. From around this sheet thickness of approx. 3 mm, TIG welding would already require a two-layer approach (with weld preparation and using fillers). The application of the keyhole technique requires:     

Investments costs special training for welders (setting parameters) only mechanised welding is possible good weld preparation required (gap: max. 1/10 of the sheet thickness) Weld positions PA, PC, PG

Due to the high energy density of plasma key-hole welding the weld preparation is different. Up to 10 mm thickness the square but joint can be selected depending on the base material type (figure 19).

Weld preparation TIG welding

Weld preparation Plasma Key-hole welding

Figure 19: Weld preparation for TIG and plasma welding In general Plasma welding is used in the flat, PA-position but other positions are possible and common.

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Welds, materials and applications of plasma welding using images

The following pictures show some applications of plasma key-hole welding.

Figure 20: Weld on stainless steel Weld surface, left Weld root, right

Figure 21: Plasma-welded longitudinal welds on pipes made from stainless steel

Figure 22: Plasma-welded stainless steel containers (Air Liquide)

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Figure 23: Plasma-welding of a pipe elbow made from stainless steel (Air Liquide)

Figure 24: Cross-sections of plasma-welded sheets of unalloyed structural steel with sheet thicknesses of 4, 6 and 8 mm

Figure 25: Plasma-welded pipes made from low-alloy steel (Air Liquide)

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Figure 26: Plasma keyhole welding Base material S1100QL t = 6 mm Plasma gas: Argon 100%, 5 l/min; shielding gas argon +10% CO2, 13 l/min Vs = 20 cm/min, gap w = 0 mm Vd = 1.20 m/min Pulsed current IP = 250 A; Base current IG = 62.5 A; Pulse frequency Pf = 15 Hz (GSI mbH, NL SLV Munich)

Figure 27: Aluminium, material 5083 Sheet thickness 6 mm Plasma-welded (DC/+) Bend test Stimulated, ductile forced rupture (Linde, Schalchen)

(photo courtesy of SPB)

Figure 28: Plasma welding of thin-walled components made from aluminium alloys

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Figure 29: Sheet thickness 9 mm Plasma gas: Argon between 1.5 -3.2 l/min Shielding gas: Argon 15 l/min Welding speed: 0.25 m/min Filler wire: 1.6 mm Ø; wire speed: 2.5 m/min Welding current 260 A AC frequency: 80 Hz; AC pulse width: 50%

Figure 30: Plasma keyhole welding on aluminium with alternating current

Figure 31: Plasma welding of housings made from aluminium alloys using robots

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Process variants Micro-plasma welding

Due to the particular course of the arc characteristic curve of the constricted plasma arc, there is still an intersection point, even at the smallest current values, with the characteristic curve of the power source and therefore a stable working point. Current intensities of approx. 0.3 to around 25 A are applied to weld workpieces in the thickness range from 0.01 to approx. 1.5 mm. With micro-plasma welding, e.g. the thinnest of foils and wire netting are welded.

Figure 32: micro-plasma arc (Linde). Figure 33 shows a micro-plasma torch and figure 34 shows some components.

Bellows made of X 2 CrNiMo 18 12

Figure 33: Microplasma torch

Wire with 0.5 mm diameter, steel X5CrNi 18 9

Figure 34: Microplasma-welded components

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Plasma-powder joining and arc surfacing

With these two processes, two separately adjustable arcs are used. One ignites between the tungsten electrode and the copper nozzle. With this arc (non-transferred), the powder supplied through or beside the nozzle can be melted in a targeted manner. The second arc ignites from the tungsten electrode right through the nozzle (transferred) to the workpiece and melts the base metal in a targeted manner (little melting of the base metal - low level of dilution). The structure of such a torch is presented schematically in Figure 35.

Figure 35: Schematic view of the principle of a torch for plasma joining and arc surfacing [1] Plasma-powder welding was developed only a few years ago. As the powder is supplied concentrically around the arc, the torch does not have to be turned in accordance with the component geometry, for example, when using robots to perform welding - in contrast to TIG welding with filler wire. The good gap bridging ability and the option to supply very defined amounts of filler material, tailored to the geometry of the component to be joined, is also advantageous. Components made from aluminium alloys, for example, are welded using this process, Figure 36.

Figure 36: Plasma-powder welding of AlMg 5

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Another weld join is shown in Figure 37.

Plasma-powder weld

Figure 37: Plasma-powder welded, circumferential weld Plasma-powder arc surfacing (also: PTA procedure “Plasma-Transferred-Arc”) has been used in manufacturing for many years. It is used primarily to create a wear- and corrosion-resistant layer. Among other things, materials are used which cannot be manufactured as wires, e.g. stellites. One particular feature of the process is the achievably low dilution of up to minimum 5% and the formation of a low heataffected zone. The deposition rate can be up to 20 kg/h. The achievable surface output is comparatively low, which is why it is the preferred option for coating smaller components (also small-volume runs due to the low heat input such as e.g. valves of large motors). The process is usually fully mechanised. Frequently used surface weld powders include wear-resistant materials and also nickel-based, cobalt-based and iron-based alloys. A typical, bead-shaped surface weld is shown in Figure 38.

Figure 38: Plasma-powder arc surfacing (SLV Halle)

1.6.3

Plasma MIG welding

This process is used both for joining and surfacing. It can yield high-quality welds on aluminium materials. This procedure improves the efficiency of MIG welding by preheating the welding wire and bringing additional heat into the component. A further advantage lies in the option offered by the plasma arc to heat up the welding point to such an extent that the addition of filler material (MIG) does not lead to a lack of fusion. The high cost (including that for two power sources) and the difficulty in operation (many setting parameters) restrict its application to special cases. Figure 39 shows the principle underlying the process.

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Figure 39: Plasma MIG welding, schematic representation (according to Baum)

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Plasma-hot-wire surfacing

Process for large-area surfacing. By supplying 2 hot wires, very high deposition rates and high surface outputs can be achieved. The basic idea of this Plasma-hot-wire process is to separate the fusing of the base metal and the melting of the filler material. The plasma torch can be operated with comparatively little power, which has a favourable effect on the heat input and thereby on the joint between the base metal and cladding as well as on the width of the heat-affected zone and the dilution. The penetration depth and surfacing thickness can be varied independently of one another via the plasma current and the hot wire efficiency and feed. It is used in particular where submerged arc welding cannot be used due to a lack of powder; thus for example in the area of materials used in off-shore technology. Figure 40 shows the process principle.

Figure 40: Plasma hot wire surfacing system, schematic representation

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Plasma soldering

With plasma soldering, the filler material is not guided live into the arc, in contrast to gas-shielded metal arc soldering. The filler material is thus melted off largely independently of the supplied energy per unit length of weld and consequently the weld geometry can be influenced within comparatively broad limits. The gap bridging ability is particularly high. Plasma soldering is used particularly in the automotive industry and is preferred here for joining galvanized sheets, because the deliberately low heat input via the plasma arc “protects” the zinc layer, Figure 41.

Figure 41: Manual plasma soldering on the door sill of a BMW car body (photo courtesy of BMW, Binzel)

Figure 42: Plasma soldered joint without fusing of the base metal (Figure: EWM) 1.6.6

Plasma spot welding

Like TIG spot welding, the torch is applied to the workpieces to be joined with gentle pressure and, with the pre-set welding time, the fusing of generally two but possibly of several overlapping sheets is performed. The application is used particularly with sheets of different thicknesses or which can only be accessed on one side. Prerequisite for a good quality weld is above all, the cleanliness of the surfaces of the components to be welded.

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The welding process is used, e.g. in the stainless steel processing industry, automotive industry, mechanical and systems engineering, foodstuffs industry, container and cabinet construction. Sheets that are galvanized on both sides can also be successfully welded whereby due to the high energy concentrations and the resultant short welding time, only minimal burn-off of the zinc layer around the welded point occurs. Figure 43 shows a torch set up for plasma spot welding. Figure 44 shows a cross-section of a corresponding weld, while Figure 45 shows spot welds on a container.

Figure 44: Cross-section of a plasma spot weld (photo courtesy of SBI)

Figure 43: Torch for plasma spot welding (photo courtesy of SBI)

Figure 45: Plasma spot welds on a stainless steel wine tank (photo courtesy of SBI)

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Bibliography

[1] [2] [3] [4] [5] [6] [7] [8] [9]

Baum Linde, Welding Shielding gases: Development, Consulting and Application Technology del Plasma, Mario Marconi, Genova 1983 DVS-Leaflet 0937 Root protection in gas-shielded arc welding Leaflet 822: The processing of stainless steel, Information centre Stainless Düsseldorf 2001 DIN 65153:1997 06 Aerospace – Acceptance testing of Plasma arc welding equipment DVS leaflet 0919 Tungsten-Plasma arc welding DVS Leaflet 0966-1 Plasma-MIG/MAG-Welding – Technical equipment DVS Leaflet 0950 Mechanised TIG- und Plasmas arc welding – Requirements of power sources and equipment technology [10] DVS Leaflett 6/2001 Arc welding [11] DVS-Leaflet 0938-1 Arc brazing – Basics, Processes, Installation requirements -Verlag Düsseldorf

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(1)

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Test questions

What feature of the plasma process gives rise to the significantly higher power density compared to e.g. the TIG welding process?

   

The use of shielding gas containing hydrogen The water-cooled copper nozzle The use of helium as the plasma gas The constriction of the arc while welding with alternating current

(2)

What materials can be welded with plasma welding (name at least four)?

(3)

Name at least five advantages of plasma welding over TIG welding.

(4)

In what sheet thickness range can plasma welding be used to create a butt joint without weld preparation?

   

0.1-1.0 mm 3.0-9.0 mm 6.0-12.0 mm 10.0-20.0 mm

(5) Which physical characteristics of the gasses used during plasma welding have a particular fluence on the formation of the plasma jet and thus on the shape of the weld pool?

    (6)

in-

The specific weight The dissociation or ionisation energy The thermal conductivity The recombination heat

How is the arc ignited in plasma welding?

   

Using high-voltage pulses between the tungsten electrode and the workpiece Through brief contact between the torch and the workpiece Using high-voltage pulses between the tungsten electrode and the nozzle to start a pilot arc Using a high-frequency pulsed current

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Electron beam material processing

Abbreviation: EB (electron beam), standardised designation 51 2.1

Basics

Figure 1: Basic design of an electron-beam unit for material processing, high voltage column for high beam power (left), sketch of a beam column (right).

An electron-beam gun essentially comprises a beam generator which comprises a cathode, Wehnelt cylinder and anode, as well as various coils for focussing and for beam deflection. Other magneto-optics are used as options to influence the beam geometry and shape. There are various options for acquiring free electrons to create the beam. The most commonly used is the directly heated cathode, whereby power flows through a cathode, heating it up so that electrons are emitted from its surface. In order to accelerate these electrons to one- to two-thirds the speed of light, a voltage of up to 150kV is created between the cathode and the anode. In order to avoid discharges between the potentials and the oxidation of the cathodes, a high vacuum exists throughout the entire electronbeam gun. This also offers the advantage that the accelerated electrons are not exposed to collisions with other particles, which would lead to the deceleration and scattering of electrons. In order to achieve energy densities >106W/cm2 and beam diameters of 100 µm, the beam is focussed through the objective lens. If the electrons then strike the predominantly metallic workpieces, they are decelerated to zero over a distance of a few µm and release their kinetic energy in the form of heat. This leads to the melting and partial evaporation of the material, but also to x-ray radiation, which at acceleration voltages >60 kV must be shielded off by surrounding the gun and chamber with lead.

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By using magneto optics, it is possible in electron beam technology to divert the beam not just slowly, for example to follow a contour to be welded, but also to change its direction with hardly any inertia. If this happens so quickly that the thermal inertia of the beam is overcome and it works on several locations practically simultaneously, this is termed multibeam technology. Here the beam can be programmed as desired, and can work on different points of the workpiece in the same way or differently.

Figure 2: Principles of multibeam technology

Nowadays, modern photo-optical systems (CCD cameras) and the signal of back-scattered electrons, familiar from scanning electron microscopes, are used to observe the processing process or positioning of the beam on the workpiece.

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Figure 3: Basic design of imaging using back-scattered electrons (left), comparison of photo-optical and electron-optical component viewing (right).

2.2

System technology

The electron beam as a software-controlled welding tool excels through its high flexibility, precision and reproducibility. These characteristics have led to this tool becoming widely used in industry. Modern electron beam systems are machines controlled by a programmable logic controller (PLC). Designs with a computerised numerical controller (CNC) are capable of automating the joining processes. Prerequisites for the operation of the electron beam machines, such as for example the process vacuum, are generated and monitored automatically by the machine. Economic and ecological advantages arise from the high energy efficiency of electron beam technology and the fact that process media are not required, e.g. shielding gas. The design of the EB machine is primarily determined by the component to be processed. Here, in addition to the dimensions, the required process times are also of relevance. Up to date, a wealth of concepts have been developed for machines which realise, in the most different of ways, the movement of the component in relation to the electron beam under vacuum.

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Chamber machine

Lock machine

Stroke machine

Lock rotary machine

Figure 4: Machine concepts

In a closed chamber (recipient) the parts are moved using kinematics relative to the electron beam. Access to the chamber is generally via a door, which extends across the entire cross-section of the chamber. The working chamber guarantees on the one hand, the mechanical stability of the machine under a working vacuum and on the other hand protects the machine operator from the x-rays created in the process. The working vacuum of the machine for most applications is in the range between ≤2 x 10 -2 and 7 x 10-4mbar and is determined by the application. The kinematics used for part movement within the chamber are very strongly determined by the purpose of the task. Typical standard solutions involve a coordinates table installed on the floor of the chamber, on which various fixtures can be mounted. The fixtures generate additional movements in order to turn, pivot or raise the parts.

3

Figure 5: Compact EB chamber machines, type K2, with chamber volume of 0.2 m and type K110 with chamber 3 volume of 10 m

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For large-volume components, the chamber principle is used with an internal EB generator. The component is positioned on a simple pallet or turning device and the EB generator is moved by means of kinematics relative to the component.

3

Figure 6: EB large chamber machine with chamber volume of 630 m (left) and EB generator on the robot arm (right)

The concept of the lock-shuttle machine is a further development in the direction of shorter auxiliary process times for the machines. This was achieved by adding a lock chamber to the working chamber. The machine thus comprises: 1. 2. 3.

Station for loading and unloading Lock chamber for evacuation and flooding Working chamber

The parts are moved between the stations in the machine on palettes or devices. Consequently there are always 3 pallets in circulation. The parts are loaded onto the pallets at the loading and unloading station. At the next lock cycle, the pallet is automatically moved into the lock chamber. Simultaneously, on the opposite track, a pallet with processed parts is moved to the loading and unloading station. After closing the lock door, the lock chamber is pumped out. At the next lock cycle, the pallet is moved into the working chamber and on the opposite track, a pallet with processed parts is moved into the lock. Once the working chamber is closed, the EB processing can start immediately. The working chamber is thus always kept under vacuum (7 x 10-4mbar). To process the parts, the pallet can be moved in the working chamber using a coordinates table over the entire ground surface of the working chamber. If further movements are required to process the parts, this can be done using kinematics in the pallet. Upon completion of the EB processing, the pallet is moved at the next lock cycle into the lock and then in a further cycle through to the loading and unloading station. The time required for the lock cycle is determined definitively by the process time required to process the parts. The key advantage of this concept lies in the fact that not only can tool-dependent auxiliary processing times be run simultaneously, but so can the times for evacuation and flooding. The auxiliary process time is thus reduced to the required driving in/out of the pallets and for opening and closing the doors. Optimal operation is possible if the process time is longer than the pumping time of the lock (approx. 60 seconds), as well as the time for loading and unloading at the first station. As there is frequently space on the pallets for more than 30 components, the auxiliary process time per component is then reduced to under one second. Furthermore, the part output of the machine can be easily planned.

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Figure 7: Lock-shuttle machine, type S20

Stroke machines work on the design principle of the chamber machines, whereby the most compact workpiece carrier possible is used as part of the working chamber. The machine has two positions, the loading position (loading and unloading) and the working position. The parts are supplied into the workpiece carrier under atmospheric conditions. Standard components are used for the supply. Depending on the type of stroke machine, up to 4 parts per workpiece carrier can be processed. Generally stroke machines are designed as single-purpose machines. The advantage of the stroke machine is that loading and unloading can take place simultaneously. This reduces the auxiliary process time to a range of less than 10 seconds. The compact design of the machine means that its required floor space is minimal. All components are mounted on a platform, which is container-compatible. The stroke machine is well suited to automation and to interlinking of production processes. The machines stand out due to their low investment, operating and maintenance costs. In order to ensure the highest level of productivity possible, it is important that the process time should be longer than the auxiliary process time. The short cycle time means that it is possible to realise modern production strategies (single piece flow), which are particularly prevalent in the automotive industry.

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The lock-rotary machine combines the efficiency of the lock-shuttle principle with that of the stroke machine. Optimised for processing small parts, the typical chamber volume requires only a few seconds. The machine has two stations: 1. 2.

Loading and unloading station (works additionally as a lock) Working station

After evacuation, the parts are brought to the working station under vacuum by means of a rotary table. In general, the machine is operated in single piece flow. Advantages of the machine lie in the shorter auxiliary process times due to making the loading, unloading and EB processing operations run in parallel.

Figure 9: EB stroke machine as individual machine (left) and integrated in an automated production line for series production of gear parts (right)

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Page 37

Process variants in addition to welding technology

Table 2: Process variants of EB material processing Sphere of application

Technical requirements

Materials

Drilling

Drilling depths: up to 20 mm Bore diameter: 0.04 - 5 mm

e.g. turbine materials special materials e.g. cast iron heat-treated steels

Surface treatment

Remelting with and without additives: The material is heated slightly above its melting temperature. As it cools down quickly, a finegrained microstructure is achieved. Segregations are removed. By adding filler material, wearresistant layers can be obtained. Remelting depths between 0.1 and 5 (10) mm

Hardenable materials Transformation hardening: Heating above austenite temperature. As the material cools down quickly, a high degree of hardness is achieved. Hardening depths of 0.1 to 1.7 mm without slight surface fusions

2.3.1

EB drilling

EB drilling operates with an energy density of 107 to 108 W/cm2 in pulsed mode. As a result, a lot of material is melted abruptly. The exit side (side where the beam leaves the workpiece) is furnished with an ancillary material which upon contact with the molten material evaporates explosively and thereby expels the material in the opposite direction out of the drill hole. Bores of up to 20 mm in depth and a diameter of approx. 1 mm can be generated. The smallest bore diameters are approx. 0.04 mm with a depth of 0.5 mm. For the technical implementation of EB drilling, a very precise focusing of the electron beam is required that must also be repeatable. An appropriate high voltage supply is required for the high drilling rate, as is a CNC controlled moving mechanism for the workpieces. There are EB drilling applications in the area of aircraft construction (gas turbine parts, combustion chamber parts) and in general process engineering (e.g. sieves, including for the paper and foodstuffs industry), as well as the drilling of highly temperature resistant centrifugal disks for manufacturing glass wool. Due to the high removal speeds, the electron beam offers above all a highly-effective procedure for drilling lots of small holes (=perforation) in thick, tough material.

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Figure 9: Spinning bowl for manufacturing glass wool (left) and combustion chamber of an engine (right)

Figure 10: Cooling hole at an angle (left) and filter for paper and pulp industry (right)

2.3.2

Surface treatment by electron beam

In surface treatment with an electron beam, the base material is either re-melted or heated above austenite temperature. As the energy is brought in only locally, big temperature gradients occur. This makes the workpiece cool down quickly and, in case of hardenable materials, leads to surface layers of high hardness that often measure less than 1 mm in thickness. Applications are to be found with smaller components that are partially exposed to high wear and tear (such as bearing bushes, etc.)

Figure 11: Phase transformation options by means of electron beam

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Figure 11: EB edge zone hardening on camshaft made of GGG 60 (left) and EB annealing of medical fracture pins (right)

Figure 12: EB coating of stellite 6 (left) and EB surfacing (right)

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Page 40

Electron beam welding

Table 2: Electron beam welding

Sphere of application

Technical requirements

Welding in vacuum

Minimum sheet thickness: Possible welding depths: Weld width/weld depth: Welding speeds:

2.4.1

Materials approx. 0.01 mm  300 mm up to 1:50 0.1 mm to 300 mm/s

Almost all metals: e.g. low-alloy steels aluminium fine-grained structural steels heat-resistant materials. Special materials (e.g., CU, W, Ti)

Process flow of electron-beam welding

Due to an energy density of more than 106 W/cm², material at the surface of the workpiece is liquefied and shortly after, it is evaporated. A vapour cavity is generated, and along this cavity, the electron beam continually penetrates deeper into the workpiece. When the workpiece is moved, the vapour cavity moves as well, and directly behind the cavity, due to the surface tension, the melted material flows together - a weld is created. The general procedure is shown in Figure 13.

Joint before welding Melting at the point of electron-beam impingement

Vapour cavity is created

Vapour and melt cavity have penetrated the workpiece

Weld seam after solidification

Figure 13: General procedure of EB welding

The width/depth relation of the weld can be up to 1/50. As a consequence, only a minimum of distortion occurs in EB welding. For this reason, it is also applied as a finishing process. However, the disadvantage is that high temperature gradients occur which may result in high hardness values. Depending on the workpiece, material and the performance of the EB machine, welding depths of up to 300 mm can be achieved. Welding speeds of 300 mm/s (18 m/min) and more are possible. The actual welding speeds specified are determined by part and joint geometry, material and quality requirements and are generally between 5 mm/s – 80 mm/s. EB welding can be applied to almost all metallic materials and usually requires no filler materials. Figures 14 to 17 show a selection of constructive designs of EB welds.

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Preparation and design of welded joints

Figure 14: Examples of straight EB welds (longitudinal weld)

Figure 15: Examples of circular EB welds (radial weld)

Figure 16: Examples of circular EB welds (axial weld)

Figure 17: EB welding of inaccessible welds

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To prepare the weld, it is essential to use machined weld flanks without an air gap. The surfaces should be cleaned as the surface elements evaporating during the welding process would cause pores due to the poor degasification conditions. Table 3: Weld preparation requirements for EB welding Criterion

Requirement

Gap

Zero gap: 0.0 mm to max. < 0.15 mm, depending on the penetration depth

Surface finish

Surface finish Ra < 3.2 µm Machining Free from oil and grease (DVS 3213) Phosphate layers, nitride layers, etc. need to be removed.

Surface layers (beads) Other

2.4.3

Workpieces must be non-magnetic/demagnetized -4 ( 20,000 h) and a compact, low-maintenance and wear-resistant structure. Properties of the fibre laser • Performances of up to 20 kW and more, subsequently extendable • The range of sheet thicknesses that can be processed with solid-state lasers is increasing • High socket output efficiency (>30%) • High beam quality • Better focussing ability and greater Rayleigh length for processing larger sheet thicknesses or to increase the processing speed • Hybrid welding processes are state of the art and can also be used with fibre lasers • Adaptation to special framework conditions is also very possible Advantages The advantages of the fibre laser are largely the same as those of the slab laser. Consequently the beam quality of the emitted radiation is up to four times better than that of a comparable Nd:YAG laser; its power thus opens up numerous fields of application in material processing, such as e.g. high-quality cutting, soldering and welding of metals. With corresponding beam widening through defocussing, the hardening of large metallic surfaces is also possible. Due to the high beam quality, comparatively large working distances are possible (e.g. metal welding approx. 1 m), which opens up entirely new opportunities in the area of automated manufacturing in 2D and 3D in hard to access areas, or in beam diversion with mirror scanners (remote welding).

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(Laser, electron-beam and plasma welding) 3.2.5

Diode laser

In a semiconductor charge carrier, photons are emitted when electrons and holes recombine. If the semiconductor material is doped accordingly (e.g. GaAs), the probability of a fluorescent transition in the contact area (pn junction) can be increased. The active area will be limited by the p and n doped areas, which have a lower refractive index, so that the generated radiation will act as it does in a wave conductor. The radiation only escapes in the longitudinal direction from the edge of the crystal (edge emitter). Depending on the doping of the semiconductor material, the wavelength of diode lasers ranges between 0.78 and 0.94 µm. To achieve a higher beam power, about 25 emitters are combined into one sub array. In turn, several arrays make the diode-laser bar with its specific beam characteristic. Depending on the cooling and assembly technique applied, from one bar a power of 10 to 40 W can be taken. At an efficiency rate of 35 to 50%, enough lost heat is still generated, that - in relation to the small ground surface of the bar - a power density in the order of kW/cm2 is to be dissipated by corresponding micro coolers. The laser beam can be focused by means of micro lenses and micro prisms. To obtain a high-power diode laser (HPDL), Figure 19, several bars are brought together to form a “stack”. Beam focusing is carried out mechanically or optically. The beam quality of the HPDL does not currently match the range of the solid-state laser or the CO2 lasers. This ultimately explains why a power density (intensity) of 5 x 105 W/cm² is currently achieved by HPDL. Figure 18 shows a comparison of beam qualities and laser powers of various laser types. Possible applications of diode lasers are to be seen in the fields of soldering (electronic components), hardening, alloying, welding of plastics and (heat conduction) welding of metals. Diode lasers are the most compact of lasers with the highest efficiency ratings and permit almost maintenance-free operation.

Figure 18: Beam quality and laser power for different types of lasers (ILT)

Figure 19: High-performance diode laser system for industrial use, ROFIN DL025 for 2.5 kW incl. mains adapter (without cooling unit)

Figure 20 shows a diode laser in use, while Figure 21 shows a cross-section of a weld created using a diode laser.

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Figure 20: Welding a Cr-Ni steel container using a 2.5 kW diode laser Photo: Fraunhofer IWS Dresden

3.2.6

SFI / IWE 1.12-1 Page 62

Figure 21: Producing a flat lap-joint fillet weld by means of a 1.5 kW diode laser Photo: Fraunhofer IWS Dresden

Summary

Laser systems essentially comprises three modules: resonator, energy supply and beam guidance. Construction types differ depending on the active medium used: Gas laser:

Electrical excitation DC or HF, gas agitation

Solid-state laser: Diode laser:

Optical pumping, water cooling

Electrical excitation, photons, compact construction form

In terms of beam guidance, light can be guided using total reflection (fibre) or via mirror systems. Fibre guidance does not work for very short or very large wavelengths (e.g. CO2 laser). The ability to focus is influenced by the beam parameters (K value, wavelength, etc.). There are options and measures which can be used to change the focussing ability and/or divergence (telescope, mode shutters etc.).

3.3

Procedure for laser material processing

3.3.1 Laser cutting Laser cutting is discussed with the cutting processes under 1.13 “Cutting, drilling and other joint-preparation processes”. 3.3.2 Welding Laser beam welding is currently already in use for many applications; knowledge about the strengths and the required framework conditions for optimum application of the procedure serves to extend its area of application in industry and crafts, Table 2.

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Table 4: “Laser cutting” technology phases Process/ Material 1. Micro, fine welding (electronics, precision engineering) a) Stainless steel b) NE metals 2. Macro, seam welds a) Unalloyed steel

b) Stainless steel

c) NE metals Aluminium Other NE metals d) Ceramic e) Glass f) Plastics

Specification

Technically possible

Technology phase Ready for Ready for Ready for pre-series mass testing production production

State of the art

Up to 1 mm Up to 10 mm Over 10 mm Up to 1 mm Up to 10 mm Over 10 mm

Up to 1 mm

Depending on the energy density, laser welding is generally subdivided into conduction welding and deep welding (Figure 22), whereby its advantages lie primarily in deep welding.

Figure 22: Principle of laser welding

Examples of laser beam welds with different laser systems and materials are shown in Figure 23.

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3-sheet joining 3 x s = 1.6 mm

Spot weld Pt-wire diameter 63 µm

Film weld 2 x s = 80 µm

Pipe/pipe edge weld 1.4301

Aluminium t = 10 mm

Fine grain steel s = 4 mm

SFI / IWE 1.12-1 Page 64

Figure 23: Examples of laser beam welds with different laser systems and materials.

Deep welding is distinguished by the formation of a vapour cavity. The laser beam can thereby penetrate deep into the material. Laser welds have a width of approx. 1 mm, even at penetration depths of 5 - 8 mm. So only a slight distortion occurs. As an example, Figure 24 shows the achievable welding speeds as a function of the laser power. The width/depth ratio of laser beam welds is between 1/5 and 1/10.

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Figure 24: Welding speed as a function of laser power

Usually no filler materials are added. The welding speeds reached are significantly above those of traditional welding processes. An upper limit due to physical effects is generally given at approx. 15 m/min. In laser beam welding, edge preparation is of particular importance. Consequently, the gap width and edge offset need to be kept within narrow limits, Figure 25.

Sheet thickness d

Gap width b

Offset e

0.5 ... 3 mm 3 ... 10 mm

0.1 d 0.05 d

0.15 d 0.1 d

Figure 25: Reference values for permissible joining part tolerances

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The effects of the gap width in the case of laser welds in the butt joint without filler materials are shown in Figure 26.

Gap: 0.1 mm

Gap: Power: Material thickness: Material:

0.3 mm PL = 10 kW s = 8 mm S355J0

Gap: 0.5 mm

Figure 26: Laser beam welds with different gap widths

3.3.3

Variants of laser welding

Procedural variants of laser welding are described below. In addition to the classic conduction and deep welding processes, hybrid and remote welding (Figures 27 and 28) are increasingly finding their way into the manufacturing industry. Hybrid welding is when two different joining procedures create the weld in one common molten pool.

Figure 27: Depiction of the principle of laser hybrid welding

The advantages of hybrid technology lie in the exploitation of the individual advantages of the processes. Thus the laser increases, in particular, the welding speed, the penetration depth, the accessibility and the stabilisation of the arc. The MIG/MAG process (for example) contributes cost-effective component preparation, a large a-size and influences the composition of the weld through the use of filler material.

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Another variant that is experiencing increasing popularity is remote welding. The prerequisite for this application is the currently available and very good beam qualities of the CO2 and solid-state lasers (Nd:YAG, slab and fibre lasers). The beam movement takes place via X-Y scanner mirrors, as have been used in laser beam inscription for decades.

Figure 28: the principle of laser hybrid welding

The advantages of remote welding include, above all, very short jump times for stitch welds, high welding speeds as well as the ability to set the focal distance for 3D workpieces using adaptive optics.

3.3.4

Surface treatment

At the moment, surface treatment with lasers is not yet implemented on a large industrial scale. Basically, a distinction is made between: 1. 2. 3. 4.

Remelting Transformation hardening Surfacing Alloying

Details with characteristic values of the different methods are shown in Figure 29.

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Figure 29: Surface treatment with lasers (source: ILV 1999)

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3.3.4.1 Remelting Thin edge areas are remelted and quickly cooled down. With some materials, an important microstructural refinement can be achieved this way. Examples from wear protection are stellites and Fe40Ni20B20-layers. The cross-section in Figure 30 shows the microstructure of a ledeburitic remelted edge area of cast iron.

Advantages: Microstructure can be adjusted in a defined manner. Applications: Surfacing with wear-resistant thin layers Figure 30: Microstructure of a ledeburitic remelted edge area of cast iron 3.3.4.2 Transformation hardening Very thin edge areas are heated above austenite temperature; but the material is not melted. Due to the material-related quick cooling, martensite is formed with high hardness values. Advantages:  No distortion  Complicated components can be hardened locally Applications:  Gear surfaces  Contact surfaces of bearings  Contact surfaces of pistons

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3.3.4.3 Surfacing The base material is melted, and at the same time, filler material (mainly in powder form) is added. Thin layers are generated upon the base material. Application, e.g. for components exposed to wear and tear. Figure 31 shows a sample application. Advantages:  Small complex components can be surfaced without distortion. Applications:  Surfacing of turbine blades  Surfacing of exhaust valves  Surfacing of log saws

Figure 29: Example of laser surfacing of a screw conveyor

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the required laser-beam power. the material to be processed. the material thickness. the ambient temperature. the efficiency.

The CO2 laser

     (5)

The CO2 laser The dye laser The excimer laser The Er YAG laser The semiconductor laser

The choice of the laser-beam source used depends on

     (4)

The radiation is monochromatic. The wavelength is in the infrared spectrum. The radiation has low divergence. The wave trains of the laser beam are in phase. The laser beam is polychromatic.

Which laser types are mainly used for material processing?

     (3)

Page 71

Test questions Name the three physical properties for generating very high energy densities in the focal point of the laser beam?

     (2)

SFI / IWE 1.12-1

is the main beam source that is used for processing tasks in production engineering. emits in the visible spectral range. is only used for welding and cutting. is mainly guided to the processing point via optical fibers. can be excited with high frequency or with direct current.

The solid-state laser (Nd:YAG)

    

is currently available with a beam power of up to 40 kW. can only be guided to the processing point via mirrors. requires helium as an internal cooling gas. can be excited by excitation lamps or diode arrays. can operate several processing stations via a multiplexer.

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Page 72

Which are the advantages of a diode laser?

     (7)

SFI / IWE 1.12-1

Excellent beam quality. Good focusing ability for micro welds. Compact construction volume compared to the Nd:YAG laser. Good absorption of the laser beam with regard to metals, compared to the CO 2 laser. Poor absorption of the laser beam with regard to metals, compared to the CO 2 laser.

Name the characteristics of laser-beam welding compared to the traditional joining techniques?

    

High demands on joint preparation. High degree of process automation possible. No filler wire can be used. Due to the energy input per length, distortion is significantly lower compared to the traditional shielding-gas techniques. The lower energy input per length results generally in low hardness penetration.

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(Laser, electron-beam and plasma welding) 3.5

Bibliography

DIN EN ISO 15609-4

Specification and qualification of welding procedures for metallic materials - Welding procedure specification, Part 4: Laser beam welding

DIN EN ISO 15614-11

Welding procedure test, electron beam welding

DIN EN ISO 13 919-1

Electron and laser-beam welded joints - Guidance on quality levels for imperfections, Part 1: Steel

DIN EN ISO 13 919-2

Electron and laser-beam welded joints - Guidance on quality levels for imperfections, Part 2: Aluminium

Part

11:

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Laser

beam

welding

Welding processes and equipment

and

Other special welding processes

Chapter 1.12-2:

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Special welding processes II

Contents 1

2

3

Arc stud welding 01 ........................................................................................................................ 5 1.1 Summary description of the procedure ..................................................................................... 5 1.1.1 Description of the general principle ................................................................................ 5 1.1.2 Component geometry and processing materials ............................................................ 5 1.1.3 Process-specific advantages and disadvantages ........................................................... 6 1.1.4 Areas of application........................................................................................................ 6 1.2 Process principle – detailed description ................................................................................... 7 1.2.1 Process variants ............................................................................................................ 7 1.2.2 Instrument technology incl. accessories ......................................................................... 7 1.2.3 Weld pool protection – expendable materials ................................................................. 8 1.3 Question .................................................................................................................................. 9 Arc stud welding 02 ...................................................................................................................... 10 2.1 Drawn-arc stud welding .......................................................................................................... 10 2.2 Arc stud welding with tip ignition ............................................................................................ 12 2.3 Requirements, load behaviour ............................................................................................... 13 2.4 Cross-section evaluation and avoiding errors ......................................................................... 14 2.5 Materials used in arc stud welding ......................................................................................... 16 2.5.1 Drawn arc ignition processes ....................................................................................... 16 2.5.2 Materials used in tip ignition welding ............................................................................ 17 2.6 Quality assurance in arc stud welding .................................................................................... 18 2.7 Special welding types and alternatives ................................................................................... 19 2.8 Standards and guidelines: ...................................................................................................... 19 2.9 Bibliography ........................................................................................................................... 19 2.10 Questions............................................................................................................................... 20 Electroslag welding (RES) ............................................................................................................ 21 3.1 Summary description of the procedure ................................................................................... 21 3.1.1 Description of the general principle .............................................................................. 21 3.1.2 Component geometry and processing materials .......................................................... 21 3.1.3 Process-specific advantages and disadvantages ......................................................... 21 3.1.4 Areas of application...................................................................................................... 22 3.2 Process principle – detailed description ................................................................................. 22 3.2.1 The sequence of electroslag welding ........................................................................... 22 3.2.2 Joint preparation .......................................................................................................... 22 3.2.3 Execution variants ........................................................................................................ 23 3.2.4 Welding Flux ................................................................................................................ 24 3.2.5 Welding parameters and process parameters .............................................................. 25 3.3 Process variants .................................................................................................................... 26 3.3.1 Electroslag welding of aluminium ................................................................................. 26

Other special welding processes

4

5

6

7

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3.3.2 Electroslag welding of circumferential seams ............................................................... 26 3.3.3 Electroslag strip cladding ............................................................................................. 27 3.4 Similar types of welding ......................................................................................................... 27 Aluminothermic welding............................................................................................................... 28 4.1 Summary description of the procedure ................................................................................... 28 4.2 Description of the general principle ........................................................................................ 28 4.3 Material, additives .................................................................................................................. 28 4.4 Process - specific advantages and disadvantages ................................................................. 28 4.5 Areas of application ............................................................................................................... 28 4.6 Process principle in railway track welding - detailed description ............................................. 29 4.7 Aluminothermic reaction......................................................................................................... 30 4.8 High frequency welding (HF welding) ..................................................................................... 31 4.8.1 Summary description of the procedure ......................................................................... 31 4.8.2 Description of the general principle .............................................................................. 31 4.9 Component geometry and processing materials .................................................................... 31 4.10 Process-specific advantages and disadvantages ................................................................... 31 4.11 Areas of application ............................................................................................................... 31 4.12 Process types – detailed description ...................................................................................... 32 4.12.1High frequency contact welding (conductive HF welding). ............................................ 32 4.12.2Induction welding (non-contact HF-/MF-welding) ......................................................... 33 Ultrasonic welding ........................................................................................................................ 35 5.1 Summary description of the procedure ................................................................................... 35 5.1.1 Description of the general principle .............................................................................. 35 5.2 Component geometry and processing materials .................................................................... 36 5.3 Process types – detailed description ...................................................................................... 36 5.3.1 Ultrasonic (continuous) seam welding, figure 38 .......................................................... 36 5.3.2 Ultrasonic ring welding ................................................................................................. 37 5.3.3 Ultrasonic plastic welding ............................................................................................. 37 5.4 Questions............................................................................................................................... 39 Friction welding (1) ....................................................................................................................... 40 6.1 Summary description of the procedure ................................................................................... 40 6.2 Description of the general principle ........................................................................................ 40 6.3 Geometry of used components and suitable materials ........................................................... 40 6.4 Process-specific advantages and disadvantages ................................................................... 42 6.5 Areas of application ............................................................................................................... 42 6.6 Process principle – detailed description ................................................................................. 43 6.6.1 Process sequences of spin/ rotary welding................................................................... 43 6.6.2 Process variations of spin welding ............................................................................... 44 6.6.3 Types of spin / rotary welding ....................................................................................... 45 6.7 Equipment and accessories ................................................................................................... 46 6.8 Consumables ......................................................................................................................... 48 Friction welding (2) ....................................................................................................................... 48 7.1 Process conditions and joint properties in spin welding .......................................................... 48 7.1.1 Preparation of parts...................................................................................................... 48

Other special welding processes

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7.1.2 Welding parameters ..................................................................................................... 49 7.1.3 Structure of a friction welded joint ................................................................................ 50 7.2 Examples of Application ......................................................................................................... 54 7.3 Process variants .................................................................................................................... 55 7.3.1 Friction stud welding .................................................................................................... 55 7.3.2 Linear friction welding and orbital friction welding ......................................................... 56 7.4 Workplace safety ................................................................................................................... 57 7.5 Questions............................................................................................................................... 58 7.6 Bibliography ........................................................................................................................... 59 8 Friction welding - friction stir welding ......................................................................................... 60 8.1 Summary description of the procedure ................................................................................... 60 8.1.1 Description of the general principle .............................................................................. 60 8.1.2 Suitable materials and the joints geometries ................................................................ 60 8.1.3 Process-specific advantages and disadvantages ......................................................... 61 8.1.4 Areas of application...................................................................................................... 62 8.2 Process principle – detailed description ................................................................................. 63 8.2.1 Machines ..................................................................................................................... 63 8.2.2 Tools ............................................................................................................................ 63 8.2.3 Welding sequence........................................................................................................ 64 8.2.4 Welding parameters ..................................................................................................... 64 8.2.5 Structure and properties of the joint ............................................................................. 64 8.3 Process variants .................................................................................................................... 65 8.4 Questions............................................................................................................................... 68 8.5 Bibliography ........................................................................................................................... 69 9 MIAB welding, diffusion welding ................................................................................................. 70 9.1 Pressure welding with magnetically impelled arc butt (MIAB) ................................................. 70 9.1.1 Process description ...................................................................................................... 70 9.1.2 Processing materials and geometries........................................................................... 71 9.1.3 Process-specific advantages and disadvantages ......................................................... 72 9.1.4 Areas of application...................................................................................................... 72 9.1.5 Welding machines ........................................................................................................ 73 9.1.6 Magnetic field distribution ............................................................................................. 74 9.1.7 Welding parameters ..................................................................................................... 74 9.1.8 Structure and properties of the joint ............................................................................. 75 9.1.9 Process variants .......................................................................................................... 76 9.2 Test questions........................................................................................................................ 77 9.3 Bibliography ........................................................................................................................... 78 10 Diffusion welding .......................................................................................................................... 79 10.1 Process description ................................................................................................................ 79 10.2 Structure of a vacuum diffusion welding machine................................................................... 79 10.3 Welding data .......................................................................................................................... 80 10.4 Weldable materials and geometries ....................................................................................... 81 10.5 Process-specific advantages and disadvantages ................................................................... 82 10.6 Applications ........................................................................................................................... 83

Other special welding processes

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10.7 Questions............................................................................................................................... 84 10.8 Bibliography ........................................................................................................................... 85 11 Cold pressure welding .................................................................................................................. 86 11.1 Summary description of the procedure ................................................................................... 86 11.1.1Description of the general principle .............................................................................. 86 11.1.2Weldable materials....................................................................................................... 86 11.1.3Application of the process ............................................................................................ 86 11.2 Detailed description ............................................................................................................... 86 11.2.1Welding units ............................................................................................................... 86 11.2.2Process principle .......................................................................................................... 86 11.2.3Operating conditions .................................................................................................... 87 11.2.4Process variants .......................................................................................................... 87 11.3 Bibliography ........................................................................................................................... 88 11.4 Questions............................................................................................................................... 88 12 Explosion welding......................................................................................................................... 89 12.1 Process principle.................................................................................................................... 89 12.2 Materials and applications ...................................................................................................... 89 12.3 Operating conditions .............................................................................................................. 89 12.4 Variants.................................................................................................................................. 90 12.5 Bibliography ........................................................................................................................... 90 12.6 Test questions........................................................................................................................ 91 13 Magnetic impulse welding and crimping ..................................................................................... 92 13.1 Applications ........................................................................................................................... 92 13.2 Welding units ......................................................................................................................... 93 13.3 Process principle of the Magnetic impulse welding ................................................................ 93 13.4 Working conditions of materials ............................................................................................. 94 13.5 Bibliography ........................................................................................................................... 94 13.6 Questions............................................................................................................................... 95

Other special welding processes 1

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Arc stud welding 01

1.1 1.1.1

Summary description of the procedure Description of the general principle

Stud welding is the joining of pin-shaped parts (studs) with flat workpieces using pressure welding (DIN EN 14610). Arc stud welding is the solidification of a relatively small melt between stud face and sheet surface, created with the aid of an arc. The joining principle of arc stud welding is displayed in Figure 1.

Figure 1: Joining principle of arc stud welding and image of ceramic ferrule stud welding

The arc can be ignited either by lifting the stud from the sheet ( drawn arc ignition) or by evaporating a defined ignition tip (tip ignition) at the contact with the sheet metal. 1.1.2

Component geometry and processing materials

Arc stud welding is usually used on studs with a round cross-section. In certain cases, is can also be used for rectangular cross-sections. In welding processes, the geometric shape is usually irrelevant, with the exception of the welding point and the stud length. Processes primarily use studs, pins and threaded bolts that manufacturers offer as DIN EN ISO 13918 standard types. Common diameters are: 2 to 25 mm in the case of steel, and up to 12 mm in the case of aluminium. The stud is attached to the sheet at a perpendicular angle. Basically, any welding position is possible (in relation to the sheet). The following limitations apply:  Welding position PA, horizontal sheet: all diameters,  Welding position PC, perpendicular sheet: diameters up to 16 mm,  Welding position PE, overhead/horizontal sheet: diameters up to 20 mm, Arc stud welding can be used on sheet metals with a thickness no smaller than 0.5 mm. There is no upper limit to the thickness of the sheet. There are however limits in the choice of material, due to the rapid melting and cooling limitations set by DIN EN ISO 14555. Regarding unalloyed steel, alloyed steels and aluminium materials, only a few commonly used materials are suitable for welding studs as per DIN EN ISO 13918, since only these can be welded in good quality to similar materials of sheet metals. Copper and brass materials can only be welded with the capacitor unloading process.

Other special welding processes 1.1.3

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Process-specific advantages and disadvantages

Arc stud welding has the following advantages: good joint quality by full surface welding, economical due to short welding time, universal applications by handy, easily transportable instrument technology, good options for the automation of the processes with high clock rates, only simple manual skills are required from the operator, no bores necessary, low heat input, resulting in almost no warping, welding onto thin plates (thinner than 1 mm) is possible without burn-through, the sheet metal only needs to be accessible from one side, good static load capacity of the joint, low investment costs. Disadvantages: limited choice of materials, limited process control, sensitive against arc-blow effect, less suitable for mechanical-cyclic loads. 1.1.4

Areas of application

Due to its highly economical nature, arc stud welding is used in countless areas of the metal processing industry. The demand for this efficient jointing method is constantly increasing around the globe. Typical areas of application include: Drawn arc stud welding:

Construction industry with steel engineering, bridge construction (composite construction, Figure 2) and facade construction, shipbuilding, power-plant construction, vehicle construction (Figure 2), mechanical engineering, container construction, insulation technology, military technology,

Stud welding with tip ignition:

Household appliances, casing construction, facade construction, vehicle industry.

Composite construction, end stud connector Ø 22 mm Figure 2: Sample applications

Vehicle body (Ø 5 to 6 mm)

Other special welding processes

1.2

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Process principle – detailed description

1.2.1 Process variants Arc stud welding offers four process variants for various applications and situations, labelled with ISO numbers 783 to 786, as detailed in Table 1. Types of arc ignition:  Arc stud welding process with drawn arc ignition, no. 783 to 785  Arc stud welding process with tip ignition, no. 786. Table 1: Types of stud welding and important parameters /3/

Capacitor discharge stud welding with drawn ac ignition

¼ d with shielding 1/8 d Both types work with direct current, but with different power sources. 1.2.2

Instrument technology incl. accessories

The following instruments are necessary for arc stud welding: Welding power supply, weld time adjustment control, manipulation device (welding gun or welding head), welding current cable and connecting clamps. Figure 3 shows switching arrangement for the different ignition methods. Depending on the process used, further simple devices are available for supporting the gun on the sheet. In ceramic ferrule stud welding, the gun is positioned into the ceramic ring on the sheet. In shielding gas stud welding, a suitable shielding gas device is used instead of the simple foot support. In the case of drawn arc ignition, conventional welding rectifiers or inverters with amperage of up to 3,000 A are used. In capacitor discharge welding, the power supply includes a capacitor battery with a capacity between 40 and 140 mF.

Other special welding processes

Drawn arc ignition processes 783 and 784

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Tip ignition process 786

Figure 3: Switching arrangement of arc stud welding

1.2.3

Weld pool protection – expendable materials

The process types differ in welding time and the requirements of weld pool protection (see Table 1 /4/). Without weld pool protection: for short welding works (< 200 ms) on steel materials, no weld pool protection is needed. Shorter weld times however result in more imperfections (e.g. pores). The fillet is often formed unevenly. Ceramic ferrule welding:

A ceramic ferrule is needed for studs with a diameter exceeding 16 mm, but it is also highly recommended for smaller diameters. Very high quality welding results can be achieved by easy repeatability, due to a comparatively longer welding time and strong melting. The ceramic ferrule is usually placed manually by the operator. Objectives of the ceramic ferrule (CF): Concentrating and stabilising the arc, shaping the welding bead and supporting the pool when welding on a vertical wall. Typical applications include: manual welding of 22 mm diameter headed stud anchors in the construction industry. Disadvantage: one ceramic ferrule is needed for each welding. This type of welding is not suitable for batch use. In the case of stud welding without a ceramic ferrule the weld metal misses the support. Therefore, when welding among adverse conditions, arc welding without ceramic ferrule is only possible for very short welding times.

Shielding gas stud welding:

Shielding gas can be used as an alternative to the ceramic ferrule. It reduces the pore formation in the weld metal. The ignition of the arc is more even and stable. The requirements are: an effective shielding gas device and a suitable shielding gas (for steel e.g. Ar+18 % CO2, for aluminium Argon or Ar-He-mix). This type of welding is highly recommended for use in batch production, and whenever high quality results are needed in short-time stud welding.

Other special welding processes 1.3 (1)

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Question Which types of arc stud welding are used widely today?

(2)

Why is arc stud becoming more popular in the whole metalworking industry?

(3)

What limitations restrict the applicability of arc stud welding?

(4)

Which conditions require or should be used for weld pool protection in arc stud welding?

(5)

In which out of positions can stud welding be used (what diameter)?

Other special welding processes 2

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Arc stud welding 02

The welding results of arc stud welding are influenced by several factors. These are the following:  the weld preparations (e.g. surfaces, studs ends, weld pool protection, ground connection, power source, manipulation device)  the welding process (e.g. gun position, current flow, right stud movement, cooling of the weld pool, effectiveness of the weld pool protection)  the choice of the (these depend particularly on the stud diameter). welding parameters - Drawn arc ignition processes: Amperage, welding time, lift height, plunging depth (projection), plunging speed - Tip ignition process: Charging voltage, spring force (impact speed), projection, and optionally: capacitor capacity and gap. 2.1

Drawn-arc stud welding

The stud is slid into the stud holder. The manipulation device is usually positioned on the workpiece using a support piece or a ceramic ferrule. First, projection P is set depending on the stud diameter, as explained in figure 4, in order to ensure a sufficient plunging. The arc is ignited by the lifting mechanism when the tip is lifted. A secondary arc is ignited first, and the main arc is ignited after lifting the bolt from the workpiece. The fusion face of the stud and the base material are melting/melted. After the welding time passed, the stud plunges into the weld pool with a low force (< 100 N); shortly after this, the current is shut off. Work sequences

Welding parameters: - current strength, - welding time, - lift, - plunging depth (projection parameter).

View of an actual welding process

Figure 4: Working sequences in drawn-arc stud welding /4/ and view of welding

Other special welding processes

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Extensive collective experience helps in setting the right welding parameters for the most frequent applications. The results (depending on the stud diameter) are included in the reference value diagrams of DVS notes 0902 to 0904. The values shown here can be used as reference values for determining the right welding parameters. Deviations are to be made due to the actual plate thickness, welding position, coating and weld pool protection. Reference values for ceramic ferrule stud welding: see Figure 5.

Lift / elevation [mm]

Example 1: Stud diameter 16 mm: current 1,200 A, welding time 0.6 sec, flat-tip bolt (with aluminium ball): lift 3 mm, projection 3.5 mm, submergence speed approx. 100 mm/s.

Lift / elevation [mm]

Example 2: Stud diameter 12 mm: current 800 A, welding time 0.4 sec, round-tip bolt (with aluminium ball): lift 1.5 mm, projection 4.5 mm or more, plunging speed approx. 200 mm/s. Figure 5: Diagram of reference values for ceramic ferrule stud welding /6/

Bolts have different tip shapes depending on the welding type. Tapered-tip bolts are usually used for longer welding times and larger melting of the sheet metal. For very short welding times, flat-tip bolts should be used to provide an even thickness of the weld metal over the whole cross-section. Some stud types have an aluminium ball (or a aluminium layer) on the tip. This helps to ignite the arc in the centre of the bolt. Besides that, aluminium also acts as a deoxidiser during the welding process.

Other special welding processes 2.2

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Arc stud welding with tip ignition

Arc stud welding with tip ignition is a capacitor discharge type of welding where the arc is ignited by a defined shaped ignition tip on the stud face. In this type of welding, the length and diameter of the ignition tip influence welding quality. The tips are usually manufactured by cold upsetting which additionally results in a raised flange, as shown in figure 6. This flange has the advantage that the weld contact area is increased as compared to the diameter of the bolt, and that the highest stress zone lays outside the welding point under mechanical load situations. No weld pool protection is required.

Figure 6: Different stud shapes with ignition tip and upset flange /5/

The welding can be carried out in two ways. Gap welding:

Before start, the bolt (and therefore, the ignition tip) is placed at a certain, adjustable distance to the workpiece (figure 4a). When the movement starts and the thyristor ignites, the bolt accelerates towards the workpiece. As the ignition tip makes contact with the workpiece, the quickly rising capacitor current melts and evaporates it instantly. This produces an arc which is able to melt the bolt and workpiece. The bolt continues to move freely forward, and finally comes to a halt in the melt of the workpiece. Upon contact with the welding pool, the arc goes out with a short-circuit. The stud and the workpiece are now connected. Welding time is approx. 1 ms. This enables the welding of aluminium materials even without the use of shielding gas.

Contact welding:

The difference to gap welding is that the stud is mounted directly on the surface of the workpiece, slightly pressed onto it by spring force. The welding process is initiated by the thyristor ignition. The following steps are similar to those described under gap welding. Welding time is 3 ms. A longer welding time improves the cleaning effect in the case of slightly oiled or zinc-coated workpieces.

Figure 7: Work sequences of arc stud welding with tip ignition /5/ Gap welding: Sequences a to d Contact welding: Sequences b to d

Other special welding processes 2.3

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Requirements, load behaviour

Depending on their application, stud welds have to bear mechanical, and sometimes thermal loads as well. Accordingly the welding point has to transmit forces and even heat. Quality requirement

Tests

Appearance ( weld shape, etc.).

Visual test

Dimensional accuracy (position, distortion, length)

Dimensional check

Strength (fracture behaviour)

Tensile test

Ductility (deformation)

Bending test

Surface imperfections (pores, cavities)

Radiographic test

Penetration (weld zone)

Macro-section

Hardening (brittleness)

Hardness tests

Constant welding data

Parameters control

Reproducibility

High number of samples

Figure 5: Quality requirements and allocation of suitable tests /6/

Visual test:

View of an aluminium welding /8/

The completeness and homogenity of the weld seam as well as spatter formation are inspected. Undercuts (incomplete fillet) cannot be accepted. Even if the weld has a good overall appearance, the load capacity can be reduced by severe internal imperfections. Visual control therefore should be regularly complemented by mechanical testing.

One of the easier workshop tests is bending. Also tensile and torque tests can be performed. Such tests may be carried out with a limited load that does not destruct the joint. Basic requirement:

when applying a severe mechanical static load the fracture should be located in the base material (bolt or sheet), outside of the weld zone, as displayed in figure 9.

Such load behaviour can be achieved if the materials weldability is good and the weld execution shows the least possible faults.

bending exceeding 60 ° should be achieved

Fracture in the stud after applying tensile test

Disconnection from the sheet

Figure 9: Images of stud welds after applying mechanical loads (image on the right: sheet thickness of 3 mm) /7/

Other special welding processes 2.4

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Cross-section evaluation and avoiding errors

Beside mechanical load testing of stud welded joints, the selected welding parameters can be excellently evaluated using cross-section images. Macro-sections reveal the penetration shape as well as any welding imperfections, such as pores, cavities, cracks and incomplete fusion in the welded piece. Relevant characteristics: consistent melting form of stud and sheet

Ceramic ferrule stud welding

Short cycle stud welding

Tip ignition welding

Figure 10: Cross-section images of different types of steel stud welds (images: SLV München)

Figure 11 shows typical welding imperfections and tips for avoiding them during ceramic ferrule stud welding.

Good

One-sided melting: Fusion problems and pores created by arc-blow effect

Broader melt zone above 16 mm diameter: Good

Insufficient plunging due to friction or short-circuits

High edge fusion when using shielding gas (6 to 10 mm diameter): Good

Burn-through: time too long, sheet too thin

Imperfection hazard in stud centre: check lift, projection and stud shape

Crack-like cavity in stud centre: lift too low

Fusion errors in the perimeter region caused by cold plunging

Pores caused by low welding current or contaminated surface Figure 11: Schematic representation of good penetration shapes and some typical welding imperfections during ceramic ferrule stud welding /6/.

Other special welding processes

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Prevention of arc-blow effect Arc-blowing effects frequently affect arc stud welding. These arc-blows usually result from the asymmetry of the magnetic fields created by the current-carrying conductors, such as the bolt and sheet metal in this case.

Cause of the arc-blow effect: one-sided current flow direction in sheet

Cause

Remedy: symmetrical ground clamping of the sheet

Remedy

Note: the arc-blowing effect is proportional to amperage and can be affected by symmetrical assembly of the workpiece clamps, or by creation of compensation mass or (for manual welding guns with an external welding cable) by turning the pistol around its perpendicular axis. This causes one-sided fusion and can increase the number of pores in the weld metal. It can be minimized by suitable use of different remedies.

Figure 12: Causes of magnetic arc blow and possible solutions in arc stud welding

The solutions shown in figure 12 can usually help to reduce magnetic arc-blow resulting in a complete weld formation. In shielding gas stud welding, arc blows can also be caused by an asymmetrical gas flow. In addition to this, pre-magnetised sheets (transport of sheet metals with supporting magnets) can also cause similar arc-blows.

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Other special welding processes 2.5 2.5.1

Page 16

Materials used in arc stud welding Drawn arc ignition processes

Drawn arc ignition welding can be used for welding unalloyed, stainless and heat-resistant steel bolts as well as aluminium or aluminium-alloy bolts. This type of welding allows joining bolts to sheets of the same material or even different material. For combinations, see table 2 /9/. Arc stud welding is characterised by a fast cooling rate of the melt after the arc is broken. This allows hardening of the solidified weld material even in unalloyed steels. Therefore, the preferable stud material is unalloyed steel S235FF with material properties as per DIN EN ISO 13918 /2/. Table 2:

Weldability of usual combinations of stud and base material in drawn arc stud welding /4/ Base metal

Stud material

CR ISO/TR 15608 Groups 1 and 2.1

CR ISO/TR 15608 Groups 2.2, 3, 4 and 5

CR ISO/TR 15608 Groups 8 and 10

CR ISO/TR 15608 Groups 21 and 22

S235 4.8 (suitable for welding) 16Mo3

a

b

b

1.4742/X10CrAl18 1.4762/X10CrAl24 1.4828/X15CrNiSi20-12 1.4841/X20CrNiSi25-4

c

c

c

-

b

a

-

-

-

b

1.4301/X5CrNi18-10 1.4303/X5CrNi18-12 1.4401/X5CrNiMo17-12-2 1.4541/X6CrNiTi18-10 1.4571/XcrNiMoTi17-12-2 EN AW-AlMg3 (-5754) EN AW-AlMg5 (-5019) 1) 2)

b/a

1)

-

2)

-

Up to 10 mm ø and with shielding gas Only in short cycle drawn arc stud welding

Explanation of the letters concerning weldability: -: not suitable for welding a: well-suited for all applications, e.g. transmission of forces b: suitable with limitations for transmission of forces c: suitable with limitations only for transmission of heat Explanation of the grouping numbers: 2 Group 1: Steels with a guaranteed minimum yield point of R eH ≤ 460 N/mm and with the following analysis values b b b in %: C ≤ 0.25, Si ≤ 0.60, Mn ≤ 1.70, molybdenum ≤ 0.70 , S ≤ 0.045, S ≤ 0.045, Cu ≤ 0.40 , Ni ≤ 0.5 , Cr ≤ 0.3 b (0.4 for casting) , b Nb ≤ 0.05, V ≤ 0.12 , Ti ≤ 0.05 Group 2.1: Thermomechanically rolled fine grain steels and cast steel with a specified minimum yield strength of 2 2 360 N/mm < ReH ≤ 460 N/mm Group 2.2: Thermomechanically rolled fine grain steels and cast steel with a specified minimum yield strength 2 of ReH> 460 N/mm Group 3: Hardened and tempered steels and precipitation hardened steels, not including stainless steels, with a specified 2 minimum yield strength of ReH> 360 N/mm Group 4: Vanadium alloyed Cr-Mo-(Ni) steels with Mo ≤ 0.7% and V ≤ 0.1% c Group 5: Vanadium-free Cr-Mo steels with C ≤ 0.35 % Group 8: Austenitic steels: Group 10: Austenitic-ferritic (duplex) stainless steels Group 21: Pure aluminium with max. 1.5% impurities or alloy content Group 22: Non-hardenable aluminium alloys Index b: a higher value is acceptable, provided that Cr + Mo + Ni + Cu + V ≤ 0.75% Index c: “Vanadium free” means that no amount of vanadium was alloyed intentionally

Note: Material groups in this leaflet differ from those described in DIN EN ISO 14555: 2006

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Other special welding processes

Page 17

When using stainless steels studs in construction supervision situations for components with predominantly static loads, the conditions should be respected as defined in the applicable licence notice of the German Construction Institute (e.g. Z-30.3-6). 2.5.2

Materials used in tip ignition welding

Stud welding with tip ignition can be used for welding unalloyed and alloyed steel studs, as well as aluminium and brass studs. The weldability of different combinations of stud and base materials is described in table 3. In this very fast welding process the sheet is melted only to a depth of 0.2 mm. This allows the welding of materials that would result in brittle alloys if melted heavily and mixed. In such fast welding processes the weld zones of the stud and sheet don't have enough time to mix. The load capacity of joints can be evaluated by the appropriate mechanical tests. Table 3:

Weldability of usual combinations of stud and base material in tip ignition stud welding /5/ Base metal

Stud material

CR ISO/TR 15608 Groups 1 to 5, 11.1

CR ISO/TR 15608 Groups 1 to 5, 11.1 zinc-coated and metallized sheets, max. coating thickness 25 µm

CR ISO/TR 15608 Group 8

Pure copper and lead-free copper alloys, e.g. CuZn37 (CW 508L)

CR ISO/TR 15608 Groups 21 and 22

S235

a

b

a

b

-

1.4301/X5CrNi18-10 1.4303/X5CrNi18-12

a

b

a

b

-

CuZn37 (CW 508L)

b

b

b

a

-

EN AW-Al99,5 (1050A)

-

-

-

-

b

EN AW-AlMg3 (5754)

-

-

-

-

a

Explanation of the letters concerning weldability: -: not suitable for welding a: well suitable b: suitable with limitations Explanation of the grouping numbers: 2 Group 1: Steels with a guaranteed minimum yield point of R eH ≤ 460 N/mm and with the following analysis values b b b in %: C ≤ 0.25, Si ≤ 0.60, Mn ≤ 1.70, molybdenum ≤ 0.70 , S ≤ 0.045, S ≤ 0.045, Cu ≤ 0.40 , Ni ≤ 0.5 , Cr ≤ 0.3 b (0.4 for casting) , b Nb ≤ 0.05, V ≤ 0.12 , Ti ≤ 0.05 Group 2: Thermomechanically rolled fine grain steels and cast steel with a specified minimum yield strength 2 of ReH> 360 N/mm Group 3: Hardened and tempered steels and precipitation hardened steels, not including stainless steels, with a specified 2 minimum yield strength of ReH> 360 N/mm Group 4: Vanadium alloyed Cr-Mo-(Ni) steels with Mo ≤ 0.7% and V ≤ 0.1% c Group 5: Vanadium-free Cr-Mo steels with C ≤ 0.35 % Group 8: Austenitic steels: d Group 11.1: Steels of group 1 , but 0.25% < C ≤ 0.35% Group 21: Pure aluminium with max. 1.5% impurities or alloy content Group 22: Non-hardenable aluminium alloys Index b: a higher value is acceptable, provided that Cr + Mo + Ni + Cu + V ≤ 0.75% Index c: “Vanadium free” means that no amount of vanadium was alloyed intentionally Index d: a higher value is acceptable, provided that Cr + Mo + Ni + Cu + V ≤ 1%

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Other special welding processes 2.6

Page 18

Quality assurance in arc stud welding

The quality of a stud weld is the result of the production process. Non-destructive tests only offer partial evaluation of the quality, and do not guarantee the detection of all imperfections. If mechanical tests with limited load are used to assess the quality of the stud weld, the cost efficiency will be reduced. Because of this, it is usual to conduct random tests by simple non-destructive tests /10/. Legally regulated area (steel constructions) Qualification for stud welding on steel structures in construction supervision situations can be obtained by service providers that hold a big (or small) “suitability certificate” in accordance with DIN 18800. To obtain this, a stud welding process test as per DIN EN ISO 14555 is carried out, in which the welding situation is only examined prior to start of production. The operators need to detect if there are any changes in the working conditions throughout the production, in order to carry out the necessary modifications. Therefore, it is essential to conduct production tests before a new shift and before a new production sequence. The extent of the procedure test of arc stud welding is shown in Figure 13. Type of the test

Procedures

Application ≤ 100°C d ≤ 12 mm

Application > 100°C d> 12 mm

all diameters (d)

Visual test: all studs Drawn arc stud welding with ceramic ferrule or shielding gas and Short cycle stud welding with drawn arc ignition

Bending test with torque wrench: 10 studs

60° bending test: 10 Studs Tensile test: 10 studs

a

b

Tensile test: 5 studs or optional radiography test: b 5 studs

-

Macro cross-section (offset by 90° through stud centre): 2 studs Capacitor discharge stud weld with tip ignition and capacitor discharge stud weld with drawn arc ignition a

b

Visual test: all studs Tensile test: 10 studs 30° bending test: 20 studs

Tensile tests are only needed when the material of the stud is from group 8 as per ISO/TR 15608 and the material of the base metal is from group 1 or 2 as per ISO/TR 15608. Only for dynamic loaded components.

Figure 13: Scope of the procedure tests for stud welding as per DIN EN ISO 14555 /1/.

A welding procedure specification (WPS) is developed for the welding task, to be authorised by an examining body. A welding procedure specification can cover a whole range of workpiece thicknesses and stud diameters. An authorised welding procedure specification is valid for an unlimited period, as long as no crucial quality-impacting changes occur and the production book is kept updated /1/. When applying the inspection criteria, one need to consider what kind of support functions are expected from the welded stud or pin. We have to distinguish between simple or secondary support functions, load-bearing support function with static or dynamic load, and pins undergoing thermal stress. Apart fromf this, the quality requirements of DIN EN ISO 3834 can be divided into a) general requirements (EN ISO 3834-2), b) standard requirements (EN ISO 3834-3) and c) elementary requirements (EN ISO 38344). The test criteria (e.g. allowed imperfection surface) decrease from a) to c).

Other special welding processes 2.7

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Special welding types and alternatives

Penetrating weld technology: Based on arc stud welding, penetrating stud welds are mainly used in Anglo-Saxon countries, as they enable a cost-effective technique for attaching a cover sheet on a steel beam via a headed stud anchor. At the welding point, the arc pierces completely through the thin cover sheet resting on the steel beam, thereby allowing the headed stud anchor to be joined directly to the steel beam. At the same time the cover plate is also joined to the fillet. Nut welding, pad welding: In this type of arc stud welding an externally produced magnetic force is used on the arc, in order to flow this around the face surface of a ring-shaped hollow body. This forced rotation produces a very even melting between the pad and the sheet metal on capsule-shaped parts with an external diameter between approx. 10 and 30 mm. Depending on the application, the sheet metal may already be prepared with a hole. Alternatives of arc stud welding: Differences arise from the heat input type /1, 3/: Resistance stud welding: Because equipment is quite complex and stationary it is not being applied to studs with a diameter bigger than 4 mm. Requires high amperage and high forces. Friction stud welding: Machines with electric motor drives are available since 1999. Studs of up to 10 mm diameter can be used. The friction connection between stud and workpiece requires an appropriate mounting facility, therefore it can only be carried out in stationary mode. Essential advantage: welding of material combinations such as aluminium bolts on steel sheets. The sequence of friction welding is explained under point 1.10. 2.8

Standards and guidelines:

DIN EN ISO 14555: Arc stud welding of metallic materials /1/ DIN EN ISO 13918: Studs and ceramic ferrules for arc stud welding /2/ DVS leaflet 0901: Stud welding processes for metals – overview /3/ DVS leaflet 0902: Drawn-arc stud welding /4/ DVS leaflet 0903: Capacitor discharge stud welding with tip ignition /5/ DVS leaflet 0904: Practical notes – arc stud welding /6/ 2.9

Bibliography

/1/

DIN EN ISO 14555: Welding, arc stud welding of metallic materials. 2006-12.

/2/

DIN EN ISO 13918: Welding, studs and ceramic ferrules for arc stud welding. 2008-10.

/3/

DVS leaflet 0901: Stud welding processes for metals – overview. 1998-12.

/4/

DVS leaflet 0902: Drawn-arc stud welding. 2000-12.

/5/

DVS leaflet 0903: Capacitor discharge stud welding with tip ignition. 2000-12.

/6/

DVS leaflet 0904: Practical notes – arc stud welding. 2000-12.

Other special welding processes

SFI / IWE 1.12-2 Page 20

/7/

Research report from project 79 of SLV München: Examination to reduce the possibility of errors in drawn arc ignition stud welding. Studiengesellschaft für Anwendungstechnik von Eisen und Stahl e.V., Düsseldorf (1983).

/8/

Welz, W., A.W.E. Nentwig and A. Jenicek: Drawn arc stud welding on aluminium materials. Aluminium 67 (1991), H. 2, pp. 153-159 and SLV note no. 90 (1991).

/9/

Trillmich R., W. Welz: Stud welding – fundamentals and applications. Fachbuchreihe Schweißtechnik Bd. 133, DVS-Verlag Düsseldorf (1997).

/10/ Trillmich, R.: Quality assurance concepts and regulations for arc stud welding. Manuskript zum Vortrag anlässlich des Seminars “Qualitätssicherung beim Bolzenschweißen” der SLV München GmbH, 12.06.1997, (1997) pp. 1-14. /11/ Hahn, O., K.G. Schmitt: Examination of affecting parameters in capacitor discharge stud welding. Schweißen + Schneiden, volume 34 (1982) issue 11, pages 521-524.

2.10 Questions (1)

What are the basic work sequences of drawn-arc stud welding?

(2)

What are the appropriate welding parameters for welding a 22 mm flat-tip headed stud of unalloyed steel using a ceramic ferrule?

(3)

What are the respective advantages of the gap welding and contact welding variations of arc stud welding with tip ignition, regarding welding time, strike speed and application possibilities (materials)?

(4)

What non-destructive tests can be applied to arc stud welding?

(5)

What are the causes of arc-blows and how can these be avoided?

(6)

Which material combinations are well suitable for arc stud welding, i.e. for transmitting forces?

(7)

What qualification prerequisites have to be fulfilled by a manufacturer concerning stud welding of steel structures in construction supervision situations?

Other special welding processes 3

SFI / IWE 1.12-2 Page 21

Electroslag welding (RES)

3.1

Summary description of the procedure

3.1.1

Description of the general principle

Electroslag welding (RES, short for resistance electroslag) is a resistance melting process with a very high melt efficiency, in which one or more wires or a strip is used as filler material. The weld pool is supported by movable shoes on both sides, see figure 13.

Slag bath

Figure 13: Electroslag welding (RES)

3.1.2

Component geometry and processing materials

-

Welding of almost of any sheet thickness,

-

Used predominantly on unalloyed and low-alloy steels.

3.1.3

Process-specific advantages and disadvantages

-

Welding the entire cross-section of the seam in one run (uphill),

-

the preparation of joints is simple using flame cutting (no limiting tolerances),

-

large melt and slag pool, with a slow solidification and cooling process: - free of pores, thanks to good degasification, - comparatively less inherent welding stresses, - lowest segregations within the welding block,

-

almost completely distortion-free welding,

-

no fusion errors and inclusions, resulting in excellent quality,

-

low, even penetration and low dilution (regarding surfacing compared to UP),

-

high effective welding speed thanks to a high deposition rate and the parallel melting of several wire electrodes,

-

significant savings in welding time and cost.

Other special welding processes 3.1.4

SFI / IWE 1.12-2 Page 22

Areas of application

ES-welding is used to create joints on thick cross-sections in a single run (> 20 mm) in an uphill position. The significant amount of weld metal enables this kind of welding to be used for surfacing of large areas (drums, pipes, containers) as well. Increasing welding applications appear in the area of the electroslag strip usage with different broad bands, for example, to the production of a wear-resistant cladding on fibre board rollers, to continuous casting rollers or also to the cladding of hydro former, pipe heads, pipe welds or in vessels. 3.2 3.2.1

Process principle – detailed description The sequence of electroslag welding

The welding process is triggered by igniting an arc between the wire electrode and the gap bottom. The arc melts the flux. Since the liquid slag resulting from this process is a better conductor than the arc, the arc extinguishes. The current now flows from the electrode through the liquid slag and the metallic melt into the base material. The slag's resistance heating melts the filler material and the seam flanks, see figure 14. The welding gap is welded bottom up, in one run and in a single step. The shoes move continuously upward, along with the melt and slag pool. Consumed slag is refilled by adding welding flux.

Figure 14: The sequence of electroslag welding (schematic)

3.2.2

Joint preparation

The gap width is about 20-35 mm. The minimum value depends on the type of wire guide, to avoid short circuits. Too wide gaps cannot be processed cost-effectively. To prevent imperfections and homogeneity issues within the joint, the weld is extended by a start plate (run-on) and a run-off plate, see figure 15. The weld has to be completed in one sequence, without interruption.

Other special welding processes

SFI / IWE 1.12-2 Page 23

Figure 15: Preparation of the joint

3.2.3

Execution variants

Different types can be distinguished depending on the process adding filler material, see figure 16 and 17.

Gap width: Position: Plate thickness: Materials:

Figure 16: Electroslag welding with non-melting wire feed and two electrodes

30 – 35 mm vertical  25-30 mm unalloyed, low-alloy and high-alloy types of steel

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Other special welding processes

Position: Plate thickness: Materials:

Page 24

vertical  15 mm unalloyed, low-alloy and high-alloy types of steel

Filler materials and auxiliary materials Wire electrodes: Strip electrodes: Plate electrodes: Depositing Wire feed: Welding flux:

2.5 – 4 mm  60 x 0.5 mm 80 x 60 to 1ß x 120 mm 10 – 15 mm  must produce a slag with high conductivity

Figure 17: Electroslag welding with depositing (melting) wire feed (channel welding)

3.2.4

Welding Flux

The type of flux used (see table 4) influences the welding results by affecting the electrical conductivity of the slag (the slag of flux B conducts better than flux A), see figure 18. Table 4: Typical flux mixtures used in electroslag welding Flux type

SiO2

Al2O3

CaO

MgO

CaF2

Na3AlF6

A

15

20

15

15

35

--

B

5

--

55

--

--

40

Influence of the flux The slag produced by flux A has a higher resistance (therefore lower conductivity), therefore causes more heating. (effect can be compared to an increase in current).

Figure 18: Influence of the flux Flux A

Flux B

Other special welding processes 3.2.5

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Welding parameters and process parameters

Regarding welding parameters, penetration depth and width is influenced both by the welding current and the voltage, see figures 19 and 20. Effects of current The penetration depth first increases as the current grows, however, decreases again as the climb speed of the pool accelerates.

low

high

middle

Figure 19: Effects of current

Effects of voltage The penetration width grows as the voltage increases.

Figure 20: Effects of voltage low

high

Table 5 shows the connections between welding parameters by considering plate thickness and number of electrodes. Table 5: Welding parameters and process parameters Plate thickness mm

Number of wire electrodes fixed weaving

mm

Current strength A

Wire 

V

Welding speed m/h

Deposition rate kg/h

Voltage

30

1

--

2.5

550

41

1.9

14.0

50

1

--

3.0

600

40

1.2

15.0

50

2

--

2.5

480 each

38

1.6

21.7

150

--

2

3.0

550 each

45

0.6

35.2

Depending on the plate thickness, 1 to 3 electrodes can be used at the same time, arranged after each other and optionally weaving alongside the plate thickness. With this option, plates of almost of any thickness can be welded in a single vertical run. The respective data are compiled in table 6.

Other special welding processes

SFI / IWE 1.12-2 Page 26

Table 6: Application areas of single-wire and multi-wire welding Electrode number

Plate thickness (mm) fixed electrode

weaving electrode

1

30 - 60

60 - 150

2

50 - 100

100 - 300

3

100 - 150

150 - 450

3.3 Process variants 3.3.1 Electroslag welding of aluminium Generally, aluminium materials can also be welded using ESW. Weldable base material for pure aluminium (The necessary strength- and deformation properties of the base materials in the weld metal have not yet been achieved for aluminium alloys.) Pool backing: graphite plates (Copper pool backing draws away too much heat, which can result in fusion faults) Welding flux composition for welding aluminium: 18.5 % NaF, 30.0 % LiF, 45.0 % NaCl, 6.5 % SiO2. Typical welding data: (aluminium 99.5 – 50 mm) Wire Electrode: 5 mm  Amperage: 1,000 – 1,100 A Arc voltage: 35 – 42 V 3.3.2 Electroslag welding of circumferential seams The ES-welding of circumferential welds requires the component to be positioned on a rotating support, so that the rotational weld can be executed in one continuous, uphill run. Starting and run-off plates are fixed on the outside and inside of the pipe cross-section, see figure 21.

Figure 21: Electroslag welding of circumferential welds

Other special welding processes

SFI / IWE 1.12-2 Page 27

3.3.3 Electroslag strip cladding Strip cladding or surfacing is an interesting type of electroslag welding. The process is shown in figure 22. It is similar to submerged arc strip cladding, but it uses different flux powders and involves a resistance process as well. The maximum welding speed is slightly higher and the dilution is slightly lower than in submerged arc strip cladding.

Figure 22: Electroslag strip cladding, process principle

Figure 23 shows strip claddings created by ES-welding.

Figure 23: ESW strip cladding (Plant image: ESAB)

3.4

Shielding gas supply

Similar types of welding

Electrogas welding (EGW) is similar to the process of ES-welding, which uses an arc to meld filler wires into a vertical joint, backed by shielding gas. Just like in other types, watercooled copper shoes keep the weld laterally contained, see figure 24. A non-contact height sensor controls the climbing speed of the welding equipment. Used electrodes: Ø1.6 mm for sheets of 8 – 15 mm thickness, Ø2.4 mm for sheets of 12 – 20 mm thickness.

Arc

Power Source

Cooling water supply

Figure 24: Electrogas welding

Deposition rate: up to app. 12 kg/h Where high-quality welds are needed, this technology is significantly cheaper and faster than MIG/MAGwelding. Applications: Ship hulls, storage tanks, vertical welds in pipes and turbine pipelines

Other special welding processes 4 4.1

Page 28

Aluminothermic welding Summary description of the procedure

Other names:

4.2

SFI / IWE 1.12-2

- Aluminothermic casting welding - TW - Thermit welding

Description of the general principle

This process is a type of cast welding. Heat is transferred by casting liquid filler material into a preformed (and preheated) welding area, fusing the joint surfaces. In aluminothermic welding this liquid weld metal is produced from a chemical reaction in a crucible. The starting materials of this reaction are aluminium powder and iron oxide. The reaction (ignition temperature approx. 1,200 °C) is started using a special igniter (magnesium chip). Aluminium starts to burn and turns into slag. The oxygen necessary for this violent reaction of the aluminium is supplied by the iron oxide, which itself is also reduced to iron. Oxygen has a higher affinity for aluminium than for iron. Due to the different densities, iron collects in the bottom of the crucible, with the aluminium slag on top. This is an exothermic process. After the welding process is completed, the sand mould and the surrounding weld seam are removed, and optionally – such as in the case of rail welding – the upper surface is grinded. The rail can be used immediately afterwards. 4.3

Material, additives

Aluminothermic welding can be used with unalloyed and low-alloy steels. By adding alloying elements (such as C, Mn, Si, Cr, Mo, V) to the reaction mixture, one can adapt the properties of the produced weld metal to the base material used in the welding. For example, adding vanadium will make the welding steel harder than the rail steel. The iron oxide and aluminium powder necessary for aluminothermic welding are usually kept granular by a binding agent, so that they remain free-flowing and don't absorb water. 4.4

Process - specific advantages and disadvantages

The welding process. - requires no electrical energy, - can be applied independently of location, e.g. on any building site of the world, - does not require complex instrumentation or costly investments. Note: If the activation energy is high enough, the substances can even ignite at room temperature and liquefy in the severe exothermal reaction. Also, because thermite does not require oxygen for combustion, the reaction cannot be stopped by sand nor by water. Water would actually aggravate the reaction, since contact with water would explosively fling out the liquid materials from the mixture and produce oxyhydrogen, an explosive mixture of hydrogen and oxygen. 4.5

Areas of application

With the TW-process fusion and surface welding can be produced whereby joint welding found the higher spreading. Because the molten liquid filler material in aluminothermic welding can be produced without external energy, this process is highly preferred in the welding of railway tracks. Aluminothermic casting welding can be applied to relatively large cross-sections as well. The aluminium redox reaction is also used to reduce metal oxides - ores, such as uranium ore, chrome oxide, silicon dioxide or manganese oxide - to their respective metals.

Other special welding processes 4.6

SFI / IWE 1.12-2 Page 29

Process principle in railway track welding - detailed description

The basic process sequences of welding rail joints are displayed and described below, see figures 25 and 26. Rail welding (e.g. T-welding, also known as self-preheating method of TW)

Figure 25: Schematic drawing of the Thermit Welding (self-preheating method)

Process sequences 1.

Aligning the rail joint and preparing the welding gap (approx. 24 - 26 mm)

2.

Assembling and securing the refractory moulds on the rail joint

3.

Pre-heating the rail joint using an autogenous welding torch (1.5 - 2 min, approx. 600 °C)

4.

Initiating the chemical reaction and pouring the steel (weld metal) which melts onto the shaped track ends and fills up the welding gap.

5.

After the liquid steel has solidified (approx. 3 - 4 min) the mould is removed and the joint is returned to its original section shape using mechanical treatment.

Other special welding processes

SFI / IWE 1.12-2 Page 30

Prepared rail joint

Attaching the moulds

Pre-heating of the joint

Attaching the reaction crucible

Cast welding process

Finished weld

Figure 26: Aluminothermic welding of railway tracks

4.7

Aluminothermic reaction

The aluminothermic reaction that enables this type of welding is described in figure 27 below.

Figure 27: Description of the aluminothermic reaction process

Other special welding processes 4.8

SFI / IWE 1.12-2 Page 31

High frequency welding (HF welding)

4.8.1

Summary description of the procedure

4.8.2

Description of the general principle

High-frequency welding can be either conductive (with contact) or inductive (without contact). Heating is caused by the resistance within the current-conducting material. Because of the high frequency used, the current flow concentrates on the joint surfaces of the parts which heats up and are welded together by an applied force. 4.9

Component geometry and processing materials

-

the welding of small wall thicknesses is particularly economical. The method can also be applied to relatively thick walls as well.

-

Pipe dimensions: Wall thickness:

diameter: 10 – 1,000 mm from a few hundredths to approx. 20 mm

Materials - unalloyed steels, even with higher carbon content, - aluminium, copper, nickel and their alloys 4.10 Process-specific advantages and disadvantages -

no wear caused to the energy carrier since it makes no contact

-

the pipe does not need to be descaled continuously, the molten zone is narrow, the base material receives only a low heat impact

-

very high welding speed

4.11 Areas of application The process is used mostly in pipe manufacturing. -

possible joint shapes see figure 28.

Figure 28: Possible joint shapes in high-frequency welding (by Hörmann)

Other special welding processes

SFI / IWE 1.12-2 Page 32

4.12 Process types – detailed description 4.12.1 High frequency contact welding (conductive HF welding). The process is shown schematically in figure 29.

Sliding contacts (fixed)

The component moves forward, while the sliding contacts are secured on the welding unit and they slide / grind along on the component surface.

Figure 29: Basic assembly of high-frequency pipe welding (by Hörmann)

Practice-relevant parameters: - usual frequency: 450 kHz -

Operating Voltage: 100 V

-

Welding current:

-

Setup of welding heads for welding outputs of 25, 60, 140 or 280 kW

1,000 to 2,000 A

The heating depth is only a few hundredths millimetre; the actual depth however is a lot bigger due to heat transfer. Figure 30 shows the process principle of HF-welding with sliding contact.

Figure 30: Process principle of sliding contact welding (by Hörmann)

SFI / IWE 1.12-2

Other special welding processes

Page 33

4.12.2 Induction welding (non-contact HF-/MF-welding) The basic working of non-contact welding is displayed in figure 31.

a)

b)

Figure 31: Process principle of induction welding of pipes(by Hörmann) a) with a coil inductor (medium to high frequency up to 450 kHz) b) with a straight inductor (medium frequency up to 10 kHz)

Induction welding is mainly used for thin-walled pipes due to the small heating depth. Losses can be reduced by introducing magnetisable materials inside the pipe. Figure 32 shows the high welding speeds which can be achieved by induction welding, depending on wall thickness and power.

Figure 20: Welding speed in induction welding, in relation to wall thickness and power

Other special welding processes

SFI / IWE 1.12-2 Page 34

Figure 33 shows the application of HF welding on thick plates.

Figure 33: Sheet metal feed (left) and welding of two parts (right), (Plant picture: SMS Meer)

Figure 34 and 35 show cross-sections of welded joints. Most important welding data are also indicated. In figure 34, the flashes on both sides have not been removed. In figure 35 any external and internal flash is directly shaved off by the unit (at a distance of app. 600 to 800 mm behind the welding point), using the residual welding heat, to keep the cutting forces low. Carbide plates are used for the shaving phase. The shavings produced on the external and internal surface are cut into chips and disposed of.

Figure 34: HF welding (material: S 355) where the flash produced on both sides has not been removed (Plant image: SMS Meer)

SFI / IWE 1.12-2

Other special welding processes

Page 35

Figure 35: HF welding (material: X 65) where the flash was directly shaved off both sides (Plant picture: SMS Meer)

5

Ultrasonic welding

5.1 5.1.1

Summary description of the procedure Description of the general principle

Ultrasonic welding (USW) joins components with the use of mechanic oscillation energy, by plastic deformation of surfaces and destruction of optional surface coatings under pressure. The periodically oscillating magnetisation of the coil core produces length changes with the same periodic movements. A high frequency electrical oscillation will therefore create a high frequency mechanical oscillation. Figure 36 shows a schematic illustration of the process principle. Sonotrode (tip) for increasing the oscillation amplitude

Oscillator (constant frequency)

Fixing of the arrangement in in the nodal point of vibration

Press /contact force

Vibration direction

Coupler

Workpiece Anvil

Amplitude

Figure 24: Schematic illustration of US welding

Other special welding processes

SFI / IWE 1.12-2 Page 36

The tangential oscillation is transferred to the workpiece. The upper workpiece thereby carries out a parallel oscillating movement on the contact surface of the bottom workpiece. At the same time, the contact force causes friction and friction heating between the workpieces. The dynamic and static forces destroy possible layers of impurities on the surface of the components; the “clean” metal layers make contact and fuse together. The welding process is facilitated by local plastic deformation and temperature increases. 5.2

Component geometry and processing materials

Figure 37 summarizes materials and material combinations (along with the parameters) which have been successfully welded in the past using ultrasonic welding. This technology enables the joining of aluminium and glass, the creation of mixed joints between metal and plastic as well as metal and ceramic.

Figure 37: Materials and material combinations suitable for US welding (by Ruge) and parameters

Ultrasonic welding enables the creation of various material combinations. However, the process can only be used on thin components. Ultrasonic welding is used for joining thin foils, sheets and wires in electrical engineering, electronics and precision mechanics. Example: Connecting (bonding) thin aluminium wires to chips, welding of contacts, seal welding of housings. 5.3 5.3.1

Process types – detailed description Ultrasonic (continuous) seam welding, figure 38

The end of the sonotrode is disk-shaped, the oscillator is rotating in point K1, the contact force is applied to the oscillation node K2. Welding speed 0.4 ... 10 m/min

Other special welding processes

SFI / IWE 1.12-2 Page 37

Figure 38: Ultrasonic (continuous) seam welding, schematic illustration of operation

5.3.2

Ultrasonic ring welding

The circular sonotrode is forced to torsion vibrations by the tangentially operating oscillator, see figure 39. Figure 39 also shows component shapes suitable for welding.

Component shapes

Figure 39: Ultrasonic ring welding, schematic illustration of operation

5.3.3

Ultrasonic plastic welding

In contrast to ultrasonic metal welding, the vibrations are directed to the weld contact area at a perpendicular angle, see figure 40. The joint arises out of the plastic state. Depending on the preparation of the joint, workpieces can also be welded in butt joints or corner joints.

Other special welding processes

Butt joint

SFI / IWE 1.12-2 Page 38

Corner joint

Figure 40: Ultrasonic plastic welding

During the ultrasonic welding of thermoplastics the generator and the oscillating unit resonate together. As the sound-radiating tip of the sonotrode reflects the mechanical vibrations, a standing wave is created which transmits energy to the components. The mechanical oscillations transferred to the workpieces under a certain contact pressure are absorbed and reflected onto the interface. The resulting molecular and interface friction produces heat. Plastic starts to soften and creates a sound barrier in the area around the joint zone as a result from the plastification and strong evaporation of the plastic layer. This sound barrier enables a high intense melting, i.e. the reaction accelerates automatically as more and more oscillation energy is transformed into heat. Inner friction also contributes to welding. After a certain stopping and/or cooling period, the welding joint is created by maintaining the contact force. Characteristics of US plastic welding Frequency (fixed value for each machine): 20… 65 kHz Amplitude:

1 ... 50 µm

Contact force:

1 ... 6000 N

Welding time:

0,005 ... 1 s

Surface preparation:

not required

Maximum temperature in the welding zone

< 60 % of the metal's melting temperature

Deformation:

< 5 % of the workpiece thickness

Settings

Other special welding processes 5.4 (1)

The gap between the adjusted bar ends may not exceed 10 mm. The welding point is surrounded by a refractory mould made of luting sand. The bar ends must be pre-heated (approx. 1,000 °C) The weld bead is hydraulically shaved off in its hot state. The hardness necessary for wear protection is ensured by cooling with water.

What is the function of high frequency in HF-welding?

     (6)

Al2O3 Fe Fe2O3 Al Mg

What working conditions are usual in thermit welding of railway tracks?

     (5)

High performance regarding deposition and surface Strong penetration by high arc energy Low dilution High quality surface (few ripples, no spatter) Risk of fusion faults and inclusions

What are the raw materials used in the thermit mixture of aluminothermic welding?

     (4)

Sheets of thickness 5 – 15 mm in flat position Sheets of thickness > 20 mm in flat position Sheets of thickness 5 – 15 mm in uphill position Sheets of thickness > 20 mm in uphill position Sheets of thickness 5 – 15 mm in downhill position

What are the characteristic properties of electroslag strip cladding?

     (3)

Page 39

Questions Electroslag welding is used for welding

     (2)

SFI / IWE 1.12-2

It ensures the safe ignition of the arc It generates a current flow close the surface with high current density. It produces a fast heating with a small heating depth It produces slow heating, with a large heating depth It enables welding with slow cooling and low hardness.

What is the maximum temperature when using ultrasonic welding on metals (TS = Melting temperature)

    

T < 0.3 TS T < 0.6 TS T > 0.6 TS T = TS T > TS

Other special welding processes 6 6.1

SFI / IWE 1.12-2 Page 40

Friction welding (1) Summary description of the procedure

Friction welding is classified to the (heated) pressure welding processes. The heat input is produced purely mechanically by frictional heat – involving a relative movement and a simultaneous force application on the joint surfaces. Depending on the type of the relative motion, we can distinguish between several process variants:  Spin /rotary welding, with a revolving relative motion (DIN EN ISO 15620, /1/)  Linear / orbital friction welding, with translationally oscillating relative movement, linear / circular  Friction stir welding, with rotating-stirring friction movement (DIN EN ISO 25239, /2/) Spin/ rotary welding is the most frequently used variant for end-to-end connections of metallic materials (figure. 41). Therefore process 42 is specified as “friction welding” (FRW) both in practice as well as in regulations (DIN EN ISO 4063 – 42, /3/), without additional indication concerning the type of relative movement. 6.2

n

Description of the general principle

F1

Figure 41: Joining process of spin/rotary welding and view of a friction-welded shaft The contacting fusion faces are heated by frictional heat. The material is plasticised, but not melted (T < Ts) in this process. The joint is created by ending the relative movement and applying a (usually) increased contact force (upsetting). Since plastic material is being displaced, the joined parts will be shortened (length allowance), resulting in a typical weld bead (flash). 6.3

Geometry of used components and suitable materials

This type of joining is preferentially used for rotational symmetric full and hollow sections, but the joining parts do not need to be rotational symmetric. It is best to use the same cross-section shapes, but this is not necessary. This means that even parts of different diameters or pipes with different wall thickness can be welded together, just like a circular steel profile to a rectangular one. As the angle-rotation can be stopped very precisely (optional: “positioned friction welding”) this technology also allows exact fitting of non-rotationally symmetrical parts.

Other special welding processes

SFI / IWE 1.12-2 Page 41

Preferred

Figure 42: Weldable cross-sections Weldable: Full cross-sections of  3 to 250 mm, Reibschweißen - Geometrievarianten hollow structural steel sections, currently to  900 gleicher x 6 mm, Durchmesser, wall thickness Rohre from 1.2 mm Wanddicke Standard: Wellen gleicher

Figure 43: Typical geometry of joints involving different diameters

Friction welding is more suitable for joining standard and special materials than traditional welding processes (fusion welding), because materials are not melted but joined in a plastic state under applied force (similar to forging). Friction welding enables the use of economical standard materials or easily produced blanks, such as bars, pipes, cast or forged parts. Steels with a high carbon content (e.g. C45, 42CrMo4) are particularly well suited for friction welding, they often don't require additional processes such as preheating or subsequent heat treatment. This technology also allows the joining of porous sintered or PM-materials. Furthermore, it can also be used to create dissimilar joints (mixed joints, where brittle, intermetallic phases are generated in fusion welding) that are usually not suitable for fusion welding, of material combinations such as steel/aluminium, aluminium/copper, titanium/steel, aluminium/ceramics, etc. Friction welding offers good weldability even for many “difficult” materials and material combinations. The applicability of friction welding mainly depends on the alloy type and physical properties (e.g. friction interface, thermal expansion coefficients, diffusion properties), but partly also on the surface area and the geometry of the interface.

Other special welding processes 6.4

Page 42

Process-specific advantages and disadvantages

Advantages

          

Disadvantages:       6.5

SFI / IWE 1.12-2

Fully mechanised or automated welding process Good reproducibility and process monitoring. Simple integration in production lines. Low materials consumption ( offers a good alternative for machining production processes by joining standard blanks such as rods, pipes or plates, etc.). No filler materials or shielding gas necessary. No harmful emissions (radiation, spatter, fumes etc.). No melt (T < Ts) and short welding time compared to the size of the joint surface, resulting in low thermal load on the material. Favourable microstructure state (forged structure) of the joint (no melt or cast microstructure). Suitable for welding “difficult” materials and material combinations. Symmetrical heating and cooling: distortion low, symmetric internal stresses. High accuracy of the connected parts. Weldability is limited to joining surfaces which are quasi rotationally symmetric. Flash formation - requires subsequent treatment (process-integration possible). In some cases, high mechanical load on material or component (upsetting force, frictional torque). In some cases, increased positioning/clamping efforts. The possibility of non-destructive tests is limited. High machine costs ( as an alternative: commissioning).

Areas of application

Different branches of industry use this process in batch production.        

Mechanical engineering: gear-wheels, shafts, hydraulic cylinders, radial pump pistons and piston shafts,rods, spindles, crank shafts, drilling pipes Automotive industry: Axle beams, exhaust valves, cardan shafts, shift rods, brake camshafts, turbochargers, gear parts, pipe shafts, sling tubes, ring links, airbag gas generators, cables Air and spacecraft construction: Rotors, turbines, shafts, combustion chamber nozzles Tools: Twist drills, cutters, milling cutters, punches, chisels, reamers, tool holders Electrical industries: Long-life soldering tips, switching contacts, cable connectors, EDM-anodes Medical technology: Rotating anode shafts for x-ray tubes, hip prostheses, bone wires Equipment and pipeline manufacturing: Equipment, pipes, flanges, fittings, valve casings, transitional pieces Construction industry: Anchors, façade anchors

Other special welding processes

SFI / IWE 1.12-2 Page 43

Figure 44: Examples of automotive applications of friction welding /KUKA Systems GmbH/

6.6 6.6.1

Process principle – detailed description Process sequences of spin/ rotary welding

One of the two tightly clamped workpieces is put into rotation. The pieces are brought together at the joining point by an axial lining mechanism.

Force F1 (frictional force) and the rotating relative motion produce friction that heats up the ends of both parts. A part of the plasticising material is pushed outside. During this, the parts shorten somewhat.

n

F1

Other special welding processes

The revolving workpiece is stopped by an additional brake or due to frictional resistance; at approximately the same time, the parts are pressed together with an increased force F2 (upsetting force). The two workpieces are now welded.

SFI / IWE 1.12-2 Page 44

F2

n=0

Figure 45: Process flow of spin welding

Welding occurs in the moment when the relative movement stops or the rotating motion is brought to a halt (revolution: n = 0). The upset pressure is maintained for a short time (upset time), depending on the area of the joint surfaces, until the pre-warmed, plastified material solidifies again during the cooling of the joint area.

6.6.2

Process variations of spin welding

Spin welding and the friction welding machines accordingly, can be grouped into two categories:  

Continuous drive friction welding, Friction welding with flywheel drive,

ISO 4063 - 421 (Direct drive friction welding) ISO 4063 - 422 (Inertia friction welding)

In the case of continuous drive friction welding (figure 46) a connected rotary drive adds energy continuously during the friction process. The friction revolution speed is usually constant, but may be variable in special cases. Heat input depends on the contact force applied, the rotational speed and friction time or potentially on the shortening caused by the friction (time or path controlled). With highperformance drives and a variable rotational speed, the process of inertia friction welding can be characterised.

1 2 3a 3b 4a 4b 5

- Drive - Break - Clamping tool, rotating - Clamping tool, not rotating - Workpiece, rotating - Workpiece, not rotating - Working (force) cylinder

Figure 46: Continuous drive friction welding

1 2

- Upsetting on rotating workpiece - Upsetting on braked workpiece

Development of process parameters in time

Other special welding processes

SFI / IWE 1.12-2 Page 45

In inertia friction welding (figure 47) the power stored in a flywheel is converted into heat. The rotating drive accelerates the flywheel to a certain preset starting speed, to be uncoupled from it shortly before the friction surfaces come into contact. No more motoric energy is added during the friction process, only kinetic energy (flywheel, speed of revolution) is transformed into frictional heat. There is a typical decrease in speed by the “self-braking” effect of the friction surfaces, until the rotational motion stops completely (n = 0). As the rotating motion stops, the contact force is increased (upsetting) or maintained. Friction time is not an adjustable machine parameter - it is the reproducible result of the kinetic energy and the friction resistance of the joint surfaces.

1 2 3a 3b 4a 4b 5

- Drive - Flywheel, variable - Clamping tool, rotating - Clamping tool, not rotating - Workpiece, rotating - Workpiece, not rotating - Working (force) cylinder

Figure 47: Friction welding with flywheel drive

Development of process parameters in time

To increase the accuracy of the process - and especially to reduce length tolerances to under +/- 0.1 mm - modern machines are equipped with a so-called “path control” function. The continuously monitored shortening of the parts is immediately compared to a “trained” reference curve, and the unit automatically corrects any detected potential deviations by regulating the contact pressure (e.g. +/- 5% of the reference pressure value). 6.6.3

Types of spin / rotary welding

Friction welding with one part being rotated and the other one being translated (standard procedure) Friction welding with one part being rotated and translated, while the other part is stationary (e.g. in the case of small friction welding machines used for friction stud welding) Rotation and translation of two joining parts against a stationary connecting part in the middle (special procedure)

Rotation of the middle connection part, with a linear movement of the two outer parts (special procedure) Figure 48:

Process types of friction welding

Other special welding processes 6.7

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Equipment and accessories

Due to their dimensions, friction welding machines are usually stationary units, carrying out the welding process fully mechanised (manual loading and unloading) or fully automated. Small friction welding machines are an exception, as they allow a more mobile used, mounted on a gantry or a carriage. The axial feed and the contact force are usually produced by a hydraulic unit, although some smaller machines can also have servo-motors or pneumatic units. Asynchronous motors are becoming more popular as rotating drives, since they are more compact, they can supply a high torque output for a wide range of rotational speed values, and they also offer a dynamic control attributes due to low dead weight.

Figure 49: Design of a continuous drive friction welding machine, horizontal construction (DVS MB 2909-2)

Machine equipment, optional parts and accessories  Control unit / controller / input/storage of welding parameters and programmes logging measured values, process control and data storage (documentation)  Tool kit Component-specific clamping tools for one welding task  Turning-off unit friction surface treatment of the revolving part before welding removing flashes after welding (alternatively: shaving or punching)  Positioning unit breaking and welding of the parts in a rotation-accurate manner loading / unloading of not round parts in the specified clamping position increasing the accuracy (decreasing axis offset)  Stationary support prevention against buckling in the case of long, thin parts for guidance of long overhanging parts during flash removal  Fume extractor unit mainly for extracting oil vapours (oil remnants, corrosion protection of steel parts)

Other special welding processes

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The choice of which friction welding machine is to be used depends on the size of the surface to be welded (mm²), the required material-specific contact pressure (N/mm²) and the resulting frictional torque (required performance of the rotating drive). Table 7 provides an overview of some widely used friction welding units from 1.5 t to 2,000 t (max. upsetting load) and the cross-section surfaces weldable with each one. Table 7: Choice of friction welding machines (differentiated by maximum contact force) Contact force max.

Classification (by max. load)

Manufacturer/country e.g.

t

Welding surface up to app. (depends on material and geometry) 2 mm

kN 15 20/ 50 200 / 450 800 / 1,250 / 3,000 10,000 2,700 / 4,000 6,800 / 20,000

1,5 2/ 5 20 / 45 80 / 125 / 300 1,000 270 / 380 670 / 2,000

100 160 / 330 660 / 3,750 6,600 / 10,000 / 24,000 85,000 19,500 / 27,000 48,000 / 145,000

Harms & Wende / D KUKA / D

MTI / USA

H&W RSM210, 1.2 t (SLV München)

KUKA RS15, 15 t (SLV München)

KUKA RS30/45, 30/45 t

MTI Model 800, 2,000 t

Figure 50: Friction welding machines - examples

Other special welding processes 6.8

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Consumables

Shielding gas is not necessary for the friction welding of surfaces that are rotationally symmetric, as the joint surfaces cover each other totally from the atmosphere. An oxidised welding flash does not affect the joint, and can be removed if necessary. Shielding gas can be required if the joint surfaces are not rotationally symmetric, with some edge areas temporarily exposed and a low amount of material displacement occurring. Shielding gas is also used in cases where flash removal is not possible and the oxidisation/scaling of the flash has to be prevented. Filler materials are not necessary for friction welding, since the original materials of both welding parts are directly joined. The proportionate shortening of the joint parts should be factored in as length allowance (“welding allowance”) in friction welding.

7 7.1

Friction welding (2) Process conditions and joint properties in spin welding

7.1.1 Preparation of parts DVS leaflet 2909-2 provides information on the preparation of the joint surfaces. Foreign materials which prevents or decreases friction heating should be removed from the joint surfaces. This includes scaling, rolling & cast skin, forging powder, bonding material, oil, grease, paint, thin oxidation layers (rust film) and other oxide layers. Hardening layers should be avoided or removed from the contact surfaces before welding. For solid shaft joints a saw section is usually sufficient (piston rods). Motor valves are usually frictionwelded without additional preparation: with a forged shaft at the valve head, and a pressed valve shaft surface. The friction welding process itself more or less takes over the task of “mechanical preparation”, like adjusting the parts to enable a total surface contact of the joint surfaces before the process starts. This however also implies a higher load for the clamping tools, the machine and the welded parts. In the case of hollow sections and wherever a high degree of accuracy is needed the contact faces are usually face-turned, as to enable that friction surface contact, pressure distribution and heat input are all immediately symmetrical at the start of the process. This benefits welding processes with short friction time, less reduction in length (bulge formation) and high dimensional accuracy. To reduce peaks in friction torque at the beginning of the process (material is cold) there are several “friction helping” options for decreasing the initial friction diameter. Examples include coned surfaces or a convex front face in the case of friction stud welding. In this way, the joint surface area increases only after plastification starts and the heated joint zone starts to shorten. This can also have a positive effect on the displacement of surface impurities, the penetration of coatings and less component shortening, e.g. when using friction stud welding on galvanised or painted sheet metals. The accuracy of clamping the joining surfaces has an influence on the dimensional stability of the friction-welded parts. Therefore for clamping of cast or forged parts, it is essential to pay attention to have suitable and even surface quality of clamping-, securing- and supporting surfaces (ridges, separation edges).

Other special welding processes 7.1.2

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Welding parameters

The main welding parameters in spin welding are:  Revolution speed [1/min], or peripheral speed [m/s] (depending on diameter).  Friction pressure pR [N/mm², bar] or axial friction force FR [kN]  Friction time [s]  Friction shortening (friction path) [mm]  Upsetting pressure pSt [N/mm², bar] or axial upsetting force FSt [kN]  Upsetting time [s]  Total shortening (friction + upsetting path) [mm] Further welding parameters result from the different process variations, such as flywheel mass, (reduced) initial friction force, breaking (time) point, upsetting (time) point, etc. Formula for conversion between pressure “p” and axial contact force “F”:

pM AM = F = pB AB

Machine: pM: Hydraulic pressure (bar), AM: effective piston surface (mm²) Component: pB: axial contact pressure (N/mm²), AB: friction surface (mm²) Conversion of pressure measurement units: 1 MPa = 1 N/mm² = 0.1 kN/cm² = 10 bar Example: If a steel shaft with a diameter of 20 mm (314 mm²) has to be friction-welded with a friction / upsetting pressure of 60 respectively 120 N/mm², then a friction / upsetting force of 18.8 or 37.7 kN is needed, or a friction welding machine with no less than 3.8 t upsetting load. The reference values of friction welding parameters published in the leaflets (tables 8 and 9) are usually only valid for certain materials or diameters. They may widely vary depending on the alloy, the heat treatment attributes and the geometry-specific deformation resistance of the material. Parts with big diameters and thick walls are usually friction-welded with higher pressure than small diameters or thinwalled hollow sections (of the same material). Table 8: Welding parameter reference values for continuous drive friction welding of parts of the same material and equal full cross-sections* (by DVS-MB 2909-2)

Other special welding processes Table 9:

7.1.3

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Welding parameter reference values for inertia friction welding of parts of the same material group, pipe sections with wall thickness of 6 - 10 mm and welding times of 0.5 - 10 s (from DVS leaflet 2909-2)

Structure of a friction welded joint

In friction welding, materials are not melted: they are joined under their melting temperature, in a plastic state. Therefore, it creates no melting zone and no solidified melting-casting structures, but rather a joining zone where the original joints of the base materials are significantly deformed – similar to forging. Bonding mechanisms include plastic base material mixing (in the case of similar materials), diffusion (depending on temperature, time and grade of deformation) and adhesion (influence of forces). Friction welded joints are expected to weld the original sections with their full surface and without defects. Usually, the joint surface is even increased by the process (diameter of the flash). The flash can be machined of in this way without undercut. Steel joints Figure 51 shows friction-welded steel shaft parts – with different diameters and with equal diameters. If the joined parts are of the same diameter and material, the created flash is symmetrical to the joint zone, and both parts are proportionately shortened to the same extent. If the joined parts are different in diameter or the materials have a different heat resistance, the shortening of the two parts will be proportionately different. If the diameters are different, a tendentiously higher “friction performance” (e.g. a higher contact pressure) is needed to create more frictional heat that compensates for the increased Reibschweißen - Querschliffe heat dissipation, in order to reach the temperature level needed for plasticising the joint zone.

Ø 16 mm / Ø 25 mm

Ø 16 mm / Ø 16 mm

pR/St = 80/160 N/mm², t = 3,7 s n = 2000 1/min, l = 7 mm

pR/St = 40/80 N/mm², t = 4,2 s n = 2000 1/min, l = 7 mm

Wellenverbindungen S235 artgleich

Figure 51: Cross-section of friction-welded steel shaft joints using S235, similar type

Other special welding processes

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Friction welding is especially well suitable for joining steels with a high carbon content, equipment and tempered steel (C45, C60, 42CrMo4). Preheating and after-heating is often not necessary, as the hardening can be limited by selecting friction welding parameters (such as a longer friction time). Due to symmetrical cooling, friction welded joints have good residual stress characteristics. Therefore, even increased hardness values can be accepted in practice, provided that other criteria (bending test, ductility) are met.

S355

C45 Figure 52: Friction-welded joint of a hollow and a solid shaft (S355/C45), external bulge removed by turning

Aluminium joints Aluminium materials have better heat transfer properties and lower heat resistance than steel. This results in a rapidly spreading softening in and around the heat affected zone. In order to maintain the required intensity of friction and frictional heat, it is important not to allow the material to plasticise too much in the friction zone. Therefore aluminium joints are welded with very short friction times (aluminium shock absorber pipes: approx. 0.5 s, aluminium auto car wheel: 1.5 s). The plastification and shortening of the parts happens almost as fast as the heat-dependent softening which comes ahead. The short friction time prevents an excessive shortening of the parts. The softened material is displaced by feeding the base material. The heat-affected zone is hereby kept small. Friction welding allows the use of economical standard materials or easily produced blanks, such as bars, pipes, cast or forged parts. Due to the low joining temperature (T < Ts), this technology also allows the welding of porous sintered, PM (powder metallurgy) or MMC (metal matrix composite, such as particle-reinforced) materials. Material combinations and mixed joints Friction welding is well-suitable for joining dissimilar materials (mixed joints) which cannot be welded by fusion, e.g. Material combinations steel/aluminium, Al/copper, titanium/steels, Al/ceramic, etc. The applicability of friction welding depends on the alloy type and its physical properties (e.g. the friction interface, different thermal expansion coefficients), but partly also on the dimension and geometry of the section surfaces to be joined. When using friction welding for mixed joints, brittle intermetallic phases can be decreased under a critical level – i.e. without adversely influencing the properties of the joint created – by the right choice of suitable alloys and by an appropriate process design (short friction times, good material displacement properties).

Other special welding processes

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Table 10: Friction-welded materials and material combinations (selection by DVS-MB 2909-1)

NOTE: This schematic table does not contain application-oriented data concerning the quality of the welding.

a) Aluminium/steel propeller shaft ( 60 x 3 mm) /BMW/

Other special welding processes

b) Copper/aluminium, for electrical connections /MTI/

c) Titanium/steel tube joint  12 x 2 mm Tensile test fracture in the steel part

Figure 53: Examples of friction-welded mixed joints

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Other special welding processes 7.2

Examples of Application

Figure 54: Sample applications

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Other special welding processes 7.3 7.3.1

Page 55

Process variants Friction stud welding

Friction stud welding (ISO 4063 - 423) is a type of friction welding and a geometric variation of rotational friction welding. It is mainly used in as an alternative to arc stud welding in the following cases:     

stud joints with high load bearing capacity stud joints with a large diameter (e.g. concrete anchors), even >25 mm stud welding without shielding gas (no risk of pore formation in aluminium) stud welding on coated or painted surfaces stud welding of materials not or hardly suitable for fusion welding and of Reibschweißen - Varianten mixed joints (e.g. St/Al, St/Cu)  socket welding (e.g. threaded sleeves)

Figure 55: Friction stud welding – schematic process

Reibbolzenschweißen The technical requirements and the applied forces are higher than in arc stud welding. It requires the joining parts to be securely fixed to each other, and therefore a backside support (accessibility of both sides needed). Friction stud welding is mainly used in situations where the application of arc stud welding is limited.

Friction stud welding of small-diameter parts (up to app.  10 mm) can be carried out using compact weld heads on gantry units or by mobile equipment. When welding threaded bolts, the transfer of forces and the momentum has to be warranted e.g. by using a suitable bolt socket.

Suspension strut mount with 3 frictionwelded steel threaded bolts Figure 56: Examples of friction stud welding

Aluminium gas pressure absorber with friction-welded aluminium stud

Aluminium ribbon cable with friction-welded steel threaded bolts /14/

Other special welding processes

7.3.2

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Linear friction welding and orbital friction welding

Linear and orbital friction welding are types of friction welding with a vibrating relative motion of the friction surfaces, using a linear or a circular oscillating movement respectively. The area of plastic welding also uses the terms “vibration welding” (umbrella term) and “circular friction welding” (the same as orbital friction- welding). Reibschweißen Varianten The relative motion is the same as used by an “orbital sander”, but with a higher drive performance, rigidity and a final centring process involved. Apart of round cross-sections, both processes allow the friction welding of not-round sections, such as a longitudinal structural profile or box profile (open or closed). It is also possible to weld several joints at the same time, within the same process.

Figure 57: Linear/orbital friction welding – schematic illustration of process (f: frequency, S: stroke)

The vibration drives used for welding metals generate the friction movement using hydraulics or a Linearreibschweißen mechanical eccentric. Plastic welding units (lower friction performance) can even have electromagnetic drives. The following welding parameters apply, depending on the friction movement:  Linear friction welding: Oscillation amplitude “S” (2x amplitude), frequency of oscillation “f”  Orbital friction welding: Oscillation circle“S” (2x oscillation radius), frequency of oscillation “f” The operating ranges of welding machines – depending on design – can reach 50 Hz / 6 mm or 100 Hz 2 mm. The oscillation causes temporary misalignment, where the edges of joint surfaces become uncovered. This necessitates the use of shielding gas, unless it is assured that edge oxides are fully discharged by material displacement. The oscillation drives have a restoring mechanism, which aligns the joint parts at the end of the process, securing the final position in the centre of the oscillation. The positioning must be carried out within a very short time frame (a few tenths of a second) in a plastic state, to avoid stress or damage to the joint and to keep edge misalignment at a minimum. Upsetting takes place after the final central position is set.

Linear friction-welded titanium turbine blades /13/ Figure 58: Applications of linear friction welding

Electric ribbon cable Al/Ms /14/

Other special welding processes

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Linear friction welding has a successful track record in the construction of aircraft engines, producing new parts as well as in repair processes. Some factors still restrict the further spreading of the process to intermetallic joints: specialized machinery is needed for the specific applications (there are no universal friction welding units), it requires complex clamping fittings and the post-processing of the flash also adds to the cost. The process is used in cases where no other production alternatives (from a technical, quality or economical aspect) exist, taking into consideration the expenditure/cost compensation of secondary effects. The joining of plastics by vibration welding has found many applications areas in manufacturing.

Vehicle intake housings

Vehicle fitting housings

Vehicle liquid tanks

Figure 59: Some application examples of vibration welding of plastics /15/

7.4

Workplace safety

The potential hazards of friction welding arise from the movement of the parts (rotation, oscillation) as well as from the clamping- and process forces generated. Attention should be paid to the appropriate use and assembly of clamping tools, mounting aids and arrestors. Operators also need to take care of the proper functioning of all protective facilities of the machine (e.g. clamping protection against crushing risks, protective doors, monitoring of limit values). Sound protection has to be provided for the use of uninsulated machines. The joining parts have to be placed and secured precisely.

Other special welding processes 7.5

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Questions

(1)

What are the 2 process variants and designs of spin welding to be distinguished according to the rotating drive?

(2)

Name the main welding parameters of spin welding

(3)

What kind of joint surfaces are preferred in spin welding?

(4)

How can you recognise a friction welding joint (without post-treatment)?

(5)

How does the weldability of the materials in friction welding compare to those in traditional fusion welding processes?

(6)

What are the reasons that enable friction welding to successfully join material combinations which are not “suitable for welding” per definition by fusion welding processes?

(7)

List 3 advantages of friction welding

(8)

List 3 disadvantages of friction welding

(9)

Name typical application areas of friction welding

Other special welding processes

SFI / IWE 1.12-2 Page 59

7.6

Bibliography

/1/

DIN EN ISO 15620 (2000): Friction welding of metallic materials.

/2/

DIN EN ISO 4063 (2011-03): Welding and allied processes - Nomenclature of processes and reference numbers.

/3/

DVS leaflet 2909, part 1 (June 2009): Friction welding of metallic materials. Processes, terms, materials. DVS-Verlag, Düsseldorf.

/4/

DVS leaflet 2909, part 2 (the 2011): Friction welding of metallic materials. Characteristics and manufacturing of joint and monitoring of the welding process.

/5/

DVS leaflet 2909, part 3 (June 1994): Friction welding of metallic materials. Characteristics and manufacturing of joint and monitoring of the welding process.

/6/

DVS leaflet 2909, part 4 (January 1999): Friction welding of metallic materials. Requirements to friction welding personnel.

/7/

DVS-leaflet 2909, part 5 (August 2005): Friction welding of metallic materials. Quality Levels for rotation friction welding.

/8/

DIN EN ISO 17660-1 (December 2006) Welding of reinforcing steel - part 1: Load-bearing welded joints

/9/

DVS Guideline 2218, Part 1 (February 1994): Welding of thermoplastic materials in the batch production. Rotation friction welding equipment -, process, characteristic

/10/ Neumann, A. and D. Schober: Friction welding of metals. Fachbuchreihe Schweißtechnik volume 107, ISBN: 978-3-87155-124-6, DVS-Verlag, Düsseldorf, 1991. /11/ Grünauer, H.: Friction welding of metals. Reihe Kontakt und Studium, Vol. 198, Expert Verlag, 1987. /12/ Vill, V.I.: Friction welding of metals. DVS-Berichte, Volume 2, DVS-Verlag Düsseldorf (1967) /13/ Raiser E., S. Kallee: “LinFric” - Entwicklung einer hydraulischen Linearreibschweißmaschine. Vortrag zum 12. Erfahrungsaustausch Reibschweißen, SLV München, 2002. /14/ S. Martens: Aluminium als elektrische Leitung im Automobil - Reibschweißen und alternative Fügetechnologien. Vortrag zum 19. Erfahrungsaustausch Reibschweißen, SLV München, 2011 /15/ L. Appel, Cramer, H.: Orbitalreibschweißen - Eine neue Schlüsseltechnologie zum Fügen metallischer Werkstoffe und Mischverbindungen. DVS reports Bd. 250 to GST 2008, pp. 155 - 161.

Other special welding processes 8

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Friction welding - friction stir welding

8.1

Summary description of the procedure

Friction stir welding (FSW) was developed and patented in the United Kingdom in 1991. Based on EN 14610:2004 /1/ friction stir welding was defined in Germany as a subtype of friction welding. The process is described as follows: “Pressure butt welding in the viscoplastic phase, where heat is generated from friction between a rotating wear-resistant tool and the workpieces.” Process name/reference number: Friction stir welding, DIN EN ISO 4063 – 43 As defined by EN 4063 friction stir welding (43) became an independent process within the umbrella group pressure welding (4), similar to friction welding (42). 8.1.1

Description of the general principle

Friction stir welding uses a rotating cylindrical friction tool with a profiled nib that has an offset, broader shoulder. The joining parts are clamped firmly onto a backing support. Under pressure, the rotating pin moves along the gap-free butt joint (see figure 21). The friction heats up the material of both parts in front of the pin, to be displaced around the nib, to be mixed behind the pin and lastly to combine into a weld seam. The solid clamping of the parts, the backing and the shoulder of the tool all help to avoid external material displacement, while the contacting shoulder also produces additional friction heat. The process works best on materials which can be plastified well by heat, such as aluminium or copper alloys. A fully mechanised process can treat sheet thicknesses of up to 20 mm in one single run (“oneReibschweißen - Varianten layered”) with a simple I-seam preparation and a low thermal load (no melt).

Friction Stir Welding (Rührreibschweißen) Figure 60: Friction stir welding - schematic process; welded aluminium section (SLV-BB)

8.1.2

Suitable materials and the joints geometries

Materials:  Mainly low melting metals, aluminium, copper, magnesium and their alloys (as well as cast and die-cast materials)  Material combinations such as Al/Cu, Al/Mg, Cu/CuSn, Cu/CuZn, Al/St, Al/Ti  Less suitable for materials with a higher melting point, such as steel, titanium and nickel (too much tool wear, potential addition of external heat, e.g. from induction)

Other special welding processes

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Geometry of joints: prior to welding

after welding

a. Combination of overlap/butt joint b. Butt joint (welded on one or both sides) c. Combination of overlap/butt joint d. T-joint e. Corner joint f. Overlap joint g. Corner joint h. Butt joint (pipe peripheral welding)

Figure 61: Geometry of joints in friction stir welding, shown before and after welding /Source: DVS AG.V 11.2, document of the presentation on friction stir welding/

8.1.3

Process-specific advantages and disadvantages

Advantages • High quality welds with good reproducibility, no pores, low tendency of cracks, low distortion • Post-treatment is unnecessary with the right backing, the weld surface can also be left intact • Joining of materials (e.g. AlLi-alloys) and material combinations (e.g. Al/St) not suitable for fusion welding • No special joint preparation is required • No filler material or support materials required • Thick walls can be welded in one run • Depending on the application, welding can be done using milling machines • Joining of several metal sheets is possible • No harmful emissions (radiation, spatter, fumes etc.).

Disadvantages • Backside support is generally required under the component (the other side has to be accessible) • The remaining end hole must be taken into account (leaving intact, filling or cutting) • Reduced possibilities for refractory materials (tool life) • Strong clamping facilities are needed at a transverse angle to the weld • A relatively low tolerance is expected concerning sheet thickness (< 0.1 - 0.2 mm) • The acquisition of strong and precise equipment is expensive • Patent licensing is required

Other special welding processes 8.1.4

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Areas of application

Processing aluminium industry • Production of aluminium panels from metals sheets and extruded profiles (External cladding and baseplates of ships, offshore drilling platforms, railway racks, cooling units) • All kinds of joint welds, production of semi-finished products Shipbuilding • e.g. FSW prefabricated panels for Fast-Ferry aluminium catamarans with a length of 60 m Railway vehicle construction • Production of large, low-distortion aluminium panels of metals sheets and extruded profiles (external cladding and baseplates of railcars and carriages) Automotive construction • Tailored blanks: Aluminium plate joints of different thicknesses for further reshaping • Battery cell connections for E-cars from Cu/Al/Cu or Ms/Al/Ms (welding of long welds, subsequent transverse cutting into contact bridges) • Aluminium loading area panels for lorries (increased payload) • Aluminium telescope tubes for lorry cranes Spacecraft • pore free welding of special spacecraft alloys (not well suited for fusion welding) E.g. tank containers of rockets and space shuttles Energy industry • Generators: Joining electric copper conductors (flat band sections) • Castor container outer cladding: Cu contact profiles (t = 50 mm) • Aluminium pipes, orbital friction-stir-welded Medical technology: • e.g. highly vacuum-tight aluminium-high grade steel connections

Panels made of extruded Al sections /www.SapaGroup.com/

Mobile crane telescope pipe, aluminium, wall thickness 6 mm, length max. 4.8 m, /HAI, Hammerer Aluminium Industries GmbH/

Figure 62: Friction stir welding of longitudinal seams on aluminium panels and profile sections

Other special welding processes 8.2 8.2.1

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Process principle – detailed description Machines

Friction stir welding is usually done using gantry units, parallel carriages, adapted CNC machines or by robots. Apart from the usual long straight welds, two or three dimensional contours can also be welded.

a) stationary FSW machine with C-frame, ESAB Legio 3UT (SLV-BB)

b) FSW gantry unit /ESAB/

c) FSW robot unit /EADS, iwb, KUKA/

Figure 63: Friction stir welding machines

8.2.2

Tools

The simplest kind of FSW tool consists of a flat welding shoulder with a cylindrical or slightly rounded welding nib (pin). The use of a tool with this geometry is restricted to thin sheets (actual thickness depends on the part's material and the process parameters). The shoulder diameter is relatively large compared to the pin length. In order to have a better hold on the plastified material during the process, the shoulder design should be slightly concave and/or equipped with a screw conveyor or other conveying aid. Concave shoulder however can only be used in friction stir welding at a slight angle. To improve the transfer of material even in the welded depth, the welding pin can be designed with a threadlike conveying aid and/or with flat or spiral milling threads (Figure 64).

Figure 64: Tool for friction stir welding /SLV-BB/

Other special welding processes 8.2.3

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Welding sequence

The welding process can be divided into three phases: 1) Submersion The rotating tool is pressed onto the workpiece under application of an axial force. The resulting frictional heat softens the material, allowing the tool to penetrate deeper into the workpiece. 2) Welding The sliding motion (welding speed) can start as soon as the tool shoulder makes contact with the workpiece surface. The shoulder of the rotating tool (rotational speed) presses onto the workpiece (welding force) during the welding process. Thereby frictional heat is not only generated by the pin but also by the friction of the shoulder on the surface. 3) Emerging At the end of the weld the rotating tool emerges from the welded part, leaving an exit hole (FSW end hole). 8.2.4

Welding parameters

The main welding parameters are speed of revolution, contact force and travel speed. e.g. for AW 6082 with a sheet thickness of 6 mm: 1,000 rpm, 25 kN (2.5 t), 1 m/min, “single run” 8.2.5

Structure and properties of the joint

• Asymmetrical cross-section of the seam • The result is a generally finer granular microstructure than the base material • Limited grain growth in the heat-affected zone • Zones: – Agitated zone with the strongest deformation zone (nugget) – Thermomechanically affected zone (TMAZ) – Heat-affected zone (HAZ) – Base material (GW)

Figure 65: Structure of the joint zone, I-joint Al/Al /SLV-BB/

Aluminium joints – Thin sheets: strength can be added in the area of the base material – Thick plates: lower feed speed → more heat energy introduced → softening increases in the heataffected zone (still, there are advantages in comparison to arc, laser or hybrid processes: fee of pores, low distortion, process stability).

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Copper joints – Strength approx. 90% of the base material – Interesting applications in electronic engineering: conductance identical to base material If penetration is insufficient and produces a root gap, fatigue strength properties can be significantly impaired by its strong notch effect. This can be prevented by a sheet thickness with low tolerances and a process management using force control. Properties of friction stir welding joints  Low joining temperature (below the solidus temperature), no melt  No major metallurgical changes, no solidification structures  Weld seams well formable after joining  Weld seams are impermeable and pressure tight (suitable for tanks)  Low distortion of the parts compared to fusion welding  Excellent mechanical characteristics under static and dynamic load, in some cases as good as the base material, depending on the alloy  Mixed joints possible

8.3

Process variants

Friction spot welding Friction spot welding uses a single- or multi-part stir tool to produce a point-based overlap joint. In both cases, double-side accessibility is required with backside support in the welding area, e.g when using a friction spot welding head. 

Friction spot welding with one-piece stir tool

Friction spot welding with one-piece stir tool uses a conventional stir tip like friction stir welding. In this case, the welding process of friction stir welding is reduced to the submersion and emersion – without any longitudinal movement on the surface. The immersion and the stirring motion generate a joint between the overlapping parts (plastic dispersion joint) around the stirring pin. When the tool is retracted, an dent of the tool remains in the joint surface. The process can be executed by simple robotic welding heads and simple stirring tools. Additional clamping is recommended in the case of rounded or warped parts, to prevent accidental damage to the still “soft” joint by the extraction of the tool. Application: “Friction spots joints” FSJ (Kawasaki) in the aluminium doors of a passenger vehicle (Mazda RX8)

Bottom sheet: aluminium 2 mm, upper sheet: aluminium 1 mm, friction tool shoulder: 10 mm Figure 66: Friction spot welding joint with one-piece friction tool

Other special welding processes 

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Friction spot welding with multi-part friction tool (FSpW - Friction spot welding)

Friction spot welding with three-part stir tool enables precise overlap joints without a remaining exit hole. A pressing unit (3) squeezes the joint zone together. Within the pressing unit, a rotating pin (1) is enclosed by a sleeve rotating in the same direction (2), but carrying out a stroke in the different direction. As the rotating pin is immersed into the material, the rotating sleeve withdraws from it and pulls the displaced plastic material upwards (4). Subsequently, the pin withdraws and the then downward directed sleeve forces the material back into the friction bore hole produced by the pin. By this, an overlap joint is created in the plastified zone, and the surface will be flat on the sheet, without an indented hole.

Principle by RIFTEC 1) 2) 3) 4)

Pin (rotating) Sleeve (rotating in the same direction, but with an opposite stroke movement) Stamp Plastified sheet material

Figure 67: Friction spot welding

Experience shows that a modified stroke sequence is more successful, where the sleeve immerses first and the pin draws back. This can create a “point diameter” equal to the diameter of the sleeve (e.g. 9 mm), independent of the sheet thickness. The immersion depth has to be set in a way that it penetrates the top sheet completely, and enters the bottom sheet a few tenth of a millimetre deep. The process can also be executed by robotic welding heads.

Cross-section of a friction spot weld AlMg3Mn0.4, t = 1.5 mm / AlSi12 (Fe), t = 2.5 mm Welding parameters: Rotational speed 2,000 rpm, immersion depth 1.8 mm, friction time 1.0 sec Figure 68: Friction spot welding head and cross-section of a welded joint (SLV München)

With a friction time of app. 1s for a 1.5 mm immersion depth the process seems quite slow compared to alternative or competing methods such as resistance spot welding, TIG and MAG spot welding or

Other special welding processes

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mechanical joining of aluminium parts. Still, it has its advantages and unique characteristic for some special applications, such as:  no special surface preparation required (oxide layers not critical)  low material impact, due to the plastic joining without melting  good weldability properties for aluminium alloys not suitable for fusion welding (air and spacecraft)  enables the welding of sheets with a considerable difference in thickness, of multi-layer sheets and of wrought/cast alloys  high load-bearing capacity of the point diameter, even on thin sheets  smaller flanges are required than in resistance spot welding  any kind of spacing allowed between spots (no parallel connection)  can easily be welded over (suitable for repairing)  no indentation on the back side (suitable as visible surface)  no foreign materials required as in certain cases of mechanical joining (recycling-friendly due to pure materials)  tool requirement is low in comparison and in itself Not all of the above listed comparisons stand for fusion welding processes. Friction spot-welding can also be used for the repair of small bored defects or for closing the exit hole of friction stir welded compounds. For this, a plug of the same material is inserted into the hole, thermoplastically stirred and bonded.

Other special welding processes 8.4

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Questions

(1)

What construction types are used in friction stir welding?

(2)

What is the basic design of a friction stir welding tool?

(3)

Name the main welding parameters of friction stir welding

(4)

Which weld shape should be considered for the butt-welding of an aluminium sheet 20 mm thick, in large quantities?

(5)

What unique feature occurs at the end of the joint and what measures does it require?

(6)

What material groups are preferred in friction stir welding?

(7)

List 3 advantages of friction stir welding

(8)

List 3 disadvantages of friction stir welding

(9)

Name typical application areas of friction stir welding

(10)

Why is it possible to create joints of dissimilar material combinations using friction stir welding which would not be “suited for welding” per definition by fusion welding?

Other special welding processes 8.5

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Bibliography

/16/ DIN EN 14610:2004 Welding and allied processes - Definitions of metal welding processes /17/ DIN EN ISO 4063 (2011-03): Welding and allied processes - Nomenclature of processes and reference numbers. /18/ DIN EN ISO 25239-1(2011): Friction stir welding — Aluminium — Part 1: Terms /19/ DIN EN ISO 25239-2 (2011): Friction stir welding — Aluminium — Part 2: Design of weld joints /20/ DIN EN ISO 25239-3 (2011): Friction stir welding — Aluminium — Part 3: Qualification of welding operators /21/ DIN EN ISO 25239-4 (2011): Friction stir welding — Aluminium — Part 4: Specification and qualification of welding procedures /22/ DIN EN ISO 25239-5 (2011): Friction stir welding — Aluminium —Part 5: Quality and inspection requirements /23/ Boywitt, R.: Grundlagen und Anwendungsbeispiele des Rührreibschweißens. Vortrag zum 18. Erfahrungsaustausch Reibschweißen, SLV München, March 2009. /24/ Storch, W., R. Boywitt: Rührreibschweißen von Kupferleitern. Paper to 18. Substitutional of experience friction welding, SLV Munich, March 2009. /25/ Ellermann, F.: Rührreibschweißen von Aluminiumprofilen bei einem Halbzeughersteller. Paper to 18. Substitutional of experience friction welding, SLV Munich, March 2009.

Other special welding processes 9

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MIAB welding, diffusion welding

9.1

Pressure welding with magnetically impelled arc butt (MIAB) MIAB welding /1/ belongs to the group of the arc pressure welding processes. Other procedure designations are also common, such as “Magnetarc-”. 9.1.1

Process description

1. Initial position The axial connecting of the clamped workpiece faces, welding current, magnetic field and shielding gas are switched on.

2. Welding start Separation of the workpieces up to a defined gap width (lifting movement), with simultaneous arc ignition.

3. Heating Moved by the magnetic force, the arc rotates along the contour, uniform heating and starting and melting of both joint faces.

4. Welding end Bringing together and upsetting the workpieces, Switching off of the welding current, the magnetic field and the shielding gas (delayed).

Figure 69: Schematic illustration of an MIAB weld (acc. to DVS Merkblatt 2934)

Other special welding processes 9.1.2

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Processing materials and geometries

Cast steel

Cast iron with spheroidal graphite

Steel, alloyed

Materials

Steel, unalloyed

The following material combinations are welded in practice under production conditions: (Weldability strongly depends on the component geometry)

Steel, unalloyed Steel, alloyed (ferrit., austen). Cast iron with spheroidal graphite Cast steel Figure 70: MIAB - materials and material combinations suitable for welding

Requirements for the joint geometries: “thin-walled closed hollow geometries”     

hollow cross section (no full cross section), thin-walled (0.7 - 5 mm) closed section (not broken orbit for the arc), electrically conductive and meltable materials, equal wall thickness (heating) as much as possible on both parts.

Figure 71: Suitable connector cross sections for the MIAB welding (acc. to DVS leaflet 2934)

According to the state of the art, pipe cross-sections with 0.7 … 5 mm wall thickness (in special cases up to 10 mm) and between 5 / 300 mm diameter or with appropriate weld length and with non-rotationsymmetrical joint surface contours, can be MIAB to welded. The joint faces should be plane parallel, bare metal (rotated or milled) and deburred.

Other special welding processes 9.1.3

Page 72

Process-specific advantages and disadvantages

Advantages:

Disadvantages:

9.1.4

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        

Precise welding - also thin pipe cross sections. Good weldability for steels with high carbon contents and machining steel. Symmetric heating, favourable residual stress state, low distortion. Low reduction of hardness and strengths with hardened or quenched and tempered steels in the heat affected zone. Low susceptibility to cracking. Relatively clean, almost spatter free process. Good potential for automation. Short welding time and short cycle times for mass production. No filler material necessarily.

   

No weldability for full cross sections. Length shortening through the upsetting process. Uneconomical for materials with increased shielding gas requirements. Low flexibility of the process (usually stationary single-purpose machines).

Areas of application

Sample applications: passenger car rear axles, drives, filter casings, water connection nipple for radiators, bicycle bottom bracket axles, gear rods for car steering elements.

Front wheel drive-shafts of passenger car Figure 72: MIAB-Examples of use

Rear drive-shafts of passenger car

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Brake- und steering parts passenger car

Passenger car rear wheel axle Figure 73: MIAB-Sample applications

9.1.5

Welding machines

For MIAB welding, stationary machines with fully mechanized process management (manual un/loading) or fully automated process management are used. The machines are generally designed as single-purpose machines for mass production. The machines can be executed in vertical or horizontal types (related to the component axis). The horizontal type is preferred due to simpler un-/loading. The most important machine components are a lift-/upsetting apparatus, the power supply(s) for welding- and solenoid coil current, and a coil system for magnetic field generation.

Figure 74: Work space of an MIAB welding machine in horizontal construction / KUKA System GmbH

Other special welding processes 9.1.6

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Magnetic field distribution

F  B  L B = magnetic flux density (radial)

L = Length of the electrical conductor (of the arc)

 = welding current (DC) (axial)

F = Force on the arc (tangential)

Figure 75: Separable outer coil system and theoretical magnetic field distribution in the welding gap

Through the superposition of an externally generated radial magnetic field in the welding gap, an arcblowing effect (magnetic force) affects the arc in tangential direction. As a result, the arc is moved in the welding gap at high speed along the front face contour (orbital frequencies e.g. 200 Hz, depending on the component). The joint faces are heated and melted at the same time. In order to keep the force orientation and the orbital movement of the arc the same, this is operated with direct current.

9.1.7

Welding parameters

Important parameters of MIAB welding are: Welding data

Adjustment range

lift (arc length) Welding current Magnet coil current arc time Upsetting time upsetting force, surface-related Shielding gas - flow rate

1.5 ... 3.0 50 ... 1,500 1 ... 25 0.4 ... 15 0.5 ... 5 15 ... 150 0 ... 15

mm A A s s N/mm² l/min

Figure 76: Setting values of the most important welding parameters (acc. to DVS leaflet 2934)

Other special welding processes

(I) (II) (III) (IV)

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Ignition- and start-up phase Warm-up phase Melting phase Upsetting phase

Additional measures to the increase the arc and process stability: 

Staggered current changes for arc and solenoid coil



Chokes for current rise and current drop

Figure 77: Current flow with MIAB welding, with and without current program (acc. to DVS leaflet 2934)

9.1.8

Structure and properties of the joint

Figure 78: Three-part drive, two-times MIAB welded, weight advantage of hollow shaft up to 40% against solid shaft /KUKA-Systems GmbH/

With press welding with magnetically impelled arc the melted mass of the fusion faces that has been created by the temperature rise, is completely upset outside. In the joining zone no solidified molten metal remains, but the non-melted, upset material of the “heat-affected zone.” So a “forged structure” is formed in the joint zone, similarly as in friction welding, with good technological characteristic values.

Other special welding processes

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 Distinct cross-section enlargement; variable by heat input and upsetting force  Top edge melting of the front faces and possible oxides completely upset outside  Joining plane with narrow strip ferrite in Widmanstätten structure by carbon burn-off  Flash mostly smoothly workable (if inside accessibility is possible

Figure 79: Macro-photo-micrograph of an MIAB joint zone, pipe wall segment, wall thickness 4 mm, SAE1040 with edge hardening

9.1.9

Process variants

Fusion welding with magnetically impelled arc (MIA) respectively. Arc welding with magnetically moved arc butt welding (DIN EN ISO 4063-185). The joint is being produced only by melting – without upsetting.

Figure 52: MIA welding - design as peripheral- and front face weld

This melting welding type did not succeed in being successfully implemented in batch production in comparison to the press welding processes. During Arc stud welding hollow cross-sections (cans, nuts, etc.) are welded on with a magnetically moved arc. The instrument technique for arc stud welding (arc welding gun) will be supplemented with a solenoid coil around the component support. With this, the rotation of the arc at the hollow section and the support part is generated and the circular joint surface is melted on both sides. The joining process is followed by stud arc welding with lower spring or spindle forces with one side design (no backside support and no back accessibility required). The exact process design allows all around density and almost spatter-free joints. Application: e. g. Exhaust threaded sleeves from alloyed steels for the absorption of an exhaust sensor.

Other special welding processes 9.2

Test questions

(1)

Which are the three most important asset components to MIAB welding

(2)

Name the main welding parameters of MIAB welding

(3)

What are the requirements for the preparation of the joint surfaces?

(4)

Which joint geometries can be MIAB welded?

(5)

Why do MIAB joints have a low distortion?

(6)

List 3 advantages of MIAB welding

(7)

List 3 disadvantages of MIAB welding

(8)

Name typical application areas of MIAB welding

(9)

Which welding current type (DC, AC) is used for MIAB welding? Why?

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Other special welding processes 9.3

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Bibliography

/1/ Leaflet DVS 2934: Pressure welding with magnetically moved arc (MIAB welding). (2001-11), DVS-Verlag, Düsseldorf. /2/ Leaflet DVS 2922: Testing welded burn stud and press stud and MBP welded joints (2001-12), DVS Verlag, Düsseldorf. /3/ Grünauer, H.: MBP welding – friction welding, a comparison. DVS reports Bd. 139 (1991), S. 43 - 49. /4/ N.N.: Einfluss der Magnetfeldverteilung auf das Schweißergebnis beim Schweißen mit magnetisch bewegtem Lichtbogen. Abschlußbericht zum AiF-Forschungsvorhaben 8241 der SLV München (1993) /5/ Tölke, P.: Schweißverfahren für das Konstruktionsschweißen von Gusseisen mit Kugelgrafit (GGG) unter Großserienbedingungen. Vortrag zum 15. Erfahrungsaustausch Reibschweißen, SLV München, 8.3.2005 /6/ Weh, W.: Magnetarc-Schweißen - das innovative Fertigungsverfahren für Chassisteile. Vortrag zur Tagung “Fügen rohrförmiger Bauteile”, SLV-München, 5.12.2005

Other special welding processes

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10 Diffusion welding Diffusion welding is a process for joining similar and dissimilar materials in solid - state (below the melting temperature: T < TS) without filler materials. The welded joint takes place by diffusion, i.e. by material transport in the atomic range. Process number and designation: DIN EN ISO 4063 - 45, Diffusion welding 10.1 Process description The joining parts are fixed in a vacuum chamber and are admitted at the bare contact points with sustained low pressure (1 - 30 N/mm²) and temperature (0.5 - 0.8 x melting temperature). Through diffusion over the contact surface(s), the components are firmly bonded in solid state (without melt).

Figure 81: Schematic illustration of a diffusion welding unit with direct resistance heating

10.2 Structure of a vacuum diffusion welding machine 1. Vacuum system The parts that are to be joined are in a vacuum chamber. A high purity of the weld areas contact is achieved through vacuum, because diffusion-obstructing pollution layers on the surface must be excluded.

2. Heating The choice of heating depends on the cross-section to be welded (stove heating, inductive heating and direct resistance heating).

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3. Pressure facility The parts that are to be welded are pressed together with low pressure during the diffusion process. The pressure has to be set to such extent that a good contact of the weld areas is assured, but so less that no plastic deformation occurs in the selected temperature. In some cases you can work even without pressure facility, if the required contact pressure is built up solely by the thermal strain of the joining parts. 10.3 Welding data      

Component dimensions:

within wide ranges arbitrary, however limited by the type of the vacuum chamber Weld contact area preparation: flat and grease-free, polished: Roughness 15 - 30 m, polishing not required Vacuum: 10-4 - 10-1 Pascal (1 N/mm² = 106 Pa = 10 bar = 10.2 kp/cm² = 7.5 x 10³ Torr) Welding temperature: 0.5 - 0.8-fold of the melting temperature (T < Ts), with different materials to be orientated to the material with lower melting temperature Welding pressure: low; 1 - 30 N/mm2 usually sufficient Welding time: classified into 3 phases: Heating, keeping the temperature, cooling - 10 min to 60 min for materials with similar expansion coefficient - longer heating-/cooling times for material combinations with strongly different expansion coefficients, e.g. 60 min heating, 30 min keeping the temp, - 150 min cooling

Welding time and temperature can be varied in relatively large limits. The higher the welding temperature, the shorter welding time. A high welding temperature possibly means a higher deformation and possibly also a de-strengthening of thermal-mechanically strengthened material states (annealing). A compromise between welding time, dimensional accuracy, and strength properties of the welded parts is to be set. Figure 82 shows a diffusion-welded joint of a high temperature lightweight construction material TiAl. Due to low ductility at room temperature and high thermal shock sensitivity, this material is not fusionweldable conventionally. The almost isotherm diffusion welding can be well applied here. Joining temperatures up to 950° correspond to tempering (not annealing) of this material. The base material properties are not impaired, not even in the area of the joining zone.

 Joining zone

a) Overview, welded round specimen b) Joining zone-edge, diffusion welded (with edge offset,  18 mm clamp inaccuracy) Welding parameters: 950°C, 180 min, 5 N/mm², < 10 -4 Torr Figure 82: Diffusion welding (similar) of titanium aluminide high-temperature material

Other special welding processes

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10.4 Weldable materials and geometries Metals and mixed (material) joints can be diffusion welded. Similar to friction welding of mixed joints, the formation of brittle intermetallic phases can be limited by selecting suitable alloys and by setting the process variables to an uncritical level, i.e. without adverse effects on the joint properties. Intermediate layers (“filler material”) can also accelerate the welding process and avoid the possible formation of brittle diffusion layers (intermetallic phases). Intermediate layers of ductile materials are also used for brittle materials or for material combinations with very different coefficients of expansion in order to improve stress equalisation. Plus, in this way the surface contact of the two joining parts is already improved with low pressing forces.

Figure 83: Successful diffusion-welded material combinations /3/

If during diffusion welding of metals being connected an eutectic occurs below the welding temperature, then it is to be labelled as diffusion soldering. Figure 84 shows typical joint geometries for diffusion welding. Large-scale or multi-surface joint geometries, also in multiply stacked arrangement, can be joined at the same time, even several joining parts in one working cycle (pallet production). This relativizes long welding times. A direct accessibility of the joint zone is not required. Depending on the joining geometry heat and force can indirectly affect the joint zones via heat transfer and force transmission.

Other special welding processes

a) Cooling duct

b) Hollow body structure

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c) Joint of metal with glass or ceramic

d) Lightweight construction honeycomb structure e) Pipe transition pieces of different materials Figure 84: Examples of joining geometries for diffusion welding

10.5 Process-specific advantages and disadvantages Advantages:  Potential for automation of the process.  High quality of the welded joint: no changes to the physical-mechanical characteristics (joining without melt)  Weldability for many materials and the mixed joint materials  welding several joints (large areas) at the same time, in a process cycle  slow heating and cooling (almost isothermal), thus reduced residual stresses, low risk of cracking, low distortion  plastic deformation low (negligibly): Processing after welding does not apply. Disadvantages:  long welding times,  complex surface preparation,  great machine expenditure,  De-strengthening of materials with heat or mechanically strengthened state is possible (annealing treatment)

Other special welding processes

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10.6 Applications The process is used in special cases, if other joint processes cannot be used, or only conditionally, e.g. according the material properties and/or the joining geometry (e.g. accessibility, cavities, etc.). Diffusion welding is especially suitable for the production of wide joints, or several joints simultaneously in one process cycle. It is used e.g. in cost-intensive industry areas, such as in aerospace, in reactor construction and in the electric industry. Example: - Welding of compact connectors at ends of upper Cu-flat band-conductor stacks - Production of cooling units with special inner channel route through stacked metal plate packages - Generative production processes through bundle-wise welding of layers-wise added structures; e.g. for the design of special cavity contours for moulds - Quasi-isothermal welding of thermal-shock-sensitive materials or mixed joints, e.g. intermetallic high temperature lightweight materials, such as titanium aluminide, copper/glass, copper/ceramic bonds.

Other special welding processes

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10.7 Questions (1)

Name the three most important asset components of diffusion welding

(2)

Name the main welding parameters of diffusion welding

(3)

What are the requirements for the preparation of the components to be welded?

(4)

What are the requirements for the post-processing of the joint?

(5)

Which welding temperature arises (related to the melting temperature)?

(6)

How can the welding temperature affect the base materials?

(7)

Which possibilities are available for the diffusion welding of mixed joints for avoiding intermetallic phases, for accelerated diffusion and for reduced residual stresses?

(8)

Name 3 important advantages and particular aspects of diffusion welding

(9)

Name the most important disadvantages of diffusion welding

Other special welding processes

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10.8 Bibliography /1/ Owczarski, U. A. et al.: Application of Diffusion Welding in the USA, Welding Journal, February 1981 /2/ Lison, R.: Diffusion welding and nuclear application example , Schweißen und Schneiden, Jg. 23 (1971) /3/ Ruge, J.: Handbook of welding technology, Vol. II. Springer Verlag, 1980. /4/ Greitmann, M. J., Wiesner, P.: Applications of Special Welding Processes - Part 2, Diffusion Welding. der praktiker, Düsseldorf 54 (2002) 9, S. 314 - 315. /5/ Wilden, J., Bergmann, J.P.: Joining of dissimilar materials of Titan- and Aluminium at low temperatures by diffusion welding. DVS reports 2004 (231), P. 312-316. /6/ Wiesner, P.: Pressure welding process. Vortragsmanusskript zum Lehrgang Fügen von Kupferwerkstoffen, Deutsches Kupfer Institut / GSI mbH NL SLV Duisburg, März 2012. /7/ NF L06-391: Welding and brazing processes in Aerospace. Joining of metallic materials by diffusion welding. Weld quality, Norm, Ausgabe: 1994-03-01, French /8/ DVS reports strip 243: Hard – and high temperature soldering and diffusion welding, ISBN: 978-3-87155-799-6, DVS-Verlag, Düsseldorf (2007)

Other special welding processes

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11 Cold pressure welding 11.1 Summary description of the procedure 11.1.1 Description of the general principle Cold pressure welding is joining of similar or dissimilar materials with high pressure without heat supply (at room temperature). Through high pressure over the yield point, plastic cold forming happens. 11.1.2 Weldable materials Plastic cold deformations are only possible with comparatively soft materials such as aluminium and copper which are the most typical materials in this process. Furthermore tin, lead and silver are also welded, among others. Hard materials can be joined together with the help of intermediate layers made of soft material, e.g. St-Al-St, St-Cu-St. 11.1.3 Application of the process For the joining of parts in electrical engineering, like production of bimetal switches, welding of conductors and in capacitor production. It is also used for joining parts in the field of precision engineering and in the packaging industry. 11.2 Detailed description 11.2.1 Welding units -

Hand pincers, for welding of smaller cross-sections, e.g. Al: 3 to 40 mm², Cu: 3 to 20 mm². Welding machines, for welding of larger cross-sections, e.g. Al: 20 to 400 mm², Cu: 20 to 150 mm². One or several repeated upsetting is common.

11.2.2 Process principle Cold pressure welding is used for butt welding or for spot-welding, as shown in Figure 85 and 86. The significant material deformation leads either to a displacement with considerable surface magnification as in butt welding, or to material compression as in spot-welding.

Figure 85: Cold pressure butt welding (acc. to Ruge)

Figure 86: Cold pressure spot-welding (acc. to Ruge)

Other special welding processes

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11.2.3 Operating conditions -

Condition: mechanical processing of the joint faces and thorough cleaning before the welding process. Destruction of the oxide layers in the welding area by surface magnification and flow processes. in butt welding, the high deformation grade manifests itself in surface magnification, and in the case of spot welding by cross-section reduction. Required surface magnification: Al - 160 %, Cu - 180 Ag - 60%. Join mechanism: Atomic attractive forces (adhesion) and location changes of mobile atoms close the surface (diffusion).

11.2.4 Process variants Cold pressure welding can be used in connection with forming processes (pulling, rolling, extrusion) as well.

Figure 87: Cold pressure welding in the drawing process (acc. to Ruge)

Figure 88: Cold pressure roll cladding

Other special welding processes

Forward - hollow extrusion

Forward - full extrusion

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Backwards - cup shaped extrusion

Figure 89: Process variants of extrusion (acc. to Ruge)

11.3 Bibliography /1/ Ruge, J.: Handbook of welding technology, Vol. II. Springer Verlag, 1980. /2/ Eichhorn, F.: Production processes in welding technology, Vol. 1, welding and cutting technology, VDI Verlag, Düsseldorf, 1983.

11.4 Questions (1)

Describe the process principle of cold pressure welding

(2)

Which properties must the materials have to be eligible for cold pressure welding?

(3)

In connection with which production processes can cold pressure welding be used?

(4)

What size of weld cross-section can be joined in cold pressure welding, using stationary machines, depending on the material?

(5)

How large must the required surface magnification be in cold pressure butt welding, and which joint mechanism is achieved with it?

Other special welding processes

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12 Explosion welding Explosion welding is also called impact or shock welding. 12.1 Process principle In explosion welding the effect of a blast wave, which emerges during the detonation of explosives (e.g. Nitropenta, Hexogen, Nitroglycerin) is utilized for the welding of overlapped workpieces. The parts to be joined are arranged parallel to each other or in case of smaller dimensions on top of each other and under a certain angle, with the help of spacers.

Plate to be cladded (base component)

Figure 90: The principle of explosive welding (cladding)

12.2 Materials and applications The process is relevant especially for metal combinations, that are not soluble among themselves, and their differences in melting temperature and deformation strength are too large which would cause brittle, intermetallic compounds. The advantage of this process is the cladding of special materials such as titanium, tantalum and molybdenum on steels, like combinations of e.g. aluminium with austenitic steel, copper with aluminium and aluminium with Inconel. The process lends itself particularly to wide surface claddings, and to coating of a carrier material (substrate), for example with a corrosion-resistant layer. Practical examples of application is the wide cladding of sheet metal of up to 40 m² width, the interior cladding of vessel barrels and vessel bottoms, and the production of pipe fittings in apparatus engineering. 12.3 Operating conditions The surfaces to be joined must be metallic clean. Type and quantity of the used explosive depend on the thickness of the overlay and the characteristics of the metals to be connected. The explosive material is brought to explosion from a line or a point. Under the progressing detonation front, the overlaid sheet is accelerated in downwards direction and is bend.

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Between the two joined parts, a liquid metal beam is formed due to the local pressure load on the material, well beyond the yield point, which is then blown out of the gap due to the pressure. In the socalled point of collision, the metals fuse and are joined together. The connection plane of both workpieces is formed to a wave-like shape.

Section: AlMg3 (t = 12 mm) on S235 (t = 40 mm)

Multi Layer explosion cladding

Figure 91: Examples of explosion-cladded join zones

12.4 Variants

Figure 64: Principle of inside pipe welding (heat exchanger)

12.5 Bibliography /1/ Richter, U.: Explosive cladding - a sensible special process for broad surface joining of metals, DVS reports Vol. 25 (1972) /2/ N.N.: The compound from vacuum, special issue GVM, Bocholt, 1990 /3/ Boes, P.J., et al.: For the explosive welding of metals, strips, /4/ Sheet metals, pipes, Düsseldorf 6 (1965)

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12.6 Test questions (1)

Describe the process of explosive welding.

(2)

Which attributes does the joining zone of an explosion-welded cladding have?

(3)

For which areas of application is explosive welding suitable?

(4)

In which process step of explosive welding is the joint generated?

(5)

Which basic operating conditions must be satisfied in explosive welding?

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13 Magnetic impulse welding and crimping The force action of a magnetic impulse causes a fast material transformation in the appropriate components. These transformations can be used for welding, crimping, shaping, and punching/perforating. In this technique, the transforming force is transferred to the component without contact. Due to missing standardisation different process designations are in use, e.g.: - MPW (Magnetic Pulse Welding) - MPT (Magnet Pulse Transformation) - EMPT, EMPW, EMPT (Electromagnetic Pulse Technology, -Welding, -Types) The solenoid impulse process is a process for mass production. It is characterized by single-purpose machines of low flexibility, singular properties regarding the joint geometry and the possible material combinations, as well as high economy due to short process cycles. 13.1 Applications At the moment, the first applications are taking place in the areas of automotive, electrical engineering and vessel construction.

Figure 93: Automotive application, solenoid-pulse-welded Al/Al AC accumulator /PULSAR/

Figure 94: Solenoid-pulse-welded cable connections /PULSAR/

Figure 95: Al/St sp-welded and St/St sp-crimped,  50 mm/ PSTproducts/

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13.2 Welding units Machines for shaping and welding with solenoid pulse technology mainly consist of a capacitor battery, which are currently driven with a charging voltage of 10 or 25 kV with powers between 5 and 100 kJ, and a solenoid coil system for generating the magnetic force effect on the component. The most common installation type is equipped with a compression coil, i.e. an external solenoid compresses an internal component. The only machine parameter is the capacitor charging voltage (e.g. 3 … 9 kV). If necessary the pulse shape can be modified with the help of an additional inverter: lower pulse frequency = more pulse width. All remaining working conditions are determined by the adaptation of the components and of the coil system: -

Component/compound geometry (diameter, wall thickness, overlapping, bevels, gap) Materials/alloys (deformation capability, el. conductivity: Surface preparation Arrangement within the solenoid coil system Design of the field-shaper within the coil for the concentration of the magnetic field on the component

Expansion coils placed inside a component part, and flat coils for the creation of axial forces are also available as further system designs. The coil system must absorb the magnetic counter acting force, and is therefore exposed to high workloads during operation. In serial operation, the replacement of the coil system has to be scheduled after every 50,000 welding cycles. In contract production, an amount of 4 20 ct “Cost Coil by weld” will be charged per weld cycle. Compared and in contrast to other welding processes, no prorated costs accrue here for filler materials, gases, etc. For complex component geometries divisible field formers can be used, or in special cases divisible coil systems as well. Special coil- and field shapers also enable the manufacturing of several joints simultaneously with one unloading pulse (e.g. Alu-caps crimped contactless and germ-free, for infusion cylinders). 13.3 Process principle of the Magnetic impulse welding In magnetic pulse welding, the effect of a pressure wave is used to weld tubular, overlapped workpieces. The pressure wave is created by a pulse-like electromagnetic induction and force action. The join mechanisms are comparable to those in explosive welding.

/PULSAR/

Figure 96: Principle of the fast electromagnetic deformation and typical joining geometries

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The following basic principle is used: A high capacitor (E = 5 - 100 kJ) is discharged very quickly (pulse duration approx. 40 s) by a solenoid coil. Thereby a strong magnetic field is created in the coil. Inside the coil there is a component part (e.g. pipe). The induced current in the pipe creates the magnetic field of the opposite direction. By the emerging magnetic force, the coil and component part are pushed away from each other. The discharging current pulse is converted into an electromagnetic force impulse. Under the precondition of a stable, deformation-free coil, the electromagnetic force causes a deformation on the component inside of the coil, in the form of a radial compression. If two joining parts are placed within the solenoid coil, e.g. two overlapping pipe endings, then the external pipe will pressed up suddenly against the inside lying pipe, and will be joined to it. For the transformed energy applies: E (kJ) = ½ Cu ² = ½ LI² 13.4 Working conditions of materials The very short pulse time (e.g. pulse width 50 s) limits the induction depth comparable to a high frequency induction. Therefore only the outside component will be radially compressed by the magnetic force, but not the internal part. Still it may be necessary to prevent a mechanical deformation of the internal part by the outside part, eventually by a sufficiently large wall thickness or a supporting device. In stable, circular coils, which operate with single or multiple loops, normally thin, tubular overlap joints are produced by non-contact compression. The material of the outside part which needs to be deformed, must be cold-ductile and a good electrical conductor (Cu, aluminium alloys are preferred) In connection with a radial air gap (acceleration path), high enough deformation and collision speeds can be achieved to produce the welded joint. The material of the internal, deformation-free joining part can be similar or dissimilar. Therefore this cold process lends itself especially to the production of mixed joints (e.g. Cu -X, Al -Y) as well. The materials remain without thermal effects, i.e without thermally caused destrengthening or hardening. With steel materials in the exterior pipe, both ductility and conductivity are decreased (induction force effect). The lower transformation rate, despite the higher pulse energy, is then too low for welding, but can be used for crimping. For that, suitable ring- or longitudinal slots are added to the inner part which has now a gap-free overlapping. Density can be achieved in crimping with additional soldering agents or bondings, if necessary. 13.5 Bibliography [1]

Zech F., Cramer H. and Appel L.: “Metallografic Investigation of MPW interfaces”, First technical conference on industrialised magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008

[2]

Kallee S.W.: “Magnetic Pulse Welding as an Enabler of Light-Weighting in the Automotive Industry”, First technical conference on industrialised magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008

[3]

Shribman, V.: “Magnetic Pulse Joining of Light Metal Castings”, First technical conference on industrialised magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008

[4]

Mussi, P.: “Magnetic Pulse Welding on Receiver Drier for heat Ventilation Air Conditioning”, First technical conference on industrialised magnetic Pulse welding and Forming, SLV Munich, July 3rd 2008

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13.6 Questions (1)

Describe the process principle of Magnetic impulse welding

(2)

In which applications can solenoid pulse technique be used?

(3)

With which instrument can Magnetic impulse welding be carried out?

(4)

With which special welding processes is Magnetic impulse welding comparable, concerning the formation of the joining zone?

(5)

Which properties must materials have in Magnetic impulse welding?

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Chapter 1.13:

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Cutting, Drilling and other edge preparation processes

Contents 1 2 3

4 5

6

Introduction ..................................................................................................................................... 3 Thermal cutting ............................................................................................................................... 3 Flame cutting................................................................................................................................... 4 3.1 Process gases............................................................................................................................ 8 3.2 Flame cutting equipment .......................................................................................................... 10 3.3 Practical application of flame cutting ........................................................................................ 14 3.4 Flame-Cutting Machines .......................................................................................................... 17 3.5 Special and Auxiliary Equipment .............................................................................................. 18 3.6 Special techniques of flame cutting .......................................................................................... 19 3.6.1 Flame Gouging ............................................................................................................ 19 3.6.2 Metal-powder oxygen cutting ....................................................................................... 20 3.6.3 Oxygen lancing ............................................................................................................ 20 3.7 Workplace safety ...................................................................................................................... 20 Water jet cutting ............................................................................................................................ 21 4.1 Principle of processing ............................................................................................................. 21 Plasma cutting .............................................................................................................................. 23 5.1 Classification and application area ........................................................................................... 23 5.2 Direct plasma cutting ................................................................................................................ 24 5.2.1 Process principle .......................................................................................................... 24 5.2.2 Process variants .......................................................................................................... 24 5.2.3 Examples of Application ............................................................................................... 28 5.3 Indirect plasma cutting ............................................................................................................. 29 5.3.1 Process variant “nozzle as anode” ............................................................................... 29 5.3.2 Process variant “auxiliary anode” ................................................................................. 30 5.3.3 Examples of Application ............................................................................................... 30 5.4 Other plasma processes .......................................................................................................... 31 5.4.1 Marking, notching and punching................................................................................... 31 5.4.2 Gouging ....................................................................................................................... 32 5.5 Gases for plasma cutting .......................................................................................................... 33 5.6 System structure ...................................................................................................................... 34 5.6.1 Guiding systems for plasma cutting .............................................................................. 34 5.6.2 Fume extractor ............................................................................................................. 36 5.6.3 Plasma power supplies ................................................................................................ 36 5.6.4 Plasma cutting torch..................................................................................................... 37 5.6.5 Longitudinal (distance) control...................................................................................... 38 5.7 Cutting speed in relation to cut quality ...................................................................................... 39 5.8 Safety ....................................................................................................................................... 40 Additional special processes - Thermal material removal by gas discharge ........................... 41

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7

8

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6.1 Oxygen arc cutting ................................................................................................................... 41 6.2 Air arc gouging ......................................................................................................................... 41 Laser beam cutting ....................................................................................................................... 42 7.1 Operations of laser flame cutting .............................................................................................. 42 7.2 Process variants....................................................................................................................... 42 7.2.1 Laser-flame cutting....................................................................................................... 43 7.2.2 Laser-fusion cutting ...................................................................................................... 43 7.2.3 Laser-sublimation cutting ............................................................................................. 43 7.3 Cutting gases ........................................................................................................................... 43 7.4 Materials .................................................................................................................................. 44 7.5 Cutting speed ........................................................................................................................... 45 7.6 Guiding systems for laser cutting.............................................................................................. 46 Test questions............................................................................................................................... 47 8.1 Bibliography ............................................................................................................................. 50

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Introduction

Before semi-finished products and products like sheet metals, profiles etc. can be processed and welded in manufacturing processes of the metal processing industry, they often must be cut accurately. For this purpose, in addition to the mechanical cutting processes, thermal cutting processes like flame cutting, plasma cutting and laser cutting are used. These three processes are widely used frequently together. All thermal cutting processes are similar in terms of point-like energy input and a high-energy cutting gas jet. The goal of thermal cutting is to produce components in such a way that they can be further processed with the lowest possible amount of rework. In addition to this thermal cutting also the “cold” waterjet cutting processing did find its specific application during the years. Because of the continuous further development of the thermal cutting processes, oxy-fuel gas cutting, plasma and laser cutting are economically competing processes today for non-alloy and low alloyed steels in the medium wall thickness range.

2

Thermal cutting

Classification of the thermal cutting processes Thermal cutting processes can be classified according to different aspects. According to the common physical processes, a distinction can be drawn between flame cutting, fusion cutting and sublimation cutting. The material is treated as follows:   

When flame cutting, it is mainly burned, and the combustion products are expelled by an oxygen jet of high kinetic density. When fusion cutting, it is mainly melted and blown out by a high-speed gas jet. When sublimation cutting, it is mainly evaporated and blown out by expansion and/or by a gas jet.

Figure 1 shows a classification of thermal cutting processes dependent on the type of energy carriers affecting the workpiece externally. Thermal Removal

by Gas

by Gas Discharge

by Beam

Oxy-acetylene flame (Oxyfuel) cutting

Plasma

Laser

Flame gouging Flame Drilling Flame Spraying

Arc with O2 Arc with air

Electron Beam Ion Beam

Figure 1: Classification of the thermal cutting processes according to DIN 2310 Part 6

According to the degree of mechanisation, we can distinguish between hand cutting (manual cutting), partly mechanical, fully-mechanised and automated cutting. DIN EN ISO 9013 describes the thermal cutting processes, in particular the evaluation possibilities of cut surface qualities. © 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

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Flame cutting

Flame cutting is a thermal cutting process of which the biggest energy part, necessary for the continuous processing, is extracted from the heat release due to the combustion of the material. The upper surface of the material to be cut is locally heated by the oxy-fuel gas flame to ignition temperature and then burned by the oxygen jet stream. The heat resulting from the combustion of the material allows a continuous combustion into the depth and feed direction. Flame cutting has the largest application in terms of workpiece thickness. Standard torches are generally suitable for the range of 3 - 300 mm, special torches up to 1,000 mm and more. Figure 2 shows flame cutting during processing.

Figure 2: flame cutting processing

Process principle of flame cutting 

Heating of the workpiece to be cut by the heating flame to ignition temperature in the area of the effective zone of the cutting oxygen jet. The ignition temperature depends on the carbon content of the steel at 1,150 °C to 1,200 °C.



Supply of oxygen and thus introduction of the combustion of the material inside of the kerf, whereby the exothermic reaction during the combustion of the material with oxygen allows considerable amounts of heat to be released.



Exhausting the burned material (the slag) from the kerf by means of the cutting oxygen jet.



The kerf is generated by the uniform movement above the workpiece.

Figure 3: Schematic representation of flame cutting processing

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Hints 

The condition of the workpiece surface (e.g. rusted, scaled, painted, etc.) influences the cutting quality and the feed rate.



The flame cutting process is interrupted, if the continuous heat transfer in the workpiece is interrupted. Consequence: Sheet metals with laminations or several sheets upon each other are not suitable for cutting without special measures.



Cutting with several torches at the same time on a machine increases the cutting capacity according to the number of torches. The special advantage of flame cutting is that several torches can be operated at the same time with a relatively low expenditure level. For example, it is possible to execute the complex double-bevel butt weld preparation process in a single operation by using threetorch units.

Preconditions for flame cutting (flame cutting conditions) To enable the start of the heat generating (exothermic) process, the material to be cut must meet the following requirements: 1. The material heated to ignition temperature must burn in pure oxygen. This requirement is met by all metals with a sufficiently high affinity to oxygen; it is met particularly well by pure iron. 2. The slag generated during the combustion must be fluid, so that it can be blown out from the kerf by the oxygen-cutting-stream. In particular chrome and silicone form viscous slag. 3. Its ignition temperature must be lower than its melting temperature. The ignition temperature of structural steels is approximately 1,200 °C, the melting point is just around 1,500 °C. Such materials can burn before they become liquid. With increasing carbon content, the burning temperature increases as well, and the melting temperature decreases. For steels of about 1.6 % carbon content, this requirement is no longer met - the material melts before it is burned. Therefore, e.g. tool steels and cast iron are not suitable for flame cutting. 4. The melting temperature of the oxides must be lower than the melting temperature of the material. Some metals and alloying elements form highly melting oxides. A typical example is aluminium. Its melting point is at 660 °C, the melting point of its oxide at approximately 2,050 °C. The oxygen jet cannot even reach the metal as it is covered by a solid oxide layer. Aluminium materials are therefore not suitable for flame cutting. It is similar with chrome, which also forms highly melting oxides. As nickel only has a low affinity to oxygen, it contributes not much to the combustion heat. This is the reason, why stainless CrNi steels are not suitable for flame cutting. Also some other alloying elements of steel such as silicon, manganese, tungsten, molybdenum and copper make - in high amounts - flame cutting more difficult. 5. The emerging oxides need to be thin fluid. If, during combustion, a slag is built that is very viscous and thus cannot be expelled from the kerf easily, flame cutting can naturally be impeded. This characteristic is also influenced by chrome and silicon. 6. The heat conductivity of the material may not be too high. Namely, if more heat is dissipated as it is added during combustion, the cutting process dies down - especially in deeper layers of the material where the heating flame does not reach. This condition applies for, for example, copper and aluminium base materials. The above mentioned conditions for flame cutting are met by non-alloyed and low alloyed steels.

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Flame-cutting suitability of materials For evaluation of the cutting ability of steel, its carbon content is primarily decisive, figure 4.

Figure 4: Detail from the iron-carbon diagram

Apart from the carbon content, the suitability of steel for flame cutting also depends on the number and amount of the alloying elements of the material to be cut. The more a steel is alloyed, the less it is suitable for flame cutting. The impact of some alloying elements can be seen from Table 1. Table 1: Influence of alloying elements on the suitability of steel for flame cutting Steel

Upper limit

Remarks

C alloyed

Up to 1.6 % C

If % C > 0.45, when no heat treatment is applied, hardening and cracking will occur

Mn-alloyed Si-alloyed Cr-alloyed W-alloyed Ni-alloyed Mo-alloyed

Up to 13 % Mn Up to 2.9 % Si Up to 1.5 % Cr Up to 10.0 % W Up to 7.0 % Ni Up to 0.8 % Mo

If % C  1.3 If % C  0.2 to 4 % Si, suitable for cutting If % C  0.2 If 0.5 % Cr, 0.2 % Ni, 0.8 % C If 0.3  % C  0.5 to 34% Ni, suitable for cutting In case of higher W-, Cr- and C content, not suitable

For roughly evaluating the suitability for flame cutting, for determining the required preheat temperature and for estimating the hardening, the carbon equivalent CEq for steels is used. CÄq= C + Mn/6 + P/2 + Cr/5 +Cu/13 + Mo/4 + Ni/15 Up to a carbon equivalent of about 0.4, the steels are suitable for flame cutting without requiring any special pre-treatment. If the value is higher, steel must be pre-heated and the cutting surfaces possibly be machined. From a carbon equivalent of 1.0, flame cutting can no longer be applied. The carbon equivalents of different steels and therefore their cutting suitability are shown in table 2.

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Table 2: Flame-cutting suitability of different steels

S235 B500 P265GH 16Mo3 GC25E S355 E360 P235G1 17MnMoV6-4 17MnCrMo3-3 22NiMoCr3-7 20MnMoNi5-5 20MnCrSiMoZr4-3 13CrMo4-5 10CrMo9-10 X2Ni9 X45NiCrMo4

0.20 0.23 0.29 0.37 0.35 0.41 0.57 0.61 0.59 0.63 0.62 0.63 0.60 0.51 0.88 0.79 1.06

X2NiCoMo18-9-5

2.20

X20Cr13

2.80

X8CrNiNb19-9

4.30

X10CrNiTi18-9

4.58

X10CrNiMoTi18-10

5.20

X15CrNiSi10-12

5.40

X3CrNiMoNb25-7

5.51

suitable for flame cutting

Carbon equivalent

not suitable for flame cutting

Steel Type

The height of the preheat temperature depends, along with the chemical composition, also on the plate thickness and the required cut quality. Table 3: Standard values for preheat temperatures in flame cutting Preheat Temperatures Carbon equivalent [%]

up to approx. 50 mm plate thickness / for separation cuts [°C]

from approx. 50 mm plate thickness / for shaping cuts [°C]

Up to 0.3

-

-

0.3 – 0.4

-

max. 100

0.4 – 0.5

max. 100

100 – 200

0.5 – 0.6

100 – 200

200 – 350

Higher 0.6

200 – 350

350 – 500

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Process gases

For starting and maintaining the flame cutting process, a high-power heating flame is required. The fuel gases which are used for flame cutting differ in flame efficiency, flame temperature, ignition and combustion rate. See figure 5. In flame cutting, hydrocarbon compounds are used as fuel gases. They are burned in two steps. Inside the primary flame, an incomplete combustion occurs due to the added heating oxygen. Through oxygen absorption from the air, the fuel gas is burned completely in the secondary flame. When flame cutting, especially the heating impact of the primary flame is of importance.

Figure 5: Flame temperature, ignition speed and primary flame efficiency of different fuel gases depending on the mixing ratio of fuel gas/oxygen

Acetylene    

The highest flame temperature and flame intensity Higher cutting output (in particular for bevel cuts) compared to other fuel gases At a constant consumption of more than 700 l/h several cylinders are to be inter-coupled to a cylinder battery; the gas discharge is therefore slightly more complex than e.g. with propane and ethylene (fuel gas C2H4, cryogenically liquefied) Tends to self-decompose at higher pressures and high temperatures

Propane   

Lower flame intensity, reduced cutting speed in particular for bevel cuts. The gas is stored in liquefied state, therefore high amounts are storable Oxygen consumption for the flame (not for the flame cutting process) is, compared to acetylene, approx. four times as high.

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Natural gas (mainly methane) 

Trouble-free supply, when it is supplied from an already existing pipe system.

Oxygen (cutting gas)   

Oxygen is required for burning the fuel gas. It starts the flame cutting process, burns the material and creates the kerf by blowing out the slag. Oxygen with normal commercial purity of 99.5% (2.5) is suitable for flame cutting.

Summary An exact description of which fuel gas is the most economic for the different flame cutting tasks is difficult, because apart from pure costs for the gas, in particular the workpiece thickness, the type of the parts to be cut - whether straight, bevel cut or contour cut - have to be considered, too. Exact indications regarding this provide only suitable cost calculations. 

Acetylene has advantages for thin plates, since the material is brought fast to ignition temperature. The attainable cutting speed is high.



For thick sheets, the flame cutting process takes longer than for thinner sheets. The heat input by a propane-oxygen flame is quite sufficient in order to get the material to ignition temperature within the available time (with lower gas costs).



Basically each fuel gas is suitable for bringing the material to ignition temperature.



The manual cutting torch is often an integrated component of welding and cutting equipment, which is common for the universally usable acetylene.

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Flame cutting equipment

Cutting torches For manual cutting, besides combined welding and cutting devices, special handheld cutting torches are used. To a hand-held cutting torch, the oxygen is fed mostly via a hose and is only separated into heating and cutting oxygen in the torch (so-called two-hose torch).

Figure 6: Handheld cutting torches

To an automatic flame cutter, the fuel gas as well as the heating and cutting oxygen are generally fed via three tubes (so-called three-tube torch). Thus the heating flame is not affected when adding the cutting oxygen.

Figure 7: Machine-cutting torch

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Mixed systems According to the mixing system of oxygen and fuel gas, we distinguish between torches with suction effect and torches without suction effect. Cutting torch with suction effect (injector or injector pipe) Example for designation: i S = 2.5 – 3.5 bar

= Injector torch, A = Oxygen pressure

= Gas type (Acetylene)

Injector type torches are torches where the fuel gas is sucked via an injector nozzle into the torch by the heating oxygen. Oxygen flows with high velocity from the pressure nozzle to the mixer nozzle. In the gap between the pressure nozzle and the mixer nozzle, a vacuum is generated which sucks the fuel gas inside. The fuel gas /heating oxygen mixture of the cutting nozzle is fed through the mixing tube. Cutting oxygen is supplied separately by the cutting oxygen tube of the cutting nozzle in all torches. An excessive heating of the cutting torch - e.g. by piercing holes or cutting thick workpieces - can ignite a backfire. The heating flame extincts, the oxy-acetylene mixture ignites in the mixing tube. The torch “whistles”. As a countermeasure, both valves on the torch shall be closed immediately and the torch be cooled since otherwise it is destroyed within a few seconds. Cutting torch without suction effect (pressure torches) Example for designation: II = APMY =

Pressure torches Gas type; this torch can, if the tip provided is suitable for the given fuel gas, be operated with acetylene, propane, methane and gas mixtures.

With this torch type, fuel gas and heating oxygen flow separately into the cutting nozzle, where the two gases are mixed. Pressure torches do not have an injector. The mixing point is in the cutting nozzle, therefore they are also called gas-mixing nozzles. Such torches are preferably used where the cut must be performed in a hot area, e.g. when cutting with a multi-torch aggregate, in case of frequent hole piercing and thick plates. Pressure torches are less sensitive to backfires than injector type torches but the risk of flashbacks is higher. Because of the missing suction effect, it happens more easily that oxygen reaches the fuel gas pipe and forms an explosive mixture there. Flame-cutting nozzles The tool for flame cutting is the cutting nozzle. Specially shaped cutting channels allow the cuttingoxygen jet to leave the nozzle with 1.5 times the speed of sound without diverging fast after the outlet. Development in the sector of flame-cutting nozzles is moving towards high speed nozzles which enables an increase of cutting speed with a better cutting surface quality at the same time. In practice, the following construction types of nozzle are used:

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Cutting, Drilling and other edge preparation processes

SFI / IWE 1.13 Page 12

Annular nozzle (two-piece) It has a ring-shaped outlet of the flame gases and thus a closed flame. So the heat is fed into the workpiece evenly around the cutting jet. This provides advantages for contour cutting. A non-centred – e.g. damaged - cutting nozzle significantly reduces the cutting performance and increases the sensitivity to flashbacks. Slot nozzle (two-piece) Slot nozzles, also consisting of two pieces, are characterized by a higher cutting performance and a good cleanability compared to the annular nozzle. The gases are mixed very well. Due to the tapered fit of the heating and the cutting nozzle, a precise arrangement of the heating flame to the cutting oxygen is achieved. Slot nozzles are often used for machine-cutting torches. Block nozzle (one-piece) Block nozzles only consist of one piece which allows a precise arrangement to the cutting-oxygen jet. The disadvantage is that, in case of damage, the entire nozzle needs to be replaced. Figure 8 shows different types of nozzles.

Heating Gas

Cutting Gas

Annular flame cutting nozzle

Slot-shaped nozzle

Block-type nozzle

Figure 8: Typical forms of flame cutting nozzle

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Welding processes and equipment

Cutting, Drilling and other edge preparation processes

SFI / IWE 1.13 Page 13

Gas-mixing nozzles for pressure torches Gas-mixing nozzles are available in one piece (for acetylene) and in multiple pieces (for slow-burning gases, e.g. propane). Gas-mixing nozzles are characterized by the fact that the gases are fed to the nozzle separately. The mixing takes place in the nozzle. Consequently, cutting nozzles for different types of fuel gas can be operated with one cutting torch. Figure 9 shows gas-mixing nozzles for acetylene and slow-burning gases

Figure 9: Gas-mixing nozzles for acetylene (left) and slow-burning gases, e.g. propane and natural gas (right)

The front sides of the cutting nozzle and the heating nozzle are flush with each other when acetylene is applied as fuel gas. When using slowly burning gases such as propane and methane, for stabilizing the flame, the front side of the cutting nozzle is usually set back towards the inside.

© 2015 GSI - Gesellschaft für Schweißtechnik International mbH Reprinting and unauthorised disclosure are prohibited and will be prosecuted in accordance with the law

Welding processes and equipment

Cutting, Drilling and other edge preparation processes 3.3

SFI / IWE 1.13 Page 14

Practical application of flame cutting

Putting the cutting torch in operation After choosing an appropriate cutting nozzle for the respective plate thickness, the parameters for the gas pressures are to be taken from the cutting table and adjusted accordingly with the torch valves being opened. When adjusting the pressures, losses up to the cutting torch must be considered. Adjustment of the heating flame     

Open the heating-oxygen valve completely, the fuel-gas valve only slightly. Ignite the mixture. First adjust fuel-gas surplus via the fuel-gas valve. Choke the fuel-gas valve until the flame cone is to be seen clearly. Open the cutting-oxygen valve and check if the cutting-oxygen jet leaves the nozzle straight and concentrically so that it does not expand or oscillates.

Due to the dimensioning of the cutting nozzles, the heating flame can be adjusted to the practical requirements. In case of rusty, scaled or primed surfaces or for bevel cutting, stronger heating flames are to be set. Starting the cutting process There are two possible ways of starting the flame cutting: start of cut at the workpiece edge and the start of cut on the workpiece surface. When cutting starts at the workpiece edge, this is brought to ignition temperature by the heating flame, the oxygen is switched on and the feeding of cutting torch takes off. The cutting torch is guided at constant distance and with a uniform feed rate over the workpiece. For cutting inner counters, the cut must be started from the workpiece surface which is called hole piercing. There, the workpiece is first preheated to ignition temperature in the area affected by the cutting-oxygen jet. Then the cutting-oxygen valve is opened slowly and the torch head shall be lifted slightly. At a low feed rate, the torch shall be re-adjusted to the correct distance. By using low cutting-oxygen pressure for some seconds, a flat groove is cut. Then the cutting-oxygen pressure is raised to the value indicated for the respective workpiece thickness, and the cutting jet pierces through the workpiece.

© 2015