WPE2 Course Notes PDF [PDF]

Zitiervorschau

EWF/IIW Diploma – Welding Processes and Equipment (Intermediate) WPE2

Training & Examination Services Granta Park, Great Abington Cambridge CB21 6AL, UK Copyright © TWI Ltd

Rev 4 February 2013 Contents Copyright  TWI Ltd 2013

EWF/IIW Diploma Welding Processes and Equipment (Intermediate) Contents Section

Subject

Pre training briefing 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

Objectives What the welding processes and equipment module is about What does this module cover? What is the final outcome I can expect? What sort of material and learning methods are used? Why is this module important to me? My company has fixed ideas, who am I to change them? My company just wants me to be IIW/EWF qualified so that I can sign the paperwork, do I really need this knowledge? What will I be able to do at the end of this course that I can’t do now? So in a nutshell, what’s in it for me?

2 2.1 2.2 2.3 2.4 2.5

The History of Welding Beginnings Manual metal arc Oxy-fuel welding Semi-automatic welding Other processes

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14

General Introduction to Welding and Joining Introduction Joining methods Welding processes Surfacing or cladding Joint configuration Types of Weld Features of the completed weld Weld preparation Types of preparation Size of butt welds Size of fillet welds Shape of fillet welds Welding position, slope, rotation and weaving Standard references IWS questions on general introduction

www.twitraining.com

Rev 4 February 2013 Contents Copyright  TWI Ltd 2013

4 4.1 4.2 4.3 4.4

Fabrication Standards Application standards and codes Approval of welding procedures and welders Quality acceptance levels for welder procedure and welder approval tests Process reference numbers Revision questions on standards

5 5.1 5.2 5.3 5.4

Weld Symbols Standards Basic representation Edge preparation symbols Weld sizing Questions on weld symbols

6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

Fusion Welding Processes Creation and protection of weld pool Direction of welding Bead Shape Electrical creation of an Arc Creation of a molten pool by resistance heating Creation of a molten pool by a power beam Heat transfer Weld pool shape Questions on fusion welding Introduction and safety

7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

Arc Welding Safety Introduction Electric shock Heat Light Fumes and gases Noise Gas handling and storage Working at height and in restricted access areas Mechanical hazards

8 8.1 8.2 8.3 8.4 8.5 8.6

Gas Welding Processes Flame formation Oxy-acetylene welding Equipment Gas bottle identity and safety Operating characteristics Equipment safety checks Questions on gas welding

9 9.1 9.2 9.3 9.4 9.5

Electricity as Applicable to Welding Introduction Automic structures Electricity generation Current, voltage, watts and resistance Direct and alternating current

www.twitraining.com

Rev 4 February 2013 Contents Copyright  TWI Ltd 2013

9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13

Transforming electricity Phase Rectification Series and parallel Inductance Capacitance Transistors and thyristors Inverters Questions on electricity

10 10.1 10.2

Arcs and Plasmas Formation and distinction Arc and plasma zone Questions on arcs and plasmas

11 11.1 11.2 11.3 11.4 11.5 11.6

Power Sources Types of power source Power source characteristics Pulsed power Slope control and gas purging Duty cycle Bibliography Questions on power sources

12 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10

TIG Welding Process Characteristics Arc Initiation Current and polarity Preparing the tungsten electrode Shielding gas Filler wires Potential defects Tungsten inclusions Advantages of the TIG process Disadvantages of the TIG process Questions on TIG

13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10

MIG/MAG Welding Process characteristics Transfer modes Welding parameters Contact tip and nozzle set-up Shielding gases Solid wire consumables Important inspection points/checks when MIG/MAG welding Summary of solid wire MIG/MAG GMAW Flux-cored arc welding Process variants Questions on MIG/MAG

www.twitraining.com

Rev 4 February 2013 Contents Copyright  TWI Ltd 2013

14 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 14.11

Manual Metal Arc (MMA) Welding History Process characteristics MMA basic equipment requirements Electrode types Setting up for welding Welding parameters Practical aspects of MMA Manufacture of MMA electrodes Storage and handling Baking electrodes Electrode classification Questions on MMA

15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12 15.13 15.14 15.15 15.16

Submerged Arc Welding History Process characteristics Power source Equipment Consumables Welding parameters Increasing productivity Constant heat input Twin wire Hot or cold wire feed Core wires Metal powder addition Increased electrode extension DCEN welding Potential defects Classification of consumables Questions on SAW

16 16.1 16.2 16.3 16.4 16.5

Electroslag Welding History Process characteristics ESW materials other than steel Current status Benefits and disadvantages

17 17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 17.9

Thermal cutting and Gouging Introduction Oxyfuel cutting Powder cutting Oxyfuel gouging MMA gouging Air carbon arc gouging Plasma arc cutting Plasma arc gouging Laser cutting Questions on thermal cutting and gouging

www.twitraining.com

Rev 4 February 2013 Contents Copyright  TWI Ltd 2013

18 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8

Plasma Arc Welding History Process characteristics Power source Torch Plasma and shielding gases Backing system Applications Benefits and disadvantages Questions on plasma arc welding

19 19.1 19.2 19.3

Welding Consumables Consumables for MMA welding AWS A 5.1 and AWS 5.5Inspection points for MMA consumables

www.twitraining.com

Section 1 Objectives

Rev 4 February 2013 Objectives Copyright  TWI Ltd 2013

1

Objectives

1.1

What the welding processes and equipment module is about Welcome to the International Institute of Welding (IIW) and European Welding Federation (EWF) approved Diploma course offered by TWI Training Examination Service (TES). Successful completion of your course leads to qualification recognised in more than 40 countries. TWI-TES also offers tuition to those who do not meet the IIW/EWF access criteria. The syllabus and expected learning outcomes are given in an IIW publication, IAB-252r8-07, of which a short version may be downloaded from either the IIW website: www.iiw-iis.org or from the EWF website: www.ewf.be. This course is designed to cover the syllabus but we emphasise that selfstudy should account for at least as much time as the lectures. Larry Jeffus (Welding – Principles and Applications) is an excellent source for basic information, with coloured easy to follow diagrams. There are good books covering the topics in greater depth: AC Davies – The Science and Practice of Welding is a classic, but now rather dated, reference. Jeffries (Welding Principles and Application) and Althouse, Turnqist, Bowditch, Bowditch, Bowditch (Modern Welding) are newer titles with good explanations. The internet is, of course, a prime source of reference, though care must be taken as anyone can set up a website and post information, not all of which is accurate. We strongly suggest that you use the technical information available from TWI’s website www.twi.co.uk/content/tec_index.html Others that you may find helpful are: www.gowelding.com www.welding-technology-machines.info www.electronics-tutorials.com With the changing face of the internet we cannot say that these sites will remain in place and as useful as they seemed when we looked at them. We recommend that you use a search engine to explore what is available for any topic that you to learn more about. We hope that you enjoy this learning experience. Good luck in the exams!

1.2

What does this module cover? We will take you from the absolute basics – defining a weld, for instance – through to quite detailed understanding of the make-up and characteristics of arcs and plasmas. You will learn the basic electricity functions applicable to welding and the relationship between such fundamentals as transformation, rectification, inductance, etc and the behaviour of a welding process.

1-1

www.twitraining.com

Rev 4 February 2013 Objectives Copyright  TWI Ltd 2013

We cover all of the commonly used processes and many of those considered advanced or specialised. The basic principles behind each process are described together with the equipment and materials necessary for a quality joint. Standards applicable to welding and symbols used on drawings to indicate specific joints are covered and safety aspects are emphasised throughout. Much of the module concerns fusion welding but solid state processes, brazing, soldering, surfacing and cutting are also dealt with.

1.3

What is the final outcome that I can expect? We emphasise that we work to an international syllabus, at one of three levels, to prepare you for examinations that will qualify you to the same level as welding co-ordinators trained in any of the countries complying with the International Accreditation Board’s requirements. Your qualification will be recognised in more than 40 countries around the world. This module prepares you for specific exams on welding processes and equipment, one of four modules that you need to achieve the end qualification. Even if you choose not to be tested in this way, your involvement in the course will have given you a much greater understanding of the most influential parameters in welding and how to exert control over them in order to achieve quality welds.

1.4

What sort of material and learning methods are used? The rest of this volume contains notes and slides that show you the depth to which we take each topic. We lecture and expect active participation. This involvement increases as you progress through the levels - we expect those at the Engineer Level to be making significant personal input into the learning process. We must point out that simply learning the notes is not enough. We make frequent reference to private study and expect you to use all facilities – library, reference books and the internet, especially the TWI website with its Job Knowledge series of articles – to give you a fuller understanding of the subject. Our lecturers and course manager are always keen to hear from you. If you have input to give, ideas for improvement, or you just have a concern over the learning or examination, please speak to us.

1.5

Why is this module important to me? All welding engineers, technologists and specialists are expected to know the fundamentals of the welding processes. There is no-one in the company with better knowledge, so if the welding operation does not go smoothly everyone will turn to the specialist, ie you, for advice.

1-2

www.twitraining.com

Rev 4 February 2013 Objectives Copyright  TWI Ltd 2013

A key decision the welding specialist must make is to determine the best process for the company to use for any application. This will require an understanding, not only of the pros and cons of each process, but also any attendant requirements necessary to make the process work efficiently. This module will give you an understanding of how each process works and the differences between them; the equipment, control and operator skill required for each and the economic factors associated with choosing a welding process.

1.6

My company has fixed ideas, who am I to change them? We’re not saying change is necessary, nor always desirable, but WL Bateman famously said: "If you keep on doing what you've always done, you'll keep on getting what you've always got." Maybe your company has got it right and wants to continue getting what it always got, but we doubt it. Everyone wants to remain competitive and seeks to improve productivity. If not, we would still see rows of scribes with quill pens rather than computers in offices. Welding is a traditional process, but the equipment and control available today make even the set-up of ten years ago obsolete. This course will place recently developed processes and newer equipment types and controls in context with traditional units. It will teach you how to judge true advances and their benefit to your company.

1.7

My company just wants me to be IIW/EWF qualified so that I can sign the paperwork, do I really need this knowledge? Companies do have short-term goals and getting someone qualified as a welding co-ordinator is an admirable one, but this shows that it is working on contracts that demand that welding is taken seriously as a special process. Having succeeded with the first of such contracts, your company will surely look to take on more. A welding co-ordinator does far more than sign the paperwork and will play a big part in determining the success of future contracts of ever increasing technological and quality demands. This module will give you the confidence to speak with authority on fabrication techniques to be used and the cost-effectiveness of welding processes at your disposal.

1-3

www.twitraining.com

Rev 4 February 2013 Objectives Copyright  TWI Ltd 2013

1.8

What will I be able to do at the end of this course that I can’t do now? This is a tricky one, as everyone has different skills coming into the course and different requirements that they wish to gain from it. However, even if you are on top of the game with regard to the applications you see every day in your job, exposure to the requirements and decisions from other quarters can only be of benefit. Who knows, maybe laser cutting or friction stir welding is the next logical step for your company with regard to cost and quality improvement. This module will give you details of a wide range of processes available for many different types of material.

1.9

So, in a nutshell, what’s in it for me? The acquisition of knowledge about your speciality is never wasted. Even if you don’t use all that you learn on this course immediately, your awareness will be raised so that you will remember where to look for information when circumstances demand it. If your company develops opportunities in applications and materials currently unfamiliar, you will be in a position to come to terms rapidly with any new approaches necessary. Whilst we recognise that you are likely to be sponsored by your company against a company objective, we should also point out that your personal development and the gaining of professional qualifications is of great benefit to you, the individual, as you follow your career path.

1-4

www.twitraining.com

Section 2 The History of Welding

Rev 4 February 2013 The History of Welding Copyright  TWI Ltd 2013

2

The History of Welding

2.1

Beginnings Forge welding, where two metallic items are heated to near melting and are then hammered together until permanently bonded has been with us at least since the early Egyptians, so the origin of welding lies back in pre-history. The invention of the processes seen today is much more recent as it required the availability of electricity or high temperature portable flames – neither of which appeared before the latter part of the nineteenth century.

2.2

Manual metal arc (MMA) The origin of MMA welding is a matter of dispute as a number of workers had demonstrated that wires could be melted and joined by an electrically generated arc. An Englishman called Wilde was granted a patent in 1865 for using electrical power to melt and join small pieces of iron, but it is generally accepted that the British and Russian patents of Bernados and Olszewaski in 1885 and 1886 describing carbon arc welding were the start point of arc welding. In fact Bernados started a company producing equipment for arc welding and cutting. In 1892, Coffin in USA and Slavianoff in Russia were granted patents for the use of metal rods in place of one of the carbons in the Bernados equipment, but it was not until Kjellberg in 1908 that the coated electrode appeared. Almost coincidentally, Strohmenger in England found that wrapping a metal rod with asbestos string stabilised the arc such that it could operate under the newly available alternating current. This led his physicist friend to term it a quasi-arc – a name he used for the consumables business he subsequently set up. These two systems of coating metal rods produced weld metal without impurities for the first time and development of the MMA electrode to the diversity we know today followed quite swiftly. Asbestos winding survived into the 1950s before awareness was raised of the health and safety issues and the electrodes were withdrawn.

2.3

Oxy-fuel welding Acetylene was a laboratory curiosity around the time of Bernados, though people were experimenting with ways of its high temperature flame. The main problem was the unstable nature of the gas; it could not be compressed without exploding. At the turn of the twentieth century, the use of acetone as a solvent for acetylene was being examined but explosions could still occur unless the liquid was absorbed into a porous material with only capillary sized storage. This was perfected by Dalén of the AGA Company in 1906 and the medium he produced is basically the same as is used today.

2-1

www.twitraining.com

Rev 4 February 2013 The History of Welding Copyright  TWI Ltd 2013

2.4

Semi-automatic welding The development of the continuous wire processes took a little longer. Bernados is credited with inventing the electroslag principle in 1908, but it was not until the 1950s that the Paton Institute in Kiev developed it as a viable welding process. The Fusarc process was the first commercially available continuous feed welding process. A central wire of maybe 4mm diameter was loosely spirally wound with two smaller wires, wrapped in opposite directions creating a diamond pattern of interstices into which electrode paste was squeezed. The outermost winding wire broke the surface of the paste at the points where it crossed over the inner thin wire so that electrical contact could be made through it and the second wire to the core. An arc could therefore by struck between the core wire and the workpiece and by loading the wire and welding head on a tractor, a long length of weld could be made. An improvement was made to the process by adding carbon dioxide shielding to augment the limited cover from the thin layer of slag formed. The Fusarc CO2 process was very successful but eventually superseded by submerged arc welding production continued in Britain until the 1980s and even later in India. In 1930 patents were granted in the US for the use of continuous wire with gas shielding provided through a concentric nozzle birth of Metal Inert Gas (MIG) welding. The inventors, Hobart and Devers, used helium and argon for shielding but because of the poor quality of the gases, the process was not an immediate commercial success. It was not until 1948 that Linde made a commercial success of the process then called (shielded inert gas metal arc) SIGMA.

Submerged arc welding (SAW) was also developed in 1930 by Robinoff of the National Tube Co in the US. The process was sold to Linde who renamed it Union-Melt. It originally used lengths of metal rod rather than a continuous wire but by the 1950s, SAW as we know it was available. Carbon dioxide was introduced as an alternative gas for MIG on steel in 1953 and, in 1958, work at TWI and simultaneously in USSR and the US, defined the short-circuiting arc under CO2.

2-2

www.twitraining.com

Rev 4 February 2013 The History of Welding Copyright  TWI Ltd 2013

Flux-cored wires were developed in the late 1950s with Bernard concentrating on gas-shielded ‘Dual-shield’ and Lincoln pioneering the nogas option Innershield.

2.5

Other processes Engineers at the Northrop Aviation Co in the US developed Tungsten Inert Gas (TIG) welding, gaining a patent in 1942 under the name Heliarc. They needed a process to weld magnesium and stainless steel with precision so used a non-consumable electrode (tungsten) and inert gas (helium) shielding, hence the derivation of the name. Helium was more readily available in the US so in the UK the process used argon and was originally known as Argonarc. Resistance welding was first used to create spot welds in the early part of the twentieth century. It quickly found use in the automobile industry where it is still used today, albeit now on the end of robot arms. Thermit welding is also more than 100 years old, being the application of a reaction between powdered aluminium and iron oxide first noted Goldschmidt in 1903. It continues as the most popular method of joining rail on-site. The 1950s and 60s saw a rush of new welding processes (FSW) As well, as those mentioned above, friction welding was invented in the USSR; electron beam was developed in France; the CO2 laser appeared with enough power to be used for cutting and welding; explosive welding and plasma welding were developed in the US; cold pressure welding was invented at GEC, in the UK and pulsed power was tried in several countries for MIG and TIG.

Since then, the pace of new process development has slowed with only the invention of friction stir welding at TWI in 1991 as commercially significant in last 40 years. The process was developed on aluminium but with improvement in the design of refractory metal and ceramic tools, FSW of steel has been demonstrated to be possible. Commercially, welding of aluminium alloys for space vehicles, aircraft, trains and boats remains its main application.

2-3

www.twitraining.com

Section 3 General Introduction to Welding and Joining

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3

General Introduction to Welding and Joining

3.1

Introduction Welding and joining, have their own terminology with abbreviations and acronyms, but these soon become familiar. In this section we give the definitions of basic terms.

3.2

Joining methods Joining is the most general term used to refer to any process or procedure by which two or more separate pieces of material are physically attached to each other so as to create a single larger piece. Joining covers welding, soldering, brazing, adhesive bonding, diffusion bonding, riveting, clinching and non-engineering processes such as sewing that will not be dealt with in these notes.

3.2.1

Welding Welding is defined as an operation in which two or more parts are united by means of heat or pressure or both, in such a way that there is continuity in the nature of the metal between these parts. Many materials such as metals, plastics and ceramics may be welded though some require the use of specific processes and techniques and a number are considered unweldable, a term not usually found in dictionaries but useful and descriptive in engineering.

The parts that are joined are termed parent material and any material added to help form the joint is called filler or Consumable. The form of these materials may see them referred to as parent plate or pipe filler wire, consumable electrode (for arc welding) etc. Consumables are usually chosen to be similar in composition to the parent material thus forming an homogenous weld but there are occasions, such as when welding brittle cast irons, when a filler with very different composition and therefore properties is used. Such welds are called heterogeneous. The completed welded joint may be referred to as a weldment.

3-1

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.2.2

Brazing A process of joining generally applied to metals in which, during or after heating, molten filler metal is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined by capillary attraction. In general, the melting point of the filler metal is above 450C but always below the melting temperature of the parent material. The composition of the filler for brazing is often very different from parent material; for instance, steel may be brazed with copper alloy filler.

3.2.3

Soldering A similar process to brazing, relying on capillary attraction to draw molten filler into a gap between parts that remain solid throughout. Solders melt at low temperatures – less than 450ºC. For steel and copper, solders are usually alloys of tin.

3.2.4

Braze welding The joining of metals using a technique similar to fusion welding and a filler metal with a lower melting point than the parent metal, but neither using capillary action as in brazing nor intentionally melting the parent metal. Basically, bulk addition of filler is made in a manner similar to welding but the aim is to melt only this consumable leaving the parent material wetted by the molten braze metal but not melted.

3.2.5

Diffusion bonding Component parts are held together with force and are heated to a temperature at which easy atomic movement makes possible the diffusion of material from one part to the other. Sometimes assisted by an interlayer placed between the two parent parts but during the process this layer is fully absorbed into the parent material.

3.2.6

Adhesive bonding Sophisticated adhesives are now available that can achieve very good joint strength in most materials. Adhesives may be single component material dissolved in a solvent for ease of application or may be two parts that interact chemically when mixed. They may set or cure at room temperature or require holding at temperature to create full bond strength.

3-2

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.2.7

Mechanical bonding Rivets, bolts, nails, etc have all been used to join materials. Merely forcing one material into another to form a bonding point, Clinching, is used for more malleable materials. These processes are not explicitly covered in the course but should not be forgotten as viable joining methods. Rivets are commonly seen in bridges and aircraft wings. Occasionally, hybrid bonding consisting of both adhesive and mechanical bonding is used for an application. An example is the creation of an aluminium alloy chassis for a sports car using adhesive and rivets.

3.3

Welding processes Welding processes fall into two groups – those in which fusion takes place and those that achieve solid state bonding. Fusion welding includes  Oxy-fuel gas welding (OFW).  Manual metal (LIC) arc (MMA).  Metal inert/active gas (MIG/MAG).  Flux-cored arc welding (FCAW).  Tungsten inert gas (TIG).  Submerged arc welding (saw).  Electroslag welding (ESW).  Laser welding (LASER is itself an acronym: light amplification by stimulated Emission of radiation).  Electron beam (EB or EBW).  Resistance; magnetically impelled arc butt (MIAB) and others. American codes and standards use different terminology and abbreviations for these processes:    

MMA – Shielded Metal Arc Welding (SMAW). MIG/MAG – Gas Metal Arc Welding (GMAW). TIG – Gas Tungsten Arc Welding (GTAW). Laser - Laser Beam Welding (LBW).

Solid state processes do not involve melting because some materials can be permanently welded together by pressure if in a suitably malleable state. This may require the application of some heat, eg forge welding as carried out by blacksmiths, and friction welding in its many forms. Explosive cold pressure and ultrasonic welding are examples of processes in which heat is not deliberately generated. The most common of the abovementioned welding processes are described in these notes and some further ones are given in the Advanced Processes notes, but neither attempts to give an exhaustive listing of all of the welding processes that have been demonstrated.

3-3

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.4

Surfacing or cladding Surfacing uses welding processes, not to join together two pieces of parent material, but to coat one with a second material that may be similar in composition is more usually different with particular properties, eg corrosion, wear or heat resistance, not possessed by the base material. Often mild steel is surfaced with stainless steel or a nickel alloy to give a layered material that has the benefit of corrosion performance on one side but at a lower cost than if the component were to be fully from the corrosion resistant alloy. There are more extreme examples of the possibilities presented by surfacing, often involving a mix of metals and ceramics. High performance internal combustion engine pistons can be produced from an aluminium alloy for lightness but coated with ceramic aluminium oxide for high temperature corrosion/erosion resistance. Glass reflectors may have a metal coating applied to one side to produce the reflective surface. These coatings may be applied by what are basically welding processes, eg SAW, ESW, MIG/MAG and friction, but may also be created by the projection of metallic or ceramic particles through high-speed flame or plasma guns. This process often referred to as spraying thinly coats to components without melting the base material. Cladding is a more general term that covers the surfacing techniques but also includes explosive and roll bonding of one plate or tube to another to create a duplex structure.

3-4

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.5

Joint configuration The table below defines some of the more common joint configurations: Type of joint

Sketch

Butt joint

T joint

Corner joint

Edge joint

Definition A connection between the ends or edges of two parts making an angle to one another of 135 to 180 inclusive in the region of the joint. A connection between the end or edge of one part and the face of the other part, the parts making an angle to one another of more than 5 up to and including 90 in the region of the joint A connection between the ends or edges of two parts making an angle to one another of more than 30 but less than 135 in the region of the joint

A connection between the edges of two parts making an angle to one another of 0 to 30 inclusive in the region of the joint A connection in which two flat plates or two bars are welded to another flat plate at right angles and on the same axis

Cruciform joint

Lap joint

3.6

Types of weld

3.6.1

Based on configuration

A connection between two overlapping parts making an angle to one another of 0 to 5 inclusive in the region of the weld or welds

Butt weld

Fillet weld

3-5

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

In a butt joint

Butt

In a T joint

In a corner joint

Slot weld Joint between two overlapping components made by depositing a fillet weld round the periphery of a hole in one component so as to join it to the surface of the other component exposed through the hole.

Plug weld Weld made by filling a hole in one component of a workpiece with filler metal so as to join it to the surface of an overlapping component exposed through the hole (the hole can be circular or oval).

3.6.2

Based on penetration Full penetration weld Welded joint where the weld metal fully penetrates the joint with complete root fusion. In US the preferred term is complete joint penetration weld (CJP, see AWS D1.1).

Partial penetration weld: Weld in which the fusion penetration is intentionally less than full penetration. In US the preferred term is partial joint penetration weld (PJP).

3-6

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.6.3

Based on composition Autogenous Fusion weld made without filler metal which can be achieved by TIG, plasma, electron beam, laser or oxy-fuel gas welding. Homogeneous Welded joint in which the weld metal and parent material have no significant differences in mechanical properties and/or chemical composition. Example: two carbon steel plates welded with a matching carbon steel filler metal. Heterogeneous Welded joint in which the weld metal and parent material have significant differences in mechanical properties and/or chemical composition. Example: a repair weld of a cast iron item performed with a nickel-based electrode. Dissimilar Welded joint in which the parent materials have significant differences in mechanical properties and/or chemical composition. Example: a carbon steel lifting lug welded onto an austenitic stainless steel pressure vessel.

3.6.4

Based on accessibility

Single side weld

Double side weld

3-7

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.7

Features of the completed weld

Butt weld

Fillet weld Parent metal Metal to be joined or surfaced by welding, braze welding or brazing. Filler metal Metal added during welding; braze welding, brazing or surfacing.

3-8

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Weld metal All metal melted during the making of a weld and retained in the weld. Heat-affected zone (HAZ) The part of the parent metal that is metallurgically affected by the heat of welding or thermal cutting, but not melted. Fusion line The boundary between the weld metal and the HAZ in a fusion weld. This is a non-standard term for weld junction. Weld zone Zone containing the weld metal and HAZ. Weld face Surface of a fusion weld exposed on the side from which the weld has been made. Weld root Zone on the side of the first run furthest from the welder. Weld toe Boundary between a weld face and the parent metal or between runs. A very important feature of a weld since toes are points of high stress concentration and often are initiation points for different types of cracks (eg fatigue cracks, cold cracks). To reduce the stress concentration, toes must blend smoothly into the parent metal surface. Excess weld metal Weld metal lying outside the plane joining the toes. Other non-standard terms for this feature: reinforcement, overfill. Note: the term reinforcement, although commonly used, is inappropriate because any excess weld metal over and above the surface of the parent metal does not make the joint stronger. In fact, the thickness considered when designing a welded component is the ‘design throat thickness’ (see Section 10), which does not include the excess weld metal. Run (pass) Metal melted or deposited during one passage of an electrode, torch or blowpipe.

3-9

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Single run weld

Multirun weld

Layer Stratum of weld metal consisting of one or more runs.

3.8

Weld preparation A preparation for making a connection where the individual components, suitably prepared and assembled, are joined by welding or brazing. Features of weld preparation: Angle of bevel Angle at which the edge of a component is prepared for making a weld. For an MMA weld on carbon steel plates, the angle is typically:    

25-30 for a V preparation. 8-12o for a U preparation. 40-50o for a single bevel preparation. 10-20o for a J preparation.

Included angle Angle between the planes of the fusion faces of parts to be welded. In the case of single V or U and double V or U this angle is twice the bevel angle. In the case of single or double bevel, single or double J bevel, the included angle is equal to the bevel angle. Root face Portion of a fusion face at the root that is not bevelled or grooved. Its value depends on the welding process used, parent material to be welded and application; for a full penetration weld on carbon steel plates, it typically is around 1-2mm (for the common welding processes). Gap Minimum distance at any cross section between edges ends or surfaces to be joined. Its value depends on the welding process used and application; for a full penetration weld on carbon steel plates, it is usually 1-4mm.

3-10

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Root radius Radius of the curved portion of the fusion face in a component prepared for a single J or U, double J or U weld. In MMA, MIG/MAG and oxy-fuel gas welding on carbon steel plates, typical root radii are 6mm for single and double U preparations and 8mm for single and double J preparations. Land Straight portion of a fusion face between the root face and the curved part of a J or U preparation. It is not essential to have a land but it is usually present in weld preparations for MIG welding of aluminium alloys.

3.9

Types of preparation Open square butt preparation

Used for welding thin components, from one or both sides. If the root gap is zero (ie if components are in contact), this preparation becomes a closed square butt preparation (not recommended due to the lack of penetration problems)! Single V preparation

The V preparation is one of the most common preparations used in welding; can be produced using flame or plasma cutting (cheap and fast). Double V preparation

3-11

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

For thicker plates a double V preparation is preferred since it requires less filler material to complete the joint and the residual stresses can be balanced on both sides of the joint resulting in lower angular distortion. The depth of preparation can be the same on both sides (symmetric double V preparation) or can be deeper on one side compared with the opposite side (asymmetric double V preparation). Usually, in this situation the depth of preparation is distributed as 2/3 of the thickness of the plate on the first side with the remaining 1/3 on the backside. This asymmetric preparation allows for a balanced welding sequence with root back gouging, giving lower angular distortions. Whilst single V preparation allows welding from one side, double V preparation requires access to both sides (the same applies for all double side preparations). Single U preparation U preparation can be produced only by machining (slow and expensive). Tighter tolerances provide a better fit-up than in the case of V preparations. Usually applied to thicker plates as it requires less filler material to complete the joint compared with single V preparation and this leads to lower residual stresses and distortions.

Double U preparation

As with V preparation, for very thick sections a double U preparation can be used. Usually this type of preparation does not require a land, except for aluminium alloys.

3-12

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Single V preparation with backing strip

Backing strips allow the production of full penetration welds with increased current and hence increased deposition rates/productivity without the danger of burn-through. Backing strips can be permanent or temporary. Permanent types are made of the same material as being joined and are tack welded in place. The main problems related to this type of weld are poor fatigue resistance and the probability of crevice corrosion between the parent metal and the backing strip. It is also difficult to examine by NDT due to the built-in crevice at the root of the joint. Temporary types include copper strips, ceramic tiles and fluxes. 3.9.1

Single plate and T joint preparations All the following preparations (single/double bevel and J) can be used on T joints as well as plate butts. Double preparations are recommended for thick sections. The main advantage of these preparations is that only one component is prepared (cheap, can allow for small misalignments). Single bevel preparation

Double bevel preparation (also referred to as K-preparation)

Single J preparation

3-13

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Double J preparation

3.10

Size of butt welds Full penetration butt weld

Partial penetration butt weld

As a general rule: Actual throat thickness = design throat thickness + excess weld metal. Full penetration butt weld ground flush

Actual throat thickness = design throat thickness

Butt weld between two plates of different thickness

3-14

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.11

Size of fillet welds

3.12

Shape of fillet welds Mitre fillet weld Flat face fillet weld in which the leg lengths are equal within the agreed tolerance. The cross section area of this type of weld can be considered to be a right angle isosceles triangle with a design throat thickness a and leg length z. The relation between design throat thickness and leg length is: a = 0.707  z or z = 1.41  a.

Convex fillet weld

3-15

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Fillet weld in which the weld face is convex. The above relation between the leg length and the design throat thickness written for mitre fillet welds is also valid for this type of weld. Since there is excess weld metal present in this case, the actual throat thickness is greater than the design throat thickness.

Concave fillet weld Fillet weld in which the weld face is concave. The relation between the leg length and design throat thickness specified for mitre fillet welds is not valid the design throat thickness is equal to the actual throat thickness. Due to the smooth blending between the weld face and the surrounding parent material, the stress concentration effect at the toes of the weld is reduced compared with the previous type. This is why this type of weld is highly desired in case of applications subjected to cyclic loads where fatigue phenomena might be a major cause for failure.

Asymmetrical fillet weld Fillet weld in which the vertical leg length is not equal to the horizontal leg length. The relation between the leg length and design throat thickness is no longer valid for this type of weld because the cross section is not an isosceles triangle. Horizontal leg size

Vertical leg size Throat size 3-16

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Deep penetration fillet weld Fillet weld with a deeper than normal penetration produced using high heat input welding processes (ie SAW or MAG with spray transfer). This type of weld uses the benefits of greater arc penetration to obtain the required throat thickness whilst reducing the amount of deposited metal needed, thus leading to a reduction in residual stress level. To produce a consistent and constant penetration, the travel speed must be kept constant, at a high value. As a consequence, this type of weld is usually produced using mechanised or automatic welding processes. Also, the high depth-to-width ratio increases the probability of solidification centreline cracking. In order to differentiate this type of weld from the previous types, the throat thickness is symbolised with s instead of a.

Double bevel compound weld A combination of butt and fillet welds used for T joints with full or partial penetration or butt joints between two plates with different thickness. Fillet welds added on top of the groove welds improve the blending of the weld face towards the parent metal surface and reduce the stress concentration at the toes of the weld. Bevel weld

3.13

Fillet weld

Welding position, slope, rotation and weaving Welding position The orientation of a weld expressed in terms of working position, weld slope and weld rotation (for further details, see BS EN ISO 6947).

3-17

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

The following table gives the commonly used terminology: Welding Sketch Definition and symbol position according to BS EN ISO 6947 A welding position in which the welding is horizontal, with Flat the centreline of the weld vertical. PA. A welding position in which the welding is horizontal (applicable in case of fillet welds). PB

Horizontalvertical

A welding position in which the welding is horizontal, with the centreline of the weld horizontal. PC Horizontal

A welding position in which the welding is upwards. PF.

Vertical-up

A welding position in which the welding is downwards. PG

Verticaldown

A welding position in which the welding is horizontal and overhead, with the centreline of the weld vertical. PE.

Overhead

A welding position in which the welding is horizontal and overhead (applicable in case of fillet welds). PD.

Horizontaloverhead

3-18

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

3.13.1

Tolerances for welding position

Weld slope

The angle between root line and the positive X axis of the horizontal reference plane, measured in mathematically positive direction (ie counterclockwise). Weld rotation

The angle between the centreline of the weld and the positive Z axis or a line parallel to the Y axis, measured in the mathematically positive direction (ie counter-clockwise) in the plane of the transverse cross-section of the weld in question.

3-19

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

Weaving Weave

This is the transverse oscillation of an electrode or blowpipe nozzle during the deposition of weld metal and this technique is generally used in verticalup welds. Stringer bead

A run of weld metal made with little or no weaving motion.

3.14

Standard references BS 499-1:2009 Welding terms and symbols. Glossary for welding, brazing and thermal cutting BS 499-2C:1999 Welding terms and symbols. European arc welding symbols in chart form BS EN 14610:2004 Welding and allied processes. Definitions of metal welding processes PD CEN/TR 14599:2005 Terms and definitions for welding purposes in relation with EN 1792 BS EN ISO 17659:2004 Welding. Multilingual terms for welded joints with illustrations BS EN ISO 6947:1997 Welds. Working positions. Definitions of angles of slope and rotation Note: This list includes the main European standards concerning welding terms and symbols but is not intended to be exhaustive. In general, national and international codes (eg ASME, ASTM, AWS, DNV) comprise specific sections or standards for definitions and abbreviations.

3-20

www.twitraining.com

Rev 4 February 2013 General Introduction to Welding and Joining Copyright  TWI Ltd 2013

IWS Revision Questions on General Introduction 1. Sketch a double bevel Tee butt weld with full penetration and superimposed mitre fillet welds. 2. Sketch a single Vee butt weld and indicate the features. 3. Sketch a double J butt weld. 4. Indicate the typical excess weld metal dimension on a butt weld in 6mm thickness material. 5. For which is MMA a abbreviation? 6. Sketch actual throat thickness compared with design throat thickness.

IWT Revision Questions on General Introduction 1. Define weld slope and weld rotation. 2. What are the positions PA, PB, PC, PD, PE, PF and PG? 3. Deduce the relationship between leg length and throat mathematically. 4. Draw a cross-section of a double V butt joint and label: a Fusion line b Toes c Root run d Reheated region of an underlying weld bead 5. Sketch a slot weld in section and in plan. 6. Define an edge joint.

3-21

www.twitraining.com

Section 4 Fabrication Standards

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

4

Fabrication Standards Application standards and codes of practice ensure that a structure or component will have an acceptable level of quality and be fit for the intended purpose. The requirements for standards on welding procedure and welder approval are explained below. The term approval is used in European standards in the context of both testing and documentation; the equivalent term in the ASME standard is qualification. A standard has also been constructed that gives a unique number to a welding process, described below. An overview of the European (international) standards used in fusion welding is given in the Appendix

4.1

Application standards and codes There are essentially three types of standards that are referenced in fabrication:   

Application and design. Specification and approval of welding procedures. Approval of welders.

There are also specific standards covering material specifications, consumables, welding equipment and health and safety. British Standards are used to specify the requirements, for example, in approving a welding procedure, they are not a legal requirement but may be cited by the Regulatory Authority as a means of satisfying the law. Health and Safety guidance documents and codes of practice may also recommend standards. Codes of practice differ from standards in that they are intended to give recommendations and guidance, for example, on the validation of power sources for welding. It is not intended that they should be used as a mandatory or contractual documents. Most fabricators will be working to one of the following:     

Company or industry specific standards. National BS (British Standard). European BS EN (British Standard European Standard). US AWS (American Welding Society) and ASME (American Society of Mechanical Engineers). International ISO (International Standards Organisation).

4-1

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

In European countries, national standards are being replaced by EN and ISO standards. When there is no equivalent EN standard, the National standard can be used for example, the BS EN 287 and BS EN ISO 9606 series replaced BS 4871 and 4872, but BS 4871 Part 3 and 4872 Part 1 remains a valid standard. Examples of application codes and standards and related welding procedure and welder approval standards are listed below: Application

Application code/standard

Welding procedure approval

Welder approval

Pressure vessels

BS 5276 BS PD 5500 ASME Section VIII

BS EN ISO 15614 ASME Section IX

BS EN 287 BS EN ISO 9606 ASME Section IX

Process pipework

BS 2633 BS 2971 BS 4677 ASME B31.1/B31.3

Structural fabrication

Storage tanks

4.2

BS EN 287 BS 4872 BS EN ISO 9606 ASME IX

BS EN ISO 15614 ASME IX

BS EN 1090 BS 8118 AWS D1.1/ D1.2/ D1.6 BS EN 12285 BS EN 14015 API 620/650

BS EN ISO 15614 AWS D1.1/ D1.2/ D1.6

BS EN 287 BS 4872 BS EN ISO 9606 AWS D1.1/ D1.2/ D1.6

BS EN ISO 15614 ASME IX

BS EN 287 BS EN ISO 9606 ASME IX

Approval of welding procedures and welders An application standard or code of practice will include requirements or guidelines on material, design of joint, welding process, welding procedure, welder qualification and inspection or may invoke other standards, for example for welding procedure and welder approval tests. The requirements for approvals are determined by the relevant application standard or as a condition of contract. The manufacturer will normally be required to approve the welding procedure and welder qualification or to have it witnessed by an independent inspection authority. Welding procedure approval test Carried out by a competent welder the quality of the weld is assessed using non-destructive and mechanical testing techniques. The intention is to demonstrate that the proposed welding procedure will produce a welded joint that will satisfy the specified requirements of weld quality and mechanical properties.

4-2

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

As shown in the table above, welding procedure approval is carried out according to the BS EN 15614 series (different parts correspond to different fusion welding processes), Section IX of the ASME Boiler and Pressure Vessel Code, other codes such as AWS D1.1 for structural welding, DNV-OS-F101 (offshore structures) include requirements for welding procedure qualification. Welder approval test Examines a welder's skill and ability in producing a satisfactory test weld. The test may be performed with or without a qualified welding procedure. Note: Without an approved welding procedure the welding parameters must be recorded. BS EN 287, BS ISO EN 9606 and ASME Section IX would be appropriate for welders on high quality work such as pressure vessels, pressure vessel piping and offshore structures. They are also used for other products where the consequences of failure, stress levels or complexity mean that a high level of welded joint integrity is essential. In less demanding situations, such as small to medium building frames and general light structural and non-structural work, an approved welding procedure may not be necessary. However, to ensure an adequate level of skill, welders are often approved to a less stringent standard, eg BS 4872. Coded welder is an expression often used to denote an approved welder but the term is not recognised in any of the standards. It is used in the workplace to describe those welders whose skill and technical competence have been approved to the requirements of an appropriate standard.

4.3

Quality acceptance levels for welding procedure and welder approval tests When welding to application standards and codes consideration must be given to the imperfection acceptance criteria that must be satisfied. Some standards contain an appropriate section relating to the acceptance levels while others make use of a separate standard. For example, in welding procedure and welder approval tests to BS EN ISO 15614-1 and BS EN ISO 287 Pt1, reference is made to BS EN ISO 5817. The application standard may specify more stringent imperfection acceptance levels, however, and/or require additional tests to be carried out as part of the welding procedure approval test. For example, for joints that must operate at high temperatures, elevated temperature tensile testing may be required whereas for low temperature applications, impact or CTOD tests may be specified.

4-3

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

Guidance on permissible levels of imperfections in arc welded joints in steel is given in BS EN ISO 5817. Production quality, but not fitness-forpurpose, is defined in terms of three levels of quality for imperfections:   

Moderate Intermediate Stringent

Level D. Level C. Level B.

The standard applies to most arc welding processes and covers imperfections such as cracks, porosity, inclusions, poor bead geometry, lack of penetration and misalignment. As the quality levels are related to the types of welded joint and not to a particular component, they can be applied to most applications for procedure and welder approval. The quality levels which are the most appropriate for production joints will be determined by the relevant application standard which may cover design considerations, mode of stressing (eg static, dynamic), service conditions (eg temperature, environment) and consequences of failure. When working to the European Standards, the welding procedure, or the welder will be qualified if the imperfections in the test piece are within the specified limits of Level B except for excess weld metal, convexity, throat thickness and penetration type imperfections when Level C will apply. Guidance levels for aluminium joints are given in BS EN ISO 10042. For the American standards ASME Section IX and AWS D1.1, the acceptance levels are contained in the standard itself. Application codes may specify more stringent imperfection acceptance levels and/or additional tests.

4.4

Process reference numbers Identify welding processes with a numerical sequence the European standard BS EN ISO 4063:2000 ‘Welding and allied processes Nomenclature of processes and reference numbers’, assigns a unique number to each of the main welding processes. The reference numbers are used as a convenient way of identifying the welding process in documentation such as welding procedure (BS EN ISO 15614) and welder qualification (BS EN 287 and BS EN 9606) records. However, full process names (or both numerical ID and process name) are often used for clarity.

4-4

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

The main arc welding process reference numbers are: 1 11 12 13 14 15

Arc welding Metal arc with gas Submerged arc Gas-shielded metal arc Gas-shielded with tungsten electrode Plasma

2 21 22 23 24 25 26 27 29

Resistance welding Resistance spot Resistance seam Projection Flash Resistance butt upset Resistance stud HF resistance Other resistance welding processes

3 31

Gas welding Oxy-fuel gas

4 41 42 43 44 45 47 48 49

Welding with pressure Ultrasonic Friction Friction stir High mechanical energy Diffusion Oxy-fuel gas pressure Cold pressure Hot pressure

5 51 52

Beam welding Electron beam Laser

6

Not used

7 71 72 73 74 75 78

Other welding processes Aluminothermic Electroslag Electrogas Induction seam Light radiation Arc stud

4-5

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

8 81 82 83 84 86 87 88

Cutting and gouging Flame cutting Arc cutting Plasma cutting Laser cutting Flame gouging Arc gouging Plasma gouging

9 91 92 93 94 95 96 97

Brazing, soldering and braze welding Brazing with local heat Brazing with global heat Other brazing processes Soldering with local heat Soldering with global heat Other soldering processes Weld brazing

The actual processes are depicted by a third digit, eg: 111 Manual metal arc welding 114 Self-shielded tubular-cored arc welding 121 Submerged arc welding with one wire electrode 125 Submerged arc welding with tubular cored electrode 131 Metal inert gas welding (MIG) 135 Metal active gas welding (MAG) 136 Tubular cored metal arc welding with active gas shield 141 Tungsten inert gas arc welding (TIG) There is then the possibility to add additional information on transfer mode, number of electrodes, added filler or hybrid processes. Transfer mode D short-circuit G Globular S Spray P Pulsed So MIG welding might be described as: BS EN ISO 4063 – 131-S Number of electrodes If multiple wires are used the number may be appended, as in twin wire MAG welding: BS EN ISO 4063 – 135-2 Additional wire C Cold wire addition H Hot wire addition

4-6

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

So submerged arc with cold wire addition is: BS EN ISO 4063 – 121-C Hybrid welding Where more than one process is used a plus sign joins the two. So fibre laser/MAG might be: BS EN ISO 4063 – 521+135-S Appendix a - European (International) standards for fusion welding

4-7

www.twitraining.com

4-8

www.twitraining.com

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

Rev 4 February 2013 Fabrication Standards Copyright  TWI Ltd 2013

IWS Revision Questions on Standards 1 What is the purpose of a welding procedure approval test? 2 What is the purpose of a welder approval test? 3 What is the difference between a Standard and a Code of Practice?

IWT Revision Questions on Standards 4 Describe the BS EN ISO 4063 system of nomenclature for welding processes. Include some examples. 5 Describe how application standards and codes of practice are used to improve weld quality and the performance of the fabrication. 6 Explain how BS EN ISO 5817 is used in approval testing.

4-9

www.twitraining.com

Section 5 Weld Symbols

Rev 4 February 2013 Weld Symbols Copyright  TWI Ltd 2013

5

Weld Symbols Weld symbols are a simple way of communicating design office details to variety of shop floor personnel eg welders, supervisors and inspectors, in a consistent manner. Non-company staff such as sub-contractors and insurers may also need to interpret the engineering drawings. It is essential therefore that everyone should have a full understanding of the system of weld symbols in use to ensure that the design requirement is met.

5.1

Standards The most common international standards for weld symbols are ISO 2553 / European EN 22553, published in the UK as BS EN 22553 and the American AWS/ANSI A2.4. Most of the details are the same, but it is essential that everyone concerned knows the standard to be used. The UK traditionally used BS 499-2 to define weld symbols which was superseded by BS EN 22553. Confusingly, the BSI still publishes BS 499-1 containing weld symbols as well as other terminology for welding and a chart, BS 499-2C that shows the symbols pictorially.

5.2

Basic representation All the standards use a reference line plus an arrow line and head placed at an angle to the reference line:

The V-shaped tail is optional as in Europe it shows the welding process with the reference numbers defined in BS EN ISO 4093. If only one process is to be used throughout the construction, this can be shown once on the drawing rather than repeated for each weld. The reference line has a parallel dotted line to show the other side. This is a refinement introduced in the European standard that is not present in the American one. In AWS A2.4, the top of the line is always the near side and information attached to the underside represents the far side. On these two lines (or two sides if a single line is used) symbols are placed representing the weld preparation on the near and, if appropriate, far side of the joint line. The arrow line can be at any angle (except 180O) and can point up or down. The arrow head must touch the drawn surfaces of the components to be joined at the location of the weld.

5-1

www.twitraining.com

Rev 4 February 2013 Weld Symbols Copyright  TWI Ltd 2013

5.3

Edge preparation symbols To the basic set-up of arrow and reference line, the design draughtsperson can apply the appropriate symbols for more complex situations. The symbols, in particular for arc and gas welding, are shown as simplified cross sectional representations of either a joint design or a completed weld, as shown below:

Supplementary symbols are added to the edge preparation to show the shape of the finished bead profile:

Aspects of welding not immediately apparent from the basic symbols can be added as complementary symbols:

5-2

111 www.twitraining.com

Rev 4 February 2013 Weld Symbols Copyright  TWI Ltd 2013

5.4

Weld sizing The correct size of weld can be applied so it is common to find numbers to the left or right of the symbol. For fillet welds numbers to the left of the symbol indicate design throat thickness, leg length, or both. Numbers to the right of the symbol show the length of the weld and where the welding is intermittent, the number of welds to be made in the location:

As per ISO 2553/EN 22553: a = Design throat thickness z = Leg length s = Penetration throat thickness

5-3

www.twitraining.com

Rev 4 February 2013 Weld Symbols Copyright  TWI Ltd 2013

The large Z symbol through the reference line to shows that intermittent weld beads are placed in a staggered arrangement on either side of the component. When there are no specific dimensional requirements specified on the weld symbol, it assumed that the requirement is for a full penetration, full length weld. Summary of information on symbols

5-4

www.twitraining.com

Rev 4 February 2013 Weld Symbols Copyright  TWI Ltd 2013

IWS Revision Questions on Weld Symbols 1 What is the symbol for: a Weld all round b Single bevel butt weld c Site weld. 2 Draw an indication for a fillet on the near side.

IWT Revisions Questions on Weld Symbols 3 What is the symbol for: a Plug weld b Concave on far side c Backing run 4 Draw an indication for intermittent concave fillets on both sides - 10 off, each 100mm long and staggered with 100mm between the weld elements.

5-5

www.twitraining.com

Section 6 Fusion Welding Principles

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

6

Fusion Welding Principles

6.1

Creation and protection of weld pool Fusion welding requires a source of heat sufficient to melt both parent plate and filler and a means of protecting the molten material from unwanted chemical reactions with the atmosphere. Heat may be provided by a flame, electric arc resistance power beam. Protection from reactions with oxygen and nitrogen in air may be by placing the pieces in a vacuum or controlled atmosphere or more usually by providing local cover from a shielding gas or flux. In some processes, such as flux-cored wire welding a combination of gas and flux may be used.

TIG welding.

MMA welding.

Welding flux operates in two ways to protect weld metal: It forms a gas around the arc that keeps air away from the pool and creates a slag that freezes (usually at a similar temperature to the metal) and protects the solidified, but still hot and reactive, metal from oxidation.

6-1

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

Gas shielding is usually provided by an inert gas, argon or helium, protecting the pool and adjacent hot metal from oxidation, but there is no protection for the still hot solid metal beyond the range of the gas flowing from the nozzle. A thin layer of oxide therefore often tarnishes MIG and TIG welds. Some metals, notably titanium, cannot accept exposure to air whilst hot, even if solidified, so require extra, trailing shields to provide gas coverage until the metal has dropped temperature considerably. Carbon and Carbon Manganese (C-Mn) steels do not oxidise rapidly so the protective gas can be active rather than inert, usually carbon dioxide or an Ar-CO2 mixture and the process is then referred to metal active gas (MAG).

6.2

Direction of welding When annual welding the torch is rarely held upright over the weld pool, it is usually inclined in the line of the welding direction, with the tip either pointing away from the previously deposited weld metal or towards it. For a right handed person, the usual method is to move the torch or electrode from right to left, with the torch/electrode pointing in the direction of travel. This is often referred to as the pushing technique and results in a fairly smooth weld profile. There are occasions where it is advantageous to weld in the opposite direction using a dragging technique, which gives deeper penetration but at the expense of a more convex weld profile. When using the oxy-acetylene process the movement is usually similar and is referred to as the leftward technique. However for oxy-acetylene pipe welding a technique known as all positional rightward may sometimes be used where the filler wire is fed into the weld behind the weld pool, allowing greater deposition (compared with leftward) but again at the expense of weld appearance, which will be coarser than a leftward weld.

Leftward and rightward welding (from BS 499-1:2009).

6.3

Bead shape If welding progresses directly in a straight line with no sideways movement, a stringer bead is laid.

6-2

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

The weld bead is the same width as the molten weld pool. If travel speed increases the weld pool will become elongated in the direction of travel and narrower in width and the resultant stringer bead will also be narrower. If the current is insufficient for the travel speed adopted, there may be only limited melting of the parent plate resulting in a bulbous cross-section bead and, in the extreme, lack of fusion.

Conversely, excessive current will lead to the pool being pushed into the surface of the plate and on freezing; grooves will be left at either side of the bead, termed undercut.

The welder can deliberately move the torch from side to side during the laying of a bead, called weaving

This has the advantage of dwelling at the edges of the bead giving more time to melt the parent plate. It can also achieve a better blend of the bead shape to the surface of the parent plate and can be used by a skilled welder to bridge larger than expected root gaps. It is for vertical up welding but care must be taken to keep the depth of bead to only a few millimetres. It is possible to use a wide, triangular weave technique when working in the vertical position; this should be exercised with caution as the very high heat input associated can cause deterioration of the mechanical properties of the parent material. It is often thought that blocking out is faster than using a stringer bead technique but this is incorrect. Deposition rate is controlled by the welding current or wire feed speed, not the movement of the torch. It is important to attempt to achieve a smooth profile change from the weld bead to the surface of the parent plate as sharp discontinuities create stress raisers from which defects such as hydrogen or fatigue cracks may initiate. 6-3

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

6.4

Electrical creation of an arc An electrically conductive path is required between the welding electrode which is connected to one pole of the power source and the parent plate connected to the other. In arc welding it is the establishment of a plasma between the two that allows the transfer of high current suitable for welding. The plasma is generated in TIG, MIG and plasma by ionisation of a shielding gas by the action of the arc. In the fluxed processes the arc first melts then ionises components of the flux to create a plasma. Welding arcs are sustained by direct current (DC) at around 20-30V transmitting currents of 80-400A. Depending on the process, operation outside of these ranges is possible, eg submerged arc may work with current up to 1000A but microplasma may use only 5A. It is also possible to stabilise an arc to run with alternating current (AC), useful for some MMA welding and for TIG welding of aluminium. The generation of these electrical characteristics will be dealt with in the section on power sources.

A DC arc has a fixed anode (positive pole) and cathode (negative pole). In TIG welding, the tungsten electrode is normally the cathode, termed DC electrode negative or DCEN, as more heat is generated at the anode giving more efficient melting of the weld pool. It is generally accepted that during current flow, the emission of electrons from the cathode has a cooling effect that helps to preserve the fine point of the tungsten electrode. For MIG welding the critical aspect is to melt and burn off the metal filler wire as efficiently as possible. Thus this process normally runs with DC electrode positive (DCEP) such that the wire is anodic and receives the greater proportion of heat. When TIG welding aluminium, DCEN is not successful as aluminium is very reactive and forms a tenacious, solid oxide over the molten pool. To disrupt this film current flow in the opposite direction is necessary such that electrons are emitted from the plate surface. This electron flow is sufficient to break up the oxide and is often referred to as a cleaning effect. However, continuous use of DCEP on TIG causes the tungsten to overheat and melt so AC current is used to give half cycle cleaning and half cycle cathodic 6-4

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

cooling of the tungsten. Thus the anode and cathode continuously exchange between the electrode and the parent plate. The ease of generation and maintenance of the arc depends on the ionisation potential of the shielding gas and on the work function (the ease with which ions can be produced) of the material (see section on arcs and plasmas). For fluxed processes, the composition of the flux is important in determining ease of arc initiation and subsequent arc stability. In MMA welding the core wire is brought into contact with the parent plate and the resultant spark starts to melt and ionise the flux thus giving a conductive path for welding current. Once commenced, the continuing dissociation of the flux components due to the very high temperature keeps the arc running in a stable manner. In submerged arc welding a small plug of wire wool held between the end of the filler wire and the parent metal is used to create a zone of high resistance. Flux is then poured over the assembly and current passed through the wire. The fusing of the wire wool commences melting and ionisation of the flux, after which progress is similar to MMA.

6.5

Creation of a molten pool by resistance heating Processes known as resistance welding, eg spot, seam and projection welding rely on the high resistance created at a metal interface for the generation of heat. When two sheets of steel are pressed together they touch only at microscopically fine points. If left with low pressure holding them together there are extremely few of these points. Application of an electric current through the sheets results in extremely high current density attempting to cross these very narrow bridges of contact. As heat is generated proportional to the square of current, very rapid melting of these points occurs and metal is ejected as spatter, or weld splash.

If the sheets are pressed together with excessive force, deformation can occur that spreads the surface into a very wide area of contact. Generation of resistance by such good contact can be limited leading to little heating effect and poor bonding. The aim is to hold parts together with force sufficient to allow significant melting of the interface without melting through to the top surface and without expelling quantities of metal in weld splash. 6-5

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

Electroslag welding also relies on resistance heating to create a weld pool, but in this case a flux bath is heated by the action of the welding current. A current-carrying filler wire or strip is fed into a flux bath, much as is the case for submerged arc welding. The initial start is with a wire wool plug as for SAW but, very soon after the initial ionisation and establishment of a current-carrying path, the molten flux pool extinguishes the arc. The flux bath is, however, conducting albeit with considerable resistance, so current continues to flow between the advancing filler and the parent plate. The heat in the flux bath melts the filler and effects transfer of the droplets to the weld pool.

6.6

Creation of a weld pool by a power beam The two principal power beams used in welding are the electron beam and the laser beams.

An electron beam is generated at a tungsten cathode according to the same principles as TIG welding. The EB gun has a surrounding anode with a small hole in it. Electrons are attracted to the anode but are travelling sufficiently quickly to pass straight through the hole. The gun and the column below it are under vacuum to avoid the electrons colliding with air molecules and electromagnets are positioned around the column to steer the electron beam onto the work piece placed in an evacuated chamber beneath the column.

6-6

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

The power of the beam is sufficient that when it hits the parent plate, it melts and vaporises the material, punching a hole right through the plate. As the workpiece is progressed beneath the beam, or the beam is rastered over the surface by the action of the magnets, the cooling metal condenses to form a weld pool that then rapidly solidifies. Light amplification by stimulated emission of radiation (laser) produces highly directional beams of photons possessing very high energy. These beams will pass through air or any other gas without absorption so no vacuum chamber is necessary. The action is very similar to EB; the impact with a metal parent plate creates a keyhole through the plate with condensation and solidification of the metal following as the beam advances.

6.7

Heat transfer The concentration of heat at the anode due to electron flow through the arc is not the only mechanism at work. The thermal conductivity of the shielding gas is also important and, as helium has greater thermal conductivity than argon, more heat is transferred to the metal and welders refer to the helium arc as being hotter. Hydrogen also has better thermal conductivity but the use of 5%H2 in Ar, for instance for improving the flow when welding nickel alloys, gives more energy for melting than would be expected from the conductivity effect alone. This is because hydrogen is a diatomic molecule (it is written H2 to show this) which dissociates in the arc and recombines at slightly less intense temperatures yielding additional energy. When fluxes are added into the mix the distribution of heat becomes very complex. very little has been written on the fundamentals of fluxed systems. Generally those fluxes bound with sodium silicate operate on DCEP and those with potassium silicate in the binder mix stabilise DCEN or AC but the heat distribution, and therefore deposition rate, do not follow the simple logic applied above to TIG and MIG. It would appear that the formulation of the coating has much to do with the final result. Welders often argue about the heat transfer merits of DCEN and DCEP for root runs and fill passes, but this is likely to be based on personal experience of particular electrodes rather than a general principle.

6.8

Weld pool shape Determining the factors affecting weld pool shape, studying their effects quantitatively and modelling the forces involved has kept researchers busy for years. No-one has yet a clear understanding of all the factors involved. Most work is conducted on autogenous TIG pools as these have fewer variables than those with metal transfer and fluxing. We aim to raise awareness of the complexity of the issues involved.

6-7

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

The initial melting of the parent plate to form a weld pool depends on the application of heat. In an arc welding process, this is transferred to the plate from the arc and plasma (see section on Arcs and Plasmas). Heat conduction into the metal plays a big part in the formation of a molten pool but is not the whole story. If it was the only factor, metals with high conductivity, such as pure aluminium, would always form small shallow pools whilst those of poor conduction would hold heat locally and create large molten pools. There is some reality in this effect but others usually mask it. Clearly convection is likely to play a part in heat transfer through a weld pool and the central region will be hotter than the outside of the pool leading to the creation of a radial, centre to outside convection current. But this is not the only force at work. An observation that may be made by the welder is that the surface of a weld pool is concave beneath the arc. This is evidence of the existence of a force from the electron bombardment – the arc force. Research work in 1980 measured this at one gram at 200A and found it to be proportional to the square of the current. So it is not a large force but may well have an effect on the weld pool shape by pushing the centre downwards. The amount of depression depends on the surface tension, which can vary. Likely to be of much greater effect is the magnetic stirring set up because the pool is a molten conductor carrying current. As noted earlier, flowing current sets up magnetic fields. Magnetic fields in turn create forces on the conductor, called Lorentz forces. The magnitude of the force is given by: F = QvB Where: F is the Lorentz force in Newtons Q is the electrical charge in Coulombs v is the velocity of the charge in m/sec B is the magnetic field in Teslas The direction of the force is given by the right hand rule. For this, you raise the thumb of your right hand and point the first finger at right angles to it. You can then hold the second finger bent to point in the third dimension at right angles to both thumb and first finger. Then, if the thumb is aligned with the current (+ to -), the first finger aligned with the direction of the magnetic field, the second finger will give the direction of the force acting on the conductor. Lorentz forces generate a through thickness stirring action, out from the centre and down the pool edges for DCEN operation.

6-8

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

Another phenomenon affecting weld pool formation is variation in surface tension. Surface tension normally decreases with increase in temperature and so the area under the anode spot, being the hottest, has the lowest surface tension on the weld pool. This creates conditions for the Marangoni Effect to take place. This is transportation of atoms under the influence of surface tension and in this case, results in radial, centre to outside circulatory motion through the thickness. Cut surface tension is affected by other factors. It has been shown that sulphur can reverse the temperature effect can create a negative Marangoni effect with the flow towards the centre. This was proposed as an explanation for cast-to-cast variation in penetration with autogeneous TIG. A negative Maragoni effect has also been cited as a contributor to the success of Activated Flux TIG (A-TIG) where the presence of flux changes the depth of penetration most markedly. Whilst this may not be the only effect from fluxing, it begins to hint at the difficulty of considering weld pool shape in a fluxed system. Conventional wisdom that more heat gives at the anode giving more melting does not predict weld pool generation accurately in fluxed systems. In MMA, one of the most noted deep penetration cellulosic electrodes, Lincoln’s Fleetweld 5P+, is recommended for DCEP operation. Furthermore all processes in which metal transfer takes places introduce a massive turbulence to the pool as each liquid filler particle arrives at its surface. There are so many factors in these systems that theory and modelling have yet to be developed sufficiently to predict behaviour.

6-9

www.twitraining.com

Rev 4 February 2013 Fusion Welding Principles Copyright  TWI Ltd 2013

IWS Revision Questions on Fusion Welding 1 What are the essential requirements for the establishment of a successful weld? 2 Describe stringer beads, weaving and blocking. 3 What is the effect of excess current? 4 List the general safety aspects required for welding.

IWT Revision Questions on Fusion Welding 5 How is the weld protected from the atmosphere in: a. Oxy-fuel gas welding b. Submerged arc welding c. MAG welding with CO2? 6 What is the difference between Lorentz force stirring and the Marangoni effect? 7 Describe the formation of a weld pool in electron beam welding. 8 How would you protect other shop floor workers working close to a welder?

6-11

www.twitraining.com

Section 7 Arc Welding Safety

Rev 4 February 2013 Arc Welding Safety Copyright  TWI Ltd 2013

7

Arc Welding Safety

7.1

Introduction Working in a safe manner, whether in the workshop or on site, is an important consideration in any welding operation. The responsibility for safety is on all individuals but especially for welders, not only for their own safety, but also to avoid endangering other people. The Welding Coordinator has an important function in ensuring that safe working legislation is in place and safe working practices are implemented. The Co-ordinator should ensure compliance with all appropriate documents, for example:      

Government legislation – The Health & Safety at Work Act. Health & Safety Executive – COSHH Regulations, Statutory instruments. British Standards – OHSAS 18001. Company Health and Safety Management Systems. Work instructions – permits to work, risk assessment documents, etc. Local Authority requirements.

There are many aspects of arc welding safety that the Co-ordinator needs to consider:       

Electric shock. Heat and light. Fumes and gases. Noise. Gas cylinder handling and storage. Working at height or in restricted access. Mechanical hazards: trips, falls, cuts, impact from heavy objects.

To find out if welders and other operatives are at risk the Co-ordinator needs to consider the working conditions. The Management of Health and Safety at Work Regulations 1999 require that employers assess the risks to health of employees arising from their work. The actions arising from the risk assessment are dictated by other more detailed regulations, eg the Control of Substances Hazardous to Health (COSHH) Regulations 2004. The following sections give guidance on risk avoidance but continuous effort on improvements to precautions and working conditions is essential for the wellbeing of the workforce.

7.2

Electric shock Contact with metal parts which are electrically live can cause injury or death because of the effect of the shock upon the body or because of a fall as a result of the reaction to electric shock.

7-1

www.twitraining.com

Rev 4 February 2013 Arc Welding Safety Copyright  TWI Ltd 2013

The electric shock hazard associated with arc welding may be either from the primary 230 or 460V mains supply or from the output voltage at 60-100V. Primary voltage shock is very hazardous because it is much greater than the secondary voltage of the welding equipment. Electric shock from the input voltage can occur by touching a lead inside the welding equipment with the power to the welder while the body or hand touches the welding equipment case or other earthed metal. Because of such hazards, only a qualified electrician should remove the casing of a welding power source. Residual circuit devices (RCDs) connected to circuit breakers of sufficient capacity will help protect personnel from the danger of primary electric shock. The transformed power is available from terminals on the front of the welding set. Heavy-duty cables are attached to these terminals to carry the welding current to the torch or electrode holder and to bring a return path from the work or metal workbench to the other terminal. This return is often referred to as the earth or ground and there may be secondary earthing arranged so that the work is at zero volts. Secondary voltage shock occurs when touching a part of the electrode circuit – perhaps the jaws of an MMA electrode holder or a damaged area on the electrode cable – while another part of the body touches the other side of the welding circuit (the work or welding earth) at the same time. Whilst most welding equipment is unlikely to exceed an OCV of 100V, electric shock, even at this level, can be serious. The welding circuit should be fitted with low voltage safety devices to minimise the potential of secondary electric shock. It is important that the welding cables can carry the maximum possible output of the welding set without overheating as overheating can damage the insulation, leading to an increased risk of electrical shock. TWI Job Knowledge No 29, available from the TWI website (www.twi.co.uk) gives more guidance on avoiding electric shock during welding.

7.3

Heat As arc welding relies on melting metal to effect a joint, it follows the metal will in part be very hot. All metals conduct heat to a greater or lesser degree so the area heated to a temperature that will cause skin burns is much larger than the weld bead itself. It is a wise precaution to assume that all metal on a welding workbench or adjacent to a site weld is hot. Temperature indicating sticks should be used to check that material is cool enough to handle. Patting metal with the bare hand to check its temperature is a surefire way of being burnt!

7-2

www.twitraining.com

Rev 4 February 2013 Arc Welding Safety Copyright  TWI Ltd 2013

The welding arc creates sparks, with the potential to cause flammable materials near the welding area to ignite and cause fires. The welding area should be clear of all combustible materials and is good practice for all personnel working in the vicinity of welding to know where the nearest fire extinguishers are and the correct type of fire extinguisher to use if a fire does break out. Welding may also produce spatter, globules of molten metal expelled from the weld area. These can cause serious burns, so protective clothing, such as welding gloves, flame retardant coveralls and leathers must be worn around any welding operation to protect against heat and sparks. It is most important that traps in clothing are avoided. Trousers should not have turnups and should not be tucked into boots – very serious injury can occur if spatter drops into the inside of a work boot. Radiant heat from welding can be quite intense, particularly when welding at high current and duty cycle is taking place. Sufficient air movement is required to keep the welder at a sensible temperature especially important when working in restricted access areas where reflected heat will intensify the effect. Welders should also take water regularly to avoid potential dehydration.

7.4

Light Light radiation is emitted by the welding arc in three principal ranges: Type Infra-red (heat) Visible light Ultra-violet radiation

7.4.1

Wavelength, nanometres >700 400-700 d tendency for surface cracks.

b

W < d tendency for centreline cracking.

c

W/d 3/2 giving sound welds.

c

Cracking can be a problem in root runs where dilution of parent plate into the weld is high giving excessive carbon content. Long and deep weld pools or welds made at high welding speeds or with high restraint and large gaps, accentuate the problem. Conversely, a combination of high arc voltage and slow welding speed can produce a mushroom-shaped weld bead with solidification cracks at the weld bead sides.

a In the root beads of a multi-run weld.

b Caused by high speed giving a long deep weld pool in first pass.

15-17

www.twitraining.com

Rev 4 February 2013 Submerged Arc Welding Copyright  TWI Ltd 2013

c Caused by high restraint and root gap.

Mushroom-shaped weld penetration resulting from high voltage combined with low speed.

Occasionally a groove may be found on the surface running along the centre of the weld which may be caused by shrinkage and is sometimes mistaken for incipient solidification cracking, but is only superficial.

15.16 Classification of consumables 15.16.1 BS EN system Of all the arc welding processes, only SAW uses two completely separate components, both may have a major effect on the mechanical properties of the weld deposit which makes specifying consumables somewhat complicated. The following covers the carbon, carbon-manganese and low alloy structural steels only. BS EN 756 is the specification for wires and wire/flux combinations in nonalloy and fine grain steels with a minimum yield strength of up to 500N/mm2 and covers the classification of wire chemical composition and the wire/flux combination. It also specifies the mechanical properties of all weld metal deposits in the as-welded condition. The classification is divided into five parts:  

Symbol indicating the process - for SAW this is S. Two digits indicating the tensile properties of either a multi-run deposit or the parent metal to be welded using a two run technique - see Tables 1 and 2.

15-18

www.twitraining.com

Rev 4 February 2013 Submerged Arc Welding Copyright  TWI Ltd 2013

Table 1. Symbols for tensile properties - multi-run technique. Symbol

Min. yield, N/mm 2

Min. UTS, N/mm 2

Min.elongation %

35

355

440-570

22

38

380

470-600

20

42

420

500-640

20

46

460

530-680

20

50

500

560-720

18

Table 2. Symbols for tensile properties - two-run technique. Symbol

Min yield parent metal, N/mm 2

Min tensile strength of welded joint, N/mm 2

-2T

275

370

-3T

355

470

-4T

420

520

-5T

500

600

Note: the two-run technique has two tensile results specified; one for the minimum yield strength of parent metal, one for the tensile strength of the welded joint. Table 3 gives the temperature at which the average Charpy V impact value of 47J may be achieved for both multi-run and two-pass techniques. The welding parameters for the test piece produced using a two-run technique must be within a range specified by the manufacturer. Table 3. Symbol for Charpy V impact properties Symbol

Temperature for min impact energy at, 47J°C

-Z

No requirements

-A

+20

-0

-0

-2

-20

-3

-30

-4

-40

-5

-50

-6

-60

-7

-70

-8

-80

15-19

www.twitraining.com

Rev 4 February 2013 Submerged Arc Welding Copyright  TWI Ltd 2013

Table 4 gives the symbols for the type of flux. Ten fluxes are listed identified by an abbreviation of the main constituents as below. Table 4. Flux type symbol. Flux type

Symbol

Manganese-silicate

MS

Calcium-silicate

CS

Zirconium-silicate

ZS

Rutile-silicate

RS

Aluminate-rutile

AR

Aluminate-basic

AB

Aluminate-silicate

AS

Aluminate-fluoride basic

AF

Fluoride-basic

FB

Any other type

Z

The final table contains a list of the chemical composition of 22 wires. The wires contain a maximum content of 0.15%C and range from plain C, through C-Mn, C-Mo, Mn-Mo to Ni and Ni-Mo. All are prefixed S followed by a number from 1-4 denoting from 0.5%Mn (1) to 2%Mn (4). The addition of nickel and/or molybdenum is denoted by the chemical symbol of the alloy addition being included. Thus an S3 wire contains 1.5%Mn, an S2Ni1Mo 1%Mn, 1%Ni and 0.5%Mo. The designation for a flux/wire combination designed to provide a multi-run weld metal with a minimum yield strength of 500N/mm2, minimum Charpy V impact value of 47J at -40°C using a Mn-Mo wire with an aluminate-basic flux would be BS EN 756 S 50 4 AB S4Mo. 15.16.2 AWS system As with the BS EN specifications for SAW consumables, the American Welding Society (AWS) system also uses a dual flux type/wire composition designation to identify the flux/wire combination that will provide the required properties. The AWS system is simpler than the BS EN but is described in four main specifications. ANSI/AWS A5.17 - Carbon steel electrodes and fluxes and ANSI/AWS A5.23 Low Alloy Steel Electrodes and Fluxes. The bare wire specifications are ANSI/AWS A5.9 Bare Stainless Steel Welding Electrodes and Rods and ANSI/AWS A5.Nickel and Nickel Alloy Bare Welding Electrodes and Rods.

15-20

www.twitraining.com

Rev 4 February 2013 Submerged Arc Welding Copyright  TWI Ltd 2013

In AWS A.5.17 the first part of the designation describes the flux type and can be up to six digits depending upon whether the flux is supplied with the tensile strength expressed in increments of 10 megapascals (two numbers where 43 represents 430MPa) or in pounds per square inch (1 digit ie 6 represents 60,000psi). The first character F identifies the consumable as a SAW flux, the next, S means the flux is made from or includes crushed slag. Omission of this letter indicates that the flux is unused and contains no crushed used flux introduced either by the flux manufacturer or the welding fabricator. The next one or two digits specify the minimum tensile strength as explained above and this is followed by A or P for whether the test results were obtained in the as-welded, (A condition) or postweld heat treated, (P condition). Digit

Test temperature °C

°F

Impact value, Joules

Z

No impact requirements

27

0

-18

0

27

2

-29

-20

27

4

-40

-40

27

5

-46

-50

27

6

-51

-60

27

8

-62

-80

27

The last digit identifies the minimum temperature at which a Charpy V impact value of 27J can be achieved as in this table. In AWS A5.17 there are a total of eleven wires, split into three groups of low, medium and high manganese. The first character, E, identifies the consumable as a bare wire electrode. If supplemented by C the wire is a composite (cored) electrode. The composition of the solid wire is obtained from an analysis of the wire. Since the composition of a cored wire may be

15-21

www.twitraining.com

Rev 4 February 2013 Submerged Arc Welding Copyright  TWI Ltd 2013

different from that of its weld deposit the composition must be determined from a low dilution weld deposit made using a specific, named flux. The next letter, L, M or H indicates a low (0.6% max), medium (1.4% max) or high (2.2% max) manganese content. This is followed by one or two digits that give the nominal carbon content. An optional K indicates a silicon killed steel. There are a final two or three optional digits identifying the diffusible hydrogen in ml/100g weld metal, H16, H8 or H4. A full designation for a carbon steel flux/wire combination could be F6P5EM12K-H8. This identifies a solid wire with a nominal 0.12% carbon, 1% manganese and 0.1-0.35% silicon capable of achieving an ultimate tensile strength of 60kpi (415MPa) and a Charpy V impact strength of 27J at -50°F (-46°C) in the post weld heat treated (PWAT) condition. The properties given by these designations are obtained from as-welded, all weld metal specimens deposited using standard welding parameters of current, voltage and travel speed. The properties achieved in a production weld may be entirely different due to the effects of dilution from the parent metal, higher or lower heat input, different wire diameters, preheat and interpass temperatures and PWAT. It is essential, that the suitability of a flux/wire combination is confirmed by procedure qualification testing. Note Flux/wire combinations supplied to the same specification designation by different manufacturers may not necessarily provide similar mechanical properties or weld cleanliness. Further reading A series of articles on SAW are available on TWI’s webste. (www.twi.co.uk/content/jk87.html; www.twi.co.uk/content/jk88.html; www.twi.co.uk/content/jk89.html)

15-22

www.twitraining.com

Rev 4 February 2013 Submerged Arc Welding Copyright  TWI Ltd 2013

IWS Questions on SAW 1 Describe the basics of the SAW process, including the use of different polarity power. 2 Describe the various types of flux and their typical use. 3 Why is travel speed an important variable? What problems may occur if it is not optimum?

IWT Questions on SAW and ESW 4 Describe the differences in fundamental operation between SAW and ESW. 5 What are the features of a basic agglomerated flux and how can it be used to help give high toughness weld metal in C-Mn steel? 6 What are the likely defects in SAW and how do you counter them?

15-23

www.twitraining.com

Section 16 Electroslag Welding

Rev 4 February 2013 Electroslag Welding Copyright  TWI Ltd 2013

16

Electroslag Welding

16.1

History Electroslag welding (ESW) is a very efficient, single pass process carried out in the vertical or near vertical position and used for joining steel plates/sections of 25mm and above. As with many welding processes, there is confusion over its invention. Many believe that a patent granted to Hopkins in the US in 1939 describes electroslag welding. He certainly had many patents for electroslag remelting but Uttrachi (www.netwelding.com/serv04.htm#Background Details) states that Hopkins’ 1939 patent for applying surfacing was based on a furnace rather than a welding heading head. What is clear is that electroslag was developed into a viable welding process by the Paton Institute in the Ukraine in the early 1950s and that a patent for the consumable guide variant was granted to Shrubsall in USA in 1957. The Paton Institute published a book entitled electroslag welding in 1959. In 1962 and 63 US patents were granted for electroslag consumable guide welding for joining rail track. Uttrachi points out that although the quality of weld metal in the railroad joints was superior to the alternative Thermite process, ESW did not catch on for this application as it took considerably longer to complete a weld. Time is of the essence when a train is due! The process was used very extensively in USA, for welding thick structural steel members in the 1960s and 70s. The Federal Highways Agency decided on the basis of laboratory tests that the very high heat input of ESW gave dangerously low toughness which led to a ban in the US of the use of ESW for many applications. The Northridge earthquake in 1994 gave a real life test to welds in highway bridges and structural steelwork. Repairs to self-shielded welds in structural steel cost over £1bn, but that not one ESW weld had required a repair. The FHA ban was rescinded in 2000.

16.2

Process characteristics Unlike other high current fusion processes, electroslag welding is not an arc process. Heat for melting the welding wire and plate edges is generated through the molten slag's resistance to the passage of an electric current. In its original form, plates are held vertically 30mm apart with the edges of the plate cut normal to the surface. A bridging run-on piece of the same thickness is attached to the bottom of the plates. Water-cooled copper shoes are placed each side of the joint, forming a rectangular cavity open at the top. Filler wire, which is also the current carrier, is fed into this cavity, initially striking an arc through a small amount of flux. Additional flux is added which melts forming a flux bath which rises and extinguishes the arc. The added wire then melts into this bath sinking to the bottom before solidifying to form the weld. For thick sections additional wires may be 16-1

www.twitraining.com

Rev 4 February 2013 Electroslag Welding Copyright  TWI Ltd 2013

added and an even distribution of weld metal is achieved by slowly oscillating the wires across the joint. As welding progresses, both the wire feed mechanism and the copper shoes are moved progressively upwards until the top of the weld is reached.

Figure.16.1. Electroslag welding.

The consumable guide variant of the process uses a much simpler set-up and equipment arrangement which does not require the wire feed mechanism to climb. The wire is delivered to the weld pool down a consumable, thick-walled tube which extends from the top of the joint to the weld pool. The original consumable guides were flux-covered which helped avoid any shorting onto the preparation sides and topped up the flux bath as material was lost by sticking to the copper shoes. This process was patented to the Linde Division of Union Carbide and subject to royalty payment, so alternatives were tried.

At TWI in the mid 1960s, experiments with bare guide tubes proved successful provided set-up was accurate so that the guide did not touch the wall during any part of its oscillation. One simple, cheap, guide tested consisted of four straight lengths of rod tacked together in a square format with sufficient space in the centre for the wire to be passed down it. This worked well if the gap was sufficiently wide but was prone to arcing onto the 16-2

www.twitraining.com

Rev 4 February 2013 Electroslag Welding Copyright  TWI Ltd 2013

side. Consumable guide ESW is often carried out without oscillation and the tubular guides can be further supplemented by additional consumable plates attached to the tube. As the thickness of plate increases, the number of wires/guides increases, approximately in the ratio of one wire per 50-75mm of thickness. Support for the molten bath is provided by two pairs of copper shoes which are moved upwards, leapfrogging each other as welding progresses. An operator is required to observe the flux bath and add more flux as the bath thins. The flux is very similar to submerged arc flux and is usually agglomerated. Slight changes in composition give the flux more fluidity so that it floods the initial start-up arc and extinguishes it. After that, heating and melting continue due to the resistive heating of the current flow through the molten flux bath.

16.3

ESW materials other than steel

16.3.1 Aluminium Uttrachi (www.netwelding.com/serv04.htm#Aluminum Electroslag) describes work at Union Carbide, Linde Division and latterly at WA Technology that demonstrated ESW being used on aluminium alloys. His narrative from the website is reproduced below.

The Consumable Guide Aluminum Electroslag Welding process was developed in the Laboratory and produced welds in 2 inch thick (50mm) and 4 inch thick (100mm) busbar material. Welds were made at a very rapid rate of vertical travel speed not possible with steel welding. A sample of a weld made with the process is shown on the left. Unfortunately the main application for the process was for joining heavy aluminum busbars. These are mostly employed in aluminum production facilities and the market for aluminum had significantly deteriorated. The development work was therefore terminated and the process was not commercialized. The demand for aluminum is now high and new plants are under construction. A company who works in the area asked if it were possible to weld over 10 inch thick by 4 foot high busbars by completing the early development work and extending it to these much thicker sections. After considerable additional development work and cost, refining the flux, welding parameters and equipment; the objective was achieved. The process was used on a production application over 10 inches thick with welds made at very high vertical travel speeds.

16-3

www.twitraining.com

Rev 4 February 2013 Electroslag Welding Copyright  TWI Ltd 2013

The photo left shows the equipment system welding a >10 inch thick section. The center photo is the finished weld. Welding speeds were very high, much higher than in steel welding. Weld surface is excellent. The photo right is a cross section showing good fusion and defect free weld. 16.3.2 Titanium A team working with Eager of MIT demonstrated the feasibility of ESW welding thick Ti-6Al-4V alloy. They used a consumable guide technique as described in a research paper published online at www.eagar.mit.edu/EagarPapers/Eagar089.pdf. In this paper they refer to early work (1957, 1962 and 1968) in USSR that developed the principle. The team showed that pure calcium fluoride was needed as flux and that this must be kept free from moisture. They found that AC power was necessary but reported the successful completion of welds in both 25 and 50mm plate. 16.3.3 Stainless steel and nickel alloys The Paton Institute in Kiev welded many materials by ESW during the early years of development of the process. Reference can be found to the possibility of welding both austenitic stainless steel and nickel alloys but there are no examples of its use commercially other than as a surfacing technique.

16.4

Current status Electroslag welding is not a major welding process because the high heat input generates large, coarse grains in the weld metal and HAZ that lead to poor fracture toughness properties in these areas. Toughness improvements can only be achieved by post-weld normalising treatment. Additionally, the near parallel-sided geometry of the weld, combined with the coarse grains, can make it difficult to identify defects at the fusion boundary by standard ultrasonic NDT techniques. Considerable interest was shown in electro slag welding (ESW) during the 1970s when ideas for increasing welding speed, such as narrow gap welding, were investigated. This was seen as an important parameter for increasing productivity and reducing heat input to improve HAZ and weld metal impact properties. 16-4

www.twitraining.com

Rev 4 February 2013 Electroslag Welding Copyright  TWI Ltd 2013

Since then little development has been done. Developments have been limited to the tuning of parameters and tailoring techniques for specific applications. ESW has considerable potential for increasing productivity, but its use has been limited because of relatively poor understanding of the process and for specific applications the significance of the fracture toughness values. As a result use of the process has been restricted to a few niche applications. In the fabrication industry, the process continues to be used for thick walled pressure vessels which are post-weld normalised and for structures such as blast furnace shells and steel ladles used at above ambient temperatures. The process is extensively used for welding railway points. It is most commonly used now with strip electrode as a surfacing technique is described in more detail in the section on surfacing.

16.5

Benefits and disadvantages The principal benefits of the process are:       

Speed of joint completion; typically 1 hour per metre of seam, irrespective of thickness. Lack of angular distortion. Lateral angular distortion limited to 3mm per metre of weld. High quality welds produced. Simple joint preparation, i.e. flame-cut square edge. Major repairs can be made simply by cutting out total weld and rewelding. Can be modified for use as a cladding technique.

The main disadvantages are:   

Grain growth giving very large grains due to very high heat input and slow cooling giving poor toughness. Process is limited to vertical or near vertical position. Difficult to examine with NDT.

16-5

www.twitraining.com

Section 17 Thermal Cutting and Gouging

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

17

Thermal Cutting and Gouging

17.1

Introduction Thermal cutting normally refers to the severing of metal, creating two pieces or a specific shaped single piece. Gouging is a particular form of cutting where the aim is to remove metal in a controlled manner to leave a groove that can act as the basis of weld preparation. In terms of the process and the fundamental principles, they are the same; only the details of the torch and parameters vary. Thermal cutting and gouging are essential parts of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected - usually by blowing it away. Flame, laser or arc processes can be used to produce rapid melting and metal removal. Thermal processes, operations and metals which may be gouged or otherwise shaped: Process operations Thermal process

Metals Primary

Secondary

Oxy fuel gas flame

Cutting Grooving Washing Gouging Chamfering

Manual metal arc

Gouging

Grooving Chamfering

Air carbon arc

Gouging

Grooving Chamfering

Plasma arc

Cutting Chamfering Grooving Gouging Washing

Laser

Cutting

Chamfering Drilling

Ferritic steels, cast iron Ferritic steels, stainless steels, cast iron, nickelbased alloys

Ferritic steels, cast iron, nickel-based alloys, copper and copper alloys, copper/nickel alloys, aluminium Ferritic steels, aluminium, stainless steels

Ferritic steels, aluminium, stainless steels and may other alloys and non metallics

Note: All processes are capable of cutting/severing operations. Preheat may or may not be required on some metals prior to gouging

General safety It should be emphasised that because cutting and gouging rely on molten metal being forcibly ejected, often over quite large distances, the operator must take appropriate precautions to protect himself, other workers and his equipment. Sensible precautions include protective clothing for the operator,

17-1

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

shielding inside a specially-enclosed booth or screens, adequate fume extraction and removal of all combustible material from the immediate area.

Gouging applications Thermal gouging was developed primarily for removal of metal from the reverse side of welded joints, removal of tack welds, temporary welds and weld imperfections.

Typical back-gouging applications carried out on arc welded joints.

Imperfection removal in preparation for weld repair.

Applications include:   

Repair and maintenance of structures - bridges, earthmoving equipment, mining machinery, railway rolling stock, ships, offshore rigs, piping and storage tanks. Removal of cracks and imperfections - blow holes and sand traps in both ferrous and non-ferrous forgings and castings. Preparation of plate edges for welding.

17-2

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

 

17.2

Removal of surplus metal - riser pads and fins on castings, excess weld bead profiles, temporary backing strips, rivet washing and shaping operations, demolition of welded and unwelded structures - site work. Removal of temporary welded attachments such as brackets, strongbacks, lifting lugs and redundant tack welds, during various stages of fabrication and construction work.

Oxyfuel cutting The oxyfuel process is the most widely applied industrial thermal cutting process because it can cut thicknesses 0.5-250mm and the equipment is low cost and can be used manually or mechanised. Several fuel gas and nozzle design options can significantly enhance performance in terms of cut quality and cutting speed.

Process fundamentals Basically a mixture of oxygen and fuel gas is used to preheat the metal to its 'ignition' temperature which, for steel, is 700-900°C (bright red heat) but well below its melting point. A jet of pure oxygen is then directed into the preheated area instigating a vigorous exothermic chemical reaction between the oxygen and the metal to form iron oxide or slag. The oxygen jet blows away the slag enabling the jet to pierce through the material and continue to cut through the material.

17-3

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

There are four basic requirements for oxyfuel cutting:    

Ignition temperature of the material must be lower than its melting point otherwise the material would melt and flow away before cutting could take place. The oxide melting point must be lower than of the surrounding material so that it can be mechanically blown away by the oxygen jet. The oxidation reaction between the oxygen jet and the metal must be sufficient to maintain the ignition temperature, ie sufficient amount of iron must be present in the steel. A minimum of gaseous reaction products should be produced so as not to dilute the cutting oxygen.

As stainless steel, cast iron and non-ferrous metals form refractory oxides, ie the oxide melting point is higher than the material and powder must be injected into the flame to form a low melting point, fluid slag. It should be noted that as the ignition temperature needs to be reached before the exothermic reaction can take place, laminated or stacked materials cannot be cut unless they are in very close contact with each other. Preheating The preheating flame has the following functions in the cutting operation:    

Raises the temperature of the steel to the ignition point. Adds heat energy to the work to maintain the cutting reaction. Provides a protective shield between the cutting oxygen stream and the atmosphere. Dislodges from the upper surface of the steel any rust, scale, paint or other foreign substance that would stop or retard the normal forward progress of the cutting action.

Purity of oxygen The cutting speed and cut edge quality are primarily determined by the purity of the oxygen stream so nozzle design plays a significant role in protecting the oxygen stream from air entrainment. The purity of oxygen should be ≥99.5%. A decrease in purity of 1% will typically reduces the cutting speed by 25% and increases gas consumption by 25%. Choice of fuel gas Fuel gas combustion occurs in two distinct zones. In the inner cone or primary flame, the fuel gas combines with oxygen to form carbon monoxide and hydrogen which for acetylene, the reaction is given by: 2C2H2+2O2

4CO+2H2

Combustion also continues in the secondary or outer zone of the flame with oxygen from the air.

17-4

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

4CO+2H2+3O2

4CO2+2H2O

Fuel gases are characterised by:   

Flame temperature - the hottest part of the flame is at the tip of the primary flame (inner cone). Fuel gas to oxygen ratio - the amount of fuel gas required for combustion varies according to whether the flame is neutral, oxidising or reducing. Heat of combustion - greater in the outer part of the flame.

The five most commonly used fuel gases are acetylene, propane, MAPP (methylacetylene-propadiene), propylene and natural gas and their properties are given in the table. The relative performance of the fuel gases in terms of pierce time, cutting speed and cut edge quality, is determined by flame temperature and heat distribution within the inner and outer flame cones. Heat distribution, Maximum flame temperature °C

Oxygen to fuel gas ratio, (vol)

kJ/m3

Primary

Secondary

Acetylene

3160

1.2:1

18,890

35,882

Propane

2810

4.3:1

10,433

85,325

MAPP

2927

3.3:1

15,445

56,431

Propylene

2872

3.7:1

16,000

72,000

Natural gas

2770

1.8:1

1,490

35,770

Fuel gas

Acetylene Produces the highest flame temperature of all fuel gases with maximum flame temperature (in oxygen) approximately 3160°C compared with a maximum temperature of 2810°C with propane. The hotter flame produces more rapid piercing of the materials with the pierce time being typically one third that produced with propane. The higher flame speed (7.4m/s compared with 3.3m/s for propane) and the higher calorific value of the primary flame (inner cone) (18,890kJ/m3 compared with 10,433kJ/m3 for propane) produce a more intense flame at the surface of the metal reducing the width of the HAZ and degree of distortion.

17-5

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Propane Produces a lower flame temperature than acetylene and has greater total heat of combustion than acetylene but the heat is generated mostly in the outer cone. The characteristic appearance for acetylene and propane flames are shown below where the propane flame appears less focused. Consequently, piercing is much slower but as the burning and slag formation is effected by the oxygen jet, cutting speeds is about the same as for acetylene. Propane has a greater stoichiometric oxygen requirement than acetylene; for the maximum flame temperature in oxygen, the ratio of volume of oxygen to fuel gas are 1.2:1 for acetylene and 4.3:1 for propane.

Oxyacetylene gas jet and nozzle design.

Propane gas jet and nozzle design.

17-6

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

MAPP Gas mixture various hydrocarbons, principally, methylacetylene and propadiene and produces a relatively hot flame (2976°C) with a high heat release in the primary flame (inner cone) (15,445kJ/m3), less than for acetylene (18,890Kjm3) but much higher than for propane (10,433kjm3). The secondary flame (outer cone) gives off a high heat release, similar to propane and natural gas. The combination of lower flame temperature, more distributed heat source and larger gas flows compared with acetylene results in a substantially slower pierce time. MAPP gas can be used at higher pressure than acetylene so can be used for underwater cutting in deep water as it is less likely to dissociate into its components of carbon and hydrogen which are explosive. Propylene A liquid petroleum gas (LPG) product with a similar flame temperature to MAPP (2896°C compared with 2976°C for MAPP); it is hotter than propane, but not as hot as acetylene and gives off a high heat release in the outer cone (72,000kJ/m3) but, like propane, it has the disadvantage of a high stoichiometric fuel gas requirement (oxygen to oxygen ratio of approximately 3.7:1 by volume). Natural gas Lowest flame temperature (similar to propane) and lowest total heat value of the commonly used fuel gases, eg for the inner flame 1,490kJ/m3 compared with 18,890kJ/m3 for acetylene so is the slowest for piercing. Selection of fuel gas Factors to be considered when selecting a fuel gas include:  Time required for preheating when starting cuts.  Effect on cutting speed and productivity.  Cost and availability.  Volume of oxygen required per volume of fuel gas to obtain a neutral flame.  Safety in transporting and handling. Fuel gas characteristics and their applications: Fuel gas

Main characteristics

Applications

Acetylene

Highly focused, high temperature flame Rapid preheating and piercing Low oxygen requirement

Rapid cutting of thin plates Bevel cuts Short, multi-pierce cuts

Propane

Low temperature flame, high heat content Slow preheating and piercing High oxygen requirement

Cutting of thicker sections (100-300mm), long cuts

MAPP

Medium temperature flame

Cutting underwater

Propylene

Medium temperature flame

Cutting of thicker sections

Natural gas

Low temperature flame

Cutting of thicker sections

17-7

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Cutting quality Generally, oxyfuel cuts are characterised by:    

Large kerf (1mm).

The face of a satisfactory cut has a sharp top edge, drag lines, little oxide, sharp bottom edge and on underside free of slag.

Satisfactory cut in the centre. Cut too slow (left) the top edge is melted and there are deep grooves in the lower portion of the face, scaling is heavy and the bottom edge may be rough, with adherent dross. Cut too fast (right); Appearance similar with an irregular cut edge. Plate thickness 12mm.

17-8

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

With a very fast travel speed the drag lines are coarse and at an angle to the surface with an excessive amount of slag sticking to the bottom edge of the plate due to the oxygen jet trailing with insufficient oxygen reaching the bottom of the cut.

Satisfactory cut in the centre. Preheating flame too low (left): most noticeable effect on the cut edge is deep gouges in the lower part of the cut face. Preheating flame too high (right): Top edge is melted, cut irregular and there is an excess of adherent dross. Plate thickness 12mm.

Satisfactory cut in the centre. Blowpipe nozzle too high above the work (left): Excessive melting of the top edge occurs with much oxide. Torch travel speed irregular (right): Uneven spacing of drag lines can be observed together with an irregular bottom surface and adherent oxide. Plate thickness 12mm.

17-9

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Advantages of oxyfuel cutting  Steels can generally be cut faster than by most machining methods.  Section shapes and thicknesses difficult to produce by mechanical means can be cut economically by oxyfuel cutting.  Basic equipment costs are low compared with machine tools.  Manual equipment  very portable and can be used on site.  Cutting direction can be changed rapidly on a small radius.  Large plates can be cut rapidly in place by moving the torch rather than the plate.  An economical method of plate edge preparation. Disadvantages of oxyfuel cutting  Dimensional tolerances significantly poorer than machine tool capabilities.  Essentially limited to cutting carbon and low alloy steels.  Preheat flame and expelled red hot slag present fire and burn hazards to plant and personnel.  Fuel combustion and oxidation of the metal require proper fume control and adequate ventilation.  Hardenable steels may require pre- and/or post-heat adjacent to the cut edges to control their metallurgical structures and mechanical properties.  Special process modifications are needed for cutting high alloy steels and cast irons (ie iron powder or flux addition).  Being a thermal process, expansion and shrinkage of the components during and after cutting must be taken into consideration.

17.3

Powder cutting Is oxygen cutting in which a suitable powder is injected into the cutting oxygen stream to assist the cutting action (definition from BS499: Part 1:1991 Section 7 No.72 008). Mild steels readily ignite in a stream of oxygen when they are heated to 700900°C, but for stainless steels, the ignition temperature is over 1500°C. The oxides formed when cutting mild steel have lower melting points than the parent metal and this facilitates a clean cut. With stainless steel, the oxide has a higher melting point than the parent metal so hampers the cutting process. These barriers to cutting can be overcome by adding materials to the cutting gas stream which either remove the oxide film or raise the reaction temperature:  

Flux injection into the cutting gas stream which chemically removes the oxides of chromium. Finely divided iron-rich powder fed separately into the cutting zone in a gaseous medium. Combustion of the iron powder increases the reaction temperature and the fluidity of oxidation products.

17-10

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

The iron-rich powder injection technique has been used for cutting copper, nickel and aluminium and their alloys and for cutting cast irons. The quality of the cut surface is, at best, equivalent to flame cut carbon steel; but with many materials, the cut quality is very poor.

17.4

Oxyfuel gouging

Oxyfuel or flame gouging offers a quick and efficient method of removing metal, principally ferritic steel and can be at least four times quicker than cold chipping operations. Particularly attractive because of its low noise, ease of handling and ability to be used in all positions. Process description Flame gouging is a variant of conventional oxyfuel gas cutting. Oxygen and a fuel gas are used to produce a high temperature flame for melting the steel. When gouging, the steel is locally heated to a temperature above the 'ignition' temperature (typically 700-900°C) and a jet of oxygen melts the metal - a chemical reaction between pure oxygen and hot iron. This jet also blows away molten metal and slag. Compared with oxyfuel cutting, slag is not blown through the material, but remains on the top surface of the workpiece. The gouging nozzle is designed to supply a relatively large volume of oxygen through the gouging jet, as much as 300 litre/min through a 6mm orifice. In oxy-acetylene gouging, equal quantities of oxygen and acetylene are used to set a near-neutral preheating flame. The oxygen jet flow rate determines the depth and width of the gouge. Typical operating parameters for achieving a range of gouge sizes are: Nozzle orifice dia, mm

Gouge dimensions, mm

Gas pressure,

Gas consumption,

Widt h

Depth

Acetylen e

Oxygen

Acetylene

02 preheat

02 gouge

3

6-8

3-9

0.48

4.2

15

22

62

600

5

8-10

6-12

0.48

5.2

29

31

158

1000

6.5

10-13

10-13

0.55

5.5

36

43

276

1200

Travel speed, mm/mi n

17-11

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its surfaces are smooth with a dull blue colour. Operating techniques The depth of the gouge is determined principally by the speed and angle of the torch. To cut a deep groove the angle of the torch is stepped up (increases the impingement angle of the oxygen jet) and gouging speed reduced. To produce a shallow groove, the torch is less steeply angled and speed increased. Wide grooves can be produced by weaving the torch. The contour of the groove is dependent on the size of the nozzle and operating parameters. If cutting oxygen pressure is too low, gouging progresses with a washing action, leaving smooth ripples in the bottom of the groove. If the cutting oxygen pressure is too high, the cut advances ahead of the molten pool, disrupting the gouging operation especially when making shallow grooves. Four basic flame gouging techniques are used in the following types of application. Progressive gouging Produces uniform grooves and is conducted in either a continuous or progressive manner. Applications include removal of an unfused root area on the reverse side of a welded joint, part-shaping a steel forging, complete removal of a weld deposit and preparing plate edges for welding.

Spot gouging Produces a deep narrow U-shaped groove over a relatively short length ideally suited to removal of localised areas such as isolated weld imperfections. Experienced operators are able to observe any imperfections during gouging, which appear as dark or light spots/streaks within the molten pool (reaction zone). Back-step gouging Once the material has reached ignition temperature, the oxygen stream is introduced and the torch moved in a backward movement for 15-20mm. The oxygen is shut off and the torch moved forwards 25-30mm before restarting the gouging operation. Favoured for removal of local imperfections which may be deeply embedded in the base plate.

17-12

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Deep gouging It is sometimes necessary to produce a long deep gouge, operations the deep gouging technique is used which is basically a combination of progressive and spot gouging.

17.5

MMA gouging Operates in the same way as the welding process–an arc is formed between the tip of the electrode and workpiece. As only the arc force ejects metal, it requires special purpose electrodes with thick flux coatings to generate a sufficiently strong arc force and gas stream. Unlike MMA welding where a stable weld pool must be maintained, this process forces the molten metal away from the arc zone to leave a clean cut surface. Cutting thin material can be achieved with these electrodes but is not very satisfactory, leaving a very ragged edge. The gouging process is characterised by the large amount of gas generated to eject the molten metal but as the arc/gas stream is not as powerful as a gas or separate air jet, the surface of the gouge is not as smooth as an oxyfuel gouge or air carbon arc gouge.

Although DCEN is preferred, an AC constant current power source can also be used. MMA gouging is used for localised gouging operations, removal of defects for example and where it is more convenient to switch from a welding electrode to a gouging electrode rather than use specialised equipment. Compared with alternative gouging processes, metal removal rates are low and the quality of the gouged surface is inferior. When correctly applied, MMA gouging can produce relatively clean gouged surfaces. For general applications, welding can be carried out without the need to dress by grinding. When gouging stainless steel, a thin layer of higher carbon content material will be produced which should be removed by grinding.

17-13

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

The main advantage of MMA gouging is that the same power source can be used for welding, gouging or cutting, simply by changing the type of electrode. Grooving electrodes, though based on mild steel core wires, are not just restricted to steels: the same electrode composition may be used for gouging stainless steel and non-ferrous alloys, in which case the cut surface must be ground after the gouging operation has been completed. Power source MMA gouging can be carried out using conventional DC and AC power sources. In DC gouging, electrode polarity is normally negative but electrode manufacturers may well recommend electrode polarity for their brand of electrodes and for gouging specific materials. When using an AC power source, a minimum of 70V open circuit (OCV) is required to stabilise the arc. Most MMA welding power sources can be used for gouging but the current rating and OCV must be capable of accommodating current surges and longer arc lengths. Electrode diameter, mm

Current, A

3.2

Gouging dimensions, mm Depth

Width

Gouging speed, mm/min

210

2

6

1200

4.0

300

3

8

1000

4.8

350

4

10

800

Operational characteristics The arc is struck with an electrode held at a normal angle to the workpiece (15 degrees backwards from the vertical plane in line with proposed direction of gouging). Once the arc is established, the electrode is immediately inclined in one smooth and continuous movement to an angle of 15-20 degrees to the plate surface. With the arc pointing in the direction of travel, the electrode is pushed forward slightly to melt the metal, then pulled back to allow the gas jet to displace the molten metal and slag. This forward and backward motion is repeated as the electrode is guided along the line to complete the gouge. To produce a consistent depth and width of gouge, a uniform rate of travel must be maintained, together with the angle of electrode: 10-20 degrees. If the electrode angle becomes too steep, in excess of about 20 degrees, the amount of slag and molten metal will increase, a result of the arc penetrating too deeply. Digging the electrode into the metal causes problems in controlling the gouging operation and will produce a rough surface profile. For gouging in positions other than vertical, the electrode is always pushed forward. With vertical surfaces, the electrode is directed and pushed vertically downwards.

17-14

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Application When correctly applied MMA gouging can produce relatively clean gouged surfaces. For general applications, welding can be carried out without the need to dress by grinding but when gouging stainless steel, a thin layer of higher carbon content material will be produced which this should be removed by grinding.

17.6

Air carbon arc gouging An electric arc is generated between the tip of a carbon electrode and the workpiece, the metal becomes molten and a high velocity air jet streams down the electrode to blow it away, thus leaving a clean groove. The process is simple to apply (same equipment as MMA welding), has a high metal removal rate and gouge profile can be closely controlled.

As air carbon arc gouging does not rely on oxidation it can be applied to a wide range of metals. DCEP is normally preferred for steel and stainless steel but AC is more effective for cast iron, copper and nickel alloys. Typical applications include back-gouging, removal of surface and internal defects, removal of excess weld metal and preparation of bevel edges for welding. Electrode A graphite (carbon) rod with a copper coating to reduce electrode erosion. Electrode diameter is selected according to required depth and width of gouge. Cutting can be precisely controlled and molten metal/dross is kept to a minimum. Power source A DC power supply with electrode positive polarity is most suitable. AC power sources which are also constant current can be used but with special AC type electrodes. The power source must have a constant current output characteristic. If it does not, inadvertent touching of the electrode to the workpiece will cause a high current surge sufficient to explode the electrode tip which will disrupt the operation and cause carbon pick-up. As arc voltage can be quite high (up to 50V), open circuit voltage (OCV) of the power source should be over 60V.

17-15

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Air supply The gouging torch is normally operated with either a compressed air line or separate bottled gas supply. Air supply pressure will be up to 100psi from the air line but restricted to about 35psi from a bottled supply. Providing there is sufficient air flow to remove molten metal, there are no advantages in using higher pressure and flow rates. Carbon pick-up Although carbon is picked up by the molten metal, the air stream will remove carbon-rich metal from the groove to leave only minimal contamination of the sidewalls. Poor gouging technique or insufficient air flow will result in carbon pick-up with the risk of metallurgical problems, eg high hardness and even cracking. Operation Typical operating data for air carbon arc gouging: Gouging dimensions, mm Width

Carbon electrode consumed, mm/min

Gouging speed, mm/min

6-7

9-10

120

609

350

7-8

10-11

114

711

9.5

425

9-10

12-13

100

660

13.0

550

12-13

18-19

76

508

8.0

300-400

2-9

3-8

100

1650-840

9.5

500

3-12

3-10

142

1650-635

13.0

850

3-15

3-13

82

1830-610

16.0

1250

3-19

3-16

63

1830-710

Current A Note DC electrode

Depth

6.4

275

8.0

Electrode diameter, mm

Manual

Automatic

17-16

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

For effective metal removal it is important that the air stream is directed at the arc from behind the electrode and sweeps under the tip of the electrode. The width of groove is determined by the diameter of electrode, but depth is dictated by the angle of electrode to the workpiece and rate of travel. Relatively high travel speeds are possible when a low electrode angle is used which produce a shallow groove; a steep angle results in a deep groove and requires slower travel speed. Note: A steeply angled electrode may give rise to carbon contamination.

Oscillating the electrode in a circular or restricted weave motion during gouging can greatly increase gouging width, useful for removal of a weld or plate imperfection that is wider than the electrode itself. The groove surface should be relatively free of oxidised metal and can be considered ready for welding without further preparation but grinding should be carried out if a carbon rich layer has been formed. Dressing may be necessary if working on crack-sensitive material such as high strength, low alloy steel. Advantages  Fast, approximately five times faster than chipping.  Easily controllable, removes defects with precision. Defects are clearly visible and may be followed with ease. Depth of cut is easily regulated and slag does not deflect or hamper the cutting action.  Low equipment cost, no gas cylinders or regulators are necessary except on site.  Economical to operate no oxygen or fuel gas required. The welder may also do the gouging (no qualification requirements for this operation, although adequate training should always be given).  Easy to operate, the equipment similar to MMA except the torch and air supply hose.  Compact, torch is not much larger than an MMA electrode holder, allowing work in confined areas.  Versatile.  Can be automated.

17-17

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Disadvantages  Air jet causes the molten metal to be ejected over quite a large area?  Because of high currents (up to 2000A) and high air pressures (80-100 psi), it can be very noisy.  Other cutting processes usually produce a better cut.  Requires large volume of compressed air.  Increases the carbon content leading to an increase in hardness in the case of cast iron and hardenable metals. In stainless steels it can lead to carbide precipitation and sensitisation. So grinding the carburised layer usually follows gouging.  Introduces hazards such as fire (due to discharge of sparks and molten metal), fumes, noise and intense light.

17.7

Plasma arc cutting

Plasma arc cutting uses essentially the same torch as plasma welding, was described in the chapter on the subject. In cutting the constricted arc issuing from the plasma orifice develops a high velocity jet of ionised gas that blows the melted metal away. A pilot arc is struck between a tungsten electrode and a water-cooled nozzle. In the transferred arc variant, a stronger arc is then developed to the workpiece, being constricted by the orifice in the nozzle. As plasma gas passes through this arc, it is heated rapidly to a temperature in excess of 20,000°C which causes huge expansion of the gas which is accelerated to near the speed of sound as it passes through the constricting orifice towards the workpiece. As the arc melts the workpiece, the high velocity jet blows away the molten metal. Where materials are electrical insulators, the nontransferred arc method is used where the arc remains within the torch as in the initial, pilot stage of the transferred arc method and the plasma jet stream travels toward the workpiece.

17-18

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Transferred arc

Non-transferred arc

Plasma arc cutting is seen as an alternative to the oxyfuel process but the important difference between the two is that while the oxygenfuel process oxidises the metal and the heat from the exothermic reaction melts the metal, the plasma process operates by using the heat from the arc to melt the metal. The ability to melt the metal without oxidation is essential when cutting metals, such as stainless steel, which form high temperature oxides and the plasma process was introduced for cutting stainless steel and aluminium alloys. The first plasma torches gave poor quality cuts and the process suffered from excessive noise and fume, especially when cutting thicker material. Over the last thirty years, it has been highly refined and is now capable of producing high quality cuts, at increased speeds, in a wide range of material thicknesses. Power source The plasma arc process power sourcemust have a drooping characteristic and a high voltage. Although the operating voltage to sustain the plasma is typically 50-60V, the OCV to initiate the arc can be up to 400V DC. On initiation, a pilot arc is formed within the body of the torch between the electrode and the nozzle. For cutting metals, the arc should be transferred to the workpiece in the so-called 'transferred' arc mode. The electrode is negative and the workpiece positive so that the majority (approximately ⅔) of arc energy is used for cutting. Gas composition In the conventional system using a tungsten electrode, the plasma is inert, formed using Ar, Ar-H2 or N2. However, as described in process variants, oxidising gases, such as air or O2, can be used but the electrode must be copper with a hafnium tip.

17-19

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

The plasma gas flow is critical and must be set according to the current level and the nozzle bore diameter. If the gas flow is too low for the current level, or the current level too high for the nozzle bore diameter, the arc will break down forming two arcs in series, electrode to nozzle and nozzle to workpiece. Double arcing is usually catastrophic with the nozzle melting. Cut quality Plasma cut quality is similar to the oxyfuel process. As the plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of the metal resulting in top edge rounding, poor edge squareness or a bevel on the cut edge. These limitations are associated with the degree of constriction of the arc, so several torch designs are available to improve this to produce more uniform heating at the top and bottom of the cut. Process variants Dual gas

The process operates in the same manner as the conventional system but a secondary gas shield is introduced around the nozzle. The benefits are increased arc constriction and more effective 'blowing away' of the dross. The plasma forming gas is normally Ar, Ar-H2 or N2 and the secondary gas is selected according to the metal being cut:   

Ferritic steel – air, O2, N2. Stainless steel – N2, Ar-H2, CO2. Aluminium – Ar-H2, N2-CO2.

The advantages compared with conventional plasma are:   

Reduced risk of double arcing.' Higher cutting speeds. Reduction in top edge rounding.

17-20

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Water injection Nitrogen is normally used as the plasma gas. Water is injected radially into the plasma arc to induce a greater degree of constriction. The temperature of the plasma is considerably increased (30,000°C) so higher cutting speeds and because of the greater constriction of the arc there is a much improved cut quality. The presence of an annular film of water around the plasma also protects the nozzle bore, reducing erosion.

The advantages compared with conventional plasma are:    

Improvement in cut quality and squareness of cut. Increased cutting speeds. Less risk of 'double arcing.' Reduction in nozzle erosion.

Water shroud The plasma can be operated with a water shroud or with the workpiece submerged 50-75mm below the surface water. The water acts as a barrier in reducing fume and noise levels. Noise levels at high current levels in excess of 115dB, can be reduced to about 96dB with a water shroud and 52 to 85dB when cutting underwater.

As the water shroud does not increase the degree of constriction, squareness of the cut edge and cutting speed are not noticeably improved.

17-21

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Air plasma The inert plasma forming gas (Ar or N2) can be replaced with air but requires a special electrode of hafnium or zirconium mounted in a copper holder. Air can replace water for cooling the torch and the use of compressed air instead of more expensive cylinder gas, makes it variant highly competitive with the oxyfuel process. A variant is the monogas torch in which air is used for both the plasma and cooling gas. Air plasma is widely applied in light engineering industries, eg cutting sheet steel of thickness 1-20mm. It is most often used on C-Mn and stainless steels but will also cut SG cast iron and non-ferrous materials. For thin section material of a few millimetres, the process is much faster than oxyfuel, but at thicknesses approaching 30-40mm, air plasma becomes relatively slow.

The cost advantages of using air in preference to expensive gases (for the plasma and oxyfuel processes) may be offset somewhat when other operating costs are taken into account. The air must be fed at a relatively high pressure (typically 150litres/min at 5bar) and clean, requiring a sizeable compressor with suitable filters for dust particles and oil. Hafnium or zirconium electrodes are expensive and their operating life can be severely shortened if there are frequent stops and starts. Low current air plasma torches, typically less than 40A, are particularly attractive for cutting thin sheet material, as compressed air is used for both the plasma forming gas and cooling the torch. As N2 and O2 suppress the formation of a series arc, compared with Ar, contact cutting can be practised with the air plasma system. The process is becoming more widely used for manual cutting thin sheet components in C-Mn and stainless steel, where contact cutting greatly deskills the operation. High tolerance plasma

17-22

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

To improve cut quality and compete with the superior cut quality of laser systems, high tolerance plasma arc cutting (HTPAC) systems are available which operate with a highly constricted plasma. Focusing of the plasma is by forcing the oxygen generated plasma to swirl as it enters the plasma orifice and a secondary flow of gas is injected downstream of the plasma nozzle. Some systems have a separate magnetic field surrounding the arc which stabilises the plasma jet by maintaining the rotation induced by the swirling gas. The advantages of HTPAC are:   

Cut quality lies between a conventional plasma arc and laser beam cut. Narrow kerf width. Less distortion due to smaller heat affected zone.

HTPAC is mechanised, requiring precision, high speed equipment. It is claimed that the cut quality lies between conventional plasma arc and laser beam cutting, but the speed is significantly lower than conventional plasma arc cutting and approximately 60-80% the speed of laser cutting. Advantages  Not limited to materials which are electrical conductors, so widely used for cutting all types of stainless steels, non-ferrous and non-conductive materials.  Operates at a much higher energy level compared with oxyfuel cutting resulting in faster cutting speed.  Instant start-up particularly advantageous for interrupted cutting; this also allows cutting without preheat.  Can be used with a wide range of materials, including stainless steel and aluminium.  High quality cut edges can be achieved, eg HTPAC process.  Narrow HAZ formed.  Low gas consumable (air) costs.  Ideal for thin sheet material.  Low fume (underwater) process. Disadvantages  Dimensional tolerances significantly poorer than machine tool capabilities.  Introduces hazards such as fire, electric shock (due to the high OCV), intense light, fumes, gases and noise levels that may not be present with other processes. In underwater cutting, the level of fumes, UV radiation and noise are reduced to a low level.  Compared with oxyfuel plasma arc equipment tends to be more expensive and requires a fairly large amount of electric power.  Being a thermal process, expansion and shrinkage of the components during and after cutting must be taken into consideration.  Cut edges slightly tapered.

17-23

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

    

17.8

Air plasma limited to 50mm thickness plate. High noise especially when cutting thick sections in air. High fume generation when cutting in air. Protection required from the arc glare. High consumable costs (electrodes and nozzles).

Plasma arc gouging Plasma arc as a gouging tool dates from the 1960s when the process was developed for welding. Compared with the alternative oxyfuel and MMA gouging techniques, it has a needle-like jet that can produce a very precise groove, suitable for application on almost all ferrous and non-ferrous materials. Process description A variant of the plasma arc cutting process, temperature and force of the constricted plasma arc determined by the current level and plasma gas flow rate, are so the plasma can be varied to produce a hot gas stream or a high power, deeply penetrating jet. This ability to control quite precisely the size and shape of a groove is very useful for removing unwanted defects from a workpiece surface.

Whilst gouging, normal precautions should be taken to protect the operator and other workers in the immediate area from the intense arc light and hot metal spray. Unlike oxyfuel and MMA, the plasma arc's high velocity jet will propel fume and hot metal dross considerable distance. When using a deeply penetrating arc, noise protection is essential. Equipment The power source for sustaining this gouging arc must have a high OCV, usually well in excess of 100V. The torch is connected to the negative polarity of the power source and the workpiece must be connected to the positive. The plasma torch is the same as used for cutting; either gas-or water-cooled and have the facility for single and dual gas operation.

17-24

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

Electrodes are normally tungsten for argon and argon-based gases. When using air as the plasma gas, special, for eg hafnium tipped copper, electrodes must be used to withstand the more aggressive, oxidising arc. Plasma and cooling gases Argon-35%H2 is normally recommended as a general- purpose plasma gas for cutting most materials. Alternative plasma gases are Ar (a colder gas, will reduce metal removal rates) and He. (Generates a hot but less intense arc than Ar-H2, can produce a wider, shallower groove). Nitrogen and air are also used as plasma gases, especially for gouging C-Mn steels. Gas costs will be substantially reduced but the groove surface profile will be inferior to that achieved with Ar-H2 gas. Air is not recommended for gouging aluminium as this requires an inert or reducing gas. Argon, nitrogen and air may be used as cooling gases. Use of argon will normally produce the best quality gouge, but nitrogen or air will reduce operating costs. Operating techniques Gouging is effected by moving the torch forward at a steady controlled rate and is carried out progressively to remove metal over 200-250mm. The jet can then be repositioned, either to deepen or widen the groove, or to continue gouging for a further 200-250mm. Principal process parameters are current level, gas flow rate and speed of gouging, which determine groove size and metal removal rate. In a typical gouging operation on C-Mn steel metal is removed at about 100kg/hr at a speed of 0.5m/min, and groove size will be around 12mm wide and 5mm deep. The torch stand-off and its angle to the surface of the workpiece have a major influence on speed of travel, groove profile and quality of surface. The torch is normally held 20mm from the workpiece and inclined backwards to the direction of gouging at an angle of 40-45°. Gouging will remove up to 6mm depth of metal in a single pass. The torch stand-off should not be less than 12mm to avoid spatter build-up on the nozzle from the molten particles ejected from the groove. At stand-off distances greater than 25mm, arc/gas forces are reduced which lessens the depth of penetration of the jet. By reducing the torch angle to the workpiece surface, the plasma jet can be encouraged to 'skate' along the surface of the workpiece; producing a shallower wider groove. By increasing the angle of the torch the plasma jet is directed into the workpiece surface, resulting in a deeper and narrower groove.

17.9

Laser cutting Background The first experiment in laser materials processing which subsequently evolved into a significant industrial process was at TWI in 1967. The team used O2 blown coaxially with a focused CO2 laser beam to cut 1mm thickness steel sheet.

17-25

www.twitraining.com

Rev 4 February 2013 Thermal Cutting and Gouging Copyright  TWI Ltd 2013

The first oxygen assist gas laser cutting.

The advantage of lasers in cutting is that the light can be focused to a very small spot size (160

DC

Method B This method uses a numerical designation from the table as listed below Symbol

Covering type

Positions

Type of current

03

Rutile/basic

Allb

AC and DC +/-

10

Cellulosic

All

DC +

11

Cellulosic

All

AC and DC +

12

Rutile

Allb

AC and DC -

13

Rutile

Allb

AC and DC +/-

14

Rutile + Fe powder

Allb

AC and DC +/-

15

Basic

Allb

DC +

16

Basic

Allb

AC and DC +

18

Basic + Fe powder

Allb

AC and DC +

19

Rutile + Fe oxide (Ilmenite)

Allb

AC and DC +/-

20

Fe oxide

PA/PB

AC and DC -

24

Rutile + Fe powder

PA/PB

AC and DC +/-

27

Fe oxide + Fe powder

PA/PB only

AC and DC -

28

Basic + Fe powder

PA/PB/PC

AC and DC +

40

Not specified

As per manufacturer’s recommendations

48

Basic

All

AC and DC +

bAll positions may or may not include vertical-down welding

Further guidance on flux type and applications is given in the standard in Annex B and C.

19-7

www.twitraining.com

Rev 4 February 2013 Welding Consumables Copyright  TWI Ltd 2013

Hydrogen scales Diffusible hydrogen is indicated in the same way in both methods, where after baking the amount of hydrogen is given as ml/100g weld metal ie H 5 = 5ml/100g weld metal.

19-8

www.twitraining.com

Rev 4 February 2013 Welding Consumables Copyright  TWI Ltd 2013

19.2

AWS A 5.1- and AWS 5.5A typical AWS A5.1 and A5.5 Specification E 80 1 8 G Reference given in box letter: A) B) C) (D For A5.5 only)

A) Tensile + yield strength and E% Code Min yield Min tensile PSI x 1000 PSI x 1000 General E60xx 48,000 60,000

17-22

E 70xx E 80xx E 100xx

17-22 19-22 13-16

57,000 68-80,000 87,000

70,000 80,000 100,000

Min E % In 2” min

B) Welding position 1 All positional 2 Flat butt & H/V fillet welds 3 Flat only Note: Not all Category 1 electrodes can weld in the vertical down position.

Specific electrode information for E 60xx and 70xx E 6010 48,000 60,000 22

V notch impact Izod test (ft.lbs) 20 ft.lbs at –20F

Radiographic standard Grade 2

E 6011 E 6012 E 6013 E 6020 E 6022 E 6027 E 7014 E 7015 E 7016 E 7018 E 7024 E 7028

20 ft.lbs at –20F Not required Not required Not required Not required 20 ft.lbs at –20F Not required 20 ft.lbs at –20F 20 ft.lbs at –20F 20 ft.lbs at –20F Not required 20 ft.lbs at 0F

Grade 2 Not required Grade 2 Grade 1 Not required Grade 2 Grade 2 Grade 1 Grade 1 Grade 1 Grade 2 Grade 2

48,000 48,000 48,000 48,000 Not required 48,000 58,000 58,000 58,000 58,000 58,000 58,000

60,000 60,000 60,000 60,000 60,000 60,000 70,000 70,000 70,000 70,000 70,000 70,000

22 17 17 22 Not required 22 17 22 22 22 17 20

C) Electrode coating and electrical characteristic Code Exx10

Coating

Current type

Cellulosic/organic

DC + only

Exx11

Cellulosic/organic

AC or DC+

Exx12 Exx13

Rutile Rutile + 30% Fe powder

AC or DCAC or DC+/-

E xx14 E xx15 E xx16 E xx18 E xx20 E xx24 E xx27 E xx28

Rutile Basic Basic Basic + 25% Fe powder High Fe oxide content Rutile + 50% Fe powder Mineral + 50% Fe powder Basic + 50% Fe powder

AC or DC+/DC + only AC or DC+ AC or DC+ AC or DC+/AC or DC+/AC or DC+/AC or DC+

D) AWS A5.5 low alloy steels Symbol Approximate alloy deposit A1 0.5%Mo B1 0.5%Cr + 0.5%Mo B2 1.25%Cr + 0.5%Mo B3 2.25%Cr + 1.0%Mo B4 2.0%Cr+ 0.5%Mo B5 0.5%Cr + 1.0%Mo C1 2.5%Ni C2 3.25%Ni C3 1%Ni + 0.35%Mo + 0.15%Cr D1/2 0.25-0.45%Mo + 0.15%Cr G 0.5%Ni or/and 0.3%Cr or/and 0.2%Mo or/and 0.1%V For G only 1 element is required

19-9

www.twitraining.com

Rev 4 February 2013 Welding Consumables Copyright  TWI Ltd 2013

19.3

Inspection points for MMA consumables Size

Wire diameter and length

Condition Cracks, chips and concentricity

All electrodes showing signs of the effects of corrosion should be discarded. Type (specification) Correct specification/code

20.1.1 E 46 3B

Storage

Suitably dry and warm (Preferably 0% humidity)

Checks should also be made to ensure that basic electrodes have been through the correct pre-use procedure. Having been baked to the correct temperature (typically 300-350C) for 1 hour and then held in a holding oven (150C max) basic electrodes are issued to the welders in heated quivers. Most electrode flux coatings will deteriorate rapidly when damp and care should be taken to inspect storage facilities to ensure that they are adequately dry and that all electrodes are stored in conditions of controlled humidity. Vacuum packed electrodes may be used directly from the carton only if the vacuum has been maintained. Directions for hydrogen control are always given on the carton and should be strictly adhered to. The cost of each electrode is insignificant compared with the cost of any repair, thus basic electrodes that are left in the heated quiver after the day’s shift may potentially be re-baked but would normally be discarded to avoid the risk of H2 induced problems.

19-10

www.twitraining.com

Welcome Welding Processes and Equipment IIW/EWF Diploma in Welding

Welcome to the Welding Processes and Equipment module of TWI’s Diploma course approved by the International Institute of Welding (IIW) and European Welding Federation (EWF).

Welcome - What this module is about TWI Training & Examination Services

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

What does this Module Cover? • • • • • •

What can I Expect?

Absolute basics – defining what a weld is. Detailed principles – how plasma is formed. Electricity – how it is used in welding. Processes – common and more specialised. Standards – briefly, those on fabrication. Symbols – how to show welds on drawings.

• Working to international syllabus. – IAB-252r8-07 (short version on IIW website www.iiw-iis.org).

• This is one of four modules each examined separately. • Qualification towards TWI Diploma. • Qualification towards IIW/EWF Diploma. – Requires entrance criteria to be met.

• Greater understanding of important aspects of welding. Copyright © TWI Ltd 2013

What Learning Methods are used? • • • • •

Copyright © TWI Ltd 2013

Example – Self-Adjusting Arc

Binder has notes and powerpoint's. Lectures given in classroom style. Extra study encouraged – necessary really. Interaction – especially for engineer. Tuition and counselling – talk to us.

Feed speed = burn off

Copyright © TWI Ltd 2013

V up, i down, burn off down. Feed speed > burn off

Wire advances, i increases until: Feed speed = burn off

Copyright © TWI Ltd 2013

1-1

Example – Laser Deposition

Why is this Module Important to me? • Welding Engineer, Technologist, Specialist must know fundamentals of processes. • Regarded as company specialist. • Choose best process for job. • Make decisions on best use of processes.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

My Company Has Fixed Ideas

I Just Need To Sign The Paperwork

• WL Bateman: If you keep on doing what you've always done, you'll keep on getting what you've always got.

• Everyone wants cost efficiency. • Today’s equipment and control make even a few years-old gear obsolete. • Future developments always seek to improve. • Your company will want you input.

• Short-term objective gaining Welding co-ordinator status is excellent. • Co-ordinator does not just sign paperwork. • Contracts need co-ordinator. • Future contracts need to be at required quality and profitable. • Co-ordinator can advise best practice and save company money.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

What Will I Do That I Don’t Now? • Tricky – all individuals coming with different backgrounds. • Depth of understanding can sort problems. • New perspectives on traditional processes experience from another viewpoint helps. • New processes detailed - could be applicable now or in future.

Copyright © TWI Ltd 2013

What’s In It For Me? • • • • •

Knowledge – better performance at job. Where to find reference material when needed. Ability to respond to changing needs. Possibility of professional qualification. More assured future with wider prospects.

Copyright © TWI Ltd 2013

1-2

Earliest Welding Welding Processes and Equipment History of Welding TWI Training & Examination Services

Forge welding • Egyptians heated iron to bright red heat and hammered pieces together to make a weld. Carbon arc Bernados and Olszewaski patented in 1885/6. Metal arc • Coffin (US), Slavianoff (Russia) gained patents in 1892 to replace one carbon with metal rod. Resistance • Thomson demonstrated principle in 1886.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Early 20th Century

1920s and 1930s

1903 • Thermit process defined by Goldschmitt. 1906 • Dalen produces porous medium for absorbing acetone so acetylene can be dissolved without explosion risk. Birth of practical oxy-acetylene. 1908 • Kjellberg coats metallic rods with lime and clay to make first non-porous MMA deposits. • Strohmenger finds that asbestos string wrapped around rod stabilises AC arc.

1920s • Fusarc process developed as first continuous feed welding process. 1930s • Patents for forerunner of SAW. Developed by Linde as Unionmelt process. • Patents for Ar or He gas shielded continuous wire process. Later developed by Linde as SIGMA.

Copyright © TWI Ltd 2013

1940s and 1950s

Copyright © TWI Ltd 2013

1960s Onwards

1940s • TIG welding invented to weld magnesium and stainless steel. 1950s • CO2 used for MAG welding. • Electroslag from USA developed in USSR. • Friction welding invented in USSR. • EB welding pioneered in France.

1960s • Laser cutting and welding developed in TWI. • Solid state power sources developed in TWI. • Pulsed power sources became available. • Explosive welding perfected. • Cold pressure welding invented in UK. 1990s • Friction stir welding invented at TWI.

• Plasma invented by Gage in the US. Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

2-1

Joining

Welding Processes and Equipment General Aspects of Welding TWI Training & Examination Services

• • • • • • • •

Welding. Brazing. Soldering. Adhesive bonding. Diffusion bonding. Riveting. Clinching. Sewing, stapling, etc.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Welding

Weldable/Unweldable

An operation in which two or more parts are united by means of heat or pressure or both, in such a way that there is continuity in the nature of the metal between these parts.

• • • •

Metals. Plastics. Ceramics. Composites.

Copyright © TWI Ltd 2013

Parts To Be Joined

Copyright © TWI Ltd 2013

Brazing

• Parent material, base material. – Plate, pipe, section. • Filler, consumable. – Electrode, wire, powder. Completed item may be called: • Joint. • Weld. • Weldment.

A process of joining in which, during or after heating, molten filler metal is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined by capillary attraction. • In general, the melting point of the filler metal is above 450°C but always below the melting temperature of the parent material.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

3-1

Braze Welding

Soldering

Joining of metals using a technique similar to fusion welding and a filler metal with a lower melting point than the parent metal, but neither using capillary action nor intentionally melting the parent metal.

A similar process to brazing, relying on capillary attraction to draw molten filler into a gap between parts that remain solid throughout. Solders melt at low temperatures - less than 450ºC. • For steel and copper, solders are usually alloys of tin.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Diffusion Bonding

Welding

A process in which component parts are held together with force and heated, usually in vacuum, to a temperature at which easy atomic movement makes possible diffusion of material from one part to the other.

Fusion • Melting of parent, filler, or usually both. Solid state • May or may not be heated, but no melting.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Fusion Welding • • • • • • • • • • •

Solid State Welding

Oxy-fuel gas (OFW). Manual metal(lic) arc (MMA). Metal inert/active gas (MIG/MAG). Tungsten inert gas (TIG). Flux cored arc (FCAW). Submerged arc (SAW). Electroslag (ESW). Electron beam (EBW). Laser. Resistance. Magnetically impelled arc butt (MIAB).

• • • • •

Copyright © TWI Ltd 2013

Forge or blacksmith. Friction – many variations, including friction stir. Explosive. Cold pressure. Ultrasonic.

Copyright © TWI Ltd 2013

3-2

Surfacing or Cladding

Joint Terminology

Surfacing • Uses welding processes to coat one material with a second, usually different with particular properties, eg corrosion, wear or heat resistance, not possessed by the base material. Cladding • More general term covering surfacing techniques and including explosive and roll bonding of one plate or tube to another to create duplex structure.

Edge

Lap

Open and closed corner

Butt

Tee

Cruciform

Copyright © TWI Ltd 2013

Butt Preparations

Copyright © TWI Ltd 2013

Single Sided Butt Preparations Single sided preparations are normally made on thinner materials, or when access form both sides is restricted.

Square edge closed butt

Single bevel

Single Vee

Single-J

Single-U

Square edge open butt

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Joint Preparation Terminology

Double Sided Butt Preparations Double sided preparations are normally made on thicker materials, or when access form both sides is unrestricted.

Included angle

Included angle Angle of bevel

Double -bevel

Root Radius

Double -Vee

Root face Double - J

Double - U Copyright © TWI Ltd 2013

Root face

Root gap

Root gap

Single-V butt

Single-U butt Copyright © TWI Ltd 2013

3-3

Joint Preparation Terminology Angle of bevel

Weld Terminology

Angle of bevel

Butt weld

Root radius

Root face Root gap Single bevel butt

Root gap

Spot weld

Fillet weld

Root face Land

Single-J butt

Edge weld

Plug weld Compound weld

Copyright © TWI Ltd 2013

Welded Butt Joints

Copyright © TWI Ltd 2013

Welded Tee Joints

Butt welded butt joint Fillet welded T joint

Fillet welded butt joint

Compound welded butt joint

Butt welded T joint

Compound welded T joint

Copyright © TWI Ltd 2013

Welded Lap Joints

Copyright © TWI Ltd 2013

Welded Closed Corner Joints

Fillet welded lap joint

Fillet welded closed corner joint

Spot welded lap joint

Butt welded closed corner joint

Compound welded lap joint Copyright © TWI Ltd 2013

Compoundwelded closed corner joint Copyright © TWI Ltd 2013

3-4

Penetration

Full penetration

Sides

Partial penetration

Single sided

Double sided

Copyright © TWI Ltd 2013

Runs

Single run

Copyright © TWI Ltd 2013

Stringer or Weave

Stringer bead

Multirun

Copyright © TWI Ltd 2013

Welding Positions

Weave

Copyright © TWI Ltd 2013

Slope and Rotation Weld slope • The angle between root line and the positive X axis of the horizontal reference plane, measured in mathematically positive direction (ie counter-clockwise).

Flat - PA

Horizontaloverhead - PD

HorizontalVertical - PB

Overhead PE

Horizontal - PC

Vertical-up - PF Vertical-down - PG Copyright © TWI Ltd 2013

Weld rotation • The angle between the centreline of the weld and the positive Z axis or a line parallel to the Y axis, measured in the mathematically positive direction (ie counterclockwise) in the plane of the transverse cross section of the weld in question.

Copyright © TWI Ltd 2013

3-5

Weld Zone Terminology

Tolerances

Face B

A

Weld metal Heat affected zone

Weld boundary C

D

Root

A, B, C & D = Weld toes Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Weld Zone Terminology

Weld Zone Terminology

Excess Cap height

Weld Width

Excess root penetration Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Toe Blend

Heat Affected Zone (HAZ) Maximum temperature

solid weld metal

6 mm

Solid-liquid boundary 80°

Grain growth zone Recrystallised zone Partially transformed zone Tempered zone Unaffected base material

• The higher the toe blend angle the greater theamount of stress concentration.

Poor weld toe blend angle 3 mm 20°

• The toe blend angle ideally should be between 20-30o.

Improved weld toe blend angle Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

3-6

Features to Consider

Fillet Weld Profiles

Fillet welds - toe blend

Fillet welds - shape

Convex fillet

Mitre fillet

Concave fillet Copyright © TWI Ltd 2013

Fillet Weld Features

Copyright © TWI Ltd 2013

Fillet Weld Throat Thickness

a Excess weld metal Vertical Leg length

Design throat Horizontal leg Length

a = Design throat thickness Copyright © TWI Ltd 2013

Fillet Weld Throat Thickness

Copyright © TWI Ltd 2013

Deep Penetration Fillet Weld

b

a a = Design throat thickness b = Actual throat thickness

b = Actual throat thickness Copyright © TWI Ltd 2013

b

Copyright © TWI Ltd 2013

3-7

Leg and Throat Relationship

Throat, a = 0.7 x Leg, z Leg, z = 1.4 x Throat, a a = z/√2 Copyright © TWI Ltd 2013

3-8

Types of Standard

Welding Processes and Equipment

• Application and design. • Specification and approval of welding procedures. • Approval of welders.

Fabrication Standards TWI Training & Examination Services

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Levels of Standards

Typical Standards

• Company or industry specific standards. • National BS (British Standard). • European BS EN (British Standard European Standard). • US AWS (American Welding Society) and ASME (American Society of Mechanical Engineers). • International ISO (International Standards Organisation).

Application

Application Code/Standard

Welding Procedure Welder Approval Approval

BS 5276 BS PD 5500 ASME Section VIII Process Pipework BS 2633 BS 2971 BS 4677 ASME B31.1/B31.3 Structural Fabrication BS EN 1011 BS 8118 AWS D1.1/ D1.2/ D1.6

BS EN ISO 15614 ASME Section IX

Storage Tanks

BS EN ISO 15614 ASME IX

Pressure Vessels

BS EN 12285 BS EN 14015 API 620/650

BS EN ISO 15614 ASME IX

BS EN ISO 15614 AWS D1.1/ D1.2/ D1.6

Copyright © TWI Ltd 2013

Welding Procedure Approval Test • Carried out by a competent welder. • Quality of the weld is assessed using NDT and mechanical testing techniques. • Demonstrate proposed welding procedure gives welded joint to specified weld quality and mechanical properties.

Copyright © TWI Ltd 2013

BS EN 287 BS EN ISO 9606 ASME Section IX BS EN 287 BS 4872 BS EN ISO 9606 ASME IX BS EN 287 BS 4872 BS EN ISO 9606 AWS D1.1/D1.2/ D1.6 BS EN 287 BS EN ISO 9606 ASME IX Copyright © TWI Ltd 2013

Welder Approval Test • Examines welder's skill and ability to make satisfactory test weld. • Test may be performed with or without a qualified welding procedure. • BS EN 287, BS ISO EN 9606 and ASME Section IX for quality work. • BS 4872 shows an adequate level of skill fro general work.

Copyright © TWI Ltd 2013

4-1

Process Terminology - BS EN ISO 4063 • • • • • • • • •

Process Terminology - BS EN ISO 4063 • • • • • •

1 - Arc Welding. 2 - Resistance welding. 3 - Gas welding. 4 - Welding with pressure. 5 - Beam welding. 6 - Not used. 7 - Other welding processes. 8 - Cutting and gouging. 9 - Brazing, soldering and braze welding.

1 - Arc welding. 11 - Metal arc with gas. 12 - Submerged arc. 13 - Gas-shielded metal arc. 14 - Gas-shielded with tungsten electrode. 15 - Plasma.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Process Terminology - BS EN ISO 4063 • • • • • • • • •

Process Terminology - BS EN ISO 4063 • • • • • • • • • • •

2 - Resistance welding. 21 - Resistance spot. 22 - Resistance seam. 23 - Projection. 24 - Flash. 25 - Resistance butt upset. 26 - resistance stud. 27 - HF resistance. 29 - Other resistance welding processes.

3 - Gas welding. 31 - Oxy-fuel gas. 4 - Welding with pressure. 41 - Ultrasonic. 42 - Friction. 43 - Friction stir. 44 - High mechanical energy. 45 - Diffusion. 47 - Oxy-fuel gas pressure. 48 - Cold pressure. 49 - Hot pressure.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Process Terminology - BS EN ISO 4063 • • • • • • • • • • •

Process Terminology - BS EN ISO 4063 • • • • • • • •

5 - Beam welding. 51 - Electron beam. 52 - Laser. 6 - Not used. 7 - Other welding processes. 71 - Aluminothermic. 72 - Electroslag. 73 - Electrogas. 74 - Induction seam. 75 - Light radiation. 78 - Arc stud. Copyright © TWI Ltd 2013

8 - Cutting and gouging. 81 - Flame cutting. 82 - Arc cutting. 83 - Plasma cutting. 84 - Laser cutting. 86 - Flame gouging. 87 - Arc gouging. 88 - Plasma gouging.

Copyright © TWI Ltd 2013

4-2

Process Terminology - BS EN ISO 4063 • • • • • • • •

Process Terminology - BS EN ISO 4063 Actual processes depicted by a third digit, eg: • 111 - Manual metal arc welding. • 114 - Self-shielded tubular-cored arc welding. • 121 - Submerged arc welding with one wire electrode. • 125 - Submerged arc welding with tubular cored electrode. • 131 - Metal inert gas welding (MIG welding). • 135 - Metal active gas welding (MAG welding). • 136 - Tubular cored metal arc welding with active gas shield.

9 - Brazing, soldering and braze welding. 91 - Brazing with local heat. 92 - Brazing with global heat. 93 - Other brazing processes. 94 - Soldering with local heat. 95 - Soldering with global heat. 96 - Other soldering processes. 97 - Weld brazing.

• 141 - Tungsten inert gas arc welding (TIG welding).

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Process Terminology - BS EN ISO 4063 Possible to add transfer mode, number of electrodes, filler or hybrid processes, viz: • Transfer mode: – – – –

D – Dip, Short-circuit. G – Globular. S – Spray. P – Pulsed.

• So MIG welding might be described as: – BS EN ISO 4063 – 131-S.

Copyright © TWI Ltd 2013

4-3

Why Are Symbols Needed?

Welding Processes and Equipment

• • • •

To avoid excessive wording on drawing. To give universally accepted description. To ensure everyone has same understanding. To achieve design requirement on shop floor.

Welding Symbols TWI Training & Examination Services

Copyright © TWI Ltd 2013

Basic Design of Symbols

Copyright © TWI Ltd 2013

Basic Symbols for Edge Preparation

Copyright © TWI Ltd 2013

Supplementary Symbols

Copyright © TWI Ltd 2013

Complementary Symbols

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

5-1

Dimensioning Fillet Welds

Symbols for Intermittent Welding

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Summary of Weld Symbols

Copyright © TWI Ltd 2013

5-2

Creation and Protection of Weld Pool

Welding Processes and Equipment Fusion Welding Principles TWI Training & Examination Services

Fusion welding • Heat to melt parent plate and filler. • Protection of melt from atmosphere. Heat • Flame. • Electric arc. • Electrical resistance. • Power beam. Protection • Vacuum or controlled atmosphere. • Shielding gas and/or flux.

Copyright © TWI Ltd 2013

Protection

Copyright © TWI Ltd 2013

Gas Shielding Inert gas • Argon – Ar. • Helium – He. • Ar-He. • Nitrogen – N2 (inert for copper, but not others). Active gas • CO2. • Ar-CO2. • Ar-O2. • Ar-H2. Copyright © TWI Ltd 2013

Flux Shielding

Copyright © TWI Ltd 2013

Leftward and Rightward Directions

• Flux may create gas to shield arc. • Flux may have ingredients that react with oxygen or nitrogen. • Flux melts and solidifies to slag that covers hot metal and excludes air.

Leftward technique

Copyright © TWI Ltd 2013

Rightward technique

Copyright © TWI Ltd 2013

1 6-1

Creation of a Molten Pool • • • •

Flame • Burning fuel gas with oxygen creates flame temperature around 3000°C. • Cannot melt refractory metals – Nb. Mo, W. • Heat transfer by conduction and small amount radiation. • Parent material and filler, if used, melt and mix in pool.

Flame. Arc. Resistance. Power beam.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Arc

Resistance

• Electrical potential ionises gas to give conductive path between electrode and work. • Arc generates plasma of ionised gas. • Temperature very high – ca 10,000°C. • Heat transfer by conduction and radiation. • Will melt all metals.

• Two sheets of metal pressed together by electrodes of Cu-Cr alloy. • Current passed between electrodes has to cross boundary between sheets. • High resistance at boundary generates heat that melts the interface. • Pressure applied to compact the molten area into a nugget.

Copyright © TWI Ltd 2013

Power Beam

Copyright © TWI Ltd 2013

Pool Penetration and Shape

• Intense, focussed beam of electrons (EB) or photons (laser) directed to joint line. • Very high temperature and concentrated beam impact area boils metal making keyhole. • At keyhole periphery metal condenses to liquid pool. • As beam progresses liquid pool follows behind keyhole creating weld.

Copyright © TWI Ltd 2013

• Gas or arc force will depress surface of pool giving some depth melt zone. • Power beam force very high creates keyhole. • Keyholing achieved with plasma welding. • Conduction transfers heat into body of metal melting a deeper shape. • Pool stirred by convection, Marangoni effect and Lorentz Forces.

Copyright © TWI Ltd 2013

2 6-2

Marangoni Effect

Lorentz Forces

• Surface tension normally reduces with temperature so least in weld centre. • Atoms are transported from low to higher surface tension zones. • Movement of atoms in liquid from centre to outside of pool called Marangoni effect. • Can be modified by composition. Sulphur reverses flow. Can give batch variation. • Pool development under flux not simple, in part due to Marangoni effect. Copyright © TWI Ltd 2013

• • • •

Current flow in conductor creates magnetism. Magnetic field induces force on conductor. If conductor is liquid, force gives movement. Right hand rule gives direction of force. For DCEN, up at centre and down at pool edge.

Copyright © TWI Ltd 2013

3 6-3

Compliance

Welding Processes and Equipment Welding Safety TWI Training & Examination Services

• Government legislation – The Health and Safety at Work Act. • Health and Safety Executive – COSHH Regulations, Statutory instruments. • British Standards – OHSAS 18001. • Company Health and Safety Management Systems. • Work instructions – permits to work, risk assessment documents etc. • Local Authority requirements.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Must Consider • • • • • • •

Electric Shock

Electric shock. Heat and light. Fumes and gases. Noise. Gas cylinder handling and storage. Working at height or in restricted access. Mechanical hazards: trips, falls, cuts, impact from heavy objects.

• Primary 240 or 460V mains. • Do not open welding equipment. • Only qualified electrician to wire or repair machine. • Secondary 60-100V high current. • Don’t touch metal parts of torch or electrode holder - certainly not when touching an earth. • Don’t work with worn cables. • Cables must have capacity for max current.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Electric Shock Assistance

Heat • • • • • • • • •

• Don't touch the person. • Keep others from being harmed. • Switch off power. • Use non-conductive pole to free the person. • Check obvious injury. • Move victim only when power off and no neck or spine injuries. Copyright © TWI Ltd 2013

Burns can be severe. Assume all metal around welding is hot. Don’t use hand pat to check. Use indicator stick. Sparks ignite flammable material - remove. Hot metal spatter gives very serious burns. Don’t tuck trousers in boots. Don’t wear turn-ups. Ventilate and cool welder in confined space. Copyright © TWI Ltd 2013

7-1

Light

Infra-Red

• Different hazards according to type. • Type depends on wavelength. • Welding creates all three types. Type

Wavelength, nm

Infra-red (heat)

>700

Visible light

400-700

Ultra-violet radiation

burn off

Wire advances, i increases until: Feed speed = burn off

Copyright © TWI Ltd 2013

11-2

Self-Adjusting Arc

Multi-Process Power Sources • Solid state control. • Inverter small size. • Circuitry to adjust between CC and CV. • Machines do all:

Feed speed = burn off

V down, i up, burn off up. Feed speed < burn off

– – – – – –

Wire retracts, i decreases until: Feed speed = burn off

MMA. TIG. MIG. Pulsed MIG. FCAW. Carbon arc gouging.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Pulsed Power

Pulsing by Wave Chopping

• Switching off or reversing polarity in programmed manner. • Useful for heat input and weld pool control. • Makes positional welding easier, eg MIG with spray transfer during peak current pulse. • Balancing melting and cleaning when AC TIG welding aluminium alloys.

i

High current

t

Low current

t

i

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Square Wave Pulse - Frequency Change

i

Ave

Square Wave Pulse - Width Change

i

t

t Ave

i

Ave

t

Ave

i

t Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

11-3

Square Wave Pulse - Width Change

i

Ave

Square Wave AC

+v e i

t Ave

t

ve

i t Copyright © TWI Ltd 2013

Synergic Control MIG

Modified Square Wave AC

• Can adjust pulse parameters – height, duration, frequency - to melt and detach one drop per pulse. • Different for each filler and each wire size. • Can programme machine with most common combinations. • Select via buttons or knob. • One-knob control.

+v e i

Copyright © TWI Ltd 2013

t

ve

Copyright © TWI Ltd 2013

One-Knob Control

Copyright © TWI Ltd 2013

Slope Control TIG

• Select material/wire/gas combination on knob in wire feeder compartment. • Adjust voltage on front panel for thickness. Copyright © TWI Ltd 2013

• Starts can have porosity and tungsten defects. • Worse if started at full current. • Start at very low current then build up. • Slope-in or slope-up. • Stops can have crater cracking. • Step down to low current before switch off. • Slope-out, slope-down or crater-fill. • Gas pre- and post-purge also help minimise defects. Copyright © TWI Ltd 2013

11-4

BS EN 60974 Label for Duty Cycle

Duty Cycle • • • • •

Heat generated by current through wires. May degenerate insulation, electrical safety. Fire hazard. Must use then allow to cool. Length of time in use in ten minutes with rest for cooling to remain within temperature limit. Duty cycle

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

11-5

TIG Basics

Welding Processes and Equipment TIG Welding TWI Training & Examination Services

Copyright © TWI Ltd 2013

Arc Starting

Equipment for TIG Power control panel

Transformer/ Rectifier

Power return cable

Inverter power source Power control panel

Torch assemblies Tungsten electrodes

Copyright © TWI Ltd 2013

Power cable

Scratch start: • Tungsten touched on workpiece. • Short-circuit starts current. • Arc established as torch lifted. • Can leave tungsten inclusions. Lift Arc: • Electronic control very low short-circuit current. • Builds to operational current as torch lifted. HF: • Superimposition of HF high voltage spark.

Flow-meter Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Polarity

Oxide Cleaning

DCEN: • Most used. • Tungsten cooled by electron emission. • Workpiece receives more heat. DCEP: • Will clean oxide from Al and Mg. • Heat tends to melt tungsten. • Can be done with water cooled torch. AC: • Usual way to weld Al and Mg to get cleaning. Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

12-1

Polarity

Current DCEN type/polarity Heat balance 70% at work 30% at electrode Weld profile Deep, narrow Cleaning No action Electrode Excellent capacity (3.2mm/400A)

Tungsten Types

AC

DCEP

50% at work 50% at electrode Medium Yes – every half cycle Good (3.2mm/225A)

30% at work 70% at electrode Shallow, wide Yes Poor (6.4mm/120A)

Pure W - green band. • Cheap, but short life. Poor arc start. W +ThO2 - yellow (1%), red (2%). • High current carrying but slightly radioactive. W + CeO2 - grey (Europe), orange (US). • Good for low current DC work. W + La2O3 - black. • Increasing use to replace thoriated. W + ZrO2 - white (Europe), brown (US). • Used for AC.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

GTAW Torch

GTAW Torch Tungsten electrode Torch cap/tungsten housing

Torch types: • Gas cooled: cheap, simple, large size, short life for component parts. • Water cooled: recommended over 150A, expensive, complex, small size, longer life of parts.

Electrode collet

Collet holder

Torch body Ceramic nozzle On/off switch

Copyright © TWI Ltd 2013

Correct Gas Flow

Copyright © TWI Ltd 2013

Shielding Gas Selection

• Too low and air can reach pool from sides. • Too high and eddies draw in air.

Copyright © TWI Ltd 2013

Argon (Ar):

He/Ar mixes:

• Suitable for welding Csteel, stainless steel, Al and Mg. • Lower cost, lower flow rates. • More suitable for thinner materials and positional welding.

• Suitable for welding Csteel, stainless steel, Cu, Al and Mg. • High cost, high flow rates. • More suitable for thicker materials and materials of high thermal conductivity. Copyright © TWI Ltd 2013

12-2

Gas Lens

Special shielding methods

Stainless steel wire sieve Thread for gas nozzle Thread for torch body

• Reduces eddies in gas flow. • Extends length of laminar flow prevents contamination. • Highly recommended for reactive metals (eg Ti, Al).

Torch trailing shield

Welding in protective tent

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Commercially Available Trailing Shields

Pipe Backing Gas Dams

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Pre- and Post-Flow

Electrode Tip for DCEN

• Gas flow is started before and continues after, welding current. • Better protection against oxidation.

2-2.5 times electrode diameter

Penetration increase

Electrode tip for low current welding Copyright © TWI Ltd 2013

Increase Vertex angle Decrease Bead width increase

Electrode tip for high current welding Copyright © TWI Ltd 2013

12-3

Electrode Tip for AC

Electrode tip ground

Grinding Tungstens

Electrode tip ground and then conditioned

• • • • • • • •

Reserve grinder for tungsten only. Use diamond or boron nitride wheels. Grind longitudinally and concentrically. Never use belt sander or sides of wheels. Do not breath grinding dust. Use exhaust system for thoriated tungsten. Tungsten splinters. Wear gloves and glasses. Use grinding wand. Electrodes get hot.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Autogenous Welding and Fillers • • • • • •

TIG can be used autogenously. Can mechanise and use more than one head. Can add filler from reel for mechanised. Manual filler - 1m rods in 5kg pack. Made to compositions in BS EN ISO 636. Stamped for identity:

Potential Defects Tungsten inclusions: • Thermal shock splinters W. • Touch start fuses spots to workpiece. • Spitting and melting can throw pieces into pool. • Very visible on radiograph but not critical defect. Solidification cracking: • Some compositions inherently crack sensitive. • Impurities often make eutectics. • Fillers designed with elements to react with impurities, eg Mn used to give high MPt MnS.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Potential Defects

Advantages of TIG

Oxide inclusions:

• Oxides contribute to lack of fusion. • No fluxing to absorb oxides. • Need to keep good gas cover to avoid oxidation of reactive metals. Diffraction mottling:

• Not real defect. • Black and white parallel lines on radiograph. • Can obscure real lack of fusion defect. Copyright © TWI Ltd 2013

• • • • • • •

No spatter, high cleanliness. Good welder easily produces quality welds. Good for penetration beads in all positions. Wide range metals, including dissimilar. Good protection for reactive. Very good for joining thin materials. Very low levels of diffusible hydrogen.

Copyright © TWI Ltd 2013

12-4

Disadvantages of TIG • • • • • •

Low deposition rates. Higher dexterity and co-ordination. Less economical for thicker sections. Not good in draughty conditions. Low tolerance of contaminants. Tungsten inclusions can occur.

Copyright © TWI Ltd 2013

12-5

MIG/MAG Welding

Welding Processes and Equipment TWI Training & Examination Services

• • • •

Also known as gas metal arc welding (GMAW). Uses continuous wire electrode. Weld pool protected by shielding gas. Classified as semi-automatic - may be fully automated. • Wire can be bare or coated solid wire, flux or metal cored hollow wire.

Copyright © TWI Ltd 2013

MIG/MAG - Principle of Operation

Copyright © TWI Ltd 2013

Process Characteristics • DCEP from CV power source. • Wire 0.6-1.6mm diameter. Gas shielded. • Wire fed through conduit. Melt rate maintains constant arc length/arc voltage. • WFS directly related to burn-off rate. • Burn-off rate directly related to current. • Semi-automatic - set controls arc length. • Can be mechanised and automated.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Wire Feeding

MIG/MAG Equipment External wire feed unit

Transformer/ Rectifier

Internal wire feed system Power control panel 15kg wire spool Power return cable

Power cable and hose assembly Liner for wire Welding gun assembly Copyright © TWI Ltd 2013

Separate feeder

Feeder in set

Copyright © TWI Ltd 2013

13-1

Feeder Drive Rolls Internal wire drive system

Types of Wire Drive System

Plain top roller

Two roll

Half grooved bottom roller

Four roll

Push-pull

Wire guide Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Roll Grooves • • • • • •

Liners for MIG/MAG

Often have plain top roll. Bottom, and sometimes top, roll grooved. V shape for steel. U shape for softer wire, eg Al. Knurled for positive feed. Care needed on tightness of rolls. – Too light – rolls skid, wire stalls. – Too tight – rolls deform wire, wire can jam.

Close wound stainless steel wire.

• If wire stops arc burns back to contact tube.

Teflon liner

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Welding Gun Types

Torch Components Welding gun assembly (less nozzle)

Swan neck

Welding gun body On/Off switch

Spatter protection

Hose port

Nozzles or shrouds

Push-pull

Spot welding spacer Gas diffuser

Copyright © TWI Ltd 2013

Contact tips Copyright © TWI Ltd 2013

13-2

Push-Pull Torch Assembly

Power Source Characteristic

Gas diffuser

Small change in voltage = large change in amperage

Contact tip

Union nut WFS remote control potentiometer

Trigger

V

Gas nozzle

i Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Self-Adjusting Arc

Feed speed = burn off

V up, i down, burn off down. Feed speed > burn off

Self-Adjusting Arc

Wire advances, i increases until: Feed speed = burn off

Feed speed = burn off

V down, i up, burn off up. Feed speed < burn off

Wire retracts, i decreases until: Feed speed = burn off

Copyright © TWI Ltd 2013

Wire Feed Speed / Current Relationship

Wire feed speed: • Increasing wfs automatically gives more current. Voltage: • In spray, controls arc length and bead width. Current: • Not separately set. Mainly affects penetration. Inductance: • In dip, controls rise in current. Lowers spatter. Gives hotter or colder welding. More info on several websites, eg: www.millerwelds.com/resources/articles/MIG-GMAW-weldingbasics Copyright © TWI Ltd 2013

500 450 400

Welding Current, A

Welding Parameters

Copyright © TWI Ltd 2013

350 300 0.8

250

0.9 1.2

200

1.6 150 100 50 0 2.5

5

7.5

10

Wire Feed Speed, m/min Copyright © TWI Ltd 2013

13-3

Process Variables

Process Variables Electrode orientation.

Arc voltage: Increasing Voltage. Reduced penetration, increased width. Excessive voltage can cause porosity, spatter and undercut.

Electrode extension.

Penetration

Deep

Moderate Shallow

Excess weld metal

Max

Moderate

Undercut

Min

Severe Moderate Minimum

Travel speed: Increasing travel speed. Reduced penetration and width, undercut.

Increased extension Copyright © TWI Ltd 2013

Shielding Gas

Copyright © TWI Ltd 2013

Transition Current Dip to Spray

Argon: • OK for all metals weldable by MIG. • Supports spray transfer, not good for dip. • Low penetration. Carbon dioxide: • Use on Ferritic steel. • Supports dip and globular, not spray. Ar based mixtures: • Add He, O2, CO2 to increase penetration. • >20Ar + He, >80Ar + O2, CO2 can spray and dip.

Material

C-Steel

Stainless Steel

Shielding Gas Wire Dia, mm Transition Current, A 0.8 155-165 0.9 175-185 Ar + 10%CO2 1.2 215-225 1.6 280-290 0.9 130-140 1.2 205-215 Ar +2%O2 1.6 265-275 0.8 120-130 0.9 140-150 Ar +2%O2 1.2 185-195 1.6 250-260

Copyright © TWI Ltd 2013

MIG and MAG Shielding Gases

Copyright © TWI Ltd 2013

Metal Transfer Modes

Metal inert gas (MIG): • Usually Ar shielding. • Can be Ar + He mixture - gives hotter action. • Used for non-ferrous alloys, eg Al, Ni. Metal active gas (MAG): • Has oxidising gas shield. • Can be 100% CO2 for ferritic steels. • Often Ar + 12-20% CO2 for both dip and spray. • Ar + O2 for stainless steel. Copyright © TWI Ltd 2013

Depending on shielding gas and voltage, metal crosses from wire to work in: • Spray mode - wire tapers to a point and very fine droplets stream across from the tip. • Globular mode - large droplets form and drop under action of gravity and arc force. • Short-circuiting (dip) mode - wire touches pool surface before arc re-ignition. • Pulsed mode - current and voltage cycled between no transfer and spray mode. Copyright © TWI Ltd 2013

13-4

Use of Transfer Modes

Droplet Growth and Detachment

Spray transfer: V > 27; i > 220. • Thicker material, flat welding, high deposition. Globular transfer: between dip and spray. • Mechanised MAG process using CO2. Dip transfer: V < 22; i < 200. • Thin material positional welding. Pulse Transfer: spray + no transfer cycle. • Frequency range 50-300 pulses/second. • Positional welding and root runs.

• Current heating wire causes melting and droplet formation. • Droplet held by surface tension and viscosity. • Droplet detachment by electromagnetic forces (Lorentz and arc forces), gravity. • Electromagnetic forces proportional to current t hence dip at low current.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Lorentz Force Pinch Effect

Dip Transfer

• Droplet stays attached and touches pool causing shortcircuit. • Current rises very quickly giving energy to pinch-off droplet violently. • Akin to blowing a fuse - causes spatter. • Droplet detaches, arc re-establishes and current falls. • Cycle occurs up to 200 times per second. Copyright © TWI Ltd 2013

Inductance Effect

Practical Effect of Inductance

Inductance slows rise of current during short-circuit. No inductance With inductance

Current

Copyright © TWI Ltd 2013

Maximum inductance: • Reduced spatter. • Hotter arc more penetration. • fluid weld pool flatter, smoother weld. • Good for thicker materials and stainless steels.

Minimum inductance:

• Colder arc used for wide gaps. • Convex weld, more spatter. • Good pool control. • Recommended on thin materials.

Time Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

13-5

Dip Transfer Attributes

Globular Transfer

Advantages: • Low energy allows welding in all positions. • Good for root runs in single-sided welds. • Good for welding thin material. Disadvantages: • Prone to lack of fusion. • May not be allowed for high-integrity applications. • Tends to give spatter.

• Transfer by gravity or short circuit. • Requires CO2 shielding. • Drops larger than electrode hence severe spatter. • Can use low voltage and bury arc to reduce spatter. • High current and voltage, so high distortion. Copyright © TWI Ltd 2013

Gas Metal Arc Welding

Copyright © TWI Ltd 2013

Spray Transfer

Spray transfer: When current and voltage are raised together higher energy is available for fusion (typically > ~ 25 volts and ~ 250 amps). This causes a fine droplets of weld metal to be sprayed from the tip of the wire into the weld pool. Transfer-mode advantages: • High energy gives good fusion. • High rates of weld metal deposition are given. • These characteristics make it suitable for welding thicker joints. • Transfer-mode disadvantages. • It cannot be used for positional welding.

• Tapered tip as anode climbs wire. • Small droplets with free flight from pinch effect. • Requires Ar-rich gas. • High current and voltage, high distortion. • Large pool, not positional. • Used for thick material and flat/horizontal welds.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Pulsed Transfer

Electronic Generation

Simplest form uses mains frequency and chops to control current.

• Now use synthesised pulsed can have height, duration and frequency control. • Droplets spray during peak current for time above threshold. • No transfer during background - current too low for dip. • Can select conditions to give single drop transfer each pulse - synergic MIG.

i

t

i

t

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

13-6

Pulsed Transfer Attributes

Setting Torch to Work Distance

Advantages:

• Good fusion. • Small weld pool allows all-position welding. Disadvantages:

• More complex and expensive power source. • Difficult to set parameters. • But synergic easy to set, manufacturer provides programmes to suit wire type, diameter and type of gas.

Contact tip

Gas nozzle Contact tip setback Electrode extension

Nozzle-to-work (stand-off) distance

Contact tip-towork distance

Arc length

Workpiece

Copyright © TWI Ltd 2013

Contact Tip to Work Distance

Copyright © TWI Ltd 2013

Contact Tip to Nozzle Distance Metal transfer mode

Metal transfer mode

CWTD, mm

Dip

10-15

Spray

20-25

Pulse

15-20

Dip Spray Spray (Al)

Contact tip to nozzle +/- 2mm 4-8mm inside 6-10mm inside

Electrode extension 19-25mm

Contact tip Electrode recessed extension (3-5mm) 6-13mm

Contact tip extension (0-3.2mm)

Set up for Dip transfer

Set up for Spray transfer

Copyright © TWI Ltd 2013

Filler Wire

Copyright © TWI Ltd 2013

Filler Wire Packaging

• • • • •

Similar composition to base material. Solid, flux cored or metal cored. FCW run in spray, give good fusion. Slag of FCW allows all-positional welding. Metal cored wires similar to solid wires, but better deposition rate. • Some FCW are self-shielded.

Layer wound on spool 15kg Machine wound on basket 16kg Pre-twisted into bulk pack 300kg

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

13-7

Filler Wire Feeding

Cast and Helix

Manufacturer needs to control: • Helix. • Cast. • Lubrication. • Stiffness.

Cast: • Pull ~ 1m wire from the reel or drum and toss it onto floor. Diameter of loop is the cast. • If cast is too small, wire will rub the walls of liner and may judder. Helix: • Loop used to demonstrate cast also shows helix. • Clip loop to single circle, hang over a horizontal bar offset between ends is the helix. • Excessive helix can give wear of contact tip and wander of the wire and arc across the bead. Copyright © TWI Ltd 2013

Lubrication and Stiffness

Copyright © TWI Ltd 2013

Potential Defects

Lubrication: • Welding wires need thin layer of lubricant to give efficient feeding through the liner. • Drawn wire has oil film left from the process. • Manufacturers control lubrication of final stages of drawing and winding to improve feeding. Stiffness: • C-Mn steel wires are in a cold-drawn state but some alloys may be annealed prior to final draw. • Al alloys, even cold-drawn, have poor stiffness so difficult to feed. May need plastic liner and even two motor, pushpull feeding system.

• Most defects caused by lack of welder skill, or incorrect settings of equipment. • Worn contact tip causes poor power pick up and this causes wire to stub into work. • Silica inclusions build in steels if poor inter-run cleaning. • Lack of fusion (primarily with dip transfer). • Porosity (from loss of gas shield on site etc). • Cracking, centerline pipes, crater pipes on deep narrow welds.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

MIG/MAG Attributes Advantages: • High productivity. • Easily automated. • All positional (dip and pulse). • Material thickness range. • Continuous electrode.

Disadvantages:

• Lack of fusion (dip). • Small range of consumables. • Protection on site. • Complex equipment. • Not so portable.

Copyright © TWI Ltd 2013

Flux Core Arc Welding

Copyright © TWI Ltd 2013

13-8

Gas Shielded Principle of Operation

Self-Shielded Principle of Operation

Copyright © TWI Ltd 2013

Benefit of Flux

Copyright © TWI Ltd 2013

FCAW - Differences from MIG/MAG

• Flux assists in producing gas cover, more tolerant to draughts than solid wire. • Flux creates slag that protects hot metal. • Slag holds bead when positional welding. • Flux alloying can improve weld metal properties. • Reduced cross-section carrying current gives increased burn-off at any current.

• Usually operate DCEP but some self-shielded wires run DCEN. • Some hardfacing wires are larger diameter - need big power source. • Don't work in dip. • Need knurled feed rolls. • Self-shielded wires use a different torch.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Self-Shielded Welding Gun Close wound stainless steel spring wire liner (inside welding gun cable)

Handle

Travel Angle

24V insulated switch lead

Conductor tube

Trigger

Welding gun cable

75°

90°

75°

Thread protector Contact tip

Hand shield

Courtesy of Lincoln Electric Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

13-9

Backhand (Drag) Technique Advantages: • Preferred for flat or horizontal with FCAW. • Slower travel. • Deeper penetration. • Weld hot longer so gasses removed.

Forehand (Push) Technique

Disadvantages: • Produces higher weld profile. • Difficult to follow weld joint. • Can lead to burnthrough on thin sheet.

Advantages: • Preferred method for vertical up or overhead with FCAW. • Arc gives preheat effect. • Easy to follow weld joint and control penetration.

Disadvantages: • Produces low weld profile, with coarser ripples. • Fast travel gives low penetration. • Amount of spatter can increase.

Copyright © TWI Ltd 2013

FCAW Advantages

Copyright © TWI Ltd 2013

Deposition Rate for C-Steel

• • • • • • •

Less sensitive to lack of fusion. Smaller included angle compared to MMA. High productivity. All positional. Smooth bead surface, less danger of undercut. Basic types produce excellent toughness. Good control of weld pool in positional welding especially with rutile wires. • Ease of varying alloying constituents gives wide range of consumables. • Some can run without shielding gas. Copyright © TWI Ltd 2013

FCAW Disadvantages

Copyright © TWI Ltd 2013

Electrogas Welding

• Limited to steels and Ni-base alloys. • Slag covering must be removed. • FCAW wire is more expensive per kg than solid wires (except some high alloy steels) but note may be more cost effective. • Gas shielded wires may be affected by winds and draughts like MIG. • More fume than MIG/MAG.

Copyright © TWI Ltd 2013

• Vertical position process. • Single pass to weld complete thickness. • Water-cooled copper shoes on either side of thick vertical plates. • Wide gap makes well in which molten pool can form. • Wire fed into well which is flooded with gas. • Shoes are leapfrogged over each other as weld rises to top of each. Copyright © TWI Ltd 2013

13-10

Tandem Welding

Adaptive Control

• Large diameter contact tip has two wires passing through it. • Single power source. • Arcs form into a single pool from each wire. • Benefit is the increased current density, eg 2 x 0.8mm dia instead of 1 x 1.6mm diameter.

Copyright © TWI Ltd 2013

• Electronic control detects arc events and machine automatically compensates. • Example is Lincoln Surface Tension TransferTM dip transfer without spatter. • Machine detects short-circuit and applies high current to pinch off droplet. • Just as it detaches, current is dropped to low value to avoid explosive collapse of droplet. • Current then immediately high to restart arc and decays to setting to start next cycle. Copyright © TWI Ltd 2013

13-11

Early History

Welding Processes and Equipment Manual Metal Arc Welding TWI Training & Examination Services

• Bernados and Olszewaski often cited as inventors from 1885 British patent but this was carbon arc welding with two electrodes. • Coffin in 1890 gained US patent for replacing one carbon with metal rod. First instance of metal transfer through an arc. • Slavianoff also suggested using metal rods. • In 1908 Kjellberg patented coated electrode – dipped in CaCO3, clay and silicate. • In 1909 Strohmenger patented asbestos wound rods, stable on AC.

Copyright © TWI Ltd 2013

Developments

Copyright © TWI Ltd 2013

MMA - Principle of Operation

• In WW1 US short of asbestos rods. Smith tried paper making first cellulosic rod. • Extruded electrodes appeared in the 1920s. AO Smith selling heavy coated rods in 1926. • Rutile tried in 1930s, for flat and horizontal welding. • Roberts made rutile VODEX (Vertical, Overhead, Downhand for Murex) in 1936. • MMA dominated welding 1940s to 1980s • Also known as Shielded Metal Arc Welding.

Filler metal core

Electrode angle 75‐80o to  the horizontal Consumable electrode

Flux coating Direction of electrode travel Solidified slag

Arc

Gaseous shield Molten weld pool Parent metal

Weld metal Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

MMA Welding

MMA Basic Equipment

Main features: • Shielding provided by decomposition of flux. • Consumable electrode. • Manual process. Welder controls: • Arc length. • Angle of electrode. • Speed of travel. • Current setting. Copyright © TWI Ltd 2013

Control panel (amps, volts) Electrode oven Electrodes

Power source Holding oven Inverter power source

Return lead Electrode holder Welding visor filter glass

Power cables Copyright © TWI Ltd 2013

14-1

Constant Current Power Source

MMA Electrode Holder

100

O.C.V. Striking voltage (typical) for arc initiation

90 80

Voltage

70 60 50 40

Normal Operating Voltage Range

30 20 10 20

40

60

80

100

120

130

140

160

180

200

Collet or twist type

Amperage

Tongs type with spring-loaded jaws

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Process Characteristics

Cellulosic Electrodes

• Straight lengths of coated electrode 250-450mm long and 1.6-6.0mm diameter. • DCEP, DCEN and AC all possible. • Coatings grouped: – – – – –

Cellulosic. Iron oxide. Rutile. Basic. With or without iron powder.

• Use industrially extracted cellulose powder, or wood flour in the formula. • Characteristic smell when welding. • Slag remains thin and friable, although arc force can create undercut and/or excessive ripple which may anchor the slag. • Strong arc action and deep penetration. • AWS E6010 types DC; E6011 run on AC. • Gas shield principally hydrogen. • Only used on C- and C-Mn steels. • High arc force allows V-D stovepiping.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Rutile Electrodes • • • • • • • • •

Rutile High Recovery Electrodes

High amount of TiO2, (rutile sand or ilmenite). Coatings often coloured. AWS type E6012 are DC; E6013 run on AC. Many designed for flat position. Fluid slag, smooth bead, easy slag removal. Need some moisture to give gas shield. Not low hydrogen. Available for ferritic and austenitic steels. Fair mechanical properties. Copyright © TWI Ltd 2013

• • • • • • •

High amount Fe powder added. More weld metal laid at the same current. Coating much thicker, forms deep cup. End of coating can rest on workpiece. Slag easy release, sometimes self-releasing. Only for flat position. These AWS E7024 have recovery between 150- 180%. • Recovery = Weld metal wt x100/core wire wt. Copyright © TWI Ltd 2013

14-2

Basic Electrodes

Other MMA Coatings

• • • • • • •

CaCO3 and CaF2 main ingredients. AWS E7015 first modern basic rods. Ran DC. Superseded by E7016 or E7018 - AC and DC. E7018 has Fe powder to help stabilise arc. E7016 good rooting and all positional. Both can give good mechanical properties. Often hybrid; small dia. no Fe powder, larger dia. increasing amounts. • Used for ferritic, stainless steels, Ni and Cu.

AWS E7028: • Basic with high levels of Fe powder added. • Flat and horizontal only. • Good mechanical properties. AWS E6020: • High levels of iron oxide. • Rare now, used for painted steel. • High arc force, relatively poor properties. Asbestos wound: • No longer permitted.

Copyright © TWI Ltd 2013

Setting up for MMA Welding

Process Characteristics

• Slag will help clean but rust and scale must be removed. For stainless and Ni wire brush. • Edge preparation usually needed: – – – –

Copyright © TWI Ltd 2013

60° for Ferritic – deep penetration rods available. 70-90° for stainless and Cu - less forceful rods. Up to 90° for Ni alloys - sluggish, viscous pool. Root gap 1-3mm for most applications.

• Arc melts both electrode and parent plate. • Flux forms gas to protect and form a plasma and slag to protect hot metal. • Short runs as finite length electrode. • Must deslag before next run.

• Good earth connection. Weld towards it on DC to minimise arc blow (or use AC).

Copyright © TWI Ltd 2013

MMA Welding Variables

MMA Welding Parameters

Open circuit voltage (OCV):

• Value of potential difference delivered by set with no load. Must be enough for specific electrode. • Electrodes labelled with min OCV, usually ~80V. Voltage:

• • • •

Copyright © TWI Ltd 2013

Measure arc voltage close to arc. Variable with change in arc length. Too low, electrode stubs into weld pool. Too high, spatter, porosity, excess penetration, undercut, burn-through. Copyright © TWI Ltd 2013

Current: • Range set by electrode, diameter, material type and thickness. • Approx 35A per mm diameter. • Too low - poor start, lack of fusion, slag inclusions, humped bead shape. • Too high - spatter, excess penetration, undercut, burnthrough. Polarity: • Can be DCEP, DCEN, AC. • Determined by operation and electrode type. Copyright © TWI Ltd 2013

14-3

MMA Welding Parameters

MMA – Parameter Setting

Travel speed: • Controlled by welder. • Often measured as run-out length as time to burn single rod fairly standard at constant current. • Too low - wide bead, excess penetration, burnthrough. • Too high - narrow bead, lack of penetration, lack of fusion, difficult slag removal.

Left to right:

• • • • • • •

Good conditions. Current too low. Current too high. Arc length too short. Arc length too long. Travel too slow. Travel too fast.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Typical Current Ranges Type

EN Specification

Cellulosic, mild steel

E38 0 C 11

Rutile, mild steel all-positional

E 35 2 R12

Rutile,mild steel high recovery, 160%

E42 0 RR73

Basic, low alloy

E69 4 Mn2NiCrMo B42 H5

Rutile, stainless

E19 9 LR12

Basic, Cu 7Sn

Dia. mm 3.2 4.0 5.0 2.0 2.5 3.2 4.0 5.0 6.0 2.5 3.2 4.0 5.0 6.0 2.0 2.5 3.2 4.0 5.0 1.6 2.0 2.5 3.2 4.0 5.0 2.5 3.2 4.0

Current Range, A 90 – 120 120 – 160 135 – 200 40 – 70 75 – 100 95 – 125 135 – 180 155 – 230 185 – 300 85 – 125 130 – 170 180 – 230 250 – 340 300 – 430 50 – 75 70 – 110 100 – 150 135 – 210 180 – 260 35 – 45 35 – 65 50 – 90 70 – 130 90 – 180 140 – 250 60 – 90 90 – 125 125 – 170

Heat Input • Total energy put in weld bead in unit time. • Calculated as: HI (kJ/mm) = 60iVk/1000S. – – – – – – – –

Where: i = current in Amps. V = voltage in Volts. S = travel speed in mm/min. k = thermal efficiency factor. k = 0.8 for MMA, MIG/MAG and FCAW. k = 0.6 for TIG and plasma. k = 1.0 for SAW.

Copyright © TWI Ltd 2013

Stringer or Weave

Copyright © TWI Ltd 2013

Multipass or Block Welding

Weave: • Lateral swings as well as moving along joint. • Useful to assist side wall fusion. • Run-out is shorter so heat input is higher. • Slows cooling rate, poorer toughness. Stringer bead: • Run weld bead in straight line along joint. • Lower heat input per unit length. • Can be too low – martensite in steel so poor toughness. Copyright © TWI Ltd 2013

• In thick material, typical bead won’t fill groove. • Move slowly allowing metal to build but limited in flat position. • Vertical, can triangular weave - into root along one edge, out along other, across face. • Block welding very high HI so poor properties. • Use multiple layers - multipass welding. • Need good cleaning of slag between runs. • Excellent properties, each bead heat treats one below. Can give with high toughness. Copyright © TWI Ltd 2013

14-4

Skip or Back-Step Welding

Preheat

• Technique to minimise distortion. • 30-50mm weld made then move ~150mm along seam and lay another short run. • Continue to end of seam. • Return to start and make 30-50mm welds in gaps. • Repeat until seam completely welded. • Large number of starts and stops may have defects like porosity or cracking.

• Ferritic steels must not have hydrogen diffusing and inducing cracking. • Can apply preheat to slow rate of cooling giving hydrogen time to be released. • Preheat may be with gas torch and large nozzle or electrically heated blankets. • Preheat specified as a minimum. Parent plate near weld must be heated. Check with probe or temperature sensitive crayons.

Copyright © TWI Ltd 2013

Interpass Temperature

Copyright © TWI Ltd 2013

Operating Factor for MMA

• In multipass welding must avoid heat build up. Can lower strength and toughness. • Maximum interpass may be specified. • Note preheat still applicable so may have minimum interpass temperature (equivalent to original preheat) and maximum.

• Welder needs time to change rods. • Also has to de-slag weld bead and grind any imperfections. • May be required to observe interpass temperatures. • Inspection will be required. • On long runs welder has to reposition. • All reduce time weld metal is deposited. • Arc time % to total time is operating factor. For MMA this is rarely above 30%.

Copyright © TWI Ltd 2013

Electrode Classification – EN499 E (for electrode) followed by 2 digit strength.

Copyright © TWI Ltd 2013

Electrode Classification – EN499 Symbol for toughness. Symbol

Temperature, °C, for average of 47J

Z

No requirement

A

+20

Symbol

Min Yield Strength N/mm

Tensile Strength N/mm

Minimum Elongation %

35

355

440-570

22

38

380

470-600

20

42

420

500-640

20

3

-30

46

460

530-680

20

4

-40

50

500

560-720

18 Copyright © TWI Ltd 2013

0

0

2

-20

5

-50

6

-60 Copyright © TWI Ltd 2013

14-5

Electrode Classification – EN499 Symbol for composition. Symbol

Electrode Classification – EN499 Symbol for flux type.

Chemical composition % max or range Mn

Mo

Ni

Symbol

No symbol

2.0

-

-

A

Coating acid

Mo

1.4

0.3-0.6

-

C

cellulosic

MnMo

>1.4-2.0

0.3-0.6

-

R

rutile

1Ni

1.4

-

0.6-1.2

RR

thick rutile rutile-cellulosic

2Ni

1.4

-

1.8-2.6

RC

3Ni

1.4

-

>2.6-3.8

RA

rutile-acid

Mn1Ni

>1.4-2.0

-

0.6-1.2

RB

rutile-basic

1NiMo

1.4

0.3-0.6

0.6-1.2

B

basic

Z

Any other agreed composition Copyright © TWI Ltd 2013

Electrode Classification – EN499 Non-compulsory symbol for recovery and polarity. Symbol

Weld metal recovery %

Current type

1

160

DC+ or DC-

Copyright © TWI Ltd 2013

Electrode Classification – EN499 Non-compulsory symbol for position. Symbol

Welding position

1

All positions

2

All positions except Vdown

3

Flat butt and fillet welds, HV fillet weld

4

Flat

5

V-down, flat butt, flat and HV fillet welds

Copyright © TWI Ltd 2013

Electrode Classification – EN499 Non-compulsory symbol for hydrogen.

Symbol H5

5 10

H15

15

Full EN499 Designation Example: E42 2 B32H5. • Basic C-Mn steel electrode. • Weld metal YS 420N/mm2. • Better than 47J at -20°C. • Recovery >105%. • AC or DCEP in all positions except V-D. • 1000A. • Can be mechanised or automatic welding. • Not self-regulating arc so must have voltagesensing WFS control. • More expensive. • Voltage from WFS control, power source controls current. • Not for high-speed welding of thin steel.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Wire • • • •

Fused Fluxes

Usually 2-6mm diameter. Copper coated to avoid rusting. 25 or 30kg coils. Can be supplied in bulk 300-2000kg.

• Original Unionmelt design - manganese, aluminium and calcium silicates. • Non-hygroscopic, no need to bake. • Good for recycling, composition doesn’t vary. • Some can accept up to 2000A. • Very limited alloying and property control. • Cannot make basic fused flux.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Bonded or Agglomerated Flux • • • • • • •

SAW Operating Variables

Powdered minerals pelletised with silicate. Baked to high temperature but hygroscopic. Flexible composition, can alloy, make basic. Can add deoxidants for good properties. Composition can vary as particle breakdown. Need to extract fines when recycling. Can add Mn and Si to weld so separate. formulae for single or multipass.

Copyright © TWI Ltd 2013

• • • • • • •

Welding current. Current type and polarity. Welding voltage. Travel speed. Electrode size. Electrode extension. Width and depth of the layer of flux.

Copyright © TWI Ltd 2013

15-3

Welding Current

Setting Current • Too high excess weld metal, increased shrinkage, more distortion. • Excessively high digging arc, undercut, burn through, narrow bead cracking. • Too low lack of fusion, poor penetration. • Excessively low unstable arc.

Controls penetration and dilution:

Copyright © TWI Ltd 2013

Current Type and Polarity

Copyright © TWI Ltd 2013

Welding Voltage

• DCEP - deep penetration; better for porosity.

• Controls arc length. • Increase gives flatter, wider bead. • Increase also in flux consumption and alloying transfer. • Increase reduces porosity. • Can bridge root gaps.

• DCEN - higher deposition rate; reduce penetration; surfacing use. • AC used to avoid arc blow; can give unstable arc. Copyright © TWI Ltd 2013

Setting Voltage

Copyright © TWI Ltd 2013

Setting Voltage

• Low voltage - stiffer arc penetration in deep groove and resists arc blow. • Excessive low voltage - high narrow bead, difficult slag removal.

Excessively high voltage: • Produces hat-shaped bead tendency to crack. • Increases undercut, slag removal difficult. • Produce concave fillet weld subject to cracking.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

15-4

Setting Travel Speed

Setting Travel Speed • Excessively high speed leads to undercut, arc blow and porosity. • Excessively low speed produces hat-shaped beads cracking. • Excessively low speed produces rough beads and slag inclusions.

Increase gives: • Low heat input. • Less filler metal applied per unit of length. • Less excess weld metal. • Smaller weld bead.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Electrode Size

Electrode Extension Increased extension: • Adds resistance. • Increases deposition. • Decreases penetration and bead width. • Helps prevent burnthrough. • Increase voltage to control weld shape. Excessive extension: • Difficult to position tip.

At same current, small electrodes have higher current density so higher deposition rates.

Copyright © TWI Ltd 2013

Depth of Flux

Copyright © TWI Ltd 2013

Effect of Electrode Angle on Bead Shape

• Influences appearance of weld. • Usually, depth of flux is 25-30mm. • If too deep: – Arc too confined so rough rope-like top surface. – Gases trapped so pool surface distorted.

• If too shallow: – Flashing and spattering. – Poor appearance and porous weld.

Copyright © TWI Ltd 2013

Penetration

Deep

Moderate

Excess weld metal

Max

Moderate

Undercut

Severe

Moderate

Shallow Min Minimum

Copyright © TWI Ltd 2013

15-5

Electrode to Work Angle

Smaller work angles reduce penetration

Typical work angle = 40°

Work Angle for Flat Fillet

Larger work angles increase penetration

Correct

Exception - when more penetration is required

Copyright © TWI Ltd 2013

Weld Backing

Copyright © TWI Ltd 2013

Starting/Finishing the Weld

Backing strip

Backing weld

Copper backing Copyright © TWI Ltd 2013

Increasing Productivity

Copyright © TWI Ltd 2013

Increasing Heat Input

There are four main ways: • Increase heat input. • Keep same heat input but add more metal. • Use narrower preparation. • Use more heads simultaneously.

Use smaller diameter wire at same current Increase current, use larger wire

Deposition rate in Kg/h at min and max current

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

15-6

Same Heat Input

Twin Wire

Increase deposition rate with: • Fine wire diameters. • Hot or cold wire additions. • Tubular flux and metal cored electrode wires. • Metal powder additions. • Increased electrode extension. • DCEN polarity.

• Two small diameter wires run through single contact tip. • 50% smaller cross-sectional area gives higher current density at the same current. • Higher burn-off so greater deposition rate. Viz: 4.0mm wire: area = π x 22 = 12.57mm2. 2 x 2.0mm wires: area = 2 x π x 12 = 6.28mm2.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Hot or Cold Wire Feed

Cored Wires

• Arc energy normally melting base material can be used to melt additional filler. • Wires are pushed through the flux into the molten weld pool. • Can use wire feeder without connecting to power source – cold wire feeding. • Can pass wire through contact tip to pick up current, resistive heating to near melting. • Gives greater deposition than cold wire before risk of lack of fusion.

• Same as MIG/MAG, sheath is conductive path for a cored wire. • Effectively much reduced cross-sectional area so higher current density. • Increased deposition rate. • Cored wires can be used in twin wire set-up to give greatly increased deposition.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Metal Powder

Metal Powder

• Arc energy will melt significant metal powder increasing deposition rate. • Dispensing equipment is cheap and simple. • Forward feed, powder supplied from bucket wheel dispenser ahead of weld. • Reduced penetration so not for roots. • Typically 5kg/hr added reducing number of weld passes by 30-50%. • Reduced flux consumption offset extra cost of powder. Copyright © TWI Ltd 2013

• • • •

Can attract powder onto wire magnetically. Carried by wire into pool through flux layer. Higher addition possible, up to 9kg/hr. Used for fillets and small dia. circumferential seams not practical for forward feed method. • For high toughness different compositions for two methods due to the different elemental recovery rates.

Copyright © TWI Ltd 2013

15-7

Metal Powder Addition Methods

Electrode Extension • Electrode extension distance wire protrudes beyond contact tip. • Resistive heating proportional to i2R. • R proportional to length of stick-out. • Longer the extension, the greater the heating and therefore melting (as much as 25-50%).

Forward feed system

Magnetic attraction method

Copyright © TWI Ltd 2013

DCEN

Copyright © TWI Ltd 2013

Narrow Gap Welding • No increase in deposition rate but significantly reduced weld volume. • Small angle preparations, typically 2-20°. • Less weld metal, less welding time to fill. • In SAW one run fuses both sides of preparation, each run laid directly on last. • Require specialised equipment, as root difficult to reach.

• DCEN gives greater burn-off at same current. • Lower penetration so not usual in butt welding. • First choice for surfacing.

Copyright © TWI Ltd 2013

Multiple Heads

Copyright © TWI Ltd 2013

Potential Defects

• • • • •

2-6 wires in line astern. Each has power source. All feed into one pool. DCEP first wire only. Trailing wires on AC to avoid arc blow. • Can have one head either side of job (eg web onto flange).

Porosity: • Oil, paint, grease, etc decompose in the arc to give elongated wormhole porosity. • Flux must be dry. Manufacturer's give drying temperatures. • Compressed air flux recovery units need dry air. • Insufficient flux burden can expose arc and pool to atmospheric contamination.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

15-8

Solidification Cracking

Solidification Cracking

• Control composition, susceptibility predictor. 230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1 • Add Mn and Si to counter C, S and P, either in wire or through flux. • Depth to width ratio important.

In the root beads of a multi-run weld

Caused by high speed giving a long deep weld pool in first pass

– W much greater than D - surface cracks likely. – D much greater than W - centreline cracks likely. – D similar to W - sound welds.

Caused by high restraint and root gap

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Solidification cracking

BS EN756 Classification

Mushroom shaped weld penetration resulting from high voltage combined with low speed.

S (for SAW) plus 2 digit strength (multirun). Symbol

Min. Yield N/mm2

Min. UTS N/mm2

Min.Elongation %

35

355

440-570

22

38

380

470-600

20

42

420

500-640

20

46

460

530-680

20

50

500

560-720

18

Copyright © TWI Ltd 2013

BS EN756 Classification

Copyright © TWI Ltd 2013

BS EN756 Classification Digits to show toughness.

S plus 2 digit strength (2 pass).

Symbol Symbol

Min Yield Parent Metal N/mm2

Min Tensile Strength of Welded Joint N/mm2

2T

275

370

3T

355

470

4T

420

520

5T

500

600 Copyright © TWI Ltd 2013

Z A 0 2 3 4 5 6 7 8

Temp. for Min Impact Energy 47J at °C No requirements +20 0 -20 -30 -40 -50 -60 -70 -80 Copyright © TWI Ltd 2013

15-9

BS EN756 Classification

BS EN756 Classification • Final table lists composition of 22 wires. • All prefixed S followed by a number from 1-4 denoting from 0.5% Mn (1) to 2% Mn (4). • Addition of Ni and/or Mo denoted by chemical symbol and whole number for %. No number means ca 0.5%. – S3 wire contains 1.5% Mn. – S2Ni1Mo has 1% Mn,1% Ni and 0.5% Mo.

Symbol for flux type. Flux Type manganese-silicate calcium-silicate

Symbol MS CS

zirconium-silicate rutile-silicate aluminate-rutile aluminate-basic

ZS RS AR AB

aluminate-silicate

AS

aluminate-fluoride basic

AF

fluoride-basic any other type

FB Z Copyright © TWI Ltd 2013

Advantages of SAW • • • • • • • • •

Copyright © TWI Ltd 2013

Disadvantages of SAW

High current density, deposition, productivity. Deep penetration so narrow grooves. Fast travel speed, less distortion. Easy deslagging. Good surface finish and fatigue properties. Mechanised, high duty cycle, low skill level. Consistent quality. Arc not visible so no UV hazard. Low fume.

• Only flat and horizontal positions. • Limited to C, low alloy, creep resisting, stainless steels and nickel alloys. • High HI can give low impact strength. • Need flux handling and recirculation control. • Difficult to apply on-site. • High capital costs. • Straight or circumferential seams only. • Needs accurate fit-up.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

History • Many cite Hopkins (US) as inventor in 1939. • Mostly electroslag remelting, even surfacing reference still remelting. • Paton Institute developed process in 1950s. • Shrubsall (US) consumable guide in 1957. • Much used in the US buildings in 1960s, 1970s. • Very poor toughness led to ban. • Earthquake 1994 showed no problem to ESW. • Ban lifted in 2000.

Electroslag Welding

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

15-10

Principle

Variants of ESW

• Thick vertical plates, square edge, large gap. • Copper shoes on either side make well. • Wire fed to bottom, usually through tube that also melts (consumable guide). • Flux covers wire end. • Initial arc melts wire and flux. • Molten flux conductive, floods arc so wire melts through resistive heating of flux. • Weld completed in single pass.

Guide tube system Consumable guide

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Process Characteristics

Materials Welded

• After initiation arc extinguishes, wire melted rapidly by resistive heating. • Welds up to 300mm made in single pass. • Copper guide tube used in standard process. Oscillated, slowly lifted as weld progresses. • Tubular consumable guide not lifted so melts into pool. Not usually oscillated either. • Very slow cooling, near equilibrium structure. • PWHT to gain properties.

• Mostly used on C and C-Mn steel. • Has been used on stainless and Ni alloys by Paton Institute. • Also claimed to weld Ti successfully. • Al is possible but not welded commercially. • Process developed for rail track joining but although better quality than Thermite did not gain favour.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Advantages and Disadvantages Advantages: • Speed ~1 hr per m whatever thickness. • No angular distortion. • Low lateral distortion. • Defect-free. • Simple flame-cut square edge. • Can be used for cladding (major application now).

Disadvantages: • Grain growth gives very large grains and poor toughness. • Limited to vertical or near vertical position. • Except cladding modification - flat. • Difficult to examine with NDT.

Copyright © TWI Ltd 2013

15-11

History

Welding Processes and Equipment Electroslag Welding TWI Training & Examination Services

• Many cite Hopkins (US) as inventor in 1939. • Mostly electroslag remelting, even surfacing reference still remelting. • Paton Institute developed process in 1950s. • Shrubsall (US) consumable guide in 1957. • Much used in US buildings in 1960s, 1970s. • Very poor toughness led to ban. • Earthquake 1994 showed no problem to ESW. • Ban lifted in 2000.

Copyright © TWI Ltd 2013

Principle

Copyright © TWI Ltd 2013

Variants of ESW

• Thick vertical plates, square edge, large gap. • Copper shoes on either side make well. • Wire fed to bottom, usually through tube that also melts (consumable guide). • Flux covers wire end. • Initial arc melts wire and flux. • Molten flux conductive, floods arc so wire melts through resistive heating of flux. • Weld completed in single pass.

Guide tube system Consumable guide

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Process Characteristics

Materials Welded

• After initiation arc extinguishes, wire melted rapidly by resistive heating. • Welds up to 300mm made in single pass. • Copper guide tube used in standard process. Oscillated, slowly lifted as weld progresses. • Tubular consumable guide not lifted so melts into pool. Not usually oscillated either. • Very slow cooling, near equilibrium structure. • PWHT to gain properties.

• Mostly used on C and C-Mn steel. • Has been used on stainless and Ni alloys by Paton Institute. • Also claimed to weld Ti successfully. • Al is possible but not welded commercially. • Process developed for rail track joining but although better quality than Thermite did not gain favour.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

16-1

Advantages and Disadvantages Advantages: • Speed ~1 hr per m whatever thickness. • No angular distortion. • Low lateral distortion. • Defect-free. • Simple flame-cut square edge. • Can be used for cladding (major application now).

Disadvantages: • Grain growth gives very large grains and poor toughness. • Limited to vertical or near vertical position. • Except cladding modification – flat. • Difficult to examine with NDT. Copyright © TWI Ltd 2013

16-2

Description of Processes

Welding Processes and Equipment Thermal Cutting and Gouging TWI Training & Examination Services

• Thermal cutting and gouging are essential parts of welding fabrication. • Thermal cutting severs metal, creating two pieces or a specific shaped single piece. • Gouging form of cutting removing metal to leave groove as weld preparation. • Torches and parameters different for each. • Material locally heated and molten metal ejected - usually by blowing it away. • Flame, laser or arc processes can be used.

Copyright © TWI Ltd 2013

Summary of Processes Thermal process Oxyfuel gas flame

Process operations Primary Secondary Cutting Grooving Gouging Chamfering

Copyright © TWI Ltd 2013

General Safety Metals

Ferritic, cast iron Ferritic, stainless, cast iron, Ni alloys Ferritic, cast iron, Ni alloys, Cu alloys, Al

Manual metal arc

Gouging

Grooving Chamfering

Air carbon arc

Gouging

Grooving Chamfering

Plasma arc

Cutting Gouging

Chamfering Grooving

Ferritic, stainless, Al

Laser

Cutting

Chamfering Drilling

Ferritic, stainless Copyright © TWI Ltd 2013

Gouging

• Cutting and gouging forcibly eject molten metal, often over large distance. • Must take appropriate precautions to protect operator, other workers and equipment. • Protective clothing, enclosed booth or screens, fume extraction, removal of all combustible material.

Copyright © TWI Ltd 2013

Typical Applications of Gouging

• Like cutting but not severing into two pieces. • Reverse side of welds, removal of tacks, temporary welds, and weld imperfections. – Repair and maintenance of structures. – Removal of cracks, blow holes and sand traps in forgings and castings. – Preparation of plate edges for welding. – Removal of surplus metal - excess weld bead profiles, temporary backing strips. – Removal of temporary welded attachments such as brackets, strongbacks, lifting lugs. Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

17-1

Oxyfuel Gas Cutting

Process Fundamentals

• Most widely applied industrial thermal cutting process. • Can cut thicknesses from 0.5-250mm. • Low cost equipment can be manual or mechanised. • Several fuel gas and nozzle design options.

• Mixture of O2 and fuel gas used to preheat metal to its 'ignition' temperature. • O2 jet then directed into preheated area. • Exothermic reaction between O2 and metal to form iron oxide or slag. • Jet blows away slag so it can pierce through the material and continue to cut.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Four Basic Requirements

Oxygen Purity

• Ignition temperature lower than melting point. • The oxide MPt must be lower than metal so that it can be blown away by jet. • Reaction between O2 and metal must give heat to maintain ignition temperature. • Minimal gas products so as not to dilute the cutting O2.

• Cutting speed and cut edge quality determined by purity of O2. • Nozzle design protects O2 from air entrainment. • Jet should be ≥99.5% O2. • Decrease in purity of 1% reduces cutting speed by 25% and increases consumption by 25%.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Fuel Gas Reactions

Fuel Gas Flame Energy

Combustion occurs in two distinct zones: • In inner cone gas combines with O2 to form CO and H2, eg for acetylene: 2C2H2 + 2O2 → 4CO + 2H2 • Combustion continues in outer zone with O2 being from air: 4CO+2H2 +3O2→ 4CO2 +2H2O

Copyright © TWI Ltd 2013

Fuel gas

Flame temp, ºC

Acetylene Propane MAPP Propylene Methane

3160 2810 2927 2834 2770

Ratio O2 to Inner gas flame energy, Kj/m3 1.2:1 18,890 4.3:1 10,433 3.3:1 15,445 3.7:1 16,000 1.8:1 1,490

Outer flame energy, Kj/m3 35,882 85,325 56,431 72,000 35,770

Copyright © TWI Ltd 2013

17-2

Acetylene

Propane

• Highest temperature so fastest preheat. • Highest heat energy in inner flame reduces HAZ width and distortion. • High flame speed (7.4m/s), good piercing. • Lowest ratio of O2.

• • • •

Highest heat energy in outer flame. Flame unfocussed, (speed 3.3m/s). Slower preheating than acetylene but effective. Once at ignition temperature, O2 reaction is same so cutting speed same.

Copyright © TWI Ltd 2013

MAPP

Copyright © TWI Ltd 2013

Cutting Quality

• Methylacetylene and propadiene. • High flame temperature (second to acetylene), good flame energy levels. • Can be readily compressed. • Choice for underwater cutting.

Oxyfuel typically: • Large kerf (1mm).

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Preheating

Cutting Speed • Left - too slow, top face melting, irregular cut. • Centre - optimum. • Right - too fast, metal and oxide not fully expelled.

• Left - too little, deep gouges low on face. • Centre - optimum. • Right - too much, top face melts.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

17-3

Advantages and Disadvantages Advantages: • Faster than machining. • Shapes can be cut economically. • Equipment costs low. • Portable equipment. • Can follow small radius easily. • Can mechanise torch for large plates. • Economical for edge preparation.

Powder Cutting

Disadvantages: • Not precision cut. • C and low alloy steel. • Fire and burn hazards. • Need fume control and ventilation. • Can give distortion and residual stress.

• Can inject flux into flame to remove oxide from stainless making cut possible. • Can inject Fe powder giving exothermic reaction makes cuts in stainless, Cu, Ni possible. • Cut quality usually poor.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

MMA Gouging

Flame Gouging • Cutting principle adapted to gouging. • Curved nozzle. • Quick, efficient removal on steel. • Low noise, ease of use, all positional. • Nozzle size changes gouge dimensions.

• Similar to welding but electrode has very high arc force to eject metal. • Used at low angle to push molten pool away from groove. • DC or AC on standard MMA power source. • Can cut thin material but poor quality. • Gouge not as smooth as gas processes. • Mild steel electrode used for all materials.

Copyright © TWI Ltd 2013

MMA Gouging

Copyright © TWI Ltd 2013

Air Carbon Arc Gouging • Arc between tip of carbon electrode and workpiece. • Metal melts and high velocity air jet blows it away, leaving clean groove. • Simple, uses MMA equipment. • High metal removal rate and gouge profile can be closely controlled. • Can be used on wide range of metals.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

17-4

Process Characteristics

Air Carbon Arc Gouging

• DCEP for steel and stainless steel. AC for cast iron, Cu and Ni alloys. • Graphite electrode with Cu coating to reduce electrode erosion. • Diameter selected for depth and width. • Molten metal/dross kept to minimum. • Standard MMA CC power source. Electrode different for AC vs DC. • Air from compressor or bottle used. Copyright © TWI Ltd 2013

Typical Parameters

Electrode diameter (mm)

Current on DC electrode

6.4 8.0 9.5 13.0 8.0 9.5 13.0 16.0

275 350 425 550 300-400 500 850 1250

Manual

Automatic

Gouging dimensions Depth (mm)

Width (mm)

6-7 7-8 9-10 12-13 2-9 3-12 3-15 3-19

9-10 10-11 12-13 18-19 3-8 3-10 3-13 3-16

Copyright © TWI Ltd 2013

Advantages and Disadvantages Carbon Gouging electrode speed consumed (mm/min) (mm/min) 120 114 100 76 100 142 82 63

609 711 660 508 1650-840 1650-635 1830-610 1830-710

Advantages: • Low equipment cost. • Economical to run. • Easy to operate. • Fast, easy to control. • Defects visible. • No slag issues. • Compact, can work in confined areas. • Use on all materials. • Can be automated.

Disadvantages: • Air jet ejects metal large distance. • Very noisy. • Cut only OK. • Needs large volume air. • C increase, grinding usually needed. • Sparks, ejected metal, fumes, noise and intense light.

Copyright © TWI Ltd 2013

Plasma Arc Cutting

Copyright © TWI Ltd 2013

Plasma Cutting Variants

• Basic process uses same torch as plasma welding. • Keyhole range plasma arc pierces metal. • Conditions set to avoid pool formation so becomes cutting tool. • No oxidation reaction, usable on any metal. • Introduced for stainless and Al. • Cut quality similar to oxyfuel. • Variants developed with different torches. Copyright © TWI Ltd 2013

Dual gas: • Second gas shield in nozzle. • Increased constriction, more effective dross removal. • Reduced top edge rounding. Water injection: • Water into plasma stream. • More constriction, hotter arc. • Increased cut speed. • Less nozzle erosion. Copyright © TWI Ltd 2013

17-5

Plasma Cutting Variants

Plasma Cutting Variants High tolerance plasma arc (HTPAC): • Attempting to rival laser cut quality. • Plasma gas swirls, constricting arc. • May use magnetic field to constrict arc further. • Narrow kerf width. • Less distortion due to smaller heat affected zone.

Water shroud or immersion: • Shroud cuts fume and noise. • Bath cuts noise 115-70dB. • No effect on top edge rounding. Air plasma: • Air as plasma gas, cheap. • Needs Hf electrode. • Manual cutting thin steel. • Can touch torch to work. Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Plasma Gouging

Advantages and Disadvantages Advantages: • Cuts non-conductors. • Faster than oxyfuel. • Instant start-up. • HTPAC has high quality cut edges. • Narrow HAZ. • Air plasma low gas) cost. • Ideal for thin sheet. • Water bath low fume 0.

• Standard torch may be used. • Air plasma also possible. • Use low angle. • Forces metal away from groove by power of plasma.

Disadvantages: • Noise can be high. • Fairly expensive. • Cut edges tapered. • Air plasma limited to 50mm thick plate. • Fume cutting in air. • Arc glare. • High consumable costs.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Laser Cutting

Laser Cutting

• First done in 1967 at TWI. • O2 jet with laser in centre. • CO2 laser then only high power, now Yb fibre or Nd-YAG possible. • Nd-YAG good for Al, Cu.

Copyright © TWI Ltd 2013

• • • • • •

Very quick, especially on thin sheet. Now used for automotive door panels. Growing use in shipbuilding. Automated with programmed pattern. Complex and very fine detail possible. Can also drill very fine holes.

Copyright © TWI Ltd 2013

17-6

Advantages and Disadvantages Advantages: • Very fast speed. • No preheating. • Readily automated and can follow three dimensional tracks. • Can cut polymers and other non-metallic materials. • Good quality squareedged kerf.

Disadvantages: • High cost of equipment. • Need to isolate personnel from laser.

Copyright © TWI Ltd 2013

17-7

History

Welding Processes and Equipment Plasma Welding TWI Training & Examination Services

• Robert Gage (Linde Division Union Carbide) gained US patent in 1955. • Described torch with W cathode enclosed in Cu bowl-shaped anode with hole in bottom through which plasma is blown. • Patent said workpiece could be in circuit more +ve than Cu anode so arc forced through. • Gage’s torch design still in use today.

Copyright © TWI Ltd 2013

Process Characteristics

Copyright © TWI Ltd 2013

Three Modes of Operation

• When workpiece connected more +ve than anode on torch variant called transferred arc and is usual array for welding and cutting. • Can operate without connection to workpiece (non-transferred arc) with plasma forced through hole by the gas pressure. • This gives lower energy heat source but not restricted by need for electrical connection. • Used for spray surfacing where torch may be moved from one component to the next.

Microplasma: 0.1-15A: • Can operate at very low current. • Arc stable even when length varied up to 20mm. • Big advantage over microTIG. Medium current: 15-200A: • Process characteristics similar to TIG. • Stiffer arc, improved penetration. Keyhole plasma: over 100A: • Powerful beam gives full penetration (like laser or EB) up to 10mm stainless steel single pass.

Copyright © TWI Ltd 2013

Plasma Welding Schematic

Copyright © TWI Ltd 2013

Power • DC constant current. • Normally DCEN to maximise heat at work. • Special torches available for DCEP for Al which needs cathodic cleaning of oxide film. • Difficult to stabilise AC arc, but square wave AC (inverter, switched DC) possible. • HF only at first start for pilot arc held in torch. Automatically transferred to workpiece when required for welding. Very reliable, eliminates HF interference. Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

18-1

Torch

Nozzle

• Larger, more complex than TIG. • Attention needed to initial set up, internal distance electrode to anode, number of parts to be adjusted. • Inspection and maintenance needed during production.

• Power determined by degree of constriction. • Choose bore diameter vs current and plasma gas flow rate. • Soft plasma, micro and medium current, large bore to minimise nozzle erosion. • For keyhole, bore, plasma gas flow rate and current selected to give highly constricted arc with power to cut through the material.

Copyright © TWI Ltd 2013

Copyright © TWI Ltd 2013

Plasma and Shielding Gases

Plasma Gas Flow

• Usually Ar for plasma gas. Can use Ar + H2 or He, hotter arc, lower electrode and nozzle life. • Shielding gas often Ar + 2 to 8%H2 gives reducing atmosphere and cleaner welds. • 75%He - 25%Ar used for shielding for Cu. • Shielding gas flow rate is not critical. • Plasma gas flow rate must be set accurately as it controls the penetration of the weld pool.

Plasma gas flow rate crucial - too low gives double arcing in torch and nozzle melting.

Copyright © TWI Ltd 2013

Electrode

Copyright © TWI Ltd 2013

Electrode Set Back

• Traditionally W + 2 and 5% ThO2. Now Ce, La doped avoid radioactivity precautions. • Tip ~15° for microplasma. • Angle increases with current, for keyhole 60-90° recommended. • For high current, tip also blunted to ~ 1mm. • Tip angle not critical for manual welding. • For mechanised, electrode condition helps determine shape of arc and penetration. Copyright © TWI Ltd 2013

• Need constant electrode position for consistency. • Guidance and special tool provided by the torch manufacturer. • Balance with other variables. • If lower plasma gas flow rates used to soften arc electrode set-back distance is reduced.

Copyright © TWI Ltd 2013

18-2

Backing System

Advantages and Disadvantages

• TIG backing bar or gas techniques can be used for micro and medium current. • For keyhole either gas backing or a grooved backing bar must be used. • Efflux plasma ~10mm below back face, so groove must be deep to avoid disturbance. • Arc instability will disturb weld pool, causing porosity.

Copyright © TWI Ltd 2013

Advantages: • Non-critical torch to workpiece distance. Useful at low current. • Welds thick material in keyhole mode. • Faster deposition rates than TIG.

Disadvantages: • More complex torch set up than TIG. • Bulky torch hinders manual use.

Copyright © TWI Ltd 2013

18-3