DAC3 March 2012 [PDF]

Zitiervorschau

IIW/EWF Diploma – Materials and Their Behaviour (Advanced) MAB3

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

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IIW/EWF Diploma Materials and their Behaviour (Advanced) Contents Section

Subject

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

Crystalline Structure Atomic structure Chemical bonding Crystal structures Atomic structure and mechanical properties Solidification of metals Recrystallisation – solid state transformation Crystal structure imperfections Types of deformation Strengthening mechanisms

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

Phase Diagrams Alloying Introduction Using phase diagrams The lever rule Solidification and microstructure Solidification and coring Age hardening Solid state transformations Introducing the iron carbon phase diagrams Advantages and disadvantages of phase diagrams

3 3.1 3.2 3.3 3.4 3.5

Manufacture of Steels Modern steelmaking Primary steelmaking Secondary steelmaking Casting Steel processing

4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11

Material testing Mechanical testing Tensile testing Cross-weld tensile testing Validity of tensile data Charpy impact testing Crystallinity and lateral expansion Fracture toughness testing Bend testing Fatigue testing Creep testing Corrosion testing

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4.12 4.13 4.14 4.15 4.16

Hardness testing Vickers hardness test Brinell hardness test Rockwell hardness test Metallographic specimen preparation

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11

Heat Treatment Purpose of heat treatment Heat treatment equipment Bulk heat treatment equipment Localised heat treatment equipment Types of heat treatment Full annealing Normalising Recovery and recrystallisation Non-equilibrium heat treatment Precipitation hardening Postweld heat treatment

6 6.1 6.2 6.3 6.4 6.5 6.6

Fe-C Steels Steel Steel terminology Weldability of steels HAZ toughness in C-Mn steels Weld metal toughness Practical consideration for improved toughness

7 7.1 7.2 7.3 7.4

Micro-alloyed/High Strength Low Alloy (HSLA) Steels Micro-alloyed steels TMCP steels Weldability of mico-alloyed steels Applications for high strength steels

8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8

Structure of the Welded Joint Regions of a weld Heat input Weld metal solidification and transformation Further issues C-Mn steel HAZ microstructures Intercritical HAZ Subcritical HAZ Microstructures of multi-pass welds

9 9.1 9.2 9.3 9.4 9.5 9.6

Cracking in welds Cracking phenomena Hydrogen (cold) cracking Solidification (hot) cracking Liquation cracking Lamella testing Reheat cracking

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10 10.1 10.2 10.3 10.4 10.5 10.6 10.7

Corrosion Definition Reactions during corrosion Galvanic series Types of corrosion Common corrosion protection techniques Pickling and passivation Corrosion testing

11 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13 11.14

Welding of Stainless Steels Stainless steels Predicting the phases in stainless steel Constitution diagrams Measuring ferrite content Austenitic stainless steel Duplex stainless steels Pitting resistance Welding duplex stainless steels Weldability problems for stainless steels Weld decay (sensitisation) SIGMA (δ) phase formation 475oC embrittlement Solidification cracking Heat tint

12 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 12.11 12.12 12.13 12.14 12.15 12.16

Surfacing Reasons for using overlays and coatings Types of protective layers Weld surfacing Buttering Arc welding surfacing techniques Laser weld deposition Explosive cladding Clad pipes Cutting clad or lined plates Dilution in weld overlays Sensitization of the substrate Welding and NDT of clad steels Consumable selection Standards Quality of control and weld overlays Thermal spray coatings

13 13.1 13.2 13.3 13.4 13.5 13.6 13.7

Creep Resistant Steels Creep resistance Creep-resistant steels Weldability of creep-resistant steels Reheat cracking Controlling reheat cracking Temper embrittlement Type IV cracking

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14 14.1 14.2 14.3 14.4 14.5

Cryogenic Steels Cryogenic properties Applications of cryogenic steels Composition of cryogenic steels Weldability of nickel steels Weldability of austenitic stainless steels

15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9

Aluminium and Light Alloys Background Work hardening alloys Heat treatable alloys Classification and temper designation of aluminium alloys Weldability of aluminium alloys Choice of filler metal Application of welding processes Titanium alloys Magnesium alloys

16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8

Joining dissimilar metals Differences in physical properties Metallurgical incompatibility Recommendations for dissimilar welding Welding two different ferritic steels Welding ferritic steel to stainless steel Welding two different stainless steels NDT of dissimilar metal weldments Using the schaeffler diagram

17 17.1 17.2 17.3 17.4 17.5

Welding other Alloys Cast iron Types of cast iron Nickel and nickel alloys Weldability problems Copper and copper alloys

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Section 1 Crystalline Structure

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1

Crystalline Structure

1.1

Atomic structure An atom is the smallest possible particle of a chemical element that retains its chemical properties. An atom contains three main particles:   

Protons. Neutrons. Electrons.

The protons and neutrons are located in the nucleus of the atom and the electrons orbit the nucleus.

The number of protons (ie atomic number Z) determines the chemical element, eg iron, Fe, has 26 protons and so its atomic number is 26. The number of neutrons determines the atomic mass of an element and atoms of the same element can have different atomic mass A. Atoms having the same Z but different A are isotopes of an element. The number of valence electrons governs the bonding behaviour.

1.2

Chemical bonding Chemical bonding is the physical phenomenon of substances being held together by attraction between atoms. Bonding can determine some of the physical properties of a substance (eg melting point and conductivity). There are three types of chemical bonding: ionic, covalent and metallic bonding.

1-1

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In ionic bonding, a metal atom donates an electron (becoming a positive ion) and a non-metal atom accepts this electron (becoming a negative ion). These ions are attracted to each other by electrostatic forces. Ionic compounds  Tend to have high melting points.  Become electrically conductive when melted or dissolved.  Are generally brittle.  Eg NaCl (salt).

In covalent bonding, atoms share one or more pairs of electrons to form a molecule. Covalent bonding occurs between non-metals. Covalent compounds  Low melting points.  Poor electrical conductivity.  Interatomic (covalent) bonds are always stronger than intermolecular bonds.  Eg CO2, H2O and N2.

Metallic bonding occurs as metal ions sit within a lattice surrounded by delocalised shared electrons. There is electrostatic attraction between ions.

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Metallic bonding accounts for many physical characteristics of metals  Thermal/electric conductivity.  Ductility (layers of atoms can slide past each other).  Strength.  Lustre.

1.3

Crystal structures A crystal is a solid in which the constituent atoms are packed in a regular order in a repeating pattern extending in three dimensions (space lattice). The smallest repeating unit of this pattern is called a unit cell. Crystals form during solidification. If there is a single centre of solidification, then a single crystal solid will form. If there are multiple centres of solidification, then multiple crystals, or grains will form. Materials containing many grains are called polycrystalline solids.

The arrangement of atoms inside the unit cell defines the crystalline structure and there are many different types of crystal systems. Three examples are body-centred cubic (bcc) crystal structure, face-centred crystal (fcc) structure and hexagonal close-packed (hcp) crystal system.

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The co-ordination number is the number of nearest neighbouring atoms to any atom located in the space lattice, eight for bcc, twelve for fcc and twelve for hcp. The co-ordination number affects interatomic distances and the ability to accommodate interstitial atoms within the crystal structure.

1.4

Atomic structure and mechanical properties Deformation in metals occurs largely by a process of slip, where atomic planes slide over each other, with the aid of dislocations. Slip occurs on the most densely packed planes. These planes are called slip planes. FCC has four close-packed planes (octahedral planes) and three possible slip directions, giving twelve slip systems. One of these planes is shown here.

Body centred cubic is less closely packed than fcc, however it has six closepacked planes (on cube diagonals) and two directions, therefore twelve slip systems. Slip, however, is more difficult than for fcc. Hcp has one closepacked plane and three slip directions giving three slip systems. Because of the limited slip systems, hcp metals tend to be more brittle. The slip plane is shown.

1-4

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Different metals have different crystal structures and the slip systems available to each give them their mechanical properties. More ductile

More brittle

fcc

bcc

hcp

Aluminium

Chromium

Cadmium

Copper

Iron and most steels

Cobalt

Gold

Molybdenum

Magnesium

Lead

Tungsten

Zinc

Nickel

Zirconium

Silver Stainless steel (austenitic)

1.4.1

Allotropes If an element exists in more than one crystal form they are called allotropes. Iron is an unusual metal in that it is allotropic, which means that it is able to exist in more than one crystal form eg above 1400°C it is bcc delta iron (), between 910-1400°C it is fcc austenite () and below 910°C it is bcc ferrite (). The transition between the two crystallographic structures occurs at a certain temperature and is called a phase change. The phenomenon of allotropism makes steel very useful but can also lead to certain problems including distortion, due to the expansion and contraction that take place during the allotropic changes. The low temperature bcc phase of iron or steel exhibits a step change in the toughness of structural steels, which is known as the ductile-brittle transition, where the material is brittle at lower temperatures. Fcc materials such as austenitic stainless steels and aluminium are used for storing cryogenic liquids such as liquid nitrogen because they do not suffer from a ductile-brittle transition.

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1.5

Solidification of metals When metals solidify:  Nuclei form when the temperature of molten metal falls to its freezing point, these are sometimes known as seed crystals.dendrites begins to form in the direction of cooling.  Secondary arms form on the dendrites.  Arms grow until they touch the next dendrite when the arms begin to thicken. Dendrites continue to grow and interlock until the grain structure is complete. Columnar grains form in weld metal as they follow the direction of heat dissipation.  Crystallisation is complete and only the grain boundaries are visible.  Impurities collect at the grain boundaries. Grain boundaries are linear imperfections where adjacent grains meet. At grain boundaries, ions are farther apart than inside the grains giving an amorphous structure.

Columnar structure of weld metal.

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1.6

Recrystallisation – solid state transformation

Grain boundaries have higher energies than the grain interiors, so a fine grain structure with a large number of grain boundaries is far from the energetic equilibrium. However, this structure has high toughness and yield strength. During recrystallisation grain boundaries disappear and the overall length of grain boundaries reduces, meaning a reduced energy, closer to energetic equilibrium. This structure also has good conductivity due to a greater amount of crystalline and less amorphous structure.

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1.7

Crystal structure imperfections The two types of crystal imperfection are point defects and linear defects. Point defects can be vacancies, interstitial and substitutional atoms. Vacancies are where an atom is missing from the atomic arrangement. Interstitials are where an atom is present between atoms in the normal crystal structure and a substitutional atom is an atom different from the matrix but which occupies a position where the matrix atom could be. In the case of steels, C and N are substitutional alloying elements, while Cr, Mn and Ni are substitutional.

Vacancy

Substitutional

Interstitial

Linear imperfections are also known as dislocations and there are two types, depending on the geometry. Crystalline materials deform by movement of dislocations along favourable crystal planes.

Edge dislocations consist of an extra half-plane of atoms.

Screw dislocations consist of a step of atoms in the crystal structure.

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If enough stress is applied to the material, it will deform by dislocation movement along planes of atoms that have a high density of atoms. Plastic deformation can be made more difficult by the presence of barriers to dislocation movement. Such barriers may be interstitial atoms, precipitates, substitutional atoms, other dislocations and grain boundaries. If deformation is more difficult, then a material is stronger.

1.8

Types of deformation Elastic deformation is when atomic bonds are stretched but not broken. Plastic deformation is when permanent deformation occurs. Atomic bonds are broken and new ones formed and dislocations are generated and move through the crystal lattice.

ELASTIC

YIELD

PLASTIC

POINT

As the temperature increases the strength decreases and the toughness increases. This occurs because dislocation movement becomes easier as the temperature increases. If the temperature continues to be increased, then grain boundaries can also move and grain coarsening will occur.

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1.9

Strengthening mechanisms Properties of pure metals can be greatly affected and thus tailored for different applications, by different strengthening mechanisms. In metals, the most common strengthening mechanisms are: alloying, work hardening, precipitation hardening, grain refinement and quenching. These are briefly described below.

1.9.1

Alloying Alloying with other elements eg adding C to Fe, or adding Si to Al is used to increase the strength of the Fe and Al matrix, respectively. Depending on the size of the atom being added, it can occupy an interstitial or substitutional position in the matrix. Both can be effective in increasing strength. Interstitial alloying elements are smaller atoms (compared to the matrix atoms) and are able to fit into the spaces between the larger atoms eg carbon or nitrogen in iron. Other common interstitial atoms in iron are hydrogen and oxygen, although these are not effective in increasing the strength of iron. Substitutional alloying elements are of similar size to the lattice and replace atoms of the lattice eg chromium, manganese and nickel in iron (and other atoms whose diameters are within 15% of each other). Both substitutional and interstitial alloying elements act by deforming the lattice, which makes it harder for dislocations to move and therefore increase the strength of the alloy.

1.9.2

Work hardening Work hardening, also called cold working or strain hardening is an important industrial process used mainly in alloys that do not respond to heat treatment, such as austenitic stainless steels and aluminium alloys. Work hardening occurs as a result of cold-working of the material, which generates and moves dislocations through the lattice. As the dislocations start interacting with each other they become increasingly entangled and further dislocation movement becomes harder, thus increasing the strength of the material. Cold work is plastic deformation imposed at temperatures and strain rates where the strain hardening is not relieved. As the deformation temperature is increased, the work hardening rate is decreased. Also, in general, hcp metals have a lower work hardening rate than cubic metals. As the material is work hardened, it becomes stronger, but also loses some of its ductility. For recovery of the ductility (at the expense of strength) the material can be recovery annealed, normalised or full annealed.

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1.9.3

Grain refinement The only strengthening mechanism which increases both the strength and toughness of a material is grain refinement. Grain boundaries are effective barriers to dislocation movement so the greater the number of grain boundaries (smaller grain size) the harder plastic deformation is. The grain size is greatly influenced by heat treatment and is thus affected by welding, maintaining a fine-grained HAZ is desirable from a mechanical property point of view and normally, excessive coarsening should be avoided through control of heat input and use of multi-pass welds.

1.9.4

Precipitation hardening Small, evenly distributed second-phase particles distributed in a ductile matrix are commonly used for alloy strengthening. Common examples of precipitation hardened alloys are aluminium-copper and copper-berillium alloys. Precipitation hardening heat treatment is carried out by solid solution annealing to put the second phase into solution, followed by quenching to obtain a supersaturated solution. A further heat treatment or ageing treatment is performed at a lower temperature than before, which allows the precipitation of a second phase to occur. For precipitation to occur, the second phase must be soluble at an elevated temperature and must show decreasing solubility with decreasing temperature. In steels, precipitation hardening is possible without an ageing heat treatment and precipitation of carbides and nitrides can be obtained by slow cooling from the temperature range where they are in solution. This is used to good effect in HSLA steels. A fine dispersion of second phase precipitates confers the highest strength increase, but ductility is reduced. If the ageing treatment temperature is too high or the time is too long then the precipitates become too large and the alloy is then said to be over-aged. Over-aged alloys are soft and ductile.

1.9.5

Quenching Quenching is the rapid cooling of a material. This is used in Fe-C alloys and steels to transform austenite to martensite. With rapid cooling from the austenite phase field (face centred cubic), the austenite does not have time to transform to ferrite and pearlite and instead the carbon atoms are trapped in special locations (octahedral sites of the body centred cubic structure) thus producing a new phase, martensite. The carbon deforms the lattice and thus greatly increases the strength of the material. The strength and hardness of martensite increase with increasing carbon content. Although very hard microstructures are possible, fresh martensite is usually brittle and has very low ductility (unless the carbon content is very low). To improve toughness and increase ductility, quenched steels are tempered to relieve stresses and allow carbon to move to reduce the lattice deformation.

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Revision Questions 1 What is the importance of dislocations? How can dislocations affect the strength of materials?

2 List three types of lattice imperfection and explain how they affect the properties of metals.

3 Describe three strengthening methods for alloys. Can these also be used for pure metals?

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Section 2 Phase Diagrams

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2

Phase Diagrams

2.1

Alloying Alloys are solutions of one or more metals or semi-metals (C and N are examples of semi-metals) in another metal. Alloys are used to obtain materials with superior properties, or to match requirements that cannot be fulfilled by pure metals alone, eg higher strength, corrosion resistance, wear resistance. The atoms of one element can dissolve in another metal to form a solid solution either substitutionally (to form substitutional solid solution) or interstitially (to form interstitial solid solution) depending on the relative sizes of the atoms. Substitution solubility occurs when the atomic radii of the solute and solvent atoms are within ±15% of each other (eg Ni in Cu). Interstitial solubility occurs when the solute atoms are much smaller than the solvent atom (eg C in Fe), so that the solute atoms can reside in the interstitial sites in the solvent lattice.

2.2

Introduction The various structures or phases present when two or more metals are mixed may be represented by equilibrium phase diagrams, where the stable phases with corresponding chemical compositions are plotted versus temperature. These are called equilibrium phase diagrams as they only apply to thermodynamic equilibrium condition of very slow cooling or heating. Equilibrium phase diagrams can be thought of as a map giving the phase or phases of the system at a given temperature and composition. Alloy phase diagrams are useful to engineers in the development of new alloys, definition of heat treatment temperatures, fabrication parameters and evaluation of performance issues such as hot cracking, pitting corrosion and so on. There are three main types of binary alloy system phase diagrams: Binary Alloy Systems

Solid Solution

Eutectic

Combination

Total solubility in liquid and solid phase

Total solubility in liquid and total insolubility in solid phase

Total solubility in liquid and partial solubility in solid phase

The simplest kind of phase diagram is where the two elements form a solid solution and there is complete solubility in the solid and the liquid phase (for example the copper-nickel system). In such an alloy system, there is a range of temperatures through which the alloy freezes, where both solid and liquid are present as a mush before complete solidification occurs.

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Solidus is the phase boundary which limits the top of the solid phase in the phase diagram. Liquidus is the phase boundary which limits the bottom of the liquid phase field. In other words, the alloy is completely liquid above the liquidus line and solid below the solidus line. The second type of phase diagram, called eutectic, is where there is total mixing in the liquid, but no mixing in the solid phase (eg the lead-antimony, or bismuth-cadmium system).

An eutectic is a mixture of two (or more) substances which liquefies at the lowest temperature of all such mixtures. It has a specific composition and it freezes at one fixed temperature. It can sometimes behave like a phase in itself and usually consists of a lamellar structure. It is possible an alloy system can possess more than one eutectic alloy. Some examples of eutectic alloy systems include that of Bi-Cd, Al-Si, Pb-Sn etc.

2-2

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More commonly, there is total solubility in liquid phase and partial solubility in solid phase (particularly where there is a high percentage of one of the elements, with a small amount of solute atoms). An example is the copper (Cu)-silver (Ag) system. Other examples include that of lead (Pb)-tin (Sn), Pb-magnesium (Mg) alloy systems. In the diagram (Cu-Ag system) the phases the regions with solid solubility of Ag in Cu and Cu in Ag are represented by the Greek symbols  and , respectively. The black lines (or the phase boundary) that represents the limit of solid solubility is also called the solvus.

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2.3

Using phase diagrams Phase diagrams can be used to: 1 Determine the phases present at different temperatures.

2 Find the temperatures at which solidification starts/ends when an alloy undergoes equilibrium cooling from its melt.

3 Determine the composition of phases at a specific temperature (T).

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4 Determine the quantity of phases present at a specific temperature (T) using the lever rule (Described in greater detail in the next section):

At 1232oC, the amount of solid phase is given by dividing the length A by the length C:  

0 0 1 0 7 6 5

6 6 6 6

%

d i l o S

︵ ︶



Therefore there is 66.7% solid at 1232oC. As the only other phase present at 1232oC is liquid the amount of liquid present at this temperature is 100-66.7=33.3%. The exact same result is also obtained by using the lever rule:  

0 0 1 7 7 5 5

2.4

0 6 6 6

% d i u q i L

︵ ︶



The lever rule The lever rule is used to predict the proportions of phases present at a specific overall composition and temperature. It can be used for solid to solid phase changes, as well as solid and liquid phase changes. The idea of the lever rule is to imagine a lever, where the quantities of the phases balance the lever arms, so in this example, the percentage of  phase needs to be greater than the percentage of  phase for the see-saw to be in equilibrium.

Before any calculations can be made a line (called a tie line) is drawn on the phase diagram to determine the percent weight of each element. This is accomplished by drawing a straight line at the temperature of interest, starting from the point of the overall composition of the alloy and extending it to the phase boundaries at each side of this composition.

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In the example given here, the tie line is drawn at 1232°C and extends from 60% Cu and 40% Ni (overall composition) to the boundaries of the solidus and liquidus lines. The percent weight of copper in the liquid at this temperature is 66%, while the percent of copper in the solid is 57%. The percent weight of solid and liquid can then be calculated using the following lever rule equations: % Solid 

A 66  60 * 100  * 100  % Solid  66.66% C 66  57

% Liquid 

60  57 * 100  % Liquid  33.34% 66  57

In fact as the total necessarily has to be 100%, when two phases are involved, the amount of a given phase is given by 100-% of other phase. In the example above, % liquid is equal to 100-66.66, which is equal to the 33.34% calculated using the lever rule.

2.5

Solidification and microstructure Two examples are given of the solidification of alloy phases and the microstructures that result. In the bismuth-cadmium system, an alloy containing 80% cadmium will start to solidify at around 300°C with the formation of crystals of pure cadmium in the liquid. The crystals will grow as the alloy cools down to 145.5°C, at which point all of the remaining liquid will freeze as a eutectic containing lamellae (layers) of bismuth and of cadmium.

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In the copper-silver system, an alloy of 30% silver will start to solidify at around 900°C with the formation of crystals of alpha phase (), which is a copper-rich solid solution of copper and silver. The  crystals grow as the temperature cools, but the composition of the solid changes as it continues to freeze. This means that the  crystals will have a composition slightly higher in copper in the centre than the outside, which is called coring. When the temperature reaches 780°C the composition of the solid  phase that is solidifying is nearly 10% silver and the composition of the surrounding liquid reaches the eutectic composition (around 60% silver). At this temperature the remaining liquid all solidifies as a eutectic of lamellae or layers of  (copper-rich) and  (silver-rich) phases.

2.6

Solidification and coring During solidification, the first solid to form (1 in the diagram) has a different composition to the last solid (5 in the diagram). Therefore chemical composition and properties vary through the dendrite. If there is a large distance between liquidus and solidus lines, or if there is a high cooling rate, then the inhomogeneity will be larger. Such inhomogeneity is often termed coring. It occurs when the cooling rate is sufficiently rapid so that significant diffusion is prevented; this results in a concentration variation in the solidified alloy. The problem can only be corrected by heat treatment for a long time at high temperature (eg 24hr at 1000°C for steels), followed by slow cooling. This allows the atoms to diffuse and homogenise the material.

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2.7

Age hardening The shape of the lines in the diagram below the eutectic temperature show that the solubility of one element in the other reduces as the temperature decreases. When the solubility limit of one element in the solid phase of the other is exceeded, particles of one solid phase will precipitate in the other solid phase, requiring slow cooling (equilibrium conditions) in order to occur. At normal cooling rates precipitates tend to nucleate on existing grain boundaries, however, these second phase precipitates can be used to age harden the alloy. Often the second phase particles are not simply the second element but an intermetallic compound of the two, for example, copper forms a compound with aluminium, CuAl2 and this is the second phase that can be used for age-hardening Al-Cu alloys.

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An age hardening heat treatment involves dissolving any second phase particles using a solution heat treatment at high temperature to take the alloy into the single phase region of the phase diagram (dot on right hand figure) and then quenching. The second stage nucleates and grows small precipitates, by using a precipitation heat treatment, heating the alloy in the two-phase region of the phase diagram, to an intermediate temperature so that there is sufficient energy for diffusion and for precipitates to form on the grain boundaries.

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Age hardening increases the hardness and tensile strength of the alloy. The size of the precipitates become coarser as the ageing temperature is increased and the ageing temperature can control the distribution of the second phase as well as the precipitate size. Having many fine and uniformly dispersed precipitates is more effective at increasing strength than having a few coarse precipitates. If heating is too prolonged or excessive, the alloy re-softens due to over-ageing of the precipitates, which become too large to be effective.

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This table summarises the age hardening procedure for the example of an aluminium alloy. Fast cooling (no diffusion)

Single phase 

 supersaturated with secondary  Heat up to 200°C (artificial ageing)

 supersaturated with secondary 

 + fine precipitates of 

Keep at room temperature for long time (natural ageing)

 supersaturated with secondary 

 + fine precipitates of 

Heat up to 500°C (over ageing)

 supersaturated with secondary 

+coarse ppts of 

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2.8

Solid state transformations After initial solidification, it is possible for further phase transformations to occur within the solid state, ie new solid phases nucleate from the existing solid phases. A similar reaction to the eutectic reaction in liquids can occur in solid state transformations, but it is known as eutectoid reaction when it occurs in the solid state.

2.9

Introducing the iron carbon phase diagram The iron-carbon phase diagram as a whole is complex but breaks down into three smaller phase diagrams the:   

Peritectic. Eutectic. Eutectoid reactions.

We are only interested in the iron end of the diagram so the compound cementite (Fe3C) that has 6.67wt% C is used as the right hand edge of the diagram. Peritectic reaction Is a three-phase reaction in which, upon cooling, a liquid and a solid phase transform to give one different solid phase. In the Fe-C system this occurs at high temperature and is characterized by the transformation of liquid plus delta ferrite to austenite. Eutectic reaction Occurs in cast irons with more than 2.1 wt% carbon as with the peritectic, the eutectic occurs at a single temperature. In the Fe-C system, the peritectic occurs at 1148oC and is characterized by the transformation of liquid to austenite plus cementite.

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The eutectoid reaction occurs at 727oC and involves the transformation of the residual austenite to a lamellar structure of ferrite and cementite, called pearlite.

The critical temperature lines A1 and A3 denote the critical temperatures for formation of certain microstructures and heat treatments. The A1 line occurs at T = 723°C and is the temperature below which austenite transforms into the eutectoid of ferrite and pearlite. The A3 line is the slope between the austenite () and ferrite/austenite (/) phase fields. The heating and cooling rates can effectively raise or lower these lines respectively, as can certain alloying elements (eg Cr, Mn).

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The room temperature microstructure of slowly cooled steel depends on the carbon content of the alloy. Hypoeutectoid steels, containing less than 0.77wt-% C will have a ferrite plus pearlite microstructure, with increasing pearlite content as the carbon content is increased, up to 0.77wt-% carbon where the microstructure will be fully pearlitic, such as in railway steels. Alloys with carbon content greater than 0.77wt-% carbon, hypereutectoid steels will have a microstructure of cementite and pearlite under equilibrium conditions with increasing cementite as the amount of carbon is increased.

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2.10

Advantages and disadvantages of phase diagrams The advantages of using phase diagrams are that one diagram is valid for an entire alloy system and it is possible to determine at a specific temperature, the phases present, their composition and the percentage of each phase for various alloy compositions (using the lever rule). It is possible to determine the melting/freezing points and solubility’s of all the alloys in the system and is useful for determining heat treatment temperatures, since it shows the phase changes that occur with a change in composition and/or temperature. However, the phase diagram does not indicate the structural arrangement of the phases ie lamellae, globules, films, nor does it indicate the structural distribution of the secondary phases ie either distributed within grains or deposited at grain boundaries. The phase diagram shows only the equilibrium (slow cooling) state, which is not representative of some production or welding thermal conditions.

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Revision Questions 1 Sketch a phase diagram for a binary alloy system showing partial solid solubility and complete liquid solubility, eg the Cu/Ag phase diagram.

2 On your sketch mark the position where an age-hardenable copper-rich alloy would need to be heated to dissolve any second phase precipitates. How would this alloy be subsequently heat treated in order to obtain a dispersion of fine second phase precipitates?

3 What phases are formed during the peritectic reaction in the iron-carbon phase diagram?

4 Describe the phase formation sequence and microstructure formation as a hypoeutectoid steel is cooled from molten.

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Section 3 Manufacture of Steels

Rev 2 July 2011 Manufacture of Steels Copyright  TWI Ltd 2012

3

Manufacture of Steels

3.1

Modern steelmaking Steel is essentially an alloy of iron (Fe) and carbon (C) and is generally produced in a two stage process known as primary steelmaking. The first stage involves the extraction of raw iron (also referred to as pig iron) from iron ore inside a vessel known as a blast furnace. In its raw form, pig iron contains high levels of carbon, sulphur and phosphorus, making it very brittle and severely limiting its usefulness as an engineering material. In the second stage of primary steelmaking, the amount of carbon and other unwanted elements in the molten pig iron is reduced using the Basic Oxygen Steelmaking (BOS) process. The resulting steel exhibits superior toughness and ductility to the raw pig iron and can be used in a wide variety of engineering applications. Engineering steel can also be produced from scrap steel in an electric arc furnace. Further reductions in carbon content or adjustments to the steel composition that may be required by the end user necessitate additional processing steps such as ladle refining and vacuum degassing and this is known as secondary steelmaking.

In an integrated steelworks the entire manufacturing process, from the extraction of iron from its ores to the production of steel, is carried out at a single production site. For more information about blast furnaces and steel manufacture see Davies: Science and Practice of Welding vol. 1 pages 65-79.

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3.2

Primary steelmaking Primary steelmaking generally refers to the extraction of molten iron from its ores followed by conversion to steel in the basic oxygen steelmaking (BOS) process, although high quality steels can also be produced entirely from scrap in an electric arc furnace. Both processes are discussed in the following sections:

3.2.1

Iron ores Most metals exist in nature as ores, which are generally metal oxides. Iron can be found in the following forms:    

Haematite (Fe2O3) is reddish grey or blackish red and contains approximately 70%Fe. Magnetite (Fe3O4) is greyish black or iron black and is a natural magnet, containing approximately 72%Fe. Limonite (Fe3O(OH).nH2O) is a hydrated iron oxide, yellowish brown and contains approximately 48%Fe. Siderite (FeCO3) is greenish grey or brown grey and contains approximately 63%Fe.

To extract the iron from the ore the oxygen needs to be removed in a process called reduction. Molten iron is initially produced in a vessel known as a blast furnace. 3.2.2

The blast furnace

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The blast furnace is a large vertical stack, roughly 30m tall, lined with refractory bricks. The temperatures within the blast furnace increase from approximately 250°C at the top to over 1900°C at the bottom. Blast furnaces tend to be operated continuously due to the cost and difficulty of stopping the process once the furnace is up to temperature and often run for several years at a time. The raw materials of iron ore, coke (which is almost pure carbon) and a flux (typically limestone) are introduced continuously at the top of the furnace adding to the furnace contents or burden. Hot air is blasted in near the base of the furnace through water-cooled nozzles known as tuyères. Blast furnace reactions begin at the base of the furnace where the temperature is 1800ºC. The coke is burnt to form carbon dioxide and carbon monoxide. C (from coke) + O2 (from hot air) → CO2 + heat CO2 + C → 2CO At the top of the furnace between 500 and 800ºC the iron ore reduces to iron oxide as it reacts with the carbon monoxide. 3Fe2O3 + CO → CO2 + 2Fe3O4 Fe2O3 + CO → 2FeO + CO2 Fe3O4 + CO → CO2 + 3FeO In the middle of the blast furnace at 1000ºC the iron oxide reduces to liquid iron. FeO + CO → CO2 + Fe

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At the bottom of the furnace at 1500ºC further reduction to liquid iron occurs. FeO + C → CO + Fe The limestone decomposes and reacts with impurities to produce the slag. For example: CaCO3 → CaO + CO2 FeS + CaO + C → CaS(slag) + FeO + CO The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form a fayalitic slag which is essentially calcium silicate, CaSiO3. SiO2 + CaO → CaSiO3 The oxygen supplied in the hot air blast supports combustion of the coke forming carbon monoxide (CO). The CO reduces the iron ore to molten iron which seeps down through the furnace burden under the action of gravity and collects at the bottom of the furnace in a structure known as the hearth. The limestone decomposes in the heat of the furnace to calcium oxide (CaO) which reacts with impurities in the ore such as silica to form a molten slag. This slag also collects at the base of the furnace above the molten iron. The molten pig iron and slag are extracted from the furnace through tap holes, with the iron being transported to the next processing step in ladles or refractory lined torpedo-shaped rail cars. As the molten iron travels down through the furnace it picks up high levels of carbon (up to 4.5%) and other impurities (approximately 1.5% silicon, 0.05% sulphur and 0.15% phosphorus) from the burden. As a result solidified pig iron exhibits low tensile strength, very low ductility and contains large amounts of dissolved gases, severely limiting its usefulness as an engineering material. Typically pig iron is converted into steel using the basic oxygen steelmaking (BOS) processes, which reduces the amount of carbon, sulphur and phosphorus dissolved in the molten metal. Prior to being transferred to the BOS vessel, the molten pig iron is frequently pre-treated with powdered magnesium, iron oxide and lime in order to reduce the levels of sulphur, silicon and phosphorus respectively.

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3.2.3

The basic oxygen steelmaking (BOS) process

Molten pig iron from the blast furnace is charged into the BOS vessel, which is lined with a high temperature resistant refractory material. Scrap steel is added to the BOS vessel prior to charging with molten pig iron in order to cool the charge and protect the refractory lining. A water-cooled lance is lowered into the converter and high purity oxygen is blown through the molten metal, causing the combustion of carbon dissolved in the metal and forming carbon monoxide and carbon dioxide gasses which escape from the top of the vessel, thereby reducing the carbon content of the steel. Other unwanted elements such as silicon and phosphorus react to form acidic oxides which combine with basic fluxes added to the BOS converter forming a slag which mixes with the molten metal during blowing to form an emulsion, thereby facilitating the refinement of the steel. After the process is complete the slag separates from the steel and floats on its surface, allowing the steel to be tapped into a ladle whilst leaving the slag in the BOS converter to be tapped off separately. A typical steel chemistry produced by the BOS process is as follows: 0.2%C, 0.2%Si, 0.8-1.0%Mn, 0.025%S and 0.020%P. While a carbon content in the region of 0.2% may be acceptable to some end users, it is often necessary to reduce the carbon content further and/or make compositional adjustments, thus requiring additional processing steps. A basic slag or lining containing lime (CaCO3) removes the Si, Mn, C, P and S from the iron. Deoxidisers such as ferro-manganese, ferro-silicon or aluminium are also required. An acidic slag or lining containing silica (SiO2) removes Si, Mn and C by oxidation, but cannot remove S & P. 2Al + 3FeO (soluble) = 3Fe + Al2O3 (solid) Mn + FeS (soluble) = Fe + MnS (slag) 2CO (soluble) + Si = 2C + SiO2 (solid)

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3.2.4

Electric arc furnace (EAF) steelmaking

The electric arc furnace offers an alternative route for the production of engineering steels to the traditional (and resource-intensive) method of reducing iron ore in the blast furnace followed by refinement in the BOS converter. The structure of an electric arc furnace used for steelmaking consists of a refractory-lined shell with a retractable roof, through which up to three graphite electrodes protrude. The primary feedstock in the electric arc steelmaking process is typically scrap steel, although some pig iron or directly reduced iron may be used. The furnace charge is melted by a high energy electric arc struck between the electrodes and the charge. Oxygen is introduced to the molten steel through a lance in order to remove carbon and other unwanted elements and flux is added to react with impurities in the steel forming a slag which floats on the molten steel. The slag layer acts as a thermal barrier, helping to prevent excessive heat-loss from the molten bath, thus allowing greater thermal and electrical efficiency. The flux is typically composed of burnt lime (calcium oxide) and dolomite (magnesium oxide) and can either be charged with the scrap or blown into the furnace during melting. Once the temperature and composition of the melt is correct, the steel is tapped into a ladle by tilting the furnace. The slag remains in the furnace to be tapped off separately. The principal advantage of the electric arc steelmaking process is the fact that new steel can be produced entirely from scrap. The process is also extremely flexible and unlike the blast furnace, can be started and stopped to suit demand. While electric arc furnaces represent a comparatively low capital investment the operating costs can be high (power rating can be up to 150,000kW).

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3.3

Secondary steelmaking After the steel is tapped from the BOS vessel or the electric arc furnace into a ladle, further adjustments can be made to the steel composition prior to casting in order to meet the specification set out by the end user. This is known as ladle refining or secondary steelmaking. Additions of desirable elements such as vanadium, titanium and manganese can be made to alter the final properties of the steel. Oxygen dissolved in the molten steel after the BOS process can be removed by adding small amounts of aluminium, which readily reacts with the dissolved oxygen to form an aluminium oxide slag which can be skimmed from the surface of the melt prior to casting. Alternatively oxygen and other dissolved gasses can be removed through a process called vacuum degassing, in which the molten steel is gently agitated under very low pressures in a specially designed vessel. Finally the molten metal can be stirred by blowing with argon gas, ensuring uniformity of temperature and composition before the steel is cast. Ladles often incorporate a method for maintaining the temperature of the melt during secondary processing, in which case they are known as ladle furnaces. Heat is typically supplied by an electric or plasma arc or by induction coils.

3.3.1

Ladle refining Ladle refining is often necessary to remove undesirable elements from the steel that can severely degrade its mechanical properties. These impurities include phosphorus, sulphur and residual elements such as oxygen and nitrogen.   

Phosphorus and sulphur are associated with hot cracking problems when welding, however they are also known to improve the machinability of a steel and are frequently added to so-called free machining steels. Residual oxygen can result in the oxidation of desirable alloying elements (eg Si, Mn) and can have a deleterious effect on mechanical properties such as notch toughness. Residual nitrogen can result in nitrides being formed and lead to strain ageing in welds. The yield strength and UTS increase, while the notch toughness and elongation decrease with residual nitrogen. Nitrogen is also a strong austenite stabiliser.

Primary deoxidisation of steel can be achieved with additions of silicon and manganese. Aluminium is a strong deoxidiser used for secondary deoxidisation and also leads to grain refinement by restricting the coarsening of austenite grains after solidification. Nitrogen can be removed through the addition of strong nitride-forming elements such as titanium (Ti), zirconium (Zr) or vanadium (V). Sulphur can be removed with calcium (Ca), or even more readily with rare earth metals such as cerium (Ce), lanthanum (La) and neodymium (Nd), which form stable sulphides with high melting points (over 2200°C) and have a small, globular shape. All of these elements readily form oxides necessitating preliminary deoxidisation prior to their addition.

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Phosphorus is removed by using a basic slag in the following reaction: P + O2 → P2O5 P2O5 + CaO → Ca3PO4 3.3.2

Vacuum Arc Melting (VAM) Vacuum arc melting or VAM is a method of forming a cast ingot with improved microstructure and properties. A consumable cast electrode is remelted using the heat from an arc struck on a starter plate. During the process impurities are removed in the vacuum. Metallurgical reactions include decarburisation to a very low degree, removal of Zn, Mg, Ca, Pb, Cu, Al, Si and Mn. No loss of Cr, Ni, Co, Mo, V or Ti occurs. For removal of S and P a slag is required. The steel has extremely low gas content and isotropic mechanical properties, better fatigue properties, improved notch toughness, greater ductility and better creep resistance. Narrow alloying tolerances can be achieved. The electrode can be prepared by vacuum induction furnace or vacuum arc re-melting. The process requires a relatively long period of time.

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3.3.3

Electroslag refining (ESR) Electroslag refining (ESR) takes place at atmospheric pressure, under a protective layer of conductive slag, with no electric arc. Excellent microstructure can be obtained due to the directional, progressive solidification of a re-melted consumable cast electrode. The resulting steel is free from segregation and non-metallic inclusions and has low gas content. It also has isotropic mechanical properties and it is possible to closely control the chemical composition through metered additions during remelting. Excellent ingot surface (smooth and defect-free) and internal soundness, means there is no need to treat the ingot, the material can be used as-cast. The process is, however, somewhat costly.

3.4

Casting Before the molten steel produced by the BOS or EAF processes can be transformed into useful products it must be cast into ingots or slabs. Casting involves the solidification of molten metal into a desired shape or profile inside a mould.

3.4.1

Ingot casting Whilst ingot casting has largely been superseded by continuous casting for volume steel production, it is still the preferred method for certain speciality, tool and forging steels. The molten steel from the steelmaking process is transferred into a refractory-lined ladle, from which it is teemed into individual ingot moulds where it solidifies. The steel in contact with the mould walls solidifies first producing a fine equiaxed grain structure or chill zone. Large columnar grains then grow inwards from the chill zone into the molten core. The last material to solidify typically forms a coarse equiaxed grain structure at the centre of the ingot. The typical as-cast crystal structure of an ingot can be seen below.

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Gasses (such as oxygen) dissolved in the molten steel can give rise to porosity or other casting defects as the material solidifies. Steels for ingot casting are typically given one of three designations depending on the level of deoxidisation they have been subjected to killed, semi-killed or un-killed.

Pipe defect

Killed steel is completely deoxidised by additions of silicon, aluminium or manganese which react with the dissolved oxygen forming oxides. Aluminium additions also react with dissolved nitrogen forming aluminium nitrides. This almost completely eliminates gas evolution during solidification. Killed steel ingots are therefore characterised by a high degree of chemical homogeneity and very low levels of porosity, however they also suffer from a high degree of solidification shrinkage leading to pipe defects which have to be cropped from the slab prior to processing. The majority of structural steels are killed.

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Semi-killed steel is produced when insufficient amounts of deoxidising agents are added prior to casting to completely remove the dissolved oxygen in the molten metal. The remaining oxygen reacts with carbon forming carbon monoxide which produces moderate levels of porosity in the finished slab. However this also counteracts the solidification shrinkage typical of killed steels, thereby reducing wastage.

Un-killed or rimming steels have little to no deoxidising agents added prior to casting. This leads to the evolution of significant amounts of carbon monoxide during solidification which results in high levels of porosity in the finished ingot. Un-killed steels are therefore unsuitable for structural applications although they may be used for cold-working applications such as wire drawing. Products fabricated from un-killed steels are also prone to developing porosity when welded. The above designations are derived from the behaviour of the molten steel when it is poured into the mould. The violent evolution of carbon monoxide from solidifying un-killed steels contrasts with the passive solidification behaviour of fully deoxidised steels hence the term killed.

Narrow end up can lead to formation of a secondary pipe.

Wide end up means no secondary pipe is formed.

Feeder head design means no pipe forms inside the ingot.

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3.4.2

Continuous casting Continuous casting involves the solidification of molten metal into semifinished billets, blooms or slabs which subsequently require significantly less processing than ingots. For this reason continuous casting has largely replaced ingot casting in modern steelmaking facilities. Molten steel is continuously tapped from a vessel known as a tundish into an open-base copper mould. The steel in contact with the walls of the water-cooled mould rapidly freezes to form a solid shell. The mould is oscillated vertically to prevent the shell from sticking to the mould walls. Below the mould exit, the thin solidified shell supports and contains the still molten core. Guide rolls below the mould continuously withdraw the shell from the mould producing a long vertical strand of solidifying material. Water sprays help to cool the strand as it passes through the guide rolls. The strand is then typically bent through 90 until it is horizontal before it is sheared or flame cut into slabs of desired length once the material has fully solidified. There is a risk of non-metallic inclusions along the centreline of the slab due to low melting point products being trapped between solidifying sides. Continuous casting represents the most cost and energy efficient method for solidifying large volumes of metal into simple shapes for subsequent processing. A high degree of automation is possible with continuous casting and the process produces a product of high quality and uniformity in a range of cross-sections.

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3.4.3

Shape casting Molten steel can be directly transformed into a finished product in a process called shape casting. Casting involves the pouring of liquid metal into a mould which contains a hollow cavity of the desired shape. The molten metal fills the cavity and solidifies following which the part can be ejected or broken out of the mould. Moulds can be permanent (eg the metal moulds used in die casting) or expendable (eg sand casting). Casting allows the production of complex shapes that would be difficult or uneconomical to fabricate via other methods. Careful design of the mould helps to prevent casting defects such as voids or shrinkage porosity from occurring in the finished products.

3.5

Steel processing The ingots and slabs produced by ingot casting and continuous casting are typically subjected to several thermo-mechanical processing steps in order to transform them into useable products and components. For example plate or strip used for structural steel or pipe, or sheet steel used in car body and white goods manufacture is produced through a process known as rolling. Other rolled products such as billets and blooms are subjected to further processing in order to produce finished products such as engine components and steel cable. These processing techniques are described in the following sections.

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3.5.1

Rolling Rolling involves the reduction in thickness of a metal feedstock by passing it between a pair of rotating rolls. The gap between the rolls is less than the starting thickness of the feedstock thus forcing the material to deform as it passes between the rolls. Traditionally rolling was carried out in two stages consisting of an initial stage which converted large cast ingots into slabs or blooms followed by a secondary rolling stage which converted the blooms and slabs into plates, sheets and other products. With the widespread introduction of continuous casting, semi-finished slabs, blooms and billets could be produced directly, thereby eliminating the requirement for the initial rolling stage for volume steel production.   

Slab is used to make plate, sheet and pipe. Typical cross-sectional dimensions: 3000 x 200mm. Bloom is used to make rolled shapes and I-beams. Typical cross sectional dimensions: 150 x 150mm. Billet is used to make bars, rods and wire. Typical cross sectional dimensions: 50 x 50mm up to 120 x 120mm.

Rolling introduces plastic deformation into the material, which, at lower temperatures, results in higher strength and hardness with a corresponding decrease in ductility and toughness. Intermediate heat treatments may therefore be used to compensate for this work hardening.

During cold rolling the as-cast grain structure is elongated and deformed or cold worked leading to an increase in hardness. The deformed material is typically subjected to an intermediate anneal at high temperature between rolling operations in order to recrystallise the sheet, restoring the equiaxed grain structure and increasing ductility. This softened product can then undergo further rolling operations.

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During hot rolling, the steel is held above the recrystallisation temperature (950-1000°C) throughout the process. After each rolling operation the distorted grains spontaneously recrystallise and grain growth occurs before subsequent rolling operations (see figure below). The result of hot rolling is steel with a refined equiaxed grain structure. Steel is then usually heat treated (normalised or possibly quenched and tempered) before being used.

Any inclusions in the steel will become elongated during rolling forming features known as stringers (see micrograph overleaf) which are not altered by annealing operations. Material containing stringers is considered to be anisotropic. It is recognised that stringers can lead to lamellar tearing when the material is welded and it is therefore desirable that they are avoided. Where material properties in the through-thickness direction are required to be guaranteed, the material is designated Z grade (where Z is the throughthickness axis in XYZ coordinates). The X direction corresponds with the rolling direction and generally exhibits the highest tensile strength, Charpy impact toughness and ductility. The Y or transverse direction tends to exhibit slightly poorer mechanical properties than the X direction, with the Z direction often showing the lowest strength and toughness. It is therefore important that the rolling direction of the parent material is considered when designing and fabricating sheet steel components.

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3.5.2

Overview of sheet steel production

The above diagram summarises the most common process route for production of coils of hot rolled steel from molten metal. It can be seen that the molten metal is initially cast into a long strand before being cut into individual slabs which are heated or soaked in a re-heat furnace at a temperature of approximately 1250°C prior to being rolled. The thickness of the steel is progressively reduced by a series of rolling operations until the thickness required by the end user is reached, upon which the steel sheet is coiled ready for delivery.

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3.5.3

Forging Forging is a manufacturing process that involves the deformation and shaping of metals under localised compressive forces. Due to its strength and resistance to deformation, steel is forged at high temperatures. This is known as hot forging. Forging techniques can be generally divided into two groups; open and closed die. Open-die forging is a basic technique for producing simple shapes using a moveable ram or hammer and a static anvil. Open-die forging is frequently used to roughly shape a component prior to closed-die forging. The closed-die forging process uses a more complex-shaped die that completely encloses the workpiece, forcing it into the desired shape. The flash generated around the periphery of the component is removed in subsequent finishing operations. Closed die forging gives accurate component dimensions, however tooling and maintenance costs can be high. Forged products typically exhibit forging lines which are revealed by etching as shown in the figure below.

Forging lines

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3.5.4

Extrusion Extrusion is used to produce a length of material with a fixed cross-sectional profile by forcing the material through a shaped die under high pressure. It is possible to extrude steels however very high extrusion temperatures and pressures are required (in excess of 1200°C and 100,000psi), with correspondingly high tooling and maintenance costs. Glass powder is used as a lubricant.

3.5.5

Drawing Drawing is a process in which the cross-sectional profile of a wire or pipe is reduced by pulling through a drawing die. Although similar in concept to extrusion, drawing differs in that the material is pulled rather than pushed through the die. Tube drawing necessitates the use of a mandrel which fits inside the die to maintain the shape of the pipe as it is drawn.

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Revision Questions 1 When would the electric arc furnace be used to produce steel in preference to the basic oxygen furnace?

2 During ladle refining what elements are added as deoxidisers?

3 How can casting defects be minimised when making ingots?

4 What are the advantages and disadvantages of the continuous casting method?

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Section 4 Material Testing

Rev 2 July 2011 Material Testing Copyright  TWI Ltd 2012

4

Material Testing

4.1

Mechanical testing Mechanical testing produces data that may be used for design purposes or as part of a welding procedure or operator acceptance scheme. The most important function may be providing design data, since it is essential that the limiting values a structure can withstand without failure are known. The materials properties that can be determined by mechanical testing include, yield strength, UTS, ductility, notch (impact) toughness, fracture toughness, crack arrest properties, hardness, corrosion, creep and fatigue resistance, physical properties (density, thermal conductivity, etc). More information on material testing can be found in TWIs Job Knowledge articles on its web site. Inadequate control of material properties by the supplier, or incompetent joining procedures and operatives are, however, equally crucial to the supply of a product that is safe in use and fit for purpose. Mechanical tests are employed to ensure that both parent material and joint properties are met. For example the tensile test may be used to determine the yield strength of a material for use in design calculations and to ensure that the material complies with the material specification's strength requirements. Mechanical tests may be divided into quantitative or qualitative tests. A quantitative test provides data that will be used for design purposes, eg tensile or CTOD tests. A qualitative test is where the results will be used for making comparisons or as a go/no go test, such as the bend test.

4.2

Tensile testing The test is carried out by gripping the ends of a suitably prepared standardised test piece in a tensile test machine and then applying a continually increasing uniaxial load until failure occurs. Test pieces are standardised so that results are reproducible and comparable. Specimens are said to be proportional when the gauge length is related to the original cross sectional area. European and ASME codes give gauge lengths of approximately 5x and 4x specimen gauge diameter respectively.

Typical tensile test specimen configuration.

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Examples of standards covering tensile testing are:    

BS EN ISO 6892-1:2009 Metallic materials. Tensile testing. Method of test at ambient temperature. BS EN 876-1995 Destructive tests on welds in metallic materials longitudinal tensile test. BS EN ISO 4136: 2011 Destructive tests on welds in metallic materials transverse tensile test. ASTM E8-09 Tension testing of metallic materials.

Both the load (stress) and the test piece extension (strain) are measured and from these data an engineering stress/strain curve is constructed. From this curve we can determine:  

  

Ultimate Tensile Strength (UTS): the load at failure divided by the original cross sectional area. In EN specifications this parameter is also identified as Rm. Yield point or yield strength: the stress at which deformation changes from elastic to plastic behaviour. Below the yield point the specimen would return to its original length if unloaded. Above the yield point, permanent plastic deformation has occurred. In EN specifications this parameter is also identified as Re. When there is no yield plateau it is possible to define a proof strength, the stress at which an arbitrarily defined, non-proportional extension is achieved. Usually the 0.2% proof strength is defined identified as Rp0.2. By re-assembling the broken specimen the percentage elongation can be measured ie how much the gauge length had extended at failure. A further parameter is the percentage reduction of area ie how much the gauge diameter has necked or reduced at the point of failure. In EN specifications this parameter is identified as Z.

Typical stress strain curve generated during tensile testing.

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Stress-strain curve with the proof stress for a fixed deformation indicated.

Schematic of a tensile specimen before and after testing showing the elongation of the gauge length and necking of the gauge diameter.

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4.3

Cross-weld tensile testing To approve a butt welding procedure, most specifications require tensile tests to be carried out. These are generally cross-weld (or cross-joint) tensile tests of square or rectangular cross section oriented across the weld, so that both parent metals, both HAZs and the weld metal itself are tested. The excess weld metal in the cap of the weld may be left in place or machined off. The specifications require only the UTS and position of the fracture to be recorded from a cross-weld tensile test. It is possible to measure yield strength, elongation and reduction of area of cross-joint specimens, but the fact that there are at least three different areas with dissimilar mechanical properties makes such measurements inaccurate and unreliable, although they are sometimes reported for information purposes. The cross-weld strength is usually required to exceed the minimum specified UTS of the weaker parent metal. In most situations the weld metal is stronger than the parent metal ie it overmateches and hence, failure occurs in the parent metal or the HAZ at a stress above the specified minimum.

Tensile test specimen

Orientation of a cross-weld tensile specimen.

4.4

Validity of tensile data Tensile samples are assumed to be representative of the bulk of the material, but this is not always the case. The tensile strength of a casting, for instance, is often determined from a specimen machined from a riser and this will normally have a grain size different from that of the bulk of the casting. In this instance an alternative approach would be to machine test samples from an appropriate thickness region of a stepped block cast from the same heat. A rolled steel plate has different properties in the longitudinal, transverse and through thickness directions. Material specifications therefore sometimes require the tensile test to be taken transverse to the rolling direction so that the steel is tested across the elongated grains which would typically be the lower strength, lower ductility direction. In some instances, guaranteed through-thickness properties are required for example in Z grade steels.

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The size of a product can also influence the properties, as during heat treatment the section thickness will affect the cooling rate, with slower cooling rates and hence softer structures, at the centre of thicker sections. This is dealt with in material standards by specifying what is known as the limiting ruling section, the maximum diameter of bar at which the required mechanical properties can be achieved at the centre. In addition to variations of the properties due to the shape of the specimens and the testing temperature, rate of loading will also affect the results; faster loading can show apparently higher tensile strength.

4.5

Charpy impact testing The Charpy impact test involves striking a standard specimen with a controlled weight pendulum travelling at a set speed. The amount of energy absorbed in fracturing the test piece is measured and gives an indication of notch toughness of the test material. The test allows metals to be classified as being either brittle or ductile. A brittle metal will absorb a small amount of energy when impact tested, a tough ductile metal a large amount of energy. Testing is generally carried out at a single temperature, for example the minimum design temperature, with triplicate tests performed. Alternatively tests can be carried out over a range of temperature to generate a transition curve. It should be emphasised that the results can usually only be compared with each other or with a requirement in a specification. Whilst they can be used to estimate the fracture toughness of a weld or parent metal, conservative assumptions are necessarily made and thus this is not a very good substitute for actual fracture toughness measurement. Examples of standards covering Charpy testing are:   

ASTM E23-07ae1 Standard test methods for notched bar impact of testing on metallic materials. BS 131-6: 1998 Notched bar tests – Part 6: Method for Precision Determination of Charpy-V Notch Impact Energies for Metals. BS EN ISO 148-1: 2010 Metallic Materials – Charpy Pendulum Impact Test Part 1: Test Method.

The standard Charpy V specimen is 55mm long, 10mm square and has a 2mm deep notch with a tip radius of 0.25mm machined on one face. In addition to a V notch, the Charpy specimen may be used with a keyhole or a U notch. The keyhole and U notch are used for testing brittle materials such as cast iron and for testing plastics.

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Charpy test specimen geometry.

Charpy test configuration.

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4.6

Crystallinity and lateral expansion As well as the impact energy, it is also possible to measure the percentage crystallinity and the lateral expansion. These can be found as a requirement for Charpy tests in some specifications. The appearance of a fracture surface gives information about the type of fracture that has occurred. A brittle fracture is bright and crystalline; a ductile fracture is dull and fibrous. The percentage crystallinity is therefore a measure of the amount of brittle fracture, determined by estimating the amount of crystalline or brittle fracture on the surface of the broken specimen. Intergranular fracture can also occur, but is not very common.

Photograph showing an untested and tested Charpy V-notch specimens.

Lateral expansion is a measure of the ductility ductile metal is broken, the test piece deforms ears being squeezed out on the side of the specimen. The amount by which the specimen expressed as millimetres of lateral expansion.

No lateral expansion brittle fracture

of the specimen. When a before breaking, a pair of compression face of the deforms is measured and

a+b = lateral expansion ductile fracture

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4.7

Fracture toughness testing A quantitative assessment of fracture toughness can be made using the crack tip opening displacement (CTOD) test. The data generated allows a fitness-for-purpose analysis to be carried out which enables a critical defect size to be calculated. So prior to fabrication, realistic acceptance standards can be set and decisions on appropriate NDE techniques and detection sensitivities can be made. Whilst the CTOD test was developed for the characterisation of metals it has also been used to determine the toughness of non-metallics such as weldable plastics. The CTOD test is used when some plastic deformation can occur prior to failure, as it allows the tip of a crack to stretch and open, hence tip opening displacement. Unlike the inexpensive 10 x 10mm square Charpy-V test piece with a blunt machined notch, the CTOD specimen may be the full thickness of the material will contain a genuine crack and will be loaded at a rate more representative of service conditions. Conventionally three tests are carried out, to ensure consistency of results, at a single temperature, for example the minimum design temperature. On occasions, testing may be carried out over a range of temperatures to generate a transition curve. The test piece itself is proportional with the length, depth and thickness of each specimen inter-related so that, irrespective of material thickness, each specimen has the same proportions. There are two basic forms namely a square or a rectangular cross section specimen. If the specimen thickness is defined as W, the depth will be either W or 2W with a standard minimum length of 4.6W. A notch is machined at the centre and then extended by generating a fatigue crack so that the total defect length is half the depth of the test piece as shown below. For example a test on a 100mm thick weld will require a specimen measuring 100mm wide, 200mm deep and  460mm long. This is an expensive operation, the validity of which can only be determined once the test has been completed.

Proportional rectangular cross section CTOD specimen.

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The test is performed by loading the specimen in three points bending and measuring the amount of crack opening. This is done by means of a strain gauge attached to a clip placed between two accurately positioned knife edges at the mouth of the machined notch (see below).

Typical test arrangement, the specimen can be easily immersed in a cooling bath. As bending proceeds, the crack tip plastically deforms until a critical point is reached when the crack has opened sufficiently to extend by ductile tearing or to initiate a cleavage crack (brittle fracture). This may lead to either partial or complete failure of the specimen. As a rule of thumb, a CTOD value of between 0.1 and 0.2mm at the minimum service temperature is regarded as demonstrating adequate toughness. The values that are required for the calculation of fracture toughness are firstly the load at which fracture occurs and secondly the amount by which the crack has opened at the point of crack propagation (see below).

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Configuration of CTOD specimen immediately prior to crack propagation.

The location of the notch in the weld HAZ or parent metal is important, as an incorrectly positioned fatigue crack will not sample the required region, making the test invalid. To be certain that the crack tip is in the correct region, polishing and etching followed by a metallurgical examination are often carried out prior to machining the notch and fatigue cracking. This enables the notch to be positioned very accurately. Similar examination should be carried out after testing, as further confirmation of the validity of the test results. In general the causes of test failure/invalidity can unfortunately only be determined once the test has been completed and the crack surface examined. The precise length of the fatigue crack is measured, this is required for the analysis, but if the length of the crack is not within the limits required by the specification the test is invalid. If the fatigue crack is not in a single plane, if the crack is at an angle to the machined notch or if the crack does not sample the correct region, the test will need to be repeated. CTOD testing is covered by BS 7448 Parts 1, 3 and 4 fracture mechanics toughness tests, BS EN ISO 15653-2010 metallic materials- method of test or determining quasistatic fracture toughness of welds and ASTM E18202009E1 standard test method for measurement of fracture toughness.

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4.8

Bend testing The bend test is a simple and inexpensive qualitative test that can be used to evaluate both ductility and soundness of a material. It is often used as a quality control test for butt welded joints, having the advantage of simplicity of both test piece and equipment. No expensive test equipment is needed, test specimens are easily prepared and the test can be carried out on the shop floor as a quality control test to ensure consistency in production. The bend test uses a coupon that is deformed in three points bending to a specified angle. In a guided bend test, the coupon is wrapped around a former of specified diameter and is the type of test specified in the welding procedure and welder qualification specifications. The former diameter is related to material thickness and the angle of bend can be 90º, 120º or 180º. The outside of the bend is extensively plastically deformed, so that any defects in, or embrittlement of, the material will be revealed by the premature failure of the coupon. A defect of 3mm or more is cause for rejection.

Below approximately 12mm material thickness, transverse specimens are usually tested with the root or face of the weld in tension. Material over 12mm thickness is normally tested using the side bend test, which tests the full section thickness.

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4.9

Fatigue testing Fatigue failure can occur at a fluctuating load well below the yield point of the metal and below the allowable static design stress, with little or no deformation at failure. The number of cycles at which failure occurs may vary from a couple of hundreds to millions. To quantify the effect of these varying stresses, fatigue testing is carried out by applying a particular stress range and this is continued until the test piece fails. The number of cycles to failure is recorded and testing is repeated at a variety of different stress ranges. By testing a series of identical specimens, it is possible to plot an S/N curve, a graph of the applied stress range, S against N, the number of cycles to failure. These can be developed for parent materials and welds. The direction of the load, environment and shape of the component all affect the fatigue life. In an un-welded component, the majority of fatigue life is spent initiating the fatigue crack, with a smaller proportion spent in the crack propagating through the structure. In a welded component, the majority of the fatigue life is spent in propagating a crack, since small intrusions at weld toes act as initiation sites for fatigue. This gives welded components much shorter fatigue lives, with some dependence on the configuration of the welded joint.

Comparative S/N curves for unwelded and welded materials.

Standards for fatigue testing include:   

ASTM E647 - 08e1 standard test method for measurement of fatigue crack growth rates. ASTM E606-04e1 standard practice for strain-controlled fatigue testing. BS ISO 12108:2002 metallic materials. Fatigue testing. Fatigue crack growth method.

For more information on fatigue testing see TWIs Job Knowledge articles 78-80.

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4.10

Creep testing Creep testing normally involves the application of a constant axial load to a test specimen in a similar manner to a tensile test. Testing usually involves tensile loading, although a compressive load could be adopted. The test specimen design is based on a standard tensile specimen, but should be machined to tighter tolerances than a standard tensile test piece; in particular the straightness and surface finish are important. The typical set-up involves a vertically mounted cylindrical or rectangular cross-section specimen with a constant load applied via means of a dead weight and lever system. A typical creep test set-up is shown schematically below.

The creep test set-up.

Tight control of temperature is required during the test and this is usually achieved with the use of a tubular furnace mounted onto the frame of the test rig. The temperature is typically thermostatically controlled to within  23C and monitored by a thermocouple attached to the gauge length of the specimen. In some instances detailed measurement of creep strain is required. This is achieved through the use of sensitive extensometers which are able to measure extensions of 10-3 or 10-4mm. The results of the test are plotted as strain versus time to give a creep curve, from which it can be seen that creep occurs in three distinct phases. In the region of primary or transient creep, the strain rate is initially high but gradually reduces as the material work hardens to a constant rate in the secondary or steady state creep regime. The rate of steady state or secondary creep at a constant stress increases with temperature.

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In the primary and secondary creep regimes a number of mechanisms are believed to operate including vacancy diffusion, dislocation movement and climb and grain boundary sliding. The steady state or secondary creep frequently forms the bulk of the creep life of a component, although at very low temperatures, only primary creep occurs and at very high temperatures primary and tertiary creep merge. At the end of the steady state region, the creep strain rate increases in an unstable manner, this is the tertiary creep stage and eventually rupture occurs. The onset of tertiary creep arises due to the formation of small cavities/voids on grain boundaries or precipitates, or localised necking of the material which, in turn, leads to an increase in the effective stress (due to a reduction in the cross-sectional area) which increases the strain rate. Growth and linkage of the creep cavities leads to the formation of cracks and ultimately leads to creep rupture.

Typical creep curve of strain versus time showing the three stages of creep.

A more simplified creep test is the stress rupture test in which the time to rupture at a specific temperature and stress is determined. In this case no measurement of strain is required, negating the need for expensive extensometry. Further cost savings can be achieved through the use of multi-specimen strings within a single furnace, with the temperature of each sample generally recorded separately via thermocouples. When a single specimen breaks, the load on all samples is removed and in most cases the equipment is designed such that failure of a specimen stops the clock and switches off the furnace. The broken specimen would then be removed from the string and the test re-started. Creep testing standards include:  

BS EN ISO 204: 2009 Metallic Materials. Uniaxial Creep Tests in Tension. Method of test. ASTM E139-06 Standard Test Methods for Conducting Creep. CreepRupture and Stress-RuptureTtests of Metallic Materials.

For more information on fatigue testing see TWIs job knowledge article 81.

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4.11

Corrosion testing Welds are often the area most susceptible to corrosion due to physical design and influencing factors such as residual stress, modified metallurgy, weld metal properties, etc. However, it is necessary to understand how parent material and weldments would perform in a particular environment. There is a wide range of corrosion testing standards, which generally involve immersing a standard size specimen in a specified environment (which may involve temperature, stress, test solution and test gas). After exposure for a given time the specimen is visually examined and sectioned for signs of corrosion damage, eg pitting or stress corrosion cracking or it may be weighed to quantify any mass lost due to corrosion. Frequently the standardised test environments are more severe than the expected service conditions. Additional customised testing may be performed to be more representative of the service conditions.

4.12

Hardness testing Hardness is the resistance of a material against penetration, measured by indentation under a constant load. There is a direct correlation between UTS and hardness. There are a number of test methods depending on the applied load and the geometry of the indenter, but the most common types are the Vickers, Brinell and Rockwell techniques but others exist including a number of micro-hardness test techniques. The Brinell test, which uses a ball indenter, is generally used for bulk metal hardness measurements since the impression is larger than that of the Vickers test, which uses a pyramid indenter. This is useful, as it averages out any local heterogeneity and is less affected by surface roughness. However, because of the large ball diameter, the test cannot be used to determine the hardness variations associated with the different regions of a welded joint, for which the Vickers test is preferred. The hardness data generated for a material or for the different areas of a weld joint can help to assess the resistance to brittle fracture, fabrication hydrogen (cold) cracking and resistance to cracking in corrosive environments, particularly those which contain hydrogen sulphide (H2S). Test reports should give hardness value, material type, location of indents (for welds), type of hardness test and load applied on the indenter and details of the standard to which testing was performed.

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Standards covering hardness testing include:    

4.13

BS EN 1043 Destructive tests on welds in metallic materials. Hardness testing, Part 1: 1996 Hardness tests on arc welded joints. BS EN 1043 Part 2: 1997 Micro hardness testing on welded joints. BS EN ISO 6506-1:2005 Metallic materials. Brinell hardness test. Test method. ASTM E18 - 08b Standard Test Methods for Rockwell Hardness of Metallic Materials.

Vickers hardness test The Vickers hardness test forces a square-based pyramidal diamond indenter into the surface of a sample using a standard load. Vickers hardness (HV) is calculated as follows: HV = 2F sin (136°/2) d2 Where: F = force (kgf) d = mean diagonal of the indent. It is usually read from a set of standard tables with this indenter configuration, the hardness value is independent of the indenter load (although operator error becomes increasingly important at low indenter loads). The diamond does not deform at high loads and so the results on very hard materials are more reliable than Brinell hardness.

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Testing arrangement for Vickers hardness measurement.

4.14

Brinell hardness test The Brinell hardness test forces a hardened steel ball indenter into the surface of a sample using a standard load. The diameter/load ratio is selected to provide an impression of an acceptable diameter. The Brinell hardness number (BHN) is calculated by dividing the load by the surface area of the impression and is often simpler to refer to a set of standard tables from which the BHN can be read directly from the impression dimension.

Ball indenter

Testing arrangement for Brinell hardness measurement.

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4.15

Rockwell hardness test The Rockwell hardness test employs a diamond cone or hardened steel ball indenter. The indenter is initially forced into the material under a preliminary minor load. Once equilibrium has been reached and with the minor load still applied, a major load is applied, which leads to an increase in the depth of penetration into the material. Once equilibrium has again been reached, the major load is removed, but the minor load maintained. Some relaxation in penetration occurs at this stage. The increase in the permanent depth of penetration that arises from the application and subsequent removal of the major load is then used to calculate the Rockwell hardness number (HR): HR=E-e Where: E is a constant, 100 for a diamond cone indenter and 130 for a steel ball indenter. e is the permanent increase in the penetration depth due to the major load. A number of Rockwell hardness scales exist depending on the indenter size and geometry and the loading levels employed.

The principle of the Rockwell hardness test.

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4.16

Metallographic specimen preparation Metallographic examination is the visual examination of specimens taken through a welded joint. Examination up to 10 times magnification is known as macro examination; when the magnification is between 100 and 1200 times it is called micro examination. The technique is used for detecting weld defects (on macro specimens), to measure grain size or the proportion of different microstructural constituents (micro) and detecting brittle structures, precipitates, etc. Examination of the microstructure can also help assess resistance toward brittle fracture, cold cracking and corrosion sensitivity.

Comparison of macro and micro examination.

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The procedure for carrying out metallographic specimen preparation is as follows: 1

Identify

Take care when identifying where to cut the specimens from. Make sure all pieces of material are identified using an engraving tool (stamping may not be suitable for small samples).

2

Record

All details of preparation to be recorded, eg in a laboratory notebook. The record may include sketches or photographs mapping the specimen removal. Use this record to file other data and test results too.

3

Cut

Must be performed mechanically, eg by band-saw or preferably by slitting wheel.

4

Mount

It is usually important to mount the specimen to polish it. Large samples may be mounted in Araldite resin, smaller ones in a hot press in Bakelite or clear resin.

5

Grind

Use wet or dry silicon carbide papers ranging from 200 (coarse) up to 1200 (fine) grit finish. This is often sufficient quality finish for a macro but not for a micro specimen, which needs polishing.

6

Polish

Use a rotary polishing wheel with diamond paste applied to a cloth base, starting with 6m paste and finishing with 3 or 1m.

7

Inspect

The specimen needs to be examined carefully during and after polishing to ensure that the last grade of scratches have been removed before polishing using a finer finish, or etching.

8

Etch

Rinse specimens in acetone or alcohol before etching. Etching may be purely chemical or (particularly for stainless steels) be encouraged by electrolytic polarisation. Immerse and/or swab the etch on to the polished surface using cotton wool (a typical etch for ferritic steels is 2% Nital and for stainless steels 20% sulphuric acid). Use a heavier etch for macro than micro examination. Rinse and dry.

9

Inspect

Ensure that the etching has brought out the features of interest.

10

Photograph

Record the investigation.

11

Storage

In a dry environment for an agreed period.

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Revision Questions 1

What can be determined from the result of a cross-weld tensile test?

2

When would Vickers hardness testing be preferable to Brinell hardness testing?

3

How do you measure lateral expansion of a Charpy specimen and what is it a measure of?

4

When would a side bend test be specified instead of a face or root bend test?

5

Describe how a metallographic specimen is prepared.

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Section 5 Heat Treatment

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5

Heat Treatment

5.1

Purpose of heat treatment Heat treatment involves the use of controlled heating and cooling to achieve desired mechanical properties (hardening, softening, improved toughness and formability) in a metal or alloy. It can also be used to improve certain manufacturability characteristics such as machinability, formability or ductility after cold working. Heat treatment temperatures for steels are derived on the basis of the iron-carbon equilibrium diagram which expresses the stability of different phases under equilibrium conditions and is a helpful tool for assessing the correct temperature for the majority of heat treatment processes. The microstructures developed under non-equilibrium conditions can be assessed by means of isothermal and continuous cooling diagrams. The most important objective of heat treatment is often to increase the strength of a material, but it can also be used to improve certain manufacturability characteristics such as machinability, formability and ductility after a cold working operation. Thus it is a very enabling manufacturing process that not only assists other manufacturing processes, but can also improve product performance by increasing strength or other desirable characteristics.

5.2

Heat treatment equipment The choice of heat treatment equipment depends upon the type of intended heat treatment ie local or bulk. Bulk heat treatment involves heating and cooling the whole piece or component to induce the desired mechanical properties whereas local heat treatment consists of heating a localised region of a component to induce desired characteristics. Heat treatment

Heating and cooling bulk specimen

Furnaces and ovens

Gas fired

Localised heat treatment

Localised heat sources Temperature control? Use thermocouples, optical pyrometers

Electric

Flame heating

Induction heating Laser heating

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5.3

Bulk heat treatment equipment Bulk heat treatment is applied to the whole component or specimen, usually by fuel fired or electric resistance furnaces. Bulk heat treating furnaces are broadly divided into batch and continuous furnaces. In batch furnaces, workpieces are manually loaded and unloaded, while continuous furnaces have automatic conveying systems for continuous throughput through the furnace. Continuous furnaces are easily automated and are thus usually the preferred choice for high volume work.

5.3.1

Fuel fired furnace: Direct fired furnaces The components being heat treated are directly exposed to the product of combustion. The environment in a direct fired furnace is more difficult to control because of the need to adjust the fuel to air ratio, too much oxygen may result in excessive scaling. In addition to a neutral environment, there must be adequate circulation of heat inside the furnace to avoid temperature gradients. Radiant-tube-heated furnaces The components being heat treated are protected from the product of combustion, as combustion is carried out inside high creep resistance metal tubes which then heat up the chamber by radiating heat.

5.3.2

Electrically heated furnaces Electrically heated furnaces are usually cleaner, generate less noise and are usually associated with cooler overall workplaces as no exhaust stacks and hoods are necessary (normally). There is also very good thermal distribution inside the furnaces. However, electrically heated furnaces might have limited cooling rates and are generally more expensive to run then fuel fired furnaces. Various types of temperature monitoring instruments such as thermocouples or optical pyrometers are used to ensure the correct operating temperature inside the furnace. It is important that thermocouples are attached to the component, not just the furnace wall, or they may not record the temperature the component is experiencing.

5.4

Localised heat treatment equipment

5.4.1

Electromagnetic induction (induction heating) Any material that is electro-magnetic can be heated by electromagnetic induction. Induction is normally used for surface hardening or tempering of steels and cast iron. Induction heating works by the induction of a rapidly alternating magnetic field around the coil, which in turn induces an electric current on the workpiece. It is the resistance of the material to the flow of the electric current which generates heat.

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Induction coil

Part to be heat

The depth of current penetration decreases as current frequency increases. Therefore, high frequency induction can be used for surface hardening and lower frequencies used for deeper heating. One of the advantages of induction heating is the fast heating and cooling rates possible with this process. 5.4.2

Flexible ceramic pad Flexible ceramic pad (FCP) heaters are assembled from interlocking sintered alumina ceramic beads which insulate stranded nickel-chrome wire. The 80/20 nickel-chrome wire gets heated due to the resistance to current flow when connected to a power source. The pad heaters are used as heating sources in furnaces and industrial process equipment operating at temperatures of up to 1100°C. In welding, flexible ceramic pads, also called heating blankets, are used for pre-heating before welding and for PWHT after welding. This is a very efficient method of preheat, however, the elements may burn out or arc during heating.

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5.5

Types of heat treatment Heat treatment consists of three stages; heating, hold period and cooling. The important parameters are heating rate, soaking temperature, soaking time (often 1h/inch for PWHT of steel) and cooling rate.

The only type of heat treatment commonly applied to welds is post weld heat treatment (PWHT), for stress relief and improvement of HAZ microstructure toughness. Other heat treatments, such as annealing, normalising, recovery, recrystallisation, quenching and tempering and precipitation hardening are generally only used for parent metals. oC

Homogenizing and hot working Austenite

Acm

910 Normalizing A3

Annealing

727

Recovery and recrystallization 600

A1

PWHT and PWHT Stress Relieve

500 0.022

0.77

2.0

Carbon content in weight %

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5.6

Full annealing Full annealing is a heat treatment whereby Hypoeutectoid steels (0.8%C) steels are heated to the lower critical temperature (A1) plus approximately 40oC to form fine grains of austenite and proeutectoid cementite, held at temperature for a time proportional to the thickest section of the material and slowly cooled to room temperature. Cooling is usually performed inside a furnace. The slow furnace cooling rates from the austenite phase field result in equiaxed and fairly coarse grained ferrite and pearlite with coarse interlamellar spacing. This microstructure results in good ductility, low strength and hardness, which are the main objectives of a full annealing and are desirable properties, for example, before cold working. Hypereutectoid steels (>0.83%C) are annealed in the austenite plus cementite phase field, promoting spheroidized pearlite and thus avoiding low toughness cementite networks on the grain boundaries from slow cooling from the austenite phase field.

5.7

Normalising The purpose of normalising is to produce a fine ferrite-pearlite microstructure and remove internal stresses introduced by heat treating, casting, forging, or forming. Normalizing differs from annealing in that the steel is allowed to cool in air, as opposed to furnace cooled as is the case for full annealing. The cooling rate depends on the mass of the component ie thin sections cool faster in air and develop finer grains than thick sections of same type of steel. Soaking temperatures for normalizing heat treatments are usually slightly higher than that for annealing. The temperature range used to normalise Hypoeutectoid (150°C. • Slow cool and PWHT.

Copyright © TWI Ltd 2012

Copyright © TWI Ltd 2012

Interpass Temperature • A temperature, specified as minimum and/or maximum for the deposited weld metal and adjacent base metal before the next pass is started. • Steels which require preheat, must be kept above minimum interpass temperature between the weld passes. • Heat input is often adequate to maintain the interpass temperature, depending on plate thickness. • Maximum interpass temperatures are imposed to limit grain coarsening, or to ensure transformation from austenite between passes. Copyright © TWI Ltd 2012

6-2

Summary of Heating Requirements

Welding Flaws in Steels C and C-Mn Steels:

Carbon Equivalent

Preheat temperature (oC)

Interpass PWHT Temperature (oC) temperature (oC)

CE ≤ 0.4

Not required

Not specified

Not required

0.4 ≤ CE ≤ 0.5

40-75°C (thick section)

100-200°C

525-650°C (thick section)

CE ≥ 0.5

150-200°C

150-300°C

550-650°C

• • • •

Hydrogen cracking/cold cracking. Solidification cracking/hot cracking. Weld metal porosity. Lamellar tearing.

Low Alloy Steels: • Same as C and C-Mn steels + reheat cracking.

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HAZ Toughness

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Effects of Composition on HAZ Toughness

• Thermal cycle and shrinkage strains affect microstructure and properties of the parent material. • Nearly always reduces toughness in the HAZ. • Lowest toughness (C-Mn steels) in: – Grain coarsened as-welded HAZ associated with the last pass of a weld (coarse and/or hardened structures). – Intercritically reheated grain-coarsened HAZ regions (martensite or coarse carbides on grain boundaries).

• Lower C or CE (typically down to around 0.3 IIW CE) generally improves HAZ toughness. • Reduction induces in hardenability. • Reduction in tendency for M-A formation. • Increased tendency for auto-tempering (increase in martensite start temperature). • But very low C or CE can result in formation of coarse microstructures at higher heat inputs.

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HAZ Toughness in C-Mn Steels

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HAZ Toughness in C-Mn Steels • Toughness is reduced with increasing C content. • Decrease in toughness due to the decrease in the martensite start (Ms) temperature with increasing C content and therefore to a decrease in the extent of auto-tempering of the martensite formed. 0

490 Calculated Ms, °C

480

28J Transition Temperature, °C

-20

470

-30

460

-40

450

-50

440

-60

430

-70

420

-80

410

-90 -100 0.05

Ms, °C

28J Transition Temperature, °C

-10

400

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0.21

390 0.23

% Carbon Copyright © TWI Ltd 2012

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6-3

Effects of Heat Input on HAZ Toughness • Generally decreases with increased heat input. – Change in transformation product from (eg) tough auto-tempered low C martensite to more brittle upper bainite/Widmanstätten ferrite. – Increase in grain size.

• Low CE steels only produce auto-tempered martensite at very low heat input. – Toughness at normal low heat input can therefore be lower than that of higher CE steels. – Toughness often better than that of higher CE steels. • Bainitic microstructures softer.

Improving Toughness in C-Mn Steels • Low carbon content. – Carbon increases yield and tensile strength. – Carbon decreases Charpy impact energy.

• Use fine grained steels. – Small grains have higher toughness. – Add Al, Ti grain refiners.

• Low heat input. – Limits HAZ grain growth and weld grain size. – But, need to avoid hard phases in HAZ.

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Improving Toughness in C-Mn Steels

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Improving Toughness in C-Mn Steels • Low carbon martensite.

• Low nitrogen content.

– Tempered martensite with low carbon has high toughness. – But, consider weldability.

– N can cause strain age embrittlement. – Mainly in root pass from N in parent steel.

• Use a clean steel.

• Add nickel.

– S,P cause low Charpy toughness. – Add Mn to form MnS inclusions. – Limit S to 350HV). • High tensile stress (>0.5 YS). • Low temperatures (2:1).

In order to avoid solidification cracking, reduce penetration and increase bead width (depth: width ratio ~0.5:1). Copyright © TWI Ltd 2012

Wrong (washed up, too wide/high and concave)

Right (slightly convex, not full width)

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9-3

Hot Cracking and Dilution

Hot Cracking and Weld Profile Arc voltage effect

Filler metals often formulated to resist hot cracking. Avoidance of high dilution welding conditions or joint geometries associated with high dilution, is advisable. Increased arc voltage

Shielding gas effect (MIG/MAG)

Argon

Argon-Helium

Helium

CO2

• Dilution also affected by welding process (MMA vs. EBW) and shielding gas (CO2 vs. Ar and Ar-CO2 mix).

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Avoidance of Hot Cracking

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Summary - Hot Cracking Avoidance

Control the travel speed

Reduce restraint

Add Mn

Decrease travel speed

Mn forms inclusions with sulphur before Fe can react (higher temperature reaction)

Reduce heat input to avoid coarse grains Improve joint fitup

Reduce heat input to lower dilution

Refine (purify) the weld metal FeS + Mn  Fe + MnS FeS + CaO  FeO + CaS

TM = 1620°C Not soluble in the weld metal separates into slag

Change joint design

Increased hot ductility

Reduce dilution (eg avoid rutile electrodes) Low S steel Reduce C%

Avoid hot cracking

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Liquation Cracking

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Liquation Cracking Mechanism

• Liquation cracking occurs at high temperature, due to shrinkage strains. • Forms between grains near the fusion line. • Caused by: – High level of impurities in weld/parent metal. – Poor cleanliness on joint prep. – High level of restraint.

• Can occur in steels • Prevalent in Al alloys, Ni alloys and stainless steels.

• Phase 1 - as a result of heat cycle, liquid films of (Fe + FeS) eutectic appear near fusion line. • Phase 2 - during cooling, liquid films of (Fe + FeS) eutectic are subjected to tension  cracking. Copyright © TWI Ltd 2012

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9-4

Lamellar Tearing

Segregation

Rolling direction

Causes of lamellar tearing: • High level of through-thickness strain. Use Z-grade material • Weld orientation. eg BS EN 10164. • Material susceptibility. Copyright © TWI Ltd 2012

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Reheat Cracking

Avoiding Lamellar Tearing Design of fillet welds to avoid lamellar tearing

• Occurs in creep resistant alloys containing >2 of: Cr, Mo, Nb or V. • Occurs in the coarse grained HAZ (occasionally in the weld metal). • Precipitates dissolved during the weld thermal cycle remain in solution when cooled. • Reheating (PWHT, service), causes them to grow, strengthen the grain and weaken the g.b. •  reheat cracking along grain boundary forms at 350-550°C.

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Reheat Cracking

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Avoidance of Reheat Cracking • Control the heat input during welding  small narrow CGHAZ; finer grain size. – But: balance the beneficial effect for reheat cracking against the risk of cold cracking. • On reheating, heat through 350-550°C as quickly as possible (over 600°C precipitates coarsen and grain strengthening reduces). – But: maximum heating rates are specified in some codes • Control amount of residual elements. • Reduce stress concentration by grinding toes (while hot for V alloyed grades); avoid backing strips and partial penetration welds.

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Copyright © TWI Ltd 2012

9-5

Assessing Reheat Cracking Sensitivity Probability of sensitivity to reheat cracking, PSR P SR = %Cr + %Cu + 2%Mo + 10%V +7%Nb + 5%Ti - 2 < 0  less susceptible to reheat cracking

P SR ≥ 0  susceptible to reheat cracking Increased sensitivity to reheat cracking 5Cr 1Mo

2.25Cr 1Mo

0.5Mo B

0.5Cr 0.5Mo 0.25V Copyright © TWI Ltd 2012

Weld Repairs • Repairs are very expensive - 3 to 10 times the cost of getting it right the first time. • If you have found cracking do you need to repair it or not? • Is the defect outside the limits in the acceptance standard? • Can it be shown to be tolerable in an ECA? • Why has the cracking occurred? How will it be avoided again after repair? • Often either the welder or fit-up problems, but could be the WPS parameters. Copyright © TWI Ltd 2012

Carrying out Weld Repairs • Removal of the flaw (grinding, machining or gouging), position of the flaw and shape of subsequent groove. • How many times can you re-weld? (often 3 max) • Already undergone PWHT? Can further PWHT be carried out after repair and still comply with the WPS? • May need to qualify the repair weld procedure. Use existing range of approval from the original WPS? • How will repair weld be inspected? Inspect after grinding to ensure the cracking is entirely removed. • Provide extra welder training and/or improve the fit-up of the joint. • Access for welding, confined spaces? Copyright © TWI Ltd 2012

9-6

Definition The deterioration of a metal due to chemical or (very often) electrochemical reactions with its environment. Common types of corrosion

Introduction to Corrosion TWI Training and Examination Services EWF/IIW Diploma Course

General corrosion

Galvanic corrosion

Intergranular corrosion

Crevice corrosion Stress corrosion cracking

Pitting corrosion

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Reactions During Corrosion

Reactions During Corrosion

Corrosion = Red-Ox reaction

For example rusting of iron or steel.

• • • •

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Iron + water + oxygen

Rust

Fe + H2O + O2

Fe(OH)2

Chemical or electrochemical reaction of iron with oxygen and water = exchange of electrons. Iron is said to be oxidised – its atoms lose electrons. Water and oxygen are said to be reduced – their atoms gain electrons. REDOX.

Reduction

Oxidation Current flow

• Oxidation = metal atoms • Reduction = oxygen atoms release electrons. accept electrons. • Anode (up direction in Greek) • Cathode (down direction in is where the oxidation reaction Greek) is where the reduction takes place  anode is reaction takes place  consumed. cathode builds-up.

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REDOX

REDOX Potential

• Oxidation is electron loss (eg iron loses electrons). 2Fe

2Fe2+ + 4e-

• Reduction is electron gain. O2 + 2H2O + 4e-

4OH-

• Corrosion is a balance of reduction and oxidation. - REDOX 2Fe + 2H2O + O2

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2Fe(OH)2 Copyright © TWI Ltd 2012

• Defined as a measure of the affinity of a substance for electrons compared with that of hydrogen (which is set at 0V). • Measured in volts. • Substances more electronegative than hydrogen have positive redox potentials  they are capable of oxidising. • Substances less electronegative than hydrogen have negative redox potentials  they are capable of reducing.

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10-1

Types of Corrosion 1 - Galvanic Corrosion

Galvanic Series Relative potentials in seawater: • • • • • • • • • • •

Platinum. Gold. 316 Stainless steel. Titanium. Nickel. Copper. Tin. Mild steel. Cadmium. Zinc. Magnesium.

Practical implications of the galvanic series: More positive REDOX potential Least active, increasingly inert  cathodic

More negative REDOX potential Most active  anodic

• The more anodic metal will corrode – use as large an anode as possible to reduce the penetration rate of corrosion. • The farther apart the two metals, the faster will be the corrosion rate – choose metals closer together to reduce the corrosion rate. • To protect against galvanic corrosion. – Electrically insulate dissimilar metals from each other. – Eliminate the electrolyte. – Use a corrosion inhibitor. – Connect a third anode (sacrificial anode).

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Types of Corrosion 2 - General Corrosion

• Corrosion attack proceeds at the same velocity over the entire surface – Rate of corrosion can be determined = mm/year, mpy. • In case of stainless steel, occurs almost exclusively in acidic or strong alkaline solutions. • Resistance against this type of corrosion is improved by increasing the Cr and Mo content in the steel or passivation in case of Cr, Fe, Ni, Ti. • For stainless steels apply pickling (to remove high temperature scale) followed by passivation (to restore Cr oxide layer). • Corrosion allowance may be specified in design. Copyright © TWI Ltd 2012

Types of Corrosion 4 - Pitting Corrosion

• Highly destructive non-uniform attack due to localised breakdown of passivity (the pit becomes the anode whilst the surface becomes the cathode = effect of differential aeration). • Results in holes in the metal. • In stainless steel occurs most commonly in chloride-containing environments or oxidising salts. • Resistance against this type of corrosion is improved by increasing Cr and Mo content; N has also a favourable influence. Copyright © TWI Ltd 2012

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Types of Corrosion 3 – Intergranular Corrosion • Corrosive attack is localised at and adjacent to grain boundaries. • Occurs in stainless steel due to Cr carbide precipitation (sensitisation); in case of welded stabilised grades (eg 321, 347) can take the form of knife line corrosion in HAZ. • Resistance against this type of corrosion is improved by lowering the C content, control the welding procedure or by addition of Ti or Nb (stabilisation).

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Types of Corrosion 5 – Crevice Corrosion

• Attack occurs in narrow crevices filled with a liquid and where the O2 level is very low (differential aeration). • Eg gasket surfaces or under bolt/rivet heads, weld toes. • Under-deposit corrosion = when corrosion occurs under nonmetallic deposits or coatings on the metal surface. • Materials resistant to pitting corrosion are also resistant to crevice corrosion. Copyright © TWI Ltd 2012

10-2

Avoiding Crevice Corrosion

Types of Corrosion 6 - Stress Corrosion Cracking

Remove crevices: • • • •

Use welds instead of bolts or rivets. Avoid non-removable backing strips. Seal welds and avoid partial penetration welds. Design vessels to avoid stagnant areas and ensure complete drainage.

Prevent the environment in the crevice becoming corrosive:

• Material can remain uncorroded generally while fine branched cracks progress through it (cracking can be either intergranular or transgranular). • In austenitic stainless steel occurs in chloride or halogen containing solutions risk increases with increasing salt concentration, tensile stress and service temperature (seldom found below 60°C). 1. Austenitic stainless steel resistance is improved by increasing Ni content. 2. Ferritic stainless steel (without Ni) is insensitive to SCC. 3. Duplex stainless steel is more resistant to SCC than austenitic stainless steel.

• Use non-absorbing gaskets. • Remove accumulated deposits frequently to allow free circulation of fluids and avoid stagnant areas. • Ensure complete drainage.

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SCC Main Contributing Factors

Common Corrosion Protection Techniques

Environment, temperature and exposure time.

Material employed (microstructure).

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Coatings: • paints, plating, weld cladding, anodising, metal spraying.

Inhibitors: • Add suitable chemicals to control the environment.

SCC

Cathodic protection: • impressed current or sacrificial anodes (eg galvanised steel).

Level and distribution of tensile stresses.

Anodic protection: • Passivating stainless steel (next slide). Copyright © TWI Ltd 2012

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Pickling and Passivation

Corrosion Testing

• A very thin layer of chromium oxide naturally grows on the surface of stainless steels = passive layer. 1) Pickling involves applying highly corrosive acids to a metal or alloy to remove areas of lowered corrosion resistance eg weld oxide. 2) Passivation involves immersion in nitric acid to regrow the passive layer by oxidation. • The passive state has a higher redox potential than the active state (active state is when corrosion is occurring). Copyright © TWI Ltd 2012

• Ranking tests – Relative corrosion resistance of materials in an environment – ASTM G48 etc.

• Electrochemical tests – Critical Pitting or Crevice Temperature – Pitting Potential – ASTM G150 etc.

• Don’t necessarily represent service conditions – asreceived/as-welded metal, environment, temperature. Copyright © TWI Ltd 2012

10-3

Stainless Steels - Properties

Welding of Stainless Steels TWI Training and Examination Services EWF/IIW Diploma Course

• • • • • • • •

Corrosion resistance. Oxidation resistance. High temperature strength. Toughness at low temperatures. Ductility/formability. Resistance to stress corrosion cracking. Strength and wear resistance (if work hardened). Addition of Cr (>13%), Ni and Mo make stainless steels more expensive than carbon steels.

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Types of Stainless Steel

Stainless steels

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Properties of Austenite and Ferrite

Ferritic

Fe-Cr, 0 susceptible to reheat cracking. PSR < 0 less susceptible to reheat cracking. K = Pb + Bi + 0.03Sb (ppm) K < 1.5 to achieve freedom from reheat cracking. Copyright © TWI Ltd 2012

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Temper Embrittlement

Temper Embrittlement Can be practically assessed by step cooling followed by Charpy test to measure: • The shift in transition temperature. • Toughness drop, eg minimum 55J at 10°C. Temperature (°C)

• In steels exposed to 350-600°C, impurity elements segregate to grain boundaries. • P, Sb, Sn and As are most damaging providing Mn and Si are relatively high. • Embrittlement is fully reversible, if steel is reheated above 600°C. • Loss in ductility of grain boundaries, in addition to grain body strengthening, leads to intergranular embrittlement.

593°C 1 h 538°C 15 h

523°C 24 h

495°C 48 h

468°C 72 h

415°C Remove from furnace Time (hours)

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13-3

Cryogenic Steels Key considerations

Cryogenic Steels TWI Training and Examination Services EWF/IIW Diploma Course

• Modification of composition to achieve properties suitable for low temperature. – Toughness is the main concern. – Grain size. – Can be further improved by alloying with Ni. • Service temperature is important. – Fine grain size allows service down to -60oC. – Nickel allows lower temperature applications. – Variation in content depending how low the service temperature is.

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Fine Grain Al-killed Steels

Features of Ni-Alloyed Steels

• Widely used for LPG carriers. • Have low CE; micro-alloyed and TMCP. Al, Ti

Increased toughness

• Very good toughness (down to -196°C for 9%Ni).

Nitrides carbonitrides

• Lower thermal expansion compared with austenitic stainless steels and Al alloys.

N

Increased strength

Fine-grained steels

• Low thermal conductivity. • Nickel is the main alloy element (up to 9%).

Normalising and tempering

TMCP process

Quenching and tempering

• Low C content (less than 0.15%).

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Effect of Ni on Steel Toughness Transition Curve Energy (J)

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Ni-Alloyed Cryogenic Steel Types Grouping system acc. ISO 15608 Subgroup 9.1: Steels with Ni  3.0%

Nickel additions

Ni alloy steels group 9

Carbon steels

Subgroup 9.2: Steels with 3.0% < Ni  8.0%

Subgroup 9.3: Steels with 8.0% < Ni  10.0% Temperature (ºC)

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14-1

Effect of Ni on Impact Toughness Transition Curve

Weldability of Cryogenic Steels • Applicable welding processes: MMA, TIG, MAG, FCAW and SAW. OFW not recommended. • Sensitive to hot cracking  limit C, P and S content; cleanliness is important. • Hydrogen-assisted cracking is an issue when not using nickel-based fillers. • Use low heat input processes in order to preserve the fine grain structure (less than 4.5kJ/mm); also to avoid HAZ softening in QT grades; pulsed welding is beneficial. Copyright © TWI Ltd 2012

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Welding 2.3% and 3.5% Ni Steels

Welding 5% Ni Steel • Pearlitic/martensitic structure (QT steel)  reduced toughness in the HAZ (layer heated over 850°C) due to increase in grain size; low hydrogen process essential • Use low heat input processes in order to preserve the fine grain structure (less than 4.5kJ/mm); pulsed welding beneficial • Interpass temperature max. 250°C; PWHT at 650°C, followed by rapid cooling • Filler materials are Ni based, eg Inconel 82, Inconel 625.

• Use basic filler materials (for good toughness and hydrogen control). • Preheat in the range 150-250°C to avoid cold cracking; exact value depends on thickness, restraint and hydrogen potential. • Interpass temperature: max 350°C for 2.3% Ni; max. 250°C for 3.5% Ni. • PWHT at 620-730°C may be used. • Matching filler materials available; Inconel type (NiCr-Fe) filler might be used for 3.5% Ni.

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Welding 9% Ni Steel

Welding 9% Ni Grades

• Very low S content (max 0.006%) to avoid liquation cracking  no lamellar tearing. • Very low P content (max 0.006%) to improve low temperature toughness. • Not sensitive to stress corrosion cracking. • Tempered (ductile) martensitic structure (max 400HV) + 5% retained austenite (QT steels); austenite reduces risk of cold cracking + Ni based filler  no preheat required for thickness up to 50 mm. • Hardness slightly increases in the HAZ near fusion line (T>900°C) but with little drop in toughness. • Use low heat input to avoid loss of toughness in the high temperature HAZ, eg pulsed MIG. Copyright © TWI Ltd 2012

• • • •

Welding process usually used: MMA, MIG and SAW Interpass temperature 250°C. PWHT not normally required. Plates must be cleaned at least 25mm from the edge (oxide layer is adherent); if flame cutting is used, heated layer must be removed by grinding. • Strongly ferro-magnetic (and also may have residual magnetism)  arc blow is a potential problem. • Avoid contact with electromagnets for moving or for NDT. • For welding, AC current and demagnetisation before welding may be required. Copyright © TWI Ltd 2012

14-2

Filler Materials for 9% Ni Steel

Welding Fine-Grained Al-Killed Steel

• Filler materials are currently only Ni-based: AWS A5.11 ENiCrMo-6 (EN ISO 14172 E Ni 6620) or ENiCrMo-3 (E Ni 6625)  under-matching on strength, high viscosity (sluggish); cleanliness very important to avoid hot cracking. • Fully austenitic filler of type 16Cr-13Ni-Mn are prohibited; disadvantage is the higher coefficient of thermal expansion than the parent metal and brittle martensite formation near the fusion line. • Smooth blending between weld face and parent metal is required to avoid stress concentration. Copyright © TWI Ltd 2012

• Increase the amount of acicular ferrite in the weld. – Avoid slow cooling rate which promotes coarser structure. – Add B + Ti to aid acicular ferrite formation; B reduces the amount of grain boundary ferrite. – Increase Mn content and add Ni (2-2.5%). – Keep C in the range 0.080.12% and O in the range 0.02-0.06%. Copyright © TWI Ltd 2012

Welding Fine-Grained Al-Killed Steel • Limit the heat input to approximately 4.0 kJ/mm to avoid grain growth; avoid use of single run procedures. • Use only basic filler materials. • Preheat helps to avoid cold cracking. • No preheat necessary for thickness 99.5% Ni • Corrosion resistant, thermal expansion and melting point similar to steel, used in chemical industry and electronics.

Monel alloy 400 - Ni + 31.5%Cu • Highly corrosion resistant in sea water, H2SO4 and HF acids.

Inconel 625 - Ni + 21.5%Cr, 2.5%Fe, 9%Mo, 3.6%(Nb +Ta) • Excellent strength and toughness from cryogenic to high temperature; oxidation, corrosion and fatigue resistant.

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Welding Nickel Alloys Welding processes available for Ni alloys: • Arc welding processes: MMA, TIG and MIG; SAW only for solid solution strengthened alloys, not ppt hardened. • Resistance welding (spot, seam, projection) • Power beam processes - EBW and LBW.

Incoloy 825 - Ni + 30%Fe, 21.5%Cr, 3%Mo, 2.25%Cu • Excellent corrosion and pitting resistance and for service in reducing acids and oxidising chemicals. Copyright © TWI Ltd 2012

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17-3

Welding Nickel Alloys

Ni Weldability Problems

• Solid solution hardened are readily welded. • Precipitation hardened grades are harder to weld. – Lower ductility, susceptibility to cracking, possibility of ageing. – Use solution annealed/overaged  weld  PWHT restore strength. • Consider the service conditions and environment. • Preheat 20°C to avoid porosity; cleanliness very important. • Shielding gases: Ar, He or Ar-He mixtures. – Ar + 5% H2 only for single pass TIG welds; H2 produces hotter arc but danger of porosity in multipass welds. • If material is in contact with caustic soda or HF acid, SCC is possible  reduce residual stress by PWHT. • Use matching filler with added de-oxidisers (Al, Ti, Nb).

Hot cracking: • Even in very small quantities (starting at 0.003%) S  hot cracking. • Mn and Nb additions in filler metals are used to combine with S. • Pb, B, P and Bi also form low melting eutectics. • Tendency for hot cracking is increased for coarser grain sizes (coarser than ASTM no. 5). • Reduce welding speed to improve depth-to-width ratio.

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Ni Weldability Problems

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Ni Weldability Problems

Porosity:

Oxide inclusions:

• Forms due to contamination with O2 and N2; also H2 from surface contamination.

• Oxides have much higher melting temperatures than the base metal  oxides trapped in the weld pool form inclusions. • Surface oxide must be removed by machining or grinding; wire brush only polishes the oxide.

• Preheat might be required in case of moisture condensation; also careful cleaning. • Avoid oxygen contamination by using welding fluxes, shielding gases and/or additions of Mn, Nb, Ti, Al  Al and Ti can form small islands/slag spots so interpass cleaning is critical.

Lack of sidewall fusion/poor blending: • Molten weld metal is very viscous  increase bevel angle; accurate weld metal placement is required.

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Copper Welding • Good atmospheric oxidation and corrosion resistance. • Copper is a very effective heat sink (high thermal conductivity). – High heat input, interpass, preheat (for >5mm thickness).

Copper and Copper Alloys

– For thick components preheat may be as high as 600°C. – Copper alloys have lower thermal conductivity (can be 70% lower).

• TIG and MIG are preferred welding processes. • He-Ar shielding gases are used instead of Ar (higher arc voltage). • To avoid porosity, use filler wires with deoxidants (Al, Mn, Si and Ti).

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17-4

Copper - Weldability Problems

Copper - Weldability Problems Porosity:

Hot cracking: • Due to O2 and impurities that form low melting point eutectics (eg Pb, Bi). • Avoidance: – Use slag/inert gases to protect molten weld pool. – Add deoxidisers (Mn, Si, Zn, Al, Ti). – Use boron-based deoxidising fluxes and forge the weld joint (to fragment the oxide layer).

• From presence/diffusion of H2 into the hot solid metal. • Water vapours are trapped inside, leading to porosity or cracking. • Avoidance: use PDO or oxygen-free grades, dry inert gases; dry electrodes and fluxes before welding.

High level of residual stresses/distortions: • Due to the high thermal expansion coefficient. • Can lead to weld/HAZ cracks. • Preheat and free expansion of components during welding are required.

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Welding Copper Alloys - Brasses

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Welding Copper Alloys - Bronzes Phosphor bronze

Brasses (Cu-Zn) and nickel silvers (Cu-Zn-Ni) Low Zn (