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U4 p1 welding metallurgy
This document discusses welding metallurgy and the structure of fusion welds. It describes the different zones that make up a typical fusion welded joint, including the fusion zone, weld interface, heat affected zone, and base material. It explains how the microstructure varies across these zones due to melting and solidification processes during welding. Factors like welding parameters, heat input, and joint geometry are described as influencing weld pool shape and grain structure. The concept of thermal severity number is introduced as a way to assess cracking susceptibility based on total plate thickness.
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U4 p1 welding metallurgy
- 1. Design and Metallurgyof Weld Joints (MEM-510) 1 - 1 Metallurgy of Weld Joints Dr. Chaitanya Sharma
- 2. Welding Metallurgy Lesson Objectives Inthis chapter we shall discuss the following: 1. Structure of fusion welds; 2. Thermal effect of welding on parent metal; 3. Effect of cooling rate; 4. Weld metal solidification and heat affected zone; 5. Heat flow - temperature distribution-cooling rates; 6. Influence of heat input; Joint geometry; Plate thickness; Preheat; 7. Significance of thermal severity number; Learning Activities 1. Look up Keywords 2. View Slides; 3. Read Notes, 4. Listen to lecture Keywords:
- 3. What Is Metallurgy? • The science of joining metals by welding is closely relates to the field of metallurgy. • Metallurgy involves science of: – Producing metals from ores, – Making and compounding alloys, – Metal reactions, Heat treatment, – Steel making and – Processing of metals e.g. Forging, Foundry etc. • Welding metallurgy can be considered a special branch, since reaction times are very small (minutes to fraction of seconds), while in other branches reactions are large (h-m). • Welding metallurgy deals with the interaction of different metals and interaction of metals with gases and chemicals of all types.
- 4. Why Welding Metallurgy? •Welding metallurgist will examine the changes in physical characteristics that happen in short periods. • The solubility of gases in metals and between metals and the effect of impurities are all of major importance to the welding metallurgist. 1 - 4
- 5. Basic Structure ofFusion Welds • A typical fusion welded joint varies in metallurgical structure due to melting and solidification with very high temperature gradient. • In general, a weld can be divided in four different zones as shown schematically in fig. namely: 1. Fusion zone, 2. Weld Interface, 3. Heat affected Zone and, 4. Base material
- 6. Structure of FusionWeld Joints • The fusion zone (FZ) can be characterized as a mixture of completely molten base metal (and filler metal if consumable electrodes are in use) with high degree of homogeneity where the mixing is primarily motivated by convection in the molten weld pool. • The main driving forces for convective heat transfer and resulting mixing of molten metal in weld pool are: (1) Buoyancy force, (2) Surface tension gradient force, (3) Electromagnetic force, (4) Friction force.
- 7. Structure of FusionWeld Joints continued… • The weld interface, (or mushy zone), is a narrow zone consisting of partially melted base material which has not got an opportunity for mixing. This zone separates the fusion zone and heat affected zone. • The heat affected zone (HAZ) is the region that experiences a peak temperature that is well below the solidus temperature while high enough that can change the microstructure of the material and mechanical properties also change in HAZ. • The amount of change in microstructure in HAZ depends on the amount of heat input, peak temp reached, time at the elevated temp, and the rate of cooling. • The unaffected base metal zone surrounding HAZ does not undergo any change in microstructure and is likely to be in a state of high residual stress, due to the shrinkage in the fusion zone. 1 - 7
- 8. Various Regions InFusion Weld & Corresponding Phase Diagram Differentzonesinasteelweldvis-à- visIron-Carbonequilibriumdiagram 1 - 8 Zone 7: UBM Zone 6: Tempered HAZ Zone 5: Intercritical HAZ Zone 4: Fine grain HAZ Zone 3: Coarse grain HAZ Zone 2: Unmixed zone + FZ Zone 1: Solidified weld γ 𝛼 + 𝐹𝑒3 𝐶 γ + 𝐹𝑒3 𝐶 L+γ Liquid
- 9. • The fusionzone and heat affected zone of welded joints can exhibit very different mechanical properties from that of the unaffected base metal as well as between themselves. For example, the fusion zone exhibits a typical cast structure while the heat affected zone will exhibit a heat-treated structure involving phase transformation, recrystallization and grain growth. The unaffected base metal, on the other hand, will show the original rolled structure with a slight grain growth. 1 - 9
- 10. Fig: Schematic illustrationof various regions in a fusion weld zone (and the corresponding phase diagram) for 0.30% carbon steel. Source: AWS
- 11. Weld Joint Structure Fig:Characteristics of a typical fusion- weld zone in oxyfuel-gas and arc welding. Fig: Grain structure in (a) deep weld and (b) shallow weld. Note that the grains in the solidified weld metal are perpendicular to their interface with the base metal (see also Fig. 10.3). (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead.
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- 14. Fig: Intergranular corrosionof a 310-stainless- steel welded tube after exposure to a caustic solution. The weld line is at the center of the photograph. SEM micrograph at 20 X. Source: Courtesy of B. R. Jack, Allegheny Ludlum Steel Corp.
- 15. The Fusion Zone •Similar to a casting process, the microstructure in the weld zone is expected to change significantly due to remelting and solidification of metal at the temperature beyond the effective liquidus temperature. • However fusion welding is much more complex due to physical interactions between the heat source and the base metal. • Nucleation and growth of the new grains occur at the surface of the base metal in welding rather than at the casting mould wall.
- 16. Weld Pool Structure •If the weld pool is quenched, its microstructures at different positions can be revealed, e.g., aluminium weld pool structure, as shown in fig. • Microstructure near the fusion line consists of – S : Solid dendrite – L : Interdendritic liquid • PMM – partially melted material & partially melted materials (PMM) and mushy zone (MZ). 1 - 16
- 17. Weld Pool Structure continued… •The mushy zone behind the shaded area consists of solid dendrites (S) and interdendritic liquid (L). • Partially melted materials (PMM) consists of solid grains (S) that are partially melted and intergranular liquid (L). 1 - 17
- 18. Weld Pool Shapeand Grain Structure • The weld pool becomes teardrop shaped at high welding speeds and elliptical at low welding speeds. • Since the columnar grains tend to grow perpendicular to the weld pool boundary, therefore the trailing boundary of a teardrop shaped weld pool is essentially straight whereas that of elliptical weld pool is curved. 1 - 18 • Axial grains can also exist in the fusion zone, which initiate from the fusion boundary and align along the length of the weld, blocking the columnar grains growing inward from fusion lines. Note: Axial grains has been reported in Al alloys, Austenitic stainless steels and indium alloys
- 19. Effect of WeldingParameters on Weld Pool Shape • As the heat input Q and welding speed V both increase, the weld pool becomes more elongated, shifting from elliptical to teardrop shaped. • The higher the welding speed, the greater the length–width ratio becomes and the more the geometric center of the pool lags behind the electrode tip. • Quenching weld pool during welding resulted in sharp pool end of a teardrop-shaped weld pool. Fig: weld pools traced from photos taken during autogenous GTAW of 304 stainless steel sheets 1.6 mm thick. Welding parameters have more significant effect on pool shape in stainless steel welding than aluminum welding. The much lower thermal conductivity of stainless steels makes it more difficult for the weld pool to dissipate heat and solidify Fig: Sharp pool end observed in autogenous GTAW of 1.6-mm 309 stainless steel I = 85A, V=10V, WS = 4.2mm/s
- 20. Effect of WeldingSpeed on Weld Structure GTAW of 99.96% aluminium at welding speed of (a) 1000 and (b) 250 mm/min Axial grains of GTAW (a) 1100 aluminium at 12.7 mm/s welding speed, (b) 2014 aluminium at 3.6/s welding speed WS 1100 mm/min WS 250 mm/min Columnar grains Columnar grains WS 12.7 mm/min WS 3.6 mm/min Columnar grains Columnar grains Axial grains Welding direction Axial grains
- 21. Effect of HeatInput on Weld Structure • A slight tendency for the element C, Mn, Si to decrease (in the composition of the weld) when the heat input increases. • Typical macro segregation of multipass weld deposited with different heat inputs
- 22. Thermal Severity Number •Thermal severity number defines the total thickness of the plate through which heat could flow away from the weld. TSN helps in detecting cracking susceptibility. • TSN is Usually given as ‘Total Thickness’ in millimeters. • The total thickness is the sum of thickness of all the paths along which heat can be conducted. • Heat flow may be along two, three and four path as shown • This method can not be applied to complex shape or made to allow effect of jigging.
- 23. • TSN canbe calculated – For bithermal welds TSN = 4 (t + b) – For Trithermal welds, TSN = 4 (t + 2b) where: t and b are thickness of the top and bottom plate • A series of plate thickness which provided varying cooling rates are tested. The crack susceptibility of the base metal – filler material combination is determined by the minimum TSN that produces cracking. • Controlled thermal severity testing is used to measure the cold crack sensitivity of steels under cooling rates controlled by thickness of the plates. • CTST specimen consists of a square plate bolted and anchor welded to a larger rectangular plate. After the anchor welds have cooled to room temperature, two test welds are made on the specimen. Fillet weld along the plate edges is controlled by the thickness of the plates and the differences in cooling rates between bithermal and trithermal welds. This test is primarily used to evaluate the crack sensitivity of hardenable steels Thermal Severity Number continued…
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