3.3 Joining of metals and alloys
3.3.3 Welding
An ideal weld would be chemically and physically indistinguishable from the bulk material: this may sometimes be approached in solid state welding, but seldom in fusion welding.
Solid state welding
The formation of a sound joint requires either chemically clean surfaces or sufficient deformation to squeeze out any contaminants such as surface oxides.
Cold pressure welding works well with ductile metals such as aluminium
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and copper, provided the surfaces to be joined are carefully prepared to be free from contamination. In hot pressure welding, the material adjacent to the weld is softened and the two surfaces are forced together, thus squeezing out any surface contaminants. The heating may be externally applied or, in friction welding, the heat is generated by rotating one surface against the other. The latter technique is widely used in the automotive industry for the manufacture of welded drive shafts.
‘Diffusion bonding’ can be employed to bond two surfaces without recourse to plastic deformation, but it is a relatively slow and expensive process which is only used in special applications.
Fusion welding
‘Welding Handbooks’ are readily available to the engineer to provide a guide to appropriate welding processes for given alloy compositions, joint design and joint size. Here we will discuss some of the factors that may affect the microstructure and properties of fusion welds.
The weld metal
The weld metal is essentially a small casting and the essentials of its structure can be appreciated by referring to our earlier discussion of the mechanism of crystallization of metals and alloys in Chapter 1. Cored columnar crystals form on the still solid component surface along the fusion line. They then grow along the direction of the steepest temperature gradient in the weld pool, as indicated in Fig. 3.31. The crystals which grow from the melt initially share the same orientation as the solid, so it is important to consider whether grain growth occurs in the component material adjacent to the fusion line, as this will influence the grain size of the solidified weld metal. In fact, the grain size in the weld is controlled by the grain size at the fusion line, since
Isotherm Rmax
Heat source
Weld centre line
90° Melt
Transition line Base metal
3.31 Showing growth of columnar crystals in the weld pool.
Metals and alloys 119 the grain boundaries there will be common to both. The region of the component adjacent to the weld is known as the heat-affected zone (HAZ).
The heat-affected zone
The temperature–distance profile across the weld is a result of the balance between the rate of heating and the rate at which heat is conducted away (the base material forming a large heat sink). A more intense heat source will give a steeper profile and the HAZ will be confined to a narrower region. The changes in microstructure that take place in the HAZ will depend on the material being welded and upon its thermal and mechanical history. For example, if the material is in the cold-worked condition, it will recrystallize and, if it is age-hardened, the precipitate distribution will be changed. Different changes will occur in different temperature regimes of the HAZ and Fig.
3.32 is a schematic diagram illustrating the possible regimes in the HAZ adjacent to a weld in a low-carbon steel. The adjacent iron–carbon phase diagram indicates the temperatures concerned in the case of a 0.15 wt% C steel and the various regimes are self-explanatory.
Residual stresses and cracking
When a weld is deposited, the volume of material being heated and cooled
Peak temperature (Tp)
Heat affected zone Solidified weld
Solid-liquid transition zone Grain growth zone
Temperature (°C) 1600 1400 1200
1000 800
600 400 200 Recrystallized zone Partially transformed zone
Tempered zone Unaffected base
material
Liquid
Liquid + γ
γ
γ + Fe3C
α + Fe3C
0.15 1.0
Fe Wt% C
3.32 Schematic diagram of the heat-affected zone of a 0.15% C steel indicated on the Fe-Fe3C phase diagram.
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is small relative to the whole assembly, so that as the weld and HAZ cool and contract they are constrained by the surrounding unaffected material. Large stresses develop, leading to local plastic deformation and, when all the structure returns to room temperature, tensile residual stresses remain in the weld. The magnitude of these stresses can be reduced by preheating the structure or the stresses relieved by a post-weld heat-treatment, neither process being straightforward for large structures.
These internal stresses can lead to cracking in the weld deposit and HAZ through several mechanisms:
(a) Solidification cracking. This occurs in the weld deposit during cooling and is found at the weld centre line or between columnar grains.
(b) Hydrogen-induced cracking in steel. Atomic hydrogen can be introduced into the weld during the welding process, its principal origin being moisture in the electrode fluxes employed, although hydrocarbons on the plate being welded is another possible source. Hydrogen can give rise to so- called cold cracking in the HAZ (underbead; root crack) or in the weld metal itself and is the most serious and least understood of all weld- cracking problems. The solubility of hydrogen in austenite is much higher than in ferrite or martensite, so that if a steel transforms from austenite on cooling, it will be highly supersaturated with respect to hydrogen.
Under these conditions, hydrogen diffuses to discontinuities in the metal such as grain boundaries and nonmetallic inclusions where it recombines to form hydrogen gas as microscopic bubbles that can develop into cracks.
(c) Liquation cracking. This occurs in the HAZ near the fusion line and is associated with the segregation of impurities such as sulphur and phosphorus to melted grain boundaries during welding. On cooling, these segregants tend to form films of intermetallic compounds and, with the development of high residual stresses, these impurity-weakened boundaries tend to rupture.
(d) Lamellar tearing. This occurs just outside the HAZ, and is commonly observed when a weld runs parallel to the surface of a plate. During the rolling of steel plate, flattened stringers of MnS or oxide–silicate phase are formed in the plane of the plate along the rolling direction. The orientation of the residual stress in such a weld is such that the weak inclusion/matrix interface can decohere and thus nucleate a crack.