Among the solid state reactions, the most important phenomenon is the formation of cold cracks or delayed cracks. This type of cracking is confined to steels that can be hardened. These steel contain a hard phase called martensite.
The cracks occur after the weld completely cools down, sometimes hours after or even weeks after welding. This is always associated with the presence of hydrogen in the weld metal.
At high temperature the steel is F. C.C. austenite, a form in which hydrogen is quite soluble. On cooling the austenite changes to pearlite or martensite, and there is drastic reduction of hydrogen solubility. In plain carbon steels this transformation takes place at a relatively high temperature (about 700°C), even if cooling is rapid, there is sufficient mobility so that much of the rejected hydrogen diffuses out of the metal. Moreover the transformation product (ferrite plus carbide) formed in the HAZ are relatively ductile and crack resistant.
A rapidly cooled hardenable steel transfoms at a much lower temperature (generally below 400°C) and often room temperature, so the hydrogen is locked into the structure which may also be hard and brittle. It is this combination that induces cracking. This has led to the development of low hydrogen electrodes. These electrodes have to be protected from moisture.
5.2.3 Macro and Microstructure of Weld, Heat-Affected Zone (HAZ) and Parent Metal
The metallurgical changes that takes place in weld and HAZ significantly affect the weld quality. The wide variety of changes that may take place depend on various factors, e. g.,
(a) the nature of the material (i. e. single-phase, two-phase)
(b) the nature of the prior heat-treatment
(c) the nature of the prior cold working
We now consider typical examples of these changes.
Let us consider the fusion welding of two pieces of a single-phase material, which have been cold worked to yield a desired orientation. These cold worked grains result in a high strength and low ductility. However, on fusion welding, a random grain growth again takes place within the melt boundary, which, in turn, results in a low strength. Within the heat affected zone, the grains become coarse due to heat input (annealing), and a partial recrystallization also occurs. In either case, the strength falls much below that of the parent material. With increasing distance from the melt boundary, the grains become finer until the heat unaffected zone with elongated grains is reached. All these changes are shown in Fig. 5.9.
Fig. 5.9 Characteristics of welded joints in pure metals.
Let us now consider a two-phase material which derives its strength mostly from precipitation hardening. In this case, the strength within the melt boundary is again too low. But, in the immediately adjacent heat affected zone, the thermal cycle results in heating and quenching followed by further aging. This aging process recovers some of the strength. The material beyond this zone is only overaged due to the heat of welding and becomes harder with the loss of strength. Hence, the strength and ductility variation near the joint are as shown in Fig. 5.10.
Fig. 5.10 Characteristics of welded joints in precipitation hardened alloy
The two examples we have considered clearly demonstrate that various types of metallurgical changes are possible during welding, particularly for complex alloys. These changes are governed by the non-equilibrium metallurgy of such alloys, and must be clearly understood to yield a satisfactory fusion weld. Also, a decision on the postwelding heat treatment to be given, must be taken to restore the desirable characteristics of the joint.