An investigation was carried out to determine the effect of rapid solidification on the weld metal microstructure of austenitic stainless steels and its implication on the ferrite constitution diagram. A wide variety of stainless steels were laser welded at different welding speeds and laser power levels. The results indicate that both weld pool cooling rate and the postsolidification solid state cooling rates have a profound effect on the microstructures. For the steels investigated, the microstructures ranged from duplex austenite (7") + ferrite (6) to fully austenitic or fully ferritic. These microstructures were found to be sensitive to both cooling rates and composition. The observed results are rationalized based on rapid solidification theory. Observations of this investigation indicate that solidification rates and postsolidification cooling rates have a profound effect on the observed microstructures, thus making it impossible to predict the microstructures of rapidly cooled weld metal from the conventional constitution diagrams. The influence of the observations made in this investigation on the Schaeffler diagram is demonstrated and possible corrections to the constitution diagram incorporating the cooling rate effects are proposed.
The kinetics of the bainitic transformation in a polycrystalline Fe-Cr-Mo-C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the transformation behaviour, with clear evidence that the stress tends to favour the development of certain crystallographic variants of bainite, even though the stress may be well below the single phase yield strength. It is concluded that the transformation is influenced significantly by stresses as low as 45 MN m-2, even though the effect may not be obvious in metallographic studies. The results are analysed and discussed in terms of the mechanism of the bainite transformation.MST/1394
The welding and weldability characteristics of single crystal nickel base superalloy PWA 1480 were investigated over a range of welding conditions and orientations using electron beam and laser welding processes. Using differential thermal analysis, the freezing range for this alloy was established and the formation of the γ/γ′ eutectic during the last stages of solidification was identified. Both electron beam and laser welds showed extensive fusion zone cracking, and the cracks were identified as solidification cracks, i.e. hot cracks. Crack free welds could be made over a very narrow range of welding conditions with preheat. Most of the microstructural features observed in the weld are similar to those previously observed in Fe–Ni–Cr alloy single crystal welds. The welds contain misoriented stray grains which playa critical role in the promotion of hot cracks in the welds. The origin of these grains is explained in terms of constitutional supercooling and the growth conditions. Detailed atom probe field ion microscopy analysis was carried out to determine the γ/γ′ compositions and the segregation at the γ/γ′ interface. The results revealed no solutal segregation at this interface, and the composition of the γ/γ′ was close to that of the equilibrium composition.
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