Modelling microstructural changes in pulsed weldments

Abstract: Microstructural changes in pulsed weldments have been studied in mild steel, a quenched and tempered steel and in two austenitic stainless steels. A heat flow model has been developed to compute the thermal cycles at different locations in the pulsed welds. It is shown that this model is useful in understanding the sequence of phase transformations in the pulsed welds. With additional information on the kinetics of phase transformations obtained by weld simulation, it is shown that a phenomenological modelling approach can be used for choosing appropriate procedures for applications where it is desired to obtain the maximum possible refinement of microstructures in the weld metal and heat affected zone. The effect of a refinement in the solidification structures in pulsed welds on the susceptibility to soldification cracking has been studied with the help of a slow-bend transvarestraint test method. This testing method is specially suited for studying the cracking tendency under dynamic conditions such as those prevailing in pulsed welds. First, with tests on constant current welds, it is shown that the susceptibility of a material to solidification cracking can be estimated in a sensitive manner. This data also provided the basis for evaluating the effects of pulsed welding on the cracking sensitivity. Work carried out on two austenitic stainless steels showed that pulsed welding leads to a refinement in the solidification structures. This was found to increase the cracking resistance only in the steel solidifying by the ferritic-austenitic mode. In case of the steel solidifying by the austenitic mode, an increase in the cracking sensitivity was observed for certain pulsing parameters. Metallographic evidence of Equation was observed in the pulsed welds of the latter steel. In the pulsed welding conditions studied, the adverse effect on the cracking resistance due to liquation seemed to override any positive effects that could result from grain refinement.