Induction-assisted laser beam welding of a thermomechanically rolled HSLA S500MC steel: A microstructure and residual stress assessment

The present work deals with the effect of different combinations of induction heating and autogenous CO2 laser welding on the gradients of microstructure, microhardness and residual stresses in butt-joints of thermomechanically processed S500MC steel grade. Five strategies were pursued by varying the inductor position with respect to the laser beam. This enabled in-line pre-, post-, and simultaneous pre- and post-heating as well as annealing of the fusion and heat-affected zones. The induction-assisted CO2 laser welding strategies were compared to individual CO2 and Nd:YAG fiber welding procedures. The results demonstrate that induction heating can be combined to laser welding in order to effectively increase the cooling times. Martensite formation could be suppressed within the fusion and heat-affected zones and smooth hardness distributions were obtained by pre-heating and combined pre- and post-heating. The tensile residual stresses are, however, still of significance because of the high transformation temperatures (>500°C) observed for the S500MC steel. This allowed for extensive thermal contraction after exhaustion of the austenite to ferrite transformation.