Effects of forming route and heat treatment on the distortion behaviour of case-hardened martensitic steel type S156

The distortion behaviour of carburised and fully heat treated Ni-Cr-Mo martensitic steel (S156) has been experimentally evaluated. Dimensional measurements of Navy c-ring distortion coupons during interrupted heat treatment process for parts manufactured from two forming routes, hot forging and machined from as received bar, was performed. Metallurgical analysis was carried out to attempt to relate the observed microstructural characteristics with measured process induced distortion. The carburisation process was found to be the most severe in terms of inducing distortion. It was found that additional heat treatments during the process results in a larger final distortion. Machining parts from forgings results in higher distortions than that of those machined directly from as received bar due to the added thermal processing history. An FE simulation of the carburisation process for a c-ring coupon is presented.