Carburizing is a method of enhancing the surface properties of components, primarily made from low to medium carbon steels, such as shafts, gears, bearings, etc. Carburized parts are generally quenched and tempered before being put into service; however, after quenching of carburized parts further annealing and hardening treatments can be employed before final tempering. This work analyses the impact of the two aforementioned heat treatment approaches on the development of subsequent microstructures and mechanical properties of hot forged 18CrNiMo7-6 steel. Moreover, this study aims to understand the impact of normalizing treatments prior to the two aforementioned heat treatment routes. Microstructural and mechanical tests were conducted on four as forged flat cylinder components that received a combination of the abovementioned heat treatments. In general, better microstructure refinement, in terms of prior austenite grain size (PAGS), was obtained for carburized parts that received the intermediate annealing and hardening treatments after quenching and prior to the final tempering. Additionally, further refinement of the martensitic pockets/blocks was observed for parts that did not receive a normalizing treatment prior to carburization. The studied heat treatments appear to have a negligible effect on the mechanical properties of the hot forged flat cylinder components.

The distortion behavior of carburized 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 carburization 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. A finite element simulation of the carburization process for a C-ring coupon is presented.