Influence of Different Pre-Heat Treatments on the Grain Size in the Core Region of Ultra-Clean Gear Steels

In a previous publication, the influence of various process steps during the case-hardening process on the resulting material properties of two distinct batches of ultra-clean gear steels were investigated. The steel batches were case-hardened in the as-delivered condition. However, in the industrial practice it is common that pre-heat treatments are done, such as annealing to a ferritic-pearlitic microstructure, or quenching and tempering, as well as forging as a process if for example larger gears are needed. It is known that the whole heat treatment process route can affect the grain size negatively. Therefore, this publication presents investigations into the effects of preheat treatments on the grain size in the core region of ultra-clean gear steels after case-hardening. The aim is to receive a deeper understanding on how the whole process route influences the grain size in the core region of ultra-clean gear steels.