Influence of Heating Rate and Soaking Temperature during Case-Hardening on the Hardness and Grain Size of Ultra-Clean Gear Steels

In the framework of a research project, the tooth root bending strength of high-strength gears is investigated. The most common type of failure encountered with these shot-peened, case-hardened gears is tooth root breakage caused by a crack initiation below the surface at a non-metallic inclusion (fisheye failure). However, it is well known that case-hardening depth and quenching grain size can have a great influence on a gear's load carrying capacity. To ensure that this research project solely investigates the influence of non-metallic inclusions, any cross-influences must be excluded or minimized, or at least known. However, in the case of such ultra-clean gear steels, no recommendations or investigations of the influence of case-hardening on material properties are currently available. Therefore, in the framework of this paper, extended investigations are conducted into the effects of different process steps during case-hardening on the resulting material properties of ultra-clean gear steels.