Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact in Carburized SAE4320 Steel

The deformation-induced martensitic transformation is a phenomenon that significantly improves the mechanical properties of steels, and is well known to be beneficial for the rolling contact fatigue (RCF) of bearings. In the present study, the characteristics of the deformation-induced martensitic transformation in the RCF of carburized, quenched and tempered SAE4320 steel were investigated in detail using scanning electron microscopy with electron backscattering diffraction and transmission electron microscopy with automated crystal orientation mapping. These analyses clarified that different variants of the extremely fine deformation-induced martensites as small as several tens of nm were formed within an austenite grain with RCF, and the martensites were speculated to have the Kurdumov-Sachs or the Nishiyama-Wasserman relationship with the retained austenite. Furthermore, the deformation-induced martensites were preferentially formed within the retained austenite grains rather than at the interface between the tempered martensite and retained austenite. This suggests that the deformation-induced martensites were formed from some localized regions that were plastically introduced within the retained austenite grains.