Mechanistic study of dark etching regions in bearing steels due to rolling contact fatigue

Dark etching region (DER) formation is the first stage of a series of subsurface microstructural alterations induced by cyclic stresses during rolling contact fatigue in bearing steels followed by the formation of low angle bands (LAB) and high angle bands (HAB). A unified formation mechanism for LAB and HAB development in bearing steels has been proposed as a series of energy build-up and release in a recent study. This paper presents the development of DER through the analysis of two different steel, 100Cr6 and 50CrMo4, at various stages in the lifetime of bearings using SEM, EBSD and nanoindentation. It is found that early stages of DER consist of patches of multiple dark etching bands orientated at three distinct orientations relative to the rolling direction. As the dark etching bands grow in density, they contribute to the refinement of the parent microstructure through fragmentation of martensite laths. The fragmentation as well as intersections of dark etching bands lead to the creation of stress points within the region that become nucleation sites for the formation of equiaxed ferrite grains through recrystallization, which has been found to be the initiation stage of LAB. Hence this study establishes a link between DER and LAB/HAB development in rolling bearings.