The origin of microstructural alterations in M50 bearing steel undergoing rolling contact fatigue

M50 bearing steel designed for aerospace applications exhibits unique microstructural alterations at the subsurface during rolling contact fatigue (RCF). In this work, three types of microstructural alterations, light etching region (LER), white etching band (WEB) and white etching area (WEA) are systematically studied. A hardness profile at the subsurface is obtained by microindentation with material softening being detected. It is found that the material softening is associated with the formation of WEBs. The microstructural nature of the three types of microstructural alterations is revealed by detailed characterisation using focused ion beam milling and transmission electron microscopy. It is found that LERs, WEBs and WEAs are a manifestation of the decay of the parent martensite and all consist of dislocation cells or fine grains, indicative of plastic deformation. Through subsurface stress field analysis, LER, WEBs and WEAs are found to be caused by different stress components. The similarities and differences of the microstructural alterations in M50 and in widely studied 100Cr6 are discussed where the stability of carbides is believed to play a key role. The difference in material response in the microstructural alterations in M50 is further analysed and modelled based on Kocks and Mecking theory, with different types of activated dislocation slip systems found to be the root cause for such difference. Based on the material response analysis for WEBs, a material softening model for M50 during RCF is established by considering residual plastic strain accumulation, yielding prediction results in agreement with experimental measurement.