Formation mechanism and microstructure evolution of light etched region during rolling contact fatigue in M50 steel

The microstructure evolution and formation mechanism of lighting etched region (LER) during rolling contact fatigue (RCF) in M50 steel are systematically investigated. The results indicates that LER is a kind of microstructure degeneration due to the accumulation of plastic strain. Finite element simulation indicates that, under 5 GPa contact pressure, the equivalent stress at the subsurface exceeds the yield strength of M50 steel and, thus, induces plastic deformation. The dislocation density and fraction of grain boundaries increase in LER and thus the grain size is refined and the micro-hardness increases. The microstructure evolution in LER during RCF is divided into four stages. Stage 1: dislocations and GBs increase significantly, while the micro-hardness remains almost unchanged with the increase of RCF cycles; Stage 2: GBs and micro-hardness increase gradually with the increase of RCF cycles; Stage 3: LAGBs evolve to HAGBs and thus the total length of LAGBs decreases and the total length of HAGBs increases. Dislocation density and micro-hardness both increase with the increase of RCF cycles; Stage 4: dislocations and GBs continue to increase with the increase of RCF cycles due to accumulation of plastic deformation, and the micro-hardness also increases.