Multiscale study on the dark-etching region due to rolling contact fatigue of 0.57C-bearing steel

The formation of a dark-etching region (DER) caused by rolling contact fatigue is a complicated phenomenon that is accompanied by macroscopic groove formation and hardness reduction, as well as microscopic texture development and carbon redistribution. To understand the mechanism of DER formation, this study employed a bearing steel with 0.57 wt% C and conducted in-depth characterisation of DER at different scales. While the accumulation of plastic strain within the material began at an early stage of the rolling cycles, DER and hardness reduction appeared only after 5 × 10 6 cycles. Inside the DER, a strong 〈 111 〉 // ND texture and a number of recrystallised cells developed, which indicated dynamic recrystallisation. Moreover, the carbon distribution changed when the DER was formed and its concentration in the matrix decreased as the number of rolling cycles increased, which was consistent with the results estimated by the dislocation-assisted tempering model. Based on these observations, it was suggested that the accumulation of plastic flow caused by repetitive rolling contacts enabled the carbon to redistribute from the matrix to carbides via dislocations and triggered recrystallisation. Hence, both carbide thickening and dynamic recrystallisation resulted in different etching behaviour and hardness reductions, which may result in the spalling of the bearing.