Process of White Etching Cracks Formation in Carburized Bearing Steel under Rolling Contact Fatigue

The purpose of this study is to investigate the mechanism of the premature failure of bearing steels in rolling contact fatigue (RCF), with a particular focus on the process of the white etching cracks (WECs). A two-roller type rolling contact fatigue test was carried out using a carburized SAE5120 steel, which successfully provides systematic sequences leading to the WECs under a contact pressure of 2700 MPa with 3.0x10(7) RCF cycles. The process of WECs consisted of crack initiation at prior austenite grain boundaries, crack propagation accompanied by WECs formation, and crack propagation without WECs. The initial stage of the RCF test resulted in the formation of acicular structures, which were caused by {110}<111> slip driven by cyclic shear stress. However, these acicular structures were found to be unnecessary for crack initiation or the formation of white etching area (WEA). Instead, it was observed that crack initiation occurred at the boundaries of the prior austenite grains. After the crack initiation, the WEA was formed around the cracks, indicating that rubbing of the crack surfaces leads to WEA formation. Stress analysis revealed that a mode-I crack was formed due to cyclic compressive stress applied by RCF. Furthermore, it was found that the crack initiation was suppressed with low amount of hydrogen content. This suggests that hydrogen accelerate the crack initiation at prior austenite grain boundary.