Multiscale characterization of white etching cracks and its adjacent white etching areas in M50 bearing steel

Premature failure of bearings under rolling contact fatigue (RCF) is often associated with the formation of white etching cracks (WECs) and its adjacent white etching areas (WEAs). The formation mechanisms of WEC/WEAs in M50 bearing steel remain highly debated. To investigate the initiation and propagation of WEC/WEAs, two types of cracks and the branching were observed. A multiscale characterization of microstructural alterations was performed, focusing on carbide dissolution, elemental redistribution, and the role of hydrogen (H) in the formation of WEC/WEAs. The results show that WEC/WEAs exhibit a markedly lower carbon content and significantly enhanced hardness as compared to the matrix. Meanwhile, atom probe tomography (APT) and high-resolution transmission electron microscopy (HRTEM) reveal the presence of dissolved carbides or newly formed clusters within WEC/WEAs, accompanied by hydrogen segregation. These findings indicate that under prolonged RCF, severe plastic deformation induces the proliferation of dislocations and hydrogen segregation around carbides. This may accelerate carbide dissolution, along with microscopic elemental depletion and diffusion. Subsequently, carbide dissolution leads to localized hardness elevation and microstructural refinement, resulting in the formation of nanocrystalline and amorphous phases. This microstructural evolution promotes crack initiation and propagation, thereby accelerating the formation of WEC/WEAs.