Mechanism of White Band (WB) Formation due to Rolling Contact Fatigue in Carburized SAE4320 Steel

Microstructural alterations such as white band (WB) including low angle band (LAB) and high angle band (HAB) following the formation of dark etching area (DEA) in bearing steels below the contact surface due to high stress rolling contact fatigue (RCF) were investigated. Although there have been many studies on the characteristics of WB, its formation mechanism has not been sufficiently clarified yet. In this paper, we analyzed the orientation of crystal constituting WB and investigated the relationship between WB and the direction of shear stress generated by rolling contact. The morphology of WB as a function of depth from surface was observed by light optical microscope, and the crystal orientation was analyzed by scanning electron microscope–electron backscattering diffraction. It was found that LAB and HAB formed at the depth where the principal shear stress and the orthogonal shear stress become maximum, respectively. Crystal orientation analysis of LAB and HAB revealed that crystal rotates under the principal shear stress at the specific depth, resulting in the formation of unique texture such as {111}<211> and {122}<411>, respectively. Thus, WBs were proved to be some kind of shear band. WB formation behavior of specimens with the different amount of initial retained austenite (γR) was compared in order to clarify sub-surface initiated spalling life improvement mechanism by γR. However, the WB formation behavior showed no difference irrespective of the amount of initial γR. This suggests that the WB formation is not directly related to sub-surface initiated spalling life.