Effect of hydrogen on butterfly and white etching crack (WEC) formation under rolling contact fatigue (RCF)

Abstract

White structure flaking (WSF) as a premature wear failure mode in steel rolling element bearings is caused by white etching cracks (WECs) formed in the 1mm zone beneath the contact surface. Hydrogen release and diffusion into the bearing steel during operation and transient operating conditions have been suggested as drivers of WSF. The presence of diffusible hydrogen in steel under rolling contact fatigue (RCF) has been shown to strongly promote the formation of WEA/WECs. However, the initiation and propagation mechanisms, as well as the thresholds for WEC formation, are not well understood. This study uses hydrogen charging of 100Cr6 bearing steel rollers prior to testing on a two-roller RCF rig to enable WEA/WEC formation under service realistic loading. This study focuses on the influence of the concentration of diffusible hydrogen, the magnitude of the contact load and the number of rolling cycles on the formation of white etching features (butterflies, WEA/WECs) which are determined by a serial sectioning process. The formation of butterflies was found to be independent of concentration of diffusible hydrogen with the test parameters used, but dependent on contact pressure and number of rolling cycles up to a threshold. WEA/WEC formation thresholds were found at certain values of the concentration of diffusible hydrogen, contact pressure and number of rolling cycles. The results also show evidence for a subsurface initiation mechanism of WECs from non-metallic inclusions. It is proposed that one mechanism of WEC formation is due to multiple linking of extended butterflies or small WECs in the subsurface to form larger WEC networks that eventually propagate to the surface resulting in WSF.