Influence of White Etching Bands Formation on Integrity of Rolling Element Bearings

The development of subsurface microstructural alterations known as dark etching regions (DERs) and white etching bands (WEBs) in rolling element bearings due to rolling contact fatigue have been investigated for the past eight decades, focusing on their initiation and formation mechanisms. They have only recently been shown to be driven by repetitive cycles of energy build-up due to micro-plastic deformation and energy release through recrystallization and recovery, which results in the formation of equiaxed and elongated ferrite grains, as well as lenticular carbides. These features develop within the bearing subsurface from DER to WEBs during bearing operation at moderate to high loads, but little evidence has been presented in the literature to understand links between DER and WEBs and the nucleation and growth of subsurface cracks. This investigation examines WEBs, including low angle bands (LABs) and high angle bands (HABs), in detail especially focusing on their late stages to understand such links. A number of techniques, including optical microscopy, scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) have been used to examine the features involved. Analysis on WEBs obtained through serial sectioning has revealed that voids initiating at the interface between lenticular carbides and equiaxed ferrite grain bands within WEBs have led to crack formation which can subsequently propagate to bearing surfaces. Interactions between WEBs and non-metallic inclusions (NMIs) are observed to lead to de-bonding of inclusions from their surrounding microstructure and void formation, which has also found to influence the integrity of the bearings at late stages. Alumina and Manganese sulphide (MnS) inclusions are the mostly observed NMIs that de-bond and develop microcracks when interacting with WEBs. These findings thus provide important insights into the link between inclusions and crack initiation and represent a further step towards a fundamental understanding of the rolling contact fatigue process.