White etching bands formation mechanisms due to rolling contact fatigue

Subsurface micro-structural changes such as dark etching region (DER) and white etching bands (WEB) which develop in bearing steels due to cyclic stresses in rolling contacts, have been studied for decades and a number of theoretical models have been proposed to explain their formation mechanisms and predict their initiation. In WEB investigations, studies have generally focused only on one subtype of WEB, e.g. low angle bands (LAB) or high angle bands (HAB), while the most recent semi-empirical model has shown to be able to predict the formation of both LAB and HAB based on observed growth patterns of ferrite grains. Following from the modelling study, this paper presents a detailed mechanistic study, showing the evolution of ferrite grains (equiaxed and elongated grains) and carbide structures in WEB formed in inner rings of angular contact ball bearings at their different life stages through SEM, EBSD and nano-indentation analysis. The results strongly suggest both LAB and HAB initiate as equiaxed ferrite grains due to recrystallization arising from energy build-up in the initial microstructure that later develops to elongated ferrite grains through a grain rotation/coalescence recovery mechanism induced from plastic deformation. The formation of carbide structures in LAB is associated with the transformation of equiaxed to elongated grains, where carbides nucleate at the edges of the elongated grains rather than at the equiaxed grain band edges as being previously suggested in literature. The newly proposed formation mechanism links LAB and HAB based on experimental findings from detailed inspection of gradual microstructural alteration sequence of LAB and HAB in rolling contact fatigue (RCF) tested bearings.