Further understanding of dark etching region and white etching bands development in bearing steels due to rolling contact fatigue

Abstract

Rolling-element bearings undergo rolling contact fatigue (RCF) due to the cyclic loading they experience throughout operation, which leads eventually to failures due to surface or subsurface initiated cracking. During the life of rolling-element bearings, bearing steels experience a number of subsurface microstructural alterations such as dark etching regions (DER) and white etching bands (WEB) that could lead to damages bearing failures. WEB formation typically includes two stages, i.e. the early formation of low angle bands (LAB) followed by high angle bands (HAB). All of these microstructural alterations show inhomogeneity in phases and properties compared with the original bearing steel microstructure, which can influence the integrity of the bearings. Despite extensive research on DER and WEB spanning back to the 1940s, the formation mechanism of these features remains debated. It is unclear how these features initiate in bearings and what drives the carbon migration mechanism during their formation. However, the latest examination on RCF tested bearing samples in this study has shown a third component, i.e. elongated ferrite grains, within WEB, that was not considered in previous models. This raises some questions on how DER, LAB and HAB initiate and evolve during bearing operation and how these three features are linked as well as how they influence the final failure of bearings. This study aims to elucidate the initiation and growth mechanisms of DER, LAB and HAB by investigating their microstructural characteristics at different stages. Bearings made from four steels including 100Cr6 martensite (two different grades of steel cleanliness), 100Cr6 bainite and a low carbon steel 50CrMo4 martensite RCF tested under contact pressures (2.9 GPa and 3.5 GPa) over a series of stress cycles, have been examined using light optical microscopy, scanning electron microscopy, electron backscatter diffraction, energy dispersive X-ray spectroscopy, transmission electron microscopy, atomic probe tomography and nanoindentation techniques. Initially, the growth pattern of each feature was investigated optically to determine their evolution under the test conditions, based on which, a semi-empirical model has been developed to predict the initiation and growth of LAB and HAB. These features have all been examined at a variety of resolutions to provide evidence for the unified mechanism. The results show that DER formation and development process in the low carbon steel 50CrMo4 is much prolonged compared to that in the high carbon 100Cr6 before LAB and HAB formation under similar conditions. Detailed inspection of the features at different stages has shown that DER initiates as groups of parallel, thin, heavily etched DER bands, with each group having a distinct directionality relative to the rolling direction. These bands are found to have led to martensite fragmentation and refinement of the parent microstructure, while the dark etching bands are also seen to break-down, leading to DER becoming ‘brighter’ at later stages. The grain refinement has been shown to contribute to high stress concentrations and recrystallization. Further development led to the formation of single equiaxed grain bands orientated at about 30 towards the contacting surface. As a form of recovery, elongated ferrite grains are found to form within the single equiaxed bands through grain rotation/coalescence mechanisms, confirmed by carbon measurements. The process is found to involve the release of carbon to nucleate lenticular carbides at the edges of the newly formed elongated ferrite grains. As more LAB form, the unstable geometries of the carbides break leading to the formation of HAB through recrystallization. Surface energy analysis has shown that the lenticular carbides structure in HAB is more stable than that in LAB. This has led to the uniform mechanisms for the global microstructural alterations of DER, LAB and HAB as a continuous cycle of energy build-up and release due to cyclic loading from small to large scales. The analysis of failed bearing samples has shown that voids develop at the interface between lenticular carbides and ferrite band in WEB, which is a plane of weakness due to the difference in their hardness. Consequently, this may initiate cracks at the interface, leading to bearing failure. WEB are also observed to have grown through non-metallic inclusions, which may have caused inclusion debonding from the steel matrix and crack initiation, leading to final bearing failures.

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ORCID for Ling Wang: orcid.org/0000-0002-2894-6784

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