A study on microstructural alterations in white etching cracks, dark etching region, and white etching bands in rolling contacts

Abstract

The formation of subsurface White Etching Cracks (WECs) is found to cause catastrophic bearing failures by spalling in their early service life. WECs cause failures in a wide range of engineering applications with a large amount being reported in wind turbines in recent years. One of the characteristics that makes WECs a unique and complex failure mode is that the cracks are accompanied by White Etching Area (WEA). Many mechanisms, including adiabatic shear bands, electromagnetic field impact, severe localised deformation, carbide dissolution, accumulation of plastic deformation, low temperature recrystallisation, and others, have been suggested as possible WECs formation mechanisms. However, due to lack of supporting evidence for altered microstructure evolvement, the WECs initiation and development mechanisms are still under debate. Other microstructure alterations forming in rolling bearings, namely Dark Etching Region (DER) and White Etching Bands (WEBs) known to be caused by Rolling Contact Fatigue (RCF) were frequently reported in literature between 1940s and 1990s, however their formation mechanisms and especially their links to WECs are also not fully understood.
This study aims to investigate the initiation and propagation mechanisms of WECs as well as DER and WEBs through detailed microstructural characterisation of these features in typical AISI 52100 through-hardened martensitic bearing steels. A combination of hardness and microscopy characterisation techniques, including Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD)
combined with Energy Dispersive X-ray spectroscopy (EDX) have been used throughout this study.
Initially, the analyses have been conducted on WECs, DER and WEBs in failed bearings from service or laboratory rigs, where all features were at their fully developed stage. The results have revealed that the microstructural alterations in WECs have many similarities to the RCF features including the manifestation of equiaxed and elongated grains in all these features. The fact that DER still contains unaltered primary spheroidised carbides (Fe, Cr)3C suggests that formation of such equiaxed and elongated grains could be the initial stage of all the microstructure alterations. It has also been observed that primary spheroidised carbides have disintegrated in the WECs and WEBs, accompanying the observation of carbon and chromium redistribution in these areas. This has led to believe that primary spheoridised carbide disintegration may play a key role in the formation and development of white etching microstructure in both WECs and WEBs.
The study was then moved on to investigating the initiation and development stages of WECs and RCF features by analysing bearings tested under controlled laboratory conditions over a sequence of rolling cycles. This enabled the chronological examination of the microstructure alterations. The results have revealed that the formation of microcracks (approx. 2-4 µm in length) in the very early stage of rolling contact may be the initiation stage of WECs developed in the bearing steel in the later stages under electrical current influence. The initial microcracking is found to be followed by crack branching and microstructural change adjacent to the cracks or formation of WEA. However, in the bearings subjected to prolonged RCF testing, it has been found that initially equiaxed grains form, followed by formation of carbon-rich area adjacent to the newly formed grains and then growth of both under continuous cyclic loading. Electrochemical processes involving the ‘bad lubricant’ and electrical current may be responsible for the initial microcracks formation, however further investigation of WECs formation under other operating conditions should be conducted to explore whether same mechanisms are involved. The detailed findings from this study provide a significant breakthrough in the understanding of WECs as well as its relation to DER and WEBs in RCF.

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

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