a b s t r a c tMartensite decay in bearing steels manifested as dark etching regions (DERs) under rolling contact fatigue (RCF) is modelled. The proposed model is established based on a dislocation-assisted carbon migration mechanism. The proposed model is capable of predicting the progress of DER formation and the corresponding mechanical property evolution with increasing number of cycles, in good agreement with the experimental data reported throughout seventy years. The effects of RCF testing conditions on DER formation are studied and a useful tool, DER% maps, is developed for illustrating the temperature, contact pressure and number of cycles for DER occurrence. Moreover, an atom probe tomography study is carried out, revealing the nature of DER ferrite and obtaining strong evidence supporting the postulated DER formation mechanism. The successful application of the dislocation assisted carbon migration mechanism to DER formation provides a plausible explanation to the phenomenon of martensite decay under rolling contact fatigue.
A processing-microstructure-performance approach is followed to study three bearing steel samples manufactured from the most frequently used continuous casting routes. The inclusion microstructures of the samples were altered by varying the metallurgy and hot working conditions. Inclusion size distribution information is obtained, showing the steel-making route that results in the highest cleanliness. 3D analysis of inclusion morphologies using electrolytic extraction indicates the irregularities on the surface to be favourable sites for crack nucleation under RCF. Flat-washer and ball-on-rod tests were conducted to study the rolling contact fatigue life of the steels, with the results from the flatwasher testing method being more representative for bearing life. This research suggests that early fatigue of bearings is governed by silicate fragmentation and late fatigue by TiN inclusions.
The formation of white etching bands (WEBs) occurs at the subsurface of rolling contact-fatigued bearing inner rings, exhibiting microstructural decay detrimental to bearing life. Despite the fact that WEBs have been observed in bearing steels for nearly 70 years, the understanding of WEB formation is still limited and mostly qualitative. Therefore, a systematic investigation is carried out in this research to reveal the evolution of WEBs with respect to the number of contact cycles. WEBs formed at different stages are reproduced by full-scale bearing RCF tests with predetermined numbers of cycles. Multi-scale characterisation techniques such as optical microscopy, micro-indentation, scanning and transmission electron microscopy and atomic force microscopy are conducted on the microstructural alterations to study the development and microstructure of WEBs. WEBs are found in the absence of dark etching regions which is attributed to the heat treatment. With an increasing number of cycles, WEBs grow in number density and in all three dimensions, and their formation is found to be controlled by the maximum shear stress component. Ferrite bands within WEBs that contain dislocation cells manifest accumulated plastic strain in the material. Based on the characterisation results, the evolution of plastic strain under RCF is quantified.
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