Improving the reliability of highly loaded rolling bearings: the effect of upstream processing on inclusions

The connection between the cleanliness of 52100 type bearing steels and their reliability has been well documented. Most research over the past 30 years has focused on sensitive compositional control during metallurgical refinement, leading to steels so clean that industrial cleanness standards are no longer suitable for quantifying further improvements. There is less literature, however, detailing the mechanism by which different impurities initiate rolling contact fatigue (RCF). Early work focused on comparing fatigue lives with cleanness ratings, which include a worst field analysis to determine the inclusion content and post-failure analysis to determine damage nucleation sites. The stress concentrations around discontinuities in the steel matrix can now however be visualised using computational modelling techniques. There is now a much clearer picture of how non-metallic inclusions (NMIs) nucleate fatigue damage by causing changes in the subsurface microstructure, including white etching regions in the form of butterflies. Size, morphology, distribution and type of inclusion are important factors for determining their role in RCF and the ability to control these variables could lead to improved performance. The inclusion character is greatly influenced by the steelmaking process, from initial melt through to casting, as well as hot deformation. While the impact of microchemical banding on RCF is not well understood, the effect of microsegregation on the phases that form can be modelled using simple calculations. Hot rolling techniques also influence the steel cleanliness, as NMIs can be plastically deformed or broken up and voids can be introduced around them, thus affecting the interface with the matrix. Understanding the microstructure evolution from materials characterisation and the ability to model the process could establish an optimum degree of rolling reduction. This could greatly aid the production of large bearings, as some manufacturers currently make them from very large ingots to achieve the necessary reduction ratio and therefore required level of performance.