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.

The connection between the cleanliness of bearing steels and their reliability has been well documented and there is a wide acknowledgment in industry that the early steps in steel processing (including secondary metallurgy, casting, homogenisation heat treatments, and rolling) have a profound effect upon the inclusion characteristics within the material. There is, however, little systematic work showing the progression of how the final bearing properties are intrinsically linked to the initial steelmaking. The effect on rolling contact fatigue of chemical segregation that leads to carbide banding in bearing steels is not well understood. A Scheil–Gulliver approach was used to investigate the extent of segregation in a 100CrMnMoSi8-4-6 bearing steel on the scale of a secondary dendrite arm and the composition variations are shown in the results. A simple finite differences model to solve Fick’s second law was then applied to establish how the composition distributions vary over time during a high temperature homogenisation treatment. The effect of hot rolling upon microsegregation is also investigated using electron probe micro analysis (EPMA). The impact of the microsegregation upon microstructure can be investigated using a thermodynamic approach to identifying the carbides that form in solute rich and solute depleted regions, and combining this with models for nucleation and growth kinetics. These microstructural variations can cause carbide bands that are present in rolled bearing steels and could lead to property variations on the micrometre scale and influence crack propagation along bands. Likewise, the effect of segregated regions upon large primary inclusions can be investigated in a similar way and it can be shown that manganese sulfides are only stable within solute rich regions. These large inclusions could act as stress raisers during rolling contact fatigue. From this work, a picture emerges of processing and property relationships, from initial ingot casting and through hot rolling. This may aid in identifying and quantifying the key processing parameters to control during early steel production that might improve rolling contact fatigue life.