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Modelling of micro-segregation in a 1C-1.5Cr type bearing steel

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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.