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The rolling contact fatigue testing of low oxygen, high quality, rolling bearing steels is a major challenge to steel makers, bearing producers and end users. Material specimen based rolling contact fatigue tests were used for a number of years, mainly as acceptance tests. Often in this type of test, the applied contact stress exceeds the stress normally found in rolling bearing applications and also exceeds the limit stress for rapid cyclic micro-plastic groove formation in the rolling contact. For these reasons the established methods have limited ability to discriminate material quality effects in modern high cleanliness rolling bearing materials. Furthermore, the results of tests based on a material specimen are difficult to translate into life calculation factors for the bearings. In this paper a novel test method is presented and used to determine the effect of steel internal cleanliness on the performance of rolling bearings. To achieve this result considerable care is required in the preparation of the test elements, the selection of the specific test conditions and in the analysis of the results. This work also led to an ability to determine the dependence of steel quality on the steel making processes. Material cleanliness is characterized using the statistics of extreme values of the micro-inclusion size population allowing the steel cleanliness quality rating to be related directly to the fatigue performance of rolling bearings through the introduction of a material cleanliness factor η. The expected bearing life performance for materials with different inclusion size ratings can thus be calculated. Results correlate well with measurements demonstrating that the quality rating of the material can now be reliably included in standard bearing life calculations.

The presence of nonmetallic inclusions in bearing steel is well described as well as the occurrence of white-etching cracks starting at the inclusions. In this paper, the role of interfacial strength in the initiation of small cracks at nonmetallic inclusion-steel interfaces that lead to white-etching crack systems is discussed. The effect of rolling contact fatigue on the steel-inclusion interface will be described to explain the early crack initiation, with a focus on manganese sulfide and complex aluminum oxide inclusions. The factors of size and shape both play an important role in the behavior of inclusions, as well as that of interfacial bonding, which will be shown to be decisive in the strength of a bearing steel. Experimental methods to determine inclusion-steel interface properties are applied, such as advanced tensile tests combined with digital image correlation, in order to show the level of bonding of nonmetallic inclusions. The experimental results contribute to the verification of inclusion modeling in rolling contact fatigue simulations.

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