A number of competing failure mechanisms are involved in bearing failure initiation. For well manufactured bearings operating under clean and well controlled running conditions, sub-surface initiated fatigue is the classical initiation form. Three mechanisms dominate the concept of sub-surface induced initiation and growth: (i) The well documented slow structural breakdown of the steel matrix due to accumulation of fatigue damage in a process superficially similar to tempering, (ii) stress induced generation of butterflies by a process enabling the growth of butterfly micro-cracks and accompanying wings at non-metallic inclusions, and (iii) surface induced hydrogen intrusion causing hydrogen-enhanced fatigue damage accumulation in the matrix. The development of butterflies as a function of contact stress, over-rolling, and non-metallic inclusion characteristics is presented, and the influence of metallurgical cleanliness and processing history on this progression is discussed. The results of laboratory conducted tests are compared to results from field applications where premature spallings have occurred. The progression from butterfly micro-cracks to extending cracks with non-etching borders has been studied. Special interest has been paid to the interaction between the non-metallic inclusion composition and morphology and their propensity to generate butterfly wing formations, as this may affect the way that inclusion harmfulness should be judged in rolling bearing steel quality assurance efforts. Complex oxy-sulfides are the main butterfly initiators in today’s bearing steels.

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Stress-controlled, low-cycle, push-pull fatigue tests were performed on three variants of the bearing steel SAE 52100 with slightly different compositions and heat treatments. The experiments demonstrated differences in the cyclic plastic behaviour of differently hardened steels (bainitically-hardened and martensitically-hardened, respectively), whereas the two martensitic variants, which differ in composition, behaved very similarly. Bainitically-hardened SAE 52100 steel exhibited initial hardening followed by cyclic softening above a stress amplitude level of 1200 MPa. In contrast, the rnartensiticallyhardened variants showed a pronounced cyclic hardening. The deformation behaviour of the martensitically-hardened bearing steel in a monotonic tensile test and during the first cycles can be well understood on the basis of the transformation of retained austenite. This process leads to an onset of plastic deformation at lower stresses compared to the bainitically-hardened bearing steel. As a result of the subsequent cyclic hardening of the rnartensitic variants, the CSS curves are almost identical for the differently hardened conditions under investigation. Additional tests under pulsating compression documented that a high negative mean stress enhances the cyclic plasticity. NOMENCLATUREE, = Young's modulus at zero stress N = number of cycles N, = number of cycles till failure R, = tensile strength R f 1 2 = 0.2% yield strength As,, = plastic strain range E, = total strain E,, = elastic strain sp, = plastic strain u = stress

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The present paper is a review of recent results on simulation of heat treatment response of bearing steels. Two aspects are considered namely firstly the evolution of the microstructure during heat treatment, especially the carbide structure and the corresponding distribution of alloying elements in the austenite around the carbides. This information is important in order to understand the hardening response of the steel. The analysis is based on thermodynamics and kinetics of multi component systems. Both single carbide sizes and realistic size distributions are considered. Secondly the evolution of the residual stresses and the distortion is considered during the full heat treatment cycle including heating and cooling. These latter simulations focus on the behaviour of full components. Special interest is devoted to origins of out of roundness of bearing rings. Factors like original geometrical imperfections, residual stresses from mechanical manufacturing steps, segregation and uneven cooling action during quenching are considered.

Crack nucleation, first spall generation, and spall growth in rolling contact fatigue (RCF) are known to be highly sensitive to the heterogeneity of the microstructure. Yet the current state-of-the-art in the design of high performance bearing materials and microstructures is highly empirical requiring substantial lengthy experimental testing to validate the reliability and performance of these new materials and processes. We have laid the groundwork necessary to determine the influence of microstructure in RCF to aid in the development and processing of bearing steels. Microstructure attributes that may control the fatigue behavior are explicitly modeled in a 41xxx steel. The methodology is demonstrated by studying the role of an aluminum oxide inclusion embedded in a matrix of tempered martensite and retained austenite. The matrix is represented by crystal plasticity, which provides more realistic accumulations of localized plastic strains with cycling compare to homogenized J2 plasticity. As a demonstration of the approach, the relative influence of the volume fraction of retained austenite on RCF is evaluated.