The objectives of this paper are to investigate the size of the maximum micro-inclusion in a railway wheel using ultrasonic fatigue testing and to evaluate the effect of the micro-inclusion on subsurface-initiated rolling contact fatigue of the railway wheel. Fatigue specimens with a large risk volume were quenched, tempered and oxynitrocarburized to increase both the strength of the matrix and the resistance to surface corrosion, which ensured that all of the specimens fractured from the internal inclusion under water cooling. The test results show that the size of the micro-inclusion obtained using the fatigue method are much larger than those obtained by a traditional surface analysis method, which means that the accuracy of the fatigue method is higher than that of the traditional method. The maximum square root of the micro-inclusion area and the maximum length of the major axis of the elliptical micro-inclusion for a full-sized railway wheel are estimated using the statistics of extreme values method to be 296 µm and 736 µm, respectively. Based on the estimated maximum micro-inclusion sizes and principles of fracture mechanics, the critical loading of the railway wheel for both subsurface-initiated fatigue and fatigue initiated at a deep-lying material defect are obtained.
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