Synergistic Evolution Law of Surface Work Hardening and Contact Fatigue in Wind Power Bearing Steel Pretreated by Ultrasonic Impact

To address rolling contact fatigue failure in 100CrMnMoSi8-4-6 wind power bearing rollers and the limitation of conventional oilstone finishing, this study applies ultrasonic impact treatment (UIT) at two static loads (500 N and 1000 N), systematically investigating effects on surface integrity. Under laboratory rolling contact conditions corresponding to the typical contact stresses encountered in actual wind turbine bearings, gradient microstructure, residual stress, hardness, secondary work hardening, and fatigue life. Results show that UIT simultaneously achieves surface smoothing and gradient strengthening; under 1000 N, roughness drops from 0.41 to 0.11 μm, residual stress reaches −842 MPa, hardness approaches 900 HV0.5, and grain size refines to 1.44 μm with increased low-angle boundaries and fragmented carbides. After rolling contact, the secondary hardening increments for the as-received specimen, the 500 N specimen, and the 1000 N specimen were 4.98%, 3.63%, and 2.11%, respectively; this indicates that the subsequent contact-induced hardening response is closely related to the initial strengthening state established by the UIT process. The 500 N specimen exhibited a significant secondary hardening capability despite having achieved initial strengthening, whereas the 1000 N specimen, although possessing a lower secondary hardening increment, achieved the highest rolling contact fatigue life under the experimental conditions of this study owing to its higher initial hardness, residual compressive stress, and more pronounced microstructural refinement. The fatigue life of the specimens treated with 500 N and 1000 N was increased by 378% and 1214%, respectively, compared to the as-received specimen. These results demonstrate that, within the range of parameters investigated, there exists a static load-dependent relationship between the initial UIT strengthening and the subsequent cyclic contact hardening; this suggests that when evaluating UIT parameters, both the initial surface condition and its subsequent evolution under rolling contact should be taken into account simultaneously. Therefore, the UIT technique holds potential as a comprehensive surface finishing and strengthening treatment method for 100CrMnMoSi8-4-6 bearing rollers; however, its optimal processing range and transferability to actual bearing raceways require further validation.