Enhanced Wear Resistance of the Ultrastrong Ultrasonic Shot-Peened M50 Bearing Steel with Gradient Nanograins

The enhancement of the wear resistance has long been important for bearing steels, hampered in part by the lack of suitable surface strengthening techniques. In this study, ultrasonic shot peening (USP) is employed to enhance the wear resistance of the M50 bearing steel by generating an ultrastrong surface layer with gradient nanograins. The effects of USP treatment on the surface integrity, microstructures, and wear behavior of M50 steel are investigated. The microstructural observation shows that the gradient nanograined surface layer is over 200 μm thick. The lath martensite is refined to nano-martensitic subgrains with few low angle boundaries in the USP-treated M50 steel. Additionally, the microhardness of the nanostructured M50 steel is increased from 795 HV to 987 HV with a penetrating depth of around 800 μm. Wear tests show that the wear rate of the USP-treated M50 steel is reduced by 50.4% under sliding conditions compared to that of the untreated samples. The significantly enhanced wear resistance of the USP-treated M50 is attributed to the ultrastrong gradient nanograined surface layer. The wear mechanism of the USP-treated M50 steel changes from oxidative wear and severe plowing wear to mild plowing wear. Additionally, the thickness of the plastic deformation layer generated during the friction process is reduced. This work provides the materials scientists and engineers with an efficient surface treatment method to enhance the wear resistance of high-strength steels.