Effect of heat-assisted ultrasonic surface rolling on the surface microstructure and tribological properties of M50 bearing steel

In this study, surface strengthening of M50 bearing steel was conducted using heat-assisted ultrasonic surface rolling (HUSR), and its tribological performance was evaluated under room temperature (RT) and 350 °C high temperature conditions. The results indicate that the optimal HUSR temperature is 150 °C, which produced a gradient microstructure with a thickness of 288.3 μm in the surface layer. This treatment increased the surface hardness by 47.7% and introduced a residual compressive stress of −484.5 MPa. Notably, compared to the untreated (UT), HUSR treatment reduced the average coefficient of friction (COF) and wear rate at room temperature by 17.11% and 46.15%, respectively; under the 350 °C condition, the two decreased by 9.43% and 66.88%, respectively. Furthermore, HUSR treatment altered the wear mechanism, transitioning the wear mechanism from severe abrasive wear to mild adhesive wear at room temperature; at high temperatures, it effectively suppressed the severe spalling behavior of the substrate, shifting predominantly to oxidative adhesive wear.