Gradient nanostructure and tribological properties of M50 bearing steel treated by ultrasonic surface rolling

In this study, Ultrasonic Surface Rolling Process (USRP) was applied to enhance the surface properties of M50 bearing steel, with a focus on improving its tribological performance through the formation of a gradient nanostructured surface layer. The results demonstrate that the USRP effectively produces a gradient nanostructured layer (300 nm in thickness) and a plastic deformation layer (45 µm). This treatment, characterized by fragmented carbides within the deformation layer, induced a compressive residual stress of −143.85 MPa and increased surface microhardness by 10.84 %. Notably, the friction coefficient and wear mass of the USRP-treated samples were significantly reduced by 16.00 % and 13.97 %, respectively, compared to untreated counterparts. Furthermore, it was observed that increasing the USRP load enhanced microhardness and rendered surface rolling textures more pronounced. The dominant wear mechanisms transitioned sequentially with increasing surface-strengthening loads: from initial abrasive wear to mild adhesive wear accompanied by tribo-oxidative protective layers, and ultimately to severe abrasive wear involving oxidation.