Abstract: Ultrasonic surface rolling extrusion (USRE) processing is applied on quenched and tempered 40Cr. Microstructure observations of USRE specimen surfaces indicate that the processing can both get nano-structured layers, with grain size of 3～7 nm, and reduce the surface roughness to 0.05 µm; plastic flow is formed from top surface to about 200 µm deep with graded distribution of grain size. Tests of mechanical properties show that microhardness of USRE specimen surface is increased by 63%; residual stress at the top surface can reach to –846 MPa and depth of residual stress layer is more than 1 mm. It has been determined by contrast wear test that USRE can reduce friction coefficient and improve the wear-resistant property.

Key words: 40Cr, Mechanical properties, Microstructure, Nano-crystal Line, Ultrasonic surface rolling extrusion processing