Improvement of Surface Properties of 30CrNi2MoVA Steel with Ultrasonic Composite Strengthening Modification

The surface roughness, surface hardness, tensile properties and friction-wear properties were characterized, in comparison with those of the traditional turned cutting, electropulsing treatment (EP) and ultrasonic surface-rolling process (USRP) sample. The surface microstructure was obviously refined after USRP and EP-USRP, with a fine-grain depth of 60 μm and 100 μm, respectively. The surface roughness significantly decreases at first, and then gradually increases after surface-strengthening modification. The lowest roughness of 0.035 μm and 0.040 μm is obtained for the USRP and EP-USRP samples, respectively, which is about 12 times less than that of the turning surface roughness of 0.421 μm. The surface hardness increases from 280 HV to 360 HV after strengthening modification. The super tensile property of 30CrNi2Mo steel is obtained for the USRP, for which the yield strength, tensile strength, elongation and yield-to-strength ratio are 743 MPa, 961 MPa, 11% and 0.773, respectively. The friction coefficients in the turning state, USRP and EP-USRP are 0.37, 0.35 and 0.4, respectively. Ultrasonic composite-strengthening modification can increases the surface hardness, and obtains gradient microstructure on the material surface, which endows the material with better surface properties.