Rolling contact fatigue behaviors of 25CrNi2MoV steel combined treated by discrete laser surface hardening and ultrasonic surface rolling

DLSH) and ultrasonic surface rolling (USR) on the material properties (surface roughness, microstructures, microhardness and residual stress) and rolling contact fatigue (RCF) behaviors of 25CrNi2MoV steel were investigated. A continuous-wave diode laser with a maximum output power of 2 kW was used to fabricate four different types of DLSH density samples with a size of F42 mm x 6 mm. The results showed that the combined USR treatment improved the surface quality (including roughness and oxide layer) of the DLSH samples, increased surface hardness, and obtained a beneficial surface compressive residual stress of up to 1240 +/- 91 MPa. The severe plastic deformation introduced a gradient nano/ultrafine grain layer on both hardened zone (HZ) and substrate zone (SZ) surfaces of DLSH group samples, and the deformation depth decreased with the increase of DLSH density within 24.3-65.3 mu m. Accumulation of plastic deformation below and around the HZ edge resulted in a gradient drop in hardness from HZ to SZ, which helps to relieve the occurrence of stress concentration at the surface HZ edge. Benefit from favorable factors, the RCF life of combined treated samples with hardened spot densities of 28%, 50%, 79% and 100% was increased by 82.2%, 123%, 143.6% and 171.9% compared with that of the untreated samples, respectively. After USR treatment, the failure mode within the SZ changed from spalling to delamination, but it remained as spalling failure within the HZ.