The tensile behavior and failure mechanism of high-strength steel under compound treatment of laser shock peening and ultrasonic surface modification

Surface modification could effectively delay the component failure process, which is vital for high-strength steel applied in heavy-duty axle and gears. In this paper, the compound strengthening treatment of laser shock peening (LSP) and ultrasonic rolling modification (USM) was conducted, to study its influence on tensile behavior and strengthening mechanism of the 25CrNi2MoV high-strength steel. Overlapped-LSP, single-LSP, and untreated samples are also compared, to investigate the strengthening process of compound strengthening. LSP treatment induces deep compressive residual stress (CRS) field, and the compound of UMS induces continuous and stable plastic deformation on surface with high surface integrity. The compound treatment enhances the surface roughness and CRS field distribution in cross-section, which greatly enhances the gradient structure and geometrically necessary dislocation (GND) distribution in surface and sub-surface. The intensive GND distribution indicates a dense accumulation of dislocations in cell, wall, tangle, and line, and promises the work hardening effect in corresponded area. For the strengthening mechanism in tensile behavior, the competition of CRS field and surface hardness greatly influences the tensile strength, plasticity, and tensile morphology. CRS field counteracts the external stress in the initial tensile stage. As the intensity of plastic deformation increasing, hardness has a greater influence in overlapped-LSP and compound samples.