Microstructural evolution, mechanical behaviors and strengthening mechanism of 300 M steel subjected to multi-pass laser shock peening

300 M steel was treated by multi-pass laser shock peening (LSP). The microstructures were characterized by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). In addition, the surface roughness, microhardness, residual stress, impact toughness and mechanical properties after LSP were measured. The results showed that high-density dislocations were generated after multi-pass LSP and the dislocation motion dominated the deformation process. The intensity of texture decreased while low-angle grain boundaries increased, and the grain refinement was not achieved after LSP. The residual stress transformed from tensile stress (+20 MPa) to compressive stress (−556 MPa) and the microhardness was 24.8% higher than the unpeened sample after LSP. Based on the linear superposition rule, a strengthening model was established to predict the contribution of dislocation and grain boundary strengthening on microhardness. Additionally, the fatigue life of peened samples increased by 165% compared with the unpeened ones. The improvement of high-cycle fatigue (HCF) properties was attributed to the rewarding compressive residual stress and microstructural evolution.