Plastic deformations of 42CrMo4 under LSPwC and subsequent cyclic stresses

42CrMo4 steel used as connecting rods is generally encountered with fatigue damage during service. Enhancing its fatigue behavior is therefore required to ensure the structural safety. In this work, the high cycle fatigue performances of 42CrMo4 steel subjected to laser shock peening without coating (LSPwC) are studied, and its plastic deformation mechanisms are revealed during the LSPwC processes and fatigue tests. The results show that 50.62% increase in fatigue lives of 42CrMo4 steel are achieved after 5 LSPwC cycles. The LSPwC triggers a hierarchical gradient structure, including a top-most thermal layer of ∼100 μm, an intermediate severe plastic deformation (SPD) layer of ∼200 μm and an inner substrate. They are formed as a result of competitive effects between the mechanical strain and thermal circulations at different depth. Under these incentives, some transformations occur at carbides and martensitic lath. The original cementite clusters are mechanical split into short-rod counterparts in the SPD layers, while turning into long-lamellar ones in the thermal layer. Compared with others, the long-lamellar cementite has stronger competence to establish intricate dislocation configurations to refine the martensitic lath, due to larger accommodation to collect mobile dislocation. However, thermal circulation raises the range of lath width. During cyclic deformation, LSPwC-induced compressive residual stress mitigates lath coarsening at SPD and thermal layers. As cycle proceeds, plastic deformation energy focuses on the broad decompositions of carbide debris predominantly, which further leads to a turbulence of lath boundaries (LBs) and subsequent LB network collapse. Instead, it contributes to defect initiations associated with carbide cluster and causes premature failure of substrate. Therefore, the fatigue properties 42CrMo4 steel are greatly ameliorated. Finally, the strength contributions of the gradient materials are estimated by applying a modified gradient strengthening strategy coupled with Williamson-Hall approach, proving that dislocation devotes most to the enhanced mechanical performance and SPD layer possesses higher strengthening contribution as compared to thermal layer.