Synergistic Effect of Conventional Shot Peening and Laser Shock Peening on Fatigue Life Enhancement of 4340 Steel

Extending the fatigue life of 4340 steel components subjected to severe cyclic is a critical scientific and technical challenge. The fatigue behavior of 4340 steel specimens treated by conventional shot peening (CSP), laser shock peening (LSP), and a combined LSP + CSP process was investigated to enhance fatigue resistance by microstructure characterization, microhardness and residual stress measurements, and fatigue tests. The results indicate that CSP alone induces severe gradient plastic deformation leading to increased microhardness but a relatively shallow compressive residual stress layer and deep hardening layer. Conversely, LSP alone generates a deep compressive residual stress layer up to 2000 μm. Remarkably, the combined LSP + CSP treatment synergistically integrates the advantages of both processes with a deep hardening layer and a deep compressive residual stress layer. Under a maximum stress of 1452 MPa, the fatigue life of specimens treated by LSP + CSP was approximately a fivefold increase compared to those treated with either CSP or LSP individually.