Synergistic Improvement of Fatigue Performance in Laser Powder Bed Fusion 316L Steel Through Heat Treatment and Low-Temperature Gaseous Carburizing

To improve the fatigue performance of laser powder bed fusion (L-PBF) 316L austenitic stainless steel, a stress-relief heat treatment (HT) followed by low-temperature gaseous carburizing (LTGC) was applied. The results show that the combined treatment effectively releases overall residual stress in the built to below 50 MPa and simultaneously introduces beneficial compressive residual stresses in the surface region of −2.8 GPa. Additionally, the formed case reduces the sensitivity for surface-adjacent defects. After treatment, the endurance limit increases from 215 to 340 MPa (~58%). The enhancement relies on the following mechanisms: (1) The mean tensile residual stress in the built is reduced after HT; (2) LTGC-induced compressive residual stress effectively suppresses crack initiation at surface defects and shifts the initiation site from the surface to subsurface porosity. Based on these findings, a fatigue life prediction model is proposed that incorporates the effects of defect characteristics and residual stress.