Improving Fatigue Properties of 316L Stainless Steel Welded Joints by Surface Spinning Strengthening

The surface spinning strengthening (3S) mechanism and fatigue life extension mechanism of 316L stainless steel welded joint were systematically elucidated by microstructural analyses and mechanical tests. Results indicate that surface gradient hardening layer of approximately 1 mm is formed in the base material through grain fragmentation and deformation twin strengthening, as well as in the welding zone composed of deformed δ-phases and nanotwins. The fatigue strength of welded joint after 3S significantly rises by 32% (from 190 to 250 MPa), which is attributed to the effective elimination of surface geometric defects, discrete refinement of δ-Fe phases and the appropriate improvement in the surface strength, collectively mitigating strain localization and surface fatigue damage within the gradient strengthening layer. The redistributed fine δ-Fe phases benefited by strong stress transfer of 3S reduce the risk of surface weak phase cracking, causing the fatigue fracture to transition from microstructure defects to crystal defects dominated by slip, further suppressing the initiation and early propagation of fatigue cracks.