Welding-Induced Heterogeneity Promotes Gradient Nanostructuring in Laser-Welded 304 Stainless Steel Joints

Laser-welded stainless steel joints usually suffer from strain localization and premature failure in the weld metal (WM) due to microstructural heterogeneity introduced during welding. In this work, surface mechanical rolling treatment (SMRT) was applied to laser-welded 304 stainless steel plates to enhance the mechanical performance of the welded joints. Laser welding introduced multiple heterogeneous features in the WM, including local Ni compositional fluctuations, nanoscale oxide particles and heterogeneous grain structures. Among them, the local fluctuation of Ni concentration is considered to play a dominant role by locally modifying the stability of γ-austenite and promoting strain-induced martensitic transformation during SMRT. As a result, the WM exhibited more severe grain refinement and a stronger gradient nanostructure than base metal (BM) under identical processing conditions. The near-surface hardness of the WM reached ~500 Hv, which was noticeably higher than that of the BM. Uniaxial tensile tests revealed that the yield strength increased from ~350 MPa to ~700 MPa, while the ultimate tensile strength reached ~1000 MPa with ~40% elongation. More importantly, the fracture location shifted from the WM to the BM after SMRT. The enhanced martensitic transformation and gradient nanostructure effectively suppressed strain localization and improved the mechanical reliability of the welded joint.