Enhanced strength-ductility synergy and hydrogen trap of 316L welds by double-pass MIG applied for liquid hydrogen storage tanks

A systematic investigation was conducted to assess the influence of double-pass MIG, triple-pass MIG, and MIG-SAW hybrid welding processes on the microstructure, cryogenic mechanical properties, and hydrogen embrittlement resistance of 316L liquid hydrogen storage tank weldments. Results demonstrate that the low-heat-input double-pass MIG (2-MIG) weld exhibits the most refined microstructure with the smallest secondary dendrite arm spacing (4.58 μm) and average carbide size (0.79 μm). Additionally, 2-MIG achieves the highest fraction of random high-angle grain boundaries (79.7%) and special grain boundaries (13.66%, including 10.1% Σ3 twins). The improved cryogenic strength-ductility balance is achieved through a coupled microstructural architecture. Low-heat-input double-pass MIG refines the solidification microstructure and promotes submicron spherical M 23 C 6 carbides, providing precipitation strengthening and more uniform strain partitioning. Meanwhile, the grain boundary character distribution (high HAGB fraction with a notable Σ3 special boundary fraction) suppresses crack propagation by reducing crack connectivity, enabling high strength without loss of ductility. Hydrogen embrittlement sensitivity assessment shows that after 48 h of hydrogen charging, thermal desorption analysis (TDS) detects a total hydrogen content of 3.698 mass ppm, with the desorption curve dominated by a high-temperature main peak characteristic of strong carbide traps. The hydrogen embrittlement indices REL and RRA of the dual-pass MIG joint are 89% and 82%, respectively, indicating only slight hydrogen embrittlement. These findings underscore that a low-heat-input, few-pass MIG strategy effectively tailors precipitate morphology and grain boundary character, providing a viable route toward superior cryogenic toughness and hydrogen resistance in liquid hydrogen weldments.