Investigation of microstructure and cryogenic impact toughness of laser-arc hybrid welded austenitic steels with temperature variation

With the advent of the hydrogen economy, ensuring the structural integrity of liquid hydrogen (LH2) storage systems operating at 20 K has become a critical challenge. This study investigates the cryogenic impact toughness and corresponding deformation mechanisms of Laser-Arc Hybrid Welded (LAHW) 308L and 316L austenitic stainless steels. Charpy V-notch impact tests were conducted at 297 K, 77 K, and 20 K to evaluate absorbed impact energy, followed by detailed microstructural characterization using EBSD and thermodynamic calculation of stacking fault energy (SFE). The results demonstrated that the 316L weld metal exhibited superior impact toughness at 20 K compared to 308L, despite a transient toughness inversion observed at 77 K. This divergence is attributed to distinct deformation mechanisms at 20 K, the 308L weld underwent excessive strain-induced martensitic transformation (TRIP), leading to brittle fracture, whereas the 316L weld activated twinning-induced plasticity (TWIP), forming hierarchical nano-twins that effectively retarded crack propagation. Ultimately, these findings confirm that the stable austenite reinforced by deformation twins in 316L provides a superior safety margin for LH2 applications. Specifically, the metallurgical reliability of 316L ensures the integrity of the weld joint, frequently the most vulnerable region, making it a more robust and reliable material choice for constructing large, scale liquid hydrogen storage vessels subjected to unexpected dynamic impacts in cryogenic environments.