Hydrogen Embrittlement and Fracture Mechanisms in Laser Beam and TIG Welded Joints of S960MC Ultra-High-Strength Steel

The application of ultra-high-strength steel (UHSS) S960MC in safety-critical sectors is significantly limited by its susceptibility to hydrogen embrittlement (HE). Since welding operations fundamentally alter the optimized microstructure of the base material, understanding the response of welded joints to hydrogen is crucial. This study presents a comparative analysis of S960MC joints fabricated by Tungsten Inert Gas (TIG) and Laser Beam Welding (LBW) to examine how distinct welding thermal cycles influence susceptibility to degradation. Utilizing Slow Strain Rate Testing (SSRT) on electrochemically charged specimens, the research aims to identify the dominant failure pathways. The results demonstrate that TIG welding, characterized by high heat input, produces a softened Heat-Affected Zone (HAZ) that is highly susceptible to Hydrogen-Enhanced Localized Plasticity (HELP). In contrast, the rapid cooling in LBW promotes the formation of a hard martensitic Fusion Zone (FZ), leading to premature and catastrophic brittle failure driven by Hydrogen-Enhanced Decohesion (HEDE) at stresses well below the macroscopic yield strength. Detailed fractographic analysis confirms the synergistic interplay between HELP and HEDE mechanisms, providing critical insights into the structural integrity of UHSS weldments in hydrogen-rich environments.