Effect of Advanced High-Strength Steel Microstructure on Hydrogen Embrittlement Behavior of Resistance Spot Welded Joints

To achieve automotive lightweighting and passenger safety, advanced high-strength steels (AHSS) have been developed with increasing strength levels. However, hydrogen embrittlement (HE) becomes more critical with increasing strength. Despite efforts to improve HE resistance through microstructural control, these steels remain vulnerable after resistance spot welding (RSW), where complex microstructures and defects are introduced. In this study, the effect of initial steel microstructure on HE sensitivity in RSWed steels was investigated using two 1.5 GPa-grade steels with identical compositions but different microstructures: complex-phase (CP) and single-phase (SP) steels. HE resistance of both base metal and welded joints was evaluated using slow strain rate tensile and slow deformation rate tensile-shear tests after electrochemical hydrogen charging. The SP steel, consisting of tempered martensite, exhibited higher HE sensitivity than the CP steel composed of tempered martensite and bainite in both base metal and welded conditions. With increasing hydrogen charging time, the crack propagation path transitioned from the fusion zone (FZ) boundary to the upper-critical heat-affected zone (UCHAZ) interior with intergranular cracking, consistent with the hydrogen-enhanced decohesion (HEDE) mechanism. In the SP steel, an additional crack propagation mode was observed at intermediate charging conditions, where cracks propagated across the FZ before transitioning to the UCHAZ, resulting in greater ductility loss. At higher charging conditions, both steels exhibited similar intergranular cracking behavior. These results suggest that the HE behavior of RSWed steels is associated with the microstructural characteristics of the UCHAZ, while differences in the initial steel conditions may also contribute to the observed HE sensitivity.