Impact of hydrogen embrittlement on the tensile-shear property of resistance spot-welded advanced high-strength martensitic steels

Martensitic steels, engineered to meet automotive weight reduction and crashworthiness requirements, are highly susceptible to hydrogen embrittlement (HE). Despite extensive research to improve HE resistance, these steels remain vulnerable to HE after undergoing resistance spot welding (RSW). HE in spot-welded steels has been largely unexplored, and its underlying mechanism remains unclear. This study investigates the HE mechanism in RSWed steel sheets by examining microstructural alterations and hydrogen trapping sites before and after RSW. Additionally, the study analyzes variations in tensile-shear test (TST) properties relative to increasing hydrogen content. In spot-welded steels, the tensile-shear strength and fracture displacement decline as diffusible hydrogen levels increase. Specifically, fracture displacement sharply declines within the first 3 h of hydrogen charging, but this loss rate significantly decreases thereafter. This slope shift is attributed to a transition in the crack propagation path. Initially, cracks propagate along the fusion zone line, featuring transgranular failure. However, beyond a critical hydrogen content, crack propagation shifts to the prior austenite grain boundary (PAGB) in the upper critical heat-affected zone (UCHAZ), exhibiting intergranular failure. Consequently, when hydrogen content surpasses this critical threshold, the PAGB in the UCHAZ is weakened due to hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). Cracks then propagate along the PAGB in the direction of maximum stress during the tensile-shear test (TST), leading to a change in the crack propagation path.