Effects of microstructural heterogeneity on hydrogen embrittlement susceptibility and hydrogen-induced cracking behavior in the heat-affected zone of Q1100 ultra-high-strength steel

Hydrogen-induced cracking (HIC) in the welded heat-affected zone (HAZ) severely restricts the application of ultrahigh-strength steels (UHSS). Microstructural heterogeneity in the HAZ causes different hydrogen embrittlement (HE) susceptibility among its subregions, but the mechanism remains unclear. In this study, the HE susceptibility in the intercritical HAZ (ICHAZ), fine-grained HAZ (FGHAZ) and coarse-grained HAZ (CGHAZ) of Q1100 UHSS were investigated. After hydrogen charging, the elongation loss indices increased in the order of FGHAZ (40.9%), ICHAZ (70.2%) and CGHAZ (85.2%). The FGHAZ showed the lowest hydrogen diffusion coefficient ( D eff ) due to fine lath martensite (LM) with dispersed hydrogen trapping sites. Additionally, its higher Schmid factor (SF) of 0.486 and lower fraction (7.3%) of Σ3 boundaries reduced the tendency for HIC propagation, resulting in the lowest HE susceptibility. The strain incompatibility between hard fresh martensite (FM) and soft tempered martensite (TM) in the ICHAZ promoted HIC initiation within FM and along FM/TM interfaces. The enrichment of Σ3, Σ11 and Σ25b boundaries within FM, together with the SF mismatch between FM and TM, further intensified local cracking. This caused the ICHAZ to have higher HE susceptibility than the FGHAZ. The CGHAZ showed the highest D eff , allowing hydrogen to migrate more rapidly to prior austenite grain boundaries and coarse martensitic packet/block boundaries. The high fraction (33.5%) of Σ3 boundaries and the low SF (0.462) promoted continuous crystallographic crack propagation, leading to the highest HE susceptibility. These findings provide insights into how HAZ microstructural heterogeneity influences the HIC behavior of Q1100 UHSS.