Mitigation of hydrogen embrittlement in ultra-high strength lath martensitic steel via Ta microalloying

Highlights: • A novel method for improving HE resistance by Ta-microalloying was proposed. • Ta concurrently increases irreversible and reversible H trap densities–weaken HEDE. • Quantitative analysis shows TaC-induced traps plays dominant role in weakening HEDE. • Ta hinders H-dislocation interaction and improves cracking resistance–impedes HELP. Hydrogen embrittlement (HE) is a key challenge limiting the utilization of ultra-high strength martensitic steel. In this work, we reported a novel method for dramatically improving HE resistance by Ta microalloying, and the significant effect of Ta on the HE susceptibility of lath martensitic steel was elucidated from the perspectives of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). As the Ta content increased, numerous dispersed nano-sized TaC precipitates were generated and the effective areas of martensite/prior austenite grain boundaries also increased, which increased both the irreversible/reversible H trap densities, impeded localized H aggregation at defects and weakened HEDE. A quantitative analysis regarding each type of H trap revealed that, compared with reversible traps provided by microstructural refinement, TaC precipitate-induced irreversible traps exhibited a dominant role in weakening the HEDE process. Additionally, in Ta-bearing steels, the resistance to hydrogen-assisted crack propagation was enhanced through the increased Σ11 boundary, increased low-angle grain boundary fraction and the reduced Σ3 boundary fraction, which combined with the suppressing role of TaC precipitates on H-dislocation interaction, impeded the HELP process. This study provided new, deep insights into the impact of Ta on HE, which has important implications for developing steels with high HE resistance.