Synergistic effects of Ta and Mo on the hydrogen embrittlement resistance in ultra-high strength hot stamping steel

Hydrogen embrittlement (HE) is a major problem that restricts the application of ultra-high strength hot stamping steels. This paper proposes a novel strategy for against HE via Ta–Mo multi-microalloying, and systematically analyzes the synergistic effects of Ta and Mo. The results show that a major fraction of Ta and a small fraction of Mo generated high-density semicoherent nanosized (Ta, Mo) C precipitates and further refined the prior-austenite-grain/martensite structure. Ta–Mo multi-microalloying resulted in significantly higher precipitate and grain boundary (GB)-induced H trap densities and a higher H trapping capacity of the precipitates than Ta alloying, thereby reducing the H diffusivity. Additionally, Ta–Mo alloying significantly increased the HE resistance through the following mechanisms: (i) Ta–Mo hindered H enrichment at the GBs by providing numerous additional H traps and increasing the GB cohesive strength through Mo segregation, inhibiting hydrogen-enhanced decohesion (HEDE); and (ii) Ta reduced the proportion of Σ3 boundaries, Mo increased the binding force of Σ3 boundaries, and (Ta, Mo) C precipitates hindered the H–dislocation interactions, suppressing hydrogen-enhanced localized plasticity (HELP). In summary, Ta–Mo multi-microalloying combines the advantages of Ta and Mo and synergistically hinders the HELP and HEDE, thereby significantly increasing the HE resistance.