Hydrogen Embrittlement Behavior of Ultra‐High Strength 31CrMoNiNbV Secondary Hardening Steel Under Different Hydrogen Charging Conditions

Hydrogen embrittlement susceptibility of a newly developed 31CrMoNiNbV steel for grade 16.9 ultra‐high‐strength fasteners is investigated through slow strain rate tensile testing under hydrogen pre‐charging and in situ immersion charging conditions. The steel exhibits exceptional mechanical properties (ultimate tensile strength of 1644 ± 5 MPa, uniform elongation of 5.1 ± 0.1%) attributed to hierarchical carbides induced secondary hardening. Under pre‐charging with 19.2 ppm hydrogen, uniform elongation decreases by 35.3%, while in situ charging by immersion in Walpole buffer solution causes a more severe elongation loss of 70.6% despite only 4.3 ppm hydrogen uptake, demonstrating the critical role of continuous hydrogen uptake. Correlative fractographic and microstructural analysis reveals that the fracture mechanism transitions from ductile (uncharged) to hydrogen‐enhanced localized plasticity‐dominated (pre‐charged condition) and hydrogen‐enhanced decohesion‐domintated (in situ immersion charged condition). Hydrogen permeation measurements further reveal that the material has a low apparent hydrogen diffusivity of 12.17 × 10 −9 cm 2 /s and the hierarchical carbides provide an irreversible hydrogen trapping density of 0.81 × 10 22 cm −3 ; however, these trapping sites could not sufficiently prevent environmental hydrogen degradation.