Influence of gaseous hydrogen on the fatigue and fracture behaviour of martensitic and martensitic-austenitic ultrahigh-strength steels

Advanced high-strength steels are generally susceptible to hydrogen embrittlement (HE). In this paper, we demonstrate that microstructural tailoring via introducing fine-scale retained austenite (RA) in otherwise martensitic structure can decelerate the diffusion of hydrogen in steel, moderate the contribution by hydrogen to fatigue life, and modify the fracture mechanism. The studied steel grades, martensitic DQ1600 and martensitic-austenitic QP1900, represent ultrahigh-strength steels with equal mechanical properties and dissimilar microstructures. Electrochemical permeation tests revealed the presence of RA facilitated hydrogen trapping. Load-controlled low-cycle fatigue tests in 10 MPa pressurized hydrogen showed HE in both steel grades, yet the influence of hydrogen on the fatigue life was significantly greater for DQ1600 than for QP1900. H-induced fracture was dominated by hydrogen-enhanced decohesion (HEDE) mechanism, and its contribution was specific to each steel grade; in both cases also hydrogen-enhanced local plasticity (HELP) mechanism was involved. In DQ1600, fracture followed martensite packet and block boundaries, while in QP1900 failure occurred along RA-enriched lath and sub-block boundaries. These unique results provide new directions for future steel development towards improved resistance against HE under dynamic loading conditions.