Investigation on hydrogen embrittlement susceptibility in martensitic steels with 1000 MPa yield strength

Abstract

Two quenched and tempered cost-saving marine steels with 1000 MPa yield strength were charged with hydrogen by the electrochemical technique to evaluate the hydrogen embrittlement (HE) behavior. The HE susceptibility index (HEI) obtained by slow strain rate tensile tests (SSRT) indicated that the HE resistance of steel A was weaker than that of steel B. Aided by fracture surface analyses, it could be determined that HE was a combination of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) model. The microstructural characteristics were examined via transmission electron microscopy (TEM) and scanning electron microscope with electron backscattered diffraction analysis (SEM-EBSD). Thermal desorption spectroscopy (TDS) was applied to explore the hydrogen trapping behavior. It was shown that compared to steel A with fine microstructure and low Σ3 fraction, steel B had excellent HE resistance, which could result from the presence of more nano-sized NbC particles. The precipitation of NbC could not only act as an irreversible hydrogen trap to inhibit the hydrogen diffusion, suppressing the HEDE mechanism, but also can restrict hydrogen–dislocation interaction and suppress the HELP mechanism. The role of the interface between carbide and matrix on hydrogen trapping was investigated by density functional theory (DFT) and showed that the hydrogen capture force of NbC was stronger than (Nb,V)C. In addition, with the coarsening of microstructure during tempering at 580 °C, the high-angle grain boundaries (HAGBs) was obviously reduced, led to the HE susceptibility increased, especially steel A.