Adverse effects of Ni on the mechanical and corrosion-induced hydrogen embrittlement properties of ultra-strong giga steel used for automotive applications

Previous metallurgical studies have reported the beneficial effects of Ni addition to steels. In this study, however, the mechanical and corrosion-induced hydrogen embrittlement (CIHE) properties of medium C-based ultra-strong steel degraded with increasing Ni content. Steel samples with a higher Ni content had a higher fraction of undissolved cementite (Fe3C) in the martensite matrix, leading to a smaller prior-γ grain size and lattice parameter, and a lower tensile strength. Even at a higher austenitizing temperature, no significant change in the tensile strength of the sample with a higher Ni content was observed. This was attributed to the additional supply of C atoms into the lattice by the further dissolution of Fe3C. The tensile test at a slow strain rate did not reveal a proportional relationship between the Ni content and the CIHE indices, but an inflection point of the Ni quantity in the steel was noted. Based on the results, effective metallurgical strategies with respect to the optimal Ni contents and austenitizing conditions for the steel are proposed.