Hydrogen Embrittlement and Local Characterization at Crack Initiation Associated with Phase Transformation of High-strength Steel Containing Retained Austenite

摘要

States of hydrogen present in high-strength steels for use as bearing steel SUJ2 and hydrogen embrittlement susceptibility were examined using thermal desorption analysis (TDA) and tensile tests. SUJ2 specimens containing the retained austenite phase (γ _(R)) in the martensitic phase exhibited three hydrogen desorption peaks in the TDA profile. Two of the peaks desorbed at higher temperatures decreased with a decreasing amount of γ _(R), indicating they corresponded to desorption associated with γ _(R). Fracture strength in the presence of hydrogen increased with a decreasing amount of γ _(R) and with an increasing strain rate. For the specimens containing γ _(R) and hydrogen, a flat facet at the crack initiation site and a quasi-cleavage (QC) fracture in the initial crack propagation area were observed on the fracture surface. Local characterization using electron back-scattered diffraction (EBSD) revealed that the flat facet on the fracture surface corresponded not to γ _(R) but to strain-induced martensite. In addition, the facet was on the {112} plane of martensite, which is the slip plane or deformation twin plane of body-centered-cubic metals. The reason for high hydrogen embrittlement susceptibility of the specimens containing γ _(R) was attributed to the strain-induced phase transformation at the crack initiation site of the flat facet and in the initial crack propagation area of the QC fracture. Furthermore, the strain rate dependency of hydrogen embrittlement susceptibility is presumably ascribable to local plastic deformation, i.e. , the interaction between dislocations and hydrogen.