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

Abstract

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.

Crack initiation and propagation areas of SUJ2 specimen with hydrogen after a tensile test at a strain rate of 2×10⁻⁶s⁻¹ at 30°C. (a) Image of crack initiation on the fracture surface observed by SEM. (b) IPF map of the flat facet at the crack initiation site on the fracture surface analyzed by EBSD. The flat facet is on the {112} plane of martensite which is the slip plane in the bcc lattice. (c) IPF map of a cross section near the flat facet at the crack initiation site analyzed by EBSD. The flat facet is on the {112} plane of martensite, which is the slip plane in the bcc lattice. (d) Phase map of a cross section near the flat facet at the crack initiation site analyzed by EBSD. The phase of the flat facet and surrounding area is not retained austenite but martensite. (Online version in color.)