Hydrogen content dependence of crack initiation and propagation behavior of hydrogen embrittlement in tempered martensitic steel

Crack initiation and propagation behavior in hydrogen embrittlement fracture of tempered martensitic steel at a low hydrogen content was compared with the results at a high hydrogen content. Notched specimens charged with a low hydrogen content of 0.18 ppm and a high hydrogen content of 5.3 ppm were stressed and unloaded immediately upon reaching the maximum stress in tensile tests. At the low hydrogen content, quasi-cleavage (QC) fracture was dominant at the notch tip, and mixed intergranular (IG) and QC fractures were observed away from the notch tip. A crack initiated in the prior γ grains at the notch tip and propagated along the {011} plane. The crack initiation site corresponded to the site of maximum equivalent plastic strain. The other crack initiating on the prior γ grain boundaries was observed at a site away from the notch tip. Microvoids were formed discontinuously inclined at about 45° to the tensile axis direction between these two types of cracks observed at the low hydrogen content. In contrast, at the high hydrogen content, cracks initiated on the prior γ grain boundaries away from the notch tip. The crack initiation site corresponded to the vicinity of the region where both the principal stress and hydrogen concentration were high. These findings indicate that crack initiation at the low hydrogen content is not necessarily consistent with the site of the maximum principal stress and the local hydrogen concentration, unlike the case of the high hydrogen content.