Effect of Hydrogen Concentration and Strain Rate on Hydrogen Embrittlement of Ultra-Fine-Grained Low-Carbon Steel

Abstract

During the last few decades, keen attention has been paid to the advanced steels with the ultra-fine-grained (UFG) microstructure manufactured by severe plastic deformation (SPD) techniques. Although these materials often demonstrate prominent mechanical properties, the detrimental environmentally induced effects, such as hydrogen embrittlement (HE), which may appear during their service life, have been just scarcely studied. In particular, the influence of the hydrogen concentration and strain rate, which are among the main factors controlling HE, in general, has not been considered in UFG ferritic steels as yet. Thus, the objective of the present study was to examine the effect of these factors on the mechanical behaviour and fracture mode of the low-alloy steel processed by ECAP in comparison with the conventionally fabricated counterparts. The ECAPed and as-received specimens of the low-alloy steel grade 09G2S were cathodically hydrogen charged at different current densities and then subjected to tensile testing at two different strain rates. The diffusible hydrogen concentration in the specimens before tensile testing was assessed by the hot extraction method. After hydrogen charging both as-received and ECAPed specimens demonstrate HE the extent of which increases with the increasing hydrogen concentration and decreasing strain rate. It is found that the ECAPed steel occludes much higher hydrogen concentration than the as-received one. At the given hydrogen concentration, the ECAPed specimens demonstrate stronger hydrogen-induced ductility loss as well as a fundamentally different fracture mode in comparison to the as-received counterparts.