Effects of carbon and nitrogen on hydrogen embrittlement of 15Cr-15Mn-4Ni based austenitic stainless steels

Abstract

Although both carbon and nitrogen are interstitial elements, they can have dissimilar effects on hydrogen behavior inside steels. In order to investigate the effects of carbon and nitrogen on hydrogen embrittlement of ASSs, 15Cr-15Mn-4Ni based ASSs were designed with varying contents of carbon and nitrogen. A C series with 0.1-0.3 weight percent (wt.%) C, a N series with 0.1-0.3 wt.% N, and a C-N alloy with 0.2 wt.% of both C and N were prepared. Hydrogen was electro-chemically charged inside the samples, and slow-strain rate tensile testing (SSRT) was adapted for the evaluation of mechanical properties. Electro-chemical permeation tests were also conducted to evaluate the effective hydrogen diffusivity in the steels.

The hydrogen-induced degradation of tensile properties was alleviated by introducing nitrogen. The increase of nitrogen content gradually decreased the diffusivity of hydrogen inside the steels. Thus, the total depths of hydrogen penetration and the hydrogen-affected zones became shallow. Grain boundaries were weakened by both hydrogen and nitrogen. Nitrogen also induced stress concentration at grain boundaries from planar slip development. Therefore, intergranular cracks were formed easily at low strain levels. With higher strain, the cracks propagated further into the grains along martensite/austenite interfaces especially in steels with less nitrogen. Although nitrogen induced active intergranular cracking, the decrease of hydrogen diffusivity and the increase of austenite stability enhanced the resistance to hydrogen embrittlement.

Carbon also decreased the degree of hydrogen-induced degradation of tensile properties, but by a different mechanism. Carbon in the range of 0.1-0.3 wt.% had a negligible effect on hydrogen diffusivity. This induced similar depths of hydrogen penetration and hydrogen-affected zone. Hydrogen caused cracks to initiate at grain boundaries on the sample surface, and to propagate inside the grains through α’ and ε martensites. The crack propagation was suspended by confronting a grain boundary, and did not reach the depth of hydrogen penetration. The carbon at grain boundaries appears to have delayed the crack initiation, and the carbon in solid solution enhanced austenite stability, which impeded the crack propagation.

When 0.2 wt.% of both carbon and nitrogen was simultaneously introduced, the tensile properties showed low hydrogen-induced degradation. Both carbon and nitrogen affected the crack initiation and propagation in the hydrogen-affected zone. Cracks initiated at the grain boundaries on the sample surface at early stage of deformation but with low density due to the strengthening effect of C, weakening effect of N, and stress concentration at grain boundaries by C+N. The cracks could not propagate actively, and followed two paths. Some cracks went along the grain boundaries and through the grains along deformation twin boundaries with no adjacent martensite, and some followed the paths where ε martensite was concentrated. This formed a mixture of intergranular and transgranular fracture. From the C-N interaction, the hydrogen diffusivity in the steel was in between those of steels with 0.2 wt.% of C or N.

This study suggests that the addition of carbon or/and nitrogen in 15Cr-15Mn-4Ni based ASSs induced different mechanisms of hydrogen embrittlement although the overall resistance to hydrogen embrittlement was enhanced with the addition of interstitial elements. Therefore, it is asserted that understanding the role of each interstitial element is important in designing structural materials for hydrogen environments.