PSI - Issue 13

Galina Maier et al. / Procedia Structural Integrity 13 (2018) 1053–1058 Galina G. Maier et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig.3. SEM micrographs of the fracture surfaces in 0V-HNS (a-c) and 1.5V-HNS (d-f): a, d – H-free specimens, b, c, e, f – H-charged ones. The higher magnification of areas circled on images (b, e) are presented on micrographs (c) and (f) respectively.

Differences in H-effect on surface embrittlement of two high-nitrogen steels can be explained taking into account their different hydrogen diffusion and trapping characteristics. Typical trapping sites in materials are dislocations, vacancies, grain boundaries, phase boundaries and precipitates (Olden, 2008). Trapping reduces the amount of mobile hydrogen and delay the hydrogen transport into specimen interior. Smaller grain size provides higher volume fraction of high-angle grain boundaries in 1.5V-HNS compared to coarse-grained 0V-HNS. According to results obtained by Cheng et al. (2018) on tempered martensite steels with different content of V (0; 0.039;0.096 wt.%), the addition of V (0.096 %) also leads to decrease of diffusion coefficient of hydrogen almost 10 times due to vanadium carbides acted as irreversible traps in steel. The “crumbling” of t he (V,Cr)(N,C) particles in H-embrittled surface layers of the 1.5V-HNS testify to the fact that “particle/austenitic matrix” interfaces accumulate hydrogen intensively, e.g., they are trapping cites for hydrogen atoms concurrent for grain boundaries. The H-assisted brittle layer is shallower in case of 1.5V-HNS in comparison with 0V-HNS. The ratio of diffusivities of hydrogen, D , in two high-nitrogen steels can be estimated according to equation (Wang et al., 2014): = 2√ , where t is the H-charging duration. Taking the thicknesses of surface layer estimated using SEM observations, the ratio of diffusivity for V-free and V-containing steels is D 0V / D 1.5V =7.3. This means that hydrogen diffusion is 7-times faster for V-free coarse-grained austenitic steel than that for fine-grained particle-strengthened 1.5V-HNS. Assuming the diffusivity of hydrogen atoms in austenite of both steels are close to each other, because of similar interstitials contents in solid solution after solution-treatment, the differences in D-values are related to higher fraction of grain boundaries and “particle/austenitic matrix” interfaces in 1.5V-HNS. Therefore, V-alloying of high-nitrogen steels provides additional trapping effect.