PSI - Issue 36

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 36 (2022) 306–312 Hryhoriy Nykyforchyn, Leonid Unigovskyi, Olha Zvirko et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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The obtained results indicated a noticeable sensitivity of the operated metal to hydrogen action whereas there was no effect of preliminary hydrogenation on the plasticity of the as-received steel (Table 2). Reduction in area was revealed to be more sensitive characteristic for the assessment of hydrogen embrittlement of metal than elongation, which is consistent with general regularities summarized by Nykyforchyn (2021). Microfractographic features of fracture of tensile specimens are illustrated in Figs. 3 and 4 for the central and outlying zones of the fracture surfaces respectively.

Fig. 3. Fracture peculiarities in the central zone of the fracture surfaces of specimens made of the as-received (a, c) and operated (b, d) steels without (a, b) and after (c, d) hydrogenation.

The as-received state of the steel without hydrogenation is characterized by ductile fracture due to microvoid coalescence (Fig. 3a); long-term operation resulted in enhancing the role of shear processes in the formation of the microvoid relief (Fig. 3b). After preliminary hydrogenation, smooth surfaces of small delaminations and parabolic dimples around fine carbides were found on the specimens in the as-received state (Fig. 3c), whereas the areas of cleavage are clearly detectable on the fracture surfaces of the operated steel (Fig. 3d). Similar effects of hydrogenation on the fracture mechanism were obtained by Merson et al. (2020) and Okada et al. (2018). Regarding the features of micro fracture in the outlying zones, a typical ductile fracture was observed in all cases, regardless of the steel state and hydrogenating conditions, with the formation of parabolic dimples which alternated with narrow bends of dimples formed by detachment (Fig. 4). However, carbides or non-metallic inclusions were detected at the bottom of most dimples on the fracture surfaces of non-hydrogenated specimens (Fig. 4a, b) and, in the case of hydrogenated specimens, special dimples with unclear traces of tearing in their centre were revealed (Fig. 4 c, d). It was suggested that these dimples may have formed due to the formation of cavities (such as small blisters) in the hydrogenated metal, in which hydrogen accumulated in the subsurface layers of the specimens during their electrolytic hydrogen charging and promoted the visualization of these cavities on the fracture surface.