PSI - Issue 36

Ihor Dmytrakh et al. / Procedia Structural Integrity 36 (2022) 298–305 Ihor Dmytrakh et al./ Structural Integrity Procedia 00 (2021) 000 – 000

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So, we have shown the formal reducing the value of the plasticity limit as the engineering macroscopic characteristic of the pipeline steel. We understand that this is disputable to define this phenomenon as the “enhanced plasticity” in the classical meani ng of this term. In our opinion, the phenomenon observed may be named as the effect of “enhanced deformability” of steel due to the specific action of the diffusible hydrogen of low concentration, which increases the material’s defectiveness.

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Fig. 6. The SEM images of the microstructure of the tested specimens ( σ = σ f ) after the different number of cycles of hydrogen charging discharging N H : a – 0; b – 2; c – 5 ( × 500). The area of observation was equal to 41300 μm 2 .

In the end, it should be noted as follows. For a possible preliminary explanation of our results, we may involve the suggestion that the presence of hydrogen can enhance the density and clustering of vacancies as it is discussed by Nagumo (2004), McLellan et al. (1997) and Sakaki et al. (2006). Here, it was shown that the vacancies can coalesce to microvoids, which in turn may combine to form larger void-like defects leading to a decrease of fracture resistance of the material. In combination with such a well-known phenomenon as hydrogen-induced stress (see, for example the works of Barnoush et al. (2010) and Wang et al. (2019), this may be some basis for a qualitative assessment of the above-presented data. The aim of the present work was only the description of the received unobvious experimental observations. However, we understand that for a detailed explanation of these phenomena and the mechanisms of their realization, further special multidisciplinary studies are required. 4. Conclusions The fact of the formal reduction of the yield stress of low-alloyed pipeline steel under the presence of the diffusible hydrogen of low concentration has been found. It was shown that hydrogen charging and discharging of the steel does not pass without a trace concerning its microstructure. Even a short-term presence of the diffusible