PSI - Issue 17

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 17 (2019) 568–575 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000

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applying strain of 10%. Consequently, the deformation aging procedure, even for a high level of applied plastic deformation, did not correctly simulate in-service degradation of the pipeline steel, therefore its usage for this purpose is limited. Applying the proposed procedure of accelerated degradation at strain of 5%, markedly reduced changes in the ratio of σ Y /σ UTS and elongation were observed than using deformation aging procedure (Fig. 2, 3a). At the same time a decrease of another characteristic of plasticity, namely reduction in area, and impact toughness as well, was significant. That is, applying the procedure of accelerated degradation with strain of 5%, the changes in the mechanical properties of the metal were similar to that of the operating metal.

Fig. 4. Changes in impact toughness KCV for the Х52 steel in the different states relative to the as-received state: after deformation aging with applied strain of 5% (1) and 10% (2), after 30 years of operation (3), after in-laboratory accelerated degradation with applied strain of 5% (4) and 10% (5). Consequently, the in-laboratory accelerated degradation procedure simulates process of in-service degradation of metal more correctly than the deformation aging procedure. First of all, this is due to the specific action of hydrogen during loading, which, in addition to intensifying deformation aging (the first stage of in-service degradation – decreasing in the relative reduction in area and elongation, impact toughn ess and increasing in the ratio of σ Y /σ UTS ), initiates also development of dissipated damaging (second stage of in-service degradation – possible increasing in relative elongation, decreasing in the ratio of σ Y /σ UTS and decreasing in impact toughness). The developed procedure of in-laboratory accelerated degradation of pipeline steels enables, on a laboratory scale, simulating of the degradation of the pipeline steel during long-term operation under simultaneous action of hydrogenation and working loading, and it makes possible to predict the mechanical behavior of pipeline steels during operation. A key role of hydrogen in degradation of gas pipeline steels was demonstrated. The procedure of in-laboratory accelerated degradation of pipeline steels consisted in consistently subjecting of the specimens to electrolytic hydrogen charging, to an axial loading up and to an artificial aging, was developed. The results of comparative study of the mechanical behavior of pipeline steels in different states, namely as-received, post-operated, aged and after in-laboratory degradation, revealed that the developed procedure of in-laboratory accelerated degradation caused the changes in the mechanical properties of the metal at the same level compared to the properties degradation due to long-term operation. The developed procedure of in-laboratory accelerated degradation of pipeline steels enables, on a laboratory scale, simulating of the degradation of the pipeline steel during long-term operation under simultaneous action of hydrogenation and working loading, and it makes possible to predict the mechanical behavior of pipeline steels during operation. 5. Summary