PSI - Issue 28

Myroslava Hredil et al. / Procedia Structural Integrity 28 (2020) 1204–1211 4 Myroslava Hredil , Halyna Krechkovska, Oleksandra Student, Oleksandr Tsyrulnyk / Structural Integrity Procedia 00 (2019) 000–000

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3.2. Structural peculiarities of degradation in pipeline steels Structural features which could be associated with such essential change in steel’s resistance to brittle fracture, were analyzed in detail for the steel 17H1S. A pronounced texture consisted of the strips of ferrite and pearlitic grains was revealed in the structure of this steel in the as-received state and after its 30-year operation. The texture was found both in the axial (Fig. 3a, c) and radial (Fig. 3b, d) pipe cross-sections. Grains of perlite and ferrite generally retained their polygonal form. Grain size was irregular, which was especially noticeable in the pipe radial direction. In both steel states (as-received and long-term operated), thinner strips of perlite (up to 12 μm) were interspersed with wider ferrite bands (up to 31 μm). Perlite strips in the axial section of the steel pipe in the initial state had a length up to 2 mm and were almost continuous (Fig. 3a). Besides, long chains (100–200 μm) of thin non-metallic inclusions (up to 1 μm), such as manganese sulfides, were observed in this plane, mostly located in ferrite layers, somewhere crossing several grains. In contrast, the continuity of perlite strips in the radial direction of pipes was often interrupted by separate ferrite grains. Therefore, a length of continuous perlite strips did not exceed 150 μm in this pipe cross-section (Fig. 3b).

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c d Fig. 3. Microstructure of the steel 17H1S in the initial state (a, b) and after 30 years of operation on the main pipeline (c, d) in the axial (a, c) and radial (b, d) cross-sections of the pipes at a distance of 3 mm from their external surface. Two peculiarities of degradation of 17H1S steel at the microstructural level were identified (Fig. 3c, d). Some grain boundaries separating adjacent bands of perlite and ferrite are insufficiently distinct after etching, obviously, as a result of their scattering, whereas other grain boundaries became remarkably clear, which indicated their especial susceptibility to etching. Besides, enhanced etching was more pronounced in the pipe axial section (Fig. 3c), whereas in its radial section, the grain boundaries between ferrite and pearlite were etched selectively and did not spread on the entire length of continuous strips of texture (Fig. 3d). The nonuniformity in etching observed in the operated steel indicates a different degree of its damaging occurred during long term operation under service loads and steel hydrogenation which is difficult to avoid. Analyzing the mechanisms of corrosion taking place inside gas pipes, Hredil and Tsyrulnyk (2010) proved that hydrogen evolution can take place in the gas mains during their long-term operation. Earlier, Tau and Chan (1996) have measured the hydrogen content in emergency gas pipelines. It reached 0.045– 0.06 at. % close to the failure zones, significantly exceeding the average hydrogen content (0.015 at. %) in the as received pipes. Somewhat less hydrogen content (up to 0.032 at. %) was also detected in other emergency pipes operated on gas mains for 20–34 years. Besides, Lee et al. (1991) calculated that the hydrogen diffusion coefficient is