PSI - Issue 26

Olha Zvirko et al. / Procedia Structural Integrity 26 (2020) 219–224 Zvirko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 1. Relative changes in mechanical properties of pipeline steels with different strength caused by in-service degradation: (a) 17H1S steel (API 5L X52 strength grade), (b) API 5L X60 steel, and (c) API 5L X70 steel. Thus, fracture toughness of pipeline steels with different strength after operation was deteriorated (Fig. 2). It should be noted that impact toughness of pipeline steels, determined on short transverse specimens, is only decreased during operation (Nykyforchyn et al. (2017b) and Zvirko et al. (2018)). However, there are some difficulties for machining short transverse specimens from pipe due to limited pipe wall thickness, and special geometry of specimens should be used, as it was proposed by Nykyforchyn et al. (2017b).

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Fig. 2. Relative changes in fracture toughness of pipeline steels with different strength caused by in-service degradation: (a) 17H1S steel (API 5L X52 strength grade) after 29-31 years of operation, (b) API 5L X60 and API 5L X70 steels after 21 years of operation. Changes in hardness of the 17H1S steel (API 5L X52 strength grade) after 28-40 years of operation is presented in Fig. 3. Hardness of both external and internal surfaces of pipes after long-term operation was decreased. Analysing the data, it can be concluded that hardness of internal surface of pipe is lower than that of external pipe surface. This can be associated with higher degradation degree of steel near internal surface of pipe due to hydrogen charging as a result of interaction with moisture condensate during operation, as it was shown by Tsyrulnyk et al. (2008). Corrosion resistance of pipeline steels with different strength was also deteriorated due to in-service degradation (Zvirko et al. (2019b)). Corrosion degradation degree of the 17H1S and X60 steels was approximately the same, while it was the highest for the high-strength X70 steel (Fig. 4). In particular, value of corrosion current density of the operated 17H1S and X60 steels was in about 2.1 – 2.2 times higher than that of these steels in the as-received state, and it was in approximately 3.3 times higher for the operated X70 steel than that for the as-received steel. That is, high strength steel, having the highest corrosion resistance in the initial state among the investigated steels, was characterised by the highest degradation degree.

3. Sensitivity of characteristics to in-service degradation of pipeline steels

Summarising, brittle fracture resistance characteristics (impact strength and fracture toughness) are characterized by the highest sensitivity to in-service degradation of pipeline steels among the mechanical properties. Therefore, these