PSI - Issue 36

Halyna Krechkovska et al. / Procedia Structural Integrity 36 (2022) 334–341 Halyna Krechkovska, Volodymyr Kulyk, Volodymyr Vira et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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Ostash 1 et al. (2007, 2013), Krechkovska 4,5 et al. (2017, 2019), Rykavets et al. (2019), Student 2 et al. (2019), Zurnadzhy et al. (2020)). Taking this into account, the impact toughness of 17H1S steel in the initial state and after its long-term operation was estimated in the temperature range from +20 to –60 °C. As in the case of tensile tests, specimens were cut at the outer (I) and inner (II) surfaces of the pipe located in its top and bottom parts. Using obtained impact toughness values the ductile-brittle transition curves of both variants of the 17H1S steel were built (Fig. 3). The relatively low impact toughness of the steel from the stock pipe was noted. Even at a temperature of +20 °C, the KCV values for this steel near the outer and inner surfaces of the pipe were practically the same (0.56 and 0.54 MJ/m 2 , respectively). At sub-zero temperatures the difference between them was also small. In addition, the impact toughness of steel is practically independent of the location of the specimens in the pipe cross-section. Moreover, this tendency persisted regardless of their tests temperature. After operation, the impact toughness of the steel became even lower, in particular, in the top part of the pipe at its inner surface, the KCV value at +20 °C reached 0.4 MJ/m 2 , and at its outer surface it was 0.36 MJ/m 2 . At the same time for steel from the bottom of the pipe, the minimum KCV value for room temperature was 0.34 MJ/m 2 . In any case, the average degradation effect at + 20 °C was ~ 30%. As expected, the effect of steel degradation was manifested by the shift of the ductile-brittle transition curves for steel from different zones of the pipe (both along the thickness of its wall and along the perimeter) towards higher temperatures (Fig. 3). This confirmed the well-known tendency towards a decrease in the temperature of the ductile brittle transition as a result of steels degradation, in particular under the influence of radiation exposure (Orynyak et al. (2015)). According to our estimates, the ductile-brittle transition temperature of the exploited steel has shifted from –40 ° C (typical for steel in the initial state) to – 30 ° C. At the same time, the impact toughness of the exploited steel at the ductile-brittle transition temperature decreased by more than 33%. In winter, such an ambient temperature is quite achievable which increases the risk of further operation of pipelines made of such steel and implies stricter control over the state of the metal during its further operation.

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Fig. 3. Ductile-brittle transition curves of the 17H1S steel in the initial state (white symbols) and after the operation on the main gas pipeline (black symbols), obtained on specimens cut out in the vicinity of the outer (I) and inner (II) pipe surfaces.

Analysis of macrofractorams of studied steel in the initial state and after the operation on the main gas pipeline, obtained after specimen test for impact toughness in the temperature range of +20 to –60 °C, indicates a difference in the relief of their fracture surfaces (Fig. 4). In particular, on the fractures of the steel in the initial state, an insignificant amount of delamination in the direction of steel rolling and a ductile fibrous relief, which dominated at the fractures, were found (Fig. 4a, b). Its fragments were partially preserved on the fracture of unused steel up to a test temperature of –40 °C (Fig. 4 c). And only at the fracture surface of the specimen, destroyed at a temperature of –60 °C, the elements of the crystal relief prevailed (Fig. 4d).