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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exclusively with the degradation of steel (Nykyforchyn 4 et al. (2021)). Indeed, in general, with an increase in the strength of materials due to the influence of any other factors (heat treatment, alloying, etc.), their plasticity decreases. Earlier, on the example of heat-resistant steels, this atypical phenomenon of long-term operated steels was associated with the fact that during active loading of a specimen with operational damage, they opened even before the localization of the deformation process (still at the stage of uniform specimen deformation) and this additionally increased the elongation of the specimen (Student 1 et al. (2012)). In this case, the elongation ceased to characterize the plasticity of the material, and began to play the role of an indicator of internal scattered damage accumulated in the metal during operation. Therefore, the obtained result of an atypical change in the characteristics of strength and ductility was considered a sign of reaching a critical state in the metal located at the outer surface of the pipe in its bottom part due to its most degradation. Despite insignificant changes in the mechanical properties of 17H1S steel due to its degradation in absolute values, their change for the exploited steel relative to the corresponding characteristics of steel in the initial state (according to the coefficient  = (P op – P 0 ) / P 0 , where P 0 and P op are the corresponding mechanical characteristics of steel in the initial state and after operation) more clearly showed certain trends (Fig. 2). In particular, the opposite tendencies were revealed in the change in both strength characteristics of steel, depending on the location of the specimens in the top or bottom parts of the pipe during operation. Namely, when they are located in the top part of the pipe, the decrease in both strength characteristics of the steel is stronger at the inner surface of the pipe, which correlates with the opposite tendency of the elongation, which decreased more strongly at the outer surface of the pipe. When the specimens were located in the bottom part of the pipe, despite insignificant losses behind the elongation index, the loss of both strength characteristics was greatest at the outer surface of the pipe. It is difficult to associate the maximum loss of steel strength from the lower part of an exploited pipe with something other than the embrittlement effect by a hydrogenative environment. With regard to the opposite tendency of changes in the loss of steel strength located during operation in the upper and lower parts of the pipe, then a possible explanation for this phenomenon could be pipe bending due to possible soil shear.

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The top of the pipe

The bottom of the pipe

I

II

I

II

 , %

0

-5

-10

-15

 UTS  YS elongation

-20

I - outer surface II - inner surface

-25

Fig. 2. Change in the state of 17H1S steel, operated for 31 years on the main gas pipeline, according to the coefficient  = (P op – P 0 ) / P 0 , which characterizes the change in its mechanical characteristics P op (strength – σ UTS and σ YS and plasticity – elongation) relative to the corresponding characteristics P 0 for steel in its original state The brittle fracture resistance of materials is considered one of the most sensitive indicators of steel degradation, which is used to substantiate their characteristics in elements of various structures Krechkovs’ka 1 et al. (2019), Filippov et al. (2013), Marushchak et al. (2014), Okipnyi et al. (2020), Zurnadzhiy et al. (2020), Krechkovska 2 (2015), Ostash 2 et al. (2007). Indeed, on the one hand, this indicator characterizes the ability of a material to fracture for a ductile mechanism, and on the other hand, its ability to resist the most dangerous brittle fracture mechanism realized practically without significant deformation. It is especially important to take into account for steels operated at sub-zero climatic temperatures. The long-term practice of operating structures indicates that steels in different structural states are characterized by different tendencies to brittle fracture, which largely determines their service life (Nykyforchyn 1 et al. (2016),