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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strongly during operation (black dots in Fig. 5a, obtained by Hredil 3 et al. (2020). To rank these properties, there was used the coefficient of change of the corresponding characteristics of the exploited steels P op relative to their inherent in the initial state P 0 . The results obtained in this study for 17H1S steel (white dots in Fig. 5a) not only proved the validity of the previously made conclusion about highest sensitivity of the ratio P op /P 0 to steel degradation, but also expanded the evidence base for this conclusion. The fractographic index α was quantitatively evaluated, which characterizes the area of brittle elements per unit area of fractures of impact specimens made of 17H1S steel, tested at room temperature (Krechkovska 6 et al. (2021)). In this case, index α was used as a quantitative fractographic parameter of the operational degradation of steel. The points obtained for the analysed steel are in good agreement with the previously obtained dependence P op /P 0 – α (Fig. 5b). The connection between the actual alpha index and the parameter of the relative change KCV op / KCV 0 , confirmed by these studies, additionally proves the legitimacy of using the previously obtained relationship for ranking the exploited pipeline steels according to the degree of their embrittlement at the current moment of their operation. Moreover, regardless of the level of strength of steels or the duration of their operation, this regularity describes quite well the change in their state at a particular moment of analysis. Consequently, according to the estimates of the area of the crystalline fracture relief of the impact specimens, it is possible to make an assumption about the risk of unpredictable brittle fracture in the process of further steel operation. However, the critical state of pipeline steels beyond this indicator remains unreasonable. Therefore, it is required to expand the range of materials under study with different duration of their operation, and especially steels from destroyed pipeline elements corresponding to a critical state of steels. 4. Conclusions The mechanical properties of 17H1S steel in the initial state and after 31 years of operation on the main gas pipeline are analysed. The strength and plastic characteristics of the steel in the initial state, determined at the outer and inner surfaces of the pipe, corresponded to the specified values. After operation, the tensile strength σ UTS of the steel decreased, while remaining within the permissible limits (except for the metal from the bottom part of the pipe in the vicinity of its outer surface). Regardless of the location of the specimens in the pipe cross-section, the value of the yield stress σ YS of the 17H1S steel after operation fell below the minimum permissible value. Both in the initial state and after operation, the elongation of the steel was higher near the outer surface of the pipes as compared to their inner surface. The tendencies of changes in the mechanical characteristics of the exploited steel relative to its corresponding characteristics in the initial state have been established. It was assumed that the maximum loss of strength characteristics of steel from the bottom part of the pipe in the vicinity of its outer surface may be associated with its stronger embrittlement under the action of groundwater and with possible pipe deformation due to landslide. Advantage of brittle fracture resistance over strength and plastic characteristics in terms of sensitivity to changes in the technical state of the metal as a result of operational degradation was confirmed. Ductile-brittle transition curves were plotted for both variants of the 17H1S steel and fractographic features of specimen fracture at temperatures from +20 to –60° C were studied. An increase in the temperature of ductile-brittle transition of steel from – 40 to –30° C after its operation has been established. From the analysis of the fractures of the impact specimens, the fraction of their brittle crystalline relief was determined. The relationship between the loss of resistance to brittle fracture and the fraction of the crystalline relief per unit area of fractures of impact specimens of a number of steels from various pipelines was supplemented with data for the 17H1S steel under study, and good agreement between all the data was obtained. References Filippov, G. A., Livanova, O. V., Chevskaya, O. N. Shabalov, I. P., 2013. Pipe steel degradation during operation and brittle failure resistance. Metallurgist 57, 612 – 622. Hredil 1 , M. I., 2011. Role of disseminated damages in operational degradation of steels of the main gas conduits. Metallofizika i Noveishie Tekhnologii 33 (Spec. Iss.), 419 – 426.