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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Tsyrulnyk 1 et al. (2004)). With regard to pre-exploited pipes, another problem is added related to the change in the mechanical properties of structural steels during long-term operation, in particular, such as impact toughness, strength and plastic characteristics. During high-temperature operation of steels (in particular, heat-resistant steels in elements of heat and power equipment), these changes are due to obvious signs of degradation of their structure associated with favourable conditions for diffusion redistribution of carbon and alloying elements and following decrease in the cohesion of adjacent grains and subgrains due to precipitation and coagulation of carbides along them (Krechkovska 1 (2008)). However, the degradation peculiarity of objects operated in climatic conditions, which include the main gas pipelines, is the non-obviousness of changes in their microstructure. Although some authors (Nechaev (2008) and Nykyforchyn 2 et al. (2019)) believe that over many years of operation, thin cementite plates can form along grain boundaries even at climatic operating temperatures, which contributes to the steel embrittlement. At the same time, the majority of researchers attribute the damage randomly scattered the cross section of the part to the most popular reasons for loss the properties of steels in service, meeting the requirements at the time of the start of operation. In particular, it has been shown by Hredil 1 (2020), Kryzhanivskyi et al. (2011), Nykyforchyn 3 et al. (2020), that damage to steels after thirty years of operation on a main oil pipeline and in an oil storage tank negatively affected not only the mechanical properties of steels, but also electrochemical and corrosion. Moreover, the degradation of the metal most intensively occurred in those areas of structures that were constantly in contact with a corrosive hydrogenating environment in the form of produced water. Therefore, it was concluded that corrosive aqueous environments should be attributed to one of the most important factors affecting the operational degradation of structural elements. They created the prerequisites for the formation and absorption of hydrogen on the surface of the stressed metal and promote its further penetration into the depth of the cross section of pipe. Corrosion damage was also found on the inner surfaces of the pipes, which indicates the effect of corrosive environment such as water condensate inside the pipes, accompanying the transportation of hydrocarbons (Tsyrulnyk 2 et al. (2008)). Macro- and micro-delaminations on the specimen fracture surfaces from exploited steels, oriented in the rolling direction, are also referred to as one of the fractographic signs of steel degradation (Lesiuk et al. (2021) and Hredil 2 et al. (2019), Zvirko et al. (2018) and Nykyforchyn 4 et al. (2021)). Their appearance indicates a decrease in cohesion of the slag inclusions or non-metallic ones (which, as a rule, stretch as strips in the rolling direction) with matrix. It is caused by the long-term influence of operating stresses in the cross-section of pipes and their hydrogenation. The interphase boundaries between them are energetically favourable traps for hydrogen adsorbed by steel over a long term of operation. An increase in the pressure of hydrogen in microvoids along the interphase boundaries contributes to final separation of inclusion from the matrix and their subsequent merging with the nearest micro-delaminations on other inclusions, many of them are located in the cross-section of the pipe. Despite a significant amount of research regarding the influence of various operational factors on the degradation of long-term exploited gas pipeline steels and the identification of its characteristic features, the generalizing patterns that determine the effect of degradation on the technical state of steels still remains at the stage of quantitative data accumulation. Therefore, the purpose of this work is to evaluate changes in mechanical properties, namely, strength and plastic characteristics and impact toughness of the 17H1S steel after its long-term operation on gas pipelines and to show the relationship between mechanical and fractographic characteristics that are sensitive to changes in the state of steel due to its degradation. 2. Materials and methods It was investigated the 17H1S steel in the initial state (from stock pipes) and after its long-term operation for 31 years on the main gas pipeline. In both cases, the pipes had a wall thickness of 10 mm. The state of both steel variants was analysed on specimens cut from the outer (I) and inner (II) surfaces of the pipes. In addition, for the exploited steel, the mechanical properties of the metal located during operation on the gas pipeline in the upper (  op ) or lower (bottom) parts of the pipe were analysed. To certify the technical state of the studied steels the flat specimens by 3 mm thick were tested in air under uniaxial tension at a deformation rate of 3.3 ∙ 10 -3 s -1 . The test results were used to determine the strength characteristics (ultimate strength σ UTS and yield strength σ YS ) and plasticity (elongation). In addition, accounting the high sensitivity to steel degradation during operation of all characteristics of brittle fracture resistance (Hredil 3 et al.