PSI - Issue 36

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 36 (2022) 306–312 Hryhoriy Nykyforchyn, Leonid Unigovskyi, Olha Zvirko et al. / Structural Integrity Procedia 00 (2021) 000 – 000

307

2

1. Introduction Solving the environmentally important problem of decarbonisation of energy sources implies using hydrogen as an alternative fuel. Practical progress in this issue is associated with the need to solve a number of tasks, one of which is hydrogen transportation in a mixture with natural gas through the existing pipeline network, discussed by Haeseldonckx and D’haeseleer (2007), Vries et al. (2017), Pluvinage et al. (2021). Accordingly, it should be assumed that hydrogen gas can be a source of hydrogenation of the pipe wall from its inner surface and, thus, affect the pipeline integrity. Nykyforchyn et al. (2021) pointed out that hydrogen plays the dual role in increasing the fracture risk. Firstly, hydrogen worsens characteristics of brittle fracture resistance, including resistance to hydrogen assisted cracking; besides, hydrogen intensifies the processes of steel degradation during long-term operation of the pipeline, also leading to a reduction in the resistance to brittle fracture. Tsyrulnyk et al. (2008), and Hredil and Tsyrulnyk (2010) noted that even refined natural gas can become a source for hydrogenation of the pipe from the inside due to electrochemical interaction of the metal with moisture condensed on the pipe inner surface. Comparative assessments of the hydrogen concentration in the as-received and operated steel by Nykyforchyn (2021) confirmed this. Thus, transporting hydrogen through pipelines could not only initiate but also intensify pipe’s hydrogenation followed by a variety of negative consequences for the pipeline integrity. This concerns the resistance of pipeline steels to hydrogen assisted cracking as one of the main characteristics of their workability under the action of a hydrogenating environment. Analyzing a possible integrity violation of gas pipelines associated with hydrogen transportation, it should be emphasized that the existing network of gas pipelines can be characterized by a certain level of operational degradation in terms of reducing resistance to brittle fracture, in general, and hydrogen assisted cracking, in particular, as it was shown in numerous researches by Tsyrulnyk et al. (2008, 2018), Maruschak et al. (2014), Zvirko et al. (2019), Okipnyi et al. (2020), Nykyforchyn 2 et al. (2021), Zvirko et al. (2021). The operational decrease in these important characteristics was associated, first of all, with the development of damages, which can be both intergranular and intragranular (Nechaev (2008), Hredil (2011), and Marushchak et al. (2019)). Hydrogen facilitates this process, thus, brittle fracture resistance deteriorates faster in hydrogenated metal. Blending hydrogen into natural gas influenced on mechanical properties of pipeline steels: their susceptibility to hydrogen-induced embrittlement increased with the hydrogen partial pressure increasing (Meng et al. (2017)) and mechanical properties of notched specimens are significantly deteriorated, as it is shown by Meng et al. (2017) and Zhou et al. (2021). The present research has been performed in the frame of the methodology proposed by Nykyforchyn et al. (2021) for experimental investigations on the effect of hydrogen transported through pipelines or its mixture with natural gas on the pipeline integrity. It is based on long-term exposure of stressed samples in a pipe with gaseous hydrogen to detect the intensity of possible hydrogenation and its impact on the metal state. The methodology has some peculiarities, namely: i) cutting testing specimens transversally relative to the pipe axis; (ii) using flat tensile specimens with the smallest possible thickness of the working part to determine characteristics of strength, plasticity and hydrogen-assisted cracking. It implies the comparison of steel properties, which determine its integrity, in the as-received state and after its operation, and also accompanying mechanical testing by hydrogen concentration measurements and fractographic analysis. The research aimed at comparing the sensitivity of the gas pipeline steel to hydrogen assisted cracking in the as-received state and after its operation. 2. Materials and methods The object of research were two pipes of distribution gas pipelines with an outer diameter of 159 mm and wall thickness of 4.5 mm made of the VST3ps rolled carbon steel, one of them was in the as-received state, and another one has been operated during 52 years. The chemical composition of the steel of both pipes is presented in Table 1. Both steel samples have ferrite-pearlite microstructure. Cementite precipitation at the ferrite grain boundaries was noted in the as-received steel, and also traces from non-metallic inclusions, whereas in the case of the operated steel – cracking at the ferrite grain boundaries.