PSI - Issue 59

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 59 (2024) 125–130 2 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Oleh Venhryniuk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000 1. Introduction Transportation of gaseous hydrogen or a mixture of hydrogen with natural gas by the existing natural gas steel pipeline increases a failure risk due to hydrogen absorption and, consequently, hydrogen-induced degradation. Hydrogen embrittlement of pipe steels is considered as one of the most dangerous factors compromising their integrity, as demonstrated by Briottet et al. (2012), Ohaeri et al. (2018), Hoschke et al. (2023), Campari et al. (2023), Dmytrakh et al. (2023), and others. Hydrogen can be evolved due to electrochemical corrosion, both from the external and internal surfaces of the pipe. External corrosion can occur as a result of the loss, either partial or complete, of the protective properties of the insulation coating. The stimulation for metal embrittlement of pipe is the specific conditions of the electrochemical interaction of the soil environment with the surface under the cathodic protection of the pipeline or under the coating with partial loss of its insulating properties (Voloshyn et al. (2015)). As for the internal surface of the pipe, condensed moisture serves as the electrolyte, in which the corrosion process occurs with hydrogen depolarization, which also acts as a source of hydrogen. Pipeline steel used for natural gas transportation would also absorb the hydrogen from the transporting gaseous hydrogen due to its dissociation. The possibility of transporting gaseous hydrogen or a mixture of hydrogen with natural gas through pipelines exacerbates the issue of hydrogen embrittlement of pipe steels, making it a priority in assessing the risks to the integrity of pipelines (Pluvinage et al. (2021), Nykyforchyn et al. (2021, 2022a, 2022b), Laureys et al. (2022), Nguyen et al. (2022)). On the other hand, impact toughness and fracture toughness are the mechanical indicators that are most sensitive to the embrittlement of steels, including that induced by hydrogen embrittlement (Gredil (2008), Tsyrul’nyk et al. (2018), Fassina et al. (2011), Hoyos et al. (2019), Zvirko (2021), Zvirko et al. (2022, 2023a)). Different methodologies are used to assess the susceptibility of steels to hydrogen embrittlement. The most common technique is the slow strain rate tension test (Meng et al. (2017), Nykyforchyn et al. (2022b)), indicating a decrease in the plasticity of a metal due to absorbed hydrogen. The sensitivity of metal to brittle fracture can be evaluated using true stress – strain diagrams and the distribution of local stresses and strains in a specimen in the vicinity of the crack tip by numerical calculations (Zvirko et al. (2023b)). Moreover, fracture toughness is a very important property for the pipeline integrity. Regarding the evaluation of fracture toughness of ductile pipe steels using linear fracture mechanics approaches, there are methodological challenges, leading to a preference for nonlinear fracture mechanics approaches, among which the J -integral method is favoured (Fassina et al. (2011), Boukortt et al. (2018), Chatzidouros et al. (2018), Сabrini et al. (2019), Kyriakopoulou et al. (2020), Phan et al. (2022), Zvirko et al. (2022)). An important factor influencing the sensitivity of the mechanical properties of steels to the hydrogen is the displacement rate of the specimens (Toribio et al. (2016), Álvarez et al. (2019) , Andreikiv and Hembara (2022)). The lower the displacement rate, the higher the sensitivity due to a more significant reduction in resistance to hydrogen embrittlement. The J -integral method, as a characteristic of resistance to brittle fracture, is among such properties. Therefore, when using the J -integral method to assess the hydrogen embrittlement of steels, it is crucial to consider the displacement rate of the specimens. As of today, the prospective use of the existing gas pipeline network for the transportation of hydrogen or hydrogen-gas mixtures is considered viable. In such cases, long-operated pipe steels may exhibit increased sensitivity to hydrogen embrittlement. In this study, considering the possibility of transporting hydrogen or hydrogen-natural gas mixtures through existing pipelines, the effectiveness of using the J -integral method has been demonstrated for assessing the influence of hydrogen on the fracture toughness of pipe steels, taking into account the

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displacement rate of experimental specimens. 2. Methodological Features of the Research

In this work, the 17H1S (Ukrainian code, equivalent to API 5L X52 strength grade) low-alloyed pipeline steel after long-term operation was investigated. The specimens were cut from the real pipe made of 17H1S steel with outer diameter D = 1220 mm and wall thickness t = 12 mm. The pipe of the natural gas main pipeline was operated for 38 years. Basic mechanical properties such as ultimate stress σ UTS , yield strength σ Y , reduction in area RA and elongation, as well as impact toughness KCV, are presented in Table 1. They meet the requirements imposed on pipe steels.