PSI - Issue 77

Olha Zvirko et al. / Procedia Structural Integrity 77 (2026) 484–489 Olha Zvirko / Structural Integrity Procedia 00 (2026) 000–000

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The fracture toughness of the X52 steel in both as-received and post-operated states showed only slight differences (Table 2): the value is slightly lower for the operated metal compared to the as-received one, both with and without hydrogen pre-charging. Such a peculiarity was associated with complex morphology of delaminations observed by fractographic analysis of the fracture surfaces of the transverse specimens (Zvirko 2 et al. (2025)). The morphological feature is inherent to the state of the metal and the testing conditions (transverse operated specimens), where the tendency to delaminate is the highest. Thus, the delamination mechanism plays two opposing roles regarding its influence on fracture toughness. It reduces the energy absorption for fracture between the structure’s fibers while simultaneously facilitating stress relaxation through the specific branching of the crack. The X67 steel was characterized by significantly higher fracture toughness values compared with the X52 steel for both studied states, as-received and post-operated. It should be also emphasized that long-term operation of the X67 steel led to insignificant differences in the fracture toughness values for different states. Thus, J cr were 331 N/mm and 319 N/mm for the as-received and operated states, respectively. This finding contrasts with the impact toughness data, which showed a noticeably lower (by 21%) values for the operated steel compared to the as-received one. Decrease in fracture toughness under ex-situ hydrogen charging was higher for the X67 steel compared with X52 steel. Decreasing displacement rate of the hydrogen pre-charged specimens from 0.5 to 0.005 mm/min led to a decrease in fracture toughness of the X67 steel from 162 N/mm to 53 N/mm and from 159 N/mm to 38 N/mm for the as-received and operated states, respectively. Therefore, crack growth resistance depends on both hydrogen charging and mechanical loading conditions. 4. Concluding remarks Aging and degradation in natural gas pipelines are ongoing issues that are likely to accelerate under hydrogen service, emphasizing the need for continued monitoring and evaluation of material performance in such environments. Two pipeline steels, X52 and X67, were investigated considering their degradation under long-term operation. Tensile properties, strength and plasticity, show minimal changes between as-received and post-operated steels, indicating stable mechanical performance after service. Impact toughness decreases after operation, notably in X67 steel, yet remains relatively high, demonstrating retained toughness despite service exposure. The long-term operation of the pipeline steels caused an increase in the susceptibility to hydrogen embrittlement, especially for the X52 steel. Insignificant effect of long-term operation of two pipeline steels, X52 and X67, on the fracture toughness determined by the J -integral method, was observed, associated with complex morphology of delaminations influenced on crack propagation. Decrease in fracture toughness under ex-situ hydrogen charging was higher for the X67 steel compared with X52 steel; however, X67 steel was characterized by significantly higher fracture toughness for uncharged and hydrogen pre-charged specimens. The displacement rate during testing significantly affects crack growth resistance of the X67 steel after hydrogen exposure, highlighting the importance of strain rate on hydrogen-induced fracture behavior. Acknowledgements The research was carried out with the partial financial support by the National Academy of Sciences of Ukraine under R&D project (Registration No. 0124U0000911) and the National Research Foundation of Ukraine under Project No 2022.01/0099. This publication is based on work supported by a grant from the U.S. Civilian Research & Development Foundation (CRDF Global). Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of CRDF Global. References Alvarez, G., Peral, L. B., Rodriguez, C., Garcia, T. E., Belzunce, F. J., 2019. Hydrogen embrittlement of structural steels: Effect of the displacement rate on the fracture toughness of high-pressure hydrogen pre-charged samples. International Journal of Hydrogen Energy 44, 15634–15643. Boukortt, H., Amara, M., Hadj Meliani, M., Bouledroua, O., Muthanna, B.G.N., Suleiman, R.K., Sorour, A.A., Pluvinage, G., 2018. Hydrogen embrittlement effect on the structural integrity of API 5L X52 steel pipeline. International Journal of Hydrogen Energy 43(42), 19615–19624.