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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repurposing existing infrastructures of natural gas pipelines for hydrogen delivery presents an attractive path toward creating the hydrogen economy (Laureys et al. (2022), Zapukhlyak et al. (2022), Kappes and Perez (2023), Zvirko 1 et al. (2025)). Ukraine plays an important role in the European Hydrogen Backbone project and has extensive natural gas transit infrastructure which could be repurposed for transporting hydrogen. A significant part of pipelines has been in service beyond their design lifetimes, and the pipe steels undergo degradation in the form of embrittlement with deteriorated mechanical properties (Meshkov et al. (2015), Zhu (2015), Zvirko (2021), Hrabovskyy et al. (2024), Nengjun et al. (2025)). Impact strength and fracture toughness are the most critical parameters employed to determine the extent of steel degradation as demonstrated by Nykyforchyn et al. (2016), Tsyrulnyk et al. (2024), Demianchuk et al. (2025), and others. The risk of integrity loss in gas transit pipelines is commonly caused by hydrogen-related effects (Hoschke et al. (2023), Zekun et al. (2025)), which can occur via numerous mechanisms such as hydrogen embrittlement and hydrogen-induced cracking. A key challenge is the correct assessment of hydrogen impact on pipeline steels. Low alloy and low-carbon steels are employed in majority of Ukrainian transit pipelines and are most vulnerable. Hydrogen exposure reduces ductility, fracture toughness, and fatigue resistance, while promoting a ductile-to-brittle fracture transition (Depover et al. (2014), Boukortt et al. (2018), Alvarez et al. (2019), Kappes and Perez (2023), Cao (2024), Tsyrulnyk et al. (2024), Chowdhury et al. (2025), Jack et al. (2025), Nykyforchyn et al. (2025), Santana et al. (2025)). Strength properties, such as tensile and yield strength, are not normally affected. Hydrogen transport therefore increases the possibility of loss of pipeline integrity, especially in pre-embrittled steels resulting from long-term operation. Fracture toughness is a critical parameter for evaluating the structural integrity of pipelines, particularly in the context of hydrogen influence on material performance. Numerous studies (Chatzidouros et al. (2014), Depover et al. (2014), Boukortt et al. (2018), Campari et al. (2023), Tsyrulnyk et al. (2024)) have investigated hydrogen embrittlement in pipeline steels using various testing methods and specimen geometries. Fracture toughness in pipeline steels is usually determined using single-edge notched tension testing (Alvarez et al. (2019), Tsyrulnyk et al. (2024)), Nykyforchyn et al. (2025)), since it best represents service conditions of pipes. Measurement of the fracture toughness of ductile pipe steels is complicated using linear elastic fracture mechanics; thus, nonlinear fracture mechanics techniques, such as the J -integral technique, are generally preferred (Fassina et al. (2011), Tsyrulnyk et al. (2024)). The diversity of test methods, involving electrochemical versus gaseous hydrogen charging and in-situ versus ex-situ testing, also underscores the importance of careful choice of methods. The fracture mechanisms under hydrogen exposure are widely studied, highlighting shifts in ductile-to-brittle transition behavior, delamination phenomena, and complex crack path deviations including branching. Pipeline steels typically have a banded ferrite/pearlite microstructure that leads to anisotropic mechanical behavior (Zvirko et al. (2021), Shinohara et al. (2022)). Tensile and yield strengths are well documented to be greater in the transverse direction compared to rolling (longitudinal) direction, partly as a result of ferrite/pearlite band compression effects. Moreover, long-term operation can amplify anisotropy, especially in resistance to hydrogen embrittlement. This study investigates the hydrogen embrittlement susceptibility and fracture behavior of two pipeline steels, X52 and X67, considering their degradation under long-term (34…38 years) service at the Ukrainian natural gas transit pipelines. 2. Materials and methods 2.1. Materials Two low-carbon pipeline steels, API 5L X52 and API 5L X67, in different states, as-received and post-operated, were investigated. The Х52 steel (Ukrainian code 17H1S, equivalent to API 5L Х52 strength grade; chemical composition in wt. %: C 0.2; Mn 1.3; Si 0.4; S≤0.04; P≤0.01; Fe balance ) was studied. Specimens were machined from two pipes (a pipe outer diameter is 1220 mm and a wall thickness is 12 mm): a reserve pipe (as-received state) and after 38 years of operation at the Ukrainian natural gas transit pipeline. The X67 steel examined in this study had chemical composition in wt. %: C 0.12; Mn 1.54; Si 0.4; S ≤ 0.01; P ≤ 0.02; Fe balance. Two pipe sections with an outer diameter of 1420 mm and a wall thickness of 18.7 mm were analyzed: