PSI - Issue 68

H. Nykyforchyn et al. / Procedia Structural Integrity 68 (2025) 861–867 H. Nykyforchyn et al. / Structural Integrity Procedia 00 (2025) 000–000

862

2

1. Introduction A global strategy for decarbonization of energy sources implies the development of hydrogen infrastructure, one of the key aspects of which is gaseous hydrogen transportation through pipelines. The existing gas trunklines thus are being repurposed for hydrogen. However, the operation time of the existing pipeline networks often exceeds their calculated lifetime. Poberezhnyi et al. (2017), Zvirko (2021), Dzioba et al. (2021), Nykyforchyn 1,2 et al. (2022), Andreikiv et al. (2023), Makarenko et al. (2024) pointed out that pipe steel is subjected to degradation, mainly towards embrittlement, indicated by a decrease in brittle fracture resistance characteristics thus leading to unexpected failures. The risk of integrity loss of gas main pipelines is typically associated with hydrogen embrittlement (Li et al. (2022, 2024), Zvirko 1 et al. (2024)). The hydrogen content is often elevated in operated steel (Hembara et al. (2023), Zvirko 1 et al. (2024)) due to microdamage development and subsequent hydrogen accumulation at the formed defects. Tsyrulnyk et al. (2010) showed that hydrogen can form at the internal surface of a pipe as a result of corrosion processes on steel in condensed moisture, saturated with corrosive species. In addition, Mandryk et al. (2022), Syrotyuk et al. (2023), and Zvirko 2 et al. (2024) stated that hydrogen transportation would facilitate pipe hydrogenation. Corrosion inside the pipe can be intensified due to the saturation of a thin condensed layer of an electrolyte with hydrogen. Steel hydrogenation can be promoted by cyclic loading (Andreikiv et al. (2024)), as a factor of protective layer breaks leading to prolonged (for years) exposure of the pipe internal surface to hydrogen. Fracture mechanics parameters, in general, are particularly sensitive while assessing the brittle fracture resistance of steel, and the hydrogen effects on it. This is primarily associated with the possibility of intensive hydrogen accumulation at the crack tip. Fatigue crack growth testing and impact testing are mostly performed. Alvares et al. (2019, 2020), Nykyforchyn 1 et al. (2022), Tsyrulnyk et al. (2004, 2024) reported that the sensitivity of the evaluated parameters can be enhanced by preliminary (before testing) hydrogen charging. Hydrogen accumulates in the vicinity of the crack tip during charging and, besides, it moves from the bulk of material to the crack tip during mechanical loading. The present research compares two states (as-delivered and operated) of pipeline steel API 5L X52 subjected to preliminary (ex-situ) hydrogen charging of various intensities to evaluate the hydrogen effect on their fracture toughness depending on the loading rate. Since the tested steel is low-strength and thus highly ductile, using the linear fracture mechanics for fracture toughness determination is not applicable. That is why the non-linear fracture mechanics approaches have been employed. 2. Materials and methods The object of research is a reserve (as-delivered) pipe and a pipe operated for 38 years at the gas main pipeline (the pipe diameter is 1220 mm and the wall thickness is 12 mm), made of low-carbon steel (Ukrainian code 17H1S, equivalent to API 5L Х52, its chemical composition in wt. %: C 0.2; Mn 1.3; Si 0.4; S £ 0.04; P £ 0.01; Al £ 0.04, Fe balance). Single-edge notched bending (SENB) specimens (Fig. 1 a ) for three-point bend testing were cut out longitudinally relative to the rolling direction (pipe axis), as shown in Figure 1 b : in the longitudinal specimen crack growth is across the fibres (L–T), a notch is made along the wall thickness, which allows averaging the results through the pipe wall.

a

b

Figure 1. SENB specimen dimensions ( a ) and scheme of their cutting from the pipe ( b ).

Fracture toughness has been evaluated by the J -integral method according to ASTM E 813. Fatigue pre-cracking was done using a facility for cantilever bending with tensile stresses in the notch zone, the stress ratio R was close to 0, and the loading frequency was 10 Hz. Pre-cracked specimens were loaded in three-point bending with a distance