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

863

3

between supports of 60 mm, corresponding to fourfold specimen height. We employed a multiple-specimen method for J -integral evaluation, testing a series of 5–7 specimens for each steel state. A displacement rate of 0.5 mm/min corresponds to the requirements and is commonly used for evaluating standard mechanical properties. However, considering the well-known effect of loading rate on the fracture resistance of material with stress concentrators in corrosion and hydrogenating media, reported by Toribio et al. (2016), Alvares et al. (2019), the hydrogen impact on fracture toughness was evaluated additionally under a reduced displacement rate by one and two orders of magnitude, i.e. v = 0.05 and 0.005 mm/min. The steel susceptibility to hydrogen embrittlement was revealed by preliminary (ex-situ) hydrogen charging of specimens in the sulfuric acid electrolyte (pH 1.0) with an addition of 10 g/l of thiourea as hydrogen recombination poison. An acidic electrolyte was chosen to avoid the formation of oxide films on the steel surface. The applied cathodic current density i cat of 1.0 mA/cm 2 , considered “intensive” in this research, is still low enough and doesn't cause blistering which was observed by Khoma et al. (2023) for low-carbon steel under i cat ≥ 2.0 mA/cm 2 in a similar electrolyte. The “moderate” hydrogenation mode ( i cat = 0.05 mA/cm 2 ) is particularly low-intensive and thus is appropriate to trace minor changes in the mechanical behaviour of material using non-linear fracture mechanics methods distinguished by their special sensitivity to the metal state. The duration of hydrogen charging was 120 h, which ensures uniform hydrogenation of a specimen along its width. 3. Results and discussion A previous research by Zvirko 1 et al. (2024) revealed a noticeable degradation of the mechanical properties of the tested steel due to operation, namely, plasticity characteristics and impact strength. In particular, KCV values for reserved and operated steel were 129 and 103 J/cm 2 respectively under testing longitudinal specimens. Concerning plasticity, the decrease in reduction in area (RA) was more significant in the operated steel, especially after specimen pre-charging. Microfractographic studies of impact specimens showed a more brittle behaviour of the steel after 38 years of operation, and a significant contribution of operational degradation to steel embrittlement. These results confirm common regularities of metal embrittlement due to its operation at the gas pipeline. 3.1. The effect of steel degradation on its crack growth resistance Comparing crack growth resistance for longitudinal specimens of two steel states (dashed lines in Fig. 2a, b) without hydrogen charging reveals a significant effect of operational degradation. In this case, fracture toughness parameters are more sensitive than RA obtained by Zvirko 1 et al. (2024): the J cr value decreases from 215.0 to 108.0 N/mm (almost twice) whereas RA changes from 71 to 66%.

•••

•••

•

•••

•••

•

•••

• •

•••

•••• •• • •

•••• •• • •

••

••

•

•

•••

•••

•••

•••

•••

•••

•••

•••

•••

•••

•••

•••

•••

•••

! •••• • •

! •••• • •

a

b

Figure 2. J – R curves of as-delivered ( a ) and operated ( b ) pipe steel under a displacement rate of 0.5 mm/min after hydrogen charging at cathodic current densities 0.05 mА/сm 2 ( 1 ) and 1.0 mА/сm 2 ( 2 ). Dashed lines correspond to J – R curves without hydrogenation.