PSI - Issue 68

Olha Zvirko et al. / Procedia Structural Integrity 68 (2025) 868–873 Olha Zvirko / Structural Integrity Procedia 00 (2025) 000–000

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charging, only a slight decrease in the reduction of area was observed for the transverse specimens made from the reserved steel. Nevertheless, the resistance to HE of the reserved steel decreased somewhat after high-intensity hydrogen pre-charging ( i = 1.0 mA/cm²) under testing the transverse specimens. Hydrogen thus enhanced steel sensitivity to structure inhomogeneities and revealed the anisotropy of HE manifestation. The operated steel demonstrated lower resistance to HE even under the moderate mode of hydrogen pre-charging (Figure 2b), with the effect becoming more significant at higher cathodic current densities. This steel was highly susceptible to HE in longitudinal and transverse specimens. Furthermore, the sensitivity to HE was more significant in the transverse specimens than the longitudinal ones under both hydrogen charging modes. Fractographic examinations of the fracture surfaces of specimens of the studied steels after hydrogen pre charging indicate the critical role of non-metallic inclusions in the HE of the steel, depending on the degradation degree and hydrogen charging conditions. The fracture mechanism of non-hydrogenated specimens was ductile, and delaminations were revealed for the operated steel. For hydrogen pre-charged steel specimens, the formation of delaminations along the rolling direction and decohesion of inclusions from the matrix facilitated by hydrogen were observed for both studied steels. Hydrogen charging under moderate mode in the operated steel causes its intergranular embrittlement (Figure 3a). In contrast, intensive hydrogen charging leads to brittle transgranular cleavage (Figure 3b), which is supposed to be associated with different transport mechanisms of absorbed hydrogen (Zvirko et al. (2024)). Thus, during the moderate mode of hydrogen charging, hydrogen diffusion along grain boundaries was dominant (Hredil and Tsyrulnyk (2010), Shirazi et al. (2023), Nnoka et al. (2024)). In contrast, during intensive hydrogen charging, it was through the grain bodies (Voloshyn et al. (2015)). Notably, the initiation sites for cleavage were delaminations, which became more pronounced with increased hydrogen charging intensity.

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Fig. 3. SEM images of the fracture surface of transverse specimens of the operated steel after hydrogen pre-charging under moderate (a) and intensive (b) modes and tensile testing in air. 4. Concluding remarks The strength of the low-carbon pipeline steel in both studied states (reserved and after 38 years of operation at the transit gas pipeline) was similar. However, the operated steel had lower plasticity and impact toughness associated with its operational degradation. A pronounced anisotropy of impact toughness was revealed. The reserved steel is not susceptible to HE under moderate hydrogen charging, and it is somewhat sensitive to it in intensive mode at the tensile test of transverse specimens. However, the HE resistance of the operated steel is significantly lower even at moderate hydrogen charging, and it further decreases as its intensity increases. The highest sensitivity to HE is observed for the operated steel when testing transverse specimens. Hydrogen-facilitated decohesion of inclusions from the matrix and formation of delaminations along the rolling direction were observed for hydrogen pre-charged steel specimens. Hydrogen charging of the operated steel under moderate mode causes its intergranular embrittlement, while intensive – transgranular embrittlement. The distinctions are supposed to be associated with different diffusion mechanisms of absorbed hydrogen.