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

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delaminations due to shear under fracture toughness testing. These delaminations divided the specimen section into thin layers with conditionally intact metal between them, and their occurrence contributed to the worsening of the metal properties.

a

b

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d

Figure 5. The fracture surfaces of SENB specimens of non-hydrogenated (a, b) and ex-situ hydrogenated (c, d) steel in the as-delivered (a, c) and operated (b, d) states (higher resolution).

Another sign of metal hydrogenation is cementite crumbling within pearlite grains, clearly visible in the operated steel (Fig. 5с, d, blue arrows). After testing specimens in both states without hydrogenation, pearlite grains are identified quite clearly (Fig. 5a, b, blue arrows). Microstructure analysis of the 17H1S steel by Krechkovs’ka et al. (2019) showed that pearlitic grains were damaged, manifested by their intensive etching and crumbling. The extensive network of interphase boundaries within pearlite grains, which are a mechanical mix of finely dispersed cementite with ferrite interlayers, contributed to the hydrogen accumulation around these inclusions leading to cementite elimination in pearlite (Fig. 5c, d, blue arrows). This fractographic feature is concerned with the influence of hydrogen absorbed by the metal during long-term operation, which leads to the fracture toughness decrease. 4. Concluding remarks The fracture toughness of pipe steel 17H1S, evaluated by the J -integral method, decreases due to pipe operation. The critical values of J -integral J c r are not always consistent with the previously revealed regularities for impact strength KCV and reduction in area RA, which indicates more brittle behaviour of the operated steel compared to the as-delivered one. Fracture mechanics indicators are significantly more sensitive to the hydrogen impact on steel than plasticity under moderate ex-situ hydrogen charging ( i cat = 0.05 mА/сm 2 ) conditions. This effect diminishes under intensive hydrogenation ( i cat = 1.0 mА/сm 2 ). The displacement rate affects the crack growth resistance of the electrochemically hydrogenated pipe steel 17H1S. The critical J -integral value is significantly decreased with the displacement rate reducing by two orders of magnitude for steel in both as-delivered and operated states. However, a further decrease in the loading rate could lead to the inversion of its effect by raising the contribution of the counteracting process of hydrogen release from metal. The fracture of both steel states without hydrogenation is ductile with a prevalence of shear in forming dimples in the operated steel. Ex-situ hydrogen charging led to occurring cleavage areas, delaminations, secondary cracking, and cementite elimination within pearlitic grains, which are more pronounced in the operated steel compared to the as-delivered one.