PSI - Issue 42

Olha Zvirko et al. / Procedia Structural Integrity 42 (2022) 522–528 Olha Zvirko / Structural Integrity Procedia 00 (2022) 000 – 000

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Microfractographic investigations of the fracture surfaces of tensile specimens (both with and without preliminary hydrogenation) were performed using scanning electron microscope Carl Zeiss EVO-40XVP. 4. Influence of long-term operation on anisotropy of strength and plasticity of pipeline steels The average values of strength and plasticity characteristics of the tested pipeline steels are presented in Tables 1 and 2.

Table 1. Mechanical properties experimentally observed for the 17H1S pipeline steel Steel state Specimen type Ultimate strength σ UTS , MPa Yield strength σ Y , MPa

Reduction in area RA , %

Elongation δ , %

620 632 637 643

392 402 485 497

72.0 54.8 64.0 43.5

23.9 25.1 19.1 18.2

As-delivered

Longitudinal Transversal Longitudinal Transversal

Post-operated

As it can be seen from Table 1, the 17H1S pipeline steel underwent strengthening during long-term operation. Thus, both strength characteristics are slightly increased. Concerning plasticity, decrease in both plasticity characteristics was observed. These peculiarities of mechanical properties changes are in accordance with that previously demonstrated for pipeline steels after different operation time, as it was reported by Zvirko et al. (2018, 2021), Marushchak et al. (2019), Zvirko (2021) and others. A variation in mechanical properties as a function of the orientation of the test specimen relative to the rolling direction of the pipe steel was also analysed. The anisotropy of strength properties is insignificant. However, the anisotropy is particularly pronounced in both plasticity characteristics, reduction in area and elongation. Table 2. Mechanical properties experimentally observed for the 20 pipeline steel from different sections of the pipe elbow Steel state Pipe elbow section Specimen type Ultimate strength σ UTS , MPa Yield strength σ Y , MPa Reduction in area RA , % Relative displacement Δ, mm Reference Straight Longitudinal 582 492 59.6 1.24 Short transversal 553 466 36.2 0.45 Degraded Tensioned Longitudinal 611 475 48.8 1.00 Short transversal 599 431 28.9 0.51 The steel from tensioned pipe elbow section was characterised by increased strength compared to the straight pipe metal regardless of the orientation of the test specimens relative to the rolling and short transversal directions of the pipeline steel. A noticeable decrease in the plasticity characteristics in the tensioned section metal in comparison with the straight one was revealed. Thus, reduction in area of the tensioned section metal was reduced by 22% and 25% for longitudinal and short transversal specimens, respectively, compared to the straight one. At the same time, plasticity anisotropy was also revealed. 5. Influence of long-term operation on anisotropy of hydrogen embrittlement of pipeline steels Following to the experimental procedure described above, the susceptibility of the investigated steels to hydrogen embrittlement was studied for the as-delivered and degraded specimens depending on their orientation relative to the rolling direction of the pipe. HES indices of the 17H1S pipeline steel and the 20 pipe elbow steel are presented in Figs. 1 and 2.