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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the longitudinal specimens, damages (delaminations, pores etc.) in in-bulk of the metal are oriented in the plane of fracture propagation, parallel to the rolling direction; therefore, they could influence on increasing elongation, as discussed by Zvirko et al. (2021). Fig. 3 shows SEM images of the fracture surfaces of short transversal specimens from the 20 pipeline steel from tensioned pipe elbow section after tensile testing in air without and after electrolytically hydrogen pre-charging. The number of dimples as ductile fracture signature in the center of the fracture surface of non-hydrogenated specimen was observed (Fig. 3a). A colony of micro-delaminations originated from non-metallic inclusions and propagated between fibres along rolling direction, and fracture of membranes between them by transgranular chipping was revealed (Fig. 3b) for the tested specimen subjected to hydrogen pre-charging electrolytically, indicating hydrogen embrittlement of the steel.

b

a

Fig. 3. SEM photographs of the short transversal specimens from the 20 pipeline steel from tensioned pipe elbow section tensile-tested in air without (a) and after (b) hydrogen pre-charging electrolytically, showing fracture features in the central section of fracture surface. 6. Summary The influence of long-term operation and hydrogenation on anisotropy of characteristics of strength, plasticity, and resistance to hydrogen embrittlement of two rolled ferritic-pearlitic pipeline steels depending on the orientation of the plane of fracture propagation in longitudinal, transversal and short transversal directions was investigated. The anisotropy of strength properties was insignificant. However, the anisotropy is particularly pronounced in both plasticity characteristics, reduction in area and elongation. Hydrogen charging of long-term operated pipeline steels significantly enhances the anisotropy of hydrogen embrittlement susceptibility depending on the plane of fracture propagation relative to the rolling direction of the pipe, and also on degree of their operational degradation. It is due to micro-delaminating between fibres and in the vicinity of non-metallic inclusions along rolling direction and fracture of membranes between them by transgranular chipping. Such anisotropy of pipeline steels, which increase with long-term operation, influences on the safe exploitation of natural gas pipelines and should be considered at assessment of the possibility of usage of the existing network for safe transportation of hydrogen or its mixture with

natural gas. References

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