PSI - Issue 33

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 33 (2021) 646–651 Hryhoriy Nykyforchyn, Leonid Unigovskyi, Olha Zvirko et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 2. Fracture surface of specimen made of the 17H1S pipeline steel after 40 years operation fractured during Charpy impact test (below – stress concentrator).

Taking into account a possibility of development of such dissipated microdamages in steels during operation, two stages of operational degradation of pipeline steels were considered by Nykyforchyn et al. (2017), namely: deformation aging and development of dissipated damages. Every stage is associated with certain features of changes in mechanical properties of steels in comparison with as-received state. During stage I, deformation aging, strength and hardness of a metal increase and characteristics of plasticity and resistance to brittle fracture decrease. Under stage II, for characterizing ductility of material, relative reduction in area should be used, because of relative elongation is significantly affected by opening of multiple microdefects during tensile loading, which leads to increasing this characteristic and incorrect characterisation of true ductility. For the same reason, strength and hardness of material can be decreased. For rolled pipes, damages are mainly associated with microdelamination along elongated structure fibres and non metallic inclusions, shown by Student et al. (2019), therefore defects are oriented in the rolling direction. The fracture behaviour of a rolled pipe steel is dependent on crack orientation and direction of crack propagation, with respect to pipe axes (longitudinal L, transversal T, and short-transversal S) as it was pointed out by Ju et al. (2007), Xue et al. (2021) and others. It was demonstrated by Ronevich et al. (2016) that fatigue crack growth rate was nearly an order of magnitude lower for cracks propagating in the X65 pipeline steel in hydrogen gas in L-S orientation (perpendicular to the banded pearlite) than that growing parallel to the pearlite fibers. When fracture propagates in cross-sectional direction S in an operated rolled steel it intersects elongated microdefects and absorbs more energy than in other directions, longitudinal or transversal. Therefore, impact toughness of L-S orientation for an operated steel is higher than that of the other orientation, and it can be even higher than that of an unoperated material, shown by Nykyforchyn et al. (2017). Hence, for proper evaluation of operational degradation of resistance to brittle fracture of pipeline steels, in particular, impact toughness or fracture toughness, specimens, for which fracture path is parallel to texture fibres, should be preferable. The same is true for assessment of susceptibility to hydrogen embrittlement of long-term operated pipeline steels caused by transport of hydrogen or hydrogen-natural gas mixture. 4. Peculiarities of experimental studies of an influence of hydrogen or natural gas-hydrogen mixture on fracture of pipeline steels A set of experimental tests is planned to be carried out for assessment of an influence of hydrogen or natural gas hydrogen mixture on fracture behaviour of long-term operated pipeline steels. According to the developed research program sensitivity of pipeline steels to hydrogen embrittlement, depending on the metal state, test conditions and other factors, will be investigated. For this purpose, hydrogenation of specimens by both electrolytical and dissociation ways will be studied. Specimens will be in-laboratory pre-hydrogenated by electrochemical method and then tested in order to study of influence of absorbed hydrogen on certain physical and mechanical properties of steels. Pre-loaded