PSI - Issue 28

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 28 (2020) 896–902 H. Nykyforchyn, O. Tsyrulnyk, O. Zvirko, M. Hredil / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Assessment of the technical condition of metal of long-term operated structures is one of requirements for substantiation of possibility of their residual lifetime extension. Considering structural steels, this approach is applicable firstly for critical structures and components in power engineering, petrochemical facilities, oil and gas transmission pipelines, bridges etc., operated for decades and even over a hundred years, for which further safe and reliable operation is especially important. The technical state of the mentioned facilities can be considered from the different points of view. On the one hand, a presence of macrodefects which can cause failure, on the other hand, a deterioration of mechanical properties mainly determining fracture resistance of a metal. The phenomenon of a worsening of metal properties with time of operation is called operational degradation of structural materials. The effect of long term operation of steels on the standard mechanical properties is most often considered, i.e. Kharchenko et al. (2014) studied the changes in strength (yield strength, ultimate strength) and plasticity (elongation, reduction in area), Meshkov et al. (2015) – their combination; Bolzon et al. (2018) evaluated mechanical parameters by indentation experiments; Krasowsky et al. (2001), Student et al. (2012, 2018), Hredil et al. (2019), and Lesiuk et al. (2020) determined characteristics of brittle fracture resistance (impact toughness, fracture toughness, fatigue crack growth rate); besides, stress corrosion cracking and hydrogen induced cracking were considered by Tsyrulnyk et al. (2004), Gabetta et al. (2008), Stasyuk et al. (2019), Zvirko et al. (2019), including the researches of Andreikiv et al. (2012) and Shtoyko et al. (2019) of the stage of crack propagation, etc. An essential part of critical structures is operated under conditions of possible hydrogenation, therefore, in this case these objects are subjected to combined action of applied working stresses and hydrogen, that is a facilitating factor of microdamage evolution, as it was shown by Nykyforchyn et al. (2019) and Marushchak et al. (2019). Such operational conditions influence primarily resistance to stress corrosion cracking and brittle fracture. In this paper damaging effect of hydrogen on pipeline steels during long-term operation is considered and general

regularities, causes and sequences of hydrogen related degradation are analysed. 2. General regularities of degradation of long-term operated pipeline steels

General regularities of operational degradation of mechanical properties of pipeline steels have been summarized by Nykyforchyn et al. (2017), as illustrated in Fig. 1. As a rule, in-bulk degradation of steels operated under ambient temperature is concerned with deformation aging (stage I). The stage of deformation aging is characterized by a rise of strength and hardness, which results in a decrease in plasticity and brittle fracture resistance. However, for long term operated pipeline steels, these regularities are violated, as it is indicated as stage II.

Fig. 1. Scheme of the two-stage in-bulk steel degradation: Stage I – deformation aging; Stage II includes two substages: IIA – disoriented dissipated damaging, and IIB – damage accumulation in the rolling direction.