PSI - Issue 47

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 47 (2023) 190–194 Hryhoriy Nykyforchyn, Olha Zvirko, Oleksandr Oliynyk et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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1. Introduction Structural steels used in many industries are required to have specific strength properties and brittle fracture resistance in order to guarantee safe long-term operation. The properties are regulated for materials in as-delivered state. If the structural steels are subjected to long-term operation under environmental and mechanical loading action, they undergo microstructure changes, which are accompanied by the mechanical properties’ deterioration, as demonstrated by Nykyforchyn et al. (2018), Chmelko et al. (2020), Čamagić et al. (2021), Vukelic et al. (2022), Dziubyk et al. (2022), Pustovyi et al. (2022), Zvirko et al. (2022a) and others. Structures are often subjected to an action of hydrogenating environments under operation. Therefore, hydrogen being present in technological solutions or evolved due to interaction between metal and surrounding environment can penetrate into a material. The detrimental role of hydrogen was analysed in numerous studies, such as presented by Dadfarnia et al. (2019), Nykyforchyn et al. (2020), Cauwels et al. (2022) and others. As a result of long-term operation of structural steels, the characteristics of resistance to brittle fracture (impact toughness and fracture toughness, usually determined by the J-integral method) decrease most intensively among the mechanical properties (Nykyforchyn et al. (2020), Zvirko (2021)). Operational degradation of steels can also be manifested in decreasing other properties, such as stress corrosion resistance, hydrogen induced cracking, corrosion or hydrogen assisted fatigue (Voloshyn et al. (2015), Lesiuk et al. (2019), Syrotyuk et al. (2021)). In our previous studies (Nykyforchyn et al. (2020), Zvirko (2021), and Zvirko et al. (2022b)) two main stages of operational degradation of steels were distinguished, namely deformation aging and development of dissipated damages. These stages are referred to two main mechanisms of degradation of steels during the long-term operation. The first stage consists in deformation hardening of the metal. If the structure is operated under an elevated temperature, then deformation hardening can transform into strain aging due to the formation of Cottrell clouds nearby the core of the edge dislocations. In laboratory conditions, a significant effect of deformation aging on mechanical behaviour of steels can be achieved within a relatively short period of time (up to 1 hour) at a temperature of 250 ºС . However, it can be reasonably assumed that in the case of long-term operation, it can be realized at much lower temperatures. Therefore, deformation aging can be considered as a generalized mechanism of operational degradation of steels at the first stage. It leads, on the one hand, to an increase in strength characteristics, and, on the other hand, to a decrease in plasticity characteristics, resistance to brittle fracture, and other properties associated with brittle fracture. The second stage of degradation is associated with the development of dissipated damages in a metal bulk, usually at the micro-scale. Such a mechanism of degradation is particularly dangerous in terms of a decrease in brittle fracture resistance and failure under the action of hydrogenating environments. In addition, in the case of a significant microdamaging, the effect of strain hardening may be levelled out according to the strength characteristics, and, regarding to elongation, the opposite effect may be observed – levelling off a decrease in plasticity according to this indicator due to the contribution of the opening of multiple defects during specimens` tensile testing. Therefore, elongation in some cases will not detect real ability to plastic deformation. The mechanism of development of dissipated damages is especially relevant for rolled steels, as shown by Nemchuk et al. (2019) and Nykyforchyn et al. (2019, 2020), since elongated texture fibres and non-metallic inclusions contribute to the development of delamination in the rolling direction. In this work, on the example of a number of long-term operated rolled carbon steels, the peculiarities of the implementation of the mechanism of development of dissipated damages and the associated effects of degradation of mechanical properties, including that

under the action of corrosive environments with hydrogenating properties, are analysed. 2. Operational damaging as the main factor of in-service degradation of rolled steels

The results of impact toughness tests of pipeline steels with different strength presented in Table showed a significant dependence of impact toughness values not only on the steel state (as-delivered of post-operated), but on the specimen orientation relative to the rolling direction (pipe axis) as well. The highest impact toughness values were obtained for longitudinal specimens regardless of the metal state. Transversal specimens are characterized by lower impact toughness values compared with longitudinal ones for both studied steels: for the as-delivered metal the difference is insignificant, while for operated one it can be 1.5 times less than the impact toughness of longitudinal specimens. This means that the anisotropy of brittle fracture resistance of rolled steels is more