PSI - Issue 51

Olha Zvirko et al. / Procedia Structural Integrity 51 (2023) 24–29

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O. Zvirko et al. / Structural Integrity Procedia 00 (2022) 000–000

fatigue strength characteristics (e. g., fatigue limit) changes to the opposite: they decrease during operation, as demonstrated by Kossakowski (2013), Beltrán-Zúñiga et al. (2022), Nykyforchyn and Zvirko (2022). It means that fatigue limit and toughness change in the same direction. Obviously, the factors that facilitate the development of damage in metal will accelerate the onset of stage II of the operational degradation of steels. These factors include hydrogen, which can be absorbed by the metal under its hydrogen charging as a result of interaction with aggressive environments. Its influence on the staging of the degradation of fatigue strength of steels is shown in Fig. 1 by a separate dependence, which illustrates a further shortening of degradation stage I. 3. Role of corrosive and hydrogenating environments in operational degradation of steels If we consider only the mechanical characteristics of steels to assess the operational degradation of steels, then corrosive environments will affect this process only due to their hydrogenating properties. In this case, possible in bulk hydrogen charging of metal and its degradation occur under the combined action of stresses and hydrogen. It should be also noted that hydrogen solubility in steels increases under cyclic loading conditions, as it was demonstrated for low alloy steel by Cabrini et al. (2019). The effect of hydrogen on the deformation aging (hardening) of steels during their operational degradation is obvious. The atomic hydrogen during the strain aging of metals is localized mainly in dislocation traps inside the grains, as demonstrated by Ebihara et al. (2020). Moreover, it was shown by Nagumo (2004) that stresses increase the ability of the metal to trap hydrogen. Therefore, a steel subjected to deformation aging traps more hydrogen as a result of a higher dislocation density. However, the most significant hydrogen effect should be expected on stage II taking into account the significant hydrogen impact on microdamage evolution. The above mentioned peculiarities of the hydrogen influence on the operational degradation of steels were discussed by Marushchak et al. (2019), Nykyforchyn et al. (2020), Zvirko et al. (2021), and they became the basis for the development of some related directions in this issue. Namely, Nykyforchyn et al. (2019a) proposed an express-method of in-laboratory degradation of steels based on the well-known method of deformation ageing by plastic deformation of specimens with their further heating up to 250 °С. The proposed procedure additionally includes preliminary (prior to plastic deformation) electrolytic hydrogen charging of specimens which intensifies metal degradation. Obviously, a similar procedure could be used also for the simulation of the degradation of steels operated under cyclic loading. Moreover, susceptibility of the operated pipeline steels to hydrogen assisted damaging is usually higher than that of the as-delivered steels, as highlighted by Zvirko et al. (2021, 2022). Such behaviour of steels can be associated with damaging or strain hardening under operation. Thus, an increase in susceptibility of steels to hydrogen embrittlement with increasing in prestrain values was reported by Han et al. (2019) and Park et al. (2019). Therefore, preliminary hydrogen charging of steels can be used for increasing sensitivity of plasticity characteristics to in-service degradation at assessment of their degradation degree, as it is schematically presented in Fig. 3. In this way, it is possible to reveal the significant difference between the properties of the materials in different states, that is, the sensitivity of estimation of the operational degradation increases.

Fig. 3. Scheme demonstrating an increase in sensitivity of plasticity characteristics to operational degradation using preliminary hydrogen charging.