PSI - Issue 72

Halyna Krechkovska et al. / Procedia Structural Integrity 72 (2025) 149–156

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1. Introduction Long-term heat-resistant steels operated in structural elements of thermal power equipment are usually subjected to the complex effects of high mechanical and thermal loads, aggressive and hydrogenative environments (high temperature water, superheated steam, and steam-water mixtures) (Nykyforchyn (2010)). As a result of this, the strength, ductility, and resistance to brittle fracture of low-alloy heat-resistant steels decreases (Tsybailo (2024), Hodžić (2010, 2011), Hu 2008)). The reduction in the mechanical properties of steel is due to structural transformations, such as the dissolution of carbides within the grains and their precipitation and coagulation at their boundaries, as well as a noticeable increase in ferrite grain size (Tsybailo (12023), Dobrzanski (2007)). Heat treatment makes it possible to effectively restore the microstructure and improve the mechanical properties of long-term operated low-alloy steels (Golanski 2007, 2008), Student (12021)). However, a certain number of large pores remain in the steel structure even after heat treatment ( Tsybailo ( 2 2023)) . The structural state of the steel of steam pipeline bends at the beginning of their operation, which depends on the heat treatment mode of the bends after their deformation under factory conditions, also significantly affects their properties (Rožnovská (2021)). Moreover, the susceptibility of steels to the negative effects of hydrogen depends on the amount of damage left after RHT. This damage reduces the resistance to brittle fracture of the bridges between adjacent defects thus promoting fracture. During long-term operation hydrogenation of the metal of steam pipes occurs. The grain and carbide sizes are among the main structural factors determining the steel's ability to hydrogenation (Ваlitskii (2009), Krechkovs’ka (2016), Tsybailo (32023)). Long-term operation of steam pipelines is associated with inevitable planned and unplanned shutdowns of the technological process, leading to significant thermal stresses in the thick-walled components (Nykyforchyn (2010)). Modelling the effect of block shutdowns by thermal cycling of samples in hydrogen accelerates structural transformations in steels, promoting the diffusion redistribution of carbon and alloying elements to grain boundaries with subsequent precipitation and coagulation of complex-alloyed carbides shown by Student (1997, 1998). In addition, even during steady-state operation, significant fluctuations in the temperature and steam pressure. Although these fluctuations remain within the limits of regulated requirements, their large number (gigacycles) contributes to the loss of cohesion of inclusions to the matrix, leading to the formation of voids (Smiyan (2021)). Due to the discrepancy between the mechanical (elastic modulus) and physical-thermal (coefficient of thermal expansion) properties of carbides and the surrounding matrix, the bond between them gradually weakens. Defects in the form of nano-sized voids are formed, which become traps for hydrogen dissolved in the metal. Hydrogen captured by such traps creates pressure in them, which promotes the final decohesion of inclusion from the matrix (Zvirko (2024)). Decogesion of carbides from the surrounding matrix also promotes the coalescence of microvoids along grain boundaries with the formation of intergranular cracks which is shown in studies by Nykyforchyn (2010), Otsuka (2007), and Qin (2017). Moreover, such structural transformations during the long-term operation of heat-resistant steels at high temperatures in a hydrogenating environment worsen their mechanical properties (Student (22021)). Under operating conditions, hydrogen can form segregations that are unevenly distributed along grain boundaries, or even and also chemically interact with carbon. As a result, microvoids filled with methane under high pressure can occur, which also negatively affects the mechanical properties of steel (Saba (2003), Louthan (2008)). All of the above processes (including hydrogenation) contribute to the accumulation of damage and thereby facilitate the manifestation of high-temperature creep of steel (Babii (2007)). The practice of operating main steam pipes and the experience of studying the mechanical properties of heat resistant steels in operation indicate that the weakest links in the entire system of pipelines are pipe bends (Tsybailo (12023)) and welded joints (Student (2006), Nykyforchyn (2007)). This is due to the creation of the most favourable conditions for steel degradation in these sections of steam pipelines due to the presence of a significant stress gradient, temperature and pressure fluctuations, heterogeneity of chemical composition, the presence of technological stress concentrators, more favourable conditions for intensive hydrogenation, etc (Nykyforchyn (2010) Vorobel (2024), Materials (2023)). To assess the applicability of RHT for restoring the structure and mechanical properties of used steels, it is necessary to focus on the study of most damaged metal elements. If the effectiveness of restoration is demonstrated on the steel of most sections of the steam pipeline, then the application of this approach will be even more successful in restoring steel used in less critical sections. Therefore, in this work, a mode of RHT is proposed, its effectiveness for restoring mechanical properties of 12Kh1MF steel after long-term operation in the stretched bend zone of the main steam pipeline at a TPP is analyzed, and the sensitivity to hydrogen embrittlement is assessed.