PSI - Issue 68

Halyna Krechkovska et al. / Procedia Structural Integrity 68 (2025) 762–768 Halyna Krechkovska et al. / Structural Integrity Procedia 00 (2025) 000–000

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pipe wall thickness (Fig. 3a, b, columns I), there is still a slight tendency for them to decrease towards the outer surface of the pipe. Applying the proposed RHT mode increased both strength indicators of the exploited steel (Fig. 3a, b, columns II). A similar increase was noted in all three analyzed zones along the pipe wall thickness. The maximal increase was observed in the samples cut near the outer surface of the pipe and reached 18.5% for σ UTS and 11% for σ YS , respectively. Thus, the application of the RHT mode demonstrated its effectiveness in restoring both strength indicators of 12Kh1MF steel, which can serve as a basis for extending its service life in critical elements of the main steam pipelines of the TPP. As for the plasticity characteristics (elongation and RA) of the long-term operated 12Kh1MF steel, the gradient of both indicators across the pipe wall thickness was even more obvious (Fig. 3c, d, columns I). Their minimum values were recorded for steel from the vicinity of the outer surface of the pipe. Its elongation was 20%, and the RA index was 22% lower than the maximum value of these characteristics in the entire section of the pipe. It was believed that the noticeable decrease in plasticity characteristics of operated steel near the outer surface of the pipe is due to its more intensive operational damage caused by creep, which is typical for this section of the pipe cross-section in the zone of tensile bending. The application of RHT to the exploited steel generally increased its plasticity characteristics (Fig. 3c, d, columns II). The maximum recovery effect was observed near the outer surface of the pipe, where its positive effect on both steel ductility characteristics exceeded 30%. A decrease in the gradient of the plasticity characteristics across the pipe wall thickness after RHT was also noted. Based on the obtained results, it can be concluded that the application of RHT enables to significantly eliminate the negative consequences of long-term operational degradation of 12Kh1MF steel. This gives grounds to propose this processing mode as an effective way to enhance the properties of steel, which is important for extending the service life of long-term operated elements of steam pipelines.

Fig. 3. Mechanical properties of 12Kh1MF steel after 2.86∙10 5 h of operation in the stretched bending zone of a thermal power plant steam pipeline (I) and after subsequent restorative heat treatment according to the optimal mode (II), determined on tangential samples cut at different levels along the thickness of the pipe wall (1 and 3 – near the outer and inner surfaces of the pipe; 2 – in the center of its cross-section): (a) ultimate tensile strength σ UTS and (b) yield strength σ YS , (c) Elongation (d) and RA and (d) impact toughness KCU.