PSI - Issue 72

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

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and therefore the most evident signs of steel degradation are observed. As for the plasticity characteristics (elongation and RA) of steel, after its restoration, the gradient of both these characteristics across the pipe wall thickness was significantly more pronounced than that observed for the strength characteristics (Fig. 1). The maximum increase in both indicators after RHT was observed near the outer surface of the pipe. In particular, elongation increased by 50%, and RA by 60% relative to the corresponding values, which are typical for the exploited steel. The effect of RHT on the elongation and RA of the metal from the central part of the pipe wall thickness and located near its inner surface was significantly less, reaching 28% and 38%, respectively. Thus, the effectiveness of applying the optimal RHT mode for restoring the strength and plasticity characteristics of the steel, which is of crucial importance for improving its creep resistance, was confirmed. This is a necessary precondition for extending the service life of long-operated steam pipeline elements. Therefore, this is a strong argument in favor of using RHT to extend the service life of steel in the bends of main steam pipelines at thermal power plants.

Fig. 1. Change in λ index, which characterizes the change in the mechanical properties of thermally restored 12Kh1MF steel, c ut from different levels along the pipe wall thickness (I and III – near the outer and inner surfaces of the pipe; II – in the center of its wall thickness), determined relative to corresponding properties of this steel after 2.86∙10 5 h of operation in the stretched bend zone of a thermal power plant steam pipeline. The change in mechanical characteristics of exploited and restored steel under the additional influence of hydrogenation. Since the long-term (tens of years) operation of TPP steam pipelines does not provide grounds to exclude the possibility of steel hydrogenation, the study of its influence as a factor of embrittlement on the mechanical properties of steels remains relevant. Therefore, it is important to consider the issue of stability of the obtained positive effect of RHT on the strength and plasticity characteristics of the restored 12Kh1MF steel after its additional hydrogenation. The mechanical properties of both hydrogenated steel variants (operated steel and after RHT), determined near samples from the vicinity of the outer and inner surfaces of the pipe, are presented in Table 2. Their comparison showed that the strength and plasticity characteristics were higher for the restored steel than for the steel after its long-term operation (regardless of the sample location). Table 2. Mechanical properties of 12Kh1MF obtained after tensile testing of pre-hydrogenated samples Sample placement in the pipe wall section State of steel  UTS , MPa  YS, MPa Elongation, % RA, %

Operated

405 480 424 491

275 309 285 325

11,4 19,2 13,9

23,6 55,3 31,1

Near the outer surface (I)

RHT

Operated

Near the inner surface (III)

RHT 60,6 To assess the effect of hydrogenation on the condition of operated and after RHT steel, the coefficient λ was used. It characterizes the relative change in each of the mechanical characteristics during tensile testing of samples in the air but after their preliminary electrolytic hydrogenation. Formula 2 was used for operated steel, which made it possible to determine the effect of additional hydrogenation, which cannot be excluded during the long-term operation 20