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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in its structure. Consequently, diffusion processes predetermine the homogenization of a solid solution of carbon in austenite, the uniformity of the distribution of alloying elements in the matrix, and the dimensions of the structural components in steel. Since the degree of completion of such transformations in austenite depends significantly on the duration of holding the samples at high temperatures, it is possible to study its effect on structural transformations in steel by varying the duration of holding. Previously, the effect of holding time of samples at a high austenitization temperature at the first stage of RHT on structural transformations in steel was investigated (Tsybailo (2024)). Considering the obtained results, the steel in use was selected for further research, which, at the first stage of restoration, was treated at a temperature of 1100 °C for 150 minutes since this duration is recommended as optimal. After the first stage of the RHT, the samples were processed according to the mode recommended for 12Kh1MF steel in the initial state (TU 14-3-460-2009, Ukraine). Therefore, in the second stage of RHT, the samples were normalized from a lower temperature of 960 °C with a holding time of τ = 30 minutes and then tempered at a temperature of 740 °C with a holding of τ = 180 minutes. The scheme presented in Figure 1 gives a general idea of the complete RHT process.

Fig. 1. The optimal mode of restorative heat treatment of used 12Kh1MF steel

Diametrically oriented templates with a thickness of 12 mm, cut from the SBZ of the steam pipeline, where the most favourable force conditions for steel degradation during operation occur, were subjected to heat treatment ( Student et al. (2018)). The structural state of the steel after operation and after RHT was assessed by grain size. The steel structure was analyzed at three levels along the pipe wall thickness in the SBZ (near the outer and inner surfaces and in the center of the pipe cross-section) using images from an EVO 40XVP scanning electron microscope (SEM). The following mechanical properties were used to certify steel after operation and after RHT: impact toughness, strength characteristics (ultimate tensile strength σ UTS and yield strength σ YS ), and plasticity ones (elongation and reduction of area). The tensile mechanical properties were determined in air on cylindrical samples with a diameter of 5 mm and five times the length of their working part. To determine the impact toughness, samples (10×10×55 mm) of the Menage type with a U-shaped notch with a depth of 2 mm and a radius of 1 mm were used. Samples for mechanical testing were made from transverse templates in the operating state or after additional RHT. They were placed at three levels along the pipe wall thickness in the SBZ (near the outer and inner surfaces of the pipe and in the center of its cross-section). Tensile tests were performed on an UME-10T testing machine at a sample deformation rate of 10 –3 s –1 . An IO-5003 pendulum impact tester was used for impact tests. 3. Results and discussion Changes in the structure of steel after restorative heat treatment. The structure of 12Kh1MF steel after operation in the SBZ of a steam pipeline and subsequent RHT was analyzed. Based on size, all analyzed grains in the structure of both steel variants were conditionally divided into three categories: small (< 10 μm), medium (from 10 to 30 μm), and large (> 30 μm). The number of grains of each size category was determined by averaging the grain sizes measured at 20 points at each level of the pipe wall cross-section (Fig.2). The number of grains of each of the three size categories (about their total number in the analyzed images of the steel structure in operation) varied depending on the level of pipe wall thickness at which the analysis was carried out (Fig. 2, columns I). In particular, the proportion of small and medium grains gradually increased from the outer to the pipe's inner surface. The number of medium-sized grains was smaller near the outer surface (44%) and largest near