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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the inner surface (53%). In the center of the pipe section, the proportion of grains sized between 10 and 30 μm reached 49%. As for large grains, their quantity varied unevenly across the pipe wall thickness. Their highest percentage content was near the outer surface of the pipe (48%) compared to the minimum and average values in the center of its cross-section (39%) and near its inner surface (44%), respectively. Such an ambiguous change in the proportion of grains larger than 30 μm across the pipe wall thickness indicates that these grains are responsible for the increase in the steel's tendency of the steel to creep after its long-term operation. After all, the most favorable conditions for creep are realized near the outer surface of the pipe, where the proportion of such grains is maximum. As grain size increases, the power of dislocation pileups at grain boundaries increases, which facilitates their penetration into neighboring grains. After RHT, the proportion of small grains increased, while the proportion of medium and large grains decreased throughout the entire thickness of the pipe wall (Fig. 2, columns II). The proportion of grains with the smallest size (D < 10 μm) was 55%, 56%, and 51% near the outer and inner surfaces and in the center of the pipe cross-section, respectively. Near the outer and inner surfaces of the pipe, the proportion of medium-sized grains (10 μm < D < 30 μm) after RHT decreased to 36% and 35%, respectively and in the center of the pipe's cross-section, its value was slightly higher (39%). The number of large grains remained almost unchanged throughout the entire pipe wall thickness, ranging from 9% to 10%. It was assumed that a significant increase in the number of small grains throughout the entire pipe wall thickness after RHT should have a positive effect on the mechanical properties of the restored steel.

Fig. 2. Relative quantity N of grains with different sizes (1 – D < 10 μm, 2 – 10 < D < 30 μm, 3 – D > 30 μm) in the structure of 12Kh1MF steel from stretched bending zone of a pipe after its long-term operation (I) and restorative heat treatment according to the optimal mode (II), determined near (a) the outer and (c) the inner surfaces of the pipe, as well as (b) in the center of its cross-section. The main structural indicator of the condition of the restored steel was considered to be the number of small grains. After the restoration of the steel in use under optimal conditions, the proportion of small grains increased, and a significant increase in hardness was expected. Thus, after the operation, the hardness in the center of the pipe cross section did not exceed 130 HB, and near both surfaces, it was even lower (120 HB) (Tsybailo et al. (2023). Such hardness did not satisfy the requirements of the industry regulatory document (TU 14-3-460-2009, Ukraine). However, the hardness of the same exploited steel after its restoration according to the optimal RHT mode increased to 170 HB, which indicates the complete restoration of its workability according to this criterion. Both grain sizes and hardness values can be directly measured on the outer surface of pipes during scheduled inspections of steam pipelines. This creates the opportunity, firstly, to control the current technical condition of heat resistant steels after various periods of their operation in TPP steam pipelines using, for this purpose, a nearly non destructive testing method. Secondly, such control will enable ranking various elements of steam pipelines according to the degree of their degradation, and, on this basis, justifying the selection of elements requiring immediate restoration of operability by the proposed RHT mode. Thirdly, it becomes possible to monitor the efficiency of using the proposed RHT mode during its pilot industrial testing using inductors with industrial frequency currents (200 Hz) for heating pipes. Finally, monitoring the change in these parameters of restored steel during its further use will enable to study their stability, which is fundamental importance for substantiating the legality of using RHT in general. Effect of RHT on the strength and plasticity characteristics of the serviced steel. The results of tensile tests of the serviced steel showed that although both strength characteristics (σ UTS and σ YS ) changed insignificantly across the