PSI - Issue 36

Halyna Krechkovska et al. / Procedia Structural Integrity 36 (2022) 43–50 45 Halyna Krechkovska, Viktor Sylovanyuk, Oleksandra Student et al./ Structural Integrity Procedia 00 (2021) 000 – 000 3 consequence, the time before the initiation of cracks in it and the overall lifetime of the structural element are reduced. 2. Materials and methods The technical state of heat-resistant 15Kh1M1F steel in the initial state (unexploited pipes from storage) and after long-term operation on the main steam pipelines of the TPP is analysed. The pipes with a diameter of 325 mm and a wall thickness of 60 mm were used. These pipes were preliminarily operated at temperatures up to 540 °C and pressures up to 24 MPa. To substantiate the choice of metal for the study, data sets on operating parameters (term of operation and the number of planned and forced shutdowns of the technological process) were analysed for six TPP units. The metal from two blocks with practically the same service life (  op  2  10 5 h), but with the greatest difference in the number of shutdowns of the technological process (due to the use of shunting mode of operation), including the number of forced shutdowns (due to abnormal situations) was selected. The total number of shutdowns of one of the analysed blocks was 501 (block No. 1), and the second one was 576 (block No.2). For research, specimens of axial orientation were cut out near the outer (I) and inner (III) surfaces of all pipes and in the centre of their cross-sections (II). The strength characteristics  UTS ,  YS of the studied steel variants were determined from the deformation diagrams of smooth cylindrical specimens with a diameter of 5 mm, obtained on a tensile testing machine UME-10T at a deformation rate of 3  10 – 3 s – 1 . For metallographic studies of the microstructure of steel, an EVO-40 XVP scanning electron microscope was used. As a quantitative structural characteristic, we used the average values of the sizes of carbides d av , estimated from the results of the analysis of at least ten digital images. 3. Results and discussion The microstructures of steel in the initial state and after long-term operation at the main steam pipelines of TPPs were analysed. The volumetric content of carbides in the unexploited steel did not exceed 0.5%. They were small carbides (their sizes did not exceed 0.05 µm) almost uniformly distributed in the steel structure (Fig. 1a). According to the fracture criter ia accepted in fracture mechanics, such isolated and, at the same time, small inclusions cannot have a significant effect on the strength of steels. Diffusion redistribution of carbon and alloying elements and their deposition in the form of carbides with subsequent coagulation along the grain boundaries occurred during long-term high-temperature operation in heat resistant steel (Fig. 1b, c). Moreover, regardless of the steel structure in the initial state (ferrite-pearlite or bainite-ferrite types), a common sign of their degradation is the relocation of carbides to the grain boundaries. As a result, the ferrite grains with carbides along their boundaries prevail in the structure of operated steel variants (Robson et al. (1996 1 , 1997 2 ), Baltušnikas et al. (2007), Student 4 (1998), Nykyforchyn et al. (2010), Ostash et al. (2009), Dzioba (2010)).

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b

c

Fig. 1. The microstructure of the 15Kh1M1F steel in the initial state ( a ) and after  2  10 5 h of operation on main steam pipelines of units No. 1 with 501 shutdowns ( b ) and No. 2 with 576 ones ( c ) during the entire period of their operation.

Analysis of the structure of 15Kh1M1F steel in the diametrical section of the operated pipes showed a significant gradient of carbide sizes along their wall thickness (Fig. 2). Accumulations of carbides were mainly observed along grain boundaries or large non-metallic inclusions. According to the results of micro-X-ray spectral analysis, these carbides had an increased content of molybdenum and chromium, which is typical for this class of steels (Pietryka et al. (2010),