PSI - Issue 59

Ivan Tsybailo et al. / Procedia Structural Integrity 59 (2024) 307–313 Ivan Tsybailo et al. / Structural Integrity Procedia 00 (2019) 000 – 000

311

5

An increase in the average size of ferrite grains in the steel structure of SZ of the bend was preceded by incomplete boundaries of smaller grains, the ends of which broke off inside the larger grains. These dangling grain boundaries were considered clear evidence of the process of merging adjacent grains with close crystallographic orientation due to collective recrystallization, in which larger grains absorb smaller ones, forming a coarse-grained structure. Since during long-term operation, the diffusion processes of alloying elements and carbon are accelerated, carbides accumulate and coagulate on some grain boundaries that are favourably oriented to external loads, while on others (less favourably oriented), they dissolve, which facilitates further dismantling of these boundaries. This was most obvious at the outer surface of the pipe in the SZ of the bend (Fig. 3). Since carbides also affect the cohesion between adjacent grains, their N d amount was analyzed depending on their size. Carbides of three categories were analyzed (diameter d < 1 µ m; 1 µm ≤ d ≤ 3 µm ; and d > 3 µm ). Their amounts were normalized by the area of the analyzed structure fragments. Carbides on the boundaries and in the body of the grains of the operated steel at three levels across the thickness of the pipe wall of the bend SZ were taken into account (Fig. 4). An ambiguous change in the number of carbides at the boundaries and in the body of grains along the thickness of the pipe wall was revealed. The maximum number of carbides of all three analyzed standard sizes (regardless of their location – inside or along grain boundaries) was observed near the outer surface of the pipe. Their minimum number was found in the centre of the section, and near the pipe's inner surface, they occupied an intermediate position. Their distribution across the thickness of the pipe wall correlates with the distribution of stresses in the pipe wall caused by thermal stresses that periodically arise, in particular, as a result of shutdowns of power units ( Krechkovs’ka et al. (2019), Krechkovska et al. (2023). Their minimum value is reached inside the pipe wall, and the maximum – is near its outer surface. This adds arguments in favour of the influence of operational factors on the redistribution of carbides in the steel structure. It is also important to note that, generally, the number of carbides of all sizes and at all levels of analysis inside the grains is significantly less than along the grain boundaries (Fig. 4a, b). This is a direct sign of redistribution of carbides at grain boundaries.

Fig. 4. The number of N d carbides of three sizes of types (normalized by the analyzed area) detected (a) inside the grains and (b) along their boundaries in the structure of the 12Kh1MF steel of the stretched zone of the bend near the outer (1) and inner (3) surfaces and in the center of the pipe section (2). At all analysis levels, the number of small carbides (d < 1 μm) inside the grains was the largest . Large ones (d > 3 µm) were the smallest (Fig. 4a). While the majority of carbides along the grain boundaries corresponded to the range of their average sizes (1 µm ≤ d ≤ 3 µm) . Carbides larger than 3 µm were mainly located at triple junctions of grains, so their number was not the highest (Fig. 4b). In any case, the most significant increase in the number of carbides along the grain boundaries was considered responsible for the decrease in cohesion between adjacent grains. As a consequence, despite saving practically almost unchanged ultimate strength of steel at all analysis levels along the wall thickness of the SZ of pipe bend, a significant decrease in the characteristics of ductility and resistance to brittle fracture of 12Kh1MF steel was revealed near its outer surface (Fig. 5).