PSI - Issue 59

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

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Ivan Tsybailo et al. / Structural Integrity Procedia 00 (2019) 000 – 000

structure, the nucleation of voids can occur earlier than in steels with a ferrite-pearlite or ferrite-bainite structure (Dzioba (2010)). In the SZ bend, pearlite areas disappear the fastest, forming a ferrite-carbide structure. As a result, the structure changes intensively in this zone; carbides are precipitated the fastest (especially along the grain boundaries), and voids are formed at the carbide – matrix separation boundary. Figure 2 shows the structure of the operated steel, in which ferrite grains with carbides inside the grains (up to 0.5 μm) and along their boundaries (2...3 μm) prevailed (Fig. 2).

Fig. 2. Microstructure of 12Х1МФ steel in the stretched zone of the pipeline bend near (a) the outer and (c) inner (b) surface of the pipe and at the centre of its section after ~28.6  10 4 h operation.

The sizes of grains and carbides were analysed at three levels across the pipe wall thickness in the SZ of the bend. Figure 3 shows the ratio (as a percentage) of the number of grains of three standard sizes in the structure of the operated steel. The database of grain size measurements at each level across the pipe wall included up to 20 different analysis locations with a total area of up to 1 mm 2 . According to the data obtained, the percentage of small (<10 µm), medium (from 10 to 30 µm) , and large (>30 µm) grains varied unequally across the pipe wall thickness in SZ. In particular, the percentage of small and medium-sized grains gradually increased from the pipe's outer to the inner surface. At the same time, the number of large grains across the pipe wall thickness changed ambiguously. Their percentage (48%) was greatest at the outer surface of the pipe compared to the centre (39%) and its inner surface (44%). At the same time, the number of medium-sized grains remained the lowest on the outer surface of the pipe (44%) and the highest (53%) on its inner surface. While in the centre of the pipe section, the number of grains ranging in size from 10 to 30 microns reached 49%. The resulting ambiguous change in the maximum grain size (D > 30 μm) along the pipe wall's thickness indicates that these grains participate in creep processes that dominate in the vicinity of the outer surface of the pipe. After all, the larger the grain size, the easier the movement of dislocations occurs. With increasing grain size, a power accumulation of dislocation increases at grain boundaries, facilitating their transition to the neighbouring grain.

Fig. 3. The number of grains N D (in percentage terms) in the structure of 12Kh1MF steel in the stretched zone of the bend near the outer (1) and inner (3) surfaces and at the centre of the pipe section (2).