PSI - Issue 36

Halyna Krechkovska et al. / Procedia Structural Integrity 36 (2022) 43–50 Halyna Krechkovska, Viktor Sylovanyuk, Oleksandra Student et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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Baltušnikas 2 et al. (2010)). And if in the initial state they, as substitution elements, were dissolved in a solid solution of iron, then after operation, due to the diffusion redistribution of elements, their content in the solid solution decreased, and complex alloyed carbides precipitated along the grain boundaries. The increase in the number of shutdowns of the technological process with the inevitable cooling of the pipeline system can significantly intensify the structural changes in the metal (Nykyforchyn et al. (2010), Krechkovs'ka 4 (2008)). In general, the maneuvering mode of operation of steam pipelines accelerates the degradation of steels, which is manifested by the loss of the cohesion of large carbides at the grain boundaries with the matrix. As a result, the scattered damage is formed at the grain boundaries. These defects along the grain boundaries become traps for hydrogen dissolved in the long-term operated steel. Getting into these traps and molizing, hydrogen creates pressure in them, which intensifies the separation of carbides from the matrix and, thereby, facilitates the merging of micro cavities along the grain boundaries. In other words, hydrogen promotes intergranular micro cracking by coalescing adjacent micro pores. It is considered that at this stage the guaranteed safety of the equipment operation is completed. The described processes depend on a number of factors: the size and shape of carbides, density of their location, inconsistency between the temperature expansion coefficients of carbides and matrix, the gradient of thermal stresses in the pipe wall during shutdowns, and the hydrogen content in the degraded metal. All these factors act selectively, and in the most favourable places for metal degradation. As a result, in spite of the deceptive preservation of the metal integrity at the macro level, in many local places there is a merging of micropores and intergranular cracking.

1,5

1

2 3

I

II

1,0

III

d av , m m

0,5

0,0

II

III

I

Fig. 2. Average size d av of carbides in 15Kh1M1F steel, measured at different levels along the wall thickness of pipes in the initial state (1) and after  2  10 5 hours of operation on the main steam pipelines of units No. 1 with 501 shutdowns (2) and No. 2 with 576 shutdowns ( 3). The d i values were measured near the outer (I) and inner (III) surfaces and at the canter of the cross-sections (II) of each of the pipes. The layout of the measurement points is shown in the figure. Consequently, carbides concentrated along the grain boundaries have a low bond with the surrounding matrix (due to a mismatch in both the type and size of their crystal lattices which contributes to the formation of pores around them (Strang et al. (1996)). This further reduces the cohesion of adjacent grains and initiates the formation of a system of intergranular micro cracks, which coalesce and form a macro crack (Nykyforchyn et al. (2010)). Such transformations can certainly have a significant effect on the strength of the material. A mathematical model for predicting changes in the ultimate strength of steel was built on the basis of an analysis of the results obtained in this work and known from the literature (Murakami (1989 1 , 2002 2 )). In the initial state, the structure of the studied steel was characterised by uniform distribution of finely dispersed carbides. Their quite strong connection with the matrix provided the steel a sufficiently high strength and ductility values. As a result of long-term high-temperature operation of steel on steam pipelines along grain boundaries, which are thermodynamically favourable places for precipitation of carbides, their precipitation and then coagulation began. The discrepancy between the type and size of the crystal lattice of carbides (or non-metallic inclusions) with the matrix promoted the decohesion with formation and accumulation of defects along their interfaces. Metallographic studies of both types of operating steel confirmed the growth of carbide dimensions in the entire pipe section (Fig. 2). In particular, in the vicinity of the outer surface of the pipes in the most degraded version of steel (after the greatest number of block shutdowns), the dimensions of the carbides increased by more than 5 times compared with that fixed in the initial state of steel. This led to a decrease in their cohesion with the matrix and facilitated the