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

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Fig . 5. Mechanical properties of 12Х1МФ steel after ~28.6  10 4 h of operation in the SZ bend of the main pipeline steam of a thermal power plant, determined by tension (ultimate strength  UTS and yield strength  YS , elongation and reduction of area) and impact test (KCU) with using of the tangential orientation specimens. Hence, the number of carbides along the grain boundaries and their sizes should be attributed to the determining structural indicators of the condition of the operated steel. The established dependences of their changes in the thickness of the pipe wall cross-section can open up opportunities for non-destructive assessment of the current technical condition of the heat-resistant steels long-term operated on the TPP steam pipeline with corresponding assessments of these indicators on the outer surface of the pipe. 4. Summary The change in the structural parameters of heat-resistant steel 12X1MF after the long-term operation (~28.6  10 4 h) in the section of the stretched zone of the bend was analyzed, namely, sizes of grains and carbides and their number in the body and along grain boundaries. It is shown that, regardless of the size of grains and carbides, their number at grain boundaries is the largest near the outer surface of the pipe. This, firstly, creates favorable conditions for creep, which occurs more easily in large-sized grains. Secondly, it reduces the cohesion of adjacent grains weakened by creep defects formed at the juncti ons of three grains on large carbides (d > 3 μm) and chains of medium- sized carbides (1 μm ≤ d ≤ 3 μm). The most significant carbide increase along the grain boundaries was responsible for decreased cohesion between adjacent grains. Consequently, despite preserving almost unchanged steel strength values at all levels of analysis along the wall thickness of the SZ of pipe bend near its outer surface, a significant decrease in the characteristics of ductility and resistance to brittle fracture of steel was revealed. References Balyts’kyi, О. I., Ripei, I. V., Protsakh, Kh., 2006. A. Reliability of Steam Pipelines of Thermal Power Plants in Long - Term Operation . Materials Science 42 (4 ), 461–465. Ва litskii, А. І. , Panasyuk, V. V., 2009. Workability assessment of structural steels of power plant units in hydrogen environment . Strength of Materials 41(1 ), 52–57. Baltušnikas, A., Levinskas, R., Lukoštūtė , I., 2007. Kinetics of Carbide Formation During Ageing of Pearlitic 12X1MФ Steel. Materials Science 13 (4), 286 – 292. Baltušnikas, A., Lukošiūtė, I., Levinskas, R., 2010. Transformation kinetics of M 23 C 6 carbide lattice parameter in low alloyed steels. Materials Science (Medžiagotyra) 16(4), 320 – 323. Ćwiek, J., Łabanowski, J., Topolska, S. , 2011. The effect of long-term service at elevated temperatures on the structure and mechanical properties of Cr-Mo-V steel Archives of Materials Science and Engineering 49 (1), 33 – 39. Dzioba , I. R., 2010. Properties of 13KhMF steel after operation and degradation under laboratory conditions. Materials Science 46 (3) , 357–364. Dobrzanski , J., Zielinski , A., Krtzon, H., 2007. Mechanical Properties and Structure of the Cr - Mo - V Low - alloyed Steel After Long - term Service in Creep Condition, Journal of Achievements in Materials and Manufacturing Engineering, 23, International OSCO World Press .