PSI - Issue 36

ScienceDirect Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2021) 000 – 000 Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2021) 000 – 000 Available online at www.sciencedirect.com

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Procedia Structural Integrity 36 (2022) 43–50

1st Virtual International Conference “In service Damage of Materials: Diagnostics and Prediction” Predicting the state of heat-resistant steel of a steam pipeline at a TPP taking into account changes in its strength due to operational damage Halyna Krechkovska a,b, *, Viktor Sylovanyuk a , Oleksandra Student a , Andriy Chornenkyi a a Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 5, Naukova St., Lviv 79060, Ukraine b Polytechnic National University, 12, Stepana Bandery St, Lviv 79000, Ukraine Abstract The microstructure of heat-resistant 15Kh1M1F steel is analysed in the initial state and after long-term operation on the main steam pipelines of TPPs. According to the criteria adopted in fracture mechanics, the low volumetric content of fine carbides, which did not exceed 0.5% in the structure of the analysed steel in the initial state, does not significantly affect its strength. However, as a result of long-term high-temperature operation, a redistribution of carbon and alloying elements occurred in its structure with the precipitation and coagulation of carbides along the grain boundaries. Large carbides concentrated along the grain boundaries have a low cohesion with the matrix, which promotes the formation of pores along their interfaces. As a result, the cohesion of adjacent grains decreases, which contributes to the formation of a system of intergranular microcracks. Merging, they form a macrocrack, which has a significant effect on the strength of the material. A mathematical model is proposed for predicting changes in the ultimate strength of steel, based on the well-known solution of the problem with tension of a body with a periodic system of collinear cracks. The relationship between the intensity of tensile forces, the distance between the cracks, their size and the size of the pre-fracture zones in the vicinity of operational defects is found. The beginning of the merging of the pre-fracture zones at the tips of the nearest defects was taken as the condition for material destruction. A dependence has been obtained that makes it possible to make predictive estimates of a decrease in the ultimate strength of long-term operation of steel on steam pipelines of different units, due to its additional damage under the influence of shutdown numbers of TPP units corresponding to this steel. To do this, it is only necessary to calculate the fraction of the steel resource loss according to normalized technological indicators, such as the operation lifetime and the number of shutdowns of the units. 1st Virtual International Conference “In service Damage of Materials: Diagnostics and Prediction” Predicting the state of heat-resistant steel of a steam pipeline at a TPP taki into cc unt change i it strength due to operation l damage Halyna Krechkovska a,b, *, Viktor Sylovanyuk a , Oleksandra Student a , Andriy Chornenkyi a a Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 5, Naukova St., Lviv 79060, Ukraine b Polytechnic National University, 12, Stepana Bandery St, Lviv 79000, Ukraine Abstract The mi rostructure of heat-resistant 15Kh1M1F steel is analysed in the initial state and after long-term operation on the main steam pipelines of TPPs. According to the criteria adopted in fracture mechanics, the low volumetric content of fine carbides, w ich did not exceed 0.5% in the structure of the analysed st el in the initial state, does not si nificantly affect its strength. However, as a result of long-term high-temperature operatio , a redistributio of carbon and alloying elements occurred in its stru ture with the precipitatio and coag lati n of carbides along the gra boundaries. Large carb des concentra ed along t e grain boundaries have a w cohesion with th matrix, which promote the formation of pores along their int rfaces. As a result, the oh sion of adjac nt gra ns decreases, which contri ut to the formatio of a syst m of intergranular microcr cks. Mer ing, they form a macrocrack, whi h a a significant effect on the st ength of the material. A mat matical mod l is propos d for predicting changes in the ultimate strength of steel, bas d on the well-known solution of t pr blem with tension of a body with a periodic system of collinear cracks. The relationship between the intensity of tensile forces, the distance between the cracks, their size and the size of the pre-fracture zones in the vicinity of operational defects is found. The beginning f the merging of the pre-fracture zones at the tips of the nearest defects was taken as the condition for material destruction. A dependence has been obtained that makes it possible to make predictive estimates of a decrease in the ultimate strength f long-term operation of steel on steam pipelines of different units, due to its additional damage under the influence of shutdown numbers of TPP units corresponding to this steel. To do this, it is only necessary to calculate the fraction of the steel resource loss according to normalized technological indicators, such as the operation lifetime and the nu ber of shutdowns of the units. © 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the conference Guest Editors

* Corresponding author. Tel.: +380-67-81-12-543; E-mail address: krechkovskahalyna@gmail.com * Corresponding author. Tel.: +380-67-81-12-543; E-mail address: krechkovskahalyna@gmail.com

2452-3216 © 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the conference Guest Editors 10.1016/j.prostr.2022.01.001 2452-3216 © 2022 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the conference Guest Editors 2452-3216 © 2022 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the conference Guest Editors