PSI - Issue 36

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

44 © 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 Keywords: main gas pipe, degradation; fractographic features Halyna Krechkovska et al. / Procedia Structural Integrity 36 (2022) 43–50

1. Introduction The problem of assessing the technical state of the metal with the subsequent determination of the operability of structures is one of the top priorities for the thermal power industry due to large-scale depreciation of equipment. The main steam pipeline of TPPs, through which superheated steam is supplied to the turbine, are especially dangerous structural elements, the damage of which primarily threaten personnel and is accompanied by significant financial losses. The key factors affecting the workability of such elements include a hydrogenating environment, high temperature and load under operating conditions (steam temperature reaches 540 C, and pressure up to 24 MPa), as well as shout-downs of the technological process due to emergency situations or the use of a shunting mode of operation of the units. The practice of high-temperature operation of elements of heat-and-power equipment indicates that heat-resistant steels degrade and lose the functional properties that ensured their serviceability at the beginning of operation (Student 1 et al. (2012), Zvirko et al. (2021), Yasniy 1 et al. (2011), Student 2 et al. (2012)). The long-term high temperature operation of this steels under high-frequency, but low-amplitude cyclic loads with a high load ratio (due to the static component of loading) intensifies this process and as a result their characteristics change. In particular, their strength (Maruschak 1 et al. (2021)) and ductility characteristics, creep resistance, especially in a hydrogenative environment Babii et al. (2007), Sidorenko et al. (1978), Ваlitskii et al. (2009), Krechkovska 1 (2016), Yasniy 2 et al. (2011), Yasniy 3 et al. (2013), and, to the greatest extent, brittle fracture characteristics such as impact toughness ( Krechkovs’ka 2 et al. (2017), Hredil et al. (2019)), fracture toughness and resistance to fatigue crack growth (Dzioba (2010), Romaniv et al. (1998), Andreikiv 1 et al. (2017), Lesiuk 1 et al. 2020)) are generally reduced . At the same time, it was shown by Krechkovs’ka 3 et al. (2019) that repeated heating or cooling of the pipeline system during start-ups or shutdowns of the units contributes to the occurrence of thermal stresses in the pipes. Their combined action with stresses from the internal vapor pressure inside the pipes can sometimes lead to critical stresses in their walls, which contributes to the formation and propagation of cracks in them. And the harmful effect of hydrogen absorbed by the metal additionally contributes to the growth of cracks to a critical length. Ultimately, this leads to degradation of structural elements due to the propagation of cracks in them until to their final destruction. However, this relatively rapid stage of degradation is preceded by a longer one. This stage is not necessarily associated with the formation of macrodefects or cracking of steels (Hu (2012), Romaniv et al. (1998), Krechkovs’ka 3 et al. (2019)). At this longer stage of degradation, the sub- and microstructure of the steel changes and, as a result, their mechanical properties deteriorate. At the same time, the visibility of the integrity of the structural elements remains, however, due to long-term operation, the metal loses its mechanical properties, which ensure its operability at the beginning of operation. A significant amount of research is devoted to the discovery of the relationship between structural changes of steels during operation and their ability to withstand the real operating conditions during the residual service life (Gianfrancesco et al. (2009), Lesiuk 2 et al. (2021), Student 3 et al. (2019), Maruschak 2 et al. (2012), Yasniy et al. (2017)). It is also known (Andreikiv 2 et al. (2007)) that with prolonged exposure to static loads and high temperatures, the strength and servicability of structural elements decreases due to the initiation and subcritical growth of cracks in them. Defects that initially exist in the working cross-sections of metal structures accelerate this process. Therefore, it is important to analyze the changes in the metal microstructure of steam pipelines during their long-term high temperature operation, which will be intensified under the influence of both hydrogen absorbed by the metal and additional tensile stresses arising during start-ups and shutdowns of units. It is known that under such conditions there is an accelerated diffusion redistribution of carbon and alloying elements, which leads to the precipitation and coagulation of carbides along the grain boundaries. Hydrogen dissolved in the metal accelerates these structural transformations and promotes the detachment of carbides from the matrix. Therefore, it is important to evaluate the size and distribution of defects caused by the decohesion of grain boundary carbides from the matrix. During long term operation, the nearest defects merge with the subsequent formation of microcracks, which primarily contributes to a decrease in the mechanical properties of the metal. In particular, the strength of the metal decreases and, as a