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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1. Introduction Operating experience of main steam pipelines shows that their damage most often occurs in places where the impact of operational factors is intensified. Steam pipeline bends at thermal power plants (TPP) are usually included in the list of such places. The heterogeneity of the metal properties from different bend zones appears already at the stage of their manufacture. It is associated with the uneven distribution of deformation along the perimeter of the bend pipe. Temperature and force operating conditions for steam pipeline (pressure up to 24 MPa, temperature up to 570 °C) and long -term exposure of stressed metal into active hydrogenation environment, which is a pair of high parameters, contribute to the degradation of its structure (Krechkovs'ka (2008), Ostash et al. (2009), Yasnii et al. (2008), Zhuravel' and Svirs'ka (2010)) and loss of mechanical properties that guaranteed its performance at the beginning of operation (Foulds and Viswanathan (1994), Hong et al. (2003), Nykyforchyn et al. (2010), Yasniy et al. (2011, 2013, 2017), Student and Krechkovs’ka et al. (2012), Student et al. (2018, 2021), Krechkovska et al. (2022)). Besides that, additional stresses arise in the steam pipeline, caused by the weight of the pipes and insulation or vibration due to pulsating steam flow and thermal stresses due to temperature differences in the pipe wall (Smiyan and Student (2021). Under such conditions, bends become one of the most vulnerable elements of the steam boiler system for supplying superheated steam from the boiler to the turbine. Therefore, special requirements are imposed on pipe bends and assessing their serviceability continues to be one of the most pressing tasks for the thermal power industry of Ukraine, with a significant equipment service life. The above determined the purpose of the work, which was to study the structural and mechanical state of metal from the stretched zone of the bend of a steam pipeline after long-term operation at a TPP, with an assessment of the indicators of structural changes and mechanical characteristics of the metal along the thickness of the pipe wall. 2. Materials and methods Heat-resistant steel 12Kh1MF (chemical composition, wt.%: 0.1 C; 0.019 S; 0.015 P; 0.26 Si; 0.54 Mn; 1.1 Cr; 0.17 V; 0.26 Mo) after ~28.6  10 4 h of operation on the main steam pipelines bend of the TPP was studied. The radius and angle of its bending were 1370 mm and 90  , respectively. The steam temperature in the pipe during operation was 545 ºС, pressure – 14 MPa. The structural and mechanical state of steel from the bend's stretched zone (SZ) was estimated at three levels along the pipe wall thickness (1 mm from the outer and inner surfaces and in the centre of its section). The degree of steel degradation was evaluated by strength (ultimate strength  UTS and yield strength  YS ) and plasticity (elongation and reduction of area RA) characteristics. Cylindrical specimens  5 mm and the length of the working part 25 mm were tested on an UME-10T tensile testing machine at a strain rate of 3  10 – 3 s – 1 . The impact toughness of steel was determined on Mesnager samples (with U-type stress concentrator) of the 10  10  55 mm size. The specimens were tested on the IO-5003 pendulum impact machine. The EVO 40XVP scanning electron microscope was used for metallographic studies. 3. Results and discussion Many scientific investigations (Balyts’kyi et al. (2006), Dobrzanski et al. (2007), Zieliński et al. (2007), Golanski and Wieczorek (2008), Ćwiek et al. (2011), Zieliński (2017), Sun et al. (2022)) are devoted to the study of structural and phase transformations occurring in heat-resistant steels after their long-term operation. It was established that during their operation, a redistribution of carbides occurred along the grain boundaries. As a result, their structure is dominated by ferrite grains with carbides along the boundaries. These processes involve diffusive redistribution of carbon and alloying elements over a distance commensurate with the grain size. As a result, the matrix solid solution is depleted of alloying elements, accompanied by the loss of functionally necessary properties of heat-resistant steels. Taking into account the significant duration of all these processes, Cr and Mo alloyed carbides are released and coagulated along the grain boundaries (as in thermodynamically favourable locations) (Baltušnikas et al. (2007 , 2010)). Moreover, the hydrogenation of steel can intensify the processes of element redistribution (Student (1998), Ваlitskii and Panasyuk (2009), Krechko vs’ka (2016), Tsybailo et al. (2023)). Over time, these carbides lose a coherent connection with the matrix due to a mismatch in their mechanical (modulus of elasticity) and physical thermal (coefficient of thermal expansion) properties. Defects are formed as nano-sized pores, which become traps