PSI - Issue 42

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Halyna Krechkovska / Structural Integrity Procedia 00 (2022) 000 – 000

Halyna Krechkovska et al. / Procedia Structural Integrity 42 (2022) 1398–1405 © 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)

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Peer-review under responsibility of the conference Guest Editors Keywords: steel; steam pipeline; degradation; fractographic features

1. Introduction Important factors influencing the degradation of low-alloy heat-resistant steels of the main steam pipelines of thermal power plants (TPPs) include high operating temperature (up to 540 o C), corrosive-hydrogenating technological environment, and cyclic and static loads in various combinations. The consequences of their impact are manifested by the transformation of the structural-phase state of heat-resistant steels (Bakic (2013, Liu et al. (2022). Many researchers reported changes in their mechanical properties under creep (Babiy et al. (2007), Student et al. (2005), tension (Dzioba 1 (2010) and Student 1 et al. (2021), impact loading ( Balyts’kyi et al. (2009) and noted the weakening of crack growth resistance of steel under static and cyclic loading (Student et al. (2012), Marushchak et al. (2007, 2021). Romaniv et al. (1998), Song et al. (2019), Rykavets et al. (2019), Ostash et al. (2007). The complex influence of the listed operational factors promotes the emergence of the critical damages in structural elements as a necessary precondition of their failure. In addition, Nykyforchyn et al. (2010) attributed the shutdowns of the technological process of electricity generation to the significant factors influencing the state of the steam pipeline steels. Krechkovs ’ ka et al. (2019) emphasized that additional thermal stresses, which depend on the heating (or cooling) rate of thick-walled (up to 60 mm) structural elements when the equipment achieves the technological operation mode (or its cooling during shutdowns), contribute to their almost no-deformation (brittle) fracture. Smiyan et al. (2021), Student et al. (2016), Krechkovska (2021), Hredil et al. (2019) noted that recently fraсtographic analysis, which is widely used to find out the causes of failure of various structure elements, is increasingly being used to assess the current technical state of operated steels. Peculiarities of macro and microfracture of pipes of steam pipelines, as well as fracture surfaces of laboratory specimens, cut out from operated metal, make it possible to assess the intensity of its damage, and in this way judge the current state of objects and establish fractographic features concerned with changes in the metal during long-term operation. The research aims at analyzing the fracture surfaces of the specimens of the long-term operated steel 15H1M1F with a different number of shutdowns after their impact testing, fracture toughness and fatigue crack growth evaluation, and also identifying and quantifying fraсtographic signs of operational degradation of the steel. 2. Materials and methods Heat-resistant steel 15H1M1F (0. 14% С; 1 .3 Cr; 1.0 Mo; 0.75 Mn; 0.1 Ni; 0.3 Si; 0.25 V; 0.012 S; 0.027% P) in the initial state (IS) and after its operation on the main steam pipelines of TPP for approx. 2·10 5 h under 545  С and pressure up to 24 М P а has been analysed. The metal from two TPP units operated for almost the same time but with a different number of shutdowns N of the technological process, N 1 = 501 (DS 1 ), and N 2 = 576 (DS 2 ) was tested. Samples for the research were cut out from spare pipes and operated pipes, decommissioned due to the detection of inadmissible surface defects. Axial specimens were taken in the vicinity of the pipe’s outer surface, where, as a rule, the most favourable conditions arise for structural transformation and damage formation due to creep. Charpy specimens 10×10  55 mm mm with V-shaped notches with a radius of 0.25 mm were used for impact tests, and SENB specimens (10  20  160 mm) were used for the evaluation of fracture toughness K c (loading by three-point bending) and also fatigue crack growth resistance (cantilever bending with the frequency of 10 Hz). Following the procedure recommended by Romaniv at al. (1998), nominal  K th and effective  K th eff (taking into account crack closure determined using the compliance method as Romaniv at al. (1983) described) threshold values of stress intensity factor (SIF) ranges were obtained from the corresponding fatigue crack growth curves. Fractographic features of the specimens after their mechanical testing were investigated using SEM EVO 40XVP.