PSI - Issue 28

2 Myroslava Hredil , Halyna Krechkovska, Oleksandra Student, Oleksandr Tsyrulnyk / Structural Integrity Procedia 00 (2019) 000–000

Myroslava Hredil et al. / Procedia Structural Integrity 28 (2020) 1204–1211 © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of MedFract1 organizers Keywords: pipeline steel;; hydrogen; degradation; fractographic features

1205

1. Introduction Degradation of structural steels during their long-term operation, especially under high temperature, is accompanied by structural transformations usually associated with diffusion of carbon and alloying elements in steels at a distance comparable to the grain size. In the case of heat-resistant steels operated under elevated (up to 570 °C) temperatures, where the diffusion coefficients of carbide-forming elements are significantly higher than that at climatic temperature conditions, the formation and coagulation of carbides along grain boundaries was observed by Romaniv et al. (1998) and Nykyforchyn et al. (2004) in low-alloy heat-resistant steels and also their welds after over 20-year operation on the main steam pipelines of TPP. Decohesion of carbides along grain boundaries from the matrix leads to the bond weakening between adjacent grains and therefore facilitates intergranular cracking due to creep. Nykyforchyn et al. (2010) and Krechkovska et al. (2019) pointed out that the process is intensified by thermal stresses in the pipe wall during shutdowns of TPP units. Earlier, Student (1998) has drawn the conclusion about the intensification of structural transformations under in-laboratory modeling the effects of shutdowns by thermal cycling of steel specimens in hydrogen. However, Student et al. (2012, 2018) showed that above mentioned structural changes did not cause a significant change in tensile strength and plasticity of operated heat-resistant steels, but their impact strength, fracture toughness and threshold values of fatigue crack growth resistance are enough sensitive to steel degradation (Student 1 et al. (2012), Krechkovska et al. (2017)). Diffusion process takes decades, especially under climatic temperatures. Concerning low-alloy pipeline steels, carbide precipitation along grain boundaries is not evidenced even after their long-term operation (30 years and more) at ambient conditions because of a low mobility of carbide-forming elements in this case. However, Nechaev (2008) suggested that very small cementite particles can form at the grain boundaries of pipe steels during their long-term operation under ambient conditions, and it was confirmed by Zorin and Tolstov (2017). Gabetta et al. (2008) and Syromyatnikova et al. (2016) shoved that mechanical properties of the operated pipe steels slightly deviated from the standard mechanical characteristics. Besides, Hredil et al. (2020) did not find substantial changes in fatigue crack growth characteristics of pipeline steels after operation. However, Krasowsky et al. (2001), Gredil (2008) and Krechkovska 1 et al. (2019) revealed a high sensitivity of impact toughness to steel degradation, like in the case of heat-resistant steels. In both cases, resistance to brittle fracture deteriorated the most due to steel operation. Thus, this work is aimed at evaluating structural and fractographic features of operational degradation of pipe steels with different strength grade after their long-term service on gas mains, and estimating the influence of the steel structure on its mechanical properties and susceptibility to brittle fracture. 2. Materials and methods Gas pipeline steels with different strength (17H1S, X60 and X70) were studied in the as-received state and after their long-term operation (30, 25 and 37 years respectively). Uniaxial tensile tests were performed in air under the strain rate of 3.3∙10 -3 s -1 , and SSRT ones in NS4 solution (composition, g/l: KCl 0.122; NaHCO 3 0.483; MgSO 4 0.131; CaCl 2 0.093) with the strain rate of 1.7∙10 -7 s -1 using smooth cylindrical specimens cut along the pipe axis. Impact tests were done on longitudinal and transversal (relative to the pipe generatrix) Charpy specimens, and the stress concentrators in the specimens were induced in mutually perpendicular directions enabling crack propagation during the test along (in transversal specimens) and across (in longitudinal ones) the pipe texture. Metallographic and fractographic investigations were accompanied with a quantification of revealed fractographic features of steel degradation using the software elaborated by Kosarevych et al. (2013) for the computer analysis of halftone images of the fracture surfaces with automatic recognition of the research objects and further determination of their geometric parameters.