PSI - Issue 21

M. Hredil et al. / Procedia Structural Integrity 21 (2019) 166–172 M. Hredil, H. Krechkovska, O. Student, I. Kurnat / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Main gas pipelines are strategic objects on which a country energy security is significantly dependent. Therefore, a very important task is to ensure the safe operation of gas mains during their long term service. Recent researches have proved that mechanical characteristics of steels of gas mains worsened due to in-service degradation (Gredil (2008), Kryzhanivs’kyi and Nykyforchyn (2011), Kharchenko et al. (2017), Maruschak et al. (2017)). One of the main characteristics for assessment of pipeline steels state is impact toughness which is commonly used for evaluation of the metal suitability for gas transportation systems as shown by Nykyforchyn H. (2013), Maruschak et al. (2016). Reduction in impact toughness of structural steels clearly indicates their embrittlement, caused by long term operation under a combined action of working stresses and other adverse service conditions, i.e. aggressive environments action, hence, a risk of brittle fracture rises. Fracture of structural elements usually occur along the path with the lowest resistance to brittle fracture (Student et al. (1999)). This path is determined by in-bulk damaging of material developed during its operation. It is known that fractography, first of all microfractography, has been successfully used for the evaluation of damaging degree of structural materials and, respectively, a technical state of operated objects (Krechkovs ’ ka et al. (2015, 2016). Mohtadi-Bonab et al. (2015). Kotrechko et al. (2004)), and also for clarification of involved fracture mechanisms (Taylor at al. (2009), Krasovskii and Orynyak (2010), Hredil and Student (2012), Maruschak et al. (2017, 2018)). Prospects of quantitative analysis are widened thanks to active development of automated methods described in Student et al. (2003), Zhuravel’ et al. (2009), Kosarevych et al. (2013), Konovalenko et al. (2019) for estimation of features of steel degradation identified on its fracture surfaces, and detection of elements of fracture surface relief with the least fracture resistance caused by degradation (Krechkovs ’ ka et al. (2015, 2016)). In this way, fracture elements can be identified at the micro level, induced by so called disseminated in-bulk damages (Hredil (2011)) which has occurred during long term operation of structural steels, since these damages visualize themselves during fracture in laboratory conditions. Their quantitative evaluation and determination of dependencies between their fractographic and mechanical characteristics for steels of different degradation degree opens prospects of using existing databases on physico-mechanical properties of steels operated in various technological processes, for assessment of their current technical state. The aim of the research is assessment of degradation of pipeline steels with different strength after their long term operation on gas mains by their resistance to brittle fracture, and detection of fractographic features concerned with their degradation. 2. Materials and methods The object of the research is pipe steels of different strength: 17H1S (analogue X52) , Х60 and Х70 in as-received state and after their long term operation (from 25 to 51 years) on main pipelines. Standard mechanical properties were evaluated under the tensile test. Cylindrical smooth specimens with working diameter of 5 mm were cut from pipe fragments in axial direction. Tests were performed in air with the strain rate of 3. 3∙10 – 3 s – 1 . Specimens were polished with sandpaper of different grit to eliminate the effect of stress concentrators and avoid essential data scattering. Impact toughness was determined by Charpy testing. Peculiarities of metal degradation were analyzed on longitudinal (steels 17H1S , Х60, Х70) and transversal (Х60) specimens. Metallographic and fractographic investigations were performed using the optical (Neophot-21) and the scanning electron (EVO-40XVP) microscopes. Quantification of revealed structural and fractographic features of degradation for the tested materials was done using software elaborated in Karpenko Institute by Kosarevych et al. (2013) for the computer analysis of halftone images (metallographic or fractographic) by automatic recognition of the objects of research interest with following determination of their geometrical parameters. 3. Results and discussion Metallographic examination of steels 17H1S and Х70 taken from pipelines showed that their structure in the as received state and after their service represents alternating strips of ferritic and pearlite grains which causes texture. Pearlite interlayers are very thin (from 1 to 5 μ m) in X70 steel. Lamellar structure of ferritic-cementite mixture inside pearlite strips is mostly difficult to identify even under a high resolution (Fig. 1).