PSI - Issue 16

Yevstakhiy Kryzhanivskyy et al. / Procedia Structural Integrity 16 (2019) 237–244 Yevstakhiy Kryzhanivskyy et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Most of the gas mains (GM) in Ukraine have been in service for more than 30 years, Kryzhanivs’kyi et al. (2015). Moreover, a possible reduction in the transit of gas from the Russian Federation to the EU requires the optimization of their operating conditions and necessitates the avoidance of negative vibrations that lead to their failure. The risk of unpredictable fracture of gas mains can be reduced significantly by diagnosing the current technical condition of pipe steels (Nykyforchyn et al. (2017), Bolzon et al. (2018)) and, if necessary, conducting repairs, Banakhevych et. al. (2014). Renovation of gas mains is a complex technical task that requires a step-by-step technology based on reasonable parameters and conditions of each operation. It is known that most cases of unpredictable fracture of gas mains are caused by defects in welds. The presence of alloying elements in the pipe metal contributes to the deterioration of welding, in particular, it may lead to the appearance of areas with an impaired resistance to brittle fracture, Kryzhanivs’kyi et al. (2013). Degradation processes that occur in gas mains over a long period of operation are the most dangerous for welds, as they cause unpredictable changes in mechanical properties (Zvirko et al. (2012)). This issue is particular relevant to new technological approaches to welding of pipelines, Matrosov et al. (2004). The occurrence of dispersed inclusions in the weld zone along with the thermal effect zone may cause damage to the metal structure and affect its resistance to fracture under different temperature and force conditions in various ways. The processes of nucleation and propagation of cracks under dynamic conditions are localized, and the ultimate state of the material is determined, in fact, by the properties of the “most dangerous areas”, which can be welds , Arabey et al. (2010) and Odesskii et al. (2016). Welds are macroconcentrators of stress. They represent a complex physical-mechanical structural-heterogeneous system (base metal – weld – zone of thermal effect), which may contain technological stress concentrators, such as cuts, bad welding, pores, that can lead to fracture, Gliha et al. (2011). Welding of gas mains is a special technological process, the final result of which is ensured by a strict adherence to technological standards, non-destructive control methods, and testing in the process of construction. Therefore, ensuring the quality and stability of the properties of welds in the construction of construction of GM from pipes of the X70 strength class, as well as increasing the efficiency and reliability of GM, is possible by a comprehensive study of the impact of technological and temperature factors on their resistance to brittle fracture. In this research, the influence of the test temperature on the impact toughness of the X70 class pipe steel is considered. Impact toughness of the pipe steel of the X70 strength class was investigated. Test specimens were cut from a pipe with a diameter of 1420 mm and a wall thickness of 15.7 mm. The steel had the following chemical composition: (C = 0.12%; Mn = 1.75%; Si = 0.40% ; S = 0.006%; F = 0.020%; V = 0.10%; Nb = 0.10%; Ti ≤ 0.035%) according to the certificate of the manufacturing plant (Khartsyzsk Pipe Plant). The scheme of cutting Charpy specimens was chosen taking into account the conditions of crack propagation in the weld of the gas pipeline. Welds were executed by special electrodes: electrode UONI 55U, Ø 3.5 mm, was used to weld the root of the joint, and electrode UONI 52U, Ø 4.0 mm, was used to fill the joint. Specimens were cut parallel to the longitudinal axis of the pipe. Impact toughness was determined on Charpy specimens measuring 10 × 10 × 55 mm with a V- shaped incision with a depth of 2 mm and a rounded radius at the top of 0.25 ± 0.025 mm. 2. Research technique

a b Fig. 1. Scheme of cutting Charpy specimens from the weld: (a) a specimen with a cut in the center of the weld; (b) a specimen with an incision along the fusion line and a zone of thermal effect.