PSI - Issue 17

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 17 (2019) 568–575 Hryhoriy Nykyforchyn, Oleksandr Tsyrulnyk, Olha Zvirko / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Natural gas transit pipelines are objects of long-term operation, which leads to aging and deterioration of pipeline steels characteristics and, accordingly, to serviceability loss. It was demonstrated by Gabetta et al. (2008), Tsyrul’nyk et al. (2008) and Nykyforchyn et al. (2010) that a destructive role of corrosion medium consisted not only in thinning of pipe wall but also in acceleration of crack growth in a pipe under mutual action of working loading and corrosion environment, which reinforces each other and leads to pipe rupture, should be considered. Thus, a metal corrosion is intensified by stress and crack growth is accelerated by corrosion medium under both static stress (stress corrosion cracking) and cyclic loading (corrosion fatigue crack growth). Therefore, a special attention of scientists is paid to these types of fracture that are the main reasons of uncontrolled failures of pipelines under operation. Soil corrosion is generally considered to be a serious problem for pipeline steel. Once insulation coating is breached, soil environment electrochemically interacts with the metal surfaces of pipe. Corrosion, especially in cracks due to local acidification of corrosion medium, may be accompanied by hydrogen evolution. Hydrogen charging of a metal from inside the pipe due to electrochemical corrosion can occur on large metal surface areas, where condensed moisture can be present, first of all, on aboveground pipeline sections. Hydrogenation can accelerate both stress corrosion cracking and corrosion fatigue crack growth. Hydrogenation of metal during corrosion process together with working stresses facilitates a development of in bulk damaging at nano- and microscales. It provokes not only a development of macrocracking in pipes but also is often the main reason of decrease of pipeline steels characteristics of brittle fracture resistance under long-time operation. Reducing the fracture and impact toughness and increasing susceptibility of pipeline steels to stress corrosion cracking, which increases significantly a risk of uncontrolled failure of gas pipelines by subcritical crack propagation, is associated with in-bulk material degradation, as it was demonstrated by Kotrechko et al. (2004), Gabetta et al. (2008) and Nykyforchyn et al. (2010), Maruschak et al. (2014), Zvirko et al. (2016), Ohaeri et al. (2018) and others. The two stages of in-bulk steel degradation, deformation aging and multiple damaging, were identified by Nykyforchyn et al. (2010) ; and two substages of the second one were distinguished by Nykyforchyn et al. (2017) : dissipated disoriented damaging and damaging oriented in rolling direction. The effect of deformation aging on mechanical properties of steel can include increased yield and tensile strength and hardness, and reduced plasticity and fracture and impact toughness. However, this tendency of changes of the mechanical properties could be violated if the metal undergoes in-bulk multiple damaging under operation. As it was reported by Nykyforchyn et al. (2017) , in such circumstances strength, hardness, reduction in area, fracture toughness of the damaged material decreased and elongation increased. The presence of delaminating cracks oriented transverse to the fracture plane can lead to increasing impact toughness of material at testing of longitudinal specimens. The in-service changes of mechanical properties of pipeline steels under operation could seriously challenge the safe operation of transit pipelines; so that this hydrogen assisted degradation effects need to be taken into account. It is very important to predict operational degradation of materials in laboratory, especially for novel materials, manufacturing or processing technologies, when there is not enough data concerning changes the initial properties of metal during operation. For example, proposing a new pipe welding technology with higher performances, higher mechanical properties, resistance to stress corrosion and hydrogen induced cracking, etc., a complex issue of stability of these properties during long-term exploitation should be considered primarily. It is obviously that the material with better initial properties (Fig. 1, material A) could degrade during the operation more rapidly than other one (Fig. 1, material B) and after certain operation time it could have worse properties than the material B with lower initial properties. For simulating changes in mechanical properties of steels due to aging, they are subjected to in laboratory procedure of artificial aging according, for example, to the normative document GOST 7268-82. The procedure consists in subjecting of specimens to an axial loading up to the axial strain of 10 % followed by heating at 250ºС for 1 hour. The applied treatment causes deformation aging of steel due to Cottrell atmosphere formation and dislocations pinning. As a result of this process (dislocations pinning) movement of dislocations hinders and strengthening the metal and decreasing its plasticity takes place. Consequently, applying such treatment, the process of deformation aging of steels is simulated.