PSI - Issue 20

Sleptsov O.I. et al. / Procedia Structural Integrity 20 (2019) 143–147

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Sleptsov O.I. et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The active migration of mining and processing industries to the northern regions of the country requires answering questions concerning behavior and service life of complex materials used for high-load equipment operating in difficult climatic conditions of Siberia and the Extreme North, and the task of creating natural gas liquefaction plants in the polar regions makes it even more complicated to answer these questions because low temperature impact on the metal is carried out not only from the external - natural side of the equipment, but also from the medium i.e. the liquefied gases. For the safe operation of such equipment, it is necessary to replace carbon and low alloy steels with more complex compositions. Pipeline systems are among the most challenging elements of low-temperature equipment relating to forecasting their safe operation period and residual life. This is due to both the complexity of the direct control of the metal of the pipeline, protected by thermal insulation and the inaccessibility of the objects of control; and also the significant number of welded joints, which are made directly during the assembly of the object. It's difficult to provide welding works which satisfy all regulatory and technical documentation requirements during installation and repair of low temperature pipelines - joint sealing, preliminary and concurrent heating, ensuring a given rate of cooling of the seam after welding or necessary heat treatment. In some cases, this leads to failure or poor quality of these operations. When analyzing emergencies at such facilities, the most frequently identified cause of failure proves to be the absence of heat treatment of the welded joint that restores the metal structure. Field joints are also areas of reduced accuracy in welding work, which leads to the occurrence of welding deformations and stresses not taken into account in the strength calculations; areas of increased defects in the macro- and microstructure of the metal. The deviation of the geometric design of the structure from the strength and stability adopted in the calculations can also change the nature of the action of functional stress and lead to additional stresses. When the thermal insulation is damaged, on the outer surface, and when corrosive condensate accumulates in the stagnation zones, on the inner surface of the pipeline, during inter-operational shutdowns in the areas of field joints i.e. in the areas of increased stress and deformations, the localized damage to the metal can occur by the mechanism of corrosion cracking - corrosion under stress (stress corrosion). Currently, a significant amount of equipment of low-temperature machines made from corrosion-resistant chromium-nickel steels of austenitic class, mainly steels of the type (08-12) Х 18 Н (10-12) T, is in operation. The operating time of this equipment is close to or exceeds the maximum allowable resource indicators. 2. Materials and method of solution The problem of corrosion damage of low-temperature equipment arising during inter-operational periods has already been discussed in a number of studies Kuzyumov A.N., Krikun V.P. and Nihaenko Yu.Ya. (1999), Ermakov B.S. and Solntsev Yu.P. (2000), Ermakov B.S., Vologzhanina S.A. (2004), and its contribution to reducing the ductility and strength of the metal of low-temperature systems is well known. However, in most of the researches, studies were conducted on the “base metal”, that is, at a sufficient distance from the heat affected zones of the welded joints. The issues of mutual enhancement of corrosion processes with simultaneous effects of welding stresses and deformations, changes in the metal structure in the heat-affected zone of low-temperature equipment and corrosive environments are much less studied and require additional research, without which it is impossible to give an unequivocal answer to the question of possibility of service life extending of low temperature piping system. Corrosion under stress, as well as intergranular corrosion, is among the most dangerous types of corrosion damaging low-temperature equipment. Intergranular corrosion is noted mainly on the equipment that is subject to technological heating during inter-operational periods. In systems where the temperature at shutdowns does not exceed the temperature of the external environment, the damages associated with the occurrence of defects by the mechanism of corrosion cracking are more often detected as noted by Ermakov B.S., Kozachenko A.V. (2001). Moreover, while intergranular corrosion affects the internal surfaces of the pipes, that are in contact with the working medium, the corrosion cracking more often develops in places where the insulation of the pipelines is damaged, that is, from the external surface of the pipelines. This paper studies the effect of tensile stresses and cold plastic deformation on the anodic process rate in the base metal and heat-affected zones of steel 08X18H10T. An increase in the rate of the anodic process indicates possible