PSI - Issue 20

G.S Ammosov et al. / Procedia Structural Integrity 20 (2019) 306–309 G.S Ammosov and M.P. Lebedev / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction For the construction of storage tanks for oil and oil products, various materials are used. However, tanks assembled from steel sheets, which constitute the main part of the structure forming the envelope, are the most common in Russia. The manufacturing of vertical steel tanks (VST) is connected with a large amount of welding. Steel-plate welded constructions are, as a rule, critical structures and controlled by Rostechnadzor (Federal Environmental, Industrial and Nuclear Supervision Service of Russia). Their manufacture and operation are regulated by standard technical documentation. According to the requirements of standards, structural components of welded joints and seams of tanks should be reliably solid and correspond to the base metal in terms of standard mechanical properties of the weld metal: yield stress, temporary resistance, relative elongation, impact strength. Nevertheless, in most cases, the fracture occurs along the seam or in the heat-affected zone (HAZ), where the technological and structural stress concentrators due to the low cold resistance of the material and the occurrence of fatigue cracks in welded joints with various defects appear what showed by Rozenshteyn I.M. (1995), Ammosov A.P. et al. (2008), Ammosov G.S. et al. (2017). The main processes that determine the high operational strength and cold resistance of welded joints in VST are structural transformations in the HAZ, which are involved in the formation of mechanical properties and their stress-strain state. 2. Research methods The structure is formed in the HAZ of welded joints of all steel during welding due to the decomposition of undercooled austenite. In the process of intensive heating with the welding arc adjacent to the weld, a section of the HAZ metal undergoes full austenization. To evaluate the structural transformations of the undercooled austenite, when simulating thermal welding cycles, continuous cooling transformation (CCT) diagrams are constructed and austenite undergoes ferritic-pearlitic, ferritic-pearlitic-bainitic, ferritic-bainitic, bainitic-martensitic and martensitic transformations depending on the cooling intensity of steel. Significant changes in the structure of the metal of the heat affected zone during the welding of low-alloyed steel involve abrupt changes in volume. In the process of structural transformations, the mechanical properties of the metal of the welded joint zones are formed. These factors determine the stress-strain state of the welded joints of the tank and its operational strength. Therefore, in these processes, the cooling rate of the weld metal and, first of all, heat input during welding are of crucial importance. For steels used in the construction of tanks in the Republic of Sakha (Yakutia) according to works by Ammosov A.P. et al. (2008), Ammosov G.S. et al. (2017), CCT diagrams or the diagram of aniothermal austenite decay were investigated and constructed by Dovzhenko V.A. et al. (1984), Ammosov A.P. et al. (1993), Zakharova I.V. et al. (2001), Seyffarth P. et al. (1983) and the ranges of permissible cooling rates ( � 6/5 ) were determined in the temperature range of 600 ... 500 °C or cooling durations ( τ 8/5) from 800 to 500 °C of the heat-affected zone in the HAZ of welded joints. The volume of deposited weld metal in products welded by manual arc welding (MAW), automatic submerged arc welding (ASF) and semi-automated arc welding (SAAW) is determined from the condition for ensuring the optimally permissible cooling rate of the HAZ metal corresponding to the CCT diagram of the particular steel and guaranteeing the required cold resistance and strength of welded joints of tanks. The calculated dependence of determining the volume of deposited weld metal during welding was obtained by Ammosov A.P., et al. (2004): � � � � н � � ��� �⁄� ���� (1) where � � is the cross-sectional area of the deposited weld metal in the given weld seam; γ is the density of the weld metal; η is the effective heat transfer coefficient; � is the arc voltage; � н is the weight of the metal deposited per ampere per hour; � 6/5 is the dependence of the cooling rate in the temperature range from 600 to 500°C.