PSI - Issue 30

Nikolay I. Golikov et al. / Procedia Structural Integrity 30 (2020) 93–99 Author name / Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction Today, the Arctic regions are of great interest owing to the large deposits of oil and gas. According to experts, 13% of the worlds untouched oil reserves and 30% of natural gas reserves can be located in the Arctic regions. In the energy sector, distribution networks are built using steel structures constructed by various welding technologies. The climatic conditions of the North and the Arctic are characterized by a long period of negative ambient temperatures. Therefore, welding of large extended objects, such as gas and oil pipelines, should be performed under any weather conditions. Thus, the majority of mechanical works is performed at low temperatures. When welding at negative temperatures characteristic of the Far North and the Arctic, the conditions for arcing change, the cooling and crystallization rate of the weld pool increases, and diffusion processes slow down significantly as shown by Golikov et al. (2019). Negative temperatures affect the strength of the welded joint as well. The accelerated cooling rate of the heat-affected zone can lead to the formation of quenching structures and can increase the critical temperature of the brittleness of the material in this zone as reported by Mao et al. (2019). Therefore, the development of the necessary specific welding technology effective at low temperatures is an urgent scientific and technical task, the solution of which determines an increase in the reliability and safety of equipment and structures operating in the North and the Arctic. As noted earlier by Saraev (2015), (2016), Saraev et al. (2017) one of the most effective ways to increase the operational performance of critical structures is to improve welding technologies by using adaptive pulse-arc welding methods, which allows controlling the micrometallurgical processes due to pulsed changes in the energy parameters of the mode. Moreover, Mirzaei et al. (2013), Saraev et al. (2016) showed in their research, that due to the programmable heat input into the welded joint zone, control of the melting and transfer processes of each drop of electrode metal, it is possible to obtain a finely dispersed structure in the zones of permanent joints of technical systems. It allows reducing the degree of residual deformation of permanent joints made of various steel grades, as well as increasing the operational reliability of metal structures and products operating at low climatic temperatures. However, the majority of the studies in this direction has been conducted regarding the positive ambient temperatures during the welding, see Chen, et al. (2020), Wang et al. (2017). For the successful implementation of the pulse-arc welding for solving the problems mentioned above, it is necessary to conduct comparative studies of the influence of pulse-arc welding modes on the weld parameters and HAZ in comparison with welding with a stationary arc. It is the main goal of the present research, the results of which are given in this article. Hence, the study of the formation specifics of the structure and phase composition of the weld metal and the HAZ using this welding technology performed at low climatic ambient temperatures is relevant.

Nomenclature BM

base metal DCW direct current welding HAZ heat-affected zone WCM welding with current modulation

2. Experimental technique, materials and equipment Investigation of the structure of welded joints of structural steels was performed on the steel of 09Mn2Si and St3sp grades, which were obtained using new grades of locally produced electrodes LB-52TRU (OOO Research and Production Center Svarochnye materialy [Welding Materials], Krasnodar) and UONI 13/Moroz (OOO Vysokie tekhnologii [High Technologies], Moscow). The chemical composition and mechanical properties of steels are presented in Tables 1 and 2. Table 3 presents welding electrodes, their chemical composition, and mechanical properties.