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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with a current modulation frequency of 5 Hz (WCM-5) and direct current (DC). The most favorable structure of the HAZ of the welded joint is obtained at a current modulation frequency of 1.67 Hz using LB 52TRU electrode. Welding was performed at -45°C while an average heat input was increased by 10%. Heat input of the facing weld was increased by 15-20% as well. Welding of St3sp low-carbon steel. Studies of the microstructure of the weld metal of all samples have revealed that the weld structure in the central part has a columnar structure with the orientation of crystallites in the direction of heat removal. It consists of ferrite, perlite, and bainitic structure (Fig. 2). The metal of the heat-affected zone of the welded joint has a Widmanstatten structure with randomly oriented crystalline grains that contain structural elements in the form of needles (Fig. 3). The growth of pearlite colonies also begins at the grain boundaries of austenite or excess ferrite. In the HAZ section of the welded joint, martensitic structures are formed starting from the level of 0.5 mm and further. Moreover, the formation of bainitic structures is observed along the boundaries of ferrite grains under the considered modes.

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b

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Fig. 2. Microstructure of the weld metal of welded joints obtained at room (a, b) and negative temperatures (c, d) at the level of the middle of the weld cross-section, x1000: a, c - WCM; b, d - DWC.

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b

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Fig. 3. Microstructure of the HAZ of welded joints obtained at room (a, b) and negative temperatures (c, d), x1000: a, c - WCM; b, d - DCW.

When welding of St3sp low-carbon steel, the volume ratio of quenching structures in the form of martensite increases significantly at low temperatures compared with positive ambient temperatures. 4. Conclusions According to the results of the conducted microstructural studies, it has been established: 1. The formation of the structure of welded joints is significantly affected by the ambient air temperature, at which the welding process is performed. During welding at low temperatures, finer-grained structures are formed owing to the accelerated cooling rate. However, the ratio of quenching structures in the form of martensite and bainite increases as well. Consequently, a decrease in the values of impact strength in the zones of weld metal and thermal influence can be observed. 2. The rate of the heat flow is averaged with an increase in the modulation frequency of the welding current. The structure of the HAZ of the permanent joint becomes the same as when welding in DC mode at a frequency of 5 Hz.