Evaluation of effect of parameters of resistance flash-butt welding of hot-rolled reinforcing steels on microstructure and mechanical properties of the welded joint

The parameters of the process of flash-butt welding of reinforcing steels determine the quality of the weld. The effect of main welding cycle parameters on the microstructure and mechanical properties of flash-butt welded 35KhG2S and 25G2S steel joints was assessed. The results showed that the weld thermal cycles are characterized by high peak temperatures and fast heating and cooling rates, and the joints include a weld interface, coarse-grained and fine-grained zones, and have a fine equiaxed grain structure containing ferrite and pearlite. The analysis of the microstructure using an electron backscatter diffraction detector showed that low-angle grain boundaries with a misorientation angle of 2 ≤ θ < 15 deg. consist of an array of dislocations. High-angle grain boundaries with a misorientation angle of θ ≥ 15 deg. can be considered as an indicator of the recrystallization degree. The mechanical history is characterized by two parameters: upsetting pressure and upsetting allowance. The combination of all welding parameters determines the quality of the welded joint of reinforcing bars. During the upsetting process, the duration of exposure to high temperatures increased from 0.4 to 0.7 s, with an increase in the allowance for upsetting from 3 to 7 mm, the length of the heat-affected zone and the zone of high temperatures impact increases. Since the distance between the point of the contact surface is constantly reduced as the sparking stage progresses, the actual temperature of the contact surface will continue to increase with an increase in the total welding time, which can lead to a greater supply of heat during the welding process. This in turn will affect the subsequent cooling rate of the welded joint, the latter will be less. According to the test results, the tensile strength of welded joints made of 25G2S steel is within 398–430 MPa, for 35KhG2S steel is 584–611 MPa, which meets the regulatory requirements for reinforcing steels.