Study on the Microstructure and Properties of Flash-Butt Welding Joints of High Nitrogen Steel

A thermomechanical coupling model for the solid-state flashing process of high nitrogen steel was established, based on finite element simulations and experiments. The effect of flash current on the microstructure and mechanical properties of the welded joint was investigated, and the temperature field of the flash-butt welding (FBW) process was simulated. The phase composition of the joint was determined according to the phase diagram and cooling curve. In addition, the joint with optimal parameters was subjected to full immersion corrosion tests. The results demonstrated that the interface structure was composed of austenite and & delta;-ferrite with a thyristor angle (flash current) of 45 & DEG;. The microstructure of the overheated zone (OZ) was composed of austenite, ferrite and a small amount of the M-2 phase, in which the heat-affected zone exhibited a single-phase austenite microstructure. The joint hardness displayed a "V" shaped distribution with the lowest interface hardness. As the flash current increased, the hardness and tensile strength of the interface area of the joint first increased and then decreased, with a maximum tensile strength of 902 MPa at 45 & DEG;. During the full immersion corrosion tests, the joint exhibited the most serious corrosion in the interface center and gradually reduced corrosion on both sides.