Numerical Simulation Of The DC Flash Butt Welding For Ultra-Fine Grain Size Steel And Prediction Of Microstructure And Property

Ultra fine grain steel is a new kind of steel which is developed on the basis of TMCP (thermo-mechanical control process) technology. The steel is provided with the characteristics of ultra fine grain and ultra purity, so the result has the excellent mechanical properties with combination of strength and toughness. DC flash butt welding is a kind of solid joining method with high efficiency. The micro characteristics of HAZ affect the weld joints greatly. Due to the intrinsic differences between this method and the method of melt welding, study on the appliance of flash butt welding with ultra fine grain steel will help to drive the research and development of new generation steel materials. Simulation of DC flash butt welding by coupling with electricity, heat, and mechanics will be instrumental in designing and drawing out technical specification.Finite element method was adopted in this paper to research on the process of the welding. The model of DC flash butt welding with coupling with electricity, heat, and mechanics was built. In addition the material physical parameter variation with temperature was considered in the model. The contact resistance and flash melt were considered in the electric-heat coupling model; rigid-viscoplastic contact problem was also considered in the heat-mechanical model. The results of the experiments show that the model simulated the actual process by flash butt welding accurately.Using the electric-heat coupling model built, this work discussed how to get the temperature field with different welding parameters and its effect conditions. The flash butt welding parameters, such as stretch length, flash model and flash velocity were analyzed quantitatively. The rule of heat producing and temperature distribution by different welding parameters in coupling with electric and heat was researched. The mainly effective factors and rules of contact forging by using the heat-mechanical coupling model were computed. By the results of simulation, we provide the reasonable measurement to select the welding parameters in flash butt welding. A parameter W was built. It can conveniently provide selection of welding parameters based on relationship between flashing velocity and initial clamp distance.Properties of the HAZ are dependent on the grain size. On the basis of study on the flash butt welding by using finite element method, Monte Carlo simulation and theoretical analysis, growth of the Austenite grain size in HAZ was simulated. For the simulation the EDB model was used to established the relation between the MC simulation time and the actual time and the current isothermal data of ultra fine grain steel was utilized. The effect of abrupt temperature gradient Austenite grain size growth in HAZ was analyzed. Also the grain growth in different weld parameters were simulated. By comparison of simulation results and the Austenite grain in HAZ in actual weld joints, the results show they are consistent. At the same time, the results show that hot pin phenomenon by temperature gradient has obstructive effect to grain growth in HAZ obviously. There is the same tendency between CGHAZ grain size and the distribution of temperature gradient. The tendency of temperature distribution has the great effect on distribution of the final grain size.The microstructures in HAZ by DC flash butt welding were discussed. Effect of the variation of micro structures in each zone of HAZ, and micro hardness by flash butt welding was analyzed. On the basis of the variation of temperature field during the process of the flash butt welding, the Austenite grain size in HAZ, cooling velocity, and co-effect of phase dynamics to structures and hardness were comprehensively considered. In addition the decomposing of Austenite diffusion reactant during cooling process was computed, by using the results of MC simulation and computing with finite element method. At the same time experimented data were calibrated. On the basis of the data calibration and verifying, the volume fraction of Ferrite and Pearlite which were...