Study Of Welding Technology And Microstructure And Mechanical Properties Of Bainitic Steel Crossing And U75V Rail Joint

The common thermit welding and flash butt welding were used in the practicalapplication of bainitic steel crossing. The bainitic microstructure decomposed ortransformed due to a large amount of heat input. Compared with base metal, the hightoughness declined seriouly, it influenced the smoothness of welding joint and the servicelife of the bainitic steel crossing would decrease. In this paper, a method of improving themechanical properties of welding joint in the bainitic steel crossing and U75V rail wasproposed by analyzing the rail thermit welding and flash butt welding.By means of analyzing the rail thermit welding with a Gleeble-3500thermal simulator,we studied the microstructure and mechanical properties of thermit welding heat-affectedzone(HAZ) of bainitic steel crossing via controlled cooling and normalizing after weldingrespectively. The result indicated that the fraction of bainitic microstructure increased inthe intercritical zone, normalizing zone and overheated zone through rapid cooling to350℃follwed by holding20min and the hardness distribution and toughness weresignificantly improved. The simulated results showed that the bainite structure can beregained in the thermit welding HAZ by900℃normalizing. After normalizing, thecomprehensive properties of the welding heat-affected zone(HAZ) were significantlyimproved. Actual welding showed that, the hardness and tensile strength in HAZ aresignificantly improved when the joint was normalized at900℃.A novel flash butt welding was proposed through analyzing the production andTemperature Time Transformation (TTT) curves of rail steel and crossing steel. Firstly,bainitic steel crossing and U75V rail were welded by flash butt welding before bainiticsteel crossing subjected to isothermal transformation, and then the whole crossingsubjected to step austempering transformation in the P+B area. Finally, the microstructureof pearlite and bainite were obtained in the rail steel and crossing steel respectively. Theresult indicated that the microstructure and mechanical properties of base steel wereobtained on both sides of weld heat-affected zone(HAZ) and the impact toughness andultimate tensile strength in the centre of weld were substantially improved after weld joint subjected to550℃×15min+340℃×60min step austempering transformation.