Study On Microstructures And Mechanical Properties Of Flash Butt Welded Joints For High Strength Steels

With energy crisis and environmental pollution problems growing, as a pillarindustry of manufacturing industries, the automobile industry is facing more and moresevere challenges. Automobile lightweight，energy saving，environmental protectionand safety have become an inevitable trend of the development of automobile industrytoday. The wheels play important roles for motility and safety of the car, and realizethe rim light is particularly important. High strength steels are important lightweightmaterials for rim manufacturing. The fine bearing performance and security advantageof high-strength steel have made it become the main production material of rims. Theflash butt welding method was widely used in rim production. With the improvementof steel strength level of the rim, new flash butt welding problems for rim productionemerged, which significantly affects the production quality and safety. Therefore,carrying out research on microstructures and mechanical properties of flash buttwelded joints has important theoretical significance and practical value.In this paper, the flash butt welding of B600CL steel and SW400steel wasstudied to use different welding process parameters. By changing flash allowance,upset pressure and upset allowance, microstructures and mechanical properties of twokinds of steel welded joints were analyzed. Their section structures andmicrostructures were observed and analyzed by using an optical microscope. Weldingzone width, coarse grain region width, crystal streamline curvature, interface regiongrain size and other important parameters were measured. Based on test results oftensile properties and fracture analysis, the effecting law of welding processparameters on the mechanical properties of joints was revealed. Finally, by usingorthogonal experiment method the optimum parameters for the flash butt weldingwere determined, based on the bending property of joints.The results indicated that for both B600CL steel and SW400steel, withincreasing the flash allowance, the crystal grain bending degree, welding zone width,overheating region width and interface region grain size increase. With increasingupset pressure, the crystal grain bending degree increases, but welding zone width,overheated region width and interface region grain size decrease. With increasingupset allowance, the crystal grain degree increases, and the welding zone width andoverheated region width decrease, but the upset allowance has not significant effecton the grain size in interface regions. For B600CL steel, increasing flash allowance results in decreasing joint strength,increasing upset pressure lead to enhancing joint strength, and the upset allowance hasnot obvious effect on the strength of joints. The thickness of SW400steel plate islarger compared with that of B600CL steel plate. Increasing flash allowance wellreduce the strength of joints. In the range of4.5mm-5.5mm of upset allowance, jointshave higher strength and ductility. With orthogonal experiment method the optimumparameters obtained are the flash allowance of16mm, upset pressure of4MPa, upsetallowance of5.5mm and upset current time of0.6s.