Study On Microstructures And Properties Of Preheated Flash Butt Welded SW400 Steel Joints

With the development of the environmental protection consciousness, low energy consumption and environment friendly economic model will be the trend in the future development. In the automotive industry, manufacturers are actively seeking new development directions of energy saving. In the various factors influencing the fuel consumption of automobiles, body weight occupies a big proportion. Lighting the weight of car bodies becomes a hot spot of the vehicle development. As a part of a car, the wheel plays an important role in vehicle body lighting. Domestic vehicles, especially the mid-range and commercial vehicles, still use steel as the main production material of the wheels. So the mainstream development directions adopted by domestic wheel manufacturers are improving the steel strength grade and optimizing the structure of the wheel design. SW400 steel, developed by Benxi Iron and Steel Co.,Ltd, is a new developed dedicated steel for rim manufacture. With advantages of good bearing and safety performance, SW400 steel becomes one of the main production materials of rims.Flash butt welding has been widely applied to the production of rims. But the flash butt welded SW400 joints still faces many problems. Cracks, difficult to repair, are easy to appear in the interface of flash butt welded joints in subsequent process. That lead to the high rejection rate in the wheel production.The preheating process can effectively improve joints’ microstructures and mechanical properties of the rims. It is an effective method to optimize the wheel production technology. This article mainly studied the following things of SW400 joints:the thermal cycle characteristics of flash butt welded joints, the effects of preheating parameters on the thermal cycle, the macro-morphology and microstructures of flash butt welded joints, the effects of preheating parameters and flash allowance on microstructuresand properties of joints. Study results shows that adding preheating process can change the thermal cycle of joints. The two preheating parameters, preheating current and time,simultaneously decide the heat input in preheating process. Increasing one or both of them can alleviate the thermal gradient, increasing the peak temperature, heating rate and lower the cooling rate and chilling effect of different regions in joins.Flash butt welded joint can be divided into the weld interface zone(WZ), the overheated zone(OZ), recrystallized zone(RZ), and partially recrystallized zone(PZ). As for flash butt welded joints of SW400 steel, the weld interface area is mainly composed of clumpy ferrite, upper bainite and acicular ferrite due to factors such as decarburization,high temperature and the thermal deformation. In overheated zone there is mainly the coarse upper bainite, with a small amount of pearlite generated. In recrystallized zone the grains are refined, mainly composed of clumpy ferrite and pearlite. The composition of the partially recrystallized zone is consistent with the recrystallized zone, but the grain size is uneven. With the increasing of preheating current and time, the peak temperature and heating heating rate of SW400 joints are increased and the cooling rate is lowered. The joints’ changes of microstructures under these conditions are the decrease of upper bainite and acicular ferrite, the increase of blocky ferrite, the coarsening grain and the more homogeneous microstructures. The moderate preheating current and time can ameliorate the microstructures of joints. But the excess of the two preheating parameters will lead to serious grain coarsening and the obvious decrease of mechanical properties. This rule is verified in tests of joints’ mechanical properties.The flash allowance in flash butt welding process also have obvious effects on the microstructures and mechanical properties of joints. After adding preheating process, the flash allowance should be appropriately decreased. Otherwise, the microstructures of joints can be seriously coarsened due to the extra heat input brought by preheating process and the properties of the joints will be lowered at the same time. The decrease of flash allowance can also save material and welding time.