Study On Microstructure And Mechanical Properties Of Bainite Steel Bifurcation And U75V Rail Welded Joints

In recent years,with the development of high-speed and heavy-duty railways in the world and the development trend of railway track lines,it is necessary to weld bainite alloy steel frogs and U75V rails in order to meet the increase of train speed and seamless railway construction.Compared with the traditional rail welding methods,flash butt the welding process is a kind of solid phase and does not need to add additional metal materials and heats to the plastic state temperature just short time and metallurgical process is simple,so the welding joints appearance quality and stability are superior to the other welding methods for joints.At the same time,it is able to ensure the bainite alloy frog and U75V rail flash welding joints performance.Generally,the welding joints need to experience medium frequency induction normalizing process.The microstructure and properties of the bainite steel and U75V rail flash butt joint before and after the fire are studied in this paper.Through the static bending,tensile,impact,falling weight,microstructure and hardness test,we analyze the medium frequency induction normalizing before and after process of bainite alloy steel frog and U75V rail flash welding joints performance.The research results show that:By comparing the microstructure and properties of the bainite steel and U75V steel flash butt joint before and after the medium frequency induction normalizing treatment,it is easy to find that the tensile strength and impact toughness of the welded joints after medium frequency induction normalizing treatment are better than unheated welded joints,and greatly improve the static bending performance of welded joints.In the same time,it greatly improves the static bending properties of welded joints,which fracture load and deflection are much higher than the ministry of railways TB/T3120-AT the rail welding-2005 standard.The highest hardness appears in the heat-affected zone and there is a softening zone without heat treatment of welded joints.After normalizing treatment process,the hardness distribution of welded joints is stable and the softening zone disappears,which has a good effect on the uniformity of the hardness distribution of welded joints.The weld area of welded joints without heat treatment are mixed microstructure of granular bainite,lath bainite ferrite bundle,massive pearlite and residual austenite.The base metal,heat-affected zone and near-seam zone of the bainitic steel are mixed structures of alternating black bain bainitic ferrite bundles,granular bainite and retained austenite and exist between the slats more fine carbides.U75V steel side base material area,heat affected zone and near seam area are coarse block pearlite and ferrite mixed structure,pearlite and ferrite after normalizing treatment of welded joints of weld zone,heat affected zone and the base area and close to seam area are small and uniform distribution of microstructure.At the same time,the carbide precipitated from the side of bainitic steel is reduced and the strength and toughness of the welded joints 'are improved.The welded joints without heat treatment were broken in the drop hammer test with the hammer height of 2.8m,while the welded joints after the medium frequency induction normalizing treatment were broken continuously,meeting the specified welding standards of TB/T3120--AT rail welding--2005 of the ministry of railways.