Grain Growth And Control During Rail Flash Welding

Seamless railway is the development trend of rail transit in recent years.Rail flash butt welding is the main welding method in the construction process of seamless railway.During the welding process,the metal near the joint is heated by the welding heat cycle,so the grains in the heated part will grow up,and coarse grains can adversely affect the mechanical properties such as strength,hardness and toughness of the joint.In China,the purpose of refining grains is mainly achieved by post-weld heat treatment.At present,some scholars have adjusted the joint grain size by changing the normalizing method,optimizing the structure of the induction normalizing coil and adjusting the induction normalizing frequency,but the use of a normalizing process will increase production processes,reduce production efficiency,and increase production costs,and improper normalizing process can even reduce the performance of welded joints,while for Europe,the United States,Japan,India and other countries,they do not need post-weld heat treatment for welded joints in the rail welding standard for seamless railway construction.With the promotion of China's “One Belt and One Road Strategy and China's High-speed Railway Going Global Strategy”,it is extremely urgent to make the rail welding standards in the construction of high-speed rails in line with international standards.At present,there are great differences between domestic and international standards.Therefore,it is necessary to conduct in-depth research on the influencing factors of grain growth and the measures to control the grain growth of welded joints,and provide support for national strategies and revised standards.In this paper,the grain growth process of U75 V,U71Mn and U78 CrV rail steels used in seamless railway construction is studied,and the grain growth model of U75 V,U71Mn and U78 CrV rail steels is established.Based on this,the influence of flash butt welding process parameters on the grain of joints was studied.The experimental contents and results of grain growth and control were as follows:(1)The relationship between the average grain size of U75 V,U71Mn and U78 CrV steels and temperature and time was obtained by austempering test,and the grain growth model was established.It is found that all of the three rail materials have a low-temperature slow growth zone and a high-temperature rapid growth zone during the heating process,wherein the grain coarsening temperature of U75 V andU78CrV is 950?,and the grain coarsening temperature of U71 Mn is 900?;(2)Under different welding processes,the length of the high temperature zone along the axis of the rail and the duration of the high temperature are different.Therefore,the specification parameters in the welding process are closely related to the grain size of the joint;(3)The grain size of the rail flash butt welding joint is affected by the amount of upset upsetting.Increasing the amount of upsetting can play a role in refining the grains.Since the upsetting process affects the grain size of the welded joint,the force and deformation of the clamp of the moving flash welder during the welding process are analyzed,and it is found that the clamp will be deformed due to plastic strain.To this end,relevant measures for the optimization of the clamp structure are proposed.