Research On New Technical Process And Methods Of Pressurized Thermite Welding

Thermit welding technology has the advantages of simple equipment, convenient operation, short operation time, less space occupation, etc. The technology is widely used in constructions of new lines and maintenance of existing lines, and is one of the most important methods of long continuous welded track through railway station laying and continuously welded rail injury repairing. However, thermit welding joints’cast structure leads to poor ductility and toughness and tends to form casting defects such as blowhole, porosity, sonim and slag inclusion, hot crack and incomplete penetration. So it is difficult to guarantee the welding quality. As an effective field welding method, recent railway development in cold regions also poses an urgent need for its application in extremely cold conditions. Thereby the research directions of thermite welding is to improve toughness, reduce the brittle fracture and other welding defects.Pressurized thermit welding technology aims to refine the coarse grain of thermit welding joints’cast structure. Based on the previous pressurized thermite welding and small sample experiments, in this paper, small sample pressurized thermite welding experiments are carried out using the rail thermit welding materials and the self-designed upsetting equipment. According to the principles of pressurized thermit welding, the three major technique parameters:weld gap, time interval before upset and upset dimension are chosen. Using the method of orthogonal experiment, an optimized group of parameters is achieved, which maximum improves the microstructure and properties of thermit welding joints. By conducting impact and hardness test, and microstructure observation, the welding quality of conventional thermit welding and pressurized thermit welding is compared. As well as the impact toughness of rail flash butt welding, gas pressure welding, thermit welding and pressurized thermit welding are compared. The factors which influence toughness and the law why pressure thermit welding can improve the performance of joints are discussed.The results show that the optimized combination of the above three parameters are:5mm for weld gap,2seconds waiting time before upset and upsetting10mm. For pressurized thermite welding, the microstructure of the weld zone is composed of symmetrical ferrite and pearlite with small grain size. While the weld zone of conventional thermit welding contains widmanstatten structure with coarse grain. Compared to conventional thermit welding, the impact energy and average hardness of pressurized thermite welding’s weld joints are increased by62.51%and22.03%respectively. The result indicates that pressurized process can reduce defects, refine grain and improve the intensity, ductility and toughness of weld metal. Through the comparison of the impact toughness of rail flash butt welding, gas pressure welding, thermit welding and pressurized thermit welding, it can be shown that pressurized thermit welding’s impact toughness is2times more than that of the thermit welding and equivalents to normalized flash butt and gas pressure welding. Therefore pressurized thermit welding is an effective method to improve the welding quality of thermit welding.