Research On Post-weld Treatment Process Of Bainite Thermit Welding Joint

In recent years,with the trend of Chinese railway towards high speed and heavy haul,higher requirements are placed on the wear resistance of rails.Research and the development of a new generation of high-strength rail has great significance.In order to apply bainite rail in continuous welded rail,many researchers have been studying the weldability and welding process of bainite rail.Thermit welding is one of the main ways of rail welding in China,and it is important to improve the quality of bainite rail thermit welding joints.Therefore,this paper studies and proposes solutions to two problems of fatigue crack of the rail head bond line and joint collapse in the bainite rail test section.At the same time,the feasibility of ultrasonic peening treatment to improve wear resistance of the joint is discussed.Aiming at the problem of fatigue crack at the location of the bond line of the bainite thermit welding joint of railhead,the joint is normalized and tempered.The effect of normalizing temperature on the hardness distribution and microstructure of joints and the effect of tempering temperature on the microstructure near the bond line are studied.The results show that the microstructure of the bainite is uniform after normalizing.As the normalizing temperature increases,the width of the softened zone increases and the position moves outward.When tempering at 400?,a small amount of black precipitates begin to appear in the weld microstructure.When the tempering temperature is higher than 450 ?,there are more black precipitates in the weld.The field results show that the tempering heat treatment can effectively solve the problem of fatigue crack in the bainite thermit welding joint at the position of the rail head bond line.Aiming at the problem of collapse of the thermit welding joint in the bainite rail test section of the Houma-Yueshan Railway,the cause of the collapse is first analyzed.Using the SEC-RC RAILSTRAIGHT to measure flatness of joints with collapse and non-collapse after 7 months,it is found that the wear of the collapse joint is greater than that of the non-collapse joint.Secondly,by calculating the primary grinding reserved amount and studying the effect of the final grinding temperature on the flatness,the "primary grinding + final grinding" grinding process is designed.Finally,joints in the bainite rail test section are polished by the optimized grinding process.The results show that the "primary grinding + final grinding" grinding process can avoid collapse and obtain an ideal flatness.In terms of improving the wear resistance of bainite thermit welding joint by ultrasonic peening treatment,the hardness distribution and flatness of the bainite thermit welding joints before and after the treatment and during service are measured and analyzed.The results show that there is no obvious change in the flatness of the joint after treatment,and the hardness of the treated area is significantly improved.The hardness of the treated area decreases rapidly after 1 to 2 days of rolling,the hardness is maintained within 2nd to 40 th days,and the hardness gradually decreases after 40 th day.Within the 2nd to 40 th days,the wear resistance of the weld and softened zone after ultrasonic peening treatment is significantly improved.In this paper,through the study of heat treatment process and grinding process,problems of fatigue cracks in the bond line of the bainite thermit welding joint and collapse are solved;For the first time,ultrasonic peening treatment is applied to on-site rail welding joints,and the feasibility of improving the wear resistance of the joints is discussed.The results of the paper have positive significance for the application of bainite rail in the seamless line.