Research On U75V Rail Flash-butt Welding Joint Based On European Standard And Its Heat Treatment

With the large-scale construction and operation of high-speed rail in China,the overall technology of China’s rail welding has reached the world’s leading level.Continuous welded rail（CWR）have been recognized by more and more countries because of its high smoothness,high reliability and less maintenance.Due to the discontinuity of welded joints in smoothness,microstructure and residual stress,they have always been the weak part of CWR.In Chinese standard of TB/T 1632.2<Rail Welding of rails Part 2:Flash butt Welding>,it requires that,flash-butt welding joints must be normalized after welding to improve strength and toughness performance,while most foreign countries do not require normalizing after welding.For example,all requirements of the European standard EN 14587 are for welded joints.At present,the formulation of the rail welding standard ISO 23300 undertaken by China is considering the inclusion of post-weld normalizing.Therefore,it is necessary to conduct induction normalizing treatment for joints that meet European standards and study the impact of normalizing treatment.In addition,for the normalized joints and welded joints,the Chinese standard and the European standard respectively require air jet cooling to strengthen the joint hardness,which can reduce the wear in the softening area of the joints and increase the service life.However,air-jet cooling will inevitably cause changes in the microstructure and residual stress of the joints.Therefore,the influence of different heat treatment on the joint microstructure and residual stress is studied to provide support for the formulation of ISO standard.In this paper,the UN5-150ZB1 mobile on-site flash welder was used to debug the welding process in line with the European standard EN 14587-2,and the 60kg/m U75V hot rolled rail was welded.The dual frequency induction normalizing treatment of the welded joint shows that the heat affected zone on the longitudinal section increases significantly due to normalizing,from 27mm～36mm to 75mm～98mm.The grain size of the weld was also extremely refined due to normalizing,the grain size was increased from grade 3.1 to grade 9.0.In addition,smaller and more evenly distributed residual stress on the welds of normalized joints.The hardness of the softened area of the rail head was reduced due to normalizing,and there was an area with hardness<0.9H_P,while the hardness of the softened area of the welded joints that meet the European standard is higher than 0.9H_P.Since both the European standard and Chinese standard air jet cooling are aimed at the rail head,the head jet cooling experiments of the U75V rail joint in the welded state and the normalized heating state were carried out respectively.The results show that:when the welded joint is subjected to air jet cooling,the hardness of the softened area increases slightly compared with the increase around the weld,and the compressive residual stress of about250MPa appears on the weld.However,when the rail head of the normalized joint is subjected to air jet cooling,it is difficult to form a residual stress exceeding 150MPa even at a large cooling rate,and the results show that when the cooling rate of t_8/5 reaches 4.7℃/s,the hardness of the rail head is almost above 0.9H_P.It can be seen from the above experiments that the air jet treatment improves the hardness of the rail head of the welded joint,which can improve the wear resistance of the joint.However,the higher residual stress formed is not conducive to the fatigue performance of the joint.Based on the above research results,the residual stress of the normalized joints under different air jet cooling methods was studied.The residual stress tests were carried out on the joints that are sprayed at the rail head,rail head and rail web,rail head and rail foot,and the full section respectively.The results show that the residual stress distribution of the full-section air jet weld is relatively uniform,and the compressive stress in the rail head,rail web and rail foot is in the range of-86.2 MPa to-20.6 MPa.In addition,the coarse grains formed by induction heating in the triangle area of the rail foot can also be refined to a certain extent due to the full-section air jet,so it is better to spray the joint on the full section.After optimizing the air jet device,the appropriate full-section air jet parameters are obtained:the surface cooling speed is in the range of 4.1℃/s～4.7℃/s,and the wind speed in front of the small holes on the air jet device is in the range of 3 m/s～4m/s,the stable air pressure is about0.16MPa.The air jet cooling starts from above 800℃,and the time for cooling to 400℃ should be 218s.