Study On Post-welding Heat Treatment Process To Refinet The Rail Base Grain Size Of Welded U71Mn Rail Joints

There is increasing requirements for the welded joints technique as the development of the high-speed and heavy-duty railway. In China, Continuous Welded Rail(CWR) is always realized via flash welding technology. When U71 Mn rail is treated by welding flash, the grain size of joint will be coarsening, leading to the welded joints fragility and hence affecting the performance and lifetime of CWR. Therefore, to meet the operating requirement, post weld thermal treatment is demand for an enhanced tenacity. This study has modified the inductor design due to the negative effect including the high temperature in bottom triangle area and grain size coursing caused by the incompletely normalizing of the side of the rail corner during post weld thermal treatment in rail welding factory. Using this new inductor to optimize the after-welding thermal treatment process of U71 Mn rail welded joints, the optimum technical parameters are proposed as follow: heating power 62 kW, heating temperature 880 ± 10 °C, heating time > 130 s, wind pressure 0.06~0.10 MPa, air spray time 120 s. Based on these process parameters, the ratio of joint surface and the base material is 1.01, grain size of the rail head, rail track and the rail bottom triangle is well refined which is up to level 9. The obtained parameters are verified in two lines of the rail welding factory and testing results for the hardness, metallographic, tensile and impact of the welding joints meet the technical requirements in TB/T 1632.2. The as-designed new-style inductor has been applied for six rail welding factories and has achieved promising practical effect for post weld thermal treatment.