Bending Deformation and Microstructure Evolution of Bainite Rail during Cooling Process after Rolling

Due to the asymmetric cross-sectional characteristics of turnout rail, the rolling, heat treatment and straightening processes are very complex and the production is very difficult. In view of the flatness and residual stress of turnout bainite rail, the CCT curve was measured by Gleeble-1500, the microstructure under different cooling rates was observed by SEM, and the simulation calculation of controlled cooling before and between phase transformation was carried out by finite element software. The results show that with the increase of cooling rate, when the undeformed austenite is continuously cooled at the cooling rate of 0.05-1 ℃/s, the microstructure gradually changes from granular bainite to martensite. When the deformed austenite is continuously cooled at the cooling rate of 0.2-2 ℃/s, the microstructure gradually changes from granular bainite to a large amount of martensite + a little of lath bainite; Controlled cooling in phase change section adopts cooling the center of rail head surface to 500 ℃, then cooling from 500 ℃ to 300 ℃ (cooling rate of rail surface is 0.4 ℃/s), and then air cooling to room temperature to obtain bainite volume of 35.6 %, The final maximum tensile stress, maximum compressive stress, bending deflection in Y direction and Z direction meet the requirements. Before phase transformation, the center of rail head surface is cooled to 500 ℃ (the cooling rate of rail head upper surface, non working side and rail waist is 3 ℃/s, and the working side of rail head is 3.5 ℃/s). After air cooling from 500 ℃ to room temperature, 99.3% bainite structure can be obtained. The final residual stress value and bending deflection meet the requirements. The purpose of reducing bending deformation and residual stress after straightening is realized by controlling cooling. © 2024 Chinese Mechanical Engineering Society. All rights reserved.