Research On Microstructure Transformation And Residual Stress Of U75V Steel Rail During Laser Additive Repairing

With the rapid development of society,railway transportation presents the characteristics of high-speed,heavy-load,and high-density transportation.The failure phenomena such as rail wear,abrasion,and spalling are becoming more and more serious,threatening the safety of railway operation and service.Therefore,timely online high-quality repair of damaged rails is of great significance.This paper focuses on the unsteady temperature field,microstructure transformation and residual stress evolution in the process of laser additive repair of U75V steel.The main contents and conclusions are as follows:(1)The U75V steel cladding test was carried out by using a hollow ring laser,the microstructure of the sample was characterized,and the distribution of the structure in the cladding layer was analyzed.The heat source equation was constructed,and the unsteady temperature field in the cladding process was simulated by the FEM,and the shape of the molten pool and the thermal cycle curve were measured by infrared cameras and thermocouples to verify the accuracy of the temperature field.Combined with the structure of the cladding layer,the influence of thermal cycle on the structure transformation was analyzed,and the temperature field was introduced into the cellular automata and phase field structure transformation model,and the intergranular ferrite at the austenite grain boundary was accurately predicted.The growth behavior of the pearlite sheet and the rapid growth behavior of the pearlite layer to the austenite phase region have successfully constructed a coupling model of the unsteady temperature field and the microstructure transformation.(2)Write the phase change subroutine based on the K-V and K-M microstructure transition models,and analyze the microstructure state in the cladding layer considering the temperature and cooling rate,and construct the influence model of the temperature field and microstructure state on the residual stress.On this basis,microstructure state and stress field was constructed.Combined with the measured residual stress values of neutron diffraction,the accuracy of the coupling model is verified.Based on the coupled model,the effects of different cladding layer heights,laser power and preheating temperature on the evolution of residual stress were further explored.The results show that as the cladding layer height increases,the changes of σMises,σx,and σy tend to be gentle,and the stress gradient decreases;as the laser power increases,the maximum stress values of σMises,σx,and σy gradually decrease,and the stress concentration area gradually decreases,reducing the stress concentration area at the start t and end point,and suppressing cracking;as the preheating temperature increases,it can promote the uniform distribution of residual stress.(3)Based on the coupling model of unstable temperature field,microstructure state and residual stress field,a optimization strategy is proposed.Strategy 1:Design the alloy composition of the cladding powder,add Co element to the powder,promote the leftward shift of the CCT curve of U75V steel,and increase the critical cooling rate of pearlite transformation;Strategy 2:Preheat the matrix to increase the base temperature,so that The minimum temperature of the thermal cycle is higher than the Ms point,which inhibits the martensitic transformation.Combining strategy 1 and strategy 2,an optimized process is proposed,adding 7%Co to the alloy powder and preheating the substrate to 350℃.The results show that with the increase of Co content and preheating temperature,the pearlite content increases,and the process optimization is beneficial to promote the formation of pearlite,which proves the feasibility of the optimization strategy.Then,the properties of the substrate,cladding layer and the cladding layer prepared after the optimized process were characterized,and the comprehensive performance evaluation was carried out.The hardness,tensile strength,and wear resistance all show that the cladding layer and the substrate have the same performance,and after the process optimization,the bonding strength between the cladding layer and the base material is significantly improved,and the laser additive repair of the U75V steel cladding layer can Meet the requirements of rail service performance.