Study On Heat Treatment Process Of Track Plate 25MnB Steel

Nowadays,25 MnB steel is the latest developed new type low carbon and alloy steel for crawler plate,which has excellent comprehensive mechanical properties after hot rolling forming and heat treatment.During heat treatment quenching cooling,controlling the cooling speed is always a concern.Due to differences in cooling velocity at different positions,predisposing to a heterogeneity phase transformation,and internal hardness or plasticity do not meet the standard requirements.If the cooling speed is too fast,it is easy to produce a large deformation,and the planeness cannot meet the requirements,reducing the product qualified rate.In this thesis,based on the temperature and time curve of heating,cooling and quenching during the heat treatment of crawler plate,and combined with the calculation results of Deform software,the surface comprehensive heat transfer coefficient of crawler board in each stage is fitted.At the same time,according to the range of heat treatment process parameters,five sets of different cooling speed were tested on the thermal simulation machine,then tempering insulation treatment in the tunnel furnace.Through the tensile experiment and the hardness performance test,the critical cooling rate that meets the performance standard is explored.The SEM was used to observe the microstructure morphology and the fracture,and finally combined with the results of XRD experiment to analyze the reasons of the performance difference.Then,with the MO-HT module of Deform software,the new kind crawler board was simulated with different cooling rate.And on the premise of satisfying the microstructures transformation and deformation,the heat treatment process of the new model crawler board was finally determined.It is found that the quenching cooling rate during heat treatment has an obvious effect on the mechanical properties and microstructure of 25 MnB steel.With the increase of cooling rate,the main is that the resistance of martensite phase transition gradually increases,and the transition of martensite Ms point temperature gradually decreases.The tensile strength and Rockwell hardness showed a gradual increasing trend,and the plasticity gradually decreased.The microstructure gradually fine and uniform distribution,phase transition generated in the martensite particle size gradually decreased,larger cooling rate reduce the carbide precipitation reaction time to a certain extent,thus hinder the carbide precipitation in the matrix,will make more alloy element solid solution in the matrix,and increase the lattice resistance.At the same time,which will produce greater quenching stress,leading to the nucleation rate of martensite rise,reducing the time of grain nucleation and grow up.It can make the effective grain size get good refinement,also improve martensite transition resistance and increase the energy needed for the plastic deformation.It further enhance the mechanical properties of sample steel,so as to meet the standard requirements,the comprehensive mechanical performance is excellent.From the whole quenching numerical simulation process,with the increase of the cooling speed,the maximum deformation dominated by the thermal stress in the first stage gradually increases,the deformation generated in the second and third stages also gradually decreases,and the deformation generated after the quenching is gradually increasing.According to the comprehensive analysis of the deformation and microstructure transition rate,the cooling speed of 30℃/s and the quenching time of20 s are the most suitable heat treatment process parameters for the new type of track,which can meet the standard while ensuring thflatness of the plate surface.Through the above research work,the rules of microstructure and performance of25 MnB crawler plate in the production process with cooling speed is summarized,and the relationship between organization,performance,quenching deformation and cooling speed in heat treatment process is established,and the conventional mechanical properties at room temperature are analyzed and tested,and a reasonable heat treatment process parameter is proposed and there are the reasonable theoretical basis and the technical specifications for the actual production and application.