Research On Microstructure Transformation And Mechanical Properties Deterioration Mechanism Of Brake Disc Material In Service

The brake disc is an important basic brake component to ensure the safety of rail vehicles.The alternating braking load will cause the material structure of the brake disc to change after long-term service,cause the mechanical properties to deteriorate,and cause hidden dangers to the service safety of the train.In this paper,the wheel-mounted cast steel brake disc of the EMU is taken as the research object,and the material structure and mechanical properties of the brake disc are characterized.The effects of material microstructure transformation and casting defects on the deterioration of mechanical properties were discussed,and the failure mechanism of the material in the hot spot area of the brake disc was analyzed.The main research contents of this paper are as follows:First,the characterization of the microstructure and mechanical properties of the brake disc material was carried out.The room temperature-high temperature tensile mechanical properties of the new cast steel brake disc material were tested,and the Ramberg-Osgood constitutive model of the material at different temperatures was established.The brake disc after service was dissected,and the microstructure characteristics and mechanical properties distribution of the material from the surface layer to the core of the brake disc were studied.The results show that the microstructure transformation of the brake disc material after service is mainly manifested by the grain refinement of the surface material of the friction surface,and the martensitic transformation in the local hot spot area.The transformation of the material structure will cause non-uniform distribution of the hardness of the friction surface.The results of the layered tensile test of the disc body show that the yield strength and elongation after fracture of the friction surface material of the brake disc after service are lower than the mechanical properties of the core and the new disc material.Secondly,the influence mechanism of microstructure transformation and casting defects on the deterioration of mechanical properties of the friction surface surface layer was studied.The in-situ tensile test of the friction surface samples of the new disc and the brake disc after service was carried out under the scanning electron microscope.The research shows that different from the fracture mechanism of the defect-free new brake disc material,the friction surface of the brake disc after service has a mixed microstructure of coarse grains and fine grains.Under the action of tensile load,local uncoordinated deformation occurs between coarse and fine grains,and micro-cracks are easily formed at the interface between coarse and fine grains.The hole casting defect is the location where micro-cracks are initiated.Under the action of tensile load,the microcracks will gradually connect with the surrounding defects and expand rapidly after the initiation,and the yield strength and elongation after fracture of the material will be significantly reduced.Finally,the thermal cycle conditions of the microstructure transformation of the brake disc material were studied,and the influence mechanism of the hot spot on the deterioration of the mechanical properties of the material was studied.According to the temperature field simulation in the braking process and the temperature measurement results of the full scale braking test.The local thermal cycle test of the brake disc material was carried out on the Gleeble test machine to simulate the generation process of the hot spot area.Based on the measured continuous cooling transition curve of the material,the microstructure characteristics and hardness distribution of the brake disc body material after thermal cycle test and after service were compared and analyzed,and the temperature cycle conditions for hot spot formation were clarified.The tensile stressstrain behavior of thermally cycled materials was studied based on the digital image correlation(DIC)tensile method.The results show that under the action of tensile stress,strain concentration occurred at the boundary of the microstructure transition zone formed by local thermal cycling,which aggravates the material.The uncoordinated deformation of the yielding process causes the material to exhibit a decrease in strength and plasticity.The research work in this paper provides a scientific basis for the deterioration mechanism of the mechanical properties of the brake disc material.The research work has a certain engineering reference value for the maintenance of the performance of the brake disc in the middle and later stages of service and the improvement of the maintenance and repair technology.