Study On Thermal Fatigue Crack Initiation Mechanism And Wear Performance Of High-speed Train Brake Disc

The brake disc is one of the more critical components of the braking device for highspeed trains,and its reliability in service has a direct impact on the safety of the train.At present,the failure of brake discs on high-speed trains mainly comes from two aspects,one is the uneven wear of the disc surface during the service process to make the brake disc brake wear intensify,and the other is the expansion of thermal fatigue cracks leading to the fracture of the disc body,so the combination of these two problems,the failure mechanism of the brake disc,friction and wear performance and thermal fatigue performance research is of great significance.This paper takes the CRH5 model brake disc as the research object,from the brake disc material’s own characteristics,the disc surface crack analysis,microstructure,phase composition,hardness,stress grain size,impact performance,tissue strength,thermal fatigue performance and friction wear performance analysis.The main research elements of this thesis are as follows:(1)Study the disc surface crack characteristics of brake discs,analyse the disc surface cracks,and also analyse the organisation of brake discs in different thickness directions to study the rules of their organisation evolution and differences in mechanical properties.The study shows that the disc surface cracks of the brake disc are mainly thermal fatigue cracks,the crack fracture oxidation is serious,the formation of cracks lags behind the evolution of the tissue.The 24 Cr Ni Mo steel evolves in the thickness direction,forming three types of tissue: surface decarburisation to form white layer tissue,heat affected zone tissue,and matrix tempered sorbite tissue.The grains crystallise at large angular grain boundaries and the formation of the white layer is caused by thermal and compressive stresses,with the hardness from the surface layer to the matrix showing a trend of first decreasing and then increasing.(2)The impact performance of the brake disc material-25 ℃ and 25 ℃ and the frictional wear performance of different tissues of 24 Cr Ni Mo steel were studied.The study shows that:the brake disc material has good impact resistance,the white layer tissue is mainly abrasive wear,the grinding depth of the disc specimen is larger;the matrix tissue is mainly adhesive wear,forming a third body film,isolating the direct contact between the disc specimen and the pin specimen,the wear performance is better than the white layer tissue,so the white layer tissue formed on the disc surface should be turned regularly,so as to reduce the grinding of the disc body.As the pressure load increases,the coefficient of friction first increases and then decreases,and the form of wear gradually changes from abrasive wear to adhesive wear.(3)The high temperature mechanical properties of the brake disc material and the hot and cold fatigue properties were studied.The results show that the strength of 24 Cr Ni Mo steel gradually decreases as the experimental temperature increases,and the material surface decarburises at high temperatures,so the elongation shows a trend of increasing and then decreasing.In the cold and hot cycle experiments,the crack extension area and extension length gradually increased with the increase of cycle cycles.Cold and hot cycles at 600 °C showed no significant changes in the tissue morphology,indicating that the 24 Cr Ni Mo steel has strong tissue stability below 600 °C.Cold and hot fatigue at 700 °C showed an evolution in the tissue morphology,with the tempered sorbite decarburising and recrystallising to form a granular structure.Crack extension occurs along the carbon-rich zone.Experimental results on thick specimens at 25-700 °C show that the increase in thickness of the specimen leads to a more difficult crack extension.