# Mechanism and Effect Factor of Toughening of High-Speed Train Wheels
## 3 Results and Discussion
### Results Discussion
Therefore, it can be inferred that during the fracture of the samples, more cracks are generated at the grain boundary between the coarse grains and fine grains, forming a fracture-concentrated crack source and leading to brittle fracture. The uniform fine crystal in the toughness fracture will produce a certain dimple region, increase the fracture energy, and have a high toughness. Further analysis by OM shows that there is no inclusion in the sample, and for the occasional abnormal reduction in toughness, combining the results of SEM and OM shows that there are a certain number of large grains in the sample, but not all large grains will lead to the reduction in toughness. Grains of different sizes have a different tendency to form concentrated crack sources, mainly due to the different interface stresses of grain sizes. The authors believe that wheel fracture is carried out along the grain, regardless of brittleness or toughness. When the size of the grain reaches a certain size difference, there is a large stress there, and brittle fracture will occur, resulting in fluctuations in toughness. The critical grain size or interface stress still needs further study. In conclusion, the relationship between grain size and toughness fluctuation is proportional to the average grain size and toughness, and there is a random fluctuation in fracture toughness. One of the principles is that there is a critical difference in grain size, which will cause a concentrated crack source during fracture, resulting in brittle fracture and an abnormal decline in toughness. [PERSON] et al. believe that the fracture toughness of the steels is not significantly altered by the heat treatments, but a decrease is registered only for ER7 steel [2]. In terms of crack propagation curves, none of the tested steels exhibited significant variation as a consequence of thermal alterations, meaning that their fracture mechanics properties are stable even under severe shoe braking conditions. But through testing and analysis, the fluctuation of fracture toughness on the wheels of high-speed trains eliminated the possibility of uneven macroperformance. It has to do with the inclusion's shape and amount. The width of the ductile laceration zone and the size of the cleavage river pattern are closely related to fracture toughness, but there is no correlation for the secondary cracks. Then, pearlite lamellar spacing and uniformity of the grain affect the value of fracture toughness. For ER7 high-speed train wheels, it is not the average grain size but the maximum grain size in the microstructure that affects the fracture toughness fluctuations. A small number of coarse grains (or abnormal grains) after the transformation of pearlite leads to a larger pearlite cluster size, which has a small hindrance to crack propagation. Therefore, in order to improve the fracture toughness of ER7 wheels on high-speed trains, it is necessary not only to refine the grain size but also to make the grain size uniform. Fracture toughness fluctuations in the same wheels are associated with the uneven pearlite block. Moreover, the coarse original austenite grains cannot be eliminated in the subsequent process, and the heat treatment process fails to effectively precipitate homogeneous ferrite.