# Mechanism and Effect Factor of Toughening of High-Speed Train Wheels
## 3 Results and Discussion
### SEM and EBSD Analysis
The smaller the lamellar spacing, the higher the strength and hardness, and the better the plasticity and toughness [29; 30; 31; 32]. The quantitative measurement results of the pearlite lamellar spacing by SEM are shown in Figures 12 and 13. The minimum distance between the pearlite lamella at different positions in sample 1 is 0.178 \(\upmu\)m, the maximum is 0.232 \(\upmu\)m, and the average is 0.210 \(\upmu\)m. There are large aggregated grains in the pearlite area, and the pearlite is mostly arranged in parallel. Figure 11: Inclusion characteristics in Sample 4. (**a**) Pure sulfide inclusion (MnS); (**b**) mixed inclusions (MnS-wrapped oxides); and (**c**) mixed inclusions (alumina and calcium oxide). The minimum distance between the pearlite lamella at different positions in sample 4 is 0.178 \(\upmu\)m, the maximum is 0.201 \(\upmu\)m, and the average is 0.189 \(\upmu\)m. The pearlite is uniform, and the pearlite is arranged in different directions. The test results show that the finer the pearlite lamellar spacing, the higher the fracture toughness KQ, and there is a linear correlation between the pearlite lamellar spacing and the fracture toughness KQ. The reason for the low fracture toughness of Sample 1 is related to the large pearlite lamellar spacing, the coarse grains in the pearlite region, and the poor uniformity of the pearlite lamellar spacing [33; 34; 35]. It is generally believed that grain size has a great influence on the toughness of steel, but under the OM, the grain size of high-speed train wheels can reach 7.5 grade or more, and some even reach 9.0 grade, but in this case, the wheel fracture toughness fluctuation is still not eliminated, nor has it improved the wheel fracture toughness value [36; 37; 38]. Therefore, general grain size measurement alone cannot explain the high and low fracture toughness. By analyzing EBSD to determine the fracture toughness of the specimens, it was found that there are large differences in ferrite orientation. It can be judged that a pearlite block of pearlite structure (the same ferrite orientation) has an obvious hindering effect on the cleavage crack. Therefore, the reason for the toughness difference can be analyzed by measuring the size of the pearlite block. The ferrite orientation of Samples 1 and 4 was determined by EBSD. The results are shown in Figure 14.  
The results show there are several large sizes (\(\geq\)20 \(\upmu\)m) of pearlite block, which points to the non-uniformity of grain size distribution. However, sample 1 has more red lines and shows more pearlite block, i.e., lower fracture toughness. It reflects fracture toughness fluctuations of the same wheels associated with the uneven pearlite block. Figure 12: Quantitative analysis of Sample 1 pearlite lamella. Figure 13: Quantitative analysis of Sample 4 pearlite lamella. The step size of the EBSD orientation analysis was 200 nm. Due to the small size of the cementite lamellar layer in the pearlite lamellar, only a small amount of the cementite phase could be seen in the EBSD image, and the distribution was irregular and scattered. The region with the same ferritic orientation can be considered to belong to the same pearlite pellet, and the direction difference between adjacent pearlite pellets is usually greater than 15\({}^{\circ}\), forming a large Angle grain boundary. There are also a large number of small Angle grain boundaries below 8\({}^{\circ}\) between the ferrite inside the pearlite pellet. In most cases, the continuous orientation difference between the ferrites in the pellet gradually disappears within the crystal. Sometimes, this small Angle grain boundary can be continuously extended to separate independent pellets. From the above results, it can be found that the average size of pearlite pellets obtained by using the grain boundary map of the EBSD orientation difference is about 10 \(\upmu\)m.