# Mechanism and Effect Factor of Toughening of High-Speed Train Wheels
## 3 Results and Discussion
### SEM and EBSD Analysis  
The microscopic appearance is shown in Figures 4 and 5. It shows that the fracture surface has three major areas: one is the prefabricated fatigue crack area, and the other is the cleavage or quasi-cleavage crack propagation area. In the middle of the two areas, there is a certain width of the transition zone, which is the ductile laceration zone. For sample 1, the fracture surface is flat, the ductile laceration zone is narrow, and the edges are not neat, even though there is almost no ductile laceration zone in some areas. The brittle zone is a line-source fracture. The pattern is cleavage, a small amount of tearing ridge, and secondary cracks; the location of the secondary crack is located in the sub-surface of the fracture, about 5-15 mm, and the cleavage pattern is extremely large and uneven. For sample 4, a large number of small areas of ductile tear were found around the cleavage plane, as shown in the yellow arrow (Figure 5). More secondary cracks were found in the crack propagation zone, as shown by the red arrow. The fracture is rough, with a wide ductile laceration zone. The fracture pattern is cleavage, with more tearing ridges and secondary cracks, and the cleavage river pattern is small and uneven. It can be seen that the cracks in the samples started from the matrix structure, and the crack growth zone was mainly composed of the cleavage planes of different sizes and the ductile tearing edges around them. Secondary cracks could be found in almost all of the samples. The most significant difference between the two samples is the width of the ductile tear zone. Using a micrometer to measure, the maximum width of the ductile tear zone of sample 1 is 10.5 um, and sample 4 is 83.4 um. There is a good relationship between the width of the ductile tear zone and fracture toughness. The results of SEM analysis show that the microfracture morphology directly reflected the fracture toughness trend of the material, and the width of the ductile laceration zone at the transition between the fatigue fracture and the tear zone is positively correlated with KQ. The worst sample has almost no obvious ductile laceration zone. There is no direct correlation between KQ and the size of the secondary crack. Samples 1 and 4 were assessed by OM. The test results are shown in Figures 6 and 7 and Table 4. The microstructure of the samples is lamellar pearlite and pro-eutectoid ferrite along the grain boundary, and grain size is fine, but no large inclusions which the test results were graded according to ISO 4967, as shown in Table 4. In the inclusion class, there is no relationship between inclusions and fracture toughness. So, it needs further analysis. For Sample 1, inclusions mainly include MnS with elongated and elliptical shapes (Figure 10a) and MnS-wrapped oxides with elongated and elliptical shapes (Figure 10b). Oxides (Figure 10c) are abundant and concentrated, with sharp edges and angles. For Sample 4, inclusions mainly include MnS with elongated and elliptical shapes (Figure 11a) and MnS-wrapped oxides with elongated and elliptical shapes (Figure 11b). Oxides (Figure 11c) are less and triangular. Combined with the analysis results of SEM and EDS, it can be seen that the big inclusion is mainly a sulfide-wrapped alumina inclusion, whose main components are Mn, S, O, and Al. It is a mature technology of inclusion modification that helps to improve the plastic and toughness of steel [24; 25; 26; 27]. In addition, there are more oxides in the sample with lower fracture toughness, and the oxides are mainly diamond- and triangle-shaped. Sharp edges and angles will lead to stress concentration, which is detrimental to toughness. The microstructure of the rim of the railway vehicle wheel is lamellar pearlite and a small amount of ferrite, and the heat treatment method is to force cooling by water to form fine pearlite on the rim [28]. The distance of the pearlite layer directly affects the mechanical properties of the wheel rim.