Microstructure analysis of grinding strengthening layer of spiral bevel gear steel

Figure 1 shows the microstructure diagram of the section of the strengthening layer after grinding of 30crmnti. It can be seen from the picture that there is no obvious white layer and dark layer structure on the surface of 30crmnti after grinding, and the main body of the structure is relatively fine martensite, which shows a “fine coarse” change trend from the grinding surface to the center with the spiral bevel gear steel. This is because Ti element in 30crmnti material plays the role of grain refinement. In addition, due to the high hardenability brought by Cr element and Mn element, the structural composition is relatively uniform. It can be seen that the structural composition of carburized layer is well retained after grinding of spiral bevel gear steel.

Further observe the microstructure of the grinding surface and center of spiral bevel gear steel. See Fig. 2.

It can be seen that the main structure of the grinding surface of spiral bevel gear steel is extremely fine acicular martensite, accompanied by a small amount of carbide and residual austenite that cannot be completely transformed into martensite. This is because the grinding heating time of spiral bevel gear steel is very short and the austenite grain size is very fine, so the martensite structure obtained is very fine. The structure of the core is lath martensite + ferrite. The reason for the existence of ferrite is that the actual quenching temperature of the core is between AC1 and AC3, and the ferrite structure cannot be fully austenitized during heating, so there is residual ferrite in the structure during cooling. Due to the good hardenability of 30crmnti material, all austenite is transformed into martensite, so there is no residual austenite in the core. In addition, the microstructure of the strengthening layer decreases gradually from the surface to the center along the layer depth, and the acicular martensite transforms into a mixed form of sheet and strip.

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