Failure analysis of 90mm low chrome grinding ball cracking (2)
2.3 Metallographic test
With the hardness mentioned above test samples, the metallographic samples were prepared near the fracture point, polished and polished without erosion at 100-fold optical. Non-metallic inclusions were detected under the microscope. The results are shown in Table 2 and Figure 2.
Table 2 shows that less than 90mm low chrome grinding ball material contains non-metal, and the inclusion is especially significant. Among them, alumina class is 2—level 0,0 of the total field of view area. The spherical oxide is 215 grades of fine series and occupies an area of 1. 17%. Non-metallic inclusions exist as an independent phase in the matrix, which seriously damages the matrix’s continuity. It is the source of the micro-cracks and causes the stress concentration, which seriously reduces the impact fatigue life of the falling ball.
The XJ G25 metallographic microscope and TCI21 metallographic image analyzer were used to measure the grain number per unit volume and the grain size grade index 100 times. The unit volume, grain number, and grain size index of the low chrome grinding ball can meet the requirements of the enterprise standard. The results are shown in Table 3 and Figure 3.
Dia90mm low chrome grinding ball is a low alloy chilled cast iron with normal structure, which is pearlite and alloy carbide. The failure part’s microstructure is also pearlite (including 64.173% of austenite transformation product) + alloy carbide (32.18%), as shown in Fig. 4. From the diagram, it can be seen that there are a lot of austenite transformation products between pearlite and carbide. The products’ color is similar to carbide, but the microhardness is smaller than carbide but larger than pearlite. The results are shown in Table 4. Besides, the carbide content in the tissue is 32. 18%, higher than the usual carbide content in the sample (29.0%-30.50%). These results indicate that the tested specimens have high surface and core hardness.