Table 1 Chemical composition of grinding balls

1.2 Foundry Technology

Raw materials used low-carbon scrap, low-carbon ferrochromium, high-carbon ferrochromium, nickel plate, ferromolybdenum, Ferromanganese, such as 75 ferrosilicons. 350kg acid medium frequency induction furnace melting, out temperature 1550-1600 degrees, pouring temperature control in 1400 degrees, final deoxidization in front of the aluminum furnace, with 0.3%-0.9% rare earth alloy modification buried; high chromium cast iron grinding ball using metal mold casting The size is 50mm, a multi-model.

1.3 Heat Treatment Process

Heat treatment adopts 8kw box-type resistance furnace and 65kw box-type resistance furnace. The grinding balls were hardened by two-stage heating, I. E. 680 °C for 2H, 960 °C for 990 °C, 1010 °C, 1030 °C, 1050 °C, 1080 °C, 1100 °C for 3H, 4H, 4H, 6h, respectively The tempering temperatures of air quenching, oil quenching and special quenching solution are 530 °C and 275 °C respectively.

1.4 Test Methods

The samples were cut in the ball body by wire cutting and processed into 10 mm *10 mm * 50 mm non-notch samples. The impact energy of sample fracture was measured by the JB5 pendulum impact toughness testing machine, and the impact fracture size of the sample was measured by the micrometer. The hardness was tested with HR-50D Rockwell hardness tester on unnotched samples or balls (two parallel surfaces were milled out by milling machine) at 5 points and the average value was calculated. The metallographic specimen was cut from the broken end of the impact toughness specimen and then polished, pre-ground and polished to form metallographic specimen.

2. TEST RESULTS AND ANALYSIS

The hardness test results under different heat treatment process are shown in Table 2, figure 1, and the impact toughness test results are shown in Table 3.

2.1 influence of heat treatment process on grinding degree and impact physical property of Grinding Ball

As can be seen from Table 2 and Fig. 1, when quenched for 3 H, no matter how the quenching temperature, the quenchant, and the tempering temperature change. The hardness of the grinding ball is less than HRC, which shows that the holding time is too short and the secondary carbide is not fully analyzed, which leads to low hardness When special quenching liquid is used as quenching medium, the hardness can reach 60HRC at 1030-1080 °C for 4 H, at 1010-1080 °C for 5 H, at 60HRC at 990 to 1050 °C, and at 60HRC AT 6H. When the ball is tempered after 530 degrees *4h, the hardness decreases by 1 ~ 2HRC, but after 275 degrees *4h, the influence of hardness value is not great. Therefore, in order to make the hardness of this kind of 50mm grinding ball reach 60HRC, the most ideal process is: quenching temperature 1010-1050 degrees, holding temperature 5-6h, quenching with special quenching medium, and 275 degrees * 4H and Also. As can be seen from table 3, impact toughness decreases with increasing hardness but varies between 5J / CM2 and 6J / CM2.

2.2 Tissue after Heat Treatment

In Fig. 2a, the grinding ball structure is martensite matrix + carbide, and some secondary carbides are precipitated on the primary austenite with more retained austenite. In Fig. 2b, the grinding ball structure is similar to the figure 2a, but there are more precipitates on the AUSTENITE. In figure 2c, the grinding ball structure mainly consists of martensite + total carbides, secondary carbides, and residual austenite, and the amount of residual austenite is relatively large. In Fig. 2d is similar to that in Fig. 2c.  But the carbides are relatively small; In Fig. 2e, the grinding ball structure is martensite + chrysanthemum-like eutectic carbide + secondary carbide + very little residual austenite; In Fig. 2f, the austenite structure has been completely transformed into Martensite The final microstructure is martensite + dispersed eutectic carbide + Secondary Carbide.

Therefore, in order to make the hardness of 50mm high chromium white cast iron grinding ball larger than 60HRC, the optimum austenitizing temperature and holding time are fixed range. Because if the temperature is lower or holding time is not enough, the white cast iron is not fully austenitized and its hardness is lower after quenching. At this setting temperature, the white CAST IRON IS IN THE SECONDARY CARBIDE DISSOLUTION STAGE AUSTENITIC carbon content and alloy elements is high, austenite stability is increased, after quenching, residual austenite quantity is increased, resulting in hardness is reduced, cooling rate on the performance is also very large, air quenching and oil quenching hardness is less than 60HRC It is caused by insufficient cooling speed and unquenched grinding ball, and its hardness is mainly influenced by the amount of residual austenite in the tissue.

3. CONCLUSION

(1) the ideal heat treatment process of the designed dia50mm  chromium cast iron grinding ball is as follows: The quenching temperature should be 1010-1050 °c, holding temperature 5-6h;

Special quenchants are used and tempered at 275 degrees.

(2) because of the high hardness of M7C3 carbides, the high chromium cast iron grinding ball is heat treated to obtain a strong and tough matrix structure dominated by Martensite

So that the ball has good comprehensive performance, hardness of more than 60HRC, impact toughness in 5J / cm2.