The hardness and impact toughness of the grinding ball after 30 Min and 50 min austenitizing austempering are equivalent. The surface hardness is about 59HRC, the center hardness is about 54.5 HRC, and the impact toughness is 7.1ー7.4 J / cm2. The surface hardness of the grinding ball is similar to that of the quenched and tempered state, the impact toughness is greatly improved, and the comprehensive properties are slightly improved.

3.2 Effect of graded austenitizing austempering on microstructure
The microstructure of the grinding ball after 50 Min austenitizing isothermal quenching is shown in Fig. 6. There are a lot of needle-like lower bainite and round granular secondary carbides in the Matrix. The graded austenitizing treatment promotes the precipitation of the Round Secondary Carbides and makes the sharp M7C3 eutectic carbides rounded and passivated, which contributes to the improvement of the macro-hardness and impact toughness of the grinding balls. Its comprehensive properties are better than those of austempered grinding ball.

4. Drop ball fatigue test of Grinding Ball
In order to simulate the impact resistance and wear resistance of high chromium cast iron grinding balls after different heat treatment under large diameter ball mill, 20 grinding balls of quenching + tempering, 40 minutes of isothermal quenching and 50 minutes of graded austenitizing isothermal quenching were respectively tested on a 5.96 m high falling ball fatigue testing machine. The results are as shown in Table 4.

The surface of the grinding ball is exfoliated and broken as shown in figure 7. Due to the poor toughness and low resistance to plastic deformation of Martensite hard matrix phase, repeated plastic deformation occurs under the repeated impact, which results in the coalescence of micro-cracks and the formation of fatigue cracks or spalling wear. In addition, the slab eutectic carbides with sharp edges and a large number of massive and Fine Secondary Carbides have a great effect on the fracture of the hard matrix, the EUTECTIC carbides are easy to break and the secondary carbides are easy to fall off under large impact The contact position between them and the Matrix is easy to produce fatigue crack . Furthermore, the phase transformation hardening of retained austenite in the Matrix also leads to the initiation and propagation of microcracks. As a result, a large number of spalling and even breaking phenomena occurred in the quenched and tempered grinding balls, as shown in Fig. 7(a).

The hardness of Austenite + Bainite Matrix obtained by isothermal quenching is close to Martensite, but the toughness is much higher than that of Martensite Moreover, the plastic deformation ability of AUSTENITE + Bainite is higher, the resistance of crack passing through AUSTENITE + Bainite is higher than that through Martensite, and the internal stress of Austenite + Bainite is lower than that of Martensite The residual stress in the structure is small, and the crack is not easy to spread, which can slow down the wear of the grinding ball [10] . In addition, after graded austenitizing, eutectic carbides with obtuse edges and round Granular Secondary Carbides can reduce the splitting effect on the Matrix. Therefore, after isothermal quenching, the impact fatigue resistance of the grinding ball is higher, only a small amount of surface spalling occurs, as shown in figure 7(b) and figure 7(c).

5.CONCLUSION

(1) After quenching + tempering grinding ball heat treatment , the high chromium cast iron grinding ball obtained martensite matrix structure, in which a large number of massive and rod-like Secondary Carbides were dispersed, the surface hardness of the grinding ball was about 60HRC, the central hardness was about 56HRC The impact toughness is about 4.1 j / cm

(2) After isothermal quenching in salt bath at 260 °c for 40 min, the high chromium cast iron grinding ball was obtained with a fine needle-like bainite as the main structure and a certain amount of round granular secondary carbides dispersed in the matrix, and the strength and toughness of the grinding ball were well matched the surface hardness is about 58hrc, the core hardness is about 55hrc, and the impact toughness is about 7.2 j / cm 2.

(3) After austenitizing at 980 °C for 150 min + 920 °C for 30 min and austempering at 260 °C for 30ー50 Min, the sharp eutectic carbides in the high chromium cast iron grinding ball were passivated and the amount of round secondary carbides increased, the strength and toughness of the grinding ball were improved, the surface hardness was kept at about 59HRC, the core hardness was kept at about 54.5 HRC, the impact toughness was kept at 7.1ー7.4 J/cm

(4) the impact toughness and anti-impact fatigue of high chromium cast iron grinding ball are much higher than that of quenching + tempering after conventional isothermal quenching process and graded austenitizing isothermal quenching process For High Chromium cast iron grinding balls used in large diameter ball mills, the isothermal quenching heat treatment process is better.