The grinding ball is the grinding media in the ball mill. High Chrome cast iron grinding ball has high hardness, toughness, and excellent wear resistance, and is widely used in metallurgical mines, building materials, cement, and electric power industries. During the running of the ball mill, the ball is subjected to both abrasive wear and repeated impact loads. With the increase of the diameter of the ball mill, the wear, flaking and cracking of the ball are becoming more and more serious, which puts forward higher requirements for the strength, toughness and impact fatigue resistance of the ball mill.

In this article, a large number of high chromium cast iron grinding balls with quenched and tempered appearance appear spalling or even breaking when used in large ball mill. Based on the analysis of the effect of heat treatment on the Matrix structure and carbide, the isothermal quenching process and the graded austenitizing isothermal quenching process have been developed. The tests were carried out under factory conditions. The grinding balls are produced by sand-coated metal mold casting with a diameter of 100mm and chemical composition as shown in Table 1.

1. Quench +Temper Process

1.1 Mechanical Properties of quenched and tempered grinding balls

The grinding ball was heated to 950 °C for 150 min, then treated by oil quenching, tempered immediately after quenching, and cooled after heated to 380 °C for 480 min. After quenching and tempering, the hardness of the grinding ball is higher, the average hardness of the surface layer is about 60HRC, the average hardness of the center is about 56.1 HRC, but the impact toughness is not high, the average value is about 4.1 j / cm2.

1.2 Micro-structure of quenched and tempered grinding ball
The microstructure of the grinding ball after quenching and tempering is shown in Fig. 1. The eutectic carbides are distributed in discontinuous block-like and lath-like shapes with sharp edges and sharp corners. A large number of fine-needle-like tempered martensite are distributed in the Matrix, as shown in Fig. 1(a). As Fig.1(b) shows. The XRD results in Fig.2 show that the microstructure of the grinding ball is martensite + eutectic carbide + secondary carbide + a small amount of retained austenite. Because of the higher hardness of Martensite and eutectic carbide, the macro hardness of quenched and tempered grinding ball is higher, but the toughness of Martensite is worse, and the toughness of the Strip eutectic carbide with sharp edges and corners is unfavorable The grinding ball is less ductile.

2.Isothermal quenching process
2.1Effect of austempering on mechanical properties of Grinding Ball
After the ball was austenitized at 950 °C for 150 Min, it was quenched in the salt bath at 230 °C, 260 °C and 290 °C for 15,20,30,40 and 50 min respectively.

The results show that the hardness of the grinding ball is better when the salt bath temperature is 260 °C, and the impact toughness of the grinding ball is best when the isothermal quenching time is 40 min. The effect of isothermal quenching time at 260 °c on the hardness and impact toughness of the grinding ball is shown in Fig. 3. With the extension of the ISOTHERMAL quenching time, the surface hardness and the core hardness of the grinding ball show a decreasing trend, but the decreasing trend is gradually smooth, while the impact toughness first increases and then decreases. After isothermal quenching at 260 °C for 40 Min, the strength and toughness of the grinding ball were greatly improved, the surface hardness decreased by about 2HRC and the core hardness decreased by about 1HRC, but the impact toughness increased from 4.1 j / cm2 to 7.2 j / cm2.

2.2 Effect of Austempering on the Microstructure of Grinding Ball
The microstructure of the ball after isothermal quenching is shown in Fig. 4, and the XRD results are shown in Fig. 5. As can be seen from Fig. 4(a), the morphology of M7C3 EUTECTIC carbides has not changed much as compared with that of quenched + tempered state, and a large amount of fine-needle lower bainite is distributed in the Matrix after 40 min of isothermal quenching, as shown in Fig. 4(b). The spheroidal secondary carbides precipitated in the tempering process are dispersed in the Matrix, and the amount is less than that in the quenching + tempering state, as shown in Fig. 4(c).

Combined with the above analysis, the bainite transformation is not complete, but only consists of a small amount of acicular lower bainite and untransformed carbon-rich austenite, while the austenite with low carbon content changes to martensite during cooling the grinding ball shows high hardness and low toughness. after isothermal quenching for 40 min, the bainite transformation is sufficient and more needle-like lower bainite is obtained. the austenite is stable because of rich carbon, and the martensite transformation does not occur when cooling. the time of isothermal quenching is further prolonged, the bainite transformation is more sufficient, the amount of retained austenite is limited, the carbon-rich degree of untransformed austenite is enough to change the driving force of bainite transformation, carbides will be precipitated, and the composition of the whole structure has little change the hardness decreases gradually and the impact toughness continues to decrease. the isothermal quenching time should be chosen to obtain stable austenite while preventing carbide precipitation in austenite. therefore, when the isothermal quenching time is about 40 minutes, the comprehensive performance of the grinding ball is the best.