Influence of Heat Treatment on Structure and Properties of High Chrome Cast Iron Grinding Ball (2)
2.2Microstructure after heat treatment
Without heat treatment, the as-cast cast iron grinding ball’s microstructure is austenite + eutectic carbide, which is heated at lower quenching temperature (such as below 950 °c). After Air Cooling, the microstructure contains austenite, eutectic carbide, and martensite; there is also a large troostite (see Fig. 2A). At higher quenching temperature (above 980 °C), the amount of troostite in the air-cooled microstructure disappears, the number of austenite increases obviously, and the secondary carbides become shorter and smaller until the uniform dispersion distribution (see Fig. 2B), which is beneficial to the improvement of properties.
Therefore, the selection of the quenching process plays a key role in the performance of cast iron grinding balls. It is reported that in the austenitizing temperature range, quenching temperature is too high, the carbon in the austenite in the higher solubility, cold to room temperature, only a part of the transformation into Martensite, the remaining austenite will remain. The quenching temperature is lower, the solubility of carbon in austenite is lower, the carbon content in austenite is lower when cooling to room temperature, most austenite transforms into Martensite, a few retained austenite.
In a process, the quenched microstructure is eutectic carbide, retained austenite, martensite and troosite. The appearance of troostite shows that the high chromium grinding ball has not been quenched, which is attributed to the low quenching temperature, cooling, and insufficient speed; with the increase of quenching temperature, the ratio of the microstructure and the troostite decreases. After tempering at 260 °C, higher hardness and impact toughness can be obtained, but the increased range is limited. The impact toughness is still low in the wear of high-stress dynamic load; the high chrome cast iron grinding ball’s performance is difficult to meet the use requirements.
In the B process, the carbide in the air-cooled quenched structure is coarser when the quenching temperature is 950 °C (Fig. 2C), while the carbide in the air-cooled quenched structure is smaller when the quenching temperature is 980 °C, the martensite Matrix is uniformly distributed (Fig. 2D), and the amount of retained austenite is increased. With the increase of quenching temperature to 1060 °C, the amount of austenite in the matrix increases to 40%, the impact toughness also increases, but the hardness decreases obviously. Therefore, the grinding ball can be quenched at 980 °C to obtain the peak hardness.
For the amount of retained austenite in the structure, many scholars believe that Austenite’s contribution to wear resistance is greater than that of Martensite to wear resistance in dynamic load wear. It is also found that 35% retained austenite is more favorable to the grinding performance. In the test, the impact toughness of high chromium grinding ball tempered at 400 °C is more than twice as high as that tempered at 260 °C, and the microstructure has not changed after tempered at 400 °C; it is considered that high-temperature tempering can completely eliminate the stress of high chromium cast iron, but after tempering temperature is higher than 450 °C, the hardness of high chromium cast iron will decrease obviously because of martensite decomposition and carbide aggregation.
It is concluded that the 980 °C × 3H air cooling + 400 °C × 3h furnace cooling process for less than 80mm high chromium grinding balls has better performance.
The 80mm high chrome cast iron grinding ball was prepared by air cooling at 980 °C × 3h and tempering at 400 °C × 3h. Then produce a batch of high-chromium cast iron grinding ball, sent to Fankou concentrator in, diameter 2. 7M × 3. 6 M ball mill, the wear is only 74 g/t, the breakage rate is 0. 56%, won the user’s praise.
(1) the grinding ball of high chrome cast iron has good comprehensive properties because of the presence of M7C3 carbide with high hardness and the basic structure of the main martensite obtained by heat treatment.
(2) the high chrome cast iron grinding ball is cooled by 980 °C × 3H air cooling + 400 °C × 3h furnace cooling.