High-chromium Cast Ball Heat Treatment Process Research (2)
2.Effect of quenching temperature on microstructure and hardness
Heat treatment process: It is difficult to quench the wear resistant cast ball, mainly because the quenching requires martensite, and the cooling rate must be higher than the critical cooling rate (VK).
During quenching, how can martensite be obtained and the effect of stress reduced? This is a major problem in the quenching process. To solve this problem, we can start from two aspects; one is to find an ideal quenching medium, the other is to improve the quenching method.
According to Austenite’s isothermal transformation curve, to quench Martensite, in fact, do not need the whole cooling process for rapid cooling. The key point is that rapid cooling is required near the tip of the c-curve nose, that is, within the temperature range of 650 ~ 550 °C, while rapid cooling is not required from quenching temperature to 650 °C and below 400 °c, especially when martensite transformation occurs below 300 ~ 200 °C, in particular, should not be rapid cooling. Otherwise, it will increase the role of internal stress.
To find out the effect of quenching temperature, the quenching experiments at different temperatures were carried out, and the experimental results were shown in the form of Fig. 2. As can be seen from the diagram, the quenching temperature has a great effect on the hardness. At the same cooling rate, the hardness curve — quenching temperature appears a peak, and the quenching temperature at the peak is the best.
If the optimum quenching temperature is understood to be the quenching heat treatment process at this temperature, the number of secondary carbides precipitated at room temperature is the most suitable, which ensures the transformation of all austenite into Martensite, the high carbon content of Martensite is ensured. If the amount of secondary carbides is more than the most suitable amount, Martensite’s carbon content will decrease, and the hardness will decrease. If the amount of secondary carbides is less than the most suitable amount, the retained austenite will also decrease the hardness.
When the quenching heat treatment process cools rapidly, the secondary carbides in the continuous cooling process will be less. To get the most suitable amount of secondary carbides, a lot of secondary carbides will be precipitated when the quenching temperature is kept; therefore, the optimum quenching temperature is relatively low. When the quenching process is very slow, a lot of secondary carbides will be precipitated during the cooling process. In order to get the most suitable amount of secondary carbides, less secondary carbides need to be precipitated for heat preservation at quenching temperature; therefore, the best quenching temperature is bound to be higher. Thus, the more secondary carbides are precipitated during cooling, the higher the optimum quenching temperature is. Therefore, the optimum quenching temperature should be increased; otherwise, the optimum quenching temperature should be decreased.
3.Effect of tempering temperature
The high chromium wear-resistant cast ball should be tempered after quenching to improve its toughness. In order to reflect the influence of tempering temperature on hardness, the following tests were carried out. A batch of quasi-90mm high chromium wear-resistant cast balls was quenched in the air at 960 °C (holding temperature for 4H), then tempered at different temperatures (holding temperature for 4H), according to the curve shown in Fig. 3, it can be seen that the hardness of the high chromium cast ball samples varies little from 200 °C to 450 °C, but decreases sharply when the tempering temperature reaches or exceeds 500 °C. The results show that the quenched high chromium wear-resistant cast ball has high tempering resistance, and the hardness of the quenched high chromium wear-resistant cast ball does not change until 450 °C.
To improve the quality of products, industrial tests were carried out in the foundry of Zhongzhou Branch of Aluminum Corporation of China. After the optimum quenching and tempering temperatures obtained in this test, the high chromium wear-resistant cast ball was ground in the Zhongzhou branch, and its ball consumption was reduced by 21% compared with the original process. Therefore, the quenching temperature control in the best temperature, you can improve high chromium, wear-resistant cast ball wear resistance.
High chromium wear-resistant cast ball hardness after air quenching is closely related to the quenching temperature, and the high chromium wear-resistant cast ball has high tempering resistance under the best quenching temperature. This is the concusion of the heat treatment process of the high chrome casting balls.