Effect of Heat Treatment Temperature on Microstructure and Properties of High Chromium Cast Iron (1)
ZHANG Xiangping, TANG Jianxin
(School of Manufacture Science and Engineering, Sichuan University, Chengdu 610065, China)
Abstract：The effects of heat treatment temperature on the microstructure and properties of hypoeutectic high chromium cast iron were studied. The results show that there are secondary carbide precipitation and re-solution with the increase in heat treatment temperature. The peak hardness of the material appears when the secondary carbide precipitates and re-solution reaches equilibrium.
Key words：heat treatment; hardness; secondary carbide
High Chromium cast iron is the third generation wear-resistant material after white cast iron, high manganese steel, and nickel hard cast iron. Its Room temperature microstructure is a high hardness (1200 ~ 1800HV) M7C3 carbides distributed uniformly on the Martensite, which are isolated from each other and do not form a network, greatly reduces the Matrix of the fracture, so that in the anti-wear at the same time there is a certain toughness. Therefore, chromium cast iron is widely used in mining, paving, construction, and electric power industries. The effect of heat treatment temperature on the microstructure and hardness of hypoeutectic high chromium cast iron has been studied, the hardening mechanism of high chromium cast iron at different heat treatment temperatures, and its effect on the hardness were analyzed.
1.Experiment material and method
The high chromium cast iron with hypoeutectic composition was selected, the Cr/C was 4.8, the carbon content was 2.8%, and the chromium content was 14%. Medium frequency induction furnace melting, sand casting. The heat treatment of casting can be divided into three temperature sections: low temperature, middle temperature, and high temperature. The specific heat treatment process is shown in Table 1. The holding time was 2h and the cooling mode was air cooling.
The cast small sample (10mm 10mm 100mm) was put into the box resistance furnace (SX-12-12), the temperature was increased at the rate of 250 °c/h, the temperature was heated to the predetermined temperature, after holding for 2 hours, the sample was cooled to room temperature The hardness of the samples before and after heat treatment was measured by HR-150A Rockwell hardness tester (after heat treatment, the samples were polished and the oxide scale was removed to ensure the test precision), and the fracture of the samples before and after heat treatment was observed by metallography The morphology of the specimen was observed by XJP-100 microscope. The microstructure and phase transformation of hypoeutectic high chromium cast iron at different heat treatment temperatures were investigated
2.Experimental results and discussion
2.1Microstructure and properties under low temperature heat treatment
The hardness of the sample before and after being treated at low temperatures (500 ~ 620 °c) is shown in Table 2. Fig. 1 shows the corresponding metallographic structure of the sample. It can be seen that the hardness of hypoeutectic high chromium cast iron after low-temperature treatment has no obvious change, only a slight decrease. The hypoeutectic high chromium cast iron with a CR/C of 4.8 was heat-treated at low temperature because the alloy and carbon content in the matrix was relatively low, it is difficult to form secondary carbides or only a small amount of carbides. The alloy elements in austenite are still supersaturated and the austenite is still very stable. The MS point is higher than the room temperature, so that only a small amount of austenite changes to martensite during cooling, and the content of retained austenite is relatively high, so the hardness after heat treatment has no obvious change compared with as-cast. As can be seen from Fig. 1, the microstructure after heat treatment has almost no change as compared with that of as-cast, which consists of precipitated austenite + martensite + retained austenite, this explains why there is no obvious change in the hardness of the sample after heat treatment.