Effect of Heat Treatment Temperature on Microstructure and Properties of High Chromium Cast Iron (2)

2.2 Microstructure and properties under moderate heat treatment

The hardness of the tested sample after heat treatment at medium temperature (740 ~ 820 °c) is shown in Table 3. The corresponding metallograph is shown in Fig. 2. It can be seen that the hardness of hypoeutectic high chromium cast iron after heat treatment at medium temperature has significant change compared with the as-cast state. At 780 °C, the hardness has reached 66.5 HRC, while at 740 °c, the hardness has no obvious change. This shows that the sample undergoes a eutectoid transformation at 780 °C. As can be seen from the metallographic diagram in Fig. 2, a large number of secondary carbides were precipitated in the sample during heat treatment. The increase of the hard phase content increased the microhardness of the material. At the same time, a large number of alloying elements were dissolved from the AUSTENITE, the content of alloying elements in austenite decreases, and the stability of austenite decreases. As the MS point rises above room temperature, martensite is quenched by Air Cooling, and the hardness of the Matrix is increased. Therefore, as shown in Table 3, the macro-hardness of the material is greatly increased

2.3Microstructure and properties under high temperature heat treatment

The effect of heat treatment temperature (860ー1100 °c) on the microstructure and properties of high chromium cast iron was studied. The microstructure and hardness are shown in Fig. 3 and Fig. 4 respectively, the results show that the hardness of the sample decreases with the increase of temperature, and it can be concluded that the hardness of the heat-treated material still decreases at a higher temperatures, the results were verified at 1100 °C. After heat treatment at high temperature, a large number of alloy elements are dissolved in Austenite, and carbon atoms combine to form secondary carbides, because the driving force of diffusion of alloy elements increases and the resistance of dislocation movement decreases, the subsequent quenching of Martensite increases the hardness of the material. However, as the temperature continues to rise, the solubility of carbon and alloy in austenite increases, and when the solubility exceeds the actual content of Austenite at a certain temperature, the precipitated secondary carbides will redissolve into austenite, the decrease of MS and MF point, the decrease of martensite content and the increase of retained austenite content lead to the decrease of material hardness at room temperature.

As can be seen from tables 2,3 and 4, the hardness of the samples decreased firstly and then increased with the increase of heat treatment temperature, and then decreased again with the increase of heat treatment temperature. The peak value of the hardness of the sample is due to the precipitation of carbon and alloy elements in the form of secondary carbides when the heating temperature is low. With the increase of temperature, the precipitation driving force of alloy elements increases, the precipitation amount of secondary carbide increases, and the hardness of the material increases correspondingly. When the temperature, beyond the eutectoid transformation, changes in temperature, precipitated secondary carbides are remelted. When the heating temperature is raised further, the solubility of carbon and alloy elements in austenite is increased, the dissolution rate of secondary carbide is accelerated, and the hardness reaches the extreme value when the precipitation rate and the dissolution rate of secondary carbide are equal. If the temperature is raised further, the dissolution rate of the secondary carbides is higher than the precipitation rate, the alloying elements in the AUSTENITE increase, the stability increases, the MS and MF points decrease, and the martensite content decreases during the subsequent cooling process, retained austenite content increased

3.Conclusion

(1) when hypoeutectic high chromium cast iron is heat-treated at low temperature, the hardness is not improved obviously.

(2) when high chromium cast iron is heat-treated, the precipitation and dissolution of Secondary Carbides will occur with the increase of temperature. When the precipitation and dissolution of Secondary Carbides are in balance, the hardness of the material will peak.