- Composition analysis of high chromium cast iron
At present, the research of high chromium cast iron focuses on hypoeutectic or eutectic. There are primary austenite dendrites in hypoeutectic high chromium cast iron, which has good wear resistance and high toughness. Based on the matching principle of high wear resistance and good toughness, through comprehensive consideration, the high chromium cast iron material is designed as hypoeutectic composition close to eutectic point, and its chemical composition analysis is listed in Table 1.
| C | Si | Mn | S | P | Cr | Mo | Cu | Ni | V |
| 2.6-3.0 | 0.5-0.8 | 0.6-1.0 | ≤0.06 | ≤0.06 | 19.0-22.0 | 1.8-2.2 | 0.5-0.9 | 0-0.5 | 0-0.5 |
- Casting process
According to the structural characteristics of the mill, the optimized liner structure is shown in Figure 1.
The liner adopts sand casting method, the open gating system has large cross-sectional area, and the molten steel filling is fast and stable. According to the structural characteristics of the casting, the riser specification is calculated by the diameter of the hot spot circle, and the top necking insulating open riser is selected as the riser. Because the liner has the characteristics of thick section, large length width ratio and heavy weight, it has large casting shrinkage and obvious hot and cold cracking tendency. In order to ensure good feeding and excellent internal compact structure of the casting, the casting process of riser + direct open chill was adopted, and the casting defects such as porosity, hot cracking, shrinkage porosity and stress deformation during the solidification process of the liner were simulated and predicted by computer simulation technology, so as to optimize the casting process.
The simulation of casting solidification process is shown in Fig. 2. It can be seen that the overall temperature field distribution of the liner casting is clear, and the solidification sequence is from bottom to top. The high temperature zone finally appears in the riser. The shrinkage cavity of the casting is mainly concentrated in the insulation riser, and there is no shrinkage cavity and other casting defects on the casting body. Therefore, the casting process design of the liner is feasible.
- Heat treatment process
The thermal conductivity of high chromium cast iron is poor, which is only 1 / 3 of that of gray cast iron. It is easy to crack when heated, and the alloy content is high, which is easy to form residual stress. Combined with the practical production experience of our company and the material characteristics of high chromium cast iron, the quenching and tempering heat treatment process was selected. The quenching process is that the casting is heated to 1 000 ~ 1 030 ℃, kept for 6 ~ 8 h, and discharged for air cooling; In the tempering process, the casting is heated to 230 ~ 250 ℃, kept for 10 ~ 12 h, and cooled along with the furnace. The heat treatment process shall be operated according to the process, and necessary protective measures shall be taken at the same time. The hardness of the test block after heat treatment is 58 ~ 60HRC and the impact energy is 4.5 ~ 4.8j. The hardness of the liner after heat treatment is 56-59 HRC.