Optimization of heat treatment process for large gray iron castings

Aging treatment is used to eliminate the stress of castings, and the main factor affecting the stress elimination of castings is the heat treatment process parameters. Stress relief annealing is a process of heating the workpiece to an appropriate temperature of 100 ℃ ~ 200 ℃ below AC1 and gradually cooling slowly after holding for a certain time. The common stress relief annealing process curve is shown in Figure 1. Generally, the heating temperature of stress relief annealing is about 550 ℃. The higher the annealing temperature, the more complete the stress relief; However, too high temperature will lead to pearlite spheroidization and cementite graphitization, which will affect its service performance. At present, the heat treatment process adopted is charging temperature of 150 ℃, heating rate of 35 ℃ / h, holding at 550 ℃ for 6 hours, and cooling rate of 25 ℃ / h. The heat treatment process curve of the original casting is shown in Figure 2.

Through analysis and discussion, due to the complex structure, large wall thickness difference and staggered thickness and thin wall of this kind of castings, it is easy to produce uneven temperature in the heating process, resulting in the generation of secondary stress and superposition of hot crack defects. The temperature rise rate of stress relief annealing is usually 60 ℃ / h, and the current temperature rise rate is 35 ℃ / h, which is more reasonable. If the temperature rise rate continues to be reduced, the improvement effect is limited, and the temperature difference time is prolonged, it may bring greater secondary stress. Therefore, the heat treatment process is optimized and gradient heating is adopted in the heating stage. The thermal conductivity of castings is the same, and the thermal resistance is directly proportional to the wall thickness. The wall thickness ratio of hot crack defects is about 3:1. The temperature difference increases with the extension of heating time. According to the calculation of thermal conductivity and thermal resistance, the temperature difference is about 5 ℃ ~ 10 ℃ per hour. Therefore, heat preservation is carried out every 3 hours to ensure that the temperature difference does not exceed 35 ℃ of the heating rate, as shown in Figure 3, at 200 ℃ 350 ℃ and 450 ℃ step insulation for a period of time to ensure the temperature balance in the body and reduce the damage caused by secondary stress.

The optimized step insulation heat treatment process was used to heat treat large gray cast iron body and other castings. There were no heat treatment cracks, and the improvement effect was obvious. At present, the curing process has been completed, the heat treatment of batch complex body castings has been completed, and the quality is all qualified.