Through the above analysis, the process was optimized: the furnace holding time was extended to about 8 h, the temperature gradient of cooling was controlled by human intervention, and the data of casting heating, holding and cooling were collected to provide data support for subsequent improvement. The optimized annealing process is shown in Figure 1.

According to the above annealing process, 24 castings with in mold cooling time of 8 h were verified by limit annealing, and 3 castings were randomly selected for residual stress detection. The detection results are shown in Table 2. The results show that the residual stress is lower than the upper limit after annealing. In order to verify the influence of sand dropping temperature on casting cracking (residual stress), the cooling state of castings in the mold was controlled by manual intervention, and then the sand dropping temperature of castings was controlled. The crack state and sand dropping temperature of castings with different cooling time were measured. The specific data are shown in Figure 2.

In order to further verify the residual stress of castings under different in mold cooling conditions, the residual stress data of castings under various conditions were also collected.

(1) When the casting is cooled in mold for 4 h, the sand dropping temperature is 850 ℃, which is higher than the phase transformation temperature. The sand dropping process of the casting is equivalent to the quenching process, resulting in large residual stress, which is the main reason for the cracking of the casting;

(2) When the in mold cooling time of the casting is more than 8 h, the sand dropping temperature of the casting gradually decreases to below 500 ℃, which is lower than the phase transformation temperature, and it is not easy to produce large residual stress, so the sand dropping cracking of the casting does not occur;

(3) It can be seen from the temperature and residual stress of the casting that the residual stress of the casting decreases with the decrease of the temperature. When the in mold holding time is more than 12 h, the sand dropping temperature of the casting is lower than 350 ℃, which is equivalent to the furnace temperature when the annealing process is used. The residual stress of the casting is relatively stable, which is lower than 300 μξ。 Although the residual stress of castings can meet the requirements by annealing in furnace, the whole annealing process lasts for nearly 27 hours, which not only seriously affects the annealing efficiency, but also greatly increases the annealing cost. The scheme is only suitable for process test, not for batch production.

Because there is an extended cooling section in the molding line, the method of in mold annealing is used to reduce the residual stress of castings, which not only reduces the production cost, but also cancels the annealing link, and the production process is smoother. Therefore, in mold annealing is used for batch production. Since the adoption of this process, tens of thousands of castings have been produced. After testing, the residual stress of each batch of castings meets the requirements of customers.