The structure of a 13 l cylinder head casting is shown in Figure 1, and the overall dimension of the casting is 1 000 mm × 400 mm × The average wall thickness is about 5 mm. The customer requires that the tensile strength of the body is more than 290 MPa and the residual stress is less than or equal to 300 μξ（ About 40 MPa), is a typical high strength thin-walled parts. The residual stress of the casting itself is high, and the intake cavity of the cylinder head is connected with the cylinder head body, so the wall thickness structure fluctuates greatly, which increases the tendency of the casting to produce higher residual stress. In the early stage of production, the author company used box furnace annealing according to the annealing curve provided by customers (as shown in Figure 2). The residual stress at different positions of the casting is measured by strain tester. The measuring positions are shown in Fig. 3.
During the first round of production, 20 castings were produced according to the normal production rhythm. After cooling in mold for 4 h, cracks were found at the inlet windows of cylinder # 1 and # 6 in 12 castings (as shown in positions ② and ③ in Fig. 3). The remaining castings were annealed according to the curve shown in Fig. 2, and the residual stress at each point was finally measured.
(1) The results show that the inlet windows of # 1 and # 6 cylinders are the places with the highest stress, which is the main cause of sand falling cracks;
(2) According to the annealing curve shown in Fig. 2, the residual stress of the casting can not meet the requirements of customers.
Based on the above situation, the possible causes of the above defects are as follows:
(1) The high temperature of casting sand dropping leads to the formation of large residual stress and even cracks;
(2) In the annealing curve, the holding time is not enough, so the residual stress of the casting is not released completely, that is, the casting enters the cooling stage, and the annealing effect is not obvious.
Therefore, the following measures should be taken:
(1) The proper cooling time in the mold was explored to obtain the proper sand dropping temperature and reduce the inherent stress of the casting;
(2) The stress of casting can be released by prolonging annealing time.