Three kinds of annealing process were selected, and the annealing time was 18 h, 11 h and 8 h, respectively. After annealing, the residual stress value detected by different processes. The test of 18 h annealing process was repeated for two rounds, and the stress at each point of the two cylinder heads was less than 50 MPa; After one round of 11 h annealing test, only one point stress slightly exceeds 50 MPa; The test of 8 h annealing process was repeated to verify that the stress of three points in two cylinder heads was far more than 50 MPa.
The effect of different annealing process on the tensile strength and hardness of casting body was studied. Compared with the same batch of cylinder head parts with internal thermal insulation process, the difference of tensile strength decrease is about 30 MPa.
Although the residual stress of castings can be reduced by adopting in mold heat preservation annealing process, the scheme is not suitable for the batch production of cylinder head.
On the basis of a large number of tests, after process optimization, the furnace annealing process shown in Fig. 8 curve is adopted to reduce the residual stress of the casting, which is more suitable for the part. After testing, the residual stress of each batch of castings meet the requirements of customers.