After pouring of large steel castings, there are great differences in the cooling rates of different parts, appearance and core of the castings, and the sand core also has a strong blocking effect on the shrinkage of the castings during cooling. Therefore, when the castings are completely cooled, large casting stress will inevitably occur inside the castings.
In addition, large castings have large cross-sectional area and slow solidification speed. Generally, the as cast structure is coarse columnar crystal, which seriously affects its mechanical properties. Therefore, large casting blanks need to go through appropriate heat treatment process. The purpose of heat treatment is also shown in two aspects: one is to eliminate or reduce the casting stress, and the other is to improve the structure to obtain satisfactory mechanical properties (such as strength, hardness, impact properties, etc.).
Generally, large steel castings have high requirements for mechanical properties and internal quality, so strict heat treatment process is usually required. The common heat treatment specifications include annealing, normalizing, normalizing + tempering, etc.