Steel castings should be used after heat treatment. Because of the casting defects such as pores, cracks, shrinkage and shrinkage, coarse grains, uneven microstructure and residual internal stress in cast steel castings, the strength of steel castings, especially plasticity and toughness, is greatly reduced.
In order to refine grain, uniformly organize and eliminate internal stress, steel castings must be treated with normalizing or annealing. After normalizing the steel, its mechanical properties are higher than that after annealing, and the cost is lower, so the application is more. However, because the normalizing treatment will cause the internal stress which is larger than the annealing, it is only suitable for the steel castings with a carbon content less than 0.35%. Because of the good plasticity of low carbon steel castings, the cooling is not easy to crack. In order to reduce the internal stress, the cast steel castings should be tempered at high temperature after normalizing. For steel castings with ≥0.35% carbon content, complex structure and easy to crack, only annealing treatment can be carried out. Steel castings are not suitable for quenching, otherwise it is very easy to crack.