The as-cast structure of steel castings is prone to serious crystal segregation and uneven structure, such as fine equiaxed crystals on the surface of the casting section, second columnar crystals and intermediate equiaxed crystals, widmanstatten structure and network cementite, which need to be eliminated or alleviated by heat treatment (solid recrystallization). The microstructure of steel castings is controlled by heat treatment to obtain the required properties. In addition, due to the difference of wall thickness and the influence of microstructure, each part of the same casting has different microstructure, and considerable residual stress needs to be eliminated. Therefore, the general valve steel castings are required to be supplied in the heat treatment state.

According to the different heating and cooling conditions, the heat treatment methods of steel castings mainly include annealing, normalizing, homogenization, quenching, tempering, solution treatment, precipitation hardening, stress relief and hydrogen removal. Main characteristics and application scope of various heat treatments.

Due to the characteristics of casting structure, the heat treatment of steel castings is often different from that of forged steel or rolled steel with similar chemical composition, and its characteristics are as follows:

① In the as-cast structure of steel castings, coarse dendrites and segregation often exist. During heat treatment, the heating temperature is slightly higher than that of forged steel parts with similar composition, and the austenitizing holding time should be appropriately extended;

② As-cast microstructure segregation of some alloy steel castings is serious, so homogenization treatment is needed to eliminate its influence on the final heat treatment of castings;

(3) The shape of steel castings is complex, and the wall thickness changes greatly, so the difference between the specimen and the actual casting in structure and performance caused by section effect must be considered;

(4) After solidification and cooling of steel castings, different casting stresses exist in different parts.