Due to the different purposes of precision castings, there are many heat treatment methods, which can be broadly divided into two categories. The first category involves castings whose microstructure will not change or should not change after heat treatment; the second category involves castings whose basic microstructure changes. The first type of heat treatment is mainly used to eliminate internal stress, which is caused by different cooling conditions during the stainless steel casting process. The microstructure, strength, and other mechanical properties do not change significantly after heat treatment.
For the second type of heat treatment, the basic microstructure undergoes significant changes, which can be roughly divided into five categories:
1. Softening Annealing: Its main purpose is to decompose carbides, reduce hardness, and improve machinability. For ductile iron, the purpose is to obtain more ferrite.
2. Normalizing: Its main purpose is to obtain pearlite and sorbite structures to improve the mechanical properties of stainless steel castings.
3. Quenching: Its main purpose is to obtain higher hardness or wear resistance while also achieving very high surface wear resistance.
4. Surface hardening treatment: Primarily to obtain a hardened surface layer, while simultaneously achieving very high surface wear resistance.
5. Precipitation hardening treatment: Primarily to obtain high strength without drastically altering elongation.
