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                                                                  Heat treatment method of precision castings
Precision castings have many heat treatment methods because of the different heating and cooling conditions. The following small series introduces its main heat treatment methods: annealing, normalizing, homogenizing treatment, quenching, tempering, solution treatment, precipitation hardening, stress relief treatment and hydrogen removal treatment.
1. Annealing: Annealing is to heat the steel castings to 20 to 30 degrees Celsius above Acs, which is suitable for all grades of steel castings.
2. Normalizing: Normalizing is a heat treatment process in which the temperature of the cast steel piece is heated to above Ac temperature, 30 to 50 degrees Celsius is kept, so that it is completely austenitized and then cooled in still air. General engineering carbon steel and some thick and complex alloy steel castings are mostly normalized.
3. Quenching: Quenching is a heat treatment process in which a steel material is heated to austenitization and then cooled in an appropriate manner after a certain period of time to obtain a martensite or bainite structure. Commonly, there are water quenching, oil quenching and air quenching. The steel castings should be tempered in time after quenching to eliminate the quenching stress and obtain the required comprehensive mechanical properties. .
4. Tempering: Tempering is the heating of the steel casting after quenching or normalizing to Ac, at a selected temperature below, after a certain period of heat preservation, cooling at a suitable rate to obtain unstable structure after quenching or normalizing. A heat treatment process that transforms into a stable structure, eliminates quenching (or normalizing) stress, and improves the ductility and toughness of the cast steel.
5. Solution treatment: solution treatment is a heat treatment process in which a casting is heated to an appropriate temperature and kept warm to sufficiently dissolve the excess phase, and then rapidly cooled to obtain a supersaturated solid solution. The main purpose of the solution treatment is to dissolve the carbide or other precipitation phase in the solid solution to obtain a supersaturated single phase structure.
6. Precipitation hardening treatment: After the casting solution is solution treated or quenched, it is incubated at room temperature or above at a suitable temperature to form a solute atomic segregation zone and/or a dispersed distribution strengthening phase in the supersaturated solid solution to harden the metal. The treatment is called precipitation hardening treatment.
7. The purpose of stress relief treatment is to eliminate casting stress, quenching stress and stress formed by machining, and to stabilize the size.
8. The purpose of dehydrogenation is to remove hydrogen and improve the ductility of the cast steel. Heat to l70 ~ 200 degrees Celsius or 280 ~ 320 degrees Celsius, heat treatment for a long time. There are no organizational changes. It is mainly used for low alloy steel castings that are prone to hydrogen embrittlement.

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