The main purpose of precision casting in the tempering process is to eliminate the stress produced during quenching and to promote the decomposition and transformation of martensite and retained austenite metastable structure after quenching. The tempering temperature of precision casting is usually under the eutectoid transformation temperature. The highest temperature is about 650 degrees, and the holding time is not less than 2 hours.
The precision casting will increase with the tempering temperature, the martensite will precipitate the carbon from the supersaturated state of carbon, which will directly form the dispersed and fine carbide. The martensite ultimately forms the granular ferrite, so the hardness of the steel will gradually decrease as the temperature rises during tempering.
The low temperature tempering temperature of precision casting is about 150 ~ 250 ℃. The tempered martensite is obtained, that is, separating a part of carbon out from the supersaturation martensite, which forms carbide around the martensite. The carbon content of this kind of steel is very little, thesupersaturation carbon in martensite is less, so tempered martensite has good toughness. The precipitation and stability of carbide, further improve the strength and hardness of steel.
The tempering temperature of the high temperature tempering of the precision casting is 500～650℃. The tempered structure is the granular sorbite. Quenching + high temperature tempering treatment process is called thermal refining. It is suitable for low alloy steel with multicomponent carbide forming element, which has high obdurability.
The electro-hydraulic cleaning is to clean the shell under the action of electric pressure. Its basic principle is the electro - hydraulic thump effect, using high pressure pulse to discharge the gas through the special electrode in the water, resulting in the large hydrodynamic force. Because the sundries of the surface of the precision casting casting is different from the vibration frequency of the metal, the purpose of sand clear can be obtained.