The tempering of stainless steel precision casting has two main purposes: one is to eliminate the stress produced during the quenching process; Bipeds promote the decomposition and transformation of martensite and residual austenite metastable structures after quenching.

Tempering temperature below eutectoid transformation temperature, usually at about 650 ℃, holding time not less than 2 hours. With the increase of tempering temperature, martensite precipitates the carbon from the supersaturated state to form fine carbides. The martensite will eventually form a massive ferrite. Therefore, the hardness of "stainless steel precision casting" steel will gradually decrease with the increase of tempering temperature.

Tempering a nots allow to ignore the question is: in the process of tempering brittleness problem, in 300 ℃ ~ 400 ℃ low temperature tempering brittleness, in 500 ℃ ~ 650 ℃ high temperature tempering brittleness. When carbon atoms are separated out and a thin layer of carbide is formed at the grain boundary, an effect called low tempering brittleness is produced. Such carbides are called e-carbides, whose structure and composition are different from those of carbides, resulting in a reduction in impact toughness, which is called low-temperature temper brittleness. When concave fire continues to heat up and forms a stable temper structure, the temper brittleness will no longer appear after secondary tempering treatment, so it is also called "irreversible temper brittleness". In 450 ℃ ~ 650 ℃ high temperature tempering brittleness of speed and cold, there are a lot of slow cooling temper brittleness, no temper brittleness of fast cooling. High temperature tempering brittleness is reversible, as long as get rid of temper brittleness of steel and then heated to 600 ℃ "(: above, and then slowly cooling, there is still a temper brittleness. Therefore, the tempering brittleness may be related to the precipitation of brittle compounds at low melting point of grain boundary. Adding molybdenum to some alloy steels can eliminate the temper brittleness. It is believed that the complex carbide formed by molybdenum and some alloying elements counteracts the adverse effect of carbide.

According to different requirements of casting properties, tempering temperature can be divided into low tempering, medium tempering and high tempering.

(1) the low temperature tempering tempering temperature 100 ℃ ~ 250 ℃, the organization of the martensite with fire, and part of the supersaturated precipitation carbon martensite, carbide martensite formation. The carbon content of this steel is very small and the supersaturated carbon in martensite is very small. So tempering martensite itself has good toughness and carbide precipitation and stability further improve the strength and hardness of steel.

(2) the middle temperature tempering temperature 300 ℃ ~ 500 ℃ tempering, the tempering organization tempered troostite, has good strength and toughness. The tempering process is widely used in low alloy steel.

(3) high temperature tempering temperature 550 ℃ ~ 650 ℃ tempering, the tempering disorganized, fine-grained sorbite. Quenching + high temperature tempering process is called tempering quenching. Suitable for low alloy steel, multi-element strong carbide forming, with high strength and toughness.