The tempering of stainless steel precision casting mainly has two purposes: one is to eliminate the stress produced during quenching; The decomposition and transformation of the metastable structure of martensite and residual austenite after quenching are promoted by two feet.
The tempering temperature is lower than the eutectoid transition temperature, usually about 650 ° C, and the holding time is not less than 2 hours. With the increase of tempering temperature, martensite precipitates carbon from supersaturated state and forms dispersed fine carbides. The martensite will eventually form massive ferrite. Therefore, the hardness of "stainless steel precision casting" steel will gradually decrease with the increase of tempering temperature.
A problem that can not be ignored is tempering: the brittleness in the process of tempering, at 300 ℃~ 400℃ low temperature area tempering brittleness, at 500 ℃~ 650℃ high temperature tempering brittleness. As carbon atoms precipitate out, a thin layer of carbide forms at the grain boundary, producing an effect known as cryogenic tempering brittleness. This kind of carbide is called E-carbide, and its structure and composition are different from that of carbide, resulting in reduced impact toughness, which is called low temperature tempering brittleness. When the concave fire continues to heat up, forming a stable tempering structure, tempering brittleness after secondary tempering treatment does not appear, so it is also called "irreversible tempering brittleness".
At 450 ℃ ~ 650℃ high temperature tempering brittleness speed and cold, there are a lot of slow cooling tempering brittleness, no rapid cooling of tempering brittleness. High temperature tempering brittleness is reversible, as long as the tempering brittleness of steel is removed and then heated to 600 ℃ "(above: and then slowly cooled, there is still tempering brittleness. Therefore, the tempering brittleness may be related to the precipitation of brittle compounds with low melting point at grain boundaries. The addition of molybdenum to some alloy steels can eliminate the tempering brittleness, that is, low temperature tempering brittleness. Experts generally believe that molybdenum may form complex carbides with some alloying elements to counteract the adverse effects of carbides.
