Attention should be paid to the tempering of stainless steel precision casting after quenching

The key to tempering of stainless steel precision casting has two purposes: one is to remove the in-situ stress caused by the whole process of heat treatment; to promote the dissolution and change of the metastable structure of martensite and residual martensite after heat treatment.
The tempering temperature is less than the eutectoid change temperature, generally about 650℃, and the maintenance time is not less than 2 hours. As the tempering temperature rises, martensite precipitates carbon from supersaturation and produces dispersed fine carbides. Martensite will eventually produce a small ferrite core. Therefore, the hardness of "stainless steel precision casting" steel will gradually decrease as the tempering temperature rises.
A difficult problem that cannot be ignored in tempering is: the brittleness problem in the whole process of tempering, the brittleness of tempering in the ultra-low temperature region of 300℃~400℃, and the brittleness of tempering at high temperature of 500℃~650℃. When oxygen atoms are precipitated, a tomographic carbide is produced at the dislocations, which will cause an effect called ultra-low temperature temper brittleness. This type of carbide is called e-carbide, and its structure and composition are different from carbides, resulting in a decrease in fracture toughness, which is called ultra-low temperature temper brittleness. When the temperature of the concave fire is continuously increased and a stable tempered structure is produced, the temper brittleness has not appeared after the second tempering solution, so it is also called "irreversible temper brittleness". At 450℃~650℃ high temperature tempering brittleness speed cooling, there are many slow refrigeration tempering brittleness, rapid refrigeration without tempering brittleness. High temperature temper brittleness is reversible. If the temper brittleness of the steel is removed and then heated to 600℃"(:, and then cooled slowly, temper brittleness will still appear. Therefore, temper brittleness is likely to be related to dislocations. It is related to the precipitation of low melting point brittle chemical substances. The addition of molybdenum to some carbon steels can eliminate temper brittleness, that is, ultra-low temperature temper brittleness. Authoritative experts widely believe that it may be that molybdenum and some aluminum alloy elements produce composite carbides, which contradict each other. The bad hazards of carbides.
According to the different regulations of forging characteristics, the tempering temperature is divided into ultra-low temperature tempering, medium temperature tempering and high temperature tempering.
(1) Ultra-low temperature tempering The tempering temperature is 100 ℃ ~ 250 ℃, using the mechanism of fire martensite, and a part of the martensite is over-saturated and precipitated carbon martensite, and the martensite of the cemented carbide tool is produced. The carbon content of this type of steel is very small, and the over-saturated carbon in martensite is not large. Therefore, tempered martensite itself has excellent ductility, and the precipitation and stability of carbides further improves the compressive strength and strength of the steel.
(2) The tempering temperature of the intermediate temperature is 300℃~500℃, and the tempering mechanism tempers the troostite, which has excellent compressive strength and ductility. This tempering process is widely used in high alloy steel.
(3) High temperature tempering temperature 550℃~650℃ tempering, tempering mechanism dispersed, coarse-grained sorbite. Heat treatment+The high-temperature tempering process is called tempering and quenching. Suitable for high-alloy steel, multi-element strong carbide forming, with high compressive strength and ductility.