There are two kinds of cold treatment: ordinary treatment - 80 and ultra-cold treatment - 190. Ultra-cold treatment, also known as cryogenic treatment I, is recently called cryogenic treatment. After quenching and cooling, the transformation of martensite structure is completed from martensite transformation point Ms to martensite transformation point Mf. However, the MF of many alloy steels is lower than room temperature, and retained austenite structure exists in the structure at room temperature. The retained austenite of this kind of steel can not be completely eliminated by tempering, and the existence of retained austenite causes the decrease of workpiece performance. In order to eliminate retained austenite, the cold treatment process is to continue the quenching process, which is to continue to cool the quenched steel below room temperature when it is cooled to room temperature.
Cold treatment can improve the hardness of the workpiece, prevent grinding cracks, improve the magnetic properties of the workpiece, reduce deformation and stabilize the size of the workpiece.
It can replace multiple tempering.
The process of cold treatment includes the temperature of cold treatment, the time of cold treatment, the way and speed of cooling and heating up, the way of charging furnace, the time interval of turning into tempering process and so on.
(1) The theoretical basis for determining the temperature of cold treatment is the Mf point of each kind of steel, which is near or far below the Mf point as the temperature of cold treatment. The temperature of cold treatment can also be determined according to the use of parts and the conditions of process equipment.
(2) The determination of the cold treatment time is the same as that of the quenched workpiece. After the workpiece reaches the cold treatment temperature, the transformation is completed. When the workpiece weight and the workpiece size are not considered, the heat preservation is not necessary. But the instrument monitors only the temperature, which does not represent that the workpiece has actually reached the temperature. Therefore, in order to make the core of the large or batch workpiece reach the actual temperature of cold treatment, it can be isothermal for a period of time.
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