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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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