Martensitic transformation is a kind of cutting deformation core, not depending on diffusion. It’s an incomplete transformation.
1.Austenite must have enough cooling rate at high temperature, so that austenite will not undergo equilibrium decomposition at high temperature.
2.When austenite is cooled to MS point, martensitic transformation will take place, and it is formed by cooling. Only when industrial freezer is used to reduce the temperature continuously, the transformation will continue to take place.
3.The MF point of most steels is below room temperature, even lower, but the martensitic transformation will not continue and will not be complete. Therefore, part of austenite will remain unchanged.
4.The martensite after transformation will hinder the continuous transformation of austenite. This is because the volume expansion occurs during martensitic transformation, resulting in great compressive stress. It is the work of the remaining austenite to overcome the compressive stress during transformation, which hinders the further transformation of the remaining austenite.
5.The decrease of MS point will increase the amount of retained austenite.
6.The faster the cooling speed of the steel, the lower the MS point, and indeed the increase of retained austenite. But the cooling rate must be increased to a certain limit, generally, there is no obvious impact.
7.The removal of retained austenite can be done by industrial freezer once, but it must be done immediately after quenching, otherwise, austenite will be stabilized.
8.Before the cryogenic treatment, a low-temperature stress relief tempering can be carried out to avoid cracking during the cryogenic treatment.
Martensitic transformation is a cooling type transformation, which is produced by the way of common grid cutting, rather than nucleation and growth. Only when the temperature is constantly reduced can new martensitic plates be formed. Because of the shear mode, the new martensite will generate compressive stress on the surrounding supercooled austenite, which makes the transformation difficult. Therefore:
1.Because the steel will not continue to change below the MF point, and it is incomplete, part of the supercooled austenite will remain unchanged. It is called retained austenite. It is due to the stress action of the new martensite sheet on the undercooled austenite, and the stabilization of austenite.
2.The industrial freezer is used for deep cooling treatment, and the temperature is controlled near the MF point. After reading for a period of time, the temperature rises to room temperature, and some of the retained austenite of the workpiece will undergo further transformation, forming martensite. Generally, the reading will increase 2 ~ 3HRC, and then the normal temperature tempering.
The removal of retained austenite will have a great effect on the dimensional stability and structural stability of the workpiece.