Cold treatment of retained austenite with industrial freezer

With the continuous development of the mechanical industry, the requirements for metal materials are becoming higher and higher. How to improve the mechanical properties and service life of metal workpieces under the premise of material and heat treatment process is becoming a problem that many people at the forefront of heat treatment industry think about and explore. After heat treatment, the hardness and mechanical properties of steel are greatly improved, but there are still the following problems after heat treatment:

Retained austenite. The proportion of austenite is about 10% – 20%. Because austenite is very unstable, it is easy to change into martensite when it is affected by external force or environment temperature. However, the specific volume of austenite and martensite is different, which will cause irregular expansion of materials and reduce the dimensional accuracy of workpieces.

The structure grain is coarse and the carbide solution is supersaturated.

Residual internal stress. The residual internal stress after heat treatment will reduce the fatigue strength and other mechanical properties of the material, and easily lead to the deformation of the workpiece in the process of stress release.

The industrial freezer deep cooling and ultra deep cooling treatment workers are considered to be able to solve the above problems.
1、 What is cryogenic treatment of industrial freezers?
The cryogenic treatment of industrial freezers is a post heat treatment process, which transforms the metal at – 100 ℃ to make the soft retained austenite into high-strength martensite, and can reduce the surface porosity and surface roughness. When this process is completed, not only the surface, but also the strength, wear resistance, toughness and other properties of the whole metal can be increased The performance index is improved, so that the die and cutter still have high wear resistance and high strength after several renovations, and the service life is multiplied. However, the service life of the cutting products without deep cooling treatment of industrial freezers will be significantly reduced after renovation. The cryogenic treatment of industrial freezer can not only be applied to the products of knife and shear, but also to the molds for making knife and shear products, which can also significantly improve the service life of molds
2、 The mechanism of cryogenic treatment of industrial freezers

(1).Removal of retained austenite:
Generally, the retained austenite after quenching and tempering is about 8-20%. The retained austenite will further martensite with the passage of time. In the process of martensitic transformation, it will cause volume expansion, which will affect the size accuracy, increase the internal stress of the lattice, and seriously affect the metal properties. The cryogenic treatment can generally reduce the retained austenite to less than 2%, and eliminate the residual austenite The effect of austenite. If there are more retained austenite, the strength will be reduced. Under the action of periodic stress, it is easy to fatigue and fall off, resulting in the suspension of nearby carbide particles, which will soon fall off with the matrix, resulting in peeling off pit, forming a surface with larger roughness.

(2).Fill the internal space, and increase the metal surface area, namely the wear-resistant surface:
The cryogenic treatment of industrial freezer makes martensite fill the internal space, makes the metal surface more dense, increases the wear-resistant area, reduces the lattice, alloy analysis results in uniformity, increases the depth of quenching layer, and not only the surface, increases the number of renovations and improves the service life.

(3).Precipitated carbide particles:
The cryogenic treatment of industrial freezer can not only reduce the residual martensite, but also precipitate carbide particles, and refine the martensite twins. Because the shrinkage of martensite forces the lattice to reduce, and drives the precipitation of carbon atoms, and because the diffusion of carbon atoms is difficult at low temperature, the size of the formed carbide reaches nanometer level, and it adheres to the martensite twins, increasing the hardness and toughness. The wear patterns of the metals after cryogenic treatment of industrial freezers are significantly different from those of the metals without cryogenic treatment, which shows that their wear mechanisms are different.

The cryogenic treatment of industrial freezer can make most of the retained austenite martensite, and precipitate carbide particles with high dispersion in the martensite. With the refinement of the matrix structure, this change can not be explained by the traditional metal science and transformation theory, nor in the form of atomic diffusion. Generally, at – 150 ℃ ~ – 180 ℃, atoms have lost their diffusion ability.

4.Reduce the residual stress;

5.Make the metal matrix more stable;

6.Increase the strength and toughness of metal materials;

7.The hardness of the metal is increased about hrc1 ~ 2;

8.Red hardness increased significantly;

Deep cooling technology of industrial freezer
Generally, the deep cooling treatment of industrial freezers is carried out according to the three stages of cooling, heat preservation and heating.

Significance of each stage
(1) Cooling stage
The purpose of slow cooling is to completely eliminate the residual stress. Because in the process of quenching and tempering, there will be residual stress in the metal matrix. In the process of transformation from retained austenite to martensite, volume expansion will also increase the residual stress. Only slow cooling can counteract the increase of residual stress and completely eliminate the residual stress. In general, people don’t pay attention to the residual stress in the matrix, but it is the residual stress in the matrix that makes the shear products crack and other defects. However, rapid cooling will increase the residual stress.
(2) Insulation stage
The purpose of heat preservation is to transform the retained austenite in the matrix into martensite as much as possible and produce carbide particles as much as possible, because the transformation process from retained austenite to martensite is a slow process, and the length of heat preservation time will affect the amount of transformation of retained austenite. At the same time, the life after deep cooling is mainly determined by the length of heat preservation time, which is usually the case In the case of 2-4 hours insulation performance has been improved, but if it is a high-quality product, it needs to use more than 24 hours of insulation time, and the increase of life is directly related to the length of insulation time.

(3) Heating stage
The main purpose of the slow heating process is to prevent the generation of residual stress.

Due to the limitation of equipment, the domestic research of deep cooling treatment generally adopts industrial freezer, that is, the workpiece is directly put into the industrial refrigerator, the holding time is relatively short, and the general holding time is consistent with the diameter (mm). This method will greatly increase the residual stress and improve the performance.

Generally, the temperature rise to room temperature can be achieved in the heating stage. If the special use of parts is considered, such as high working temperature, it can be increased to + 160 ℃ slowly.

From the mechanism of deep cooling, it can be seen that the above process curve has little to do with the material and size of the material, but the effect after treatment is different due to the material factors. Almost all foreign tools, cutting tools, measuring tools, etc. adopt this process for deep cooling treatment.

The best time for cryogenic treatment:
It is generally believed that the effect of cryogenic treatment should be the best within two hours of quenching, because the retained austenite will gradually change to martensite with the passage of time, and the transformed martensite will be solidified, thus the ability of carbide precipitation will be reduced.

Factors affecting the effect of deep cooling

(1).The same cryogenic treatment process has different effects due to different materials

(2).The same cryogenic treatment process has different effects due to different workpiece shapes

(3).The lower the temperature, the better the effect

(4).The longer the time, the better the effect

(5).After cryogenic treatment, the corrosion resistance of the material is improved

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