Low-temperature treatment is a sub-zero process, which is carried out immediately after quenching and is carried out by continuous tempering. The ultra-cold treatment has a significant improvement in the wear strength of the workpiece and has a breakthrough benefit for the stability of the mold size.
With the continuous development and innovation of the machinery industry, the requirements for the precision of metal materials are getting higher and higher. How to improve the mechanical properties and service life of metal workpieces is a technology that the manufacturing industry wants to breakthrough. After the heat treatment process, the hardness and mechanical properties of the steel are greatly improved, but after the heat treatment, the following problems are still encountered:
- Residual Austenite. The ratio is about 10%-20%. Because Austenite is very unstable, it is easy to transform into Martensitic when subjected to external force or environmental temperature changes, which will cause irregular expansion of the material and reduce the dimensional accuracy of the workpiece.
- The structure has coarse grains, and the material carbides are solid solution and 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 deformation of the workpiece during the stress release process.
And through the application of cryogenic and ultra-cryogenic treatment technology, it will be improved.
What is the cryogenic treatment?
Low temperature treatment is a metal treatment that enhances and enhances the mechanical properties of metal materials by using low temperature. Placing the workpiece at a temperature below -190°C can improve properties such as wear resistance and stability.
Cryogenic treatment is a post-heat treatment process that treats the metal at -100°C so that almost all the soft residual Austenite is converted into high-strength Martensitic, and can reduce surface looseness and surface roughness. When this process is completed, it will not only the surface can almost increase the strength, wear resistance, toughness, and other performance indicators of the entire metal, so that the molds and tools will still have high wear resistance and high strength after several refurbishments, and the life will be doubled. increase. The service life of knife and scissors products that have not undergone cryogenic treatment will be significantly reduced after refurbishment. Cryogenic treatment is not only applied to knife and scissors products, but also can be applied to the molds for making knife and scissors products, and can also significantly increase the life of the mold.
Principles of cryogenic treatment technology
-
Eliminate retained austenite:
Generally, the residual Austenite after quenching and tempering is about 8-20%. The residual Austenite will be further Martensitic over time. During the Martensitic transformation process, it will cause volume expansion, which will affect the dimensional accuracy and make the lattice interior the increase in stress seriously affects the performance of the metal. Cryogenic treatment can generally reduce the residual Austenite to less than 2% and eliminate the influence of the residual Austenite. If there is more residual Austenite, the strength will be reduced. Under the action of cyclic stress, it is easy to fatigue and falls off, causing nearby carbide particles to hang in the air, and quickly fall off with the matrix to produce spalling pits, forming a relatively rough surface.
-
Fill the internal gaps to increase the metal surface area, namely the wear-resistant surface:
Cryogenic treatment enables Martensitic to fill the internal voids, make the metal surface denser, increase the wear-resistant area, smaller crystals, increase the depth of the quenching layer, increase the number of renovations, increase the stability, and increase the life of materials and objects.
-
Precipitation of carbide particles:
Cryogenic treatment not only reduces the residual Martensitic but also precipitates carbide particles. The shrinkage of martensite during the cryogenic process forces the reduction of the crystal lattice and drives the precipitation of carbon atoms. Moreover, because the diffusion of carbon atoms is difficult at low temperatures, the size of the carbides formed is up to Nanometer level and attached to the Martensitic twin belt, increase the hardness and toughness. Cryogenic treatment can transform most of the residual Austenite into Martensitic, and precipitate carbide particles in Martensitic, accompanied by the refinement of the matrix structure.
- Reduce residual stress
- Make the metal matrix more stable
- Increase the strength and toughness of metal materials
- Increase the metal hardness by about HRC1～2
- Increased redness
Cryogenic process: Cryogenic treatment is carried out in three stages: cooling, heat preservation, and heating.
-
Cooling stage
The purpose of slow cooling is to eliminate residual stress. Because in the process of quenching and tempering, residual stress will be generated in the metal matrix. In the process of residual Austenite transforming to Martensitic, the volume expansion will also increase the residual stress. Only by slow cooling can the increase in residual stress be offset. And eliminate residual stress. The residual stress in the matrix is generally ignored, but it is the residual stress in the matrix that causes defects such as cracks in the knife and scissors products. The rapid cooling will increase the residual stress.
-
Insulation stage
The purpose of heat preservation is to make all the residual Austenite in the matrix turn into Martensitic as much as possible, and produce as many carbide particles as possible, because the process of residual Austenite to Martensitic is a slow process, and the length of the heat preservation time will affect the residual austenite. The amount of the transformation of the intensification and the life after deep cooling is mainly determined by the length of the holding time. Normally, the performance of holding for 2 to 4 hours has been improved, but if it is a high-quality product, you need to use 24 For the holding time of more than hours, the life-span improvement multiple is directly related to the length of the holding time.
-
Heating stage
The main purpose of the slow heat process is to prevent the generation of residual stress.
The best time for cryogenic treatment
It is generally believed that cryogenic treatment should have the best effect within two hours of quenching the workpiece, because the residual Austenite will gradually transform to Martensitic over time, and the transformed Martensitic will solidify, thereby reducing the ability to precipitate carbides.
Factors affecting the cryogenic effect:
- The same cryogenic treatment process has different effects due to different materials
- The same cryogenic treatment process has different effects due to different workpiece shapes
- The lower the temperature, the better the effect
- The longer the time, the better the effect
- After cryogenic treatment, the corrosion resistance of the material is improved
Application of cryogenic treatment technology:
Cryogenic treatment can increase the wear resistance of tools and molds, and the cryogenic treatment process is applied to molds, knives, scissors, and measuring tools. Many professional cryogenic treatment plants use ready-made processes to perform cryogenic treatment for customers to improve the quality of their products.
