The role of cryogenic treatment equipment in heat treatment
2021-04-06 14:41:30
Cryogenic treatment is currently called cold treatment, ice-cold treatment (within -78℃), cryogenic treatment (-130℃——-180℃), and ultra-cryogenic treatment (-180℃——-196℃) in China.
The mechanism of cryogenic treatment equipment in heat treatment is as follows:
1. Improve the hardness and strength of the workpiece
Cryogenic treatment is a continuation of quenching in a sense. By lowering the temperature, materials whose Mf point is below normal temperature continue to complete the transition from A to M. For example, some high-carbon high-alloy steels and low-alloy carburizing steels have relatively low Mf points. After conventional quenching, the retained austenite can reach 25% or even higher. By continuing to transform, it can usually increase to RC1-3 degrees, and sometimes even HRC5-6 degrees.
In the martensite matrix, nano-level ultra-fine carbides are precipitated, which slightly improves the hardness of the workpiece. Qian Shiqiang of Shanghai Jiaotong University used 16Mn experiment to basically ignore the influence of paralytical transformation. Cryogenic hardness increased the HRC by about 1.5 degrees, indicating that the dispersed ultrafine carbides played a role of dispersion strengthening on the organization.
According to the material manual, when the material hardness is above HRC50, we can see that the tensile strength of chromium steel, chromium vanadium steel and chromium molybdenum steel usually increases by about 30MPA every time the hardness increases by 0.5 degrees of HRC.
Improve the overall hardness of large parts and increase the depth of hardened layer.
2. Ensure the dimensional accuracy of the workpiece
Retained austenite is an unstable structure, which will be partially transformed into martensite with the passage of time and the action of external force, which affects the dimensional accuracy of the workpiece.
Specific volume of austenite: 0.1212+0.0033(%C) w(C)%=0-2
The specific volume of martensite: 0.1271+0.0025 (%C) w(C)%=0-2
Under normal circumstances, according to the above formula, the volume of all austenite transformed into martensite will increase by about 4%. Assuming that there is 10% paralympics after heat treatment, the volume will be 0.4% if all transformed into martensite. Expansion. Cryogenic treatment can control the Paralympics to less than 5% or even 3%, and the remaining austenite is difficult to transform in the subsequent processing and use, so the accuracy of the workpiece can be greatly improved.
Point defects and vacancies in the material can also cause dimensional changes during the use of the workpiece.
Through the cryogenic treatment at extremely low temperature, the original point defects are driven to produce plastic rheology and vacancy defects are reduced. The positron submergence technique was used to measure the change in vacancy table concentration after cryogenic. The positron lifetime before cryogenic was 298s and after cryogenic was 163.4s. It can be seen that cryogenic treatment reduces nearly half of the vacancy concentration, thus improving the workpiece’s Strength and dimensional stability can even avoid the formation of vacancy groups and cause microcracks. (At the same time, this is also the reason for changing the resistivity of some non-ferrous metals.)
3. Improve the wear resistance of the workpiece
The hardness has increased, and high-hardness martensite is obviously more wear-resistant than austenite. For materials like W18CR4V\CR12, when the hardness is increased from HV600 to HV800, the relative wear resistance is increased by approximately 15% to 20%.
A large number of ultra-fine carbides precipitated and dispersed on the martensite dislocation lines and twinning bands play a role in pinning and supporting the martensite matrix. Even if cryogenic cooling sometimes does not significantly increase the hardness of the workpiece, we can Obviously see the improvement of its abrasion resistance.
For high-carbon and high-alloy workpieces, a 50% increase in wear resistance is quite common.
4. Improve the impact toughness of the workpiece
The low-temperature shrinkage causes the material lattice constant to shrink, and nano-level ultra-fine carbides are precipitated on the martensite matrix. At the same time, the axial ratio of the martensite decreases. The experiments of Chen Changfeng and Li Shiyan verify that T12 undergoes cryogenic treatment (-196℃ heat preservation). After 10 hours), its martensite axial ratio stabilized at 1.027, while the un-cryogenic ratio was 1.038. In another cryogenic experiment of high-speed steel W6CR5MO4V2 by Li Xiong and Li Shiyan, the martensite axial ratio was changed from 1.022 not cryogenically reduced to 1.014 after cryogenic temperature (-196°C for 12 hours)
In the same experiment, the carbon content of martensite is reduced from 0.49% to 0.32%, the width of martensite twins without cryogenic cooling is 20-70nm, and the width of martensite twins after cryogenic cooling is less than 10-20nm, and the grains are obvious. Refinement. And the size of carbide particles precipitated on the twinning zone is 6-10nm. The finer the grain size, the greater the amount of strengthening brought about by the fine grains.
Cryogenic cooling reduces the vacancy concentration and the reduction of point defects is also one of the reasons for the improvement of the impact toughness of the workpiece.
5. Improve the internal stress distribution of the workpiece and increase the fatigue strength
Cryogenic treatment redistributes the internal stress of the workpiece by refining the structure, dispersing carbides, reducing point defects and other changes, and changes some stress states.
When the workpiece rises from the cryogenic temperature and is tempered at the normal temperature, it is equivalent to a temperature difference close to two hundred degrees to relieve stress. In a sense, the whole process effect of long-term slow-speed deep cooling is similar to artificial aging.
To remember the effect of cryogenic treatment to relieve stress, it is very important to control the cooling rate and heating rate in the cryogenic process.
6. Improve the corrosion resistance of the workpiece
Cryogenic treatment refines the grains, and the refinement of grain boundaries can reduce the chance of intergranular corrosion.
The excess free energy is transferred, and the two-phase potential difference between austenite and martensite is reduced, and the chance of electrochemical corrosion is reduced. l Reduce the internal stress of the organization, make the organization more uniform, and reduce the chance of stress corrosion.
Reducing the vacancy concentration can effectively reduce the chance of hydrogen accumulation in the tissue.
Other small aspects include: reducing the risk of wire cutting or grinding cracking of the workpiece after heat treatment, improving the polishing performance of the workpiece, and improving the coercivity of the workpiece. For some mold workpieces that require three tempering, cryogenic treatment can also reduce one tempering process.
2. The role of cryogenic treatment
The improvement of cryogenic treatment for ferrous metals is mainly in the following aspects:
1. Increase the service life of high-speed steel cutting tools, cutting tools, and measuring tools;
2. Increase the life of carbide tools and cutting tools;
3. The service life of cemented carbide drill bits and drilling tools is improved;
4. The performance improvement of diamond products, such as the improvement of the thermal stability of synthetic diamond, the improvement of the performance of synthetic diamond mining drill bits, diamond Φ105mm saw blades, etc.;
5. Improved top hammer performance of diamond hot press;
6. The size of the assembly parts of precision machinery is stable;
7. The performance of carbon fiber yarn is improved;
8. The service life of grease nipple, spring, gear and bearing is improved;
9. The service life of hot work molds and cold work molds in the machinery manufacturing industry is increased.
