What is vacuum heat treatment technology?

What is vacuum heat treatment technology?

Vacuum heat treatment technology mainly refers to a new type of heat treatment technology combining vacuum technology and heat treatment technology. Among them, the vacuum environment of vacuum heat treatment refers to an atmosphere environment lower than one atmospheric pressure, including low vacuum, medium vacuum, high vacuum and ultra-high vacuum. Therefore, vacuum heat treatment actually belongs to atmosphere controlled heat treatment.

Vacuum heat treatment means that all and part of the heat treatment process are carried out in vacuum state. Vacuum heat treatment can achieve almost all the heat treatment processes that can be involved in conventional heat treatment, but the quality of heat treatment is greatly improved.

Compared with the conventional heat treatment, the vacuum heat treatment processing technology can realize no oxidation, no decarburization and no carburization at the same time, and can remove the phosphorus chips on the surface of the workpiece, and has the function of degreasing and degassing, so as to achieve the effect of surface bright purification.

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Classification of vacuum heat treatment technology

Vacuum high pressure air cooling quenching technology

At present, the technology of vacuum high-pressure air-cooled quenching has developed rapidly, and new technologies such as negative pressure high flow rate air-cooled, pressurized air-cooled, high-pressure air-cooled and ultra-high pressure one air-cooled have emerged successively, which not only greatly improves the ability of vacuum air-cooled quenching, but also has the advantages of good surface brightness, small deformation, high efficiency, energy saving and pollution-free after quenching.
The purpose of vacuum high pressure air-cooled quenching is to quench and temper materials, to solve and age stainless steel and special alloy, to carburize and carbonitriding, to sinter in vacuum, to cool and quench after brazing.
When high-pressure nitrogen is used for cooling and quenching, the cooling load can only be loose. High speed steel can be hardened to 70-100mm, and high alloy hot work die steel can be hardened to 25-100mm.
When quenched with 1000kPa high pressure nitrogen, the cooling load can be dense, which is about 30% – 4O% higher than that of 600kpa.
When using 2000kpa ultra-high pressure nitrogen or the mixture of helium and nitrogen for cooling and quenching, the cooling load is dense and can be tied together. Its density is 80% ~ 150% higher than that of 600kpa nitrogen cooling. It can cool all high speed steel, high alloy steel, hot work die steel, chromium steel and many alloy oil quenched steel, such as large-scale manganese steel.
The cooling capacity of double chamber air-cooled quenching furnace with separate cooling chamber is better than that of single chamber furnace of the same type. The cooling effect of the two chamber furnace cooled by 200KPa nitrogen is equivalent to that of the single chamber furnace cooled by 400kPa nitrogen. But the operation cost and maintenance cost are low. The level of China’s basic material industry (graphite, molybdenum, etc.) and supporting components (motors) need to be improved. Therefore, while improving the quality of 600kpa single chamber high-pressure vacuum protection, the development of double chamber pressurized and high-pressure air-cooled quenching furnaces is more in line with China’s national conditions.

Vacuum nitriding technology

Vacuum nitriding is to use a vacuum furnace to heat the stainless steel pipe fitting and flange as a whole, fill a small amount of gas, produce living nitrogen atoms in the low-pressure state to infiltrate and diffuse into the steel to achieve hardening; while ion nitriding is to rely on the active N ion produced by the glow discharge to bombard and only heat the surface of stainless steel pipe fitting and stainless steel flange, and generate nuclear compounds to achieve hardening.
During vacuum infiltration, exhaust the vacuum furnace to a higher vacuum degree of 0.133Pa, raise the workpiece to 530-560 ℃, simultaneously send ammonia or compound gas, and accurately control the feeding amount of various gases. The furnace pressure is controlled at 0.667pa, and the low-pressure state can accelerate the gas exchange on the workpiece surface. The active n elements (or N, c) come from chemical reaction and ammonia gas. After holding for 3-5h, use the furnace for idling Sex gas for rapid cooling. The hardened layer with the depth of 20-80 μ m and the hardness of 600-1500 HV can be obtained by using different materials.
Vacuum nitriding is called vacuum exhaust nitrocarburizing, which is characterized by the activation and purification of metal surface by vacuum technology. In the whole heat treatment process of heating, heat preservation and cooling, impure trace gas is discharged and pure composite gas containing active substance is sent, which makes it possible to adjust and control the structure of surface layer, improve the quality and efficiency. X-ray diffraction analysis shows that the compound layer in the vacuum nitriding layer is ε single-phase structure without other brittleness, so it has high hardness, good toughness and good distribution. The hardness of “white layer” single phase ε compound layer is related to the material composition. The higher the chromium content, the higher the hardness. When chromium content is 13%, the hardness can reach 1200hv; when chromium content is 18% (mass fraction, the same for the rest), the hardness can reach 1500hv; when chromium content is 25%, the hardness can reach 1700hv. The wear resistance of the single phase ε compound layer without brittle phase is higher than that of the gas nitrocarburizing structure. It has excellent anti friction burn, anti thermal bonding, anti deposition and anti melt loss properties. However, the existence of the “white layer” is also detrimental to some molds, pipe fittings and flanges, which is easy to cause cracks in the early forging die and pinholes in welding repair. Another advantage of vacuum nitriding is that there is almost no compound layer (white layer) but only 0.1-1mm diffusion layer structure by controlling the type and amount of compound gas containing active substance sent into the furnace. The reason may be formed after the vacuum furnace exhausts to 0.133Pa, and the other reason is the structure formed by the diffusion of the composite gas with active substances into the steel in a short time. This kind of structure has the advantages of heat shock resistance and crack resistance. Therefore, for the hot working die with high temperature tempering, such as the high-speed steel die, the comprehensive properties of high surface hardness, good wear resistance, good heat shock resistance, crack resistance and toughness can be obtained; however, when there is only diffusion layer structure, the die’s bite resistance, deposit resistance and melting loss resistance are not good enough. Due to the different service conditions and performance requirements of dies or mechanical pipe fittings and flanges, the structure and performance of the surface layer must be adjusted during the surface heat treatment. Vacuum nitriding can improve the performance of precision gears, wear-resistant and corrosion-resistant mechanical pipe fittings, flanges and springs, etc. Besides, it can also be widely used in tools and dies.

Vacuum high pressure air cooling isothermal quenching

Large workpieces with complex shapes are easy to deform or even crack when they are cooled continuously and rapidly from high temperature. In the past, it can be solved by salt bath isothermal quenching. Can air-cooled isothermal quenching be carried out in a single chamber vacuum high pressure air-cooled furnace? In a single chamber high pressure air-cooled quenching furnace with convection heating function, the results of quenching two groups of φ 320mm × 120mm carbon structural steel by different cooling methods are compared. One group is the result of continuous cooling with high-purity nitrogen under 600kpa pressure after heating at 1020 ℃ (the wind direction alternates up and down and switches once in 40s). The other is to cool the surface and the center of the sample at 370 ℃. From the comparison of the two curves, it can be seen that the time when the core temperature passes through 500 ℃ (half cooling time) is only about 2min different. It takes 27 minutes from the beginning of controlled cooling on the surface to the temperature near 370 ℃. Therefore, it is feasible to carry out isothermal air cooling quenching in a single chamber vacuum high pressure gas quenching furnace.

Vacuum cleaning and drying technology

Some heat treatment can not be separated from the cleaning and drying process, especially oil-cooled heat treatment, the cleaning and drying task is more arduous and more difficult. The best cleaning agent in the world is halogen series. Among them, trichloroethane and Freon have been banned because they are substances that destroy the ozone layer. Other halogens are also restricted because they are harmful to the ecological environment, people and animals. Therefore, all countries are studying various alternative cleaning and drying technologies.

Treatment technology of vacuum heat treatment technology

Development of vacuum heat treatment technology

The development of heat treatment is accompanied by the development of mechanical manufacturing industry. Mechanical manufacturing puts forward higher requirements for heat treatment, and the heat treatment of mold is the most technical part of heat treatment.
As we all know, die heat treatment is to develop the potential of die material and improve the performance of die. The performance of the die must meet the following requirements: high strength (including high temperature strength, cold and hot fatigue resistance), high hardness (wear resistance) and high toughness, as well as good machinability (including good polishing), weldability and corrosion resistance.
The biggest influence on the life of the die is the design of the die (including the correct choice of materials), the material of the die, the heat treatment of the die, the use and maintenance of the die, etc. If the design of the die is reasonable and the material is high quality, the quality of heat treatment directly determines the service life of the die. At home and abroad are trying to use more advanced heat treatment means to improve the performance of the mold and extend the service life of the mold. The vacuum heat treatment is one of the more advanced methods in mold heat treatment. Therefore, from the perspective of mold heat treatment, the status of heat treatment processing equipment, heat treatment process, production process control is particularly important. The advanced nature of equipment is the premise to ensure the realization of advanced technology. Vacuum high pressure gas quenching furnace is the most ideal equipment to realize vacuum heat treatment. The vacuum furnace has the effect of non decarburization and non oxidation, with uniform temperature, controllable heating and cooling speed, and can realize different process. Because there is no pollution in the vacuum furnace, it is internationally recognized as “green heat treatment”. There are 2-20bar vacuum high pressure gas quenching furnaces in the world, which can fully meet the requirements of vacuum heat treatment of dies.
In the process of die heat treatment, the process parameters used also have a crucial impact on the die performance: it includes heating temperature, heating speed, holding time, cooling mode, cooling speed, etc. The correct heat treatment process parameters can ensure the best performance of the die, otherwise, it will produce bad or even serious consequences. Practice shows that good structure can be obtained by correct heat treatment process, and good structure morphology can guarantee excellent mechanical properties. The proper process can effectively control the deformation and cracking of the die during heat treatment. It is found from practice that the difference between the surface temperature and the core temperature (the heating and cooling non-uniformity) is the main factor that causes the die deformation during the process of heating and cooling. (the vacuum furnace has the ability to control the heating speed and cooling speed). Different processing methods can make the mold meet different use conditions and different performance requirements.
All in all, the vacuum high pressure gas quenching process has the advantages of free control of heating and cooling speed. Different process parameters can be compiled to obtain the expected microstructure and properties.

Process principle of vacuum heat treatment technology

  • (1) Phase transformation characteristics of metals in vacuum. The thermodynamics and kinetics of solid phase transition do not change much in the vacuum which is only 0.1MPa away from atmospheric pressure. The principle of solid-state phase transformation at atmospheric pressure can be used to formulate the vacuum heat treatment process. It can refer to the data of various types of organization transformation under normal pressure.
  • (2) Vacuum degassing can improve the physical and mechanical properties of metal materials.
  • (3) Vacuum degreasing.
  • (4) Evaporation of metal: when heated in vacuum, elements on the surface of the workpiece will evaporate.
  • (5) With the function of surface purification, heating without oxidation and decarbonization can be realized.
  • (6) The degree of vacuum required for non oxidizing heating of metals.

Heating characteristics of vacuum heat treatment technology

  • (1) Advantages of vacuum heat treatment. Vacuum heat treatment is a non oxidation heat treatment technology which is widely used in parallel with controllable atmosphere, and it is also one of the main signs of the advanced degree of heat treatment production technology. Vacuum heat treatment can not only realize no oxidation and no decarburization of steel parts, but also realize no pollution and less distortion of workpieces, so it belongs to clean and precision production technology. It has become an irreplaceable advanced technology in die production.
  • (2) Vacuum heat treatment process. Small distortion of workpiece is a very important advantage of vacuum heat treatment. According to the experience at home and abroad, the distortion of vacuum heat treatment is only 1/3 of that of salt bath quenching. It is of great significance to study the vacuum heating method and the distortion law of various materials, pipe fittings and flanges with different complexity, and to simulate them by computer. The uniformity of air flow during vacuum heating, atmospheric pressure or high pressure air cooling has a great influence on the hardening effect and quality dispersion of pipe fittings and flanges. It is of great significance to study the air circulation law in the furnace by means of computer simulation for improving the furnace structure. Vacuum carburizing is the most possible way to realize high temperature carburizing. However, long-time heating at high temperature will make the austenite grain size of most kinds of steel grow very large. It is necessary to study the influence of high-temperature carburizing and reheating quenching on the properties of materials and workpieces for the optimization of vacuum carburizing, cooling, heating quenching process and equipment. In recent years, there is a trend of research and development of combustion vacuum furnace using gas fuel in the world. It is too difficult to use gas fuel to heat in vacuum furnace. Although there is a view of saving energy, it is not necessarily an important development direction.
  • (3) Vacuum heat treatment furnace. Modern vacuum heat treatment furnace is a cold wall furnace which can heat components in vacuum and then quench them in oil or in atmospheric pressure and pressure gas. Research and development of this type of equipment is a comprehensive, interdisciplinary work involving many scientific and technological fields.

Advantages of vacuum heat treatment technology

Vacuum heat treatment technology has been well applied in the vacuum heat treatment and vacuum chemical heat treatment of various die steel, stainless steel, bearing steel, carbon steel, cemented carbide, alloy steel, high alloy steel and other important pipe fittings and flanges, because vacuum heat treatment has many advantages:

Can realize bright quenching

When the vacuum degree in the furnace reaches 1-10-1pa, the surface of the workpiece achieves the goal of non decarburization and non carburization, thus realizing the goal of bright quenching.
In 1983, Hengyang Textile Machinery Factory purchased a double chamber vacuum quenching furnace from capital machinery factory (now Shenjian) for bright quenching of GCr15 bearing steel pipe fittings and flange ingot bottom. The technical requirements of the ingot bottom are very high. The inner top of the ingot bottom does not need any machining after cold stamping R, and no decarburization and other defects are allowed on the surface after quenching. After quenching in vacuum furnace, the quality is well guaranteed and the operation conditions are greatly improved.
In addition, it is also widely used in precision small and micro (bearing inner diameter < 9mm) bearings. The quality of bearing rings made of stainless steel and bearing steel after vacuum quenching is very good, because there is no decarburization and Carburization on the surface of rings after vacuum quenching, and the materials of rings can be degassed, and the impurities can be reduced to less than 1.5ppm, so that the hardness distribution is uniform and abnormal The bearing has high elastic limit and contact fatigue strength, prolongs the service life of the bearing, and improves the market competitiveness.

Degassing effect

When the vacuum degree is lower than the corresponding decomposition pressure of metal or alloy, its oxide will decompose, form free oxygen and be discharged out of the vacuum room immediately, so that the internal quality of the workpiece can be improved.
Because the billet of stainless steel pipe fittings and flanges will inhale a small amount of hydrogen, nitrogen, carbon monoxide and other gases in the atmosphere during smelting, resulting in tiny pores, which are easy to produce defects such as hydrogen embrittlement, cemented carbide. In the process of pressing, there will also be air in, so for the die steel, the residual gas in the workpiece can be removed by vacuum technology, so that the structure can be pure. For cemented carbide, the residual gas in the workpiece can also be removed by vacuum treatment.
Vacuum furnace is widely used in small and medium-sized enterprises and professional tool factories, especially in the private enterprises, the proportion of vacuum furnace used in the quenching of tools, tools, electric tools and other workpieces is also increasing.

It can reduce the machining allowance after heat treatment

Vacuum heat treatment is widely used for vacuum quenching and vacuum carburizing of oil pump nozzle coupling parts because there is no oxidation and decarburization phenomenon, with little distortion and clean and bright. In this way, it can avoid the phenomenon of black structure (decarburization) on the surface of the inner hole of coupling parts caused by salt bath furnace sometimes due to unclean cleaning of residual salt or protective atmosphere.
For the needle valve body of carburizing steel, the required carburizing layer of 0.5-0.9mm can be guaranteed by vacuum carburizing, but it is difficult to be guaranteed by controlled atmosphere furnace carburizing. For precision hydraulic parts such as plunger pump, vacuum quenching can reduce distortion, improve wear resistance and ensure the stability of workpiece quality. It has been several decades since the application of vacuum technology to quench the coupling parts of oil pump nozzle and hydraulic parts of plunger pump, which has been widely recognized in improving product quality.

Cleaning process

Vacuum heat treatment is a clean heat treatment process, which plays an important role in the whole heat treatment process. It can improve product quality, change the appearance of heat treatment workshop, and improve the operating environment. It is a technology worthy of promotion and application.

Advantages of vacuum carburizing

The product quality of vacuum carburizing is better than that of controlled atmosphere furnace carburizing. For gear carburizing to control quenching deformation, after carburizing in controlled atmosphere furnace, it is necessary to use quenching press to carry out side quenching and side correction. Sometimes the quality fails to meet the technical requirements. Moreover, the uniformity of carburizing layer and carbon concentration distribution is difficult to be guaranteed. Internal oxidation often occurs, resulting in non martensitic black With the appearance of color structure, the productivity is sometimes affected, so the carburization of small and medium-sized gears and gear shafts in the parts of motorcycles, automobile reducers, automobile engines and other parts with high quality requirements is necessary.
In the enterprises with conditions, it has gradually changed from controlled atmosphere carburizing to vacuum carburizing. Low pressure vacuum carburizing and high pressure air quenching technology can be used for carburizing of gears and gear shafts with higher quality requirements. It has been gradually applied in China.

Medium for vacuum quenching

The auxiliary materials for vacuum quenching include charging frame (basket), charging plate, nitrogen and quenching medium. The charging frame (basket) and charging plate need to be made of 40cr25ni20 and other heat-resistant steel materials. It is recommended to use 99.995% liquid nitrogen for nitrogen, because the purity of nitrogen can be guaranteed by using liquid nitrogen, and the operation and maintenance are more convenient. The quenching medium needs special vacuum quenching oil, which is suitable for use under the condition of lower than atmospheric pressure. It has short distillation range, low saturated vapor pressure, strong oxidation resistance, is not volatile, and is easy to vacuum. Good brightness, easy to clean, less oil consumption. It has the advantages of good thermal oxidation stability, stable cooling performance and long service life. Therefore, after quenching, the workpiece has good hardness uniformity, less distortion and is clean and bright. Kerun company has very mature experience and friendly after-sales service. It can be customized according to different specifications of workpieces, and provide three kinds of vacuum quenching oil with different cooling speed, fast, medium and slow.

Main application of vacuum heat treatment technology

After vacuum heat treatment, the tube and flange have small distortion, high quality, flexible operation and no pollution. Therefore, vacuum heat treatment is not only a necessary means of heat treatment for some special alloys, but also has been applied in heat treatment of general engineering steel, especially for tools, dies and precision coupling parts. After vacuum heat treatment, the service life is greatly improved compared with that of general heat treatment. For example, the service life of some dies after vacuum heat treatment is 40-400% higher than that of the original salt bath treatment, while the service life of many tools can be increased by 3-4 times. In addition, the vacuum furnace can work at a higher temperature, and the workpiece can maintain a clean surface, so it can accelerate the adsorption and reaction process of chemical heat treatment. Therefore, some chemical heat treatment, such as carburizing, nitriding, chromizing, boronizing, and multi-element co infiltration, can get faster and better results.

Difference between vacuum heat treatment and ordinary heat treatment

Vacuum heat treatment is a new type of heat treatment technology combining vacuum technology and heat treatment technology. The vacuum environment of vacuum heat treatment refers to the atmosphere environment lower than one atmospheric pressure, including low vacuum, medium vacuum, high vacuum and ultra-high vacuum. Vacuum heat treatment actually belongs to atmosphere controlled heat treatment. Vacuum heat treatment means that all and part of the heat treatment process are carried out in vacuum state. Vacuum heat treatment can achieve almost all the heat treatment processes that can be involved in conventional heat treatment, but the quality of heat treatment is greatly improved. Compared with the conventional heat treatment, the vacuum heat treatment can achieve no oxidation, no decarburization, no carburization, can remove the phosphorus chips on the surface of the workpiece, and has the function of degreasing and degassing, so as to achieve the effect of surface bright purification.

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Moreover, the price of vacuum heat treatment is high because of the expensive equipment and more energy consumption; the weight of each furnace is small because of the small vacuum cavity, which is easy to vacuum; the workpieces after vacuum heat treatment have stronger oxidation resistance and corrosion resistance than the ordinary workpieces, and the microstructure is more compact.
With the continuous progress of related technologies, not only the vacuum heat treatment equipment has made great development, but also the related supporting facilities have made great progress. At present, China has been able to produce various kinds of vacuum pumps, vacuum valves, vacuum measuring instruments, etc., and even some matching products that could not be produced independently before have also been produced by itself (such as insulation layer and heating elements made of metal and graphite materials used in the furnace), and their performance and quality have been close to or reached the level of similar products abroad. The rapid development of related industries has effectively promoted the overall progress of China’s vacuum heat treatment level.
Although the foreign vacuum heat treatment technology and related equipment started earlier, and the imported vacuum heat treatment furnace also maintains a comparative advantage in performance, but this does not mean that the imported equipment is better than the domestic equipment in usability. China has its own unique national conditions and special customer needs. With the breakthrough of domestic vacuum heat treatment furnace in many key technologies, some vacuum heat treatment equipment specially developed for China’s national conditions and special customer needs has more effectively promoted the development of relevant application fields.
For example, in view of the deficiency of heat shock resistance, cold resistance and heat alternation resistance of domestic furnace liner materials of single chamber vacuum high pressure gas quenching furnace, a vacuum double chamber high pressure gas quenching furnace is specially developed according to the actual situation of our country, which not only greatly improves the efficiency of vacuum gas quenching, but also effectively saves energy, and greatly extends the service life of the equipment.
For example, in view of the needs of special customers such as aviation, aerospace, military industry, etc., titanium alloy and some rare alloys can achieve the required performance. The vacuum water quenching heat treatment furnace has been developed, which fully meets the special needs of customers in these industries.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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