Vacuum heat treatment

1. Vacuum high pressure air cooling quenching technology

At present, vacuum high-pressure air-cooling quenching technology is developing rapidly, and negative pressure (<1×105Pa), high-flow rate, air-cooling, pressurization (1×105~4×105Pa), air-cooling, high-pressure (5×105~10× 105Pa) air cooling, ultra-high pressure (10×105~20×105Pa) air cooling and other new technologies, not only greatly improve the vacuum quenching capacity, but also the surface of the workpiece after quenching has good brightness, small deformation, and high efficiency and energy saving , No pollution and other advantages.

The purpose of vacuum high pressure air quenching is the quenching and tempering of materials, the solid solution and aging of stainless steel and special alloys, ion carburizing and carbonitriding, as well as vacuum sintering, cooling and quenching after brazing.

When quenching with 6×105Pa high-pressure nitrogen, the load to be cooled can only be loose, high-speed steel (W6Mo5Cr4V2) can be hardened to 70-100mm, high-alloy hot-work die steel (such as 4Cr5MoSiV) can reach 25-100mm, High alloy heat-treated gold cold-work die steel (such as Cr12) can reach 80-100mm.

When cooling and quenching with 10×105Pa high-pressure nitrogen, the cooling load can be intensive, which is about 30%-40% higher than the load density when cooling with 6×105Pa.

When cooling and quenching with 20×105Pa ultra-high pressure nitrogen or a mixture of helium and nitrogen, the cooling load is dense and can be bundled together. Its density is increased by 80%~150% compared with 6×105Pa nitrogen cooling. It can cool all high speed steel, high alloy steel, hot work die steel and Cr13% chromium steel and more alloy oil quenched steel, such as larger Size of 9Mn2V steel.

The cooling capacity of a dual-chamber air-cooled quenching furnace with a separate cooling chamber is better than that of the same type of single-chamber furnace. The cooling effect of a 2×105Pa nitrogen-cooled double-chamber furnace is equivalent to that of a 4×105Pa single-chamber furnace. However, operating costs and maintenance costs are low. The level of my country’s basic materials industry (graphite, molybdenum materials, etc.) and supporting components (motors) needs to be improved. Therefore, while improving the quality of 6×105Pa single-chamber high-pressure vacuum protection, the development of double-chamber pressurized and high-pressure air-cooled quenching furnaces is more in line with my country’s national conditions.

2. Vacuum high pressure air cooling isothermal quenching

Larger workpieces with complex shapes are prone to deformation and even cracks when they are continuously rapidly cooled from high temperature. In the past, it can be solved by salt bath austempering. Can air-cooled isothermal quenching be carried out in a single-chamber vacuum high-pressure gas-cooled quenching furnace? Figure 1 shows two sets of φ320mm×120mm stacked carbon in a single-chamber high-pressure gas-cooled quenching furnace with convection heating function The results of structural steel quenching with different cooling methods. A set of curves in the figure is the result of continuous high-purity nitrogen cooling at a pressure of 6×105 Pa after heating at 1020°C (the wind direction alternates between up and down, switching once in 40s). The other group is controlled cooling at 370°C on the surface and core of the sample. From the comparison of the two sets of curves, it can be seen that the time (semi-cooling time) for the temperature of the heart to pass through 50O ℃ is only about 2 minutes. It takes 27 minutes from the beginning of controlled cooling on the surface to the temperature of the core reaching around 370°C. It can be seen that it is feasible to perform isothermal gas quenching in a single-chamber vacuum high-pressure gas quenching furnace.

3. Vacuum nitriding technology

Vacuum nitriding is the use of a vacuum furnace to heat the steel parts as a whole, fill with a small amount of gas, and produce active nitrogen atoms in a low pressure state to infiltrate and diffuse into the steel to achieve hardening; while ion nitriding is the activity generated by glow discharge N ions bombard and only heat the surface of steel parts, and a chemical reaction occurs to form nuclei to achieve hardening.

During vacuum infiltration, exhaust the vacuum furnace to a higher vacuum of 0.133Pa (1×10-3Torr), then raise the workpiece to 530~560℃, and at the same time feed in the ammonia gas mainly containing active material A variety of composite gases, and precise control of the feeding amount of various gases, the furnace pressure is controlled at 0.667Pa (5Torr), after holding for 3 to 5 hours, the inert gas in the furnace is used for rapid cooling. Different materials, after this treatment, a hardened layer with a penetration depth of 20-80μm and a hardness of 600-1500HV can be obtained.

Vacuum nitriding is called vacuum exhaust nitrocarburizing. Its characteristic is to activate and purify the metal surface through vacuum technology. In the entire heat treatment process of heating, heat preservation and cooling, the impure trace gas is discharged, and the pure composite gas containing the active material is sent in, making it possible to adjust and control the surface layer phase structure, improve quality, and increase efficiency. . X-ray diffraction analysis confirms that after vacuum nitriding, the compound layer in the nitriding layer is ε single-phase structure, and there is no other brittle phase (such as Fe3C, Fe3O4), so the hardness is high, the toughness is good, and the distribution is good. The achievable hardness of the “white layer” single-phase epsilon compound layer is related to the material composition. The higher the Cr content in the material, the higher the hardness. When Cr is 13%, the hardness can reach 1200HV; when Cr is 18% (mass fraction, the same), the hardness can reach 1500HV; when Cr is 25%, the hardness can reach 1700HV. The wear resistance of the single-phase epsilon compound layer without brittle phase is higher than that of the gas nitrocarburizing structure, and it has excellent resistance to friction and burn, resistance to heat bonding, resistance to welding, and resistance to melt loss. However, the existence of this “white layer” also has disadvantages for some molds and parts. It is easy to cause cracks in the forging die at the early stage of forging, and pinholes are easily generated during welding repair. Another advantage of vacuum nitriding is that by controlling the type and amount of the composite gas containing activated substances fed into the furnace, it is possible to obtain a structure with almost no compound layer (white layer) but only a diffusion layer. The reason may be formed after the vacuum furnace is exhausted to 0.133Pa (1×10-3 Torr). Another reason is the structure formed by the diffusion of the composite gas with the active material into the steel in a short time. The advantages of this structure are excellent thermal shock resistance and crack resistance. Therefore, for hot work molds that are subjected to high temperature tempering, such as high-speed steel or 4Cr4MoSiV (H13) steel molds, the comprehensive properties of high surface hardness, good wear resistance, good thermal shock resistance, crack resistance and toughness can be obtained. ; But when there is only the diffusion layer structure, the seizure resistance, welding resistance and melt loss performance of the mold is not good enough. Due to the different service conditions and performance requirements of molds or mechanical parts, it is necessary to adjust the structure and performance of the surface layer during surface heat treatment. In addition to the application of vacuum nitriding to molds, it has obvious effects on improving the performance of precision gears, mechanical parts that require wear and corrosion resistance, and springs. The materials that can be processed are also relatively wide.

4. Vacuum cleaning and drying technology

At present, some heat treatments are inseparable from the cleaning and drying process, especially various heat treatments that require oil cooling. The cleaning and drying tasks are more onerous and more difficult. The best cleaning agent in the world is the halogen cleaning agent. Table 1 shows the ratio of halogen-based cleaning agents used in developed countries such as Japan. Among them, trichloroethane and freon have been banned because they are substances that destroy the ozone layer of the atmosphere. Other halogen-based substances are also restricted from being used because they are harmful to the ecological environment, humans, and animals. So all countries are studying various alternative cleaning and drying technologies.

Vacuum water cleaning and drying technology is the mainstream of alternative technology development, and its principles are steam distillation and vacuum distillation. The so-called steam distillation refers to a method in which volatile components such as oil are distilled out together with water while blowing water vapor onto a workpiece with water-insoluble oils and other substances. When this method is applied, the vapor of volatile components such as oil is generated simultaneously with water vapor, so the boiling point of the oil is lowered. This is to use the azeotropic method to make the oil and the additives form a low-boiling substance, making it possible to clean at a lower temperature below the tempering temperature. The so-called vacuum distillation is because when steam distillation is carried out under normal pressure, the vapor pressure of the oil is very low, and the amount of volatile oil accompanying the water vapor is small. If steam distillation is performed under vacuum, the boiling point of the oil will be further reduced. Using steam distillation and vacuum distillation at the same time, the high boiling point heat treatment quenching oil can be cleaned and dried at the tempering temperature. This method is suitable for use in large and medium-sized enterprises and mass production assembly lines, and the efficiency is relatively high. The disadvantage is the high cost of recovery, treatment and recycling of cleaning fluid.

Vacuum oil cleaning and drying technology refers to the use of light solvent oil with a relatively high vapor pressure to dissolve and clean the cutting oil, stamping oil, and quenching oil attached to the workpiece with a relatively low vapor pressure; then heat and decompress the solvent The oil is evaporated and separated to achieve the purpose of cleaning. The choice of medicinal solvent oil should be as high as possible, the viscosity should be low, it has a strong ability to dissolve the grease attached to the workpiece to be cleaned; it has no corrosion to the workpiece, and the cost is low. The 190 and 20O solvent oils introduced in my country’s national standards can be used as a reference when selecting.

Vacuum heat treatment process of mold

The development of heat treatment is accompanied by the development of the mechanical manufacturing industry, and mechanical manufacturing has put forward newer and higher requirements for heat treatment. The heat treatment of molds is the most technical part of heat treatment.

As we all know, mold heat treatment is to develop the potential of mold materials and improve mold performance. The performance of the mold must meet: high strength, (including high temperature strength, resistance to cold and heat fatigue), high hardness (wear resistance) and high toughness, and also requires good machinability, (including good polishing ) Weldability and corrosion resistance, etc.

The biggest influence on the life of the mold is the design of the mold (including the correct choice of material), the material of the mold, the heat treatment of the mold, and the use and maintenance of the mold. If the design of the mold is reasonable and the material is high-quality, the quality of the heat treatment directly determines the service life of the mold. At present, both at home and abroad are trying to use more advanced heat treatment methods to improve the performance of the mold and extend the service life of the mold. Vacuum heat treatment is one of the more advanced methods in mold heat treatment. Therefore, from the perspective of mold heat treatment, the state of heat treatment processing equipment, heat treatment process, and production process control are particularly important. The advanced nature of the equipment is the prerequisite to ensure the realization of advanced technology. Vacuum high-pressure gas quenching furnace is the most ideal equipment for vacuum heat treatment. The vacuum furnace has the effects of non-decarburization and non-oxidation, with uniform temperature, controllable heating and cooling speeds, and can realize different processes. The vacuum furnace is internationally recognized as “green heat treatment” because it has no pollution. Now there are 2-20bar vacuum high-pressure gas quenching furnaces in the world, which can fully meet the requirements of vacuum heat treatment of molds.

In the process of mold heat treatment, the process parameters used also have a crucial impact on mold performance: it includes heating temperature, heating rate, holding time, cooling method, cooling rate, etc. The correct heat treatment process parameters can ensure the best performance of the mold, otherwise, it will produce undesirable and even serious consequences. Practice has shown that the correct heat treatment process can obtain an excellent structure, and an excellent structure can ensure excellent mechanical properties. Appropriate processing methods can effectively control the deformation and cracking of the mold during heat treatment. It has been found from practice that during the heating and cooling of the mold, the difference between the surface temperature of the mold and the temperature of the core (non-uniformity of heating and non-uniformity of cooling) is the main factor causing mold deformation. (Vacuum furnace has the ability to control heating rate and cooling rate). Different process methods can make the mold meet different conditions of use and different performance requirements.

From the perspective of the service life of the mold, meeting the requirements of hardness is only one aspect of meeting the technical requirements of the mold, and it also has some performance requirements that are not easy to measure, such as strength and toughness. The quality of the mold cannot be determined by the hardness index. It is impossible to use the hardness measurement method to finally test the service life of the mold. As a special process (ie special process), heat treatment can only pass process verification and performance experiment. , Confirm reasonable process parameters, and strictly implement the confirmed process parameters (process control) to ensure the reliability and stability of product quality. A large amount of data shows that the cold-punching dies processed by vacuum heat treatment have less deformation and rarely occur wire-cutting cracking and abrasion. The die-casting mold adopts advanced technology to reduce the cracking of the mold and the phenomenon of mold sticking in use to a certain extent.

All in all, the vacuum high-pressure gas quenching process has the advantages of free control of heating and cooling rates, and different process parameters can be programmed to obtain the expected metallographic structure and performance.

Application of vacuum heat treatment technology

After the parts are subjected to vacuum heat treatment, the distortion is small, the quality is high, and the process itself is flexible and pollution-free. Therefore, vacuum heat treatment is not only a necessary means for heat treatment of certain special alloys, but also used in the heat treatment of general engineering steels, especially tools, molds and precision couplings. After vacuum heat treatment, the service life is longer than that of general heat treatment. Improvement. For example, after vacuum heat treatment, the life of some molds is 40 to 400% higher than that of the original salt bath treatment, and the life of many tools can be increased by about 3 to 4 times. In addition, the vacuum heating furnace can work at a higher temperature, and the workpiece can maintain a clean surface, which can accelerate the adsorption and reaction process of chemical heat treatment. Therefore, certain chemical heat treatments, such as carburizing, nitriding, chromizing, boronizing, and multi-element co-infiltration can get faster and better results.