Comparison between vacuum heat treatment and traditional heat treatment
- What is heat treatment
- Heat treatment process
- Technical characteristics of heat treatment
- Classification of heat treatment processes
- Surface quenching and tempering heat treatment
- Local quenching heat treatment
- Chemical heat treatment
- Common problems in heat treatment
- Heat treatment deformation
- Surface decarburization
- What is vacuum heat treatment
- Technical classification of vacuum heat treatment
- Vacuum high pressure gas quenching technology
- Vacuum nitriding technology
- Vacuum high pressure gas cooled isothermal quenching
- Vacuum cleaning and drying technology
- Treatment process of vacuum heat treatment
- The principle of vacuum heat treatment
- Heating characteristics of vacuum heat treatment
- Main applications of vacuum heat treatment
- Comparison between vacuum heat treatment and traditional heat treatment
Heat treatment is a kind of metal hot working technology which is used to obtain the expected structure and properties of materials under solid state by heating, holding and cooling. From the stone age to the bronze age and the iron age, the role of heat treatment is gradually recognized.
Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterrupted. Heating is one of the most important processes in heat treatment. There are many heating methods for metal heat treatment. The earliest use of charcoal and coal as heat source is near the application of liquid and gaseous fuels. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be heated directly or indirectly by heating molten salts or gold and even floating particles. When metal is heated, the workpiece is exposed to air, which often leads to oxidation and decarbonization (that is, the reduction of carbon content on the surface of iron and steel parts), which has a negative effect on the surface properties of parts after heat treatment. Therefore, metals should normally be heated in controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected or heated by coating or packaging. Heating temperature is one of the important technological parameters of heat treatment process. Selecting and controlling heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the purpose of the treated metal material and heat treatment, but generally it is heated above the phase transition temperature to obtain high temperature structure. In addition, it takes a certain time to change. Therefore, when the surface of the metal workpiece reaches the required heating temperature, it is necessary to maintain the temperature for a certain period of time, so that the internal and external temperatures will be identical and the microstructure will be transformed completely. This time is called the holding time. When using high-energy density heating and surface heat treatment, the heating speed is very fast, generally there is no holding time, and the heat preservation time of chemical heat treatment is usually longer.
Cooling is also an indispensable step in the heat treatment process. The cooling method is different because of different processes, mainly controlling the cooling rate. The cooling rate of general annealing is the slowest, the normalizing cooling rate is faster, and quenching is faster. But there are different requirements due to different steel grades. For example, hard steel can be hardened by normalizing cooling rate.
- 1. Normalizing: heat the steel or steel parts to a critical temperature of AC3 or ACM for a certain period of time and then cool down in the air.
- 2.Annealing: heating the hypoeutectoid steel workpiece to 20 to 40 degrees above AC3 for a period of time, then heat treatment slowly with the furnace (or buried in sand or lime) to below 500 degrees in air.
- 3. Solution heat treatment: the alloy is heated to high temperature single-phase region to maintain constant temperature, so that the surplus phase is fully dissolved into the solid solution, and then cooled rapidly to obtain the heat treatment process of supersaturated solid solution.
- 4. Aging: when the alloy is subjected to solid solution heat treatment or cold plastic deformation, its properties change with time at room temperature or slightly above room temperature.
- 5. Solution treatment: sufficient dissolution of various phases in the alloy, strengthening solid solution and improving toughness and corrosion resistance, eliminating stress and softening so as to continue processing.
- 6. Aging treatment: heating and holding at the temperature of the precipitated phase, so that the precipitates can be precipitated and hardened, and the strength will be increased.
- 7. Quenching: cooling austenitizing steel and cooling at proper cooling rate, so that heat treatment processes such as martensite and other unstable structural changes occur in the whole or certain range of the cross section.
- 8. Tempering: heating the hardened workpiece to a suitable temperature below AC1 for a certain period of time, then cooling it with the required method to obtain the required heat treatment process for the structure and properties.
- 9. Carbonitriding of steel: carbonitriding is the process of simultaneous infiltration of carbon and nitrogen into the surface of the steel. Traditionally, carbonitriding is also known as cyanidation. It is widely used in mesophilic gas nitrocarburizing and low temperature gas nitrocarburizing (gas nitriding). The main purpose of mesophilic gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low temperature gas nitrocarburizing is mainly nitriding, and its main purpose is to improve the wear resistance and bite resistance of steel.
- 10. Quenching and tempering treatment (quenching and tempering): commonly used heat treatment combined with quenching and tempering is called quenching and tempering treatment. Quenching and tempering treatment is widely used in all kinds of important structural parts, especially those connecting rods, bolts, gears and shafts under alternating load. After tempering and tempering, the microstructure of tempered Soxhlet is obtained, and its mechanical properties are better than that of normalizing Soxhlet with same hardness. Its hardness depends on the tempering temperature at high temperature and is related to the tempering stability of steel and the section size of workpieces, generally between HB200 – 350.
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment does not change the shape and overall chemical composition of the workpiece. Instead, it changes the microstructure of the workpiece or changes the chemical composition of the surface of the workpiece, giving or improving the performance of the workpiece. It is characterized by improving the internal quality of the workpiece, which is generally not visible to the naked eye.
In order to make metal workpieces possess the required mechanical, physical and chemical properties, heat treatment process is essential, in addition to the rational selection of materials and various forming processes. Steel is the most widely used material in the mechanical industry. The microstructure of iron and steel is complex and can be controlled by heat treatment. Therefore, the heat treatment of steel is the main content of heat treatment. In addition, aluminum, copper, magnesium, titanium and other alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance.
Metal heat treatment technology can be broadly divided into three categories: integral heat treatment, surface heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling method, each big category can be divided into several different heat treatment processes. Different heat treatment processes can be used to obtain different structures and different properties of the same metal. Steel is the most widely used metal in industry, and the microstructure of steel is the most complex. Therefore, there are many kinds of heat treatment processes.The whole heat treatment is a metal heat treatment process to heat the whole workpiece and then cool it at an appropriate speed to obtain the metallographic structure needed to change its overall mechanical properties. The overall heat treatment of steel consists of four basic processes: annealing, normalizing, quenching and tempering.
Surface quenching and tempering heat treatment is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Hardness testing can be done with Vivtorinox hardness tester or Rockwell or surface Rockwell hardness tester. The selection of test force (ruler) is related to the depth of effective hardened layer and the hardness of workpiece surface. There are three kinds of hardness gauges. 1. Vivtorinox hardness tester is an important means to test the hardness of workpiece surface. It can test the surface hardened layer from 0.05mm to 100kg with the test force of 0.5 ~. The accuracy is correct, and it can distinguish the tiny difference of hardness of heat treated workpiece surface. In addition, the effective hardened layer depth is also determined by the Vivtorinox hardness tester. Therefore, it is necessary to have a Vivtorinox hardness tester for surface heat treatment or surface heat treatment. Two. The surface Rockwell hardness tester is also very suitable for testing the hardness of the surface hardening workpiece. The surface Rockwell hardness tester has three kinds of rulers to choose from. All kinds of hardened parts with effective hardening depth exceeding 0.1mm can be tested. Although the accuracy of the surface Rockwell hardness tester is not as high as that of the Vivtorinox hardness tester, it has been able to meet the requirements as a testing method for the quality management and qualification inspection of the heat treatment plant. Besides, it also has the characteristics of simple operation, easy to use, low price, rapid measurement and direct reading of hardness value. The surface Rockwell hardness tester can be used for rapid and non-destructive detection of batch surface heat treatment parts. This is of great significance for Metalworking and machinery manufacturing plants. Three. When the surface heat treatment hardening layer is thicker, the Rockwell hardness tester can also be adopted. When the thickness of heat treatment hardening layer is between 0.4 and 0.8mm, the HRA scale can be adopted. When the hardened layer thickness exceeds 0.8mm, the HRC scale can be adopted. The three hardness values of Vivtorinox, Rockwell and surface Rockwell can be easily converted to each other and converted to standard, drawing or user required hardness values. The corresponding conversion tables have been given in international standard ISO, American Standard ASTM and China standard GB/T.
Local quenching heat treatment
Parts with higher local hardness require partial quenching heat treatment such as induction heating. Such parts are usually marked on the blueprint to indicate the location of localized quenching and heat treatment and the local hardness values. Hardness testing of parts must be carried out in designated areas. The hardness tester can be tested by Rockwell hardness tester. The hardness value of HRC can be tested. For example, the hardness of heat treatment layer is relatively shallow, and the surface Rockwell hardness tester can be used to test the HRN hardness value.
Chemical heat treatment
Chemical heat treatment is the infiltration of one or more chemical elements into the surface of the workpiece, thus changing the chemical composition, structure and properties of the workpiece surface.After quenching and low temperature tempering, the surface of the workpiece has high hardness, wear resistance and contact fatigue strength, and the core part of the workpiece has high strength and toughness.
From pipe fitting and flange Microstructure overheating after quenching can be observed on rough edges.But the microstructure must be observed to determine the extent of its overheating. If the coarse needle like martensite is found in the quenched structure of GCr15 steel, the overheated structure will be quenched. The reason may be the overheating caused by the high temperature of quenching or heating and too long holding time. It may also be due to the serious zonal carbides in the original structure, and the local hyperthermia caused by the coarse needle like local martensite formed in the low carbon zone between the two zones. Retained austenite increased and dimensional stability decreased in overheated structure. Because of the overheating of the quenched structure and the coarse crystal of the steel, the toughness of the parts will decrease, the impact resistance will be lowered, and the life of the bearing will also be reduced. Overheating can even cause quenching cracks.
Quenching temperature is low or poor cooling will produce more than the standard provisions of the microstructure in the microstructure, known as the sub thermal structure, which makes the hardness drop, wear resistance sharply reduced, affecting the bearing life of idler accessories.
High or cooling too fast, thermal stress and metal mass volume change when the tissue stress is greater than the steel fracture strength; the original defects on the working surface (such as surface micro cracks or scratches) or internal defects of steel, such as slag, serious non-metallic inclusions, white spots, residual shrinkage, and so on, form stress concentration during quenching; serious surface decarburization and carbide segregation; insufficient tempering after parts quenching or not timely tempering; the former process causes too much cold punching stress, forging folding, deep cutting tool marks, oil groove sharp edges and so on. In conclusion, the causes of quenching cracks may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, and the fracture is straight. It is usually vertical cracks or ring cracks on the bearing rings; the shape of the bearing ball is S, T or ring. The microstructure characteristics of quenched cracks are no decarburization on both sides of the crack, which is obviously different from forging cracks and material cracks.
Heat treatment deformation
When the tube and flange are heated, there are thermal stress and tissue stress. The internal stress can be superposed or partly offset. It is complex and changeable, because it can change with heating temperature, heating speed, cooling mode, cooling rate, shape and size of the parts, so the heat treatment deformation is inevitable. Understanding and mastering its variation rule can make the deformation of pipes and flanges, such as the ellipse of the rings, size increase, etc., be placed in a controllable range, which is conducive to production. Conduct. Of course, mechanical collision during heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improved operation.
If the pipes and flanges are heated in oxidizing medium during the heat treatment process, the surface will oxidize, which will reduce the carbon content of the parts surface and cause surface decarburization. The depth of the decarburization layer exceeds the final processing allowance and the parts will be scrapped. The surface decarburization depth can be measured by metallographic method and microhardness method. The method of measuring the microhardness distribution curve of the surface layer is the criterion and can be used as an arbitration criterion. The phenomenon of insufficient local hardness of roller pipe and flange surface caused by insufficient heating of soft spot, bad cooling, improper quenching operation and so on is called quenching soft spot. Like surface decarbonization, it can cause a serious drop in surface wear resistance and fatigue strength.
Vacuum heat treatment is a new type of heat treatment technology combined with vacuum technology and heat treatment technology. The vacuum environment of vacuum heat treatment refers to the atmosphere environment below atmospheric pressure, including low vacuum, medium vacuum, high vacuum and ultrahigh vacuum. Vacuum heat treatment is also controlled by atmosphere controlled heat treatment. Vacuum heat treatment means the whole and part of the heat treatment process is carried out in vacuum, and the quality of heat treatment is greatly improved. Compared with conventional heat treatment, vacuum heat treatment can achieve no oxidation, no decarburization and no carburizing. It can remove phosphorus from the surface of the workpiece and remove the fat and degassing, so as to achieve the effect of surface bright purification.
Vacuum high pressure gas quenching technology
At present, the development of vacuum high pressure gas quenching technology has developed rapidly, and new technologies, such as negative pressure, high flow rate gas cooling, pressurized gas cooling, high pressure gas cooling, ultra-high pressure and one gas cooling, have been developed successively. Not only has the vacuum gas cooling quenching capability been greatly improved, but also the surface brightness of the workpiece has been improved and the deformation is small after quenching.
The purpose of vacuum high pressure gas quenching is hardening and tempering of materials, solid solution and aging of stainless steel and special alloys, ion carburizing and carbonitriding, vacuum sintering, cooling and quenching after brazing.
When cooled by high pressure nitrogen, the cooling load can only be loose. High speed steel can be hardened to 70 to 100mm, and high alloy hot working die steel can reach 25 to 100mm.
When cooled by 1000kPa high pressure nitrogen, the cooling load can be intensive, and the load density will be increased by about 30% ~ 4O% when cooled by 600kPa.
When cooled by 2000kPa superhigh pressure nitrogen or helium and nitrogen mixture, the cooling load is dense and can be bundled together. Its density is 80% to 150% higher than that of 600kPa when nitrogen is cooled. It can cool all high-speed steel, high alloy steel, hot working die steel and chrome steel and more alloy oil quenched steel, such as larger size manganese steel.
The cooling capacity of the double chamber gas cooled quenching furnace with a separate cooling chamber is better than that of the single chamber furnace of the same type. The cooling effect of 200kPa nitrogen cooled double chamber furnace is comparable to that of 400kPa single chamber furnace. But the running cost and maintenance cost are low. As China’s basic materials industry (graphite, molybdenum materials, etc.) and supporting components (motors) and other levels to be improved. Therefore, in improving the quality of 600kPa single chamber high voltage vacuum protection, the development of double chamber pressurized and high pressure gas quenching furnace is more in line with China’s national conditions.
Vacuum nitriding is to use the vacuum furnace to heat the steel parts and fill up a small amount of gas, and produce nitrogen atoms to infiltrate and diffuse into the steel under low pressure. The ion nitriding is bombarded by active N ions produced by the glow discharge and only heating the surface of the steel parts.
When the vacuum is used to permeate the atmosphere, the vacuum furnace is discharged to a higher vacuum degree of 0.133Pa, and the workpiece is raised to 530~560 degrees. At the same time, it is sent to ammonia or compound gas. The feed quantity of various gases is precisely controlled. The furnace pressure is controlled at 0.667Pa. The low pressure state can accelerate the gas exchange on the surface of the workpiece. The active N element (or N, C) comes from the chemical reaction and ammonia, and after holding the insulation for 3 to 5h, the inert gas in the furnace is used for rapid cooling. After treatment, the hardened layer with a depth of 20~80 m and a hardness of 600 ~ 1500HV can be obtained.
Vacuum nitriding is called vacuum exhaust nitrocarburizing, which is characterized by activating and purifying metal surface by vacuum technology. In the whole heat treatment process of heating, insulation and cooling, impure trace gases are discharged, and clean composite gases containing active substances are sent to make possible the adjustment and control of the phase structure of the surface layer, the improvement of quality and the improvement of efficiency. X ray diffraction analysis confirmed that after vacuum nitriding treatment, the compound layer in the infiltrating layer is a single phase structure with no other brittleness, so it has high hardness, good toughness and good distribution. The hardness and material composition of the “white layer” single-phase EPI compound layer are related. The higher the chromium content in the material is, the higher the hardness is. When chromium 13%, the hardness can reach 1200HV; when chromium content is 18% (mass fraction remaining the same), the hardness can reach 1500HV; when chromium content 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 nitrocarburizing structure, and it has excellent friction and burn resistance, thermal bonding, anti cladding and anti melting properties. However, the existence of the “white layer” also has some disadvantages for some dies and parts, which makes the forging die crack at the early stage of forging. Another advantage of vacuum nitriding is that by controlling the type and quantity of compound gases in the furnace, the structure with almost no compound layer (white layer) and only 0.1-1mm diffusion layer can be obtained. The reason may be caused by the exhaust from the vacuum furnace to 0.133Pa, and another reason is that the composite gas with active substances diffused into the steel in a short time. The advantages of this structure are excellent heat shock resistance and cracking resistance. Therefore, the high temperature tempering hot work die, such as high speed steel mold, can obtain the comprehensive properties of high surface hardness, good abrasion resistance, good heat shock resistance, cracking resistance and toughness, but only when the diffusion layer is organized, the mold’s anti occlusive property, melting resistance and melting loss can not be good enough. As the service conditions and performance requirements of moulds or mechanical parts are different, it is necessary to adjust the microstructure and properties of the surface layer when surface heat treatment is carried out. Vacuum nitriding is applied to moulds and dies. It has obvious effect on improving the performance of precision gears and mechanical parts and springs such as wear-resistant and corrosion resistant materials.
Large workpiece with complex shape can easily produce deformation or even crack from rapid cooling at high temperature. Salt bath isothermal quenching has been used in the past. Is it possible for gas cooled isothermal quenching in a single chamber vacuum high-pressure gas quenching furnace? In a single chamber high pressure gas quenching furnace with convective heating function, the quenching results of two sets of carbon structural steel with 320mm 120mm 120mm stacking were quenched with different cooling methods.A group is heated at 1020 C, and is continuously cooled by high purity nitrogen at 600kPa pressure (the wind direction is alternating between the upper and the lower, and the 40s switches once). The other group is controlled cooling of the surface and core of the sample at 370 C. From the comparison of the two sets of curves, it can be seen that the temperature of the heart is only about 2min at 500 degrees (half cooling time). From the surface control cooling to the temperature of the heart reaches 370 degrees, it needs 27min. Therefore, it is feasible to conduct isothermal gas cooling quenching in a single chamber vacuum high pressure gas quenching furnace.
At present, some heat treatment is also inseparable from the cleaning and drying process, especially the various kinds of heat treatment of oil cooling. The task of cleaning and drying is heavier and more difficult. The most effective cleaning agent in the world is halogen cleaning agent. The proportion of halogenated cleaning agents used in developed countries, such as Japan, is shown in Table 1. Among them, three chloroethane and Freon are prohibited from using ozone depleting substances. Other halogen compounds are also restricted because of their harmful effects on the environment, people and animals. So various countries are studying various alternative cleaning and drying technologies.
Technological development of vacuum heat treatment
The development of heat treatment is accompanied by the development of the machinery manufacturing industry. Mechanical manufacturing has put forward higher and higher requirements for heat treatment. The heat treatment of mould is the most technical part in heat treatment.
As we all know, mold heat treatment is to play the potential of mold materials, improve the performance of molds. The performance of the mold must be satisfied: high strength (including high temperature strength, cold and hot fatigue resistance), high hardness (wear resistance) and high toughness, and also requires good machinability, including good polishing, weldability and corrosion resistance.
The most important influence on die life is mold design (including correct material selection), material of mould, heat treatment of mould, use and maintenance of mould. If the design of the mould is reasonable and the material is of high quality, the quality of the heat treatment directly determines the service life of the die. At present, more advanced heat treatment methods are used at home and abroad to improve the mold’s performance and prolong the service life of dies. Vacuum heat treatment is one of the most advanced methods in die heat treatment. Therefore, from the mold heat treatment, the state of heat treatment equipment, the process of heat treatment and the control of production process are 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 for vacuum heat treatment. The vacuum furnace has the effect of no decarbonization and no oxidation. It has uniform temperature, controllable heating and cooling speed, and can realize different process. Vacuum furnace is internationally recognized as “green heat treatment” because it has no pollution. At present, there is 2-20bar vacuum high pressure gas quenching furnace in the world, which can fully meet the requirements of vacuum heat treatment for moulds.
The process parameters used in mold heat treatment also have a crucial influence on the die performance. It includes heating temperature, heating speed, holding time, cooling mode and cooling speed. The correct heat treatment parameters can ensure the best performance of the mold, otherwise, it will produce bad or even serious consequences. Practice shows that proper heat treatment process can obtain excellent microstructure and excellent microstructure to ensure excellent mechanical properties. The appropriate 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 during the heating and cooling process is the main factor causing the deformation of the die. (vacuum furnace has the ability to control heating speed and cooling speed). Different processing methods can make the die meet different requirements and different performance requirements.
All in all, the vacuum high-pressure gas quenching process has the advantages of free heating and cooling control, and can produce different technological parameters and get the desired metallographic structure and properties.
- (1) the phase transformation characteristics of metals in vacuum. The thermodynamics and kinetics of solid-state phase transition do not change when vacuum is only in the range of 0.1MPa with atmospheric pressure. When formulating the vacuum heat treatment process rules, the principle of solid phase transformation can be completely based on atmospheric pressure. It is entirely possible to refer to data of various types of tissue transformation under atmospheric pressure.
- (2) vacuum degassing, improving physical and mechanical properties of metal materials.
- (3) vacuum degreasing.
- (4) evaporation of metals: heating in vacuum and evaporation of elements on the surface of the workpiece.
- (5) surface purification to achieve less oxidation and less no decarbonization heating.
- (6) the vacuum degree required by metal to achieve no oxidation heating.
- ( 1) the superiority of vacuum heat treatment. Vacuum heat treatment is a widely applied and non oxidized heat treatment technology, which is closely related to controlled atmosphere. It is also one of the main signs of the advanced degree of heat treatment technology. The vacuum heat treatment can not only achieve no oxidation and no decarburization of steel parts, but also realize the pollution-free and less distortion of the workpiece, so it also belongs to the category of clean and precision production technology. At present, it has become an irreplaceable advanced technology in tooling 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 workpiece vacuum heat treatment is only 1/3 of the heating quenching of salt bath. It is of great significance to popularize the vacuum heat treatment technology to study the vacuum heating method of various materials and different complicated parts and the distortion rules under various cooling conditions.The uniformity of air flow in vacuum heating, atmospheric pressure or high pressure gas quenching has a great influence on the hardened effect and quality dispersion of parts. It is of great significance to improve the furnace structure by computer simulation. Vacuum carburizing is the most possible way to achieve high temperature carburizing. However, the long time heating at high temperature will make the austenite grain size of most steels grow very large. It is necessary to study the effect of high temperature carburizing on steel and reheating quenching on the properties of materials and workpieces. It is necessary to optimize vacuum carburizing, cooling, heating quenching process and equipment. In recent years, there are international developments in the development of combustion type vacuum furnaces using gaseous fuels. It is difficult to use gas fuel to heat in vacuum furnace. Although there is energy conservation theory, it is not necessarily an important development direction.
- (3) vacuum heat treatment furnace. The modern vacuum heat treatment furnace is a cold wall furnace that can be used for vacuum heating of components and then quenched in oil or quenched in atmospheric and pressurized gases. Research and development of this type of equipment is a comprehensive, interdisciplinary, and involves many fields of science and technology.
After vacuum heat treatment, the parts have small distortion and high quality, and the process itself is flexible and pollution-free. Therefore, vacuum heat treatment is not only a necessary means for heat treatment of some special alloys, but also used in heat treatment of general engineering steels, especially tools, molds and precise coupler. After vacuum heat treatment, the service life is much higher than that of general heat treatment. For example, the life of some dies after vacuum heat treatment is 40 to 400% higher than that of the original salt bath treatment, while the life of many tools can be increased by about 3~4 times. In addition, vacuum heating furnace can work at higher temperature, and the workpiece can keep clean surface, thus accelerating the adsorption and reaction process of chemical heat treatment. Therefore, certain chemical heat treatments, such as carburizing, nitriding, chromizing, boronizing, and Multicomponent Infiltration, can achieve faster and better results.
Vacuum heat treatment can achieve almost all heat treatment processes, such as quenching, annealing, tempering, carburizing, chromizing and nitriding. In quenching process, gas quenching, oil quenching, nitriding and water quenching can be realized. Compared with ordinary heat treatment, it has the following advantages.
1. No oxidation, no decarburization and no carburizing have good protective effect on the inside and surface of the workpiece.
Oxidation causes the metal surface to lose metallic luster, the surface roughness increases, and the accuracy decreases. And the oxide scale on the steel surface is often the cause of quenching soft spot and quenching cracking. Oxidation reduces the strength of steel and decreases other mechanical properties. Decarburization (see Fig. 1) refers to the phenomenon of decreasing carbon content on the surface when steel is heated.
Fig. 1 decarbonization
Under normal circumstances, oxidation and decarburization of steel at the same time, decarburization layer due to oxidation, carbon content will significantly reduce the hardness, wear resistance and fatigue properties of steel, high speed steel decarburization will reduce red hardness (as shown in Figure 2).
Fig. 2 Decarburization of high speed steel
The vacuum heat treatment, because the metal is heated under a certain vacuum degree, the workpiece avoids contacting with oxygen, the workpiece does not oxidize, no decarbonization, and can get bright surface and better heat treatment quality. At the same time, in the vacuum state, there will be no reduction reaction, and there will be no carburizing phenomenon. The advantages of vacuum heat treatment can be more clearly seen after adding protectant to the surface of the workpiece and comparing with the gold phase diagram during vacuum quenching. As shown in Figure 3 and Figure 4,
Figure 3. After 40Cr carburizing, quenching and tempering treatment, martensite 3 grade, retained austenite 2 grade, a small amount of granular carbide 1 grade, the surface and sub surface martensite organization thickness is different.
Fig. 4. 40Cr vacuum quenching microstructure is medium carbon martensite and a small amount of black strip tempered martensite. Uniform heating, no oxidation and decarbonization on the surface.
2, improve the overall mechanical properties, degassing and promote the purification of metal surface.
The vacuum has obvious degassing effect on liquid metals, and also has a good elimination effect on dissolved gases in solid metals. The most harmful gas in metals is hydrogen. When vacuum heating is applied, the hydrogen fast in metals and alloys can be reduced to the minimum, eliminating hydrogen embrittlement, thereby improving the plasticity, toughness and fatigue strength of materials, and improving the overall mechanical properties of workpieces. When the metal and alloy are heated in vacuum, if the vacuum is lower than the decomposition pressure of the corresponding oxide, the oxide will decompose, and the free oxygen formed will be discharged into the vacuum chamber immediately, so that the surface quality of the metal will be further improved, and even the surface will reach the activation state and play a role of purification.
3. Small workpiece deformation.
In general, the processed workpiece is heated by the heat radiation inside the furnace, and the internal and external temperature difference is small and the thermal stress is small. As a result, the deformation of the parts processed by the vacuum heat treatment process is small, at the same time, it is heated in vacuum and quenched in vacuum, so that the workpiece can be transported in the air in hot state. The salt bath treatment and the atmosphere protection treatment are heated in the oxygen environment, but are still quenched in the air or the quenching medium containing oxygen molecules, thus reducing the artificial processing deformation. For example, the wire rolling wheel made of Cr12MoV material is treated by vacuum heat treatment and salt bath respectively. The vacuum heat treatment is 70% smaller than the salt bath treatment, and the qualified rate of the product is high.
4, reduce the volatile gold content of workpieces.
The vacuum heat treatment can be heated and vacuum quenched under controlled atmosphere, thereby reducing the volatilization of the gold containing elements and ensuring the heat treatment quality of the workpiece.
5. The price of vacuum heat treatment is high.
Because the equipment is expensive, and the energy consumption is much; the weight of each furnace is small because the vacuum chamber is small and easy to vacuum.
6. The workpiece has strong oxidation resistance and corrosion resistance.
The workpiece after vacuum heat treatment is more resistant to oxidation and corrosion resistance than ordinary heat treatment, and the microstructure is more compact.
The vacuum heat treatment furnace has the advantages of high thermal efficiency, rapid heating and cooling, good stability and repeatability. The working environment is good, the operation is safe, and there is no pollution and public hazard.
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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