Vacuum carburizing in one article
I. Low-pressure vacuum carburizing equipment
Vacuum carburizing is also a kind of carburizing, but the performance of the equipment is different, of course, the process is also very different. Vacuum carburizing is divided into vacuum carburizing oil quenching furnace and vacuum carburizing gas quenching furnace according to different quenching methods. From the appearance of the vacuum carburizing oil quenching furnace and vacuum oil quenching furnace similar, vacuum carburizing gas quenching furnace and vacuum high-pressure gas quenching furnace similar. After the addition of carburizing function, more gas supply system, gas flow control system, carburizing pressure control system and changes to the heating system, these system additions and changes, most of them are inside the equipment, so the appearance of the vacuum carburizing furnace and vacuum furnace is not very different.
The specifications of vacuum carburizing furnace are 644, 755, 966, 1077, 1288, and larger standard models such as 1500*1500*800mm, corresponding to the loading capacity of 150 kg, 300 kg, 500 kg, 750 kg, 1000 kg, 4000 kg. basically based on the double chamber oil quenching furnace can be used as the production platform of vacuum carburizing furnace Basically, the double chamber oil quenching furnace can be used as a platform for vacuum carburizing furnace.
Low-pressure vacuum carburizing is gas carburizing under less than one atmosphere pressure. The pressure of carburizing is generally 200-2000pa, and the most practical application is 200-500pa. Low-pressure vacuum carburizing adopts the pulsed process mode, so the carburizing medium is high-purity acetylene, and the diffusion medium is high-purity nitrogen.
Second, the advantages and disadvantages of low-pressure vacuum carburizing
Low-pressure vacuum carburizing parts have the general advantages of vacuum heat treatment, compared with ordinary carburizing parts have more of the following advantages.
1. Good surface quality: vacuum carburizing surface does not oxidize, do not decarburize, can maintain the original color of the metal; does not produce internal oxygen (black tissue), which helps to improve the fatigue strength of the parts; can greatly improve the reliability and service life of the product. Vacuum carburizing, there is no contact with oxygen, so the defects generated by oxygen are all avoided in vacuum carburizing.
2. Can deal with the complex shape of the parts, the workpiece deformation is small: vacuum carburizing workpiece heating, heating speed continuous control, can reduce the internal and external temperature difference of the workpiece, deformation is small; carburizing is completed, quenching method for vacuum quenching, significantly reduce the quenching deformation of the workpiece; reduce the amount of post-processing, save processing costs.
Appropriately slow down the heating speed, can effectively reduce the workpiece deformation. The heating speed of vacuum carburizing furnace can be controlled, and the heating speed can be adjusted according to the complexity of the workpiece. 3.
3. More uniform carburizing layer depth: Carburizing gas is introduced only after the workpiece is heated to a uniform temperature, which ensures the synchronization of the starting carburizing point of large and small workpieces, which is the basis of uniform carburizing layer. This is the basis of uniform carburizing layer. It is difficult to ensure this in conventional gas carburizing and multi-purpose furnace. The vacuum has a purifying effect on the surface of the workpiece, which is conducive to the adsorption of carbon atoms by the workpiece.
Conventional carburizing and multi-furnace carburizing, when exhausting, there is no obvious boundary between catching gas and carbon potential establishment, small parts arrive at temperature first and start carburizing first, and the starting point of carburizing is different for large and small parts. The carburizing starting point of low-pressure vacuum carburizing is the same, first heated to temperature, all workpieces to temperature and uniform temperature, and then start to pass acetylene carburizing, so the uniformity of carburizing layer of large and small carburized parts is the same.
4. Surface carbon content is easy to control: vacuum carburizing surface carbon content does not need to be controlled by carbon potential, and the precise control of surface carbon content can be achieved by controlling carburizing pressure and carburizing gas flow. The principle of vacuum carburizing has been different from traditional gas carburizing, and there is no concept of carbon potential.
5. Large range of carburizing temperature: from low temperature carburizing to the highest carburizing temperature can reach 1050℃, which can greatly save the process time for deep carburizing. It is more conducive to complete the carburizing process of special steel grades. Low-pressure vacuum carburizing of the same material in the range of 880-1000℃, with the increase of carburizing temperature, the carburizing speed increases continuously. 980℃ carburizing speed can reach twice of 920℃. Vacuum high temperature carburizing can carburize special materials, such as martensitic stainless steel, ferritic stainless steel, and H13, Cr12MoV, etc. For these materials, it is another type of carburizing, i.e. carbide precipitation type carburizing
6. Stable carburizing quality: After the process parameters are set, the whole carburizing process is controlled by microcomputer and the process parameters are recorded. The control system can precisely control the carburizing process, monitor and record the operation condition of the equipment, reduce the unfavorable factors in the process, make the heat treatment workpiece have good repeatability and stable quality.
7. Wide range of application: vacuum carburizing can achieve good carburizing layer for parts with blind holes, deep holes and slits or parts with poor or even difficult carburizing effect of ordinary gas carburizing such as stainless steel.
8. Safety and environmental protection: low-pressure vacuum carburizing equipment with ordinary vacuum equipment as the platform, has all the advantages of existing vacuum heat treatment equipment for environmental protection, the production process without oil and smoke, no open flame, safe, environmental protection, pollution-free, clean working environment.
9. High production efficiency: low-pressure vacuum carburizing realizes high temperature and high-speed carburizing, which makes the production cycle shorten significantly and saves time cost effectively.
The advantages of low-pressure vacuum carburizing are obvious, but the disadvantages, there must be, the first purchase cost of equipment is relatively high.
Third, the process of vacuum carburizing.
1, carburizing parts cleaning is mainly to prevent contamination of vacuum quenching oil and vacuum pump. If only oil, can not be cleaned. Can not have dust, debris, cutting fluid and other impurities.
2, on the tooling. Select the right tooling, using the appropriate way to load the furnace, can effectively reduce the deformation of the workpiece, improve the quenching quality of the parts. Avoid uneven local quenching due to the tooling.
3, furnace loading. Workpiece into the cold room, cold room vacuum, open the insulation door, the workpiece transferred to the carburizing hot room (vacuum carburizing gas quenching furnace without transfer), ready to heat.
4、Heating. Develop the process based on the amount of furnace and the shape of the workpiece to select the appropriate heating rate, as far as possible to use segmental heating, insulation, so that all workpieces uniform temperature.
5 Carburizing. Vacuum carburizing is the use of pulse carburizing. For example, first carburize for three minutes, then diffuse for 8 minutes, then carburize for three minutes, diffuse for 8 minutes, and so on. The whole process is composed of several segments. The number of segments, carburizing temperature, and time determine the depth of the carburizing layer.
6, cooling and holding quenching
After the process quenching operation and ordinary vacuum equipment operation in the same way. The need for secondary quenching, the use of cooling insulation normalizing, followed by high-temperature tempering, and then heating quenching. Vacuum carburizing oil quenching furnace, cold chamber with oil quenching and air cooling function. Gas cooling pressure of 2 bar, the cooling rate is slightly greater than normalizing. Vacuum carburizing gas quenching furnace has all the functions of high pressure gas quenching furnace.
Four, vacuum carburizing process parameters setting
Low-pressure vacuum carburizing process parameters to be set are process mode, temperature rise rate, holding temperature, carburizing temperature, carburizing pressure, gas flow, gas pressure, quenching temperature, quenching mode, quenching time and many other data.
The carburizing temperature is determined by the material, mainly to avoid overheating.
The gas flow rate is determined by the surface area of the loaded workpiece
The larger the surface area is, the higher the gas flow rate should be.
Carburizing pressure is determined by the material and shape of the workpiece
The gas supply pressure is generally at 0.2mpa
Carburizing time is determined by carburizing temperature and depth of carburizing layer.
V. Carburizing quality inspection.
The quality of carburizing can be tested according to the quality of carburizing. Mainly surface hardness, heart hardness, hardness gradient, metallographic organization, oxidation and other standards.
Vacuum carburizing parts without cleaning oil will not affect the uniformity of carburizing. However, the bottom of the heating chamber will have oil. After the door of the heating chamber is opened, there will be oil flow. This is because the oil is heated and vaporized, diffused to the wall of the hot chamber, due to the cooling water in the sandwich, the oil and gas coalesced and flowed to the bottom of the hot chamber. It is better to clean if you have conditions.
Our company has accumulated many years of experience in vacuum carburizing, and based on our large amount of data accumulation, we established a mathematical model and finally developed a low-pressure vacuum carburizing simulation software.
Low-pressure vacuum carburizing uses acetylene as carburizing medium, and with proper gas flow and process control, there is basically no carbon black. Vacuum carburizing workpiece, all surfaces should be processed. The non-machined parts will affect the carburizing quality and have carbon black.
Acetylene does not crack immediately after it reaches the hot chamber, but only after it reaches the surface of the workpiece. The high temperature workpiece is the catalyst for acetylene cracking. Acetylene is very active, and then in a very short time, the surface of the workpiece can reach the maximum carbon content of the material austenite at the carburizing temperature, so the carburizing time is short and diffusion is carried out later. When the carbon content of the surface is reduced, acetylene carburizing is passed again. The whole carburizing diffusion process, are always pass gas, the vacuum pump are always pumping, through the pressure sensor, according to the gas flow, control the vacuum pump pumping speed, to achieve smooth pressure.
In vacuum carburizing, materials containing more strong carbide forming elements are carbide precipitation carburized.
Certain materials, such as 1Cr17 after vacuum carburizing, can reach 3-4% carbon content on the surface, and carbon and cr form a large number of fine diffuse distribution of carbide, and the hardness and wear resistance increase greatly. However, the corrosion resistance of the material is much reduced.