Introduction of vacuum heat treatment technology and vacuum furnace work

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Vacuum heat treatment is a comprehensive technology that combines vacuum technology and heat treatment. It means that all and part of the heat treatment process is carried out under vacuum. my country divides vacuum into low, medium, high and ultra-high vacuum. At present, the working vacuum degree of most vacuum heat treatment furnaces is 1.33~1.33×10ˉ3;Pa. Vacuum heat treatment can almost realize all heat treatment processes, such as quenching, annealing, tempering, carburizing and nitriding. In the quenching process, gas quenching, oil quenching, nitrate quenching, water quenching, etc. can be realized, and vacuum brazing, Sintering, surface treatment, etc. The vacuum heat treatment furnace has high thermal efficiency, can achieve rapid heating and cooling, can achieve no oxidation, no decarburization, no carburization, can remove phosphorus chips on the surface of the workpiece, and has the functions of degreasing and degassing, so as to achieve the effect of surface bright purification. Generally speaking, the processed workpiece is heated slowly in the furnace, the internal heat temperature difference is small, the thermal stress is small, and the deformation is small. The product qualification rate is high. It can reduce costs and has a degassing effect, thereby improving the mechanical performance and service life of the work. Good working environment, safe operation, no pollution and pollution. There is no danger of hydrogen embrittlement for the processed workpiece, and the surface hydrogen embrittlement of titanium and refractory metal shell is prevented, and the stability and repeatability of the vacuum heat treatment process are good. With this series of advantages, the development of vacuum heat treatment equipment and technology has been paid more and more attention and more and more widely used.

Generally, metal materials are heated in an air furnace. Due to the presence of oxygen, water vapor, carbon dioxide and other oxidizing gases in the air, the oxidation of these gases with metals can easily cause oxide films or scales on the surface of the heated metal, which completely loses the original Metallic luster. At the same time, these gases also react with the carbon in the metal to decarburize the surface. If the furnace contains carbon monoxide or methane gas, it will also increase the carbon on the metal surface. For the very active chemical properties of Ti, Zr and the insoluble metals W, Mo, Nb, Ta, etc., heating in an air furnace will not only generate oxides, hydrides, and nitrides, but also absorb these gases and feed them into the metal. Diffusion seriously deteriorates the performance of metal materials. These ills such as oxidation, decarburization, recarburization, gas absorption and even corrosion are sometimes unavoidable when heated in a controlled atmosphere furnace or salt bath furnace. In order to solve this problem, the usual method is to leave a machining allowance before the heat treatment of the workpiece, and then process to remove the oxidation and decarburization layer after the heat treatment.

Safe operation

1. Vacuum heat treatment furnace is an advanced heat treatment equipment that has been greatly developed in recent years. The workpiece is heated and cooled in an ultra-low pressure space. It has the advantages of good quality, energy saving, safety and less pollution.

2. The heating element of the vacuum furnace uses low-voltage electricity, but the power supply voltage is still 380V, and safe electricity is still needed during operation.

3. When the vacuum furnace is manufactured, it should be ensured that it does not leak air or water. The vacuum furnace body and lid are sealed by rubber parts, so a water barrier is required for cooling. If water leaks into the furnace, the temperature of the furnace is very high and it will cause an explosion. The workpieces processed by the vacuum furnace should be cleaned of oil stains on the surface while avoiding the oil vapor from the quenching oil tank from entering the furnace. The mixture of oil vapor and air will form an explosive atmosphere. Once there is an open flame or electricity, it will explode. Therefore, the furnace door should be closed after charging the vacuum furnace, and the furnace can be energized and heated after the furnace chamber is evacuated.

Work characteristics

(1) Water-cooling device, vacuum furnace shell, furnace cover, electric heating element conduction treatment (water-cooled electrode), intermediate vacuum insulation door and other components, all work under vacuum and heating. Working under such extremely unfavorable conditions, it is necessary to ensure that the structure of each component is not deformed or damaged, and the vacuum sealing ring is not overheated or burned. Therefore, each component should be equipped with water cooling devices according to different conditions to ensure that the vacuum heat treatment furnace can operate normally and have sufficient service life.

(2) Using low voltage and high current: In the vacuum container, when the vacuum degree is in the range of a few Torr to 1×10-1 Torr, the energized conductor in the vacuum container will produce glow discharge under higher voltage. In the vacuum heat treatment furnace, severe arc discharge will be generated, which will burn down the electric heating element and heat insulation layer, causing major accidents and losses. Therefore, the working voltage of the electric heating element of the vacuum heat treatment furnace generally does not exceed 80-100 volts. At the same time, effective measures should be taken in the structural design of the electric heating element, such as avoiding sharp parts as much as possible, and the distance between the electrodes should not be too small to prevent the occurrence of glow discharge or arc discharge.

(3) Most heating and heat insulation materials can only be used in a vacuum state: the heating and heat insulation linings of the vacuum heat treatment furnace work under vacuum and high temperature, so these materials must be high temperature resistant, low vapor pressure, and good radiation effect , The thermal conductivity is small. The anti-oxidation performance is not high. Therefore, the vacuum heat treatment furnace widely uses tantalum, tungsten, molybdenum and graphite as heating and heat insulation materials. These materials are very easy to oxidize in the atmosphere, therefore, conventional heat treatment furnaces cannot use these heating and heat insulating materials.

(4) Strict vacuum sealing: The vacuum heat treatment of metal parts is carried out in a closed vacuum furnace. Therefore, the original air leakage rate of the furnace can be obtained and maintained to ensure the working vacuum of the vacuum furnace and ensure the quality of vacuum heat treatment of parts Has a very important meaning. Therefore, a key problem of the vacuum heat treatment furnace is to have a reliable vacuum sealing structure. In order to ensure the vacuum performance of the vacuum furnace, a basic principle must be followed in the structural design of the vacuum heat treatment furnace, that is, the furnace body should be airtight welded, and at the same time, the furnace body should be opened as little as possible or no holes, and used less or avoided. Sealed structure to minimize the chance of vacuum leakage. The parts and accessories installed on the vacuum furnace body, such as water-cooled electrodes and thermocouple lead-out devices, must also be designed with a sealed structure.

(5) High degree of automation: The reason why the degree of automation of the vacuum heat treatment furnace is higher is that the heating and cooling of metal workpieces require dozens or even dozens of actions to complete. These actions are carried out in a vacuum heat treatment furnace and cannot be accessed by operators. At the same time, some actions such as the completion of heating and holding, the quenching process of the metal workpiece requires six actions and must be completed within 15 seconds. To complete many actions under such rapid conditions, it is easy to cause operator tension and constitute misoperation. Therefore, only a high degree of automation can accurately and timely coordinate actions according to procedures.

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