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Vacuum Heat Treatment Furnace of Metal Parts

Metal heat treatment process for heating metal workpiece under 1 atmosphere (i.e. negative pressure). At the end of 1920s, with the development of electric vacuum technology, vacuum heat treatment technology appeared, which was only used for annealing and degassing. Due to the limitation of equipment, this process has not made great progress for a long time. In the 1960s and 1970s, gas-cooled vacuum heat treatment furnaces, such as cold-wall vacuum oil quenching furnaces and vacuum-heated high-pressure gas quenching furnaces, were successively developed, which made the vacuum heat treatment process get new development. Technological advances in Carburizing in vacuum and carburizing, nitriding or other elements under vacuum plasma field have further expanded the application scope of vacuum heat treatment.


Characteristics: The heat treatment of metal parts in vacuum can prevent oxidation decarbonization and have degassing effect, but metal elements may evaporate.


Preventing oxidative decarbonization: The heating chamber of vacuum heat treatment furnace is close to the vacuum state when it works. There are only trace carbon monoxide and hydrogen. They are reductive to the heated metal and do not react with oxidative decarbonization. At the same time, they can reduce the formed oxide film, so the surface of the heated metal workpiece can keep the original metallic luster and good surface properties. 


Degassing effect: When metal parts are heated in vacuum, harmful gases in metal, such as hydrogen and oxygen in titanium alloy, will escape at high temperature, which is beneficial to improving the mechanical properties of metal.


Metal element evaporation: Each element has its own vapor pressure. If the pressure in the environment is lower than the vapor pressure of some element, the element will evaporate. In vacuum heat treatment, the vacuum or temperature should be chosen according to the vapor pressure of alloy elements in steel to avoid evaporation of alloy elements.


Process: Vacuum heat treatment furnace can be used for annealing, degassing, solid solution heat treatment, quenching, tempering and precipitation hardening. It can also be used in chemical heat treatment after proper medium is introduced.


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Vacuum annealing, degassing and solid solution treatment are mainly used for workpieces with high purity or surface quality requirements, such as softening and stress removal of refractory metals, solid solution treatment of stainless steel and nickel-based alloys, degassing treatment of titanium and titanium alloys, annealing of soft magnetic alloys to improve permeability and coercivity, and bright carbon steel, etc. Bright annealing of low alloy steels and copper. There are two kinds of quenching in vacuum: gas quenching and liquid quenching. Gas quenching is to cool the workpiece by filling high purity neutral gas (such as nitrogen) into the cooling chamber after vacuum heating. It is suitable for gas quenched materials with low critical cooling rate of martensite such as high speed steel and high carbon high chromium steel. Liquid quenching is to heat the workpiece in the heating chamber, then move it to the cooling chamber and fill it with high purity nitrogen gas and send it to the quenching tank immediately for rapid cooling. If high surface quality is required, tempering and precipitation hardening of workpiece after vacuum quenching and solution heat treatment should still be carried out in vacuum furnace.


Vacuum carburizing is to put the workpiece into the vacuum furnace, vacuum and heating, so that the furnace is purified, after carburizing temperature into hydrocarbons (such as propane) for carburizing, after a certain period of time to cut off the carburizing agent, and then vacuum diffusion. This method can realize high temperature carburizing (1040 C) and shorten carburizing time. There is no internal oxidation in the carburized layer, and there is no problem that the carbon content on the surface of the carburized layer is lower than that of the secondary layer. It can be vacuum carburized by pulse mode to obtain uniform carburized layer through blind holes and small holes. After vacuum heat treatment, the applied parts have small distortion, high quality, flexible operation and no pollution. Therefore, vacuum heat treatment is not only a necessary means for heat treatment of some special alloys, but also applied to heat treatment of general engineering steels. Especially, the service life of tools, dies and precision coupling parts after vacuum heat treatment is much longer than 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, and the service life of many tools can be increased by about 3-4 times. In addition, the vacuum heating furnace can work at a higher temperature, and the workpiece can maintain a clean surface, thus accelerating the adsorption and reaction process of chemical heat treatment. Therefore, some chemical heat treatments, such as carburizing, nitriding, chromizing, boronizing, and multi-element co-infiltration, can achieve faster and better results.


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