Vacuum heat treatment is a comprehensive technology combining vacuum technology and heat treatment, which means that all or part of the heat treatment process is carried out in a vacuum state. The vacuum heat treatment furnace has high thermal efficiency, can achieve rapid temperature rise and drop, 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 bright and clean surface.
The development of space technology, aircraft manufacturing and electronic technology has promoted the rapid development of vacuum heat treatment technology. Vacuum heat treatment technology is not only applied in the above-mentioned cutting-edge scientific and technological industrial departments, but also popularized in all departments of the machinery industry. This is because the workpiece subjected to vacuum heat treatment has the advantages of no oxidation, no decarburization, no carburization, bright surface, reduction and omission of grinding, good wear resistance and long service life. It can also degrease and degas the workpiece.
(1) Water cooling device, furnace shell, furnace cover, electrothermal element guide treatment (water cooling electrode), intermediate vacuum insulation door and other components of vacuum heat treatment furnace work in vacuum and heated state. When working under such extremely unfavorable conditions, it is necessary to ensure that the structure of each component is not deformed and damaged, and that the vacuum sealing ring is not overheated and burned. Therefore, each component should be equipped with water cooling device according to different conditions to ensure that the vacuum heat treatment furnace can operate normally and has sufficient service life.
(2) Low voltage and high current: in the vacuum container, when the vacuum emptiness is within the range of several torr to lxlo-1 Torr, the electrified conductor in the vacuum container will produce glow discharge under high voltage. In the vacuum heat treatment furnace, serious arc discharge will occur, which will burn the electric heating elements and insulation layers, resulting in major accidents and losses. Therefore, the working voltage of the electric heating element of the vacuum heat treatment furnace is generally not more than 80-100 volts. At the same time, effective measures should be taken in the structural design of electric heating elements, such as avoiding pointed parts as far as possible, and the spacing between electrodes should not be too narrow to prevent glow discharge or arc discharge.
(3) Most of the heating and heat insulation materials can only be used in the vacuum state: the heating and heat insulation linings of the vacuum heat treatment furnace work in the vacuum and high temperature, so the requirements of high temperature resistance, low steam pressure, good radiation effect and small thermal conductivity are put forward for these materials. The requirements for oxidation resistance are not high. Therefore, tantalum, tungsten, molybdenum and graphite are widely used as heating and insulation materials in vacuum heat treatment furnaces. These materials are easily oxidized in the atmosphere, so these heating and insulation materials cannot be used in conventional heat treatment furnaces.
(4) Strict vacuum sealing: it is well known that the vacuum heat treatment of metal parts is carried out in a closed vacuum furnace. Therefore, it is of great significance to obtain and maintain the original air leakage rate of the furnace and ensure the working vacuum of the vacuum furnace to ensure the quality of vacuum heat treatment of parts. 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 as few or no holes as possible, and the dynamic sealing structure should be less or avoided to minimize the chance of vacuum leakage. Components and accessories installed on the vacuum furnace body, such as water-cooled electrodes and thermocouple lead-out devices, must also be designed with sealing structures.