The insulation layer of the WCZ-30 vacuum furnace is made of carbon fiber (graphite) felt with a thickness of 40 to 60 thick. The carbon fiber felt is resistant to high temperatures, light weight, good thermal insulation, and easy to vacuum; however, it is conductive and easily oxidized; it is widely used. In the vacuum furnace as a thermal insulation layer; Carbon fiber is of light weight; under the vacuum condition, some short-fine fibers will float; due to the electrostatic charge on the surface of the insulating porcelain tube (high-purity aluminum oxide) attracts floating fibers and adheres to the porcelain tube The surface also forms a fibrous membrane, causing a short circuit in the porcelain tube.
The problem of the adsorption of carbon fibers on the surface of insulating porcelain tubes has existed for a long time. However, in the past, the number of vacuum furnaces was small and the utilization rate was low. This problem was not prominent and caused no attention. According to data, some domestic and foreign manufacturers of vacuum furnaces apply fire-retardant paint on the surface of carbon fiber to separate the carbon fiber from the furnace and have better anti-flying effect. The carbon fiber (graphite) felt coating technology is protected by some vacuum furnace manufacturers abroad as a patented technology.
Part of the metal elements is heated and evaporated. Most of vacuum heating parts attached to the surface of the porcelain tube are alloy steel or high-alloy steel. Due to high heating temperature and long time, many alloy elements evaporate and some adhere to the surface of the porcelain tube to form metal. The layer reduces the resistance of the ceramic tube.
The higher the content of the low-melting metal element in the heated workpiece, the higher the possibility of evaporation and the evaporation rate; the higher the heating temperature, the easier the alloy element evaporates. When the vaporized metal element is cooled by the furnace, some of the vaporized metal elements are condensed on the surface of the ceramic tube to form a metal thin film. When the thickness of the metal thin film exceeds a critical thickness, the electrical resistivity of the thin film is greatly reduced. Once the insulation resistance is lower than the requirements of the operating procedures, the furnace must be overhauled to restore the resistance value to the factory level.
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