In recent years, with the vacuum heat treatment furnace level improving, the vacuum heat treatment won the favorite of some professional people because it has a series of advantages such as degassing, degreasing, anaerobic and automation and so on , but the vacuum heat treatment furnace has a high standard to electric heating element, the deformation, fracture and volatilization of high temperature electric heating element are the important factors of restricting the development of vacuum furnace. In order to solve this problem, we look to the "black gold" graphite. Graphite has the spotless advantages compared with other metal material ,so the graphite, as the heating element, almost can be used in different types of vacuum heat treatment furnace.
The advantages of graphite as the electric heating elements in vacuum furnace:
High temperature resistance: the melting point of graphite is 3850±50℃, the boiling point is 4250℃, even it is fired in the ultra high temperature by electric arc, its weight loss is very small, the thermal expansion coefficient is very small, the strength of graphite will enhance with temperature increased, in 2000℃, the graphite strength improve a times.
The conductive, thermal conductivity: the conductivity of graphite is 100 times higher than that of general non metallic minerals. Its thermal conductivity is also higher than steel, iron, aluminum and other metal materials. The thermal conductivity decreases with the increase of temperature, even in skyscraping temperature, graphite become an insulator. Graphite can conduct electricity because each carbon atom in graphite will only form 3 covalent bonds with other carbon atoms , each carbon atom still retained 1 free electrons to transfer the charge.
The lubrication performance:the lubrication performance of graphite depends on the size of graphite flake, the bigger the scale, the smaller the coefficient of friction, the better the performance of the lubricant.
The chemical stability: the graphite has good chemical stability, acid resistance , corrosion resistance and solvent resistance at room temperature.
The plasticity: the toughness of graphite is good, it can be rolled into thin sheet.
The thermal shock resistance: the graphite can’t be damaged and withstand the severe changes in temperature when we use it at room temperature. the volume of graphite change is not large and has no cracks when the temperature change suddenly.
In the vacuum furnace design and processing, we need to consider the resistance of the electric heating element with a smaller change in temperature, the resistivity is stable, so the graphite is used as the preferred material.