Selection of electric heater for vacuum furnace

Electric heater is an important part of vacuum resistance furnace, which determines the working capacity and working period of vacuum furnace. Now let’s briefly introduce the electric heating elements of the vacuum furnace.
(1) The forms of heat transfer from the electric heating element to the workpiece are different. The electric heating element of the vacuum furnace transfers heat to the workpiece through radiation, so when other conditions are the same, the temperature of the vacuum furnace is higher than that of the non vacuum furnace. However, the temperature difference is significant only in the low-temperature furnace.
(2) The conditions of interaction between electric heating elements and furnace atmosphere are different. The working condition of electric heating elements in vacuum furnace is better than that in other states, especially in non vacuum furnace with oxidation atmosphere. Therefore, the requirements for oxidation resistance of electric heating elements in vacuum state are not high. In addition to the chemical reaction between the atmosphere in the furnace and the electric heating element when the electric heating element in the electric heater of non vacuum furnace works, some electric heating materials, especially rare metals, become brittle and easy to be damaged due to the absorption of a large amount of gas during heating, but these metals can work reliably for a long time under vacuum.
(3) The volatilization rate of electric heating element materials is different. Electric heating materials are easy to volatilize in the vacuum furnace. As the temperature of electric heating elements increases, its vapor pressure also increases, so its service life decreases. From this point of view, vacuum also brings adverse effects to electric heating elements.
For low and medium temperature vacuum furnaces (the service temperature is not higher than 1000 ℃), nickel chromium alloy and iron chromium aluminum alloy are generally used as electric heating elements; For high-temperature vacuum furnaces, especially those with a temperature greater than 1200 ℃, the electric heating elements used are pure metals such as aluminum, tungsten, tantalum and graphite. Non metallic electrothermal materials, such as silicon carbide, its binder will decompose under vacuum, and aluminum silicide will soften when it exceeds 1300 ℃, so these two electrothermal materials are rarely used in vacuum furnaces