Heating element of vacuum resistance furnace

The heating element of a vacuum resistance furnace is the main component, which generally determines the working capacity and life of the furnace. The heating element converts the electrical energy introduced into the furnace into thermal energy to ensure that the furnace space reaches the predetermined temperature. The working conditions of the heating element in a vacuum resistance furnace are different from those of a non-vacuum resistance furnace in the following ways:

1 The form of heat transfer from the heating element to the charge;

2 The conditions for the interaction between the heating element and the atmosphere in the furnace;

3 The volatilization rate of the heating element material.

The heating element of a vacuum resistance furnace transfers heat to the charge only by radiation. Therefore, when all other conditions are the same, the temperature of the heating element in a vacuum resistance furnace is higher than that of a non-vacuum resistance furnace. However, the temperature difference is only significant in a low-temperature furnace, where the temperature of the heating element in a low-temperature furnace is much lower than the allowable limit temperature. Therefore, there is actually no effect on the service life of the heating element.

From the perspective of the interaction with the atmosphere in the furnace, the working conditions of the heating element in a vacuum resistance furnace are better than those in a non-vacuum resistance furnace, especially much better than those in an oxidizing atmosphere in the furnace. A heating element working in an oxidizing atmosphere should be resistant to oxidation, but there is no such requirement for the heating element in a vacuum resistance furnace.

The harmful effect of atmosphere on the heating element is not only due to the chemical reaction between the gas and the heating element, but also because some materials, especially rare metals, become brittle and quickly damaged due to the absorption of a large amount of gas when heated. These metals can work reliably for a long time under high vacuum.

Vacuum also has adverse effects on the work of the heating element. The problem is that as the temperature of the heating element increases, the vapor pressure also increases. At a certain temperature and vacuum degree, the volatilization rate of the heating element often reaches a large order of magnitude, and the service life becomes shorter, which is very uneconomical.

In low-temperature vacuum resistance furnaces, the commonly used heating element materials are mainly nickel-chromium alloys or iron-chromium-aluminum alloys. In high-temperature furnaces, molybdenum, tantalum, tungsten, graphite, etc. are mainly used. The structure of nickel-chromium alloy and iron-chromium-aluminum alloy heating elements is exactly the same as that in non-vacuum furnaces. The heating element is easy to weld and bend, so it is not difficult to manufacture and connect. The application range of nickel-chromium and iron-chromium-aluminum heating elements in vacuum resistance furnaces is below 1000℃.