Heat treatment in vacuum furnaces is characterized by special conditions with regard to the design of the furnaces as well as the control of temperature and vacuum level during the heat treatment. The design of the furnaces generally depends on the size of the load, the pressure and temperature to be attained and the medium to be used in cooling the load.
The main parts of a vacuum furnace include: vessel, pumping system, hot zone and cooling system.
Vacuum furnace vessels can be grouped into so-called hot-wall and cold-wall designs. A typical hot-wall furnace has a retort that is commonly metallic or ceramic, depending on the temperature. The heating system is usually located outside of the retort and consists of resistive heating elements or an induction coil. Limitations of this retort-type furnace are the restricted dimensions of the heating zones and the restricted temperature range of the metallic retort, usually limited to 2000°F (1100°C) maximum. With cold-wall furnaces, the vacuum vessel is cooled with a cooling medium (usually water) and is kept near ambient temperature during high-temperature operations.
In comparison to the hot-wall furnace, the features of the cold-wall furnace are:
- Higher operating temperature ranges with 2400°F (1315°C) standard and 3000°C (1650°C) or higher practical
- Lower heat losses and less heat load released to the surroundings
- Faster heating and cooling performance
- Greater temperature-uniformity control
The construction of the pumping system depends on the following factors:
- Volume of the vessel
- Surface area of the vessel and the type of furnace internals
- Outgassing of the workload and related fixturing
- Time required for evacuation down to the final pressure