A vacuum furnace is a type of furnace in which the product in the furnace is surrounded by a vacuum during processing. The absence of air or other gases prevents heat loss from the product through convection, and removes a source of contamination. This enables the furnace to heat materials (typically metals) to very high temperatures, and to carry out processes such as brazing, sintering and heat treatment with high consistency and low contamination.
Some of the benefits of a vacuum furnace are:
- Uniform temperatures in the range 1,100–1,500 °C (2,000–2,700 °F)
- Temperature can be controlled within a small area
- Low contamination of the product by carbon, oxygen and other gases
- Quick cooling (quenching) of product.
- The process can be computer controlled to ensure metallurgical repeatability.
Heating metals to high temperatures normally causes rapid oxidation, which is undesirable. A vacuum furnace removes the oxygen and prevents this from happening.
An inert gas, such as Argon, is typically used to quickly cool the treated metal back to non-metallurgical levels (below 400 °F [200 °C]) after the desired process in the furnace. This inert gas can be pressurized to two times atmosphere or more, then circulated through the hot zone area to pick up heat before passing through a heat exchanger to remove heat. This process continues until the desired temperature is reached.
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A common use of a vacuum furnace is for the heat treatment of steel alloys. Many general heat treating applications involve the hardening and tempering of a steel part to make it strong and tough through service. Hardening involves heating the steel to a pre-determined temperature, then cooling it rapidly.
Vacuum furnaces are ideal for brazing applications. Brazing is another heat-treating process used to join two or more base metal components by melting a thin layer of filler metal in the space between them.
A further application for vacuum furnaces is Vacuum Carburizing also known as Low Pressure Carburizing or LPC. In this process, a gas (such as acetylene) is introduced as a partial pressure into the hot zone at temperatures typically between 1,600 and 1,950 °F (870 and 1,070 °C). The gas disassociates into its constituent elements (in this case carbon and hydrogen). The carbon is then diffused into the surface area of the part. This function is typically repeated, varying the duration of gas input and diffusion time. Once the workload is properly "cased", quench is induced typically using oil or high pressure gas (HPGQ) typically, nitrogen or for faster quench helium.
This process is also known as case hardening.