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Vacuum Induction Melting
Vacuum casting (vacuum induction melting – VIM) was developed for processing of specialized and exotic alloys, and it is consequently becoming more commonplace as these advanced materials are increasingly employed. VIM was developed to melt and cast superalloys and high-strength steels, many of which require vacuum processing because they contain refractory and reactive elements such as Ti, Nb and Al. It can also be used for stainless steels and other metals when a high-quality initial melt is desired.

As the name suggests, the process involves melting of a metal under vacuum conditions. Electromagnetic induction is used as the energy source for melting the metal. Induction melting works by inducing electrical eddy currents in the metal. The source is the induction coil, which carries an alternating current. The eddy currents heat and eventually melt the charge.

The furnace consists of an airtight, water-cooled steel jacket capable of withstanding the required vacuum for processing. The metal is melted in a crucible housed in a water-cooled induction coil, and the furnace is typically lined with suitable refractories.

Metals and alloys that have a high affinity for gases – in particular nitrogen and oxygen – are often melted/refined in vacuum induction furnaces to prevent contamination/reaction with these gases. The process is therefore generally used for processing of high-purity materials or materials with tight tolerances on chemical composition.

Q: Why is vacuum induction melting used?

A: Vacuum induction melting was originally developed for processing of specialised and exotic alloys and is consequently becoming more commonplace as these advanced materials are increasingly employed. While it was developed for materials such as superalloys, it can also be used for stainless steels and other metals.
How does a vacuum induction furnace work?
Material is charged into the induction furnace under vacuum and power is applied to melt the charge. Additional charges are made to bring the liquid metal volume to the desired melt capacity. The molten metal is refined under vacuum and the chemistry adjusted until the precise melt chemistry is achieved.
What happens to metal in a vacuum?
In particular, most metals form an oxide layer on any surface that is exposed to air. This acts as a shield to prevent bonding. In the vacuum of space, there is no air so metals wouldn't form the protective layer.

Advantages of VIM Melting
Depending on the product and metallurgical process, vacuum levels during the refining phase are in a range of 10-1 to 10-4 mbar. Some of the metallurgical advantages of vacuum processing are:
Melting under oxygen-free atmosphere limits formation of non-metallic oxide inclusions and prevents oxidation of reactive elements
Achievement of very close compositional tolerances and gas contents
Removal of undesired trace elements with high vapor pressures
Removal of dissolved gases – oxygen, hydrogen, nitrogen
Adjustment of precise and homogeneous alloy composition and melt temperature
Melting in a vacuum eliminates the need for a protective slag cover and decreases the potential of accidental slag contamination or inclusions in the ingot
For this reason, metallurgical operations such as dephosphorization and desulphurization are limited. VIM metallurgy is primarily aimed at the pressure-dependent reactions, such as reactions of carbon, oxygen, nitrogen and hydrogen. The removal of harmful, volatile trace elements, such as antimony, tellurium, selenium and bismuth, in vacuum induction furnaces is of considerable practical importance.

Exact monitoring of the pressure-dependent reaction of excess carbon to complete the deoxidation is just one example of process versatility using the VIM process for production of superalloys. Materials other than superalloys are decarburized, desulfurized or selectively distilled in vacuum induction furnaces in order to meet specifications and guarantee material properties. Because of the high vapor pressure of most of the undesirable trace elements, they can be reduced to very low levels by distillation during vacuum induction melting, particularly for alloys with extremely high strengths at higher operating temperatures. For various alloys that must meet the highest quality requirements, the vacuum induction furnace is the most suitable melting system.

The following methods can be easily combined with the VIM system to produce clean melts:
Atmosphere control with low leak and desorption rates
Selection of a more stable refractory material for crucible lining
Stirring and homogenization by electromagnetic stirring or purging gas
Exact temperature control to minimize crucible reactions with the melt
Suitable deslagging and filtering techniques during the casting process
Application of a suitable launder and tundish technique for better oxide removal.