Vacuum melting, casting and re-melting equipment have been implemented in huge numbers over the recent years mainly with an intention to try and eradicate impurities from the process wherever possible.
Vacuum induction melting (VIM) has some specific advantages including, gas elimination, chemical composition control, process control and more.
In recent years the world of metallurgy has seen a massive growth in installations of new melting, re-melting and casting equipment under vacuum. This development is driven by various factors, but mainly by the increasing demand from the aerospace and power turbine industries, which pursue the simple philosophy: “Impurities that are not generated do not have to be removed.”
This means, especially for materials which are used in rotating parts under high thermal stress, that cleanliness is very important and influences the lifetime of such parts. For example, low cycle fatigue (LCF) properties of turbine disks can be directly related to both non-metallic inclusion content and inclusion size of the material. In aircraft and land based gas turbines, most parts and components (eg, turbine blades and vanes, turbine disks, cases, shafts, bolts and combustors) that undergo high thermal stress during operation, are made of superalloys with different amounts of alloying elements.
Most of these alloying elements have a high affinity for oxygen, nitrogen and hydrogen, therefore, during melting of such alloys under air, formation of oxides and/or nitrides will occur. These oxides have a dramatic influence on mechanical properties of the materials. To minimize or avoid the formation of inclusions, it is therefore necessary to protect the melt from contact with air.
Vaccum Induction Melting (VIM) is the melting of metals by induction done under a vacuum. This process first became important in the1950s; As a result of VIM, the jet engine made the great advance in performance and durability which has been so important to both military and commercial aviation.
The specific advantages of vacuum induction melting include:
- Elimination of gases - under the very low pressures obtained, .000001 atmosphere, undesirable gases and potentially harmful volatile elements are eliminated from the charged raw materials as melting occurs
- Close control of chemical analysis - exceptional and reproducible control of reactive element containing compositions is possible because of the lack of atmosphere
- Superior process control – independent control of pressure, temperature and inductive stirring provides an exceptional opportunity for developing melt practices specifically tailored to alloy composition and desired properties
- Slag free melting - 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
- Melt protection - high vacuum prevents deleterious contaminating reactions with atmospheric gases
What is Vacuum Induction Melting Used For?
Some applications of vacuum induction melting are:
- Refining of high purity metal and alloys
- Electrodes for remelting
- Master alloy stick for processes such as investment casting
- Casting of aircraft engine components
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Figure 1: Processing routes for superalloys and high quality steels