Vacuum induction melting
Vacuum induction melting (VIM) utilizes electric currents to melt metal within a vacuum. The first prototype was developed in 1920. One of the few ways to induce a current within a conductor is through electromagnetic induction. Electromagnetic induction induces eddy currents within conductors by changing the magnetic field. Eddy currents create heating effects to melt the metal. Vacuum induction melting has been used in both the aerospace and nuclear industries.
The process was invented in Hanau, Germany in 1917. Heraeus Vacuumschmelze and Dr. Wilhelm Rohn applied for a patent on vacuum melting on 12 January 1918 and were granted a German patent DE 345161. E.F. Northrup built the first prototype of a vacuum induction furnace in the United States of America in 1920. Medium frequency furnaces were seen soon afterwards in England and Sweden in 1927. The process was initially developed to refine certain special metals such as cobalt and nickel. As these metals and alloys became more common, the process of VIM became more widely used. VIM now helps to melt a variety of metals for aircraft and nuclear applications.
Vacuum induction melting uses currents within a vacuum to melt metal. VIM involves placing a core-less induction furnace into a vacuum chamber. The heat used to start the melting process comes from an induced current called an eddy current. The melting and casting operations are then carried out at low pressures to control the entire alloy chemistry process.
Eddy currents make the vacuum induction melting process possible. These eddy currents are induced currents in a conductor created by a changing magnetic field. The metals begin to melt after heat is created from the eddy currents. The process becomes simplified to a matter of changing the magnetic field within a conductor.
VIM was usually used for refining high purity metal and alloys. VIM is now becoming more popular due to the variety of uses now available. The commercial production of nickel titanium alloys utilizes VIM to its full extent. Other examples of using VIM include melting steels for nuclear applications, cobalt alloy for medical applications, and both high purity copper alloy and clean magnetic alloy for high permeability. The process of VIM can even begin electro-slag remelting, which can remelt electrodes.
VIM is the most widely used process for the production of nickel titanium alloys. When producing nickel titanium, the process of VIM using a hot graphite crucible sometimes causes carbon contamination which causes some of the titanium ions to form titanium carbide (TiC). These TiC particles change the transformation temperature along with the strength of the alloy. This is one drawback with the process of vacuum induction melting in graphite.
- Mühlbauer, Alfred (2008). History of Induction Heating and Melting. ISBN 978-3-8027-2946-1.
- "Vacuum Induction Melting and Casting". azom.com.
- Patent DE 345161, Vacuumschmelze and Dr. Wilhelm Rohn, Verfahren zum Vakuumschmelzen und Vergueten von Metallen und Legierungen, priority date 12 January 1918, published 12 June 1921.
- Nayan, Niraj; Govind; Saikrishna, C.N.; Ramaiah, K. Venkata; Bhaumik, S.K.; Nair, K. Suseelan; Mittal, M.C. (2007). "Vacuum induction melting of Ni Ti shape memory alloys in graphite crucible". Materials Science and Engineering: A. 465 (1–2): 44–48. doi:10.1016/j.msea.2007.04.039.