ASM Handbook, Volume 15: Casting Copyright © 2008 ASM International® Volume 15 Handbook Committee, p 1-8 All rights reserved. DOI: 10.1361/asmhba0005200 www.asminternational.org Vacuum Induction Melting MELTING UNDER VACUUM in an induc- the melting process. Accordingly, the vacuum- The charge generally consists of three tion-heated crucible is a tried and tested process melted superalloys (compared to EAF/AOD- portions: in the production of liquid metal. It has its origins melted alloys) are improved in fatigue and in the middle of the 19th century, but the actual stress-rupture properties. A virgin portion, which consists of material technical breakthrough occurred in the second Control of alloying elements also may be that has never been vacuum melted half of the 20th century. Commercial vacuum achieved to much tighter levels than in EAF/ A refractory portion, which consists of those induction melting (VIM) was developed in the AOD products. However, problems can arise virgin elements that are strong oxide formers early 1950s, having been stimulated by the need in the case of alloying elements with high vapor and have the tendency to increase the to produce superalloys containing reactive pressures, such as manganese. Vacuum melting elements within an evacuated atmosphere. The also is more costly than EAF/AOD melting. Crucible process is relatively flexible, featuring the inde- The EAF/AOD process allows compositional Power pendent control of time, temperature, pressure, modification (reduction of carbon, titanium, supply and mass transport through melt stirring. As such, sulfur, silicon, aluminum, etc.). In vacuum VIM offers more control over alloy composition melting, the charge remains very close in com- and homogeneity than other vacuum melting position to the nominal chemistry of the initial processes. charge made to the furnace. Minor reductions Vacuum induction melting can be used to in carbon content may occur, and most VIM advantage in many applications, particularly in operations now include a deliberate desulfuriza- the case of the complex alloys employed in tion step. However, the composition is substan- aerospace engineering. The following advan- tially fixed by choice of the initial charge Induction coil tages have a decisive influence on the rapid materials, and these materials are inevitably increase of metal production by VIM: higher-priced than those that are used in arc-AOD. To vacuum pumps Flexibility due to small batch sizes Fast change of program for different types of steels and alloys Easy operation Process Description Fig. 1 Basic elements of a vacuum induction melting Low losses of alloying elements by oxidation furnace Achievement of very close compositional tolerances A VIM furnace is simply a melting crucible Precise temperature control inside a steel shell that is connected to a high- Low level of environmental pollution from speed vacuum system (Fig. 1). The heart of dust output the furnace is the crucible (Fig. 2) with heat- Removal of undesired trace elements with ing and cooling coils and refractory lining. high vapor pressures Heating is done by electric current that passes Removal of dissolved gases, for example, through a set of induction coils. The coils are hydrogen and nitrogen made from copper tubing that is cooled by water flowing through the tubing. The passage Vacuum induction melting is indispensable of current through the coils creates a magnetic in the manufacture of superalloys. Compared field that induces a current in the charge Shunt inside the refractory. When the heating of to air-melting processes such electric arc fur- Heating naces (EAF) with argon oxygen decarburization the charge material is sufficient that the charge coil (AOD) converters, VIM of superalloys provides has become all molten, these magnetic fields Brick a considerable reduction in oxygen and nitrogen cause stirring of the liquid charge. The opti- crucible contents. Accordingly, with fewer oxides and mal induction coil frequency for heating the Cooling nitrides formed, the microcleanliness of vac- charge varies with the charge shape, size, coil uum-melted superalloys is greatly improved and melt status (liquid or solid). Older equip- compared to air (EAF/AOD)-melted superal- ment used a single frequency, but newer Ground detection loys. Additionally, high-vapor-pressure ele- power supplies are able to be operated at var- ments (specifically lead and bismuth) that may iable frequencies and are adjusted throughout Fig. 2 Schematic of vacuum induction melting enter the scrap circuit during the manufacture the melt to obtain the most rapid heating/melt- crucible (shell, coil stack, backup lining, and of superalloy components are reduced during ing conditions. working lining) 2 / Vacuum Induction Melting Bulk charger master melt, may use single-piece crucibles. Refractory brick linings are usually two layers. Melting Crucible (pouring) The backup lining protects the induction coil chamber in the event of a failure of the outer or working lining. The working lining is the primary inter- Cover (removed) face with the metal and is replaced when Launder erosion of the lining becomes excessive. Tundish car Operating Refractory life is also affected by the expansion platform of the refractory during the repeated melting Mold cycles. Refractory brick is chosen with regard chamber Electric room to resistance to erosion and expansion. Com- mercially available refractory brick may be Power incompletely sintered and expands during use, supply causing loss of crucible integrity. The refractory material used for the crucible lining is based on oxides such as Al2O3, MgO, CaO, or ZrO2 (Table 1). The lining is almost always rammed and sintered; prefabricated Molds Shop floor brick is used in larger furnaces. Dried silicate, combined with small oxide additions, appears to be very suitable for crucible lining because Mold car of its thermal characteristics. Because of an Vacuum system irreversible thermal expansion of 8% above 1000 C (1830 F), a high densification of the lining takes place during sintering. For this rea- son, this active lining is suitable for foundries. Fig. 3 Schematic of a top-opening, double-chamber vacuum induction melting furnace The behavior of the lining refractory with regard to stability at high temperature under Table 1 Typical refractories used to line vacuum induction melting crucibles vacuum must also be considered. The melting crucible material is not inert and Maximum melt temperature Refractory density 3 3 is actually another source of oxygen and other Refractory C F g/cm lb/in. Resistance to thermal shock Applications impurities, depending on refractory type and MgO 1600 2910 2.8 0.101 Good Superalloys, high-quality steels condition. Therefore, both melt refining temper- Al2O3 1900 3450 3.7 0.134 Good Superalloys, high-quality steels ature and refining duration are carefully scruti- MgO-spinel 1900 3450 3.8 0.138 Poor Superalloys, high-quality steels Al2O3-spinel 1900 3450 3.7 0.134 Relatively good Superalloys, high-quality steels nized. Proper melt stirring is integral to the ZrO2 2300 4170 5.4 0.195 Poor Superalloys, high-quality steels deoxidation process and must be optimized Graphite 2300 4170 1.5 0.054 Excellent Copper, copper alloys through proper furnace power frequency and application procedure to prevent refractory lin- ing erosion, a potential problem particularly solubility of oxides and nitrides in the virgin Older VIM furnaces may have been designed during the controlled but more vigorous CO charge as single-chamber systems with the mold put boiling portion of the process. A revert (or scrap) portion, which consists of inside the furnace before the beginning of the Process Sequence. Figure 4 shows a typical both internal and external scrap that previ- melt. The molten charge is then poured into the process profile for the VIM of nickel- and ously has been vacuum melted mold inside the furnace. Single-chamber furnaces cobalt-base superalloys. Before operation or at thus must be opened after each heat to extract the the completion the preceding heat, the melt Vacuum-melted scrap has already had its gas molds and put in the new molds. Most furnaces chamber is isolated from the mold chamber content reduced to levels consistent with vac- have some system of large vacuum locks for trans- and the vacuum integrity of the furnace is veri- uum production. Scrap, however, has the possi- ferring the prepared molds into the melt chamber. fied. Specific practices with regard to vacuum bility of having become contaminated during In double-chamber furnaces (Fig. 3), there is a measurements will differ in detail. The deterio- the production process, and care (expense) must separate chamber for the molds. The molten metal ration rate of the vacuum is a measurement of be taken in the segregation and preparation of is transferred via launders (refractory-lined steel the inherent vacuum integrity of the furnace. scrap materials for vacuum melting. troughs) to a refractory tub (tundish). Some sys- This integrity cannot be measured by vacuum In most VIM furnaces there is a vacuum lock tems are designed to pour directly from the cruci- alone, because the large pumping capacities of bulk charger located directly over the crucible ble into the tundish. the pumps used in VIM are able to achieve (Fig. 3). Charge material may be added to the The tundish contains a considerable volume of low vacuum pressures even with significant heat through the bulk charger while melting is metal and allows residence time for entrained slag leakage into the furnace. It is considered bad in process in the crucible. The material to be to float to the top of the tundish and be removed practice to continuously draw air into the vac- added is placed in bottom-opening buckets, from the pour stream. The pour stream exits the uum furnace and across the melt. placed in the bulk charger, and the charger is bottom of the tundish. The pour time is regulated Immediately after ensuring that the furnace is evacuated.