Vacuum Induction Melting

Home , Decarburization, Induction furnace

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 reason, 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 lining 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. The valve isolating the charger from by pour temperature and the nozzle diameter of vacuum-tight, the virgin portion of the charge is the melt chamber is opened, and the bucket is the tundish. The typical tundish is designed to pro- placed into the VIM furnace first. This may be lowered to a point close to the crucible top vide a low-velocity flow path from the point of done by opening the furnace or, more commonly, and the bottom opened so as to drop the charge entry of the metal to the bottom nozzle. by charging the furnace through hoppers lowered material into the crucible. In constant operation, Refractories. Crucibles with a capacity of through a large vacuum lock (bulk charger) if a furnace is not to be opened to the atmo- approximately 4500 to 22,500 kg (10,000 to located over the crucible. The furnace is capable sphere, all charge material for a heat will be 50,000 lb) are generally built up from refractory of quickly pumping down to or maintaining vac- added by this process. brick. Smaller furnaces, used for production of uum levels below 100 mm (and often into the Vacuum Induction Melting / 3

105 1000

Installed power 1200 104 100 Pyrometer temperature 1100 1500 2732 1000 103 1400 10 2552 Melting power

900 F

1300 Њ Pressure 2372 C Њ 800 rise test 100 1200 1.0 2192 erature, 1100 700 2012 p Casting end 1000 600 Casting start 10 0.1 1832

900 rometer tem

1652 y Melting power, kW 500 All liquid P Melt chamber pressure, mbar Melt chamber pressure, Pa Pyrometer temperature, 800 1472 1.0 400 Additions/Sampling 0.01 Pressure rise test 300 Additions 200 Sampling 200 − 392 0.1 10 3 100 100 212 1. Charge 2. Charge 3. Charge − 0.01 0 0 10 4 32 01 2 3 4 5 6 7

Melting Refining Superheating Casting Time, h

Fig. 4 Typical vacuum induction melting protocol for nickel- and cobalt-base superalloys

<10 mm) range. The virgin material is melted by Process Controls. Process computerization Determination of the process steps, such as application of current to the induction coils sur- and automation continue to enhance better completion of the refining period and the rounding the refractory crucible. reproducibility of the melts and computer mod- suitable time for tapping When the virgin material has been eling. Using charge and alloy calculations, it is Advance detection of leaks in the furnace completely melted (all molten), it outgasses. easy to achieve the required chemical analysis chamber, cooling water system, or hydraulic The outgassing is monitored until it is com- with minimal cost. For this application, in the system plete. The outgassing is a response of the gas ideal case, the computer is linked to the analy- Monitoring of the state of degassing of the in solution in the melt to the low partial pres- sis system computer so that the additives can crucible refractory lining sures of gases in the vacuum chamber. Some be calculated and, if necessary, weighed and degassing is accomplished because carbon in added directly after the analysis. If this alloy Remelting by VIM for Shape Casting. Vac- the charge will form CO with the oxygen and calculation is laid out as a charge calculation, uum furnaces for precision casting are used to also be evolved from the melt. The progress the charges and the amounts of scrap necessary remelt alloys that have already been treated in of degassing is followed by measuring the can be optimized. The complete calculation vacuum. The remelted alloy is then precision leak-up rate at set time intervals. When a con- system enables a calculation of the alloying ele- cast into preheated ceramic molds. Heating and stant leak-up rate is obtained, this indicates that ments, starting with the amount of scrap needed melting in such furnaces almost always takes degassing (refining) of the melt is complete. and proceeding to the necessary alloy addition place by induction, although some electron beam After degassing is complete, the reactive and at the end of the refining period. melters and vacuum arc furnaces are used. Elec- revert charges are added to bring the charge to A computer-controlled mass spectrometer tron beam furnaces offer the advantage of its planned weight. A ladle sample is taken after system, specially developed for the VIM pro- ceramic-free melting, but they do not permit a all additions are complete. Based on the analysis cess, can make a significant contribution to sufficiently accurate and reproducible tempera- of this sample, trim additions are made to bring the optimization of the melting process and its ture control for charges of more than approxi- the melt into a very precise compositional range. economics. Using a computer evaluation of mately 5 to 7 kg (11 to 15 lb). Vacuum arc Because there are no ongoing chemical changes the gas composition in the furnace chamber, furnaces are used for titanium precision casting. in the melting, as there are in EAF/AOD, the com- information about the state of the degassing Remelting with VIM furnaces for precision positional requirements of a melt may be met as process and of the chemical reactions in the casting is done with two-chamber furnaces closely as allowed by the reproducibility of chem- melt can be continuously obtained. (Fig. 5a), where the melting crucible and mold ical analysis. After the trim additions have been The melting process can thus be controlled are in separate chambers. The mold chamber is made, the temperature of the heat is brought pre- more exactly. Process parameters that can be preferably placed under the melting chamber so cisely to the desired point, and the heat is poured. closely controlled include: that, in connection with another chamber for The heat, although produced in vacuum, will still the charge or melting material, semicontinuous have generated significant amounts of slag from Determination of the correct time for the operation with minimal cycle times is possible. the products of deoxidation, desulfurization, and addition of chemically active elements and For cobalt- or nickel-base superalloys, the melt- the deterioration of the refractory crucible lining. the sequence of addition of such elements ing chamber pressure is of the order of 0.01 Pa 4 / Vacuum Induction Melting

(104 mbar, or 7.5 105 torr), while the mold A typical small casting produced by this Ceramic foam filters are used in some master lock would be evacuated to approximately 5 Pa method is a turbocharger wheel for automotive metal operations to remove relatively large melt (0.05 mbar, or 0.038 torr) within 1 min. engines (Fig. 5b). Automotive turbocharger inclusions by means of entrapment. Foam filters In addition to the casting of equiaxed, direc- wheels weighing 400 to 500 g (0.9 to 1.1 lb) are most effective where extremely high pour tional solidification, and single-crystal blades for are produced in automatic precision casting rate conditions and gross cleanliness problems gas turbine engines, vacuum precision casting machines. Total cycle time, including 40 s for prevail. Filter performance often varies because can be used for large parts (melt weights up to melting, is typically 90 to 100 s. In the produc- of the occasional use of filters with poor 1000 kg, or 2200 lb) or for the large-scale production of such small parts, the crucible is often mechanical strength and/or thermal shock resis- tion of small parts. Examples of large parts pro- integrated into the mold. Casting yields of 80 tance. Foam cell particle breakage often results duced using vacuum precision casting include to 90% are obtained. from handling during shipment or tundish compressor sections of jet engines and structural Filters. One of the most critical stages with installation and, if undetected, results in filter parts for stationary turbines. respect to cleanliness is the pouring of the melt. particulate in the alloy bar stock and subse- quently cast components. Optimized VIM tech- nology and practice without filters provide a clean alloy, without the inherent risks asso- ciated with filter use when applied to master metal production.

VIM Metallurgy

Vacuum induction melting is often done as the primary melting operation followed by secondary melting (remelting) operations such as electroslag remelting (ESR) and/or vacuum arc remelting (VAR).Various processing routes following VIM are illustrated in Fig. 6. Some superalloys are produced by a triple-melt sequence (VIM/ESR/VAR), where the VIM ingot is typically referred to as an electrode (a) (b) for subsequent ESR and/or VAR operation. Fig. 5 Shape casting with vacuum induction melting, (a) Computer-controlled vacuum furnace with mold chamber. The casting weight can vary from a few kilo- (b) Precision-cast turbocharger wheels for automotive engines. From left: mold with integrated crucible, bar grams to 30 Mg (33 tons), depending on stick, cast part, machined turbocharger wheel whether the VIM furnace is being used for

Fig. 6 Potential processing routes for products cast from vacuum induction melting (VIM) ingots or electrodes. VAR, vacuum are remelting; ESR, electroslag remelting; EB, electron beam; HIP, hot isostatic pressing. Source: Ref 1 Vacuum Induction Melting / 5 precision casting or for the production of ingots Suitable deslagging and filtering techniques loss is realized because of scavenging asso- or electrodes for further remelting. during pouring ciated with the CO bubbles, and a slight sulfur There are many different metallurgical factors Conception of a suitable tundish and launder reduction may occur during the CO supersatu- that arise with induction furnace melting. The system for good oxide removal ration stage via sulfur dioxide (SO2) evolution. crucible material has an extraordinary effect on Minor tramp elements such as lead, silver, bis- the metal/slag reaction because the ceramic outer For particular applications, however, the muth, selenium, and tellurium are partially eva- wall reacts with the liquid metal and with the quality of the material produced by VIM will porated during this period as well as throughout slag. In a VIM furnace, slag would be transported not be sufficient to satisfy the highest quality the entire refining process (Ref 3). Some unde- to the crucible wall by the characteristic bath requirements with respect to cleanliness and sirable elements, however, such as arsenic and movement. The result is that the slag solidifies primary structure. In this case, the VIM-pro- tin, must be controlled through raw materials at the wall and therefore has an insufficient reac- duced material must undergo secondary melting selection because they are not removed by tion with the melt. It is therefore beneficial to use processes (Fig. 6). vacuum refining. a slag that is more or less saturated with the oxi- VIM Refinement Process. The primary pro- Bath refining from a somewhat vigorous CO des of the crucible lining in order to minimize cess of melt refinement is the removal of melt- boil is undertaken at a temperature and duration heavy slag attack of the lining. contained oxygen by means of a reaction with long enough to reach the so-called system equi- In contrast to the ladle metallurgy processes carbon to form carbon monoxide (CO). The reac- librium conditions, the assurance of which is with EAF, the crucible wall lining is susceptible tion occurs most readily at or near the melt sur- provided by the attainment of consistent fur- to significantly higher erosion than the brick wall face, with the reaction kinetics being affected nace leak-up rates. At this point, there is the of a ladle. For this reason, metallurgical opera- by crucible geometry and melt stirring. The CO addition of the refractory charge, which tions such as dephosphorization and desulfuriza- bubbles form along the walls and, sometimes, includes elements with reactivity toward oxy- tion are limited. The metallurgy of VIM is bottom of the melt/lining-refractory interface gen (for example, aluminum, titanium, zirco- primarily limited to the pressure-dependent reac- (Fig. 7). This occurs preferentially at small cre- nium, and hafnium) that were withheld from tions, such as carbon, oxygen, nitrogen, and vices existing in the lining, with the bubbles the virgin charge because of their reactivity. hydrogen, and the evaporation of undesired ele- growing during movement toward the molten Revert charge is also added. Once the boiling ments with high vapor pressures, such as copper, metal/vacuum interface (Ref 2). Actual bubble subsides, surface desorption of additional CO lead, bismuth, tellurium, and antimony. formation is dependent on the number of gas occurs, and it is during this nonboiling period Cleanliness can be significantly improved if a molecules present, the pressure in the liquid at that nitrogen removal (desorption) is most reactive liquid slag capable of absorbing oxides the level of the bubble, the temperature of the effective (Ref 4). and sulfides is in contact with the melt. Vacuum gas, and, for very small bubbles, the interfacial Trace Element Removal. The removal of induction furnaces are generally not operated tension between the gas and the liquid metal. undesired volatile trace elements, such as arse- with active slags. Therefore, reaction products Following formation, bubble growth and nic, antimony, tellurium, selenium, bismuth, can precipitate only on the crucible walls, and mass transport within the liquid toward the liq- and copper, from the vacuum induction furnace the melt may not be as clean as with other pro- uid/vacuum interface is dependent on: is of considerable practical importance. These cessing methods. elements must be held to very low concentra- Because the various alloys produced in vac- The quantity of the dissolved gas tions to avoid the risk of premature part failure, uum induction furnaces must meet the highest The decreased pressure exerted on the bub- particularly for the production of superalloys quality requirements and because the vacuum ble as it rises in the melt for critical applications, such as jet engine induction furnace is primarily a consolidation The bath temperature parts. Figure 8 shows the influence of some unit and only secondarily a refining unit, the fol- The time it takes for the bubble to rise trace elements on the stress-rupture properties lowing methods are used to produce clean melts: through the melt to the surface, which, in of alloy 718. Because of the high vapor pressures of most of the undesirable trace elements, turn, is a function of melt stirring Selection of a more stable refractory mate- The pressure above the melt they can be kept to very low levels by distilla- rial for the crucible lining tion during melting under vacuum. The interfacial tension between the bubble Rinsing of the melt with inert gas and the liquid metal Figure 9 shows how the different trace ele- Minimizing the contact time of the melt in ments behave under vacuum. In a nickel-chro- the crucible The relatively vigorous, but controlled, portion mium melt, arsenic, antimony, and tin cannot Exact temperature control to minimize cru- of the boiling process results in the greatest be reduced over the gas phase, while copper, cible reactions with the melt CO removal. Concurrently, a slight nitrogen lead, selenium, tellurium, and bismuth can be reduced to a level far below 10 ppm. Nitrogen Degassing. Nitrogen gas can be decreased because its solubility at constant temperature is directly proportional to its partial pressures. As long as no strong nitride-forming elements are present in the melt, there is no difficulty in reducing the nitrogen content to 20 ppm (Ref 7–9). When nitride-forming elements such as chromium, vanadium, aluminum, and titanium are present, the activity of the nitrogen is very low; therefore, removal of nitrogen under high vacuum is difficult. If low nitrogen contents are desired in alloys containing nitride-forming elements, raw materials low in nitrogen must be used. Figure 10 shows the reduction in nitrogen levels obtained in the VIM of a die steel. From an average level of approximately 400 ppm at Fig. 7 Vacuum induction refining process the beginning of treatment, the nitrogen has 6 / Vacuum Induction Melting

200 99.9 99.5 99

160 97.5 After treatment 95 Silver 90 Selenium 80 120 70 Before treatment 60

Lead Frequency, % 50 40 80

Rupture life, h 30 Concentration, % 20 Bismuth 10 40 0 1.0 2.0 3.0 4.0 5.0 Hydrogen, ppm

Reduction in hydrogen content of 0 Fig. 11 01020304050 60 70 80 90 X38CrMoV51 die steel after vacuum induction degassing Impurity content, ppm

Fig. 8 Effect of trace elements on the stress-rupture properties of alloy 718. Test conditions: 650 C (1200 F), 690 MPa (100 ksi). Source: Ref 5 prevent refractory lining erosion, a potential problem particularly during the controlled but more vigorous CO boiling portion of the process. (0.5) 99.9 Arsenic The ﬁnal attainable oxygen content for a 99.5 given carbon content is directly proportional Tin 99 After treatment 97.5 to the CO partial pressure. The theoretical 0.1 95 With argon bubbling equilibrium values for oxygen are below 1 ppm in carbon-containing iron or nickel melts Antimony 90 Without argon bubbling at 1600 C (2910 F) and at a pressure of 0.1 80 Pa (103 mbar, or 7.5 104 torr). However, 70 the actual values are up to 1 order of magnitude

Frequency, % 60 Selenium 50 higher. These higher oxygen contents result 40 from impurities in the crucible lining, crucible 0.01 30 Before treatment outgassing and leakage, and, above all, the fact 20 Copper that the reaction does not reach equilibrium 10

Concentration, % with decreasing oxygen content because of the 0 100 200 300 400 500 difficulty of CO nucleation. Nitrogen content, ppm In addition, the hydrostatic pressure of the liquid metal in the melt must be taken into Bismuth Reduction in nitrogen content of X38CrMoV51 Lead Fig. 10 account because only a relatively small bath −3 (Fe-0.38C-5.2Cr-1.3Mo-0.4V-1Si-0.4Mn) die 10 steel after vacuum induction melting processing surface area is in direct contact with the pre- Tellurium vailing vacuum pressure. However, the influence of the liquid-metal hydrostatic pressure − (3ϫ10 3) can be minimized through the use of additional 020406080100 120 shows the hydrogen contents of a die steel agitation effects. Time, min before and after vacuum treatment. The CO reaction occurs in two stages It is evident from Fig. 11 that final hydrogen (Ref 1). The first stage is boiling, that is, the Fig. 9 Evaporation of trace elements from Ni-20Cr contents below 1 ppm can be routinely formation of CO bubbles within the melt along melts under vacuum. Source: Ref 6 achieved. The reduction in hydrogen during with a strong bath agitation as a result of this the degassing treatment amounts to approxi- gas formation. The second stage is desorption, mately 80%. Argon purging had no perceptible in which no more CO bubbles form inside the been reduced to a level of 50 ppm. Additional influence. The bath agitation in the case of this melt, and CO formation takes place only at argon purging during melting of the steel furnace, which is operating at normal fre- the bath surface. did not improve nitrogen removal. In the case quency, is sufficient for hydrogen removal. Deoxidation with carbon is also a decarburi- of iron-nickel melts, however, the additional Similar results were also obtained for the VIM zation reaction, which is used for the produc- argon purging reduces nitrogen to much lower processing of superalloys. tion of low-carbon high-chromium steels. By values. Deoxidation of the melt in the vacuum induc- using the CO reaction under vacuum, very low Hydrogen Degassing. Like nitrogen, hydro- tion furnace can be done via the generation of CO carbon contents can be achieved without a gen can be decreased via the gas phase because gas (C + O ! CO). The removal of oxygen from noticeable loss of chromium. The oxygen con- its solubility is directly proportional to its par- the melt as CO is favored by decreased melt tent in such alloys is higher because of the tial pressure. The solubility of hydrogen at chamber pressure, elevated bath temperature, decrease in oxygen activity due to chromium. atmospheric pressure and at 1600 C (2910 and increased carbon activity (Ref 10). Proper In this case, precipitation deoxidation using alu- F) in iron or nickel melts is in the range of melt stirring is integral to the deoxidation process minum, silicon, or titanium must be applied. 30 ppm. During VIM, the hydrogen can be and must be optimized through proper furnace The CO partial pressures at which the lining removed to very low concentrations. Figure 11 power frequency and application procedure to oxides are reduced by carbon in the melt are: Vacuum Induction Melting / 7

Carbon monoxide partial pressure Melting Bath Agitation. As noted, melt agi- vacuum. This ensures that the admitted purging Lining material kPa torr tation is an important process factor. Agitation gas flows through the melt and does not take is caused by the induction coil itself, depending the path of least resistance through the lining CaO 0.04 0.3 on the power input and the installed frequency. of the crucible. ZrO2 0.13 1.0 MgO 0.53 4.0 Stirring is directly proportional to the amount of Both agitation processes offer advantages Al2O3 0.53 4.0 induced power and inversely proportional to the with regard to higher reaction rates, correct SiO2 81.1 610.0 square root of the frequency. Therefore, a more temperature adjustment, homogenization with Source: Ref 1 intense stirring for a given size furnace occurs simultaneously lower erosion of the crucible at a higher power and a lower frequency. lining, and better cleaning of the melt. Figure It is evident that CaO represents the stable Bath agitation from the induction coil is basi- 14 shows the three different agitation methods crucible material, while SiO2 is reduced at rela- cally limited to the meltdown period. It can be operating in a typical VIM process sequence. tively high pressure. Apart from that, SiO2 can done with the induction coil alone only if there The agitation effect at 200 to 500 Hz in be reduced by alloying elements such as man- is a high-power input and therefore strong bath the melting phase assists in shortening the ganese, chromium, aluminum, titanium, or zir- heating. This high temperature increase is not melting time. In the refining and superheating conium. This can cause a heavy chemical always metallurgically desirable. For this rea- periods, it is better to use electromagnetic agita- erosion of the lining, accompanied by an unde- son, two agitation systems are offered in state- tion at 50 to 60 Hz or argon purging through a sired silicon pickup in the melt. Because of of-the-art vacuum induction furnaces: porous plug. these undesired reactions with a silica lining, a spinel-forming basic refractory material is used Electromagnetic agitation with an additional for the melting of high-grade steels and superal- coil Production of Nonferrous Materials loys. The spinel formation during sintering Agitation by argon purging through the bot- leads to volume growth; therefore, as with the tom of the crucible Apart from melting high-grade steels and oxidic material, a densification of the rammed superalloys, VIM is being increasingly used The difference between the electromagnetic lining takes place. for the production of nonferrous metals and agitation by a separate coil and the agitation The vacuum induction degassing and alloys. Table 2 shows some examples for possi- caused by the electromagnetic forces of the main pouring (VIDP) furnace (Fig. 12) is a varia- ble use in nonferrous metallurgy. coil lies in the fact that the bath can be more vig- tion of the conventional VIM furnace. The Aluminum alloys with additives such as zir- orously stirred without a temperature increase. VIDP furnace design employs a modular con- conium, titanium, beryllium, cerium, tellurium, A second transformer would be needed. cept that allows for connecting it to casting and cadmium must be melted under vacuum units for ingot casting, horizontal and vertical Another method of bath agitation is derived or under inert gas atmosphere because of their continuous casting, or powder production. from the principle of argon purging usually high reactivity with air and, in some cases, their Because of the smaller volume of the VIDP fur- applied in ladle metallurgy. Argon purging has toxicity. Aluminum-lithium alloys are also can- nace compared to the VIM furnace and signifi- a long history of use in atmospheric melting didates for VIM processing. cantly lower desorption and leakage rates, it is furnaces. A characteristic of this purging is that Copper Alloys. The production of high- possible to obtain very low pressures with the porous plug is not in direct contact with the purity copper having less than 2 ppm O can lower pumping capacity. The lower part of the liquid melt. Instead, the plug is covered with be accomplished only in a vacuum induction furnace can be decoupled and replaced rapidly; refractory material identical to that used for furnace. Oxygen content influences the electri- therefore, the VIDP furnace enables faster the crucible lining (Fig. 13). The basic differ- cal conductivity of copper alloys; the lower replacement of different furnace vessels with ence is that the porous plug is in direct contact the oxygen content, the higher the electrical changes of alloy. with the melt and is suitable for operating under conductivity (Ref 11). For the production of

Crucible lining

Exchangeable porous plug

Basic set

Fig. 12 Schematic of the vacuum induction degassing and pouring furnace Fig. 13 Argon purging system for vacuum induction melting furnaces 8 / Vacuum Induction Melting oxygen-free copper, melting and casting must elements with high vapor pressures. In nonferrous from copper scrap by using the vacuum to evapo- be carried out under vacuum. metallurgy, this effect is used for the distillation of rate volatile elements such as lead and zinc Selective Evaporation of Alloying Ele- metals—for example, for the separation of (Ref 12). ments. The useofvacuummetallurgy isprimarily lead and zinc in lead reﬁning, in zinc production, linked with degassing and decarburization. A side and for the reduction of magnesium and effect of these treatments is the evaporation of nonalkali metals. Similarly, copper can be reﬁned ACKNOWLEDGMENTS Portions of this article were adapted from: A. Choudhury and H. Kemmer, Vacuum Melting and Remelting Processes, Casting, Vol 15, ASM Handbook, ASM International, 1988, p 393–425 M. Donachie and S. Donachie, Superalloys: A Technical Guide, ASM International, 2002 G.L. Erickson, Polycrystalline Cast Superal- loys, Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990

REFERENCES 1. J.W. Pridgeon et al., in Superalloys Source Book, American Society for Metals, 1984, p 201–217 2. D. Winkler, Thermodynamics and Kinetics in Vacuum Metallurgy, Vacuum Metal- lurgy, O. Winkler and R. Bakish, Ed., Else- vier, 1971, p 42–54 3. Evaporation of Elements from 80/20 Nickel-Chromium During Vacuum Induc- tion Melting, Transactions of the Vacuum Metallurgy Conference, American Vacuum Society, 1963 4. V.M. Antipov, Reﬁning of High-Tempera- ture Nickel Alloy in Vacuum Induction Furnaces, Stal0, Feb 1968, p 117–120 5. W.B. Kent, Int. Voc. Sci. Technol., Vol 11 (No. 6), 1974, p 1038–1046 6. P.P. Turillon, in Transactions of the Sixth International Vacuum Metallurgy Conference (Boston), American Vacuum Fig. 14 (a) Melting and (b) stirring modes of the vacuum induction melting process Society, 1983, p 88 7. G.A. Simkovich, Int. Met., Vol 253 (No. 4), 1966, p 504–512 8. H. Katayam et al., in Proceedings of the Table 2 Possible applications for vacuum induction melting in the processing of nonferrous Seventh International Conference on Vac- metals and alloys uum Metallurgy (Tokyo), The Iron and Metal or alloy Metallurgical results Achieved by Steel Institute of Japan, 1982, p 933–940 Aluminum, aluminum Low oxygen content, effective removal Vacuum tank degassing without additions 9. A. Choudhury et al., World Steel and alloys of oxides of polluting gases or materials Metalworking Manual, Vol 9, 1987–1988, High-purity metals Oxygen-free copper (2 ppm O) Melting and pouring under steady, p1–6 controlled atmosphere or vacuum; gas 10. D.R. Gaskell, Introduction to Metallurgical stirring; carbon as reducing agent “New” alloys Al-Mg, Al-Ce, Al-Zr, Al-Hf alloys; Furnace and casting system under inert Thermodynamics, Scripta Publishing, 1973, copper alloys with beryllium, cobalt, gas atmosphere; vacuum-tight, highly p 268–273 titanium, zirconium, and lithium; automated furnace 11. O. Kamado et al., Method of Producing avoidance of lithium losses in Electrical Conductor, European Patent production of Al-Li alloys Metal production and Production of calcium, barium, lithium, Thermic/alumino-thermic vacuum 0121152, 1986 recycling strontium, and magnesium; recycling of distillation; leaching with zinc and 12. J.G. Kru¨ger, Proceedings of the Fifth Inter- hard metals; puriﬁcation of alloys or subsequent vacuum distillation national Vacuum Metallurgy Conference melts; removal of zinc and lead melting; vacuum distillation (Munich), 1976, p 75–80