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VOl. 3,NO.3, 1987 Published by the Center for Metals Production - An EPRl Sponsored R&D Appllcatlons Center

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Vacuum Induction MeltingTechnologyA unique electrical methodfor productionof hiah aualitv and reliable alloys neededin critical applications.Vacuum induction melting (VIM)is themelting of metals by induction doneunder a vacuum. This process firstbecame important in the1950s; whenheat resisting alloys or superalloys,which contam large amountsof reac-tlve elements such as alumlnum ortitanium, were found to have superiorservice life and rellabllity when meltedin a vacuum. As a result of VIM, thejetenglne made the great advance inperformance and durability which has

FANLO W PRESSURECOMPRESSOR

been so important toboth.,militaryandcommercial aviation(Figwe 1 ) .Thesuccess in theproductlon of superal-loys byVIM for the gas turbine indus-try stimulated interestin VIM for themanufacture of other crltical applica-tion alloysfor use in nuclear reactorsand electronics. Inaddition, the lowmaterial lossesof this process madeitthe economic melting techniqueforsome high-value alloys which did notrequire superior cleanliness.

IN'LETCASEFigure 1 Vacuum melted titanium fan blades and superalloy turbne blades are essentralto the highperformance of modern jet engines. This Pratt and WhitneyPW 4000 is used to power some recentmodels of the Boeing 747.

AdvantagesThe specific advantagesof vacuuminduction meltmg include:

Elimination of gases - under the.very low pressures obtained,torr or .000001 atmosphere, unde-sirable gases and potentially harmful volatile elements are eliminafrom the charged raw materials asmelting occurs.H Close control of chemical analysis- exceptlonal and reproduciblecontrol of reactive element contain-Ing compositionsis possible be-cause of the lack of atmosphere.Superior process control - inde-pendent controlof pressure, tem-perature and inductive stirringprovides an exceptionalopportu-nity for developing melt practicesspecifically tailoredto alloy compo-sition and desired properties.Slag free melting - melting in avacuum eliminates the need for aprotective slag coverand de-creases the potentialof accidentalslag contamination or inclusionsinthe ingot.Melt protection - high vacuumprevents deleterious contaminatinreactions with atmospheric gase

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Process DescriptionAlthough the VIM processis rather charged while ingots cool at the end ofsimple, the equipmentis quite sophisti- a cycle. An argon system is also at-cated when compared to other meltingtached to the melt chamber to permitinstallations. All components require any back-fill operations required or forcareful maintenance,testmg and Cali- emergency conditions.A plan view schematic of a typicalbration to assure optimum production. Thepumping system designcommercial VIM facilityis given inFiaure2.

considerably, depending upon thefurnace volume, typeof alloys to be

Samplecup 0 ChamberMold Control StatlonHeaterCeramicsFurnace Primaryand

SecondaryTundishes

Molds-

rnFurnace PurnplngSystemI 1Flgure 2 Vacuum mducf/on melbnglacrl,t/es requlres elecfrrury/of melfmg, vacuum pumpm g.rnPrtgas genefar/on. and usuallymold preheafmg.

As shown, the furnace consistsof a melted, pumping speed deisred, andmelt chamber, mold chamber, pump- the ultimate vacuum required ina typi-ing system, power supplyand a con- cal system; mechanical forepumps,trol station. blowers, and vapor pumps are com-bined in sequence to develop the de-The melt chamber contains the refrac- siredvacuum at reasonablespeeds

induction coil which supplies bothmelting and stirring energy. Bulkcharging, containers for late additions,

tory melt crucible surroundedby the with the lowest operating costs, Cham-ber pressureduring a typical meltcycle is shown in Figure 3.

The power supply for most largefur-naces IS a motor generator systemcapable of dellvering about4 .2 mega-watts at180hertz.The control statlon contains all valveactuators, sensor instruments andrecorders for control of the process.Melt observationis conducted directlythrough shielded sights or by vicameras and recorders.The major power-consuming ancil-laries are furnaces forheating molds,tundishes, other ceramlcs, and samplecups. These furnaces can be heatedby oil, gas, or electricity. However,electricity offers a dlstinct advantage freedom from chemical deposits. Forexample, minute tracesof sulfur orcarbon deposited on sample takerscan give false chemlstry IndicatioSpecific melt cycles and related procdures are considered proprietary byalloy producers. However, the follgeneral process steps usually occStep One.After raw materials arecharged, leak rates are taken periodcally during the pump down to assuresystem Integrity. Melting poweris thenapplied.Step WO .Pressure usuallyis erraticduring the melt down, increasing adecreasing as various materials meltAt times itis necessary to partiallybackfill the melt chamber with control boilingif it becomes too vigor-ous. When the charge becomesallmolten the pumprng continues untilit is

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In theorder of to torr and anearly constant leak rateIS attained.The constant leak rate lmpiles nearcompletion of deoxidation.Step Three. Reactive alloy additionsare made at this point.A small pres-sure riseIS always notlced as the reac-tive elements outgasand melt.Step Four. Samples are takenandanalyzed to assure the melt is withinspecification.Step Five. After chemlstry approvalthe pour chamberis opened.A pri-mary tundishis positioned to allowmetal to flow from the melt chamberInto the mold chamber. The primarytundish contains ceramic damstocontrol metal flow andto hold backpieces of furnace skullor ceramics thatmay break free from the meltcrucible.Metal flows through the primarynozzleInto the secondary tundish whichcon-tains additional flow control dams,weirs, and pour nozzles and directsthe metal Into the seriesof molds in thechamber. A slight pressure riseis ob-served as the pouring system out-gasses.Step Six. Upon completion of thepour, the tundishes are dralned andrepositioned in themold chamber. Theair lock is closed and the next cyclecan be started in the melt chamber.When the metal is frozen, themoldchamber vacuum is broken and thechamber is opened.At this time thefilled molds are removed and the .chamber is prepared for the next cy-cle.

Vacuum induction meltmg mainlyisused in the manufactureof superal-loys. These are usually nickel, cobaltor iron-based alloys offeringhighstrength at elevated temperatures. VIMis also used to melt special copperbase, uranium,and other unique ma-terials. Oftenit is combined with otherprocesses inproducing premium highperformance parts. Figure4 shows thevariety of process steps that can befollowed after vacuum induction melt-ing.Superalloys comprlse the majorityofVIM production. About 60 to 100 mil-lion pounds are produced per yearmainly dueto the demand for aircraftengines.

About 80% of all superalloys are usedin aircraft and aerospaceapplcatlons.The next largest useis in power gener-ation, 13%. The petrochemcal Indus-try and miscellaneous applicatlonsaccount for the remaining7% .Total electric power consumption perunit of productis high for vacuuminduction melting becauseof the smaquantities produced and the energyrequired to produce a vacuum. Whileno exact figures are available, edgeable individualsestimate the con-sumption rate is twice that of normalInduction meltingor about 1200 kwhDer ton.

I VacuumInductionI Melting I

Precisio n ForgingrII I IWWI]Elecrroslas E!ec:ron Beam

Finlshmg

Investment of PartsProductionCas tin g From Powder

Flguro 4 Metals fhafare vacuum mduc:.cn me:!su are formed !nto parts byprecwon forgmgmesfmen1casrmg. or powder metallurgy: e c n n m e s

Related TechnologiesOther premium melting processes remelting similar to VAR but done us- Inclusion particles. Light low densityoften used in conjunction with vacuum ing a slag father than under a vacuum. inclusions are vaporized. This tech-induction melting include vacuum arc While less expensive than VAR, itis not nique IS useful for obtaining the high-remelting (VAR). Inthisprocessan . aseffectivewith somealloycomposi-estqualitymaterials.ingot isselectively zone melted from tions. Electron beam cold hearth re-the bottom to top to remove impurities. melting is done by passing an electronThis is a common practice in both beam in a vacuum over a chargeto besuperalloy and titanium production. melted. The molten metal flows overaElectroslag remelting(ESR) is selective dam which traps heavyhigh density

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References:

The Electric Power ResearchInstitute (EPRI)conducts a tech-nical research and developmentprogram for theU S . electricutility industry. Through planning,selection, funding,and manage-ment of research projects carriedout by contracting organizations,EPRl promotes the developmentof new and improved technolo-gies to help the utility industrymeet presentand future electricenergy needsin environmentallyand economically acceptableways. EPEl conducts researchon all aspects of electric powerproduction and use, includingfuels, generation, delivery, en-ergy management and conser-vation, environmentaleffects,and energy analysis.

CMP-1087-00801987 Center for Metals Production

TheCenter fo r Metals Production(CMP) IS an R& D applcation centersponsored by the Electrlc Power Re-search Institute (EPRI) and adminis-teredthroughMellon Institute ofCarnegle Mellon University. CMPSgoal IS to develop and transfer techni-cal mformation that improves thepro-ductlvlty and energy efficiency ofUS. primary metals producing com-panles (SIC 33). Areas of concentra-tion are reductlonlsmelting;reflntng/remelting;thermal & me-chanical processing; and surfacecondltloninglprotection.EPRlRobert Jeffress, Project ManagerCMPJoseph E. Goodwill, DirectorRichard M. Hurd, ChairmanSteelmaking ProcessesJames M. Hensler, ManagerofTechnical ProjectsJohn Kollar, Manager ofCommunicationsW. J. Boesch, Consultant

Vacuum Melting of Metals: State-of-the-Art Assessment EPRl ReportNO.EM-4132,July 1985.Induction M elting of Meta ls: State-the-Aft Assessment EPRl Report No.EM-4508,April 1936.

LEGAL NOTICEThis report was prepared and sponsored Center tor Metals Productlon (CMP) Nelthemembersof CMP nor any person actlng othelr behalf (1) makes any warranty expresseor implled. wlth respectto the useof anymtormatlon. apparatus. method. or processdisclosed In thls report or that such usinfringe privately owned rights; or(b) assumesany habilltles wlth respectto the useof, or tordamages resultlng from the useof. any Information,apparatus. method. or processdisclosedIn thls report

Center for Metals ProductionMellon Institute4400Fifth AvenuePittsburgh, PA 1521341 2-268-3243