techcommentarytechcommentaryVacuum Furnaces for Heat Treating, Brazing andSintering

IntroductionThere are a wide variety of electric andfuel-fired furnace types used for materialsprocessing. Each furnace type hasadvantages and disadvantages dependingon the process being conducted. Selectionof the furnace type depends on the typeand volume of material being processed,the process economics, and the user’spreferences. One furnace type gainingincreased usage is the electric vacuumfurnace (Figure 1). The connected electricload for this type of furnace is typicallyin the range of 40 kW to 300 kW.

Electrically heated vacuum furnacetechnology has eliminated environmentalproblems associated with salt bathfurnaces. Electric vacuum furnaces alsoprovide more precise control thantraditional gas-fired furnace technology.In addition, more demanding qualityrequirements for high temperatureprocessing, such as that imposed by theaerospace industry, have pressured theheat treat industry for continuouslyimproved reliability and repeatability.Electric vacuum furnaces can providethis level of quality. Also, the automotiveindustry’s switch to lighter, strongermaterials has created a need for higher

processing temperaturesthat are more efficientlyaccomplished by electricvacuum furnaces. Further-more, the food and medicalinstrument industries havebecome attracted to the“bright” non-oxidized finishon parts heat treated byvacuum furnaces, whicheliminates a cleaning step.

ApplicationsApplications for these high-temperature electrically heatedvacuum furnaces include:

• Hardening♦ Tool Steels

– Chromium, Tungsten, andMolybdenum hot-work steels(H series)

– Tungsten and Molybdenumhigh-speed steels (T and Mseries)

• Annealing♦ Stainless Steels

– Conventional, Stabilized,Low-carbon, and High-nitrogengrades

♦ Nickel Alloys– Inconels and Hastelloys

• Solution Treating♦ Superalloys

– Iron-base, Nickel-base, andCobalt-base

• Brazing♦ Base metals containing more than a

few percent of aluminum,titanium, zirconium, or otherelements with particularly stableoxides.

• Sintering♦ Tungsten Carbide♦ Metal Injection Molding♦ Ceramics

These applications are found in theindustries listed in Table 1.

Process FundamentalsThe process of vacuum heat treatingbegins with the placement of theworkpieces in a basket or on a platentable. The basket or table is lifted intothe furnace and placed onto the furnacehearth using a forklift or integral gantrysystem. The furnace door (or bottom ofthe furnace) is then closed and the air inthe chamber is evacuated using vacuumpumps. The temperature of theworkpieces is raised by the heatingelements to the prescribed level duringthe ramp-up phase (Figure 2). Thisprocess temperature is held for someperiod of time to allow the energy radiatedto the part surface to conduct throughoutthe workpiece (to achieve equilibriumtemperature) and to facilitate metallurgicalchanges which are time dependent. After“soaking” at this high temperature, theworkpiece temperature is rapidly decreasedin a process phase known as quenching. Awide range of metallurgical properties ofthe workpiece can be achieved with smallvariations in the temperature-time profileduring quenching. After quenching iscomplete, workpieces are often subjectedto a second heating and cooling cycle at aconsiderably lower temperature to temperthe workpiece. Tempering results in atrade off of one desirable material property(say, increased toughness) with anothermaterial property (say, decreased strength).

Table 1. Industries Using Vacuum Furnaces

SIC Industry Description

3491 Industrial Valves

3499 Powder Metal Products

3533 Oil and Gas Field Machinery

3544 Cutting Tools and Dies

3556 Food Products Machinery

3714 Motor Vehicle Parts

3724 Aircraft Engines

376 Aerospace

3841 Surgical and MedicalInstruments

Figure 1. Vacuum Furnace Setup

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Essential aspects of the heattreating of metals includecontrolling the uniformity oftemperature from the surfaceto the center of the part andprotecting the surface of themetal at high temperatures bycontrolling the surroundingatmosphere. A temperatureprofile that is not uniformwithin a part can cause crackingfrom thermal shock or distortionstresses. Nonuniformtemperature also can result innonuniform transformation ofthe metal and, thus, nonuniformproperties of the metal throughout thepart. Improper protection of the partsurface can result in a buildup or scalethat must be mechanically or chemicallyremoved in a subsequent operation afterheat treatment. Certain atmospheres athigh temperatures can also leach outmetal alloying elements such as carbonfrom the part surface.

Technical ConsiderationsThe primary components of a vacuumfurnace include the exterior chamber, thehot zone, work-holding fixtures, heatingelements, vacuum pumps, controls, and aquenching system. The exterior chambersurrounds the hot zone, fixtures, heatingelements, and, sometimes, the quenchblower fan. The quenching systemrapidly lowers the temperature of heatedparts using inert gas flow. The hot zoneconcentrates the heat on the workpiecesand away from the chamber wall.

roller-hearth, and pusher-typefurnace designs can be adaptedfor vacuum furnaces. Verticalfurnaces are preferred when longand slender parts (that woulddistort from gravity) are insertedor to facilitate large, heavy loads.

Chamber Insulation

Hot-wall vacuum furnaces werethe first chamber type to bedesigned. Heating elementsinside a refractory insulatedchamber surround a vacuum-tight retort in which the parts

are placed. Slow heating and coolingcapabilities relegate this design totemperatures below 1,800°F (980°C),similar to gas-fired vacuum furnaces.

Cold-wall vacuum furnaces incorporatewater-cooled coils on the furnace walland doors (Figure 5). The temperatureof the external vessel is maintained nearambient, allowing large units to beconstructed since the vessel strength is

Chamber Design

Vacuum furnaces are available forworkloads from several pounds tohundreds of tons. Chamber sizes rangefrom 1 ft3 (0.028 m3) to hundreds ofcubic feet. Vacuum furnaces can begrouped into one of three basic designs:

• Horizontal-loading, or box furnaces(Figure 3)

• Top-loading vertical, or pit furnaces

• Bottom-loading vertical, or bellfurnaces

Horizontal loading designs are usuallycylindrical shells with hinged circularconvex doors on each end sealed byO-rings. Many of these furnaces areequipped with special lifting andtransfer forks permanently aligned withthe chamber. Some horizontal-loadingdesigns incorporate multiple chambersdesigned to internally transfer workloads(Figure 4). The conveyor, walking beam, Figure 3. Horizontal Vacuum Furnace

Figure 2. Heat Treating Temperature Profile

Time

Ramp-up

Austenitizing

Quenching Tempering

Tem

pera

ture

°F

(°C

) 1,500(816)

1,000(538)

500(260)

Heat Treating

Heat Treating is a process in which metal is heatedand cooled under tight controls to improve itsproperties, performance, and durability. Heat treatingmay be used for a variety of purposes, it can

• Soften the metal to improve formability(annealing and stress relieving)

• Make a part harder (austenitizing followed byquenching) to withstand tough service

• Increase the part strength (solution treatingfollowed by precipitation or aging)

• Put a hard surface on relatively soft components(case hardening) to increase abrasion resistance

• Toughen brittle parts (tempering).

Brazing & Sintering

Brazing joins metals by heating them in thepresence of a filler metal having a meltingtemperature above 840°F (450°C), but belowthe melting temperature of the base metalsbeing joined. The filler metal distributes itselfbetween the closely fitted surfaces of the jointby capillary action. Parts made by compactingmetal powder followed by thermal processing(sintering) are called powder metal parts.Sintering is the process wherein compactedpowder particles develop metallurgical bondingand further densify under the influence of hightemperatures (below the melting temperatureof any constituent in the material).

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not compromised. In addition, thevacuum itself acts as an insulator becausethe thermal conductivity of a vacuum isessentially zero.

Hot Zone

The hot zone in a furnace surrounds theheating elements and the hearth assemblyon which parts are placed (Figure 6). Itkeeps the heat concentrated on the partsand away from the chamber wall. Hotzones are either an “all-metal” or“radiation shield” design or an “insulatedtype” design. Insulated hot zones areusually rectangular boxes constructedwith a rigid graphite board, a graphite-feltmaterial, or a combination of board andfelt. Insulation materials, by their verynature, are hygroscopic (i.e. they absorbwater) and therefore limit vacuums to areduced level. Although they are energyefficient, insulation materials do notreadily give up heat and limit quenchingrates.

All-metal hot zones consist of multipleconcentric layers of thin, heat-resistingsheet metal that reflects energy back intothe hearth. The vacuum in the chamberprevents loss by convection and permitsthe use of multiple radiation shields ofvery low mass. The advantages of

all-metal hot zones include:

• Maximum quench rates

• Ultra-high achievable vacuum levels

• Excellent temperature uniformity

• Faster heating rates (due to theirhigh ratio of radiating surfaces)

• Accelerated pump-down rates, and

• Titanium processing ability (nocarbon contamination concernsfrom graphite insulation).

Pure molybdenum is the most widelyused refractory metal for hot zones.However, pure molybdenum componentsrecrystallize after the first process runand grain growth takes place, makingthem susceptible to physical damage dueto loss of ductility. In addition, puremolybdenum deteriorates upon contactwith “dripping” nickel braze alloy run-off.Alternative molybdenum alloys includeTitanium Zirconium (TZ) andMolybdenum-Lanthanumoxide (ML)alloys with higher recrystallizationtemperatures and higher strength. Onepowder metallurgy product, PM2000, isan oxide-dispersion-strengthened (ODS)superalloy that offers advantages forfurnace brazing applications.

Fixtures

The workload in most vacuum furnacesis placed on a tray or in a basket madefrom molybdenum, austenitic stainlesssteel, or Inconel. The work-supportingtray usually rests on a graphite or metallichearth consisting of three or fourhorizontal bars supported by piers risingfrom the furnace shell below. It isessential that metallurgical reactionsbetween the hearth and the work

tray/basket be taken into consideration.Thin sheets or tubes of ceramic materialare often used to separate the fixtures.

Heating Elements

Heating elements must not deteriorateat high temperatures, must radiateconsiderable amounts of energy, andmust be easily replaced. Heating elementmaterials for high temperature vacuumfurnaces include refractory metals(tungsten, molybdenum, and tantalum)and pure graphite (solid bar or wovencloth) as shown in Figure 7.

Molybdenum is the most widely usedrefractory metal element and undergoesa large increase (500%) in electricalresistivity between room temperatureand normal operating temperatures.Tungsten is capable of withstandinghigher operating temperatures. Graphiteis less expensive than metallic resistorsand has up to a 20% decrease in electricalresistivity as it heats. Low voltage(generally, <70 volts) power supplies arenormally used because a high electricalpotential can produce short-circuitingof the elements by ionizing the residualgases in the chamber. Rated power rangesfrom 40 kW to 300 kW.

Vacuum Pumps

Pumping systems are usually dividedinto two subsystems: the roughing pumpand the high-vacuum pump. Pumps areclassified as mechanical, diffusion, orcryogenic types and are often used incombination. The vacuum system shownin Figure 8 consists of a diffusion pumpwhich is initially isolated from themechanical roughing/forepump byclosing both the high-vacuum valveand foreline valve. When the chamberpressure is sufficiently low, the roughingvalve is closed and the other two valvesopened to allow the diffusion pump toachieve the final vacuum level.Figure 6. All-metal Hot Zone

Figure 7. Heating Elements

Figure 4. Multiple Chamber Vacuum Furnace

Load/Preheat High Heat Cooling/Unload

Load Transfer TransferVacuum Unload

EvacuatePreheatDegasDewaxDebind

SinterBrazer

Heat TreatHot Press

Diffusion Bond

Gas QuenchPost Process

Figure 5. Cold-wall Vacuum Furnace

Radiation Shields

Heating Elements

InternalGas-DistributionSystem

Quench GasReturn Path

Hearth

Autoclave Door

Water-JacketedFurnace Wall

High VelocityQuench Gas

Turbo Blower

Internal HeatExchanger

High HPMotor

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The level of vacuum achieved can bestated in several different units ofmeasure. The standard absolute pressureof the atmosphere can be expressed inthe following units:

1 atm = 760 torr = 760 mm Hg(height of a mercury column)

= 29.21 in. Hg= 14.696 psia= 1 x 105 Pa

The general range of vacuum achievedfor vacuum furnaces is shown in Table 2below.

Mechanical pumps are positive-displacement pumps that use pistons orrotating vanes to compress and expel air.Oil-sealed rotary mechanical pumps canachieve a soft vacuum of 0.025 torr(3.3 × 10-3 kPa) (25 µm Hg). A rotaryvane booster pump connected upstreamfrom a rotary piston pump can achieve avacuum of 0.01 torr (1.3 × 10-3 kPa). Oilvapor diffusion pumps can achieve a hard

vacuum of 10-4 torr (1.3 × 10-5 kPa). Toachieve the highest vacuum level(10-6 torr) (1.3 × 10-7 kPa), a cryogenicpump is used in combination with amechanical forepump.

Oil vapor diffusion pumps allow airmolecules to randomly diffuse into thethroat of the pump where a preferreddirection of motion is imparted bymomentum transfer. The diffusion pumpconsists of a boiler for vaporizing specialhighly stable liquids with very low vaporpressure, concentric vapor chimneys that

rise out of theboiler, and awater-cooledpump housing.Vaporized pumpoil is sent up thechimneys andthen abruptlyejecteddownward bythe chimney

nozzles. Molecules from the vacuumchamber are caught in the downwardstream of heavy oil molecules anddirected to the mechanical forepump.Pump vapor is condensed on the cooledinner walls and is returned to the boiler.Cryogenic pumps condense gas moleculeson refrigerated surfaces at -320°F(-195°C). These pumps are regeneratedperiodically by heating the condensing

surfaces to vent theaccumulation ofcondensed gases.

Process ControlInstrumentation

Recent advances invacuum furnacetechnology have beenmost apparent inprocess control

instrumentation. The trend is to use“open architecture” personal computer-based software rather than proprietaryhardware to control industrial processessuch as heat treating. Temperaturesensing is usually accomplished bythermocouples made from nickelmolybdenum, platinum rhodium, ortungsten-rhenium. Pressure sensing isaccomplished by measuring a physicalcharacteristic of the gas that bears a directrelationship to the pressure (e.g. thermalor electrical conductivity). Proportioningcontrollers are used to bring the parts upto temperature and avoid temperatureovershoot due to the thermal inertia ofthe furnace and parts. The proportioningcontroller responds to an error signalcomparison (set-point vs. actual) byconsidering the size of the error signal(Proportional band or gain), the errorsignal size plus the length of time it hasexisted (Integral reset), and the errorsignal speed of change (Derivative rate).Thus, the controller is often referred toas a PID, or three mode, controller.

Quenching

Rapidly lowering the temperature ofheated parts by contact with a quenchantis just as important as the method ofheating. The trend is to replacequenching in liquid media (salt baths,oil, water, etc.) by inert gas quenching.Increasing quench gas pressure is morecost effective than increasing gas flowto achieve higher cooling rates. Anothertrend is to use gas blends of eithernitrogen or argon with helium atincreasingly higher pressures. The relativecooling rates and cost of hydrogen,helium, nitrogen, and argon are plottedin Figure 9. For a given furnace, blower,and gas pressure, helium will cool a givenworkload twice as fast as nitrogen but iseleven times as expensive!

Table 2. Vacuum Ranges for Furnaces

Vacuum Level Torr kPa

Rough 750 to 1 100 to 1.3 ×10-1

Soft 0.1 to 10-3 1.3 × 10-2 to 1.3 × 10-4

Hard 10-4 to 10-6 1.3 × 10-5 to 1.3 × 10-7

Figure 9. Effect of Gas Type on Relative Cooling Rate and Costs

0

2

4

6

8

10

12

Relative Cooling Rate Relative Cost

Hydrogen

Helium

Nitrogen

Argon

Figure 8. Typical Vacuum System

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Quenching in high temperature vacuumfurnaces is accomplished by backfillingthe chamber with an inert gas to promotethe convective transfer of heat from theparts. There are three primary factorsthat govern heat transfer in vacuumfurnaces: the gas cooling or heat transfercoefficient, H; the difference betweenthe temperature of the load and therecirculated gas; and the surface area ofthe load exposed to the gas. Quenchinggas is recirculated through a water-cooledheat exchanger. The temperaturedifference and surface area factors remainalmost constant for a specific application.The cooling coefficient is a measure ofthe rate of heat removal per unit area perdegree of temperature. It is proportionalto the gas type, quench pressure andblower horsepower (an indirectapproximate of gas velocity).

Relative values of heat transfercoefficient (H) for various gas blowers(in horsepower) are shown in Table 3.If all other factors for a given furnaceare constant, doubling fan power willincrease cooling performance by roughlyonly 20%. Changing from one coolinggas to another, however, also necessitatesa change in the blower fan to maintain aconstant horsepower. For example, afurnace backfilled with helium instead ofnitrogen would only load the blower to14% of full current. The net effect wouldbe an improvement of only 25% incooling performance rather than 200%.

A doubling of gas quench pressure willboost cooling performance by about 40%.Quenching systems with 2, 4, 6 and10 bar pressures (200, 400, 600, and1,000 kPa) are readily available. One bar(100 kPa) is slightly less than onestandard atmosphere of absolute pressure.

Systems with 20 bar (2,000 kPa)capability are just starting to appear.Helium-argon or helium-nitrogenmixtures are routinely used by agrowing number of heat treaters.Hydrogen gas is explosive and mustbe used with extreme care.

Advantages

Vacuum furnaces remove almost all ofthe components associated with normalatmospheric air (O

2, N

2, and water vapor)

from the heating chamber before andduring heating, thus offering thefollowing advantages compared tonon-vacuum furnaces:

• Almost complete removal of oxygenand water vapor

• Extraction of deleterious gases,surface contaminants, andprocessing lubricants

• Achieving a vacuum is often fasterthan purging with inert gases

The advantages of an electrically heatedvacuum furnace compared to gas-firedvacuum furnaces include:

• Much more energy efficient at hightemperatures

• More precise ramp-up and cool-downtemperature control

Electric vacuum furnace processing is alsohelping to achieve long-term industrygoals. The heat treating industry’s desirefor the future was published by ASMInternational as the Heat Treating IndustryVision 2020 which included the followingindustry goals:

• Reduce energy consumption by 80%

• Approach near zero emissions

• Reduce production costs by 75%

• Reduce processing times by 50%

Vacuum furnace processing has improvedperformance in each of these areascompared to gas-fired furnaces.

Competing ProcessesThe higher the temperature of a metal,the more sensitive it is to reacting withthe atmosphere around it. Direct fuel-fired furnaces require a supply of oxygenin the heating chamber to supportcombustion and introduce water vaporinto the chamber. An atmosphere withoxygen promotes oxidation (corrosion,

rusting and scaling), tarnishing, anddecarburization of ferrous alloys heated toelevated temperatures. Indirect-fired gasfurnaces using a muffle/retort canincorporate a neutral atmosphere ofnitrogen gas to protect the workpiece, butheat transfer is uneven and inefficient,especially at high temperatures.

The greatest growth in applications forvacuum furnaces is coming fromhardening, brazing, and sinteringapplications above 1,800°F (980°C).The energy efficiency of gas-firedfurnaces decreases rapidly at temperaturesabove 1,800°F (980°C), significantlyincreasing operating costs.

Salt baths are still considered competitionto vacuum heat treating. A number ofheat treaters have invested in up-gradingtheir salt bath installations to complywith current EPA/OSHA requirements.This is due mainly to the reluctance ofjob shop customers to invest in theresearch necessary to prove to themselvesthat a switch of technology will notadversely affect their parts. Salt bathtechnology is still useful for someapplications, however, each year fewerand fewer salt baths remain in operation.

Fluidized-bed furnaces offer anothermethod of high-temperature heattreatment. This method uses a solidrather than a liquid or gas for the heattransfer medium. The furnace iscomposed of a layer of permeable particlesof an inert refractory (aluminum oxide orsilica sand) in a container that is heatedand agitated by a flowing stream of gas.The gas flow lifts the particles andimparts to them a violent turbulentmotion. Although the particles areactually solid, the fluidized bed simulatesthe motion of a liquid. Heat is transferredfrom the particles to an immersed objectat a rate two to ten times higher thanprovided by normal convection orradiation. The heat source can be gas orelectric.

Economic ConsiderationsThe following example calculates thetotal cost (excluding overhead andmaintenance) for vacuum annealing at asoak temperature of 1,900°F (1038°C).Tables 4, 5, and 6 show the specificationsused for this calculation.

Table 3. Effect of Blower Horsepower onCooling Coefficient

Blower power, Relative coolinghp (W) coefficient, H

50 (37,300) 0.85

100 (74,600) 1.0

150 (111,900) 1.1

200 (149,200) 1.2

300 (223,800) 1.3

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TC-113555

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The total cost for this example is shownin Table 7 below. Note that no overheadis included in the labor cost and noroutine maintenance costs are included.In this example, the $37.30 total electricpower cost represents nearly 25% of thetotal fixed plus variable costs to operatethis vacuum furnace for heat treating.

SummaryHigh temperature (>1,800°F or 980°C)vacuum furnaces are an excellentreplacement for salt bath heat treating,brazing, and sintering applications. Gas-fired furnaces cannot operate efficiently at

Table 4. Vacuum Annealing Equipment Specifications

225 kW hot zone power supply capacity

20 hp mechanical pumping motor

75 hp gas recirculating fan motor

7.5 hp vacuum booster motor

18 kW diffusion pump heater

2 kW isolation transformer for controls

Total electric load = 322 kW (1 hp = 0.75 kW)

Table 6. Vacuum Annealing Cost Specifications

$350,000 installed cost for furnace

$0.06/kWh electricity cost (1 hp = 0.75 kW)

$15/hr direct labor rate (no overhead)

$0.55/100 scf for nitrogen gas

$20,000 fixtures cost over 5 years

$75,000 hot zone replacement every 4 years

Table 7. Total Cost for Vacuum Annealing Cycle (4 hr)

Furnace ($12.65/hr* x 4 hr) $50.60

Electric: Heat-up 16.20Soak (hold) 3.40Quench (cool) 6.75Various components 10.95

Direct Labor 45.00

Process Gas 2.75

Fixtures, baskets, grids 3.50

Water System 1.00

Hot Zone ($4.15/hr* x 4 hr) 16.60

TOTAL $156.75

*Installed cost financed over 4-7 years at 10% interest rate

these high temperatures. Electric powercosts can represent one-fourth to one-third of the total costs to operate avacuum furnace for heat treating. Onceinstalled, vacuum furnaces areincreasingly utilized for treating partspreviously processed by gas-fired furnaces.Electric vacuum heat treating furnacesalso contribute to meeting the long termgoals of the heat treating industry.

Sources used in this issue ofTechCommentary were:

ASM Handbook, Heat Treating, Volume4, 1991.

George C. Carter “Optimizing GasQuenching.” Advanced Materials andProcesses, February 1996.

A. Bruce Craven “Energy Dominates inJob Costing for Vacuum Furnaces.”Industrial Heating, February 1996.

Klaus H. Hemsath “Heat-treating in the21st Century - the Role of Gas-FiredVacuum Furnaces.” Metallurgia, January1999.

Thomas P. Farrell “Technically AdvancedMaterials for Vacuum Furnace HotZones.” Metallurgia, March 1998.

Table 5. Vacuum Annealing Process Specifications

20°F (11°C)/minute heating ramp @ 80% capacity

300 CF quench gas volume requirement

1/2 hour soak time

2 hour quench with nitrogen