HEAT PROCESSING · (6) · ISSUE 2 · 2008 1

REPORTSVACUUM TECHNOLOGY

IntroductionSingle atmospheric heat treating fur-naces that separate the heat processfrom the quench process have beenavailable since the first atmospherebased internal quench furnaces wereintroduced in the mid 1900s. Similarly,vacuum furnaces that separate the heat-ing processes from the oil quenchprocesses have been on the market fordecades. It is only in recent years, sincethe viable commercial development ofacetylene based vacuum or low pressurecarburizing, that the technology reaf-firmed a 1994 ALD development byincorporating high pressure gas quench-ing into the two chamber vacuum fur-nace scenario.

This two chambers or “cold chamberquenching” concept titled DualTherm®

(Fig. 1), integrates the benefits of vac-uum processing and high pressure gasquenching however takes each to itslogical conclusion by applying threebasic rules:

• When at vacuum and at temperature,stay that way.

• When quenching, quench only whatyou need to quench.

• Load movement mechanisms don’tbelong in either process environ-ment.

Intuitively, it makes sense to separatethe processes of heating, cooling andload movement. Having a specificdesign to accommodate the unique

requirements of each function providesthe best opportunity for to get the mostfrom each function.

Staying at steady temperature and vac-uum provides numerous advantagesincluding:

• Workloads can heat up as fast as thematerials are able to absorb the heat

• Energy is applied to heat the load,not the hot zone and insulation foreach cycle

• No thermal cycling of the hot zonemeans it will last years beyond that ina conventional batch vacuum furnace

• Much higher efficiency hot zonedesigns can be employed since nocooling system is required

Dual vacuum furnaces with separated heating and cooling processes areproviding several advantages when compared to single chamber vacuumfurnaces. This includes faster heating up, less energy consumption, morequenching severity and less maintenance. Accordingly a new, “secondgeneration” of dual vacuum furnace for low pressure carburizing andhigh pressure gas quenching, DualTherm®, was developed using proventechnologies from the ModulTherm® series of vacuum furnace systems.The first furnace was recently installed at a commercial heat treater andis now in three-shift-operation.

Dual chamber vacuum furnace for Low Pressure Carburizing (LPC)and High Pressure Gas Quenching (HPGQ)Klaus Löser, Karl Ritter, Bill Gornicki

Fig. 1: Dual chamber vacuum furnace, type DualTherm®

HEAT PROCESSING · (6) · ISSUE 2 · 20082

VACUUM TECHNOLOGYREPORTS

• Hot zone is not stressed by high-pres-sure/high-speed gas flow duringquenching process resulting in signif-icant extension of life-time of hotzone

• Lower energy levels are required dueto higher efficiency hot zones

Cold chamber quenching advantagesinclude:

• Greater cross sections of loweralloyed materials can confidently bequenched

• Ultimate control of gas distributionfor a more uniform quench

• Simplified baffling system enablesalternating gas flow paths for quenchdistortion control without interfer-ence from load handling or loadheating equipment

• Fastest backfill speed to maximumpressure is possible without fragilehot zone concerns

• Separated load movement mecha-nism advantages include:

• Mechanism sees hot zone tempera-ture only when loading/unloadingheat chamber.

• Mechanism does not provide obsta-cles for quench gas flow paths.

• Motors and switches are located out-side the vacuum environment

When comparing to standard, singlechamber batch vacuum furnaces,DualTherm® furnaces stand alone.Focusing the entire quenching effort onthe workload enables quenching ofcomparatively lower allowed metals.Concerns of imploding hot zones are

eliminated making a 3 second backfill to20 bar routine thereby getting thequench speed “ahead of the curve”.Further, the hot zone insulation erosionproblems due to high velocity coolinggas flows don’t exist in this configura-tion. Naturally, this significantly reducesroutine hot zone maintenance and elim-inates the “grit blasting” of the work-load during quench.

Further, while LPC is possible in a stan-dard batch vacuum furnace, it is only amatter of time before the heatexchanger behind the hot zone isplugged with soot. Then of course the

quench motor is also susceptible to car-bon soot. Not to mention the carbonsoot (being blown around within thehot zone) will cause vacuum leaks in thedoor seal. Is soot finding its way into thequenching motor? How long will it takeuntil 30 or more heating element sup-ports short to ground? These furnaces,by nature must include cooling gaspaths within the hot zone for coolinggas flow. It is well known that the car-bon from the LPC process builds up onthese “cold spots” within and aroundthe hot zone. These furnaces alsorequire a more complex heating elementarrangement and support systems thatare prone to electrical shorting. Somemanufacturers are actually endorsing“air burn-outs” of graphite hot zones.Surprisingly, it’s the same manufacturersthat have cursed air in graphite hotzones since their inception. Endorsed bythe manufacturer or not, this will signif-icantly limit hot zone life.

The latest generation of DualTherm®

was developed using proven technolo-gies from the ModulTherm® series ofvacuum furnace systems. With morethan a hundred treatment chambers inoperation worldwide, these “systemmodules” have withstood the test oftime and proven their abilities.

Plant assemblyAs said, the DualTherm® vacuum fur-nace is a two-chamber vacuum heatFig. 2: Schematic sectional view of DualTherm®

Fig. 3: Comparison of Heating-up characteristics using vacuum and convective heatingprocesses

HEAT PROCESSING · (6) · ISSUE 2 · 2008 3

REPORTSVACUUM TECHNOLOGY

treatment system. A schematic sectionalview of the system is shown in Fig. 2.

The heating chamber is equipped with a“tight” hot zone (no cooling pathsneeded) encompassing multiple layersof graphite/ceramic insulation. Thegraphite heating elements are arrangedaround the sides of the workload ensur-ing fast and uniform heating of theworkload to a guaranteed +/-5°C uni-formity. Naturally, all modern day vac-uum furnaces provide the option forconvection heating and DualTherm® isno exception. For this purpose, afterevacuation, a carrier gas (nitrogen) is fedinto the heating chamber; this is circu-lated during heating.

Convective heating offers several advan-tages including:

• Lower temperature gradient on indi-vidual components and within theworkload

• Faster heating rates; particularlyeffective on densely packed work-loads

• Uniform heating throughout theheating event

Fig. 3 shows a comparison betweenvacuum and convective heatingprocesses. A densely packed load ofbolts with 25 mm Dia. was heated to890°C. Heating curves were recorded bymeans of a furnace tracking systemusing thermocouples in the core ofbolts, equally distributed within theload. The heating curves for bolts withmaximum and minimum heating ratewere recorded for both heating tech-nologies. As shown in Fig. 3 convectiveheating leads to a significant improve-ment in heating uniformity while thetotal heating time could be reduced by30%.

The treatment chamber hearth is of sili-con carbide (SiC) featuring high wearresistance even at very high tempera-tures. There are no moving parts in theheating chamber, apart from the con-vection fan (made of CFC material) forthe convective heating option.

For transporting workloads within theDualTherm® a set of telescoping forksare housed in the transport area. Thedrive mechanisms and sensor units aswell as the transport system sensorguides are all located outside the vac-uum environment for maintenanceease.

For quenching the workload a modularchamber was developed; capable ofbeing adapted to the requirements ofthe specific customer components. Thequenching chamber can be operated atpressures up to 20 bar.

Quench gas circulation is accomplishedto the specific needs of the user. Theprocess needs are identified and thequench system configuration is deter-mined using:

• One or two quench fans

• One or two heat exchangers

• Dynamic quenching capabilities

• Reversing gas flows, particularly use-ful in distortion control for partweights in excess of 1 kg

• Variable fan speeds

These options provide a means tospecifically provide the system requiredto meet the process needs. And ofcourse, these capabilities can be retrofit-ted later as the user’s needs evolve.

An example for the positive effect ofreversing gas flow technology is demon-strated in Fig. 4. A load of truck gearswas gas quenched from 820°C with 18bar Helium by using gas flow from topto bottom and alternatively by usingreversing gas flow. The cooling curvesdetermined in the mid-tooth of gearslocated in the top and in the bottom ofthe load are shown in the upper graph(a). Being quenched from top to bottomthe gears in the top layer showed a

Fig. 4: Comparison of different gas quenching technologies and resulting hardness distribu-tion in a load of truck gears

HEAT PROCESSING · (6) · ISSUE 2 · 20084

VACUUM TECHNOLOGYREPORTS

faster cooling rate as compared to partsin the bottom layer, as one wouldexpect due the heating of the gas pass-ing the load from top to bottom. Byusing reversing gas flow technology thecooling was much more uniform show-ing almost no difference between gearsin the top and bottom layer of the load.

The different cooling characteristicsresulted in different hardness distribu-tions in the tooth of the gears as shownin the lower graphs in Fig.4. By usingreversing gas flow technology the toothroot hardness spread of the gears (b)improved from 29 HV to 11 HV. For themid-tooth hardness (c) there is almostno difference between top and bottomloaded gears as would expected by thecooling characteristic shown above (a).

For high productivity maximum load sizeand load weight is of importance.DualTherm® offers an increased loadheight of 750 mm as compared to the600 mm standard size, which can beused for additional 25% more load or toallow for especially long parts, i.g. gearshafts. While the standard load weightof this furnace category is limited to 500kg, DualTherm® offers to load option-ally up to 1000 kg.

Plant operationThe DualTherm® heating chamberremains at the user defined processtemperature during operation and,except during convection heating, isalways under vacuum. Shutdown is eas-ily initiated by disabling the heat andleaving the unit under vacuum (eitherpumping or off). When needed, heat upto temperature is achieved in approxi-mately 30 minutes.

At the start of the process the quench-ing chamber is used as a “vacuumlock”, in which the air (it’s been openedto atmosphere) is removed by simpleevacuation. Then, via the transportmechanism, the workload is chargedinto the treatment chamber for its userdefined thermal process.

Following heat treatment, the transportmechanism retrieves the workload fromthe treatment chamber and places it inthe quench chamber. This transport isaccomplished very quickly to ensureminimal workload temperature lossprior to quench. Once in the quenchchamber, high pressure gas quenchingtakes place as controlled by the applica-ble recipe (Fig. 5). In addition to maxi-mum quenching, it is also possible touse staged quenching, known as“Dynamic Quenching” where the circu-lation fan speeds as well as the chamberpressure are controlled during quench-ing.

Following high pressure gas quenchingthe components are clean, dry andbright and need not be cleaned.

ProcessesThe DualTherm® can be used forannealing at temperatures up to1250°C as well as for low pressure car-burizing up to 1050°C. When equippedwith multiple quench gases (i.e. nitro-gen, helium, argon) it is also possible totreat tool steels, rapid machining steelsor Powdered Metals (PM) in addition tolow alloyed tempering steels and casehardening materials.

The DualTherm® is also capable of:

• Vacuum brazing processes

• Annealing under convection up to950°C

• Annealing with a partial pressure upto 1250°C

• Hardening at temperatures to1250°C with different gases and witha maximum quenching pressure ofup to 20 bar

• Low pressure carburizing with highpressure gas quenching

• Low pressure carbonitriding

• Vacuum brazing with subsequent fastcooling

System control and operationFor "operating and observing" theDualTherm® includes a swing armmounted panel PC conveniently posi-tioned near the workload entry door. Atthis PC, all treatment recipes areentered, stored and executed. Oftenrequired, user specific data (load num-ber, part numbers, quantities) can beentered for each specific workload.

The PC is also the storage facility for allhistorical data including process para-meters and fault messages. Naturally,process information can be viewed inreal time and historical trends via thegraphical user interface.

Finally, the PC can be networked tomost factory systems. The informationcan also be backed up to a CD/DVDdrive. As would be expected, all furnacemovements and functions are PLC con-trolled.

Maintenance and serviceThe DualTherm® designers placed par-ticular importance on maintenanceease. All components to be serviced arepresented for easy access. For internalmaintenance a service door is providedat the rear. After the heating chamber ascooled down, and following the subse-quent release to atmosphere, the servicedoor can be opened providing easyaccess to all hot zone components.

Additionally, without the repeated ther-mal cycling (up and down) required of asingle chamber furnace, the hot zonehas to be maintained far less. One morenicety is that the fragile hot zone is iso-lated from the operator. There is nomore risk of damage due to an operatorerror.

Fig. 5: Gas quenching of a load of gear parts in the cold quenching chamber ofDualTherm® a) shortly after the start of the gas circulator and b) 5 s later

HEAT PROCESSING · (6) · ISSUE 2 · 2008 5

REPORTSVACUUM TECHNOLOGY

The vacuum pump set is fitted directly tothe furnace chamber. The pump oil is fil-tered allowing long intervals betweenoil changes, even for carburizationprocesses.

SummaryThe compromises required to own andoperate single chamber vacuum fur-naces are now being fully appreciated.This is a result of the needs for higherquenching pressures and the growingindustry requirements of vacuum car-burizing (LPC).

Separations between cooling and heat-ing processes provide the best opportu-nity for the highest performance. Spe-cific designs for each function enablethis peak performance.

When considering utilities, it’s far lessexpensive to maintain a furnace at oper-ating temperature than reheat the entire

hot zone and insulation pack with eachcycle. Also, when quenching one canget as much as possible from thequench gas choosen. This can best bedone with a chamber designed specifi-cally for quenching workloads. Thedesign results in a significant reductionof quench gas consumption. Both aresolid reasons making the DualTherm®

concept far more environmentallyfriendly.

This new, “second generation”DualTherm® was first installed late in2007 at Härte- und OberflächentechnikChemnitz, Branch Works Hohenstein-Ernstthal. In this plant, in three shiftoperation, gears and pinions forhydraulic transmission systems are car-burized at low pressure and hardenedby high pressure gas quenching.

The DualTherm® is easily integrated intothe production environment furtheradvancing it as the vacuum furnacedesign for the future. !

Dr.-Ing. Klaus Löser

Tel. +49 (0)6181 / 3073366dr.klaus.loeser@ald-vt.de

ALD Vacuum Technologies(Germany)

Karl Ritter

Tel. +49 (0)6181 / 3073279karl.ritter@ald-vt.de

ALD Vacuum Technologies(Germany)

Bill Gornicki

Tel. +1 (248) 668-4130 WGornicki@ALD-Holcroft.com

ALD-Holcroft Vacuum Technologies Co., Inc. Michigan, USA