The furnace atmosphereplays an important role in meeting heat treatedpart requirements. Control systems for these atmospheres are an integral part of the qualitysystem leading to the production of high-quality,high value-addedcomponents.

Rolf Andersson, Javier Canala, Torsten Holm**, and Maria KöhlerLinde Gas, Lidingö, Sweden

**Member ASM International and member,ASM Heat Treating Society

Heat treatment is one of manyprocess steps involved in themanufacture of an automo-bile, which adds huge value

to the final product. Even small heattreating process improvements at rel-atively low cost create a high value.For instance, heat treatment accountsfor approximately 6% of the produc-tion cost for a gearbox, but increasesthe value of the component by at least25%, probably more.

Several types of heat treatment areinvolved in automobile productionincluding carburizing (gears andtransmission parts, for example) andfinal heat treatment operations suchas hardening, carbonitriding, nitro-carburizing, and nitriding. Thecommon element for these final heat treatment processes is that they sig-nificantly increase the performance-to-weight ratio of parts (e.g., theweight of a gear box would be at leastfive times greater if not carburizedand hardened). Different annealingoperations are used for semifinishedproducts, such as forgings, strip, tube,and wire that will be used in differentautomotive parts. These processesoccur earlier in the process chain. Ad-

ditional heat treatment processes in-volved include brazing (e.g., coolers)and sintering of powder metallurgyparts.

Freedom from decarburizationand oxidation during annealing andhardening, carbon potential and casedepth control in carburizing, andproper wetting in brazing are exam-ples for which the atmosphere con-trol is crucial. Some further examplesof the importance of atmosphere con-trol are shown in the table.

Process Product Issue Benefit

Annealing Side impact Avoid Strength, safetydoor beams decarburization

Annealing Steel strip for chassis Surface cleanliness Surface finish

Carburizing Gears Case depth control Durability, safety

Hardening Bearings Avoid Strength, safetydecarburization

Nitrocarburizing Crankshafts Compound Durability, safetylayer control

Brazing Cooler Achieve good Durability, wetting reliability

Sintering Hub synchronizers Carbon control Durability, safety

ATMOSPHERECONTROLCLOSE

TRANSLATES TOHIGH-QUALITY HEAT-TREATED AUTOMOTIVE COMPONENTS

Gears being loaded into a pusher furnace

HEAT TREATING PROGRESS • MARCH/APRIL 2006 29

Fig. 1 — Benefits of using proper atmosphere control in heat treating automotive parts

Dynamic Atmosphere ControlA dynamic control system can

recognize furnace atmosphere dis-turbances and automatically adjustthe gas mixture or the gas flow intothe furnace according to set points,which ensures that required qualitystandards are met during heat treat-ment. Different atmosphere distur-bances may occur in the furnace,such as from furnace leaks, duringloading and unloading, etc. Figure2a shows how a constant flow rateof the gas into the furnace may re-sult in varying atmosphere compo-sition. By comparison, Fig. 2bshows how flow adjustments makeit possible to continually maintainthe atmosphere composition at setpoint. Dynamic control has an im-portant impact on quality of treatedparts and on total gas consumption.The blue sections of the curve in

Fig. 2b illustrate potential gas sav-ings.

The features of the dynamic con-trol system are:

• Control of furnace atmospherecomposition

• Safety purging procedures andprocess alarms

• Purging function to protect ana-lyzing instruments

• Optional function for data log-ging

• Optional function for remote su-pervision

Examples of systems and relatedresults for annealing, and carbur-izing, and brazing are describedbelow.

Annealing of Steel TubesIncreasing demand by the automo-

tive industry for improved partquality in regard to carbon controlwas the motivation to developLinde’s Carboflex® dynamic controlsystem for steel tube annealing basedon the combination of nitrogen andendo gas atmospheres (Fig. 3). Twodifferent gas streams (nitrogen andendo gas) can be adjusted independ-ently, providing the flexibility toproperly adjust the atmosphere com-position. This flexibility meets thedifferent requirements that the work-load encounters during the annealingpass through the furnace.

The control cabinet contains a PLCfor control and a PC with a touchscreen as the HMI. Setting atmos-phere parameters, handling recipes,setting alarms, start up and stop op-erations, calibrating analyzers, andviewing actual furnace atmospheredata are made from the PC touchscreen.

The system offers:• Accurate control of tube surface

carbon content• Optimized recipes that adapt the

proper gas flow and atmospherecomposition to a given tube dimen-sion and tube quality requirements

• Reduced rework costs associatedwith oxidized, decarburized, and dis-colored tubes

• Special idling programs to min-imize gas consumption during off-production periods

• Significantly lower gas consump-tion compared with exogas systems

Figure 2 — Comparison of fixed gas flow(a) and dynamically controlled gas flow (b)and the relationship to the atmosphere com-position

a

b

Figure 3 — CARBOFLEX gas system with nitrogen and endogas supply and atmospherecontrol based on gas analysis

Figure 4 — Design of an atmosphere control system including analysis of CO and CO2

30 HEAT TREATING PROGRESS • MARCH/APRIL 2006

Carburizing in a Pusher FurnaceAccurate atmosphere carbon po-

tential control in carburizing furnacescan be used to shorten carburizingtime (increase productivity), to de-crease the scatter in final carbon con-centration and case depth, and totailor the carbon concentration depthcurve. This can be further enhancedby using an improved atmosphereanalysis. It is common to base atmos-phere control on the assumption thatCO and H2 concentrations are con-stant. Carbon potential control is ac-cordingly based on the actualanalysis of only one atmosphere con-stituent (typically CO2 concentration)or the oxygen potential measuredusing an oxygen probe. However,this does not take into account the re-lationship between CO concentrationand carbon potential. For control ofCO2, the relationship is expressed by:

Cpot =constant (vol% CO)2

(vol% CO2)

Consider, for example, a situationwhere the atmosphere carbon poten-tial is controlled at a temperature of930°C (~1700°F) by CO2 analyses tobe 0.80%C, with the assumption thatthe CO concentration is 20 vol%. Ifthe actual CO concentration is 18vol% instead of 20 vol%, the resultingcarbon potential at 930°C will not be0.80, but will be 0.68%C instead. TheCarboflex system (Fig. 4) avoids thisinaccuracy by adding the COanalysis as a controlling value. Avari-ation of the atmosphere compositionalong the furnace in this type of fur-nace is not uncommon, which makesit even more important to analyze theCO to obtain acceptable accuracy inthe carbon potential control.

In addition, a simulation programto calculate carbon concentration pro-files from process recipes is used tominimize the carburizing time. Com-bining this with accurate atmospherecontrol (Fig. 4) resulted in shorterprocess times due to the improvedprecision in obtaining target casedepths and surface carbon concen-trations, postponement of invest-ments in new furnaces due to approximately 15% increased carbur-izing capacities, and the possibilityto tailor hardness/residual stress pro-

files for optimum fatigue properties[1].

Brazing of Steel PartsBrazing stainless steel injectors and

fuel pipes for cars typically is con-ducted in a mesh belt furnace (Fig. 5)using a hydrogen-nitrogen atmos-phere. AHydroflex® dynamic atmos-phere control system consists of anatmosphere supply system for ni-trogen and hydrogen, an analyzingsystem (sampling flow train and an-alyzing instruments), gas flow con-trol, and PC or PLC and related soft-ware and a touch screen as thehuman machine interface (HMI).

The analyzing instruments and theHMI are located in the control cab-inet (Fig. 6). Agas sample taken fromeach furnace zone is pumped to thecontrol cabinet to measure oxygen,dew point, and hydrogen.

Two control loops of the system areshown in Fig. 7. Total nitrogen-hy-drogen gas flow and the nitrogen-hy-drogen ratio are controlled by theoxygen potential of the atmospherein the heating zone (calculated fromthe measured dew point and hy-drogen concentration). Nitrogen flowis controlled by the oxygen content(ppm O2) in the cooling zone.

An increase in the actual atmos-phere oxygen potential above the setpoint first increases the total nitrogen-hydrogen flow. However, if the setpoint is not reached, the nitrogen-hy-drogen ratio is reduced (increasedshare of hydrogen). In the coolingzone, an increase in the atmosphereoxygen content (ppm O2) increasesthe nitrogen flow. The set points foroxygen potential and ppm O2 in eachof the control loops are defined fromtheoretical and experience values.

The system has extra functions for

Figure 5 — Parts entering a brazing furnace

Figure 6 — HYDROFLEX control cab-inet including different components and an-alyzers

Figure 7 — Illustration of control loops

HEAT TREATING PROGRESS • MARCH/APRIL 2006 31

safety and operational purposes,which can be integrated into thesystem. The following are examplesof possible safety sequences:

•Low furnace temperature alarmshuts down the system and initiatesnitrogen purging.

• Low nitrogen pressure (nitrogensupply) shuts down the system

• Extinction of safety flames at fur-nace entrance or exit shuts down thesystem, but nitrogen flow remains

• Oxygen concentration higherthan 800 ppm results in a soundalarm and initiates nitrogen purging

• A dew point higher than +18°C(+64.5°F) results in a sound alarmand initiates nitrogen purging

• Low nitrogen flow results in asound alarm

• Low hydrogen flow results in asound alarm

Experienced benefits of the Hy-droflex system include:

• Process stability and reactionagainst external atmosphere distur-bances

• Less scrap and rework • Recipes having process set points

for required parts quality• Optimum gas consumption• Increased furnace use due to

quick start up• User-friendly screen interface

showing different views and accesslevels

• Easy implementation at existinginstallations and furnaces

• Possibility to customize views,specific functions, etc., according torequirements

Brazing of AluminumBrazing aluminum coolers and air-

conditioning equipment is com-monly performed in continuous fur-naces using a pure nitrogen furnaceatmosphere. A flux is used to im-prove wetting. The amount of fluxneeded can be minimized by properatmosphere control; that is, bykeeping the oxygen and the water-vapor concentrations in the atmos-phere below certain maximum limits.Both oxygen and water vapor enterthe furnace atmosphere throughleaks and furnace openings. Figure8 illustrates the importance of atmos-phere control for brazing quality,showing the brazing result for an at-mosphere having less than 50 ppmoxygen, but with a varying dewpoint.

Lowering the dew point has a con-siderably positive affect on braze-ability. For the lowest flux load (1.1mg/m2), the dew point must belower than -45°C (-50°F) to achieve“very good” wetting and braze-ability. For the highest flux load (3.2mg/m2), a dew point of -35°C (-13°F)is sufficient to obtain good braze-ability. Tests using an atmospherecontaining 400 ppm oxygen resultsin “bad” or “poor” brazeability forall combinations of flux loads and at-mosphere dew points. Dynamic at-mosphere control following the prin-ciples described produces highquality brazing results with min-imum flux consumption and gas sav-ings on the order of 20 to 35%.

Figure 9 — Schematic of data collection system

Figure 8 — Relationship between brazeability and atmosphere concentrations of O2 andH2O. Source: Ref. 2

Lowering the dewpointhas a considerably

positive affect on thebrazeability of

aluminum.

34 HEAT TREATING PROGRESS • MARCH/APRIL 2006

Data CollectionData collection is an additional op-

tion that can be added to the dynamiccontrol systems. With the system,data from the central factory super-vision system are combined withlogged process data from the controlsystem. The combined data arestored and can be viewed and eval-uated for statistical analyses forprocess improvement. The historicaldata also are used for traceability andto minimize the number of partsscrapped after furnace disturbances.Figure 9 shows a principal set up fordata collection.

The process control PC receivesidentification data for a specific batcheither from a bar code reader ormanual input. The data-collectiondatabase stores the input data (in-cluding date and time) and matchesit with order details and heat treat-ment recipe for the order from thecentral supervision system. When theheat treatment process is started,process data are logged and addedto the database. The data registeredcan be viewed online on the processdata screen (Fig. 10).

The collection of process data isstopped when the batch leaves thefurnace, and all information for a spe-cific batch is stored first on theprocess PC and later transferred tothe central supervision system. Thesystem matches each batch with itscorrect information even if the oper-ation is interrupted and batches areinterchanged or divided.

Remote SupervisionRemote monitoring of processes

has long been available. The tech-nique in combination with dynamicatmosphere control systems opensnew possibilities for heat treatment.The responsibility of supervision (thecorrection of irregular operation andmaintenance) can easily be trans-ferred to an external party at a remotesupervision center. (The party as-suming this responsibility must ofcourse have the required heat treat-ment expertise.)

Following is an example of remotesupervision of a hardening line. Thehardening atmosphere itself is locallycontrolled by a dynamic Carboflexsystem, but the system and atmos-

phere supply are remotely monitoredand supervised.

Figure 11 shows the system com-munication set-up. The signals forgas-flow, pressure, temperature, etc.from the endo gas generator and ni-trogen supply are sent through a se-cure internet connection to the re-mote supervising center. Thesupervision center is responsible forproper actions taken by the serviceengineers when needed within aspecified time. The system is in thiscase not actively controlled remotely,only supervised. All alarms areshown on the remote screen and theyalso appear as SMS messages sent tothe service engineer on duty.

Figure 12 shows a remote screenview during supervision. The displayshows the pressure of the incomingnatural gas and air mixture; endo gas

generator (catalyzer) data for tem-perature; CO, CO2, and CH4 concen-trations; and dew point. Other viewsshow set points, nitrogen pressure,need for back up gas, etc. Data arecollected and stored as describedabove.

To ensure gas delivery, an addi-tional endo gas generator was in-stalled as back up, as well as gascylinder storage with carbonmonoxide and hydrogen gas. Theseback up supplies enter automaticallyif the regular supply fails and enablesthe gas supplier to guarantee a con-tinuous ready-made furnace atmos-phere supply 24 hours a day, sevendays a week. The gas-supplier is inthis case also the owner of the gener-ators and the nitrogen supply tank.

Remote supervision is a tool thatalso enables an external party having

Figure 10 — CARBOFLEX data collection screen

Figure 11 — Remote Internet supervision set up

HEAT TREATING PROGRESS • MARCH/APRIL 2006 35

the required heat treat-ment expertise (in thiscase the gas supplier) tobe responsible for thefurnace operation andmaintenance. This newconcept for supply andremote monitoringopens possibilities for theheat treater to out-sourcepart of its operation.

SummaryControl systems for heat

treatment are integral parts of thequality system and are used to con-tinually improve process capabilities(e.g., reducing tolerances, etc.), pro-ductivity (reduced process time,labor, storage, etc.), safety (handlingof flammable gases etc.), and down-stream operations (easier grinding,surface coating, etc.), parts proper-ties (strength, dimensions, scatter,etc.), as well as to provide input fordevelopment (new materials, newspecifications, etc.). Benefits of suchsystems include increased product

value, reduced manpower needs, re-duced rework costs, process docu-mentation, minimized gas consump-tion, failure tracking, and partly (butnot fully) compensation forlack of specialist competence.

Acknowledgement: The authors grate-fully acknowledge the valuable adviceon preparing this article from Kaj Svart-ström (formerly plant manager at Volvo).

References1. Holm, T., Arvidsson, L., and Thors, T.,IFHT Heat Treatment Congress, Florens,19982. Claesson, E., Engström, H., Holm, T., andSchölin, K., VTMS Conf., London, May 1995

For more information: Dan Moderick,applications engineer, Linde Gas LLC;tel: 216-573-7843; fax: 216-573-7873; e-mail: dan.moderick@us.linde-gas.com;Internet: www.linde-gas.com. This articleis based on “Atmosphere Control in HeatTreatment for Creation of Quality andValue in Automotive Materials and Com-ponents,” a paper presented at the ASMHeat Treating Society Conference in Sep-tember 2005.

Figure 12 — Example of remote monitoringscreen

36 HEAT TREATING PROGRESS • MARCH/APRIL 2006