roller hearth furnaces for heat treatment in hot f

5/23/2018 Roller Hearth Furnaces for Heat Treatment in Hot f

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Special offprint from Praxishandbuch Thermoprozesstechnik[Practical Handbook ofThermoprocessing Technology], Vol. II, 2nd edition (2011)

pp. 455-476 Vulkan-Verlag Publishers Essen/Germany

3.1.3.10 Roller hearth furnaces for heat treatment in hot-form hardening applications

Harald Lehmann

Lightweighting, in conjunction with improved vehicle safety, is an overriding engineering objectivein today's quest for more fuel efficient cars. Enhanced occupant safety, and especially superiorcollision performance, is a further major consideration. The automotive industry endeavors toreconcile these requirements through the use of, e.g., new materials and manufacturingprocesses. At present, an increasingly important role is being played by boron manganese steels,Fig. 3.149, which are used to produce safety-relevant structural auto body parts such as, e.g., side

impact protection members, A- or B-pillars, door frame reinforcements and sills, Fig. 3.150 andFig. 3.151, using a hot-forming process also referred to as press hardening or hot stamping. This

process comprises two major steps. One consists in heat treating the sheet metal to obtain anaustenitic structure, the second is a metalforming operation with rapid quenching which impartsmartensitic properties to the pressed part. The quenching process takes place inside the typically hydraulic press. The part is cooled inside the indirectly water-cooled die so as toobtain a martensitic microstructure, Fig. 3.152 and Fig. 3.153.

Fig. 3.149: Elongation vs. yield strength of flat steel products (source: Salzgitter Flachstahl)


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Fig. 3.150: Range of hot-form hardenedparts (source: Internet)

Fig. 3.151: VW Passat incorporating

hot-form hardened steel parts. Afterthe crash test at 75 km/h,the doors

could be opened without problems

(source: VW Kassel).

Hot form hardening process types

In hot-form hardening, a basic distinction must be made between two processes today. Theindirect method consists of punching a panel from a coil, cold-forming it, and then hardening this

preformed part in a heat treatment cycle.

Fig. 3.152: Red-hot center pillar in thedie before pressing (source:AP&T

Group)


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Fig. 3.153: TTT diagram of 22MnB5 (source: Internet)

Following this heat treatment, the still-hot product is placed in a hydraulic press where it is hot-form hardened in an indirectly water-cooled die. Thereafter, the parts are trimmed and thensandblasted to remove residual scale, Fig. 3.154.

In the direct process, a panel is likewise punched out from the coil but does not undergo any pre-forming. Instead, the sheet metal is placed in the furnace right away. After the heat treatment thehot panel is fed to the hydraulic press where it is hot-form hardened in the indirectly water-cooleddie. Thereafter, the pressed parts are trimmed once again. Sandblasting is skipped in most cases

because, for one thing, the parts are normally AlSi-coated which protects them against oxidation.Uncoated parts will not usually need to be sandblasted because they are processed so soon afterthe heat treatment that no significant surface oxidation can take place, Fig. 3.155.

Fig. 3.154: Principle of indirect hot-form hardening (source: Schwartz)


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Fig. 3.155: Principle of direct hot-form hardening (source: Schwartz)

Heat treatment of coated and uncoated blanks / parts

Hot-form hardening has been employed in high-volume automotive manufacturing for only a few

years. However, the principle of hot-form hardening has been known for decades, e.g., in the

production of spades where the front end of the blade is hot-form hardened. Due to the highstrength it imparts, a growing number of automotive body parts are made by hot-form hardening.Initially, walking-beam furnaces were used for the heat treatment. These, however, had the majordrawback of being not sufficiently gas-tight, which resulted in more or less intense oxidation of theproduct despite the use of large amounts of protective gas. Accordingly, steelmakers were looking

for a coating of their sheet metal which would avoid the surface oxidation of parts or blanks (i.e.,scale formation) during the heat treatment step, Fig. 3.156.

Uncoated tunnel after

hot-form hardening

Coated tunnel after

hot-form hardening

Fig. 3.156: Comparison between

a coated and uncoated part

(tunnel) after hot-form hardening

(source: VW-Kassel)


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Controlled Atmosphere Gas Tight Roller Hearth Furnaces

The Innovative Approach to Heat Treatment

Hot form hardening, hardening, annealing and sintering under protective or

reactive gas in Schwartzroller hearth furnaces -convincing to the last detail Easy integration into automatic lines High flexibility in terms of material flow and

process parameters

Uniform heating due to continuous materialflow

High availability due to mature furnace technologyCeramic conveyor rollers, radiant tubes and product trays

ensure high operating r eliability

Schwartz roller hearth components are inexpensive andparticularl ymaintenance friendly

Benefit from our extensive experience!

schwartzGmbHEdisonstrae 552152 SimmerathGermany

Phone + 49 ( 0) 2473 / 94 88-0Telefax + 49 ( 0) 2473 / 94 88-11E-mail: [email protected]: www.schwartz-wba.de

The solution of choice consists in an AlSi coating applied by a hot-dip process. This AlSi coatingalso offers the advantage of providing enhanced anticorrosion properties. On the other hand, it has

the following drawbacks:

High raw material cost

A process window of 57 minutes is required in the heat treatment to allow the AlSi material to

diffuse into the steel matrix, thereby achieving its anticorrosion effect.

During the heat treatment of AlSi-coated sheet metal, the AlSi coating will go through a fusion

phase which results in intense thermochemical attack on the ceramic conveying rollers of the

roller hearth furnace.

AlSi-coated blanks cannot be processed by the indirect method as the coating would get

damaged in the pre-forming step. Despite these disadvantages, AlSi coated parts are cost-efficient and are becoming increasingly

widespread as a result.

Roller hearth furnace

Over 80% of all heat treatment systems in use in hot form hardening applications are roller-hearth

furnaces. This furnace type has gained acceptance because of its high process reliability and

availability rates. Moreover, the beneficial effect of the conveying rollers toward uniform heating of

the sheet metal deserves to be noted.
mailto:[email protected]:[email protected]:[email protected]://www.schwartz-wba.de/http://www.schwartz-wba.de/http://www.schwartz-wba.de/http://www.schwartz-wba.de/mailto:[email protected]


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Fig. 3.157:Heat transfer resistance of rollers [3.26]

The additional heating effects acting on the blank are illustrated in Fig. 3.157 [3.26].

In hot-form hardening, roller-hearth furnaces are employed for both the direct and the indirectprocess types. The basic furnace structure is shown in Figs.3.158 and 3.159

It is true that users have repeatedly criticized these furnaces for their allegedly excessive length.However, it should be noted here that furnace length is essentially determined by two factors:

1. For AlSi-coated sheet metal, steelmakers specify a process window of at least 300 seconds.This process window must be ensured to allow the coating to diffuse far enough into the steelmatrix so that it can deliver its subsequent corrosion protection properties plus good weldability.

It has been demonstrated that equipment used only for uncoated blanks can be built up to 30%shorter than roller-hearth furnaces used also for AlSi-coated blanks.

2. Cold-rolling creates residual stresses in the sheet metal. These may cause the blank to becomedistorted when heated rapidly. Accordingly, the temperatures in the front zones of the roller-hearth furnaces must not be set too high.

Fig. 3.158: Roller hearth furnace for the direct process, commonly without product trays (source:Schwartz)


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Fig. 3.159:Roller hearth furnace for the indirect process, always with product trays (source: Schwartz)

Regardless of the above, roller-hearth furnaces are used because they achieve unsurpassed plantavailability and process reliability levels.

Roller-hearth furnace for the indirect process

Since the indirect process relies on pre-formed parts, these must pass through the furnace on

product support trays to accommodate their complex shape. Moreover, the furnaces are equippedwith entry and exit lock chambers since the parts are heat-treated in their uncoated state in theindirect process. In order to prevent oxidation of the part surface (scale formation), a furnace ofthis type needs to run with a controlled atmosphere (i.e., protective gas). The entry and exit locks

prevent air from entering the furnace.Furthermore, a furnace of this type needs a return conveyor for the product trays so that these can

be run in a closed loop. The furnace may be heated with electric power, natural gas, or using a

hybrid system. Ceramic conveying rollers are used in a furnace of this type. Only the entry and exittables and the tray return conveyor are fitted with metal rollers, Fig. 3.160.

Fig. 3.160:Roller-hearth furnace for the indirect process using support trays under shop assembly(source: Schwartz)


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The disadvantages of this furnace type can be summarized thus:

Up to 50% more installed heating power, given that the trays will cool down by up to 180 C

during their return travel. Accordingly, these trays, which have a mass of over 80 kg, need to bere-heated time and again. At an average press cycle of 20 sec. per tray, an aggregate 14 metric

tons of steel must thus be heated to raise its temperature by approx. 180 C every hour. The trays themselves are high-wear components, i.e., they have a fairly short service life due to

the cyclic thermal loads to which they are subjected. Very high maintenance costs must hencebe anticipated since the purchase price of the trays is considerable due to the high-grade steelsfrom which they are manufactured.

Advantages include the following:

Since the trays may also carry non-preformed parts, it is possible to load them with AlSi-coated

blanks as well. These will not come into contact with the ceramic conveying rollers in this case,and the much-feared thermochemical attack on the ceramic rollers is avoided.

The technology allows parts of highly complex geometry to be subjected to a heat treatmentprocess prior to hot-form hardening. However, this geometry is limited by the tray dimensions.

This type of furnace is used to heat-treat hollow axle beams, which are likewise hot-form

hardened in an indirectly cooled press after this heat treatment. The dimensions of these tubularmembers do not permit them to pass through the furnace resting directly on the conveyingrollers.

Roller-hearth furnace for the direct process

This furnace eliminates the need for product trays altogether. It is therefore of somewhat morestraightforward design than an indirect process furnace. The sheet metal is placed directly on theconveying rollers and passes through the furnace in this manner, Fig. 3.161. The furnace can besubsequently converted to product tray operation if necessary.

Furnaces of this type can be run with or without protective gas. Here, too, the furnace enclosure is

of gas-tight welded construction. The changeover from protective gas to air mode and back can bemade very quickly, even while the furnace is running e.g., during a change of tooling. Thefurnaces are heated by means of gas, electric power, or a hybrid system.

Fig. 3.161:Roller-

hearth furnace for the

direct hot-form

hardening process

(source: Schwartz)


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One key drawback of this furnace type is that AlSi-coated product will rest directly on the ceramic

conveying rollers, resulting in intense thermochemical reactions between the AlSi coating and theceramic rollers. This effect calls for relatively frequent roller replacements which may impact plantavailability.

The advantages of this furnace type consist in its high flexibility regarding load patterns, for unlikea furnace using product trays, it does not impose any fixed load size. Their installed heating poweris lower than that of a direct-process furnace.

Roller-hearth furnaces with ceramic rollers for hot-form hardening applications

The heating of AlSi-coated product by the direct process is becoming increasingly widespread. Still,

it has been found that a very strong thermochemical reaction occurs when the AlSi-coated sheetmetal comes into direct contact with the rollers during the heat treatment, Fig. 3.162.

Fig. 3.162:Thermochemical attack on a conveying roller (source: Schwartz)

The rollers employed today are either hollow and formed of sintered mullite (3Al 2O3.2SiO2) or solid

and made of fused silica material (> 99 % 2SiO2). Fused silica rollers containing over 99 % SiO2have a service temperature limit of around 1100C but tend to become deformed under their ownweight at approx. 700 to 800 C.

Rollers made of sintered mullite can withstand

temperatures up to 1350 C under load without sufferingany significant deformation.

A significant advantage of both materials lies in their

high thermal shock resistance. On the other hand, bothhave a high affinity to reacting with molten aluminiumand may form diverse aluminium silicate or even silicidecompounds in this case.

As the AlSi coating melts during the heat treatment,Fig. 3.163, this molten metal is free to react with theroller material. In the course of this reaction the melt willpenetrate into the porous rollers and solidify there,causing a substantial partial density difference in theroller material that will result in failure (fracture) of the

roller.Fig. 3.163: Molten AlSi-coating reacting withthe conveying roller(product above, roller

below) (source: Technical University of Aachen,Department of Ceramic Engineering)


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Fig. 3.164: Principle of thesol-gel method (source:Technical University ofAachen, Department ofCeramic Engineering)

Approach #1: coating

Relying on the sol-gel process, Fig. 3.164, a suspension was developed for application to the

conveying rollers by spray-coating. From a total of around 40 different suspensions, three wereselected for further investigation, Fig. 3.165. From Fig. 3.166 it is evident how the roller coatingprotects the roller matrix by preventing it from reacting with the molten aluminium. Figs. 3.168 and

3.169 show the results of a large-scale industrial trial. In this setup, around 40,000 AlSi-coatedblanks where heat-treated over a 3-month period. AlSi melt attack on the roller is significantlyreduced.

Fig. 3.165:0-sample = uncoated roller sample (source: Schwartz)

Fig. 3.166: Deposit build-up during corrosion testing (source: Technical University of Aachen, Dept. ofCeramic Engineering)


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Fig. 3.167: Green abrasion resistance measuring device(source: TechnicalUniversity of Aachen, Dept. ofCeramic Engineering)

Fig. 3.168:Coated rollers after

passage of approx. 40,000 AlSi-

coated blanks through the furnace

(source: Schwartz, Kirchhoff)

Fig. 3.169: Area of transition

between coated and uncoated

rollers after heat-treatment ofapprox. 40,000 AlSi-coated

blank (source: Schwartz,

Kirchhoff)

Approach #2: alternative materials

Rollers were made from a non-oxidic, aluphobic ceramic material. After 1,500 hrs. of contact withAlSi-coated blanks, an intense build-up of deposits was noted, Fig. 3.170. However, it could beshown by microscopic examination that no significant reaction with the roller matrix had takenplace, Fig.3.171. Moreover, the material was found to exhibit a very high thermal shock resistanceand good high-temperature strength (mullite: approx. 10 MPa at 900 to 1000 C; non-oxidicmaterial: approx. 180 MPa at 900 to 1000 C).

Coated rollers

Uncoated rollers


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Fig. 3.170: Roller made of non-oxidic,aluphobic material

(source:Schwartz)

Fig. 3.171:Microscopic examination of a roller made of non-oxidic aluphobic material (source:

Technical University of Aachen, Dept. of Ceramic Engineering)

It emerged that the deposits could be removed with a grinding machine upon removal of the rollerfrom the conveyor assembly. Thereafter, the roller was fit for service again. Another conceivablesolution lies in the use of a roller cleaning machine that grinds away deposits with the conveyor inoperation; a machine of this type has already been developed.

The disadvantage of this roller material is its very high price. Thus, a single roller may be 15 to 20

times more expensive than a mullite one having the same dimensions. Another drawback is the

high thermal conductivity which results in high heat loading of the roller's bearing assembly.In dealing with roller-hearth furnaces for hot-form hardening lines, three main issues meritparticular attention.


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been requesting that the furnaces be equipped with an air drying unit.

Protective gas atmosphere: On principle, it may be stated that controlled atmospheres are madeup of neutral or inert components. However, for reasons of process technology, they may alsocontain active constituents. The neutral / inert part is nitrogen, whereas the active constituents arehydrogen and carbon monoxide, cf. Table 3.10 from [3.27]. The reason for using a protective gas

is that the sheet metal is to be prevented from oxidizing.

Table 3.10: Types of protective gas [3.27]

Chemical

constituents

DX or Exo

or CCHN

NX orMono CHN

or CHN

HNX orMono HN

or HN

AX or H75

or cracked gas

Oxidation (i.e., scale formation) takes place according to the following reactions[Me = metal]:

2Me + O2 2MeO (3.6)Me + H2O MeO + H2 (3.7)Me + CO2 MeO + CO (3.8)

The less noble a metal and the higher the temperature, the faster will be the oxidation rate (3.6).

Hydrogen is actually classified as a combustible material (fuel). Its pronounced desire to bindoxygen via combustion is utilized to maintain the furnace atmosphere as oxygen-free as possible.In addition, metal oxides can be reduced by this hydrogen. As is evident from (3.7), the reaction

takesplace in both directions, allowing us to conclude that the amount of hydrogen should notexceed a given limit if an undesirable re-oxidation is to be avoided. The desire of metals totransform themselves into metal oxides in the presence of steam varies with product compositionand temperature.


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Since protective gases comprising CO and CO2 also include hydrogen-containing portions, the

latter must be taken duly into consideration as well. These gases interreact in accordance with thewater-gas reaction

CO2 + H2 CO + H2O (3.9)with the associated equilibrium constant

KW =

pCO

pH2O

pCO2

pH2

(3.10)

From the above it is evident that the pH2O/pH2 ratio must be controlled so that the requisite gas

constants can be determined from appropriate tables, Fig. 3.172, [3.27]. Oxidation can only occur

if the gas equilibria at the relevant temperature no longer prevail at the product surface.

Fig. 3.172: Metal - metal oxide

equilibria in the CO/CO2andH2/H2O mixture [3.27]


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Table 3.11: Effect of gas constituents on the metal surface

Table 3.11 describes the effects of individual gas constituents on the metal surface.

In hot-form hardening applications it is standard practice to rely on an endothermic gas generated

by catalytic conversion of a mixture of natural gas and air, or propane and air. This conversion

requires a certain minimum temperature; the heat required to reach it is extracted directly from the

furnace, Fig. 3.173.

By setting the air/gas mixture correctly, the pH2O/pH2 andpCO/pCO2ratios can be adjusted such that

the atmosphere ceases to be oxidizing. The rule is as follows: pH2O/pH2 < 0.7 and pCO/pCO2 < 0.5.

With the appropriate settings it is possible to produce approx. 3 m of endothermic gas from 1 m

of natural gas.

A protective gas can also be generated by injecting methanol with nitrogen directly into the

furnace. At temperatures over 750 C, the methanol will become dissociated according to the

reaction formula

CH3OHCO + 2H2 (3.11)By setting the mixing ratio correctly, the N2, CO and H2concentrations in the protective gas can be

adjusted such as to correspond to the composition of the endothermic gas. Another option is to

generate a protective gas containing CO, CO2, H2and H2O by direct injection of a natural gas /

nitrogen mixture into the furnace. However, the natural gas portion must be 5% in this case.The choice of protective gas will depend on the cost of sourcing nitrogen and natural gas. Thecatalytic method of generating the protective gas involves greater maintenance cost and effort, butit relies virtually on air and natural gas alone. Nitrogen is used merely to purge the furnace systemat temperatures below the critical level of 750 C. The direct injection of nitrogen / natural gas (ormethanol) involves much less maintenance but has a higher nitrogen demand.

Fig. 3.173: Catalytic generation of endo gas (source: Schwartz)


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Issue #2

Furnace heating: In the roller-hearth furnaces described herein, a surface is heated soas to emit thermal energy, thereby heating the furnace and the product (in this case, blanks).

Heating of this surface is performed electrically and/or by combustion of natural gas.

Indirect gas heating

Since gas combustion produces a very moist exhaust gas, Fig. 3.174, an indirect heatingprocess needs to be adopted. This is because hydrogen tends to get bound up in themicrostructure of the blanks, with the potential result of subsequent hydrogen embrittlement.Therefore, single-end radiant tubes are used. The surface of a gas-tight single-end radiant tube isheated by exhaust gases passing through its interior.

The single-end radiant tubes are made of materials such as heat-resistant steels and SiSiC. The

metal radiant tubes become bent over time due to their own weight. For this reason the tubes mustbe rotated through 180at regular intervals while they are in service. As can be seen in Fig. 3.175,the straight radiant tubes are equipped with a flame tube. Due to the high thermal loadsinvolved, virtually all flame tubes are made of ceramics (SiSiC). The flame tube consists of multiplesegments to enable it to adapt to the deformation of the single-end radiant tube, Fig. 3.176.

Fig. 3.174: Composition

of exhaust gas from the

combustion of natural gas

[3.28])

Fig. 3.175: Schematic diagram of a single-end radiant tube burner (source: Schwartz)


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Fig. 3.176: SICAFLEX flexible

segmented flame tube (the designationis a trade name registered by LBE)

(source: Elster, LBE)

Single-end tubes are increasingly made of ceramic material (SiSiC). The advantage of such tubes

is that they will not get deformed at temperatures below the maximum service temperature(1350C), i.e., the laborious process of periodically turning the radiant tubes over is eliminated.Another benefit is the markedly higher surface watt density. Heat-resistant steels can be loaded to

20 to 38 W/cm2 depending on the radiant tube surface temperature. A SiSiC tube can handle loads

up to > 50 W/cm2 as long as the maximum service temperature of the SiSiC material is notexceeded. As a result, the number of radiant tubes in the furnace may be reduced within certain

limits, this limit being defined by the maximum tube-to-tube distance that must not be exceeded iftemperature uniformity problems within the furnace are to be avoided. The diameter of the single-end radiant tubes may likewise be reduced, which will save tube weight and cut the price of eachtube. In some cases it may be preferable not to utilize these advantages, at least not to the fullextent, for the reasons stated below:

Full utilization of the high surface watt density will drive up the exhaust gas temperature,

possibly past the maximum service temperature of the metal recuperator.

The use of ceramic (SiSiC) recuperators is an option. However, for reasons of manufacturing

technology, the heat exchanging surface area of these recuperators is markedly smaller thanthat of their metal counterparts. Accordingly, they cannot transfer as much heat for air-preheating, which drives up the exhaust gas heat loss and reduces the combustion efficiency.

Some furnaces use single-end radiant tubes with built-in recuperative gas burners. Theircombustion efficiency amounts to approx. 70 to 72 %.The gas burners are fitted above the rollerconveyor, with the ceramic radiant tubes hanging freely suspended. Further gas burners aremounted underneath the roller conveyor, these are equipped with steel radiant tubes. The radianttube ends rest on brackets in order to minimize sagging of the tubes.

The speed of the combustion air fan is controlled via a variable-frequency drive unit so that the fancan be run at optimum speed at all times, in accordance with its performance curve.

Direct electrical heating

With this heating type, electric resistance heating is employed. Since electric heating involves no

exhaust gas, the product can be heated directly. Metal heating elements are mounted on aceramic support tube. The heat conductors are made of Cr-Ni or Cr-Ni-Al alloys, Fig. 3.177.

Fig. 3.177: Electric heating elementsin operation (source: Schwartz)


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Advantages

Low investment cost, operation on mains voltage, long service life in thyristor-controlled mode,repair-weldability by the user in case of damage, straightforward installation, no expensive gas / airstation required, favourable energy price in countries/regions where no gas or oil is available orwhere electric power is a low-cost resource (e.g., Scandinavia).

Disadvantages

Clear limitation of heating power due to low surface watt density, sensitivity to some protective gas

atmospheres (damage due to excessively low dew point, chemical attack by evaporating drawingoils adhering to the product), costly add-on equipment (e.g., thyristors) if used for furnace control,etc. Electric power is a secondary energy and hence, usually much more expensive than the useof primary energy such as natural gas.

Hybrid heating

In a furnace of this kind the entry section is indirectly gas fired while the rear furnace sectionrelies on direct electric heating. With such continuous furnace systems, most of the heatingpower is needed at the entry because the trays or panels must be heated to their finaltemperature as quickly as possible. Further down the furnace the products remain to be heated

but with a little gradient over time so as to make up for radiation losses, Fig. 3.178.

As the electric heating is thyristor-controlled, temperature fluctuations in the rear furnace section

can be kept very small (2.5C). Another advantage of this heating type is its investment cost,which is lower than that of a 100% gas-fired furnace. Maintenance costs, too, are lower becausethe electric heating system is more maintenance-friendly than an indirect gas heating solution.

Issue #3 Furnace runout section

At the end of the roller-hearth furnace, an exit roller table is provided from which the products arepicked up. The trays (in an indirect-process furnace) or blanks (in the direct process) are movedonto this table once the heat treatment is complete.

Fig. 3.178: Schematic view of a hybrid heating system (source: Schwartz)


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It is important, for one thing, that the parts should be moved out of the furnace as quickly as

possible. On the other hand, the trays or blanks must be positioned very accurately on the exitroller table so that the parts/blanks can be placed in the press dies without any need for positionadjustments.

The exit roller table is equipped with steel rollers. These are driven via sprockets and a chain by a

frequency-controlled geared motor. Additional doughnut rings are mounted on the steel rollers toprevent the hot blanks from cooling down to fast on the cold rollers.

Load presence on the exit roller table is monitored by a light grid. At the end of the exit roller table,"stoppers" may be fitted to stop the trays or blanks before they are centered. These "stoppers"typically have a lifting device whereby "off-spec" blanks can be discharged into a scrap container.

Below the exit roller table, an air-powered or servomotor-driven centering device is installedwhich positions the trays or blanks as they leave the furnace. This centering device compriseslinear motors and the necessary centering lugs on the left and right-hand side of the exit roller

table. The system can attain centering tolerances within 1.0 mm. Moreover, discharge speedsof > 2500 mm/s are easily achievable today, Fig.3.179a throughe.

a) b)

c) d)

Fig. 3.179 a - e: Examples of exit roller tables(source: Schwartz)

e)


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Other furnace and heating configurations for hot-form hardening applications

Chain carrier furnace with trays

This furnace type has been found to give particularly good results where galvanized parts(cathodic/anodic corrosion protection) are to be heat-treated prior to hot-form hardening. It is

suitable for treating preformed parts (indirect process). Moreover, this furnace can be used to heat"prefinished" products, i.e., parts to be hot-form hardened to within defined tolerances so as toeliminate the need for trimming. In a system of this kind, special product support trays are runthrough the furnace on chains which move outside the heated furnace chamber, Fig. 3.180athrough d. These trays are of a special design which enables them to accommodate diverseparts without any time-consuming set-up operations. Moreover, the trays ensure that the parts willexit the furnace in exactly the same position in which they entered it. This gives a very high degree

of accuracy in placing the parts in the downstream hot press. In addition, the tray system allows

engineers to design heat treatment lines with a high useful furnace width, reduced length, andfaster cycle times (down to 12 s). The systems currently in use are 24 m long and achievethroughputs of up to 4.5 metric tonnes/h. This furnace type offers the following advantages:

Substantially increased useful furnace width (> 3000 mm).

Clearly reduced furnace length (resulting from the foregoing), despite faster cycles.

Form-fit tray passage through the furnace, eliminating product offset at the furnace exit (at pointof transfer to the press).

No need to position trays at the furnace exit (at point of transfer to the press), permitting shortercycle times.

Since this furnace type can also be used for AlSi-coated sheet metal, the problem of thermo-

chemical attack on conveying rollers (see description above) will not occur.Furnaces can also be built without the above-described tray return system. In this case, the heat-upsection of the furnace will be located where the tray return system would be placed, and the traysare lowered to floor level by a heated tray lifting device to be fed to the second furnace sectionwhere the parts are heated to the austenitizing temperature.

a) b)

c) d)

Figs. 3.180 a - d: Trays at the

furnace entry and return

conveyor (source: Schwartz)


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22In a system of this type, the furnace entry and exit points lie on the same side but on differentlevels.

Undisputable benefits of this design include the following:

Further reduction in furnace length

Prevention of unnecessary heat loss from trays on the return conveyor.

In addition, a tray set-up area can be provided on a second level.

Multi-tier multichamber furnace

This design provides for a chamber furnace having multiple heating chambers. Such furnacesare loaded and unloaded via a high-speed feeder system. For an explanation of their operatingprinciple refer to Figs.3.181a through d.

One major advantage of this furnace type is that it requires clearly less space than a roller-hearthunit. It also provides a high level of flexibility, for in the case of one chamber failing, the other chambers

a) b)

c) d)

Figs. 3.181 a - d: Schematic drawings of the multi-tier

multichamber furnace (source: Schwartz, AP&T Group)


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can simply remain in operation. A disadvantage of this equipment type lies in its high maintenance

demand, especially of the door systems. Every chamber has its own door that must be openedand closed at short cycles, and this configuration poses quite a mechanical challenge. A further

major issue is the controlled atmosphere. Whenever a door opens, a gaping orifice measuringbetween 0.5 and 1 sq. m. is formed. Due to the temperature differences there will be an ingress ofair into the furnace. In order to prevent this, a "sealing curtain" must be installed which in turnrequires large amounts of nitrogen. Each chamber must be provided with its own protective gasgenerator, and each chamber needs its own oxygen monitoring system.

Moreover, the product feeder system is fairly complex in terms of automation technology.

Once all these challenges are resolved, this furnace design can be a very good alternative to aroller-hearth system for users having limited space available.

Inductive and conductive-type heating strategies

Developers are continuously striving to introduce further heating methods into industrial hot-formhardening environments. Most of these projects examine the feasibility of conductive and induction-based heating. However, both systems are increasingly running up against limitations. By way ofexample, let us consider the heat treatment of AlSi-coated sheet metal.

Due to the extremely fast heating rates provided by an induction furnace, the AlSi coating will melt

very rapidly as well. Molten metal has been observed moving along the Joulean fields and thendripping off the blank edges. Moreover, edge overheating occurs and irregularly shaped blanks willbe heated in non-uniform patterns at the same time.

Another problem with induction heating technology lies in the process window required for the

AlSi-coated panels. At temperatures over 600C, the Curie point is exceeded and proper, if any,magnetic field coupling will no longer be ensured.

It is true that product has been successfully induction-heated on a laboratory or R&D scale. To thisday, however, no industrial-scale induction heating solutions providing appropriate heat-treatmentcycles have been demonstrated, nor have such systems been shown to attain the necessaryflexibility. In a production system, loading patterns vary very rapidly, i.e., a user may heat only B-pillars for a few hours, then switch to a mix of A-pillars and roof reinforcing members, followed by amix of bumpers and A-pillars, and so forth.

The same problems faced by induction technology are also encountered by conductive heating

systems. In an isolated case or on an R&D scale this heating strategy may work, but here again,nobody has yet presented a furnace demonstrating the same production-scale flexibility and

availability as the roller-hearth systems employed to date.

Literature

[3.26] Specht, E.; Holzapfel, K.-U.: Wrmebergang zwischen Transportrolle und Gut imRollenofen[Heat transfer between conveyor roller and load in a roller furnace ],GWI48 (1999), pp. 275-280

[3.27] Nassheuer Taschenbuch fr Schutzgastechnik und Industrieofenbau [Nassheuer's Handbook forProtective Gas Technology and Industrial Furnace Conststruction] 1984, Vulkan-Verlag, Essen

[3.28] Pfeifer, H.: 19. FOGI Seminar [Seminar of the Industral Furnace Research Association], Aachen,

March 2003

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