next-generation hpq vacuum furnace v - · pdf fileapplication of the practical experience...

Application of the practicalexperience gained in thedevelopment of single-chamber HPQ furnaceshaving radial gasquenching systems todouble- and multi-chambersystems enables significantimprovement in gas coolingrate and uniformity, as wellas the design of heatingchambers.

J. KowalewskiSeco/Warwick (Tianjin) IndustrialFurnace Co. Ltd.China

M. Korecki J. OlejnikSeco/Warwick SAPoland

acuum furnaces with highpressure gas quenching(HPQ, or high-pressurequench) have been used formore than 25 years for heat

treatment of tool steels, HSLA steels,and other metal alloys. Today, standardHPQ furnaces are designed withcooling pressures of 6, 10, 15 and 25bar. HPQ furnaces have been inte-grated as an industry standard becauseof ease of operation, cycle automation,reduction of part distortion, and thedevelopment of new processes such aslow-pressure (vacuum) carburizing(LPC). This paper discusses 20 yearsof global experience with radial designfurnaces and new process applications.

Selective QuenchingWith the development of the HPQ

process, metallurgists have been inter-ested in quenching uniformity, partic-ularly for densely packed charges andlarge-size parts such as molds and diesmade of hot-work tool steels. HPQ fur-naces are designed with two alterna-tive methods of cooling gas flow: ra-dial (with gas inflow from thecircumference of cylindrical heatingchamber wall) and axial (along the hor-izontal or vertical heating chamberaxis).

Vacuum furnaces with radial nozzletype quenching have been successfullyused worldwide for more than 30years, while those having axial typequenching with reversible gas infloware a standard design in common usein Europe. The axial type design hasheating chamber size limitations be-cause of poor cooling temperature uni-formity and a slower cooling rate.There have been various attempts toimprove the axial design, however, ithas limitations that cannot be over-come. The radial design, on the otherhand, can be configured to increase thespeed and uniformity of quenching,especially for furnaces with largerheating chambers. As early as 20 yearsago, radial type furnaces with largerheating chambers were used for hard-ening of large dies.

The first radial design was devel-oped and patented in Europe in 1988.

(Patent PL 156044). According to thepatent, the cooling is divided into sixcontrol zones within the singlechamber furnace. A manifold encir-cling the heating chamber is dividedinto six inflow zones, with three con-trollers built into the inlet pipe. Theinlet was controlled by thermocouplesdistributed over the charge in these sixzones. The system was an excellent so-lution for furnaces with a 2-barquenching system. Other furnace man-ufacturers now have similar arrange-ments, providing a controlled supplyof gas to separate zones (four, for ex-ample) via the divided manifold sup-plied from external pipelines.

The design becomes impractical ina HPQ class furnace with quenchingpressure of 10 to 16 bar (Fig. 1). A sig-nificant increase of the cooling ratecan be achieved by increasing the gaspressure during cooling and properdesign of the cooling gas flow. Sucha design should also provide for uniform quenching and/or its con-trolled run, which now becomes more important.

NEXT-GENERATION HPQ VACUUM FURNACE

HEAT TREATING PROGRESS • SEPTEMBER 2008 39

V

Typical HPQ furnace with 16-bar gas quenching.

Fig. 1 —- HPQ furnace with axial/radial quenchingsystem.

New Gen Vac.qxp 8/27/2008 8:01 PM

Design SolutionsTwo successful furnace configura-

tions are the horizontal VPT furnaces(Fig.1) and elevator hearth VVPT(EH)

furnaces. The proprietary furnace de-sign, in use since 1995, uses the con-cept of assisted convection heatingby closing the cooling nozzles of theconvection heating system. This de-sign has a proven track record inmore than 200 furnace installations(2 to 20 bar) with cylindrical radialheating chambers, including applica-tions in “dirty” processes wheremetal sublimation deposits appear innozzles and other cool sections of thehot zone.

In the horizontal VPT furnace withradial gas flow (Fig. 2a), one operatingsystem is used for closing nozzlesarranged 360 degrees around the loadin the cylindrical heating chamber.This design is very effective for heattreating densely packed and solidloads like molds and dies in general-purpose vacuum furnaces. However,to ensure optimum heat treatment offlat, long workpieces, (plates, for ex-ample), dual nozzle operating sys-tems are applied where the user closesthe side nozzles (Fig. 2b). By applyingthis method, the axial quenching ef-fect is obtained in the heatingchamber with radial gas flow. Theheating chamber of a HPQ 900 × 800× 1,500 mm (36 × 32 × 60 in.) furnacewith 16-bar quenching shown in Fig.1b is equipped with a dual nozzleclosing operating system.

Figure 3 shows a HPQ furnace witha 900 × 800 × 3,600 mm (36 × 32 × 140in.) heating chamber designed for heattreating cutters made of AISI S5 steel.The furnace is designed with a 16-barradial/axial gas quenching system.Figure 4 shows a furnace installationwith a quenching system equippedwith two blowers (installed axially)with 350-kW rated motors.

The gas inlet system design is alsoin use in elevator hearth furnaces.The user can program inflow ofcooling gas onto the load either in ra-dial mode or from the hearth(bottom) plane with a gas outletthrough top plane, which is a typicalfeature of the axial quenching system.Furthermore, in this application, vol-umetric gas flow in axial modethrough nozzles arranged in thehearth is comparable with volumetricgas flow through the nozzlesarranged in the cylindrical wall andin the hearth as used in the radialmode. Figure 5 shows diagram of thesystem and the 72 in. × 72 in. furnace.The furnace was provided with a 20-bar gas quenching system with a 400-kW blower motor.

40 HEAT TREATING PROGRESS • SEPTEMBER 2008

Fig. 2 — Radial gas flow (a), and axial gas flow (b).

Fig. 3 — HPQ furnace with 36 × 32 × 140 in. work chamber designed for thermal treatment of knives (cutters) made of AISI S5 steel.

Fig. 4 — View of the 36 × 32 × 140 in. HPQ furnace

(a) (b)


Quenching RateThe heat transfer coefficient (α), a

basic parameter on which the quenchrate depends, is dependent to an equalextent on the pressure (ρ 0.7) and onthe linear velocity of gas (ω 0.7) flowingalong the charge surface. This is illus-trated by the equation:

α ~~ C • ω 0.7 • ρ 0.7(W/m2oK)

The equation shows that to achieve ahigh quenching gas rate other than therelatively easy increase of quenchinggas pressure, careful design of the fur-nace system is equally important. Inpractice, this involves a number of de-sign steps including:

• Selecting the appropriate powerrating of the blower motor

• Designing a furnace with low re-sistance of gas circulation

• Designing a suitable distributionof the quenching gas inlet nozzlesusing the radial quenching system

• Determining correct positioningof the hot gas outlet orifice from theheating chamber into the heat ex-changer zone

In this design, the hot gas outletzone in the form of a large-size holepositioned in the charge center line(Fig.1b) allows axial gas flow over theentire load length, thus ensuring highlinear velocities in each part of a load.These are the most optimized designconditions to reach the highestquenching rates and to ensure highquenching uniformity within entireload volume.

These furnaces are particularly suit-able for quenching loads containingHSLAsteels, low-alloy cold-work toolsteels, and molds and dies made ofH13 steel according to the require-ments of NADCA/GM DC-9999-1, aswell as for hardening of steels aftervacuum carburizing. The furnaces aredesigned with 10-, 12- 16-and 25-bargas quenching systems.

Quenching rate parameters in suchfurnaces are defined by the heattransfer coefficient (α) or more fre-quently by the λ coefficient defined fortesting loads consisting of steel bars,for example. Quenching tests of a verydensely packed load consisting of steelbars with gross weight 450 kg (990 lb)in a standard 24 × 24 × 36 in. furnaceunder 10 bar N2 pressure shows thatλ= 0.7, while for a densely packed loadof 300 kg (660 lb) gross weight, λ = 0.6.Test results also show that for partswith larger cross sections, differencesin packing density have less of an ef-

fect on quenching rates (Fig. 6). Fur-thermore, in furnaces with largerheating chambers, achieving the pa-rameters mentioned above is more dif-ficult, while in furnaces with smallerheating chambers (e.g., 16 × 16 × 24in.), more dynamic quenching condi-tions can be achieved. Quenching ratesmeasured in the core of test barsachieved in a 24 × 24 × 36 in. furnaceat 700°C (1290°F), which are suitablefor hardening of certain grades of car-burizing steels, are shown below

Bar diameter, mm Quench rate, °C/s

25 5.6

50 4.0

100 2.3

The heat transfer coefficient (α) meas-ured in these tests is in the range of 600W/m2 K. This value is suitable forhardening a number of HSLA steelsand cold-work tool steels (e.g., O1, S1).However, it is too low to use single-chamber furnaces for hardening ofcommon industrial carburizing steelgrades after vacuum carburizing dueto nonuniform hardenability.

Grossman quenching intensityfactor H for gas quenching in stan-dard Seco/Warwick HPQ furnaces isin the range of 0.10 to 0.15, while forquenching oils used in typical sealedquench furnaces, the range is 0.25 to0.35. Therefore, single-chamber fur-naces are already used for hardeningof certain materials with limited crosssections with lower requirements asto core hardness (DIN 3990 – MLrange). For more restrictive require-ments (e.g., MQ1/MQ2), such furnacesare not used. In certain applications,furnaces with separate 20-barquenching chambers where an H


Fig. 5 — Elevator furnace (72 in. diam. × 72 in. high)with axial/radial quenching.

Radial

Axial

25 mm diameter400 kg netλ = 0.72



Fig. 6 — Quenching test results for densely packedloads of different size bars.


factor of 0.2 is obtained are used in-stead of oil quenching furnaces. Sucha cooling system enables successfulhardening of above-mentioned steelswith limited cross sections.

New Generation HPQ Furnaces forVacuum Carburizing

Seco/Warwick developed a new-generation of single-chamber vacuumfurnaces for applications wherevacuum furnaces with separatequenching chambers have been usedpreviously. The family of furnaces in-cludes single-chamber furnaces withsizes 16 × 16 × 24 in., 24 × 24 × 36 in.,and 36 × 32 × 48 in., with 25-bar he-lium or hydrogen quenching. Theyalso provide for operation under 12to 16 bar nitrogen pressure. Testswere performed in furnaces with 16× 16 × 24 in. (Fig. 7) and 24 × 24 × 36in. work zones. Heat transfer coeffi-cients measured for oil, nitrogen, he-lium, and hydrogen are shown inTable 1. Figure 8 presents thepumping system of the 24 × 24 × 36in. furnace adapted for quenchingunder 25 bar hydrogen .

With these results, single-chamberfurnaces can be competitive withdouble-chamber designs, as well as

with quenching in oil. The advantageof a single-chamber furnace is the ca-pacity to apply intermittent hardeningat the Ms (martensite start) tempera-ture to equalize the temperature overthe workpiece cross sections andcharge (patent pending) before en-tering the area of martensite transfor-mation.

The result is minimization of distor-tion. The method has been docu-mented for oil (stop at 250°C) and gasquenching. For this purpose, the Con-Flap (convection-assisted heatingsystem) in Seco/Warwick furnaces isused where cooling nozzles are closedwith flaps immediately after the re-quired temperature is reached on theload surface, and opened after the coreand surface temperatures reach an ac-ceptable level within the martensitetransformation area. This system re-duces the distortion of heat treatedparts significantly.

Double- and Multi-Chamber Furnace Systems

Figure 9 shows an installation of adouble-chamber furnace used forvacuum carburizing and gas quench-ing at pressures up to 20 bar.Quenching in a separate ColdKamchamber allows for higher quenchingrates compared with standard single-chamber furnaces, where duringcooling stage, the heating chamberwalls and internal furnace equipmentis cooled together with the load.

Such furnaces are mainly used forheat treating HSLA steels, low-alloycold-work steels, and vacuum carbur-izing with subsequent gas hardening.The furnace allows for hardening at cooling rates up to 15°C/smeasured in core of 25-mm diameterbars making up the test load. Thesystem shown in Fig. 9 is designedwith two powerful gas blowers thatforce the axial type inflow of gas tothe load space from the top with alinear velocity of about 15 m/s (fornitrogen).

Just as in case of single-chamber


Fig. 7 — HPQfurnace with 16

× 16 × 24 in.work chamber.

Fig. 8 - Pumping system for 25 bar hydrogen.

Table 1 — Quenchant heat transfer coefficients for different furnace sizesHeat-transfer

coefficient, W/m2K

15-bar 25-bar 25-bar Furnace size Oil nitrogen helium hydrogen

Oil bath 1,200 - 1,500

16 × 16 × 24 650 1,500 2,000

24 × 14 × 36 550 1,250 1,650

36 × 32 × 48 500 1,150 1,500


DICTRA is a unique software package for accurate simulations of diffusion controlled transformations.

Diffusion controlled phase transformation kineticsHomogenizationCarburizing & DecarburizingNitriding and carbonitridingMicrosegregation during solidificationCoarsening / Dissolution of precipitates InterdiffusionAnd more...Databases for Fe-, Al- and Ni- based alloys

Thermodynamic and Diffusion Simulation Software Thermo-Calc Software

Thermo-Calc is a powerful software tool for thermodynamic calculations.

Stable and meta-stable phase equilibriumAmounts and composition of phasesPhase transformation temperatures Scheil solidificationThermochemical property data for phasesPhase diagrams (isothermal, isoplethal sections for multi-component multi-phase systems)Databases for Fe-, Ni-, Al-, Ti-, Mg- based alloys,solders, refractory oxides, and more…

Rest of the World:Email: [email protected]: +46-8-545 959 30Fax: +46-8-673 37 18http://www.thermocalc.com

USA, Canada and Mexico:Email: [email protected]: (724) 731 0074Fax: (724) 731 0078http://www.thermocalc.com

Please visit us at Booth 504 MS&T 2008

Pittsburgh, PA Oct 7-8, 2008

furnaces with axial gas flow design,the separate gas cooling chamberswith axial gas flow are better suitedto uniform quenching of loads con-sisting of rods, other long workpieceshaving complex cross sections, andpinions. However, the experience ofSeco/Warwick and others shows thatsimilar hardening-distortion results(but better quenching rates) can beachieved in furnaces (quenchingchambers) with radial gas flow pro-vided that suitable volumetric ex-change of the circulating quenchinggas and careful distribution of coolinggas nozzles is considered.

For a nozzle cooling system, thesame gas volume flows throughsmaller load cross section in a radialsystem than in an axial system (Fig.10) resulting in higher linear flowrates. Seco/Warwick used this expe-

Fig. 9 - Double-chamber 24 × 24 × 36 in. furnace with 20-bar quenching.

Fig. 10 - Flow area through useful space (a) axial, (b) radial.(a) (b)



rience in the development of a newgeneration of separate ColdKamquenching chambers, achievinghigher cooling rates, extendingprocess capabilities, and improvingquenching uniformity. This design isused in double-chamber furnaces andmulti-chamber systems.

Another improvement (patentpending) enhances performance usinga rotary pulsing system incorporatinga 1/4 gas inlet surface. It facilitates pen-etrating the charge volume duringquenching and supports uniform andrapid quenching even with a high-density packed charge.

The new chamber design enableshigher linear gas velocities to over 20m/s for nitrogen, thus yielding newprocess applications. Additionally, thisdesign allows for constructing of multi-chamber systems with much longerheating chambers, such as 36 × 30 × 48in. This new option provides higher ef-ficiencies in vacuum carburizing sys-tems. Because of the problems dis-cussed above with gas flow uniformity,there are no axial-type chambers withsizes over 24 × 36 in. available in in-dustrial practice. Figure 11 shows a typ-ical design of Seco/Warwick multi-chamber system with four processchambers 24 × 24 × 36 in.

Double-Chamber Oil QuenchVacuum Furnaces

The implementation of a new gen-eration of single- and multi-chambersystems with separate quenchingchambers have not eliminated thelimitations in workpiece cross sec-tions, mainly in the case of standardcarburizing steel grades. Such limita-tions defined by various standards(e.g., BAC 5617) still exist also forHSLA steels. For these applications,a new series of double-chamber oilquench vacuum furnaces, includingvacuum carburizing furnaces, areavailable with a range of work-spacedimensions including 18 × 20 × 24 in.and 30 × 24 × 48 in. (400 × 500 × 600and 760 × 600 × 1,200 mm) as shownin Fig. 12. HTP

For more information: Beth Ryan, Mar-keting Communications, Seco/WarwickCorp. 180 Mercer St., PO Box 908, Mead-ville, PA 16335; tel: 814-332-8347; fax: 814-724-1407; e-mail: [email protected];Web site: www.secowarwick.com.

Fig. 11 —Seco/Warwick

multi-chamber system(Traveler Rotary).

Fig. 12 —Double-chamber

oil-quench furnaces 18 ×

20 × 24 in.(top) and 30 ×

24 × 48 in.(right) work

chambers.



next-generation hpq vacuum furnace v - · pdf fileapplication of the practical experience...

Documents