Distortion Control of Transmission Components byOptimized High Pressure Gas Quenching

Volker Heuer, Donald R. Faron, David Bolton, Mike Lifshits, and K. Loeser

(Submitted June 14, 2012; in revised form September 22, 2012; published online April 26, 2013)

The paper presents how the ‘‘Dynamic Quenching’’ and ‘‘Reversing gas flow’’ processes are successfullyapplied on internal ring gears and planetary gears for a 6-speed automatic transmission. The specificchallenge was to reduce distortion in such a way that subsequent machining operations are entirelyeliminated. As a result of extensive development in the quenching process, it was possible to meet the designmetrological requirements. The internal ring gears have been in continuous production since 2006. Byutilizing the special CFC fixtures and quench methodology of ‘‘Dynamic Quenching,’’ the customer wasable to achieve the design intent, while eliminating all machining operations of the ring gears followingLPC/HPGQ. Subsequent testing and monitoring over a 2-year period progressively demonstrated thatconformance. Therefore, quality inspection was reduced accordingly.

Keywords automotive, carbon/alloy steels, gear components,heat treating, high pressure gas quenching, lowpressure carburizing

1. Introduction

Proper distortion control has become even more importantthan in previous days. To answer the demand for fuel-efficientvehicles, modern transmissions are built much lighter. There-fore, the components of the transmission exhibit less wallthickness, which makes them more sensitive to distortion.Distorted gear components cause noise in the transmission, canrequire post-heat treatment machining processes, and may evencreate problems during transmission assembly. Therefore,distortion control has become more important than ever.

By applying the technology of Low Pressure Carburizing(LPC) and High Pressure Gas Quenching (HPGQ), heattreatment distortion can be significantly reduced. LPC is acase-hardening process which is performed in pressure of onlya few millibars using acetylene as the carbon source in mostcases. During HPGQ, the load is quenched using an inert gasstream instead of liquid quenching media. Usually, nitrogen orhelium is used as the quench gas.

With an optimized distortion control, it is possible tosimplify the process chain significantly. Figure 1 shows howthe process chain can be simplified, if the specified geometric

values of the components can be guaranteed after gasquenching. If the simplified process chain can be applied, thenthis will result in lower costs per part, lower throughput times,and lower energy consumption during production. Since thereis no need to dispose any oil after the quench and since cleaningoperations after the quench are unneeded, the simplified processchain is much more environmentally friendly as well.

2. Distortion Mechanisms and High Pressure GasQuenching (HPGQ)

The relevant mechanisms that cause distortion of componentsduring heat treatment have been described extensively in theliterature (Ref 1). Three different types of stresses in the materialcontribute to distortion: Residual stresses (Ref 1), Thermalstresses, and Transformation stresses. They are influenced by partgeometry, steel grade, casting, forging, machining, etc., and theydepend on the heat treatment. If the total stress in the componentexceeds the yield stress, then distortion of the component takesplace. The numerous potential factors that are influencingdistortion have been published by Walton (Ref 2).

The technology of High pressure gas quenching (HPGQ)offers tremendous potential to reduce heat treatment distortion.Conventional quenching technologies such as oil or polymerquenching exhibit inhomogeneous cooling conditions. Threedifferent mechanisms occur during conventional liquid quench-ing: film boiling, bubble boiling, and convection. Resultingfrom these three mechanisms, the distribution of the local heattransfer coefficients on the surface of the component is veryinhomogeneous. These inhomogeneous cooling conditionscause tremendous thermal and transformation stresses in thecomponent and subsequently distortion. During HPGQ, onlyconvection takes place, which results in much more homog-enous cooling conditions (Ref 3). Significant reductions ofdistortion by substituting Oil quench with HPGQ have beenpublished (Ref 4). Another advantage of HPGQ is thepossibility to adjust the quench intensity exactly to the needed

This article is an invited paper selected from presentations at the 26thASM Heat Treating Society Conference, held October 31 throughNovember 2, 2011, in Cincinnati, Ohio, and has been expanded fromthe original presentation.

Volker Heuer and K. Loeser, ALD, Hanau, Germany;Donald R. Faron, General Motors, Pontiac, MI; David Bolton andMike Lifshits, ALD Thermal treatment, Port Huron, MI. Contacte-mails: dr.volker.heuer@ald-vt.de, donald.r.faron@gm.com, anddr.klaus.loeser@ald-vt.de.

JMEPEG (2013) 22:1833–1838 �ASM InternationalDOI: 10.1007/s11665-013-0547-6 1059-9495/$19.00

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severity by choosing quench pressure and quench velocity.Typical quench pressures range from 2 to 20 bar. The gasvelocity is controlled by a frequency converter and typical gasvelocities range from 2 to 15 m/s depending on the partgeometry and the steel grade of the component. Typical gasesapplied for HPGQ are nitrogen and helium (Ref 5).

3. Furnace Equipment and Fixturing for DistortionControl

The design of the gas quenching chamber is of key importanceto minimize distortion. The chamber needs to provide a high gasvelocity to insure that the core-hardness specification is met andthe chamber needs to provide a very uniform distribution of thegas velocity to minimize the spread of distortion within the load.Intensive numerical flow calculation (CFD studies) and exper-imental studies led to the design of the quenching chamber of theModulTherm� system (Ref 6), see Fig. 2.

As in the case of liquid quenching, proper fixtures andoptimized loading of the parts are important for gas quenchingtoo. As an alternative to alloy fixtures, carbon compositematerials, e.g., CFC, were introduced for use as fixtures in heattreating applications. Figure 3 shows an example of a load onCFC fixturing. The major advantage is that CFC fixtures do notshow deformation during the heat treatment process, therebyassuring optimum positioning of the parts. This has a signif-icant positive effect on distortion control.

4. Reversing Gas Flow

Modern gas quenching chambers offer the possibility toreverse the direction of the gas flow during quenching. This‘‘Reversing gas flow’’ means that the flow of gas is alternatedback and forth from ‘‘top-bottom’’ to ‘‘bottom-top’’ (Fig. 4).By alternating the gas flow direction, there is less difference inthe cooling curves of parts placed in different layers. Thisreduces the spread of distortion inside the load.

5. Dynamic Quenching

To achieve optimum quenching results with respect tomicrostructure, hardness, and distortion, the gas quenchingparameters need to be well controlled. And, to further reducedistortion, a quenching process has been developed where thequenching parameters, gas pressure and/or gas velocity, arestepwise varied during quenching, see Fig. 5. This process,called ‘‘Dynamic Quenching’’ is typically divided into threesteps (Ref 7):

step 1 high quenching severity until a certain part temperatureis reached

step 2 quenching severity is reduced for a set time to allow fortemperature equalization in the part

step 3 quenching severity is increased again until the end ofthe quenching process

Fig. 1 Conventional and new process chain for the manufacturing of gear components

Fig. 2 ModulTherm� heat treatment system with gas quenchingchamber

Fig. 3 Load of internal ring gears on CFC fixturing

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The control system of the quenching chamber allows tocontrol the different quenching steps of ‘‘Dynamic Quenching’’in a very accurate way with high reproducibility. Optimumresults are achieved when using helium. The light quenchinggas helium can be decelerated and accelerated very preciselyfor optimum distortion control.

6. Application of ‘‘Dynamic Quenching’’ in SerialProduction

The ‘‘Dynamic Quenching’’ process was applied on theReaction internal gears of a 6-speed automatic transmission. Anintensive process optimization program was started before thestart of production. The specific challenge was to optimizedistortion control of the gears to such an extent that hardmachining is completely eliminated and the process chain issimplified as postulated in Fig. 1.

The furnace supplier and transmission manufacturer workedin close cooperation on the optimization program and success-fully implemented a Dynamic Quench practice for the start ofproduction in 6/2006. This low distortion process consists ofLPC using acetylene as the carburizing source and HPGQ usinghelium as the quench medium with an optimized Dynamicquenching process. The application of this optimized process and

the use of CFC fixtures made subsequent machining operationsafter heat treatment unnecessary as postulated in Fig. 1.

The Reaction Internal gears have an outer diameter of152 mm, 103 internal teeth, and are made of 5130 material. Apicture of the part is given in Fig. 6. The case-hardening depth,CHD, after heat treatment is specified as 0.3-0.6 mm and surfacehardness is specified as 79-83 HRA. The geometry after heattreatment is specified with a maximum circularity of 150 lm.

The results of a detailed distortion study from serial produc-tion are presented in the following. 48 pre-measured parts wereequally distributed into different layers of a standard productionload and were geometrically inspected after heat treatment. Oneproduction load consists of 120 pieces treated in 10 layers, seeFig. 3. To cover all ‘‘extreme’’ positions in the load, it was madesure that parts from all 8 corners and parts from the middle of theload were geometrically inspected. All measurements wereperformed with a CNC analytical gear checker.

The circularity of the 48 measured pieces before and afterheat treatment can be found in Fig. 7. The helix angle variation(Vbf) for each part is shown in Fig. 8. As shown in Fig. 7 and8, the achieved level of distortion is very small. The maximumchange in circularity amounts to 41 lm and the average changein circularity is only 7 lm. All circularity values after heattreatment are well below the specified maximum of 150 lm.The helix angle variation after LPC and HPGQ is below thespecified maximum as well. The average change during heat

Fig. 4 ‘‘Reversing gas flow’’ as applied in the ModulTherm� quenching chamber

Fig. 5 Schematic illustration of dynamic quenching for specimenof different sizes

Fig. 6 Reaction internal gear (d = 152 mm, 103 internal teeth)

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treatment in helix angle variation is 10 lm for the left flank and11 lm for the right flank. Measurements of two associatedinternal gears in the six-speed transmission, the Output andInput Internal gears, showed similar results.

Serial production of the 6-speed transmission started in 6/2006. Initially, all Internal ring gears from each load werechecked for excessive distortion with a so-called ‘‘Rollchecker.’’ A ‘‘Roll Checker’’ is an automated measurementsystem utilizing a rolling master held by a pivoting yoke. Theyoke enables the roll master spindle to move in the lead andtaper directions as the rolling master rotates in tight mesh withthe test component. Separate transducers located in the gimblehead monitor lead and taper travel. As distortion proved to bevery consistent, the transmission manufacturer decided to

abandon the 100% inspection of all parts. Since 10/2008, onlytwo gears per load are being measured with a CNC analyticalgear checker. One part from the top corner (TC) and one partfrom the bottom middle (BM) are being inspected in each load.These two positions were chosen to cover the ‘‘extreme’’positions in the load.

Results from distortion monitoring since 8/2008 until 3/2010are presented in Fig. 9. The diagram shows the helix anglevariation of Reaction Internal gears after heat treatment (Vbf)averaged per month of production. Additionally, the number ofinspected loads per month is indicated in the diagram. The totalnumber of inspected loads from 8/08 until 3/10 was 229 loads.Figure 9 demonstrates that the distortion values in serialproduction are very stable. This was achieved with the help of

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Fig. 8 Helix angle variation of reaction internal gears before and after heat treatment (LPC and HPGQ with dynamic quenching)

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the optimized LPC and HPGQ process in combination with thestable manufacturing process chain of the components.

7. Application of ‘‘Reversing Gas Flow’’ in SerialProduction

The ‘‘Reversing gas flow’’ process was applied on the FDPinion planetary gears of another 6-speed automatic transmis-sion. The gears are made of 5120 material, with an outerdiameter of 31 mm, a height of 32 mm, and 24 external teeth.One load consists of 1056 pieces treated in 9 layers, see Fig. 10.

Figure 11 shows the improvement that was achieved whenintroducing the ‘‘Reversing gas flow’’ process. When applying‘‘unidirectional gas flow,’’ the gas is flowing only from top tobottom through the load. With ‘‘Reversing gas flow,’’ the flowof gas is alternated back and forth from ‘‘top-bottom’’ to‘‘bottom-top,’’ as illustrated in Fig. 4. As shown in Fig. 11,with ‘‘unidirectional flow,’’ the parts in the middle and top layerof the load exhibit excessive distortion. With ‘‘Reversing gasflow,’’ the helix angle variations were significantly reduced,e.g., for the right flank of the gears from the top layer, themaximum helix angle variation was reduced by 61%.

With the optimized ‘‘Reversing gas flow’’ process, it is notnecessary to machine the teeth after heat treatment. Only thebores and faces of the gear are machined after heat treatment.

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Fig. 10 FD Pinion planetary gear (d = 31 mm, 24 teeth) and production load (9 layers with 1056 pcs total)

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8. Necessary Steps for Low Distortion HeatTreatment

The necessary steps that need to be accomplished to providelow distortion values are given in Fig. 12.

The design of the gas quenching cell is of key importance tominimize distortion. The chamber needs to provide a veryuniform distribution of the gas velocity to minimize the spreadof distortion within the load. Another important factor is properfixturing. Modern CFC materials (carbon reinforced carbon) arewell suited as fixture materials for gas quenching.

Further improvements can be achieved using ‘‘convectiveheating’’ for homogenous temperature distribution followed bycarburizing and the application of ‘‘Dynamic Quenching’’ pro-cesses. However, if the components contain high residual stressfrompriormanufacturing processes, it is impossible to achieve lowdistortion values during heat treatment. Therefore, an optimizedand stablemanufacturing process chain includingmelting, casting,cutting, soft machining, etc., is mandatory to create low levels ofresidual stress in the components before heat treating.

9. Conclusions

LPC and HPGQ processes were successfully applied on gearcomponents of 6-speed automatic transmissions. Helium waschosen as quench gas and optimized ‘‘Dynamic quenching’’and ‘‘Reversing gas flow’’ technologies were applied fordistortion control.

For relatively large diameter/thin wall Internal Ring gears,‘‘Dynamic Quenching’’ is used in serial production. A distor-tion study was conducted to analyze the geometric stability ofReaction Internal gears during heat treatment. The maximumchange of circularity during heat treatment was 41 lm and theaverage change of circularity was as low as 7 lm. All values

after heat treatment are well below the specified maximum. Thelevel of distortion stays at such a low level that subsequentmachining operations are entirely eliminated. This led toenormous cost savings for the transmission manufacturer.Production of the gears started in 6/2006. Results fromcontinuous distortion monitoring during production prove thatthe distortion of the LPC and HPGQ process is very stable.Therefore, in 10/2008, it was decided that the quality inspectionwill be reduced drastically, which resulted in significant costsavings.

For a planetary gear (FD Pinion gear), densely packed forgeometry, the ‘‘Reversing gas flow’’ technology is used inserial production. Compared to ‘‘unidirectional gas flow,’’ thehelix angle variations were substantially reduced. With thisoptimized ‘‘Reversing gas flow’’ process, it is not necessary tomachine the gear teeth after heat treatment.

References

1. K. Heeß, Maß- und Formanderungen infolge Warmebehandlung vonStahlen. Expert Verlag. 3. neu bearbeitete Auflage, 2007, ISBN-10: 3-8169-2678-9

2. H. Walton, Dimensional Changes during Hardening and Tempering ofThrough-Hardened Bearing Steels, Quenching and Distortion Control(Conference Proceedings), ASM International, 1992, p S. 265–S. 273

3. A. Stich and H.M. Tensi, HTM 50, 19954. H. Altena, F. Schrank, und W. Jasienski, Reduzierung der Formanderung

von Getriebeteilen in Gasaufkohlungs-Durchstoßanlagen durch Hoch-druck-Gasabschreckung, HTM 60, Vol. 1, 2005, p S. 43–S. 50

5. V. Heuer and K. Loeser, Low pressure carburizing for transmissions. In:Gear solutions July 2009

6. K. Loeser, G. Stueber, G. Welzig, and V. Heuer, United States Patent No.US 6,913,449 B2, Apparatus for the Treatment of Metallic Workpieceswith Cooling Gas, 5 July 2005, Stich, Tensi, HTM 50, 1995

7. V. Heuer and K. Loser, Entwicklung des dynamischen Abschreckens inHochdruck-Gasabschreckanlagen. Mat.-wiss. u. Werkstofftech., 2003,34, p 56–63

Fig. 12 Necessary steps for low distortion heat treatment with LPC and HPGQ

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