The Fundamentalsof Gear PressQuenching

L E.."Skilp" J'ooesLindberg Technical Be Management Servi,ees, IGIfOUp

Charlotte. NC

Ab tract: Most steel gear applications requireappreciable loads to be applied that will resultin high bending and compressive stre e. Forthe materia] (steen 10 meetthese performancecriteria. the gear must be heat treated.Associated with this thermal processing is dis-tortion, To control the distortion and achieverepeatable dimensional tolerances, the gearwill be can trained during the quenching cycleof the heat treatment process. This type of fix-ture quenching is the function of gear quenchpressing equipment.

IntroductionTo understand press quenching, we must

first under tand the phenomenon of quenching

Volume

Transfurrnatinnto ma.rtensite

32 GEAR TECHNOlOGVFig. 1 - Valume changes due to structural transfermatinn,

and the re I:IIt3nt Ires e within the gears thattranslate into dimensional changes or distor-lion. When we refer to the basic definitions of"press" and "quench," we see that

Quench "is th imnree ion of metal into aliquid to extract h at"; and that

Press "i to exert a teady force or pressureon a component to achieve a desired shape."

Therefore, gear press quenching can bedefined as "heating a gear to the austeniticcondition and then immersing the gear into aliquid to achieve metallurgical transformation,while at .the sam. time constraining the gear tohold size and shape."

The most important thing to rememberabout gear pre, quenching is that its purposeis only to can trnin she part and to u e onlyenough force to counteract the parts' transfer-marion stres e . Press quenehing is notdesigned to reshape or forge gear into theirfinall shape.

Causes of DistortionsGear distortions can be caused by several

factors and in any number of combinations. ]nthe heat treating process, the gear is subjectedto both thermal expansion and contraction.Non-unifcrmity in heating and cooling cancause stresses in a part. which, if large enough.can cause deformation. Consideration mustalso be given to the phase changes involved inhardening teel that re ult in expan ion andcontractions, Variations of this stress withinthe gear will cause deformation, Material com-position variations resulting from the steel pro-

duction process, grain flow variations causedin forging, and stres es due to pre-heat. treatmachining can all affect the dimensional stabil-ity of a part during hardening operations.

The magnitude of the distortion in heattreating is affected by the cooling rate, methodof cooling, transformation temperature andpart geometry. To minimize distortion, it isbest to use a slow cooling tate and a methodthat will maintain a uniform temperaturethroughout tile part.

It is critical !hat the transformation tempera-ture be consistent throughout the piece, so thatduring cooling the entire piece transforms atone time. However, a carburizedand hardenedgear that ha a composition gradient and,hence, a transformation temperature gradient,is more susceptible to distortion. (See Fig. l),

Finally, the shape of a. part strongly influ-ences the degree of distortion it may undergo.Parts that are large and thin are more suscepti-ble to distortion than parts that are compactand massive. Therefore, a complex gear also isa more viable candidate for press quenchingthan is a simple one.

Revie' or Quenching and SystemsFundamentally. quench systems, regardles .

of the quench media - oil, water, polymer orbrine - are similar. Agitation, temperature con-trol and quench ant contamination control areof primary importance. Since the primaryquench medium used in gear press quenchingis oil, the emphasis win be focu ed here.

The quenching of steel provides the rapidcooling that is required to convert the austeniteto hard martensite, The prime factors thatdetermine the quench speed required are steelchemistry, part geometry and designed func-tion of the part.

Too low a quench rate will result in softparts. Too high a quench fate may result inexcessive distortion and cracking.

Quenching has three distinctive stages:I. Vapor blanket stage - Delta temperature

creates an insulating vapor in slow cooling.2. Vapor transport stage - Liquid breaks

through the vapor barrier; rapid heat transfertakes place.

3. Liquid stage - Cooling is taking place byconduction and convection. At this stage, thepart temperature is above the boiling point ofthe quenchant, but the rate of flow prevents the

760c (1400 FI6500 c mODO FI5400 c (10000 FI4250 (BODOFI315c (600" FI

----------------------- --- -- -

Fig. 2 - Temperatures during quenehinq.

L.IE. Jonesis rill! Manager of theTechnical & Management' Services Group.Lindberg Heal TreatingCompany. He is amemeber of ASM. theAerospace Council(SAEIAMC) and thelntemaiionai Federaiionof Hear Treaters and theauthor arid editor ofnumerous books andpapers OT! heat treatingand metellurgy.

M' ... RCH/~PRIL 19~' 33

quenchant from boiling. (See Fig. 2).The control of the vapor blanket stage is

critical to the control of a consistent coolingrate for uniform heat extraction from the gear.Therefore, proper agitation Of rapid movementof the quench ant is essential to prevent forma-tion of vapor bubbles around the gear whichinsulate it and would result in irregular surfacehardness and excessive distortion. As a rule ofthumb, proper quenchant movement is definedas a volume flow of oil equal to two to threetimes the surface area of the gear in a oneminute period.

To summarize, a proper quench systemmust have control of the temperature of thequench media, adequate quenchant movement,and consistent quenchantchemistry ..

The Mechanics of the Press QuenchThere are several manufacturers of quench

presses, such as QPS, Inc., Gleason Works,Inc., Klingelnberg. Oerhkon and JennyPresses, to name just a few. All quench pressdesigns must incorporate a sufficient quanti-ty of" quench oil, variable quenchant flowrate. quenchant flow direction. die holdingpressure, die contact points. and cyclic flowand pressures. The various press designseither use pneumatics or hydraulics for thepressure systems.

Our focus on a typical press quench opera-tion wHl feature a press that uses hydraulics formechanical movements and pressing force.(See Fig. 3).

Basically. this type of quench press con-sists of a very rigid rectangular-box type con-struction. The base is a tank which acts as areservoir for the quenching oil. ]t also servesas a support for the lower die. The upper por-tion of the machine contains the upper dieram assembly, hydraulic units, and the elecm-cal panel. The opening at the front allows fullaccess for changing the upper dies; the lowerdie table moves outward and inward for load-ing and unloading.

During operation, the component to bequenched is removed from a furnace (usually IIpusher-type continuous or rotary-hearth-type),and placed onto the lower die in the "out" posi-tion. The automatic cycle moves the loadedlower die as embly into the center section ofthe machine and centers the die location. Next,the upper ram assembly descends, with anexpander centering the pan just prior to theinner and outer dies' location on their respec-tive pressure points. The inner die. outer dieand expander havecompletely independentpressure controls, (See Fig. 4). A circularguard completely encloses the upper dies toform a quench chamber which is affixed to theupper ram for movement. The chamber fillswith oil flow from the lower die once the diesare in position. A variation in oil flow is con-trolled by three solenoid valves. Anotherunique feature is the ability to pul: ethe variousdie components (that is, maintain die contacton the component and cycle the pressure every

-----

fig. 3 - Schematic - lypical quench press.

34 (JEA~ TECHNOLOGV

two' econds or so). Also, the lower die is cam-adjustable .. so the die rings can be rai ed orlowered to compensate for a predeterminedamount of component dishing or camber.(See Fig. 5),

The loading of the hot components can beeither manual or robotic. The transfer mecha-nism is critical to assure a minimum of partheat loss from the furnace lathe die. Thetemperature of the gear at the time of enter-ing the quench is a determining factor of thefinal gear size,

The press quenchant ystem typically con-sists of the holding tank within the press, anexternal oil reservoir, circulation pump, healexchanger, and internal heating source. Theoil reservoir typicaUy holds one and one-halfto two times the holding capacity of the pressitself. (See Fig. 6)..

Within the quenchant system, oil tempera-ture is maintained within 5F plus or minusduring operation. The operating oil tempera-ture for most gear press quenching operationsis between 130F and 145F.

Centrol (If the Gear PressQuenching Process

This review of the control process willfocus on carburized, case-hardened gears dueto their high susceptibility to distortion, Whenexamining a press quenching operation, COI1-sideration must be given to part geometry,chemistry of the component and the volumetricchange resulting from the hardening process ..Also, it must be remembered that the quenchpressing operation will only round up and f13.I-ten the hot plastic gear. but will not changetooth shapes. With this in mind, it is imperativethat the gear's prior manufacturing operationsare closely controlled to assure consistent pressquench results. The gear coming out of thepre s operation can only be as good as the geargoing in, The variables that must be controlledbefore heat treatment are:

Gear material composition, Grain direction and size, Prior thermal processing (normalizing). Machine stock removal, Cold work. stresses, Tolerances and sizes of reference surfaces.Once the gear enters the heat treat process,

the procedures employed must be consistent.repeatable and closely controlled. The vari-

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ables that must be controlled in caJ'burizingare:

Furnace temperature uniformity for heat-ing rates and temperature distribution,

Control surface carbon content and repeat-able diffusion gradients,

Gear fixturing during carburizing, Coating rates after carburizing, Repeatability of temperature, uniformity,

time at temperature and control cooling rates ifannealing operations are used.

The reheating operation requires clo e con-

Pressure

fig. 4 - Die systems and ram pressure.

) IAdjusting CAM for dish and camberFig. 5 -lower quench die systems.

36 GEAR. TECHNOLOGY

trol of: Temperature uniformity, Time at temperature. Fixturing and furnace loading, Atmosphere control or consistent stop-off

procedures for surface integrity, Gear handling from the furnace to the

press: Pickup contact and transfer time .To successfully operate thistype of press

quench, the factors must be regulated: Quality of quench oil supplied,

Duration of quenching, Quench oil temperature, Direction of quench oil flow. Pressure applied to hold the component, Location of component holding points.In setting up tbis ki nd of press quench

machine for operation, it is well recognizedthat the actual pressures and lime settingmust be developed for each eornponem bytrial and error. The following are generalguidelines:

I. A fast quench or high oil flow is favoredin the first stage to reduce the temperature ofthe part to the transformation range as quicklyas possible. However. the flexibility availablein the choice of duration and amount of oilflow permits a compromise between the needfor a fast quench to ensure satisfactory hard-ening response and the desirability of a slow-er quench to minimize distortion.

2, The low flow rate in the second stageallows the temperature through the variouscross sections to equalize. Thus, further cool-ing and marten sitic transformation takesplace with reduced internal stresses.

3. The third state returns to high oil flowwhen the component has nearly completed!transformation. The high quenchant flow ratecools the part for operator handling.

4. In setting the pressures for the inner andouter dies, it is recommended that minimumlevels be used. Use only the amount of pres-sure necessary to true up the component(high pressure settings can result in excessivedistortions).

The variables within the acnial press opera-tion are:

Die contact, Die oil flow, Oil flow rate and Delta temperature,

Smooth movement of lower table, Die closure rate, Consistent ram pressure on all die rings, If pulse pressure is used, repeatable time

for on and off cycles, Tooling car,e - nicks. burrs, scale or dirt

should be avoided so nor to interfere with themechanical truing,

The control. of gear press quenching opera-tions cannot be i elated for control of gear dis-tortion. The final results of the process aredependent on all prior operations, Therefore. ifthe gear manufacturing process is well docu-mented, and the overall process is in control.from the raw material stage to press quenchingoperation, repeatable results will be achievedwith predictable size changes.

Gear Press Quench InstrumentationAs the demand On today's industrial quality

systems and the need for system documenta-tion and repeatability inerea e, more interest inpress instrumentation systems is being generat-ed. To date, development of instrumentationsystems for press quenching has ranged frommechanical gauge to complex computer inte-grated systems. However simple or complexthe instrumentation is, the Information collect-ed must be analyzed and correlated to the over-all prior manufacturing process to gain controlof the quench press operation.

A completeinsnumentarion system willdocument the following:

The location of the lower table, Time measurement of the cycles indicat-

ing travel time, air lockup, cycle delay, troke,cycle quench on, pulse on and off,

Linear measurement of stroke movement, Quenchant flow rate, Quenehant oil. temperature control, Quenchant Delta temperature through the

die chamber, Hydraulic system pressure, Die position indicators. Non-contact temperature sensing of part

temperature entransfer, Hydraulic sensitivity for dimensional

measurement of the gear,.' Contact sensing device for measurement

of part cooling rate,With tabulation of the above data and

immediate analysis, adjustments to the press

'\ Tempature

\-------,----,-,r-----t--

IiII

L -O~e'iters Jl" ~'?: ."..,,&~~.

I

1Outlet

Drain

Oil Reservoir

0\Pump

Fig. 6 - Quenching recirculation svstem.

----------

Fig. 1-1ypical die and quench chamber.-------

quench operation can be made on-line, thusreducing rejected parts and increasing the pre-dictability of the final as heat treat size,

Aeknowledgement: Presented at the SME Conference.Heal Treating of Gears, August 26. 1993. Livonia. MI. /993_ Reprinted wid: permission.

MARCH/APRll1990 37