IIII IHIEATTREATINIG _

Selection for High Performance GearsGerald J Wall

lntrodoctionThe selection of the heat treat process

and thecongruent material required forhigh performance gears can become veryinvolved

A wide variety of options are avail~able and many issues must be consid-ered The application requirements mu t

take into consideration the service condi-

necessary The material election andprocessing sequence must take into con-siderarion lite mass of the pan and thehardenability of the material a theseinfluence the depth of hardening in thecore treating process Heavier sectionedparts and coarser pitches require higherharden ability materials or premachiaingfor proper core treating response Goodhardening respon e is also needed whencore treating prior to nitriding becausethe tempering temperature must be atleast 50degF higher than the nitriding tern-perature or typically at least L050degF toinsure dimensional stability durrngnitriding

Case Hardening Processes for HighPerformance Gears

High performance gear generallyrequire case hardening to produce therequired mechanical properties Contourand teeth-to-tooth progressive inductionhardening carburizing and hardeningand nitnding are therefore preferredwhile processes such as spin name hard-ening and conventional single-shotinduction hardening are not capable ofproducing the required properties

Each ofthe above processes is unique

Fcigure1-An etchedl section ola middotgear that hasbeen casa Ihardened by progressive inductionhardening

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tionsin whichthe gears will operate thepeeds and power robe transmitted and

the de ign We This data will in tumdefme the sizeand shape of the gears and

I~ along with the load carrying capacityrequired and the volume to be produced

II

III

allow material selection in conjunctionwith the choice of heat treatment toachieve the required mechanical proper-lies

The improved mechanical propertiesachieved by case hardening dictate tharvirtually all high performance gears are

Figure 2-A load of Ilargemiddot gears are uansfelrJedinto a furnace for carbulizing

processed in this manner Still we willinclude through hardening for coretreat-ing 11lour discussion becau e it irequired to achieve the desired propertieswhen induction hardening and nitridiing

Through Hardening GeneralRequiJlements for Core 11reating

Figure J-A load of large pinions are transfenedinto afumllce for carburizing Core treating prior to induction hard- and has its awn set of pluses and minus-

ening and nitriding is used to obtain both es relative to each other Contour andhe desired microstructure for subsequentprocessing and the required mechanicalproperties in the finished part Jn mostcases core treating is accomplished byhardening and tempering the parts prior togear cutting A hardness range of 2S-32HRC (269-302 HBN) is commonly pec-ifiedin order to obtain good mechanical

tooth-to-tooth inductionharmiddotdening in-vel ve heating and hardening only a smaJIlportion of the gear 0 size change anddistortion are relatively mall especiallywhen compared with tho e producedduring hardening of a carburized partContour induction hardening is suitableonly for small- to medium-sized gears inhigh volume application properties without severely sacrificing

machinability In contrast tooth-to-rooth inductionFigure middotl-Carbu rizmiddoted 18ars are transfemld fromthe lIeatingcliamber into the quench section for However higher core hardnesses are hardening is cost effective only for lowIhardening u ed where increased strength level are volume applications Also not aU parts26 MARCHAPRIL 2003 bull GEAR TECHNOLOGV bull wwwgeartechnolo(Jycom bull IwwwpowertrtlnsmfssiolJcom

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IFigun 5-A lugol carbulized pinion is traos-terred 101the oillque nch nlk for I arillaning

can be hardened by either type of induc-tion hardening due to their -ize andshape and neither type of inductionhardened gears has quite as high a loadcarrying capability as carburized andhardened gears

For some applications nitriding isthe heat treatment of choice becausevery good mechanical properties areachieved Also it produces distortionand size change so small and predictablethat generally hard finishing is notrequired itriding however involveslong cycle time and is therefore verycostly when trying 10 obtain deeper ca e

Figure 6-Carburjzed Case Depth Requirements ICOUrtBSYof AGMAI

through each tooth space Controlling the lower cost grades such as 15B41 wepower input and scanning speed progres- utilized where tile materials hardeaabil-sively harden the active profile and root ity is matched to the specific partarea of each adjacent tooth face to thedesired depth Since the size of the air gapbetween the inductor and the tooth sur-

requirements and large enough quantitiesare used so that availability i not anissue

Contour Induction HardeningFor higher volume applications of

small- to medium-sized gears ill the 8--4DP range contour induction hardening isa viable option and call compete withcarburized gearing Contour inductionhardening i similar in appearance 10 theclassic single shot spin induction

face is critical a custom inductor is oftenrequired for each gear design For fine- tomedium-pitch gears this process is gen-erally perfonned with the part submergedin a qaenchant

As each tooth space must be individu-ally canned the floor-to-floor time forhardening a gear in this manner is rela-tively long when compared to contourhardening where all the teeth are hard-

i process where the gear is rotated in anI induction coil to heat the teeth and then

i ened at one time Offsetting this is the irequired carburizing and hardening is i versatility and size range capability of the Iiusualy relied upon because itproduces i tooth-to-tooth proces Gears from 10-1excellent mechanical properties and is I) DP are routinely hardened in thi manner isuitable for low and high volume appli- To determine the case depth and pattern cation Size change and di tortion call I produced it is necessary to cross-section II

Geral~dJ Wolfbe erious problems but they can be I several teeth from a hardened gear in con- i is (l gearileal treat consultamdealt with trast to being able to check a test piece I who specializes iI gear faillln

Tootb-to-Tootb Progressive Induction i when carbunzing or nitriding (see Fig 1) analysis and materials A metal-

H d A core treated medium-carbon alloy lurgical engineer Wolwasprf-

Tooth-to-too~nin hardening L steel uch as 4142 4[50 or 4340 is gen- viously president aTire Cincinnati Steel Treating Co of

often used for loWIO medium volume I erally specified for most applications Cincinnati OH He has mort

requirements when Ihe ize or de ign of j with the choice dictated by the diametral than 30 years of experience ill h d si f h Til I middothmiddot the Ileal trearing industry Hetile part allow this proce s to be used pile an SIZe0 u e gear e arger t e spell lIe lasl 25 of IllOse ye(lr~

In tooth-to-tooth induction hardening an i part and coarser the pitch the higher the 01 Cincinnati Steel Treating

inductor with a profile matching the hardenability reqaired for good hardening specializing ill heat treatingshape of the tooth space is traversed response In higher volume applications gears

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depths required for coarser pitchesBut where the best performance is Quenched in place In most ca es how-

ever this through harden rather thancase hardens the teeth The contourinduction hardening process has beendeveloped ro produce a heal pattern that

follow the profile of the teeth and pro-duces a true case hardened tooth This isgenerally accomplished through the useof high intensity multistage heatingwhere the teeth are first preheated usinga low frequency and then the tooth sur-(aces are rapidly heated witll a burst ofhigh power high frequency energy

The nature of contour induction hard-ening requires thai the tooling procesparameters and equipment be tailored to

HEATTREATING _

each application Multiple high-peweredpower supplies with very sophisticatedcontrol ystems are necessary Thimakes the process capital-intensive andeldom practical for lower volume appli-

cations but justifiable for higher vol-umes Another advantage it has over all

other gear hardening processes is that itlends itself to integration into manufac-turing cell especially when combined

with induction tempering

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Carburizing and HardeningOf the ca e hardening proce ses car-

burizing and hardening i the mo Iwide-ly used process today for heat treatinghigh perfermance gears It is adaptable toboth high and low volume requirementsas well as to both large and small parts(see Figs 2 and 3) And with the properchoice of materials and processing tech-niques it will produce the highe t allow-able contact and bending stress ratingsobtainable

o achieve the desired result it isirnpestant that the material selection be

in accord with the nature and size of thegear as well a it erviee requirementsAll carburizing grades are low carbonalloy steels but coarser pitch heavierections and more severe service necessi-

tate using higher hardenability higheralloy and more expensive materialsCommon grades in increasing harden-ability are 4118 8620 8822 43204820 9310 and 17CrNiMo6 Thesematerials areal 0 produced in differentquality level to comply with the differ-ent AGMA grade requirements depend-ing on service requirements

It is recommended that prier tomachining the lower alloygrades be

normalized and the higher alloy gradesbe normalized and tempered or normal-ized quenched and tempered to obtainimproved machinability a well a morereproducible size change and dn rortionre ults during sub equent carburizingand hardening

From a manufacturing standpoint the

figurel-A load 0middot1 diese11 engine gears isremovedlfrom the fu~n_le after nitriding

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_--------HEATTREATING _

OQ01 2 3 5 7 0010 2 3 5 7 0100Total lBSlB depth he bullbull IOOiles

Figurel 8--Nitrided tau IlJepthRequirements (Courtesy 0middot( AGMAImo t seriou problem with carburized of theseproces es require handling eachgearing i the size change and distortion part individually they often ignificanllythai occur during hardening Thi i increase proce sing co t

very design-dependent Gears that have Th depth of the carburized case mustnonsyrnmetrical C[(10SS ection or are be in a cord with the diametral pitch andheavily relieved for mass reduction will service requirements of the gear Coarserpresent more problems than relatively pitches and more severe applicationsolid symmetrical part require deeper casesigure 6 hows the

While proper fixutring must be AGMA chart for determining the effee-employed during earbunzing to mini- tive case depth to be used for a givenmize creep-induced distonion pan ori-entation during hard ning is most criticalin order to obtain a unjform quenchQuenching in hOI (250-350degF) oil mar-quenching and high-pres ure gasquenching can often be employed toreduce the size change and distortion butince Lhey ltICe lower quenche it is

often neee ary to use a higher harden-ability grade than nece ary with con-ventional oil quenching ( ee Fig 4 and5)

When the de ign of the gear i suchthat exce ive distortion is produced dur-ing free quenching technique uchasplug andor press quenching should beused For ize and shape control of thln-

alled gears and internal spline plugquenching i recommended Howe verwhere the movement of the whole gearmust be controlled pres quenching isnece sary This require th availabilityof asuitably sized quench pre sand gen-erally custom tooling for holding the partduring the quenching prooe s Since both

diarnetral pitch The effective case depthis typically defined as the depth at whichthe hardness has decreased to 50 HRCand is measured on a repre entative lestsample or a sectioned part

itridingNilridingis another case hardening

process that has been gaining populnrityforuse on high performance gearing dueto advance that have been made in bothgas and ion nitriding Nitriding producea hard urface with excellent wear prop-erties improved cerro ion resistance andvery good high-cycle fatigue propertic This is a rather unique proces in thar itisperformedat a relatively low tempera-ture in the 950-1OSOdegF range andquenching is not involved Since the basematerial does not go through any structur-a] change size change and dis tnrtion arevery low and rarelyi any finishing nee-e ary after nitriding (see Fig 7) Theprocessing lime required however is rel-atively long especially for coal er diame-teal pitches where deeper co e depths are

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HEATTREATINGI _

I

Tabl1-Allowabl lIesign Stress fOf Steall Gaars I

II Contact BendingHeat Treatment Malariel Minimum Strass Stress

Surface [KSI] [KSIIHardness GR GR2 GII3 GRI GJIl GR3

Through Hardened 414n4340 32 HRC 127 140 I 36 4843 HRC 60 75 43 58

Induction Hardened I 104fyen4340 MHRC 175 195 II 45 55Ca rburized and

II

I

Hardened 1182019310 58 HRC 1110 225 215I 55 65 75

NitrideII 414014340 845 HR15N 155 168 180 31 50NitrallQY 135M 9OHR15NJ 170 183 195 36 50

31CIMoV9 lOHR15N --li 196 216 52 60

Table compiles datafrom various tables and graphs in ANSIIAGMA 2OO1-C95Tooth-la-tooth and contour induction hardening with ANSIAGMA type A patternMinimum surface hardness of 55 HRC is acceptable for GAl all other grades raqulra aminimum of 58 NRCbullbull Basically thesame as 25~Cr wiUlout AIThe seven bending 51Iess values for nitrided materials Bre based on a core hardness01300 HB

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Three different types of medium-car-bon alloy reels are used for nitrided gearsdepending on the ervice requirement InaU case bull core treating is employed 10 pro-duce both the desired strength and thestructure needed for good nitriding

response When the service requirementsace not too severe 4140 or 4340 i typical-

ly specified For more demanding applica-

tionsit is necessary 10 use a grade such asNitralloy 135M or 31CrMoV9 that is

especially designed for nitriding iguTe 8shows the AGMA chart for determiningthe total nitrided case depth required for

different diarnetral pitchesSummary

The purpose of this paper was to give a

general overview of the different heat treat

proce ses alld material used in hlgh per-formance gearing and thus help guide thedesigner and process engineer in makingthe best election for a given job Iable ]outline values of contact and bending

allowable stre ses for each of the afore-mentioned hardening processes For full

data and detail ee ANSIIAGMA 2001-

C9S0

This arficle was given as a presentation atthe Heat lireating amp Hardeningl of GearsZOOZTechnical Program in Nashville bullTNon January 29 2002 thaI program wasorganized by Ihe Society lid ManufacturingEngineers

Table 1 is al revision 10 materiallpresent-ed in ANSJlAGMA 2001-amp95 The originallmateriai is printed with permission ot Utecopyrlight holder~ the American IGearMainufacturersAssociation 150D KingStreet Suite 201 Alexandria Virginia22314 Statements presented are lhose ofthe author and may not represenUhe IPosii-lion of die American IGear ManufacturersAssociationTen Us What YDU Think Visit wwwtlffechnologycomtobull Rate tIIiamelebull Requm more infonnatiDnbull Collllet the aulhot Of lizltiCNI MIlionedbull Make IUllntiOftOr call 1841 437 to bilk to one of OW eelilDnl

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