_------------ HEAT TREATING FOCUS _

The Submerged InductionHardening of Gears

DW Ingham and G Panish

Fig 1-TypicallwdenjJg pattern

IFig 2-The induclor

This article is based onpapers prelliDusly presentedin Heal Treatment ~fMetals19981 arId 19982 pub-lishedb The Woy on HealTreatment Cemre AstonUniversity Asto TriangleBirmingliam B4 7ET UK

Tnis article was also pre-sented a the 1999 FallTechnical Meeting of theAmericai GetU MtlnuJilctur-us Association

Prifltedwith permissionof the copyrigld Iwlder tIeAmerican Gear Manufac-turers Association 1500King Street Suite 201Alexamlria Virginia 22314Copies of the paper areavailable from theAssociation~ Statements pre-sented in this paper are thoseof the Authors and may notlepllesent tlmiddote position oropinio 0 tire AmerictmGear Mallufactz4rers Associa-tiOIl

IntroductionThe tooth-by-tooth sub-

merged induction hardeningprocess for gear tooth surfacehardenmg has been successful-ly performed al David Brownfor more than 30 years Thatexperienc~acked up by in-depth research and develop-ment-has given David Brown

engineers a much greaterunderstanding of and conti-dence in the results obtainable

from the process Also field

28 MARCHAPRIL 2001 bull GEAR TECHNOLOGV bull wwwgflllrflChflologycom bull wwpawstransmlssioncom

experience and refinement ofgear design and manufacturingprocedures to accommodatethe induction hardeningprocess now en ure lilial gears

so treated are of guaranteedquality The processs purposeis to produce a continuous

hardened layer wllicll extends

alongthe tootIJlenglb and fromthe [Ooth tip down its flankaround tile fillet and root areaand up tile opposite flank to Olenexl tooths tip (Fig I) and toensure the depth of the hadLened zone is sufficient so the

subsurface high tooth stressesare cOlltainedin thehighstrength regions

In the submerged toolh-by-tooth process the inductor(Fig 2) which has essentially

the same shape as the spacebetween two adjacent gearteeth is energized and tra-versed along the tooth spaceheating and austenitizill1g the

neighboring tooth surfacesincludingthe root-fillets as itgoes The heating operationOCCUfi below the quenehantssurface so a soon as theinductor has moved on tt isreplaced by the surroundingqueachant thus healing and

quenching are localized pro-gressive bulland of short duration

The heated and quenchedzone is so localized that distor-tion and growth problemswhich tend to plague carburizecase hardening are essentiallyavoided High urface hardness

bull 11HEAT TREATING FOCUS 11 _

energy by the 14] workhead itransformer in the gear han- i

Idling machine

The water-cooling tank sup- I

plies three recireulatory line

erator water-cooling tank oil- supplies etccirculation tank and oontrol Over many years Davidconsole Brown performed research

The gear handling machine projects onlhe proce s be ides Fig 5-Typical indllctoMD-worlIlJlace COllpling

and surface compressive resid-ual stresses imparted by meprocess dramaticaUy improvethe contact and bending fatiguestrengths

This article dealswith manyaspects of the proees itsellfdescribes problem areas con-siders applications and discuss-es the products properties andquality

The Induction HardeningProcess

At David Brown the fre-

quency used for gear inductionhardening is 96 kHz and therange of tooth sizes processedis 8 to 38 module Figure 3is aschematic drawing of the facil-ity which is adjacent to a gen-

rigidly supports the gear accu-rately rotating aligning andindexing it durillgprocessingThe water-cooled inductor issecured to a workhead trans-former that is mounted OD acarriage in the gear handlingmachine (Fig 4) The work-head transfonner can be set totraverse a distance of morethan one meter on linear bear-ing tracks The inductors actu-al travel length is controlled bypreset Limit switches Themachine is meant for theprocesss submerged versionwith the inductor at the bottomcenter position Consequentlymuch of the handling equip-ment is in an open tank filledwith quenchanr during process-ing and drained for loading andsetting up

The generator which pro-vides lip to 75 kW cenvertsjhemain power supply of 380 V50Hz to a medium frequency(96 kHz)1 supply at a nominalvoltage of 500 V That is trans-formed [0 a supply of 50-V

a) to the inductor which is ~

I II

I r---------------------------------------~I

ellergy requirements Conse-quently the shaping of theinductor (Fig 5) i important tooptimize the coupling The inductor is designed for rigidi- II

ty to ensure accurate geometri-cal positioning I

[

Research has shown the I

heating effect is controilled by I Ij

the inductors design The i

David Brown design includes (I

two copper sides connected bya copper bridge along the root iTaennocouples in the body of Il1g7-TJpical bamprdeningplttern

capable of some heating via itsown resistance and by radia-lion ampom tlte workpiece duringprocessingb) to the qllencnants heatexchanger andc) to the generator and theworkheaduansformer

The control console man-ages the induction hardeningprocess by control of the indue-tor traverse speed inductorenergizing and de-energizingquenchant flow cooling water

pr-oduction hardening Con-sequently relationships be-tween hardening parametersand hardened depthpatternhave been establi hed eliminat-ing the need 10 establish pa-rameters on separate test pieces

The process is controlled byseveral signi ficant parameters bullthese being

1) The Inductor WorkpieceGap The space between theinductor and the gear tooth iscritical The surface-to-volumeratio differences around thetooth profile demand different

089 mm

(learTooth

165mm

85moo35kWpower150mmmintraverse speed

900

800

I 700u

e600

~500~4()()1

Io300 I

200 I100 Leading edge 01inductor raac lies measurement position

4 6 8 10 12 Ti14 16 18 20 22 24 26 2830 32IITle seconlls __

Fig hAn example ol the temperature distributionl within 61geertooli1l during Iniinduclor pass Tb depth and localionohl1e Ilimperaluruen-sor is indicated againstleach temperatllra Icurve

700-----------------60

- _-6DO

uaIlbull 45 1I

400 -4118~3G

33

Distance from surface mm

wwwpow rtrensmtsstooeom wwwmiddotge rtecnnologycom bull GEAR TECHNOLOGY MARCHAPRIL 2001 291

_-----------HEATTREATINGFOCUS------------+400 I

+300

I

+2001II

I a tooth being hardened haveI shown a typical temperature profile (Fig 6) On the mid-

I flank position two tempera-ture peaks are experienced

coinciding with passage of themductors copper sides In the

root position a single peak isfound associated with the cop-per bridge

David Browns practice

involves the exclusive use ofnumerically controlled rna-chine shaping of inductor

blanks The use of accurateshaping means it is only neces-

sary for the operator to ensurethat the inductor is aligned

central to the tooth space andthat the root gap is correct

When that is done the induc-

tor-to-workpiece gap at otherpositions around the inductorwill be correct

2) The Power As power is increased the depth of heating

1 is increased for a tooth size IIj naturally follows that the largerI

the tooth size the larger the ~ power requirementsi 3) Inductor TraverseI Speed Traverse speed deter-

mines the depth of heating byallowing more time for heatdiffusion Sufficient time

should be available to allowtransformation to au stern teoResearch by a dilatometrystudy showed that for an817M40 (4340) steel in the

quenched and ternperedcondi-tion three seconds wererequired to achieve carbonsolution and that a degree ofcoarsening with a slight reduc-tion of hardness took placeafter nine seconds Thereforeheating times within the three-to nine-second range are nor-

mal for the process whichmeans that if the inductor hasan effective length (timeabove AC3) of 18rnm the tra-verse speed range will need 10

+100

Surl~ hatd~ 750HV

~82middotmoduie

3CrIMQ 6teeltempered a1200C

~II------- -

+300

+00 I

5 6 7 a aDistance ITom ilUrfllC mm

~ 630HVSAE 4J40 steel

tempered at BOC

700

+600 - Surface hardl1ess 650HV

30 MAHCHAPRI L 2001 bull GEAH TECHNOLOGY bull WWW~9Iut9chnologycom bull wwwpow6rtramiddotnsmissoncom

+ltWO -

+300-

1 200

~ SAJ UWsteelIl00ot-_tL_e_m_pe~~_-at~2OOC__- ---~f--=-- ---~

2 3 8 9IDistance from

surfacemm

fig S-lxamples Df residual stI1esses through an induction-hardened case Tesllocations shown AJrow denoles approximate effective case deplb (10 450 HVI

be in the approximate range of

125 mm per minute to 350mm per minute

4) Quenching and coolingjets Surrounding the mountedinductor are a) the fore and

an qnenchant curtain jetswhich help stabilize the vaporphase that occupies the cou-pling space and hasten andcontrol the quenching and b)the side sprays-also curtainjets-which play on the tooth

top edge and adjacent flank to

control the heating pattern onthe tooths top and the

amount of back-tempering onthe adjacent tooth addendum

The settings for those jets

and the quantities of quench-ant flowing through them are

important5) Power switching When

a tooth space is to be hardened

the inductor is automaticallyadvanced into the tooth space

to a distance equal to about hal1the inductors length Ar thatpoint the inductor is energized

and-after a hort dwell at theentry-the inductors traverse

along the tooth pace com-mences Similarly at thetooths exitend the inductorstops dwells and Is de-ener-

gized That generally en uresa satisfactory hardening pat-

tern at the tooth ends Butexperience has shown that onoccasions the exit pattern

could be improved by cancel-ing the dwelJ and running

through 011 full power or byrunning through and de-ener-gizing during the exit Thoseare minor adjustments aimedto ensure a good product

Steels For InductionHardening

At David Brown we adopt-ed the policy of using mediumcarbon a1Joy steels of the 4340type composition for inductionhardened applications The

produce u pre-tempered sur-face hardness of more than 57HRCand a tempered surface Ihardnes of typically 55 HRC

iWith today inherently cleansteels the material basic Iqllality i nO1a problem for the )lIardening proces

The gear blanks are Ithrough hardened and rem- Ipered either as forgings or j

afler rough machining ITempering hould be II ed to ieliminate residual sire se in Ii

the gear therefore high tem- peringte mperatures (gt600DC)

should be u ed The re ultingtempered marten ilic micro-structure is mo t uitable forinduction hardening becau e itis homogeneous with re peetto carbon and the carbides

particle size is small whichfavors easy soluI1011during theshort induction heating periodie 3-10 seconds The as- I

hardened and tempered fltrength need not exceed

8boul 1000 Nmm2bull

Therefore gear cutting and I

other machining operation

are not difficuh to perfonn 1

R uUing Properties1) liardlltss Figure 1

shows a typicalltardness di 1Ii- ibution Induction hardened ur- Ifaces for which the carbon

content is nominally 040 C iu 1Ia1ly have hardne S value Iof more than 55 HRC and lipto 60 HRC a hardened iTempering al 200-250degC Ireduce hardne liightly 10

about 54-57 HRC Two fea-tures hould be noted an addedplateau of hardne (brokenIi lie) and a trough in the curvejust below lhe case-core june-tion The first feature which isoccasionally observed mayrelate to the extent of carbonsolutionand the degree of ear-

bon homogeaiaation in the all tenite phase noting that for Ia steel such as 4340 it will take Iaboutthree econds to dissolve the carbides but more time to

-500

u

~I

leanmiddotalloygear mel

-200

achieve a modest degree ofhomogenization Solution andhomogenizati n are betterserved by having the fine car-bide characteri tic induced by j

previous hardening andtem- Ipering The trough at the hard- Iened zones end i anributedto ihort-term tempering The end i

denotes where tile (e~peratllre Idue to induction heating bad Iattained the A I value of say I725degC But if he steel was pre- iviolJsly tempered at 650degC the ~core immediately beneath the Icase will have experienced i

heating within the 650-725degC j

range and hence some addi-tional tempering

2) MicmstTuctures Aninduction-hardened low-tem-perature tempered materialshardened layer usually consistsof fine tempered martensite and the tructure has II much

relined allsteniticgrain-sire-Ithough that i not usually apparent Process parameters iare selected [0 avoid develop- I

f menl or coarse marten middot1tICrnicrostructures which can j Fig 11J--Eftecto~UIIplring 0l11111rtaC81 residual com rressivllIrnS1II

negalive1y influence the hard- Iened layers toughness

All induction hardened 1layers microstructure does not i

always appear marten itic but itempered structure though

much finer Still induction

hardenings hardness valuesare typical of the marlensitic i

condition

3) Resi4ual StressesHealing of a steel surface by Iindllction currents will be iaccompanied by thermal Iexpansion and a superimposed i Fig 11-Eflecllol tempering Ionthe hardness profile 0 an inductiDn hlrdnedl gearcontraction when the material t middotIooth Uanl(SAEI140steel)

wwwpow9rlrIlISminioflcom wwwgBSrlllchnoiogycom bull GEAR TECHNOLOGY MARCHAPRIL 2001 31

sometimes tends to resemblethe original quenched and

500

Figl~Hec1 01CIS - depth onlmiddotlUJ1acemidual colIPrmivl Ibtnm

-600

-500

-400 -

045-050 ClMrVTI staal90 minute soak period

-100--a 100

Tempering Temperauira middotC

700-

600

comiddot -ci 500c -Ec ~I

300

2DO -o 2 3 4Distancelrom surface mm

5

_-----------HEATTREATINGFOCUS------------+400 I

+300

I

+2001II

I a tooth being hardened haveI shown a typical temperature profile (Fig 6) On the mid-

I flank position two tempera-ture peaks are experienced

coinciding with passage of themductors copper sides In the

root position a single peak isfound associated with the cop-per bridge

David Browns practice

involves the exclusive use ofnumerically controlled rna-chine shaping of inductor

blanks The use of accurateshaping means it is only neces-

sary for the operator to ensurethat the inductor is aligned

central to the tooth space andthat the root gap is correct

When that is done the induc-

tor-to-workpiece gap at otherpositions around the inductorwill be correct

2) The Power As power is increased the depth of heating

1 is increased for a tooth size IIj naturally follows that the largerI

the tooth size the larger the ~ power requirementsi 3) Inductor TraverseI Speed Traverse speed deter-

mines the depth of heating byallowing more time for heatdiffusion Sufficient time

should be available to allowtransformation to au stern teoResearch by a dilatometrystudy showed that for an817M40 (4340) steel in the

quenched and ternperedcondi-tion three seconds wererequired to achieve carbonsolution and that a degree ofcoarsening with a slight reduc-tion of hardness took placeafter nine seconds Thereforeheating times within the three-to nine-second range are nor-

mal for the process whichmeans that if the inductor hasan effective length (timeabove AC3) of 18rnm the tra-verse speed range will need 10

+100

Surl~ hatd~ 750HV

~82middotmoduie

3CrIMQ 6teeltempered a1200C

~II------- -

+300

+00 I

5 6 7 a aDistance ITom ilUrfllC mm

~ 630HVSAE 4J40 steel

tempered at BOC

700

+600 - Surface hardl1ess 650HV

30 MAHCHAPRI L 2001 bull GEAH TECHNOLOGY bull WWW~9Iut9chnologycom bull wwwpow6rtramiddotnsmissoncom

+ltWO -

+300-

1 200

~ SAJ UWsteelIl00ot-_tL_e_m_pe~~_-at~2OOC__- ---~f--=-- ---~

2 3 8 9IDistance from

surfacemm

fig S-lxamples Df residual stI1esses through an induction-hardened case Tesllocations shown AJrow denoles approximate effective case deplb (10 450 HVI

be in the approximate range of

125 mm per minute to 350mm per minute

4) Quenching and coolingjets Surrounding the mountedinductor are a) the fore and

an qnenchant curtain jetswhich help stabilize the vaporphase that occupies the cou-pling space and hasten andcontrol the quenching and b)the side sprays-also curtainjets-which play on the tooth

top edge and adjacent flank to

control the heating pattern onthe tooths top and the

amount of back-tempering onthe adjacent tooth addendum

The settings for those jets

and the quantities of quench-ant flowing through them are

important5) Power switching When

a tooth space is to be hardened

the inductor is automaticallyadvanced into the tooth space

to a distance equal to about hal1the inductors length Ar thatpoint the inductor is energized

and-after a hort dwell at theentry-the inductors traverse

along the tooth pace com-mences Similarly at thetooths exitend the inductorstops dwells and Is de-ener-

gized That generally en uresa satisfactory hardening pat-

tern at the tooth ends Butexperience has shown that onoccasions the exit pattern

could be improved by cancel-ing the dwelJ and running

through 011 full power or byrunning through and de-ener-gizing during the exit Thoseare minor adjustments aimedto ensure a good product

Steels For InductionHardening

At David Brown we adopt-ed the policy of using mediumcarbon a1Joy steels of the 4340type composition for inductionhardened applications The

produce u pre-tempered sur-face hardness of more than 57HRCand a tempered surface Ihardnes of typically 55 HRC

iWith today inherently cleansteels the material basic Iqllality i nO1a problem for the )lIardening proces

The gear blanks are Ithrough hardened and rem- Ipered either as forgings or j

afler rough machining ITempering hould be II ed to ieliminate residual sire se in Ii

the gear therefore high tem- peringte mperatures (gt600DC)

should be u ed The re ultingtempered marten ilic micro-structure is mo t uitable forinduction hardening becau e itis homogeneous with re peetto carbon and the carbides

particle size is small whichfavors easy soluI1011during theshort induction heating periodie 3-10 seconds The as- I

hardened and tempered fltrength need not exceed

8boul 1000 Nmm2bull

Therefore gear cutting and I

other machining operation

are not difficuh to perfonn 1

R uUing Properties1) liardlltss Figure 1

shows a typicalltardness di 1Ii- ibution Induction hardened ur- Ifaces for which the carbon

content is nominally 040 C iu 1Ia1ly have hardne S value Iof more than 55 HRC and lipto 60 HRC a hardened iTempering al 200-250degC Ireduce hardne liightly 10

about 54-57 HRC Two fea-tures hould be noted an addedplateau of hardne (brokenIi lie) and a trough in the curvejust below lhe case-core june-tion The first feature which isoccasionally observed mayrelate to the extent of carbonsolutionand the degree of ear-

bon homogeaiaation in the all tenite phase noting that for Ia steel such as 4340 it will take Iaboutthree econds to dissolve the carbides but more time to

-500

u

~I

leanmiddotalloygear mel

-200

achieve a modest degree ofhomogenization Solution andhomogenizati n are betterserved by having the fine car-bide characteri tic induced by j

previous hardening andtem- Ipering The trough at the hard- Iened zones end i anributedto ihort-term tempering The end i

denotes where tile (e~peratllre Idue to induction heating bad Iattained the A I value of say I725degC But if he steel was pre- iviolJsly tempered at 650degC the ~core immediately beneath the Icase will have experienced i

heating within the 650-725degC j

range and hence some addi-tional tempering

2) MicmstTuctures Aninduction-hardened low-tem-perature tempered materialshardened layer usually consistsof fine tempered martensite and the tructure has II much

relined allsteniticgrain-sire-Ithough that i not usually apparent Process parameters iare selected [0 avoid develop- I

f menl or coarse marten middot1tICrnicrostructures which can j Fig 11J--Eftecto~UIIplring 0l11111rtaC81 residual com rressivllIrnS1II

negalive1y influence the hard- Iened layers toughness

All induction hardened 1layers microstructure does not i

always appear marten itic but itempered structure though

much finer Still induction

hardenings hardness valuesare typical of the marlensitic i

condition

3) Resi4ual StressesHealing of a steel surface by Iindllction currents will be iaccompanied by thermal Iexpansion and a superimposed i Fig 11-Eflecllol tempering Ionthe hardness profile 0 an inductiDn hlrdnedl gearcontraction when the material t middotIooth Uanl(SAEI140steel)

wwwpow9rlrIlISminioflcom wwwgBSrlllchnoiogycom bull GEAR TECHNOLOGY MARCHAPRIL 2001 31

sometimes tends to resemblethe original quenched and

500

Figl~Hec1 01CIS - depth onlmiddotlUJ1acemidual colIPrmivl Ibtnm

-600

-500

-400 -

045-050 ClMrVTI staal90 minute soak period

-100--a 100

Tempering Temperauira middotC

700-

600

comiddot -ci 500c -Ec ~I

300

2DO -o 2 3 4Distancelrom surface mm

5

produce u pre-tempered sur-face hardness of more than 57HRCand a tempered surface Ihardnes of typically 55 HRC

iWith today inherently cleansteels the material basic Iqllality i nO1a problem for the )lIardening proces

The gear blanks are Ithrough hardened and rem- Ipered either as forgings or j

afler rough machining ITempering hould be II ed to ieliminate residual sire se in Ii

the gear therefore high tem- peringte mperatures (gt600DC)

should be u ed The re ultingtempered marten ilic micro-structure is mo t uitable forinduction hardening becau e itis homogeneous with re peetto carbon and the carbides

particle size is small whichfavors easy soluI1011during theshort induction heating periodie 3-10 seconds The as- I

hardened and tempered fltrength need not exceed

8boul 1000 Nmm2bull

Therefore gear cutting and I

other machining operation

are not difficuh to perfonn 1

R uUing Properties1) liardlltss Figure 1

shows a typicalltardness di 1Ii- ibution Induction hardened ur- Ifaces for which the carbon

content is nominally 040 C iu 1Ia1ly have hardne S value Iof more than 55 HRC and lipto 60 HRC a hardened iTempering al 200-250degC Ireduce hardne liightly 10

about 54-57 HRC Two fea-tures hould be noted an addedplateau of hardne (brokenIi lie) and a trough in the curvejust below lhe case-core june-tion The first feature which isoccasionally observed mayrelate to the extent of carbonsolutionand the degree of ear-

bon homogeaiaation in the all tenite phase noting that for Ia steel such as 4340 it will take Iaboutthree econds to dissolve the carbides but more time to

-500

u

~I

leanmiddotalloygear mel

-200

achieve a modest degree ofhomogenization Solution andhomogenizati n are betterserved by having the fine car-bide characteri tic induced by j

previous hardening andtem- Ipering The trough at the hard- Iened zones end i anributedto ihort-term tempering The end i

denotes where tile (e~peratllre Idue to induction heating bad Iattained the A I value of say I725degC But if he steel was pre- iviolJsly tempered at 650degC the ~core immediately beneath the Icase will have experienced i

heating within the 650-725degC j

range and hence some addi-tional tempering

2) MicmstTuctures Aninduction-hardened low-tem-perature tempered materialshardened layer usually consistsof fine tempered martensite and the tructure has II much

relined allsteniticgrain-sire-Ithough that i not usually apparent Process parameters iare selected [0 avoid develop- I

f menl or coarse marten middot1tICrnicrostructures which can j Fig 11J--Eftecto~UIIplring 0l11111rtaC81 residual com rressivllIrnS1II

negalive1y influence the hard- Iened layers toughness

All induction hardened 1layers microstructure does not i

always appear marten itic but itempered structure though

much finer Still induction

hardenings hardness valuesare typical of the marlensitic i

condition

3) Resi4ual StressesHealing of a steel surface by Iindllction currents will be iaccompanied by thermal Iexpansion and a superimposed i Fig 11-Eflecllol tempering Ionthe hardness profile 0 an inductiDn hlrdnedl gearcontraction when the material t middotIooth Uanl(SAEI140steel)

wwwpow9rlrIlISminioflcom wwwgBSrlllchnoiogycom bull GEAR TECHNOLOGY MARCHAPRIL 2001 31

sometimes tends to resemblethe original quenched and

500

Figl~Hec1 01CIS - depth onlmiddotlUJ1acemidual colIPrmivl Ibtnm

-600

-500

-400 -

045-050 ClMrVTI staal90 minute soak period

-100--a 100

Tempering Temperauira middotC

700-

600

comiddot -ci 500c -Ec ~I

300

2DO -o 2 3 4Distancelrom surface mm

5

To give you a broader viewWeve consolidated all of our North

American gear cutting technologies Into oneindependent organlzation And moved theentire operatlonto al new technology centerin the Detrelt areal at the hub of globalpowertrain engineerilng

No dealers No distrlibutors Rather careerspecialists seasoned in every aspect of inter-nationall gear production requirements andsolutions

The new Gear Technology Center Is yoursinglesource for tlhe broadest arirayof gearproducnon technol~ogy lin the world Fromour patented high-speed dry cuttingmachines and hobs through gear shapersshavers and honing machines to completestand-alone gear production centers Alii

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Ilfyour current manufacturingchallenges require the detailed focusot thewo~Ids foremost authority on gear produc-tlon give us 81 cell After all were in theneighborhood Gear Technology Center

_ _ Division of Mltsubishl Intelil1atlonol COl11Orotlon Detroit Branch

46922 Ube~ lOrveWi_xom MJchlgon48393(248) 669-6 36(248) 669-0614 FAXwwwmltsublshlgearoentercoml

CIRCLE 175

_------------BEATTREATIING FOCUS _~passes through the austeniteI transformation temperatureI range As a result yielding may occur somewhere in the heated layer probably close 10 the~eventual caselcore junctioni and will contribute to the resid-i ual stress distribution But the stresses development will bei mainly due to the martensitic

i transformationi Martensite formation in the induction heated and quenchedf layer involves a volume

I increase above that of the~ underlying core material plac-

I I ing the har~ened surface i a state of residual compressionI which is balanced by residual~tension imhe core just beneathIIhe case (Fig 8) The changeI from compression to tensioni occurs at a depth where thei hardness is about 40 HRC Butj unlike the carburizing and1 hardening process whichi transforms the core before thej carburized layer an inducrionI heated surface layer will losel heat during quenching to theI quenchant and by conductionr into the workpieces cooler body The outcome is a resid-i ual stress distribution where the compressive stresses in the hard case may have a highi value at some distance from the surface but stiU within the cases harder part Even so thei amount of surface com pres-[ sian is determined by the

t hardened layers depth The core tensile residua] stressesi which peak just below theI hardened layer need to be~ carefully considered by gear designers notingthat a deeperi case will push the offendingi residual tensile peak deeper to1 where the applied bending stresses are of a low orderi That feature results in the

fig Ulh bending fatigue strength of indulrtor-h~rdened 8Ilm31lIIodlllegear_teeth 1 specification of a higher case(vanous steels) (a) centaur hardened (b) flank bardened Broken hnes denole scatter i d ~- h th ~ -rl d 1 I dband lor DIN3990 - teptn an wou oe emp aye34 MARCHIAPRIL 200t bull GEAR TECHNOLOGYmiddot wwwlIeermiddotechn0ollycom bull wwwpow8rrransmissoncom

Fig 12-EHecl oithe amount of flank hardening on thelbendinglll fatigue stIlmgtb 01gear leth S1eel 1055C core strength 880 Nlmml

1200 1000 IE

~800

zii600

la)Contourhardened

200

-- ------

IIlIFllmklhardened

for carburized case depthsThe magnitude of the sur-

face residual stresses devel-oped during induction harden-ing is thought to be related tothe depth of hardening (Fig9) though tne stresses aremodified by tempering asFigure 10 illustrates Temper-ings effect on the hardness ofan AISI 4140 induction hard-ened gear tooth surface isshown iiii Figure ] I notingthai the most used temperingtemperature range for induc-tion hardened gears is200-250degC

4) Bellding fatigue lt iscrucial thaI the entire surface ofpoundhe toolh roosfi llet region ishardened A mis ed area in thatregion either along the filJet orat the tooth end will lower thebending fatigue trength orne25 compared with the toothsstrength before induction hard-ening (Ref 3) Baumganl (Ref4)confmned the 25 loss (Fig12) With adequate rootrfillethardening the fatigue strengthwill be 60 (0 70 of that of acarburized gear (Ref 3) whenthe surface hardness and thecase depth are within reason-able limits ie 590 Hv to 650Hv and minimum fillet casedepthmodnie ratio is 025 to030

Fatigue tests employing abeam type test piece withmachined notches to simulatea 29 module gear tooth with astre S concentration factor of14 produced fatigue limitvalues of 510 Nlmm2 for a055 C plain carbon steel527 Nlmm2 for a 050 Cchromium-vanadium steeland 564 Nmm2 to 630 Nmm2

for steels 4140 and 4340 Thetrendwas that the fatigue limitro e with core strength (772Nlmm2 to 1020 Nmm2)which perhaps reflected each

_____ HEATT1R~ATING FOCUS __ _steels resistance to significant yielding under load Po1 aror i- - I

test (Ref 3) on 8 mm module Igears produced the re unshOWD in Figure 13 for full I

tooth pace induction hard-ened and nank induction hard-ened teeth

S) Conlllci JaJjgue~ A sur-faces contact fatigue strengthIs related to its tensile strength I

and the urface materials hard- ness Contact fatigue tests using discs and having no I

nnentional sliding uggested ]that induction hardened sur- 1

faces had pining fatigue

trengths of about 80 of that I

of carburized and hardenedurfaces (Fig 14) Winter and i

Weiss confirmed hat eoserva- tion (Ref 3) With actual gear jtests they concluded that Iinduction hardened gears had l85 of the contact fatigue 1

strenlh of their case hardened Icounterpart Their recommen- i

dation nol to exceed 55 HRCurface hardness forlhe we of

tooth bending strength is inline with current practice not-ing that their contact fatigueplots shown in Figure IS rep-resent surface hardnesses of 52HRCand 6] HRC When thesurface hardness was 6] HRCthe contact fatigue trength wascomparable to thai of a easehardened gear of the same sur-face hardness Unfortunatelywilli such urface hardne sorne tooth bending failures

occurred with the inductionhardened gears In other tests(Ref 5) 011 gears of about 6]KRC the induction hardenedgear had a life (to tbe onset ofpitting) Ibat was 17 timesihatof a case hardened gear Againsome induction hardened gearsexperienced tooth breakagewhich may confirm Willter andWeiss recommendation Butduring contact fatigue tests

Through-II rdenedand tempelld straIghtcarbon and alloy steel

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1600

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FVA oil 31PittingD CraterinI[J Pittilg and crg1ering

101 10Number 01 load cycles

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fig ls-tDMaCf migus resistlnce 0iDiluctlOD-IInl8IId I lJetllnm agalndcamurizod teeth(~ lal42CrM04 steall52 HRC (bliCk 45 steelJ(il HRC Performance ofcase hardenedleeth shown shaded

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Fig 17-Overfleating andllmBH[ng (II overlleating (x450) (bl melting (xl0IU leI overmiddotheatingl with IItrace ot melting (x540l Slssl BUM4D

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using narrow faced gearsresults inlootlt breakage frac-tures initialed attbe early con-tact damage on the tooth Hanks

PitflillsInduction hardellinglias

problems ln the wrung beattreaters hands the results canbe di astrou But a number ofproblems havebeen recog-nized and eliminated duringDavid Browns years of experi-ence That recognition provid-ed insight and a clearer under-standitng of the process

J) Baek temperil~g Thehardening ofa single toothmeans the tooth surface attainsa temperature in excess of720degC The quencham re-moves much of the heat bursome heat conducts throughthe looth That heat can-par-ticularly with small pitchteelh-resull in back-temper-ing of the adjacent previouslyhardened tooth

Back tempering is con-trolled by side cooling jets

which are positioned tolmpinge omhe adjacent (oohstop edge and direct flow downits flank (Fig 16)A considera-lion is hal tile adjacent toothsees the conducted heat a lit-tle later than the heated toothsurface and therefore the ide

I jets need to be longer than theinductor

A sm311 amount of soften-ing by back tempering isalmost inevitable and hoeld

be accepted inlhe gear designIt is inherent in the process that311the teeth exceptme 1 tonewill experience the back-tern-per effect and that one tooth(the fIrst) will have two flankswhich experience the effect

2) Root and FlankCraccig Tooth root andorflank cracking has never reallybeen II problem with the ub-merged toolh-by-tooth process

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using quenching oil as thecoolant

The tooth-by-tootltinduc-Lion hardening process in otherorganLzations had an early his-tory of tooth cracking prob-lems (Ref 1) usualliy via theuse of steel having too high acarbon content andor too lowa hardenability togethmiddot r withIh use of higher quench rates

3) Melting and Over-healing IT the local tempera-ture become too high due tofior example too close a cou-plelhere will bea risk of sur-

face overheating or meltingOverlleating produces 3 coarsemartensitic miceostrucnire inthe as-quenched surface Amelted area produces a surfacelayer with a dendritic structare

and a sublaycr of overheatedmaterial (Fig 17) Such OCCUl-

rences are to be avoidedalthough localized occurrences

at tooth end run out can bedressed to remove Iheeffects

4) Unhardened areasFlgure 18 bows examples ofinduction hardened gearswhere small areas are leftunhardened

In (a) an inductor did notdwell at either end of a geartooth causing a small area athumbnail to receive insuffi-cient heating to effect harden-ing To correct that fault atten-tion must be given to how farthe inductor is introduced intothe tooth space before energiz-ing and how long It dwells therein he energized state beforetarting its heating traverse

Such a defect may invitefaligue cracking during service

In (b) a poorly shaped ordamaged inductor led to a nar-row band of uahardened sur-face at the tooth filIet WithinIlhe hardened mface~ tileresidual stresses are compees-sive But in unhardened areas

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such as those hownfhere willi be tensile residual stresses of a t=s~middotal~I~ill bave a vel) poor bendingfatigue strength

In (c) iasutficiemaneation

to process parameters led to thehardened layer being hin or

missing near the 1ooths end Itis nermal forthe end hardened

pattern to differ a little fromthat furtheralong the tooththere tends to be a smalamoun of case thinning nearthe exit end at a poiru midway

up Ute tooth face as the toprow in Figure 18 shows Inextreme circumstances the

thinner area may break out tothe surface

One very important factorin relation to tooth end harden-ing problems is having the COf-

rect tooth end shape chamfersand beveled edge

5) uneven hardening pat-

terns Uneven hanlelling pat-terns are mainly due to IXgtOrpositioning of the inductor inthe tooth space or to a lack ofinductor rigidity Poor inductor

alignment also causes unevenhardening

6) Distortion alldgrowthShape and volume changes areno as a rule viewed as being

ignificant to induction harden-ing Still it is good to keep inmind that they do OCCIlf

though generally to small

degrees and good to knowwhere the potential problemareas might be Tooth profilemovements due W induction

hardening are ilJu trated inFigure 20 where the shapechange is less than 0012 mm

Gear rims might also have aslight tendency to take on adisbola shape when the gear

diameter al me ends of the

teeth is greater than the mid-face width The extent of theshape change is affected by rimthickne s and tooth face width

the thinner the rim and thegreater the face width thegreater the risk of that form ofdistortion Therefore thedesigner must take mat intoaccount at an early tage ofdesign Given that tendency it

is not advisable to inductionharden gear rims shrunk onto

a center Welded fabricationgear construction on the otherhand is suitable

The ends of small- andintermediate-sized teeth whichare required to be inductionhardened should be generous-ly radiused On the other hand

1Mgt pinion teeth for which adeep case is specified andwhich are not planned to beflank ground should betapered about 01 mm over the

end 120m of flank at bothends of each flank amiddot well ashaving a 3 mm radius at theedges That is done to counter

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Table l-AGMA gear ratmqs lor vanous heat-treated conditions

_-----------_1 HEATTREATINGFOCUS1 _

Gear tooth size AGMA rating (kWf

Through Through I c rbonhardened hardened Nitrided Indudion ass-10 UTS of to UTS of 4140 hardened hardeMd

772Nlmm2 1158N1mm2

OPModule

2 127 1529 33 36 (225 10 J 5666 72 833 85 5 94 111 121 14 635 116 218 26 283 3296 423 I 387 729 845 949 1118 318 886 167 19_ 218 25612 212 282 530 615 691 81520 127 I 163 2192 2558 2815 3390JO 065 3521 6636 772l 8680 10258

Results based on 25 pinion teeth running against 75 wheel teeth helicafwith face width 04 x centres

Table 2-ApprolHiate heal treatment r~

Gear tooth size AGMA rating (kW)

IndCtion Induction CarbonModule OP Nitrided hardened hardened case-

at 50kHz etlOkHz hardened2 127 bull bull25 10 bull bull3 85 bull bull 635 bull bull6 423 bull bull bull 8 318 bull bull bull bull12 212 bull 20 127 bull 30 065 bull bull

Run-outof hardened layer

Regionof

conductedheat

Hardened Isyer

Tensilesurfacestress

Compressivesurface stress

Fig19-Residual stress althe edge 101a hardened IlyerI~)

Profile beforehardeningl

Profile afterhardening

IFig2O-I)istortillnl in Iooth fOl1lllc81lSedilby Ihl1dening (greilly uIggallledl

the minor growth that can wheel with a carburized andoccur at the flank ends due toinduction hardening therebycausing hard meshing poinls in

critical areas Helical gearteeth need to be more gener-ously rounded at the acuteangles edge the amountdepending on tooth size

ApplicationsInduction hardening joins

an array of heat treatmentprocesses available to thedesigner A process compari-SOil of the AGMA 218 gear rat-ings for a range of gear toothsizes is shown in Table L Itcan be seen that carburized andcase hardened gears providethe best ratings for both toothdurability (contact fatigue) andtooth bending fatigue gearproperties But for the largertooth sizes induction harden-ing provides a significantadvantage over nitriding orthrough hardening

The different heat treatmentprocesses tend to suit a rangeof tooth sizes Table 2 providesan overview of the data Tooth-by-tooth induction hardeningis suited to relatively largeteeth-or 10kHz frequencyfrom 8 module to 3D module

Induction hardening

because it requires a high levelof technical and manual skill issuited to larger gears whicll-by their size and weight-areexpensive tocarburize

Induction hardening mightI I I be beneficial when distortioni and growth due to carburizingI and hardening is large enough

to require excessive amounts of corrective flank grinding with a corresponding thinning of thei case and the risk of grinding

steps at the tooth filletInduction hardening can be

best used by ensuring a goodcombination with the matinggear ie an induction hardened

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hardenedpinion or a throughhardened wheel with an indue-tion hardened pinion

After an induction harden-ing process is chosen the engi-neer should design the gearaccordingly

1) Double helical gearsshould have a gap between thetwo helixes into which theinductor can pass when it hascompleted a tooth traverseModern bobbed gears willhave that anyway and gearsthat need to be finished by geartooth flank grinding win have asubstantial gap

2) Uthere is a shoulderadjacent to the end of the gearportion there should be a radi-al gap between the ends of theteeth and the houlder

3) A generous root filletradius should be included andnarrow tip widths should beavoided

Typical David Brown appli-cations for induction hardened

gears arebullMill pinions on girth gear

driven rotating roms where thepinion mates with a cast steelwheel (Fig 21)

bull Heavy-duty crane traveldrive gearing where the neededcontact accuracy bybeavilyloaded carburizedl gearing can-not be achieved in a continuous-Iy flexing gear case (Fig 22)

bull Sugar mill drive gearswhere price competitiveness iscombined with heavy torquetransmission (Fig 23)

bull Steel mill applications inboth rolling mill main drives(Fig 24) and in shear applica-tions (Fig 25) where eachtooth frequently feels heavyshock loads as well ascoileruncoiler boxes

bull Cement mill drives wherehigh torques are continuouslyapplied for long periods (Fig 26)

liig 22-Helvy-duty cnlO81r8vel drivBgeLring

Applications for inductionhardened gears include a vari-ety of applications where theeconomic balance requires ahigh strength 1hr~ughllard-ened wheel and consequentlyan even higher duty pillion orwhen the ratings demand a car-burized pinion but not a car-burized wheel

The whole [age of indu -trial gear drives can benefitfrom properties produced bythe process

ConclusionsThe submerged tooth-by-

tooth induction surface harden-ing proces for medium andlarge gear manufacture hasbeen used successfully byDavid Brown for more thanthree decades WI comes into itown for gears that cannot besurface hardened by other

method because of the gearsoverall size or because oftoothize considerations Also it can

compete with other processesfor which strength require-ments are too severe forthrough hardened gears bUI fallhart of the strengths from car-

burizing am hardening For gears hardened by the

process the surface strengthproperties (bending and 0011-

tact fatigue) are much higher(typically 40) than the high-est practicable through hard-ened gear but marginally lessthan carburized case hardenedgears (typically another 20higher) Also through hard-ened gears at the high strengthlevels must use low temperingtemperatures which can resultin retention of internaJ stressesresidual from the quenchingprocess The intemal tensilestress can combined withapplied service load be detri-mental to gear life

Consequently with suitablegear de ign modifications thesubmerged induction harden-ing process serves as an alter-native to either through harden-ing or carburizing

Contact fatigue strengthrelates to surface hardnessTherefore given adequate casethickness one might expect aninduction hardened gear to be

Austempered Ductile Iron (ADOoutperfonns steel as demonstrated in theseroad test results on bypoid gem)Switching from steelto AustemperedDuctile Iron (ADI)will also add thesebenefits

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Fig23-Sugar mill drives

Fig 2~Sleel mill applications

Fig26a Fig26b

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fairly comparable to at carbu- Geoffrey Panishrized gear of the same designand surface hardness Heartesting seems to support thatand il is common for a carbur-

ized pinion to be run with all

induction hardened wheelInduction hardening had

problems in the past in manyinduction hardening plants theproblems still abound Butlearning from experience andunderstanding the processquality control techniques Call

be established that minimizethe likelihood of process relat-ed service problems

The process is gear toothfriendly Finally a moredetailed technical appraisal ofthe process was published inRefs 6 and 7

ReferencesI Bojnican V Problems ofHardeni ng Gear WheelIndentations Int Symp forMetallurgy and Heat TreatmentWarsaw 19672 Shepelyakovskii K Z and I NShklyarov Strength and Enduranceof Heavily Loaded Machine PansHeat Treated by Various MethodsMetat I Teml Obra Metallov No7 July 1968(2)3 Winter H and T Weiss The LoadCarrying Capacity of Induction andFlame Hardened Gears PaT IFlank Strength Antriehstechmk 27No 10 1988 (57) Pan 2 RootStrength Amriebstechnn 27 No 12198B (45)4 Baumgartl E Influence of theVarious Methoos of Surface HeatTreatment on the Behaviour inService of Hardened Gear TeethInl Symp for Metallurgy and HealTreatment Warsaw 19675 Townsend 0 A lUI7Jl and MChaplin The Surface Fatigue Lifeof Contour Induction Hardened ArSI1552 Gears AGMA TechnicalPaper 95FTM56 Parrish G nw Ingham andChaney The Submerged InductionHardening of Gears Pari 1 HeatTreatment ajMetas 19981 pgs 1middot87 Parrish G DW Ingham andChaney The Submerged InductionHardening of Gears Pan 2 HeatTreatment of Metalf 19982 pgs43-508 Jacobsen M Designers Controlof the Manufacturing Process Part3 of Gear Design Series AutomotiveDesign Engineering 1969

is a gear consultant wUh 42 yearsof work experience in metallurgy27 of them ill gear metallurgy Hewas head of metallurgical researchand development at lkTvid BrownGear Industries Ltd where heworked for 15yelllS specializing in

gear heal treatment processes andmaterial properties Also he waschief metallurgist and deputy quali-ty manager at British JeffreyDiamond Dresser UK where heworked for J 2 years specializing ingear heal treatment processing andquality

IDavid W Inghamis a manager at David BrownSpecial Products Ltd with respon-sibility jor providing gears gear-boxes and service in the UnitedKingdom and South America Ametallurgist he joined DavidBrown in 1970 starting ill metal-lurgical research and developmentIn the 1970s and 1980s he worked011 projects that included develop-mlmts in submerged inductionhardening He is a member of theInstitute of Materials and a char-tered engineer

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