This paper was

presented at the

JSME International

Conference on

Motion and Power

Transmissions,

MPT2001,

in Fukuoka, Japan,

in November 2001.

Carburizing of Big Module andLarge Diameter Gears

LJ. Cheng, K.H.Wu, S.H. Vue and IC. Kuo

arburized gear have higher trength andlonger live compared with induction-hardenedor quench-tempered gears. But in big modulegears, carburizing heat-treatment becomes time-can uming and expensive and sometime cannot

Figure l-e-Conventionai ga -carburizing furnaces.Maximum mea urements of a gear can be 1.3 m indiameter, 2 m in height and 5,000 kg ill weight.

'arrierga is endogas, enriching gas is propane.

Figure 2-New gas-carburizing furnaces. Max-imum measurements of a gear can be 2.5 m indiameter, 2.5 m in height, and 15,000 kg in weight.Carrier gas is a mixture of methanol and nitrogen,enriching gas is propane.

Figure 3-lndllction hardening of tooth flank only. The gear has 92 teeth, eachone 405 mm long, a module of 36.4, an outside diameter of 3,115 mm and ismade of SCNCrM2A, a JT material specification.

achiev good hardne s due to the big mas -effect.Al 0, it i n t easy to reduce distortion of gearduring heat treatment.

In order to achieve good iurface hardnes of thecarburized layer, it i neces ary to preci ely controlthe carbon content d.istribution. The carburizingtime i much longer for big module gear due todeeper case-depth requirement. So, after carburiz-ing, it i ne e ary to have intermediate cooling toIe than 650"C to get finer grain ize and betterme hanical propcrtie . By using a carburizing fur-nace equipped with a fast cooling function, work-pieces can proceed through intermediate coolingand heat up again to quench temperature inside thearne furnace. Thi can remarkably reduce the total

heat-treatment time, as well as co Is.

The di tortion that comes from heat treatmentuna oidable. But, by improving the arrange-

ment of a load of parts. we can reduce di tortionand thereby reduce manufacturing co ts.

Introductionurface hardening technology i very important

to in reasing th durability and trength of big mod-ule gears. thereby reducing the ize and weight oftho gears and their gear reducers and, further-more, reducing the weight of their whole machines.But, in past years, rno t of the big module gears orlarge diameter gears were de igned as either castedor welded and were heat-treated by either normaliz-ing r through-hardening. After years of running,pitting or wear becomes erious and finally vibra-tion and tooth breakage can happen and stop a pro-duction line.

Until recently, we at Formo a Heavy Indu trieCorp. of aiwan u ed either carburizing or indue-lion hardening, but the company was limited tocarburizing gear with diameters of at mo t 1.3 m.To reduce manufacturing co ts and increa e thecarburizing capability for big module, large diam-eter gea ,we recently in talled a new et of fur-nace that can carburize gears with diameters ofup to 2.5 m.

The new et of furnaces is equipped with a newcarburizing proce s and technology in order to pro-duce higher quality carburized gears. The computer

36 SEPTEMBER/OCTOBER 2002 • GEAR TECHNOLOGY· www.geBftechnology.com • wWW.powBrtfsnsmlss/on.com

imulation program can control the carbon corn ntprofile in order to achie e good hardne and thefast cooling function within a nitrogen atmo pherecan h rten the total h at-treatm nt tim .

Indued n Hardeningears with diameters bigger than a carburizing

furnace hould be indu ti n-hardened, Becau ofthe big diamete ,teeth are hardened one by one.There are two way to harden the t th: "tooth flankhard ning only" and ' th hardening." Figure3 h w the treatment of tooth flank hardening, and

igure 5 .h w the treatment 0 tooth root hardening.Figures 4 and 6 'how the macrostructure and

hardne. s di tribution curve of the hardened layer.fr m tooth flank hardening and tooth root harden-ing, re pecti ely. Figure 7 h w the tooth profilafter induction hardening of the gear hown n

igure 5. That gear h been tooth root inducti nhard n d. The re ult how that the tooth pre ureangle decrea d lightly. B au only one toothhas been heated while the gear ther teeth remaincold during induction hard ning, the geometri ataccuracy and tooth lead are alma t unchanged dur-in tooth-by-tooth inducti n hardening.

Carb riziCarburizing via mallfumace with traditional

process and carrier ga generator. Figure h wsthe typical carburizing proce. of a 1.3 m furnacefor about a 3 mm case depth. During carburizing,

rmo Heavy Indu trie use carrier g prduce by an endog gen rat r. Al ,the carbonpotential i enriched by propan gas and controlledb an 0 ygen nor. t r carburizing, the ork-pi e mu t be taken ut of the arburizing fum cand pUI into a tempering furnace for intermediatec ling, then air cooled t r m temperature. After

ling, the workpie e will be painted with anti-carburizing paste, then put int the carburizing fur-nace again to heat up to quenching temperature.

igure 9 ho the load being taken out of th car-burizing fuma e, re d f r qu nching.

Carburiting of lar, e diameter gears wah newprocess furnace. Figure 10 h w the typical ar-burizing prace s for a ut a 5 rom case depth. hecarrier gas com from a mixture of liquidmethan Iand pure nitrogen. The carbon potential imeasured by a yg n r and controlled iap pane an enri bing g and air a dilutinggas. In thi case, we d n't need any g general r~ r our carrier gas. The arburizing intermediatecooling and quen hing p edure are continu usand d n't invol e taking th load out of the furna e.

igure 11 how the 1 ad being taken out of the car-burizing furnace. ready f r quen bing.

Figure 4-Te t piece showing profile of tooth-flankhardened gear in Figure 3. Hardness of materialbefore hardening: HB 250-265. Hardnes of sur-face after hardening: HRC 52-55. Thickness ofhardened layer: about 4.7 mm.

Figure 5-Induction hardening of tooth flank androot, The gear ha 108 teeth, each one 182 mmlong, a module of 25. an out ide diameter of 2,776mm and i made of M440, a }IS material speci-fication.

Re uks of carburising. igure 12 and 13 h wthe rna ro tructure and hardne di tributi n 0 atrue- ired t t piece from the load hown in igu9 that was carburized via a 1.3 m furn . 111m ured ca d pth i bout 3.5 rom. The hanln •within 0.2 mrn of th urfa e i· lower. this decar-burizing effect comes from taking workpieces utof the furnace when ding interm diare ling.

igure 1.4 hows the micro. tructure of the hardenedlayer; the micro tructure is fully marten iii .

L.J. Cheng1.1 superintendent of thgear plant oj FormosaHeavy Industries Corp ..located in Taiwan, Republicof China. The gear plumde igns and manufacturespreci ion gears, large-diameter and heal'y.dlltygear reducer. for appli u-tWIU ill the steel, ugar;

cement, rubber and pla ucsindustries. as well as otherindustries.

K.H. Wuis senior vice president ofFormosa Heavy lndustnes,a subsidiary of tn roll-

glomerate FormosaPlastics Group.

S.H. Yueis plant manager ofFormosa Heal lndu tries'gear plant,

lC. Kuo; a sistant plant managerof Forma a Heal')'Industri .\' g('or plan I.I

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IiiIiIIIiiIIII

6 iiI

Figure 6-Test piece showing profile of tooth-root hardened gear shown in I

Figure 5. Hardness of material before hardening: HB 250-265. Hardnessof surface after hardening: HRC 52-55. Thickness of hardened layer:about 2.7 mm,

800

oo 234

Depthl'rom surfaee (mm)

IT-l"

I

50jLmL--..J

Figure 7-Tooth profile after induction hardening of tooth flank and root ofgear shown in Figure 5.

nhrCarburizing

840°C

~Ol/~~cJoling

Airl·coo mg

·cQhrA'. Ir .Tempering co01lng

Figure 8-Carburizing process of conventional furnace, with maximum geardiameter of 1.3 m,

Figures 15 and 16 show the macro tructureand hardness distribution of a true-sized te t piecefrom the load hown on Figure 11,which was car-burized with a new 2.5 m furnace. Since the inter-mediate cooling occur inside the carburizing fur-nace and there is nitrogen gas protection at lesthan 750°C, the surface of workpieces will notoxidize or decarburize; 0 the near- urface hard-ness is better than it would be if carburized via a1.3 m furnace. Figure 17 shows the microstruc-ture of thi test piece; it i fully martensitic.

The distortion of carburized gears i a problemof great concern. According to our experience,the lower side of the gear, which touche the oilfir t, will hrink and that side's diameter will besmaller than the upper side's and, therefore, thegear will be a little bit conical. Can sequently, thelead will change drastically. Figur s 18 and 19show the profile and lead of the gear hown inFigure II before and after heat treatment.

Figure 20 shows one example of load arrange-ment; there are 20 gear in one load. After heattreatment, we measured the geometric a curacyand hardnes of every gear. In order to decrea ethe distortion of gears, the arrangement of work-piece during heat treatment is very important.We put plates under and above the gear todecrease their distortion. For small or ring-typegears, we overlap gear, as Figure 20 show . Forbigger gears, we have to separate them by acharging jig, a Figure 11 shows. The jig betweengears will reduce the conical distortion fromquenching. Figures 21 and 22 are the re ults oftwo loads arranged a shown in Figure 20 withthe same heat treatment condition . Figure 21shows the re .ults of normal quenching. andFigure 22 shows the results of turning the loadwhen quenching in the oil. The latter ha bettergeometric accuracy and more homogeneou hard-ne .

Performance of Carburized GearsThe performance of carburized or induction

hardened gears used in a sugar plant is hown in

~292~967

Figure 9-JlIst carburized gears are removed from conventional furnace before quenching. Each gear has 73 teeth, each one 213 mmlong, a module of 12, an outside diameter of 967 mm and is made of SCM420H, a JIS material specification. Hardness of carburizedteeth is liRC 58-59, case depth is 3.5 mill.38 SEPTEMBER/OCTOBER 2002 • GEAR TECHNOLOGY· www.gssrtechnology,com • www.powsrtransmission.com

124 hrCarburizilg Dill'coo I

Airt,coo mg

Figur I{}-Carburi:;ing process ofa newfurnace, with maximumgear din", I r of 1.5 m.

800

600

~:: 400

1 6

Figure 14-J/icroUruclure ofcarburized layer ofgear test piecehown in Figur 12.

Figure 15-Macro tructur ufKear te t piee car-buri-ed with Ihgears. hown inFigur 11. Th testpi c I,as.5 I I!III,amodule of 20 and ismade 0/ f./201l,a}1 material sp •ification.

Figure17-Mlcro 11'11 ture ofcarburized layer 0/gear test piehown in Figur I .

7

J1

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Figure 19-Tooth profile wid lead of tile same gear from Figure18 after carburizing in a new proce s furnace.

aclt-13 =-.--12---~~.-11~""''"-''''''""

aclt·lO~•••• ·9 ~•••. 8~••• -1 ~ ••• ~

abcd.fah

1"•.20-----1" •• ·19--••• ·18 ~~--- e-

••• -11 -----1"1t-16~

1t·15 --_... • "T.•• 14~~

abcdefah

Figure 2o--Arrangement 0/ gear. be/ore carburizing ill a lIewproces furnace. Each load consists of 20 gears. Each gear has107 teeth, each one 160 mm long, a module of9 all outside diam-eter of 980 IIUIIand is made 0/ l7Cr iMo6, a DIN material .pec-ification. Ca e depth of gears is 2.8 mm.

lumlng the load whenquench Into 011

hanging rod

.-6•• .s.-4.·3

go.-21:.·1

.-20------·19 --~ .•~--.--·18 •••••••_-17-----

.·16 ~•.:':'.- •.•..•.•.•.•.-

.·'5~"T.•••. ,~~Xmmabcdar,b

Figure 22- ompari Oil 0/ gear flatne and hardne of anoth-er load of gears arranged as shown ill Figure 20 and turned whilequenching ill oil.

40 SEPTEMBER/OCTOBER 2002 • GEAR TECHNOLOGY • www.gearlechnofogy.com.wwwpowertransmiss;on.com

Figure 23 and 24. Figure 23 how an open gearwith normalization and a pinion with quen hing andtempering. They have operated for three millingeason, and the gear tooth urface is already pit-

ting because the surface hardness i too low.Figure 24 show an open gear with inductionhardening and a pinion with carburizing. Theya) 0 were operated for three milling eason.Since the hardnes wa rai ed for both gear andpinion, the urface i in very good condition andthe gear and pinion Ii e can be longer.

onclusionInduction-hardened gear ha e les lead and

profile di tortion compared with carburized ones,but they have Ie . contact and bending strength.The induction hardening of big gear' require pe-cial induction coils and more operators. Of thetwo type of induction hardening tooth root hard-ening re ult in more bending trength than toothflank hardening, but the former require moreadvanced technology.

arburized gean. ha e higher tooth urfacehardne s than induction-hardened gears. Al 0, thehardne di tribution and micro tructure of car-burized gears are more homogencou than theyare in induction-hardened one. Therefore, carbur-ized gear have higher contact trengtb and bend-ing trength compared with induction-hardenedgears.

The distortion of carburized gears i biggerthan that of induction-hardened gear .. When thediameter of a gear become bigger, it i neces aryto increa. e the gear' ca e depth to compen atefor the bigger di tortion, Thi will increa e theheat-treatment time, weU a the grinding timeafter heat treatment. We have reduced the distor-tion of gears by better arranging workpiece in aload and turning the workpiece during quench-ing. We will continue to reduce di tortion throughfurther te ling and data collection. 0

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Figure 23-Dpen gear and pinion used ill a sugarplant and made by another gear manufacturer, afterthree milling seasons in operation. The pinion(upper) has 22 teeth, each one 710 mm long, a mod-ule of 40 an out ide diameter of 982 mm and ismade of quenched and tempered SCM440, a )ISmaterial specification. The gear (lower) has 95 teeth,each one 700 mm long, a module of 40, all. outsidediameter of 3,858 mm and is made of an unknownnormalized material.

Figure 24-0pen gearand pinion with thesame dimension a inFigure 23, used in thesame ugar plant andmade by FormosaHeavy Industries, afterthree milling seasons ill

operation. The pillion(upper) is made ofSCM420H, a jT mater-ial specification, carbu-rized to HR 57-59,witll case dept" of 4.5mm. Tile gear (lower) ismade of CM440, a 11material specification,tooth flank arid rootinduction hardened toHRC 52-55.

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