Gear Hardness Technology

1 Gear TechnicallGro1up,SanDiego. CAIntroduction:In avery general sense, increasingthehardnessof asteel gear increases the strength of the ,foreach process thereis backgroundinformationon each of the processes covered. Tneach section discussed. Typical processes are presented alongwith commentson variableswhich affect theresult. By reviewing the capabilities and processes,it is possibletodetermine the limits to each this article several Hardness scalesare mentioned. The abbreviationsfor these scalesare as follows:BHN -Brinell Hardness numberKHN - Knoop Hardness numberHRC - Rockwell "C" scaleHV - Vickers Hardness numberPreliminary Heat Treatment ProcessesThere are several heat treatmentsperformedduring the manufacnsriugprocess which are in-tended to condition the metal for these are essential processes they willbedescribed isa process in is heated and then slowly cooled in the furnaceto 600 F (316 C).

2 Full annealing involves heatingto a temperature above the upper critical (A3point).This will result in softening the part and improvingthe annealing involvesheating the part to a temperature above the below the A3 point. Finally there is subcriticalannealing. which heats the part to just below thefirst transformationtemperature (AI).' as in temper-SmaxPmax' ' .. TECHNOLOGYing, and low-coolsit,just as in full annealing is often done to stabilize thestructure prior a process whichinvolves heating the part to above the upper criticalas in annealing, :till or agitated air. N onnalizing is donetorelieveresidual stressesinagear blank andfordimensionalstability. A normalizedpartis very machinable, burwillbeharder than if it were relieving is heatingtobelow the lower intempering,andcooling prima-rily to relieveinternalstresses.

3 This process issometimes called process HardeningThroughhardeningrefers to heat treatmentmethods which do not This termdoes not imply thatthe Hardness is uni form through-outthegear tooth. Since the outside of a gear iscooled faster than the inside, there willbeagradientin the Hardness . The achievable Hardness is basedon the amount of carbon in the steel . The depth ofhardness depends on the hardenability of the the purposes of this article, we will concen-trate on the q uench and temper process. This methodis used to obtain thefinalcore propertieof thematerial for gears which are either cased or notcased, When this process is used to develop thecore properties for nitrided gears,itisdone priorto the nitriding cycle.

4 When it is used to harden acarburized gear,itis done after the gear has beencarburized ..Por gear which are not cased, the loadcarrying capacity of a gear is dependent on the corehardness of the material. (The capacity of case-hardenedgears is primarilydependenton casehardness).Itis generally accepted to use the hard-ness value measured at the root diameter in thecenter of the tooth. when making on theIoadingthe gear must handle,'Quench time to500of.,,Quench ..__s_ec_o.,..n_d_s__-IEffective-nenNole s:1,. Astructure not quenchedcuuclull martensitewill notbefixed up by tobeexpected414043401----1-----'1----'1- -------------------12. Material in the"F'situation could be tem-pered to meet about300 Brlnellminimum.(The "asquenched" Hardness wouldbeabove' 300 HB.)

5 Excellent(over 90% martensite)A25 BOReasonably good(martensite and some other transformation prooucts)B802003. Material in the"P'situation would probablyfall to meet Charpy V notch and ductility require-ments normally expected for a good steel . Inaddition. the fatigue strength would be good. but may be acceptable(martensite. bainite. pearlite. and perhaps some free ferrite)c200600,4. Poor quenchingresults can result Iromthingslike an Improperprior structureorthe' wrong austenitizing' temperature.(A Wquench is notthe only reason for a poor SlruC1ure.)Poor. usually nol acceptable for high performance parts(low in martensite. with much bainite, pearlite, and tres ferrile)300D1000 Very poor. usually not acceptable(pearlite. free ferrite. some bainite, maybe some martensns)F8007000it is often necessary to increase the Hardness of thesteel.

6 According to AGMA standards,Ia gear witha Hardness of400 BHN, which has a design life of107cycles can handle as much as 20% more loadthan a gear whichishardenedto300 BHN. Forhardnessesabove400 BHN the capacity increaseswithrespectto pining resistance. but the capacitydecreases with respect to bending strength, whichdeteriorates because the tooth becomes brittle,Though a greatdeal ofattention is giventothehardness of the material, is importanttounder-stand that the microstructure, upon which the hard-ness depends,iswhat really matters. Although in-depth discussionof microstructureis beyond thescope of this article. it is wonh mentioning that thedegree of martensitic structure is one of the primeindicators of a material's quality. AGMA 2004-B892does a goodjob of identifying other micro-structural!

7 Aspects that must 'be most gear heat treatments, through hard-ening is a process which can be performed eitherpriorto orafterthegear teetharecut. Thehardnes sis achieved by heating the material to the austeniticrange (usually to about 1500-1600"F)and thanquenching and tempering. For ituarions when theteetharecut after the material has been a considerationin deter-mining the Hardness . For the mostpart,conven-tional gear cutting processes (nabbing. )are capableof cutting materialswithhardnesses of up to 400 BHN ..Though 400 BHNis machinable,gear teeth are much easier tomachine when thehardness islower. Therewillbedi tortionifthehardeningisdone after theteethare cut The teeth may have to be finish-machinedto achievetherequired harden a part.

8 By this process,the partis heatedtothe austenitic range, a tem-perature that varies, depending on the carbonandalloy content, within the range of about1500-1600"P (81S-870 C).In this state the steel be-comes austenite,whichis a term for the solidsolution of carbon in fcc iron.' Then thepartisrapidly quenchedin oil(or sometimeswater) totransformthe austeniteinto is too slow, the structure will not be fullytransformedto martensite. The resulting micro-structurewillthen contain what are called tran for-mati on products, such as ferrite, bainite, cementite. The properties of Hardness , tough-ness, ductility,and strength are dependentonthe transformationproducts whicharepresent,Therateofcooling whichmust be achieved toproperly transform the steel to martensite andmini-mize the percentage of transformation products isdependent on the chemitryofthealloy being amount and type of alloying elementsinthesteel determine its hardenability , hardenability is a measure of the relative depthto which hardnessiachieved for a given quenchrate and sectionthicknes.

9 Inother words, a mate-rial with a high hardenability , which1 Squenched atthe same rate as a part of the same size. but with lowhardenability, will have hard material alloying elements whichhave of thesteelare manganese, chro-mium, nickel, and molybdenum. TableJis a tableshowing several alloyreelswhich are commonlyused for through hardened gears. A material suchas AISl4140is considered to be a low alloy teeland has rather poor hardenabil ity. A material suchas AISI4340is consideredtobe rich alloy steeland has much. better hardenability ..Oncethepart has been quenched,itneeds to the President ofDudley DTO is aconsulting firmspecializing ingearing and relatedmachine Smi,this a projectengineerwll'hDudley LI'9 9 229- -Table 3 - ApproximateCarbon Contentto Develop MaximumHardnessinICarburizedCase of Nickel Alloy Steels--!

10 ICarbon ContentMaximumIfor CStee'ITypeOuenchin91 CycleHardness,%HliIf' .. ,654817 (+ Ni, Mo (SAE EX-1) to reduce the brittleness and toughen thesteel, since quenched martensite is hard, but alsobrittle. Tempering through hardenedpartsigener-allydone at400tolOOO F (205to 450 C)~oraperiod of one or more hour. depending on the sizeof the toughness, but also lower the quench and temperprocess is limited only by thesize offurnaces andquench tanksavailable,Today, this is as large asseveral meters. From a practical standpoint. themajor limitationcomefrom theabilitytoquench gears fatenough to obtain an accept-able cases,particu-larlyw:ith lean alloy steels, it is just impossibletoquench large gears fast enough to tile comparison of timerequiredto achieve levels of metallurgical qualitybetweenAfSI 4140, a leanalloysteelwithpoorhardenability, and AI;S~4340.)