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Selecting the Best Carburizing Method for the Heat ...

Selecting the BestCarburizingMethodfor the heat treatment of GearsDaniel H. HerringThe HERRING GROUP, J. BreuerGerald D. LindellMetal Improvement Company, IncTwin Disc, compromisebetween cost andperformanceisachievedbyatmospherecarb urizing, thepresent dayde factostandardprocessing methodused inthe gear industry. Atypical workloadis shown in indications are, however, that the greatestpotential for future growth will come in shows a load of gears readyto be charged into a typical vacuum Method of Carburizing has been shown toofferproven metallurgical and the industry to stay competitivebothtechnologieswill be neededin the future. This isto insurethatthe challenges posed by everincreasingperformance requirements in smallerpackages and bya new generation of materialsand manufacturing methods can be paramount importancetodayis lowering unitcost that can only be achieved by improveddimensional control andmorecost effectivemanufacturing benefits achievedby vacuum carburizingcan be realized in highvolume, critical component Carburizing has proven itself a robustheat treatment process a

Selecting the Best Carburizing Method for the Heat Treatment of Gears Daniel H. Herring The HERRING GROUP, Inc. David J. Breuer Gerald D. Lindell

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Transcription of Selecting the Best Carburizing Method for the Heat ...

1 Selecting the BestCarburizingMethodfor the heat treatment of GearsDaniel H. HerringThe HERRING GROUP, J. BreuerGerald D. LindellMetal Improvement Company, IncTwin Disc, compromisebetween cost andperformanceisachievedbyatmospherecarb urizing, thepresent dayde factostandardprocessing methodused inthe gear industry. Atypical workloadis shown in indications are, however, that the greatestpotential for future growth will come in shows a load of gears readyto be charged into a typical vacuum Method of Carburizing has been shown toofferproven metallurgical and the industry to stay competitivebothtechnologieswill be neededin the future. This isto insurethatthe challenges posed by everincreasingperformance requirements in smallerpackages and bya new generation of materialsand manufacturing methods can be paramount importancetodayis lowering unitcost that can only be achieved by improveddimensional control andmorecost effectivemanufacturing benefits achievedby vacuum carburizingcan be realized in highvolume, critical component Carburizing has proven itself a robustheat treatment process and a viable alternative toatmosphere manufacturers ofheavy duty.

2 Off-road transmissions and relatedequipment such as Twin Disc Corporation havefound numerousbenefitsin substitutingvacuumcarburizing with high gas pressure quenchingforeither atmosphere or vacuum Carburizing with oilquenching technology. This paper will presentscientific data in support of this 1:Load ofProductionGears(650 lbs net)in Position forLoading into anAtmosphereCarburizing Furnacefollowed by Oil 2: Load ofProductionGears(650 lbs net)in Position forLoading into aVacuumCarburizingFurnace followedbyHigh GasPressureor OilQuenchingHISTORICAL BACKGROUNDIt is unfortunate that atmosphere and vacuumtechnologyare viewed as competitors instead ofas complements toone another. Theexisting usversus them mentalitycreated by constantnegativecomparisonshas hurt both 1960stheneed forbetteratmospherecontrol prompted aseries ofR&D efforts to finda solution.

3 One of theseled to the developmentofvacuumcarburizing,viewed as an alternativeto atmosphere Carburizing providingenhancedmetallurgical properties and shorter cycle , it was promoted within the heattreatment industryas a panacea for all theproblems of atmosphere Carburizing , and, thus, vacuum Carburizing proved to be a robusttechnology at that time, it is generally believedthat a significant portion of today s installedequipment base would use this technology. Itsfailure to achieve commercial success can bedirectly related to reliability and cost. Thecreation of soot was the Achilles heelof vacuumcarburizing. Perhaps a moreaccurate statement isthattheequipmentdesignsandprocessparam etersof vacuum carburizingwere notoptimized,andthetechnology s capabilitiesoversold to the heat -treating problems have been addressedandsooting is no longer a limitation of the ,controls,andprocessing methods assure excellentup timeproductivity and high volume TRANSMISSION COMPONENTST heapplication andmanufactureof high qualitytransmissiongearingusedindemandin gapplicationssuch as shown in Figures 3 and 4requirecareful consideration of a number ofcritical factors including: component design;materialselection; heat treatmentmethod.

4 Andtheinfluenceofpostheattreatmentmanufa cturing is subject to both sliding and rollingcontact stresses on the gear flanks in addition tobending stress in the tooth roots. The mostdesirable gear properties to meet these twocriteria would be hardened gears for strength andcontact propertieswith residual compressivesurface stress for bending fatigue 3: Typicalheavy dutyTransmissionUsedforAirport Fire is a majorcause of failure in failures fall into two classes:toothrootbending fatigue and tooth flank contact relatedfailures. In this work residual stress andmicrohardness testing were used as indicators tocompare the atmosphere and vacuum 4:Heavy DutyMarineTransmissionTransfer greater the magnitude and depth ofcompressive stress the greater the ability toimprove fatigue properties.

5 A high compressivestress value at thesurface helps the layer the greater the resistance tocrack growth for longer periods of stress values are an important factor infatigue critical components. Residual stresses stresses are desired as they oppose theapplied, repetitive, and undesirable tensile stressthat causes fatigue failure. X-ray diffractionmethods allow measurement of residual the purposes of this investigation, thevacuum and atmosphere Carburizing processeswere studied using x-ray diffraction techniquesand microhardness measurements. Specimens ofAISI8620materialweremanufactured,carbu rizedbythedifferentmethodsandsubjected to identical post heat treatmentoperations. Grinding and shot peening wereselected as of a steelsurface is both a functionof the rate of carbon absorption into the steel andthe diffusion of carbon away from the surfaceand into the metal.

6 Once a high concentration ofcarbon has developed on the surface, duringwhat is commonly called the "boost stage", theprocess normally introducesa "diffuse stage"where solid state diffusion occurs over time. Thisstep results in a change in the carbonconcentration, or carbon gradient between thecarbon rich surface and the interior core of themetal. The result is a reduction of the carbonconcentration at the surface while increasing thedepth of carbon stress results from the delayedtransformation and volume expansion of residual compressive stress through thecase hardened CarburizingAtmosphere Carburizing is an empirically based,time-proven process in which a carbon-richatmosphere surrounding a workload is used tochemically react with the surface ofthe parts toallow an adequate quantity of carbon to beabsorbed at the surface and diffused into atmosphere Carburizing parts are heated toaustenitizing temperaturein anEndothermic orequivalentatmosphere containingapproximately40% hydrogen, 40% nitrogen, and 20% carbonmonoxide.

7 Small percentages of carbon dioxide(up to 1 1/2%), water vapor (up to 1%), andmethane (up to 1/2%) along with trace amountsof oxygen are also present. This "neutral" or"carrier gas" atmosphere is generally consideredneither Carburizing nor decarburizingto thesurface of the order to perform the Carburizing processenriching gas is added to the carrier gasatmosphere. The enriching gas is usually eithernatural gas which is about 90- 95% methane(CH4)orpropane(C3H8).In atmospherecarburizing it is common practice to begin theflow of enrichment gasjustafter the furnace hasrecovered setpoint. This practice contributes tocase non-uniformity as various parts of theworkload are not uniform in temperature andcarburize at different water gas reaction(Equation 1) is importantin the control of the atmosphere carburizingprocess.

8 Instruments such as dew point analyzersmonitor the H2O/H2ratio ofthis equation whileinfrared analyzers and oxygenprobeslook + H2O = CO2+ H2(1)Inatmospherecarburizing,intergranul aroxidationis one of the phenomena taking placeas a result of theconstantchanges occurringinthe furnace can be explained by considering analternative form of the water gas reaction(Equation 2). Here we see thatthe transfer ofcarbon in atmospheres containing CO and H2isconnected with a transfer of oxygen, giving riseto an oxidation effect in steel with alloyingel ements such as silicon, chromium, andmanganeseCO + H2= [C] + H2O(2 )Figure5shows results from an actual gearsample that was atmosphere 5: Pitch Line & Root Comparison:Atmosphere Carburized(Oil Quenched) GearResults showcarburization to an effective casedepth (50 HRC) of ( ) in the rootand ( mm) at the pitch diameter.

9 Ofgreater significance isthe value for the depth ofhigh hardness( 58 HRC), ( ) at both the gear tooth pitch line and this depth the hardness values resultsare typical of the vastmajority of carburized gears currently in of atmosphere Carburizing include: The lowest initial capital equipmentinvestment cost. Capability of high volume output using awide variety of equipment styles, types, andworkload sizes. Furnace types include box,pit, mechanized box (integral- and sealed-quench furnaces), pusher, conveyor (meshbelt and cast link belt), shaker hearth, rotaryhearth, rotary drum (rotary retort),andcarbottom. Adequate process control; that is; all of theprocess variables are understood and reliablecontrol devices are available to provideameasure of process repeatability.

10 Capable of being easily automatedwithrecipe and/orpart-number control of heattreat cycles. Well-understood process problems allowingtroubleshooting based on an establishedtheoretical and empirical knowledge last point is very important. Often in the realworld, cost or other considerations mean thatproblems cannot be avoided, but it is the abilityto quickly and easily address the issues thatarise, which dictates the success of a giventechnology. This certainly is one of biggestadvantages of atmosphere of atmosphere carburizinginclude: A requirement of knowledgegained throughempirically methods isrequired to achieverepeatable results. This is due to a widevariability in the type of equipment, itsoperation, maintenance and constantlychanging process conditions. The need to condition equipment if idledor shut down prior to processing work.


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