1、02FTM7Selecting the Best Carburizing Methodfor the Heat Treatment of Gearsby: G.D. Lindell, Twin Disc, Inc., D.J. Breuer, Metal ImprovementCompany, Inc., D.H. Herring, The HERRING GROUP, Inc.TECHNICAL PAPERAmerican Gear ManufacturersAssociationSelecting the Best Carburizing Method for the HeatTreatm
2、ent of GearsG.D. Lindell, Twin Disc, Inc., D.J. Breuer, Metal Improvement Company, Inc.,D.H. Herring, The HERRING GROUP, Inc.Thestatementsandopinionscontainedhereinarethoseoftheauthorandshouldnotbeconstruedasanofficialactionoropinion of the American Gear Manufacturers Association.Copyright 2002Ameri
3、can Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 2002ISBN: 1-55589-807-61Selecting the Best Carburizing Method for the Heat Treatment of GearsGerald D. Lindell, Twin Disc, Inc.David J. Breuer, Metal Improvement Company, Inc.Daniel H. Herring, The HERRI
4、NG GROUP, Inc.ABSTRACTA very good compromise between cost andperformance is achieved by atmospherecarburizing, the present day de facto standardprocessing method used in the gear industry. Atypical workload is shown in Figure 1.Figure 1: Load of Production Gears (650 lbs net)in Position for Loading
5、into an AtmosphereCarburizing Furnace followed by Oil Quenching.All indications are, however, that the greatestpotential for future growth will come in vacuumcarburizing.Figure2showsaloadofgears readytobe charged into a typical vacuum carburizer. Thismethod of carburizing has been shown to offerprov
6、en metallurgical and environmental benefits.For the industry to stay competitive bothtechnologies will be needed in the future. This is toinsurethat thechallenges posedby everincreasingperformancerequirementsinsmallerpackagesandbyanewgenerationofmaterialsandmanufacturingmethods can be met.INTRODUCTI
7、ONOf paramount importance today is lowering unitcost that can only be achieved by improveddimensional control and more cost effectivemanufacturing methods. The benefits achieved byvacuum carburizingcanberealizedinhighvolume,critical component manufacturing.Figure 2: Load of Production Gears (650 lbs
8、 net)in Position for Loading into a Vacuum CarburizingFurnace followed by High Gas Pressure or OilQuenchingVacuum carburizing has proven itself a robust heattreatment process and a viable alternative toatmosphere carburizing. Gear manufacturers ofheavy duty, off-road transmissions and relatedequipme
9、nt such as Twin Disc Corporation havefound numerous benefits in substituting vacuumcarburizing with high gas pressure quenching foreither atmosphere or vacuum carburizing with oilquenching technology. This paper will presentscientific data in support of this choice.HISTORICAL BACKGROUNDIt is unfortu
10、nate that atmosphere and vacuumtechnology areviewedas competitors insteadof ascomplements to one another. The existing usversus them mentality created by constantnegative comparisons has hurt both technologies.Inthe1960s theneedforbetter atmospherecontrolprompted a series of R materialselection;heat
11、treatmentmethod;andtheinfluenceof post heat treatment manufacturing operations.Gearingis subject tobothslidingandrollingcontactstresses on the gear flanks in addition to bendingstress in the tooth roots. The most desirable gearproperties to meet these two criteria would behardened gears for strength
12、and contact propertieswith residual compressive surface stress forbending fatigue properties.Fatigueis amajor causeof failurein gears. Fatiguefailures fall into two classes: tooth root bendingfatigue and tooth flank contact related failures. Inthis workresidualstressandmicrohardnesstestingwereusedas
13、indicatorstocomparetheatmosphereand vacuum carburizing processes.The greater the magnitude and depth ofcompressive stress the greater the ability toimprove fatigue properties. A high compressivestress value at the surface helps the componentresist crack initiation. The deeper the compressivelayer th
14、e greater the resistance to crack growth forlonger periods of time.Figure 3: Typical heavy duty Transmission Usedfor Airport Fire Vehicles.Figure 4: Heavy Duty Marine TransmissionTransfer Gears.Residual stress values are an important factor infatigue critical components. Residual stresses areadditiv
15、e with applied stress. Compressive residual3stresses are desired as they oppose the applied,repetitive, and undesirable tensile stress thatcauses fatigue failure. X-ray diffraction methodsallow measurement of residual stress levels.For the purposes of this investigation, the vacuumand atmosphere car
16、burizing processes werestudied using x-ray diffraction techniques andmicrohardness measurements. Specimens of AISI8620 material were manufactured, carburized bythe different methods and subjected to identicalpost heat treatment operations. Grinding and shotpeening were selected as representative.CAR
17、BURIZING PROCESSESCarburizing of a steel surface is both a function ofthe rate of carbon absorption into the steel and thediffusion of carbon away from the surface and intothemetal. Onceahigh concentrationof carbonhasdevelopedonthesurface, duringwhat iscommonlycalled the boost stage, the process nor
18、mallyintroduces a diffuse stage where solid statediffusion occurs over time. This step results in achange in the carbon concentration, or carbongradient between the carbon rich surface and theinteriorcoreofthemetal.Theresultisareductionofthe carbon concentration at the surface whileincreasing the de
19、pth of carbon absorption.In the carburization process the residualcompressive stress results from the delayedtransformation and volume expansion of thecarbon-enriched surface. This induces thedesirable residual compressive stress through thecase hardened layer.Atmosphere CarburizingAtmosphere carbur
20、izing is an empirically based,time-proven process in which a carbon-richatmosphere surrounding a workload is used tochemically react with the surface of the parts toallow an adequate quantity of carbon to beabsorbed at the surface and diffused into thematerial.In atmosphere carburizing parts are hea
21、ted toaustenitizing temperature in an Endothermic orequivalent atmosphere containing approximately40% hydrogen, 40% nitrogen, and 20% carbonmonoxide.Smallpercentagesofcarbondioxide(upto11/2%),watervapor(upto1%),andmethane(upto 1/2%) along with trace amounts of oxygen arealso present. This neutral or
22、 carrier gasatmosphere is generally considered neithercarburizing nor decarburizing to the surface of thesteel.In order to perform the carburizing processenriching gas is added to the carrier gasatmosphere. The enriching gas is usually eithernaturalgaswhichisabout90- 95%methane(CH4)or propane (C3H8)
23、. In atmosphere carburizing it iscommon practice to begin the flow of enrichmentgas just after the furnace has recovered setpoint.This practicecontributes tocase non-uniformity asvarious parts of the workload are not uniform intemperature and carburize at different rates.The water gas reaction (Equa
24、tion 1) is important inthe control of the atmosphere carburizing process.Instruments such as dew point analyzers monitorthe H2O/H2ratio of this equation while infraredanalyzers and oxygen probes look at the CO/CO2ratio.CO + H2O=CO2+H2(1)In atmosphere carburizing, intergranular oxidationis one of the
25、 phenomena taking place as a result ofthe constant changes occurring in the furnaceatmosphere.This canbeexplainedby consideringanalternativeformofthewatergasreaction(Equation2).Herewesee that the transfer of carbon in atmospherescontainingCOandH2isconnectedwithatransferofoxygen,givingrisetoanoxidati
26、oneffectinsteelwithalloying elements such as silicon, chromium, andmanganeseCO + H2=C+H2O(2)Figure 5 shows results from an actual gear samplethat was atmosphere carburized.Results show carburization to an effective casedepth (50HRC) of 0.030 (0.76mm) inthe root and0.052 (1.33 mm) at the pitch diamet
27、er. Of greatersignificance is the value for the depth of highhardness (= 58 HRC), namely 0.014 (0.35 mm) atboth the gear tooth pitch line and root. From thisdepth the hardness values quickly diverge. Theseresults are typical of the vast majority of carburizedgears currently in service.Advantages of
28、atmosphere carburizing include:G The lowest initial capital equipment investmentcost.4Figure 5: Pitch Line that is; allof thepro-cessvariablesareunderstoodandreliablecon-trol devices are available to provide a measureof process repeatability.G Capable of being easily automated with recipeand/orpart-
29、numbercontrolofheattreatcycles.G Well-understood process problems allowingtroubleshooting based on an establishedtheoretical and empirical knowledge base.The last point is very important. Often in the realworld, cost or other considerations mean thatproblems cannot be avoided, but it is the ability
30、toquickly and easily address the issues that arise,which dictates the success of a given technology.This certainly is one of biggest advantages ofatmosphere carburizing.Disadvantages of atmosphere carburizing include:G A requirement of knowledge gained throughempirically methods is required to achie
31、ve re-peatableresults.Thisisduetoawidevariabilityin the type of equipment, its operation, mainte-nance and constantly changing process condi-tions.G The need to condition equipment if idled orshut down prior to processing work.G The need for large material allowances forpost-processing operations du
32、e to accuracyand finish requirements. Case depths typicallyarespecifiedinwideranges(e.g.0.030to0.050in. (0.75 to 1.25 mm) to compensate for cycleinduced variability.G Case depth quality issues; the best part of thecaseoftenis lost duetotheamount of stock re-moval required.G The need to constantly be
33、 concerned aboutsafety andfirepreventionissues (e.g., firefromcombustiblegasesandquenchoils,hotcontactsurfaces and pinch points).G Theneedtomonitor environmentalpollutionis-suesincludingairquality(forpotentiallyhazard-ousgases,suchasCOandNOx), waterquality(for contaminationconcerns suchas oil, miner
34、-als, etc.), waste disposal (quench oils). andsafety issuesG Processing techniques that produce non-uni-formity of case and carbon profiles throughoutthe gear geometry (tip-pitch line-root).It is important to note that a great deal of thenon-uniformity of case depth can be avoided ifadequatesoaktime
35、attemperatureisusedorifloadpreheating techniques are employed.Vacuum CarburizingVacuum carburizing is a proven method of purecarburizing and pure diffusion in which carbonpenetrates into the surface of the steel beingprocessed without interference from externalinfluences such as gas chemistry or sur
36、facecontaminants.5Vacuum carburizing is a modified gas carburizingprocess in which the carburizing is done atpressures far below atmospheric pressure (760Torr). The typical pressure range for low pressurevacuum carburizing is 1-20 Torr.The advantage of this method is that the steelsurface remains ve
37、ry clean and the vacuumenvironment makes thetransfer carbonto thesteelsurface faster (higher carbon flux values) sinceatmosphereinteractions suchas foundin thewatergas reaction do not take place. In addition nointergranular oxidation can occur.The carbon produced by the breakdown of thehydrocarbon g
38、as introduced into the chamber isfree to penetrate into the surface of the steel whilethehydrogenandresidual hydrocarbonbyproductsare removed from the system by the vacuumpumps.The hydrocarbon gases currently being used forvacuum carburizing are acetylene (C2H4), propane(C3H8) and to a lesser degree
39、 ethylene (C2H4).Methane (CH4) is essentially non-reactive at theselow pressures, unless the temperature is near1900 _F (1040 _C).In vacuum carburizing the breakdown ofhydrocarbon gases involve non-equilibriumreactions.Thismeansthatthesurfaceofthesteelisvery rapidly raised to the saturation level of
40、 carbonin austenite. By repeating the boost and diffusesteps desiredcarbon profileand casedepth canbeachieved.Depending on the type of hydrocarbon gas used,carbonisdeliveredtothesteelsurfaceviareactionssuch asC2H2 2C + H2(1)C3H8 C + 2CH4 (2a)C3H8 C2H4+CH4 C+2CH4(2b)C3H8 C2H2+H2+CH4 C+2CH4(2c)C2H4 C+
41、CH4(3)Thecontrolof thelowpressurevacuum carburizingprocess is on a time basis. The carbon transferrates are a function of temperature, gas pressure,and flow rate. Simulation programs have beencreated to determine the boost and diffuse times ofthe cycle.Figure 6 shows results from an actual gear samp
42、lethat has been low pressure vacuum carburized.Results show carburization to an effective casedepth (50HRC) of 0.040 (1.00mm) inthe root and0.052 (1.33 mm) at the pitch diameter. Of greatersignificance was the value for the depth of highhardness (= 58 HRC), namely 0.032 (0.80 mm) atboth the gear too
43、th pitch line and root.The overall case depth of maximum hardness forthe vacuum carburized part is noticeably deeperthan the atmosphere carburized part in Figure 5.One also observes a far greater consistency in theroot and pitch line hardness through the depth ofhigh hardness.Figure 6: Pitch Line &
44、Root Comparison: Vacuum Carburized (Oil Quenched) Gear6Figure 7 below shows an actual gear sample thathasbeenvacuumcarburizedandhighgaspressurequenched.Theseresults,whencomparedtoFigure5 and Figure 6 allow us to conclude that a moreuniform case depth has been developed betweenthe gear pitch line and
45、 root.This is due in large part to the absence of a vaporlayer in gas quenching resulting in a more uniformquenching rate in the gear tooth and root profiles.Advantages of vacuum carburizing include:G Absence of intergranular oxidation.G Capability of higher temperatures due to thetype of equipment
46、and the nature of the pro-cess.G Process andcycleflexibility allowsawidervari-ety of materials to be processed.G Processing methods produce more uniformcase and carbon profiles throughout the geartooth geometry (tip-pitch line-root).G Easy integration into manufacturing. The pro-cess is clean, safe,
47、 simple to operate and easytomaintain. Also, workingconditions areexcel-lent (that is, therearenoopenflames, heatandpollution).G Full automation capability using recipe or part-number control of heat treating cycles.G Precise process control achieved using com-puter simulations, which allow adjustme
48、nts toestablished cycles.G Consumption of energy by the equipment andprocess only when neededdue tothe natureofthe vacuum operation.G Typically less distortion results provided ade-quate measures are taken in loading.Disadvantages of vacuum carburizing include:G Higher initialcapital equipment cost
49、thanatmo-sphere carburizing equipment.G Part cleanliness is more critical in order toachieve desired results.G Empiricalprocess control, which requires proc-essing loads to determine optimum settings orto fine tune simulator.G Formation of soot and tar, which occur due tothetype,pressure,andquantityofhydrocarbongas introduced.It is important to note that research during the pastsix years has succeeded in finding combinations ofpressure, gas type, and flow parameters tominimizesootandtarformationandeliminatethesefactors as a concern in the vacu
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