1、12FTM21AGMA Technical PaperTypical Heat TreatmentDefects of Gears andSolutions Using FEAModelingBy Z. Li, and B.L. Ferguson,Deformation Control Technology,Inc.Typical Heat Treatment Defects of Gears and Solutions UsingFEA ModelingZhichao (Charlie) Li, and B. Lynn Ferguson, Deformation Control Techno
2、logy, Inc.The statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractSteel gears are heat treated to obtain enhanced properties and improved service performance. Quenchhardeni
3、ngisoneofthemostimportantheattreatmentprocessesusedtoincreasethestrengthandhardnessof steel parts. Defects seen in quenched parts are often due to high thermal and phase transformationstresses. Typical defects include excessive distortion, surface decarburization, quench cracks, large graingrowth, a
4、nd unfavorable residual stresses. Gear geometries with large section differences may suffer highstress concentrations and crack during quenching. Surface decarburization before quenching may lead tohigh surface residual tension and possible post heat treatment cracking. In this paper, the commercial
5、 heattreatment software DANTE is used to investigate three examples of heat treatment defects. Improvedprocesses are suggested with the help of modeling. The first example is an oil quench process for a largegear. Peeling cracks were observed on the gear surface during grinding of the quench hardene
6、d gears.Computer modeling showed that surface decarburization was the cause. The second example is a pressquench of a large face gear. Unexpected large axial bow distortion was observed in quenched gears, andcomputermodelingindicatedthatanincorrectpressloadanddiesetupwerethereasons. Thethirdexamplei
7、s an in-process quenching crack caused by high concentrated tensile stress from unbalanced temperatureand phase transformations in a spiral bevel pinion gear. The quenching process was modified to solve theproblem. This example also emphasizes the need for heat treatment modeling in gear design to r
8、educe thepossibility of heat treatment defects. The three examples illustrate how to effectively use heat treatmentmodeling to improve the quality of the gear products.Copyright 2012American Gear Manufacturers Association1001 N. Fairfax Street, Suite 500Alexandria, Virginia 22314October 2012ISBN: 97
9、8-1-61481-052-03 12FTM21Typical Heat Treatment Defects of Gears and Solutions Using FEA ModelingZhichao (Charlie) Li, and B. Lynn Ferguson, Deformation Control Technology, Inc.IntroductionDuringheattreatmentofsteelgears,thermalgradient,phasetransformation,andtheresultantinternalstressinteract with e
10、ach other to contribute to distortion and residual stress in the quench hardened parts. Bothdistortion and internal stress during quenching are complicated and not intuitively understandable in mostcases, which make process troubleshooting and improvement difficult. To reduce the machining cost afte
11、rheat treatment, minimum distortion is preferred. Minimum distortion can be obtained by tuning up the heattreatmentprocessparameters,suchasheatingandcoolingrates,andcarburizationschedules,etc.,althoughsuchtuneups areusually costly andtimeconsuming. Theincreasingdemand of gear performancerequiresthe
12、designer to take advantage of the favorable residual stresses obtained from carburized and quenchingprocesses. Toachievethesegoals,computermodelingisbeingmorewidelyusedintheheattreatindustrytooptimizetheheattreatmentprocess1-3. DANTEisacommercializedheattreatmentsoftwarebasedonthefiniteelementmethod
13、4. Itcanbeusedtopredictthephasetransformations,deformation,residualstresses,hardness, and distortion for heating, carburization, cooling, and tempering processes.Surface decarburization affects the surface hardness achieved by the quenching process. Many heattreatersbelievethatthedecarburizedlayerca
14、nbegroundofftoregainhighersurfacehardnessandtheeffectof decarburization is totally eliminated. Decarburization also affects the surface residual stresses 5.Favorable residual compression is expected on the surface of steel parts from carburization and quenchingprocesses. Decarburization can shift th
15、e surface stress from compression to tension, and the effect on thedepthoftensilestressisnormallydeeperthanthedepthofthedecarburizedlayer. Agrindingprocessmaynotable to effectively remove the surface tension and regain favorable residual compression. Computermodelingcanbeusedtounderstandtherelations
16、hipbetweenthedepthofdecarburizedlayeranditseffectonresidual stresses.A pressquenchisoftenusedtoreducethedistortionof gearslarger thaneight inchesindiameter. Thepressquenchis morecomplicatedthanthetraditional oil quench, anddistortioncanbeaffectedsignificantly by theselection of the quench press, die
17、 geometry and quenchant channel design, and the process setup.Excessive distortion and quench cracks are often seen in gears with large section differences. Nonuniformphase transformation between thin and thick sections can lead to stress concentration in a gear during thequenching process, and exce
18、ssivestress cancrack thepart at the worst, or distort the part at theleast. Thequenchingprocess canbemodifiedtoreducethepossibility of crackingor tocontrol sizeand shapechange.Quenching is a highly nonlinear process due to phase transformations and plastic deformation. Effectivecomputer modeling is
19、required to understand the part response during quenching before solutions to prob-lems can be obtained 6. A gear can rarely be designed with perfectly uniform section, soknowledge of theeffects of gear geometry on potential heat treatment defects is critical for the gear designer. Heat treatmentmod
20、els canbeusedinthegear designprocess toreducecost, improvequality, aswell asshortenthedesigncycle.Effect of decarburization on residual stressesA ring gear made of AISI 4320 was carburized, quench hardened, tempered at a relatively low temperature,andthenfinishedbyagrindingprocess. Peelingcracksonth
21、etoothfacewereobservedduringthegrindingofthehardenedgears. Theaxialheightoftheringgearis650mm,theinnerdiameteris950mm,thetipdiameteris 1300 mm, the root diameter is 1210 mm, and the gear has 60 outer straight teeth. Because of the geargeometry and the observed cracking mode, a plane strain FEA model
22、 of a single tooth with cyclic boundary4 12FTM21conditions was created to investigate the causes of cracking. Figure 1 shows a CAD model of this ring gearandthefiniteelementmodelcreatedforheattreatmentsimulations. Tomodelthedecarburizationeffect,veryfine elements are required in the shallow surface
23、to ascertain the carbon, temperature, phase, and stressgradientsduringquenching. PointAinFigure 1islocatedrightonthetoothsurface,andpointBislocatedata6mm normal depthfrom thesurface. Thematerial responsealong thestraight lineAB is used toinvestigatetheeffectofdecarburization. Thegearfacefromtiptoroo
24、tcoolsatdifferentratesduringquenching. Contourplots of carbon, temperature, metallurgical phases, and stresses are used to understand the part response.Modeling results indicate that the grinding cracks are caused by high residual tensile stresses in the toothsurface due to decarburization.The carbu
25、rization temperature was 950 C, and a boost/diffuse process was used to expedite the carbondiffusion rate. The boost step was 35 hours with a 1.0% carbon potential, and thediffuse stepwas 15hourswith a 0.85% carbon potential. At the end of the carburization process, the predicted carbon distribution
26、 isshownby the curvewith solidmarkers inFigure 2a. At hightemperature whenthe gear is entirely austenite,itssurfacemaydecarburizeinanatmospherecontainingoxygen. Anoxidizingatmospherecanbefoundinapoorly maintained furnace. During the transfer from furnace to quench tank, gears are exposed to air. For
27、large parts, the transfer time in air is usually longer than for small parts, which makes the decarburizationeffect more significant due to the reaction time between the oxygen and surface carbon. The transfer timefrom furnace to the quench tank is 2 minutes. However, an oxygen atmosphere in the fur
28、nace can causesurfacedecarburization. Inthisstudy,itisassumedthattheringgearisexposedtoanoxygenatmospherefor10 minutes before being quenched, which is a combined effect of furnace atmosphere and air transfer. Thesurfacecarbonisassumedtodropto0.4%duetothecarbon-oxygenreaction. Thecarbondistributionwi
29、thinthe decarburization zone is shown by the curve with hollow diamond markers in Figure 2a. The depth ofdecarburizedlayer is about 0.10mm using0.65%C as theinitial threshold. Withoutdecarburization, thepre-dictedaxial andtangential stressesafter quenchingare160MPaand350MPaincompression,respectively
30、.With decarburization, the axial and tangential surface stresses are 400 MPa and 250MPa in tension. Thechangeinaxialresidualsurfacestressduetodecarburizationis560MPa. Thetensilestresses willcontributeto the cracking probability during the grinding process.Figure 1. Gear geometry and finite element m
31、odel5 12FTM21a) b)Figure 2. a) Carbon and b) residual stress distribution after quenchDuring quenching, both the thermal gradient and phase transformations contribute to the stress evolution inthe part. Contours of carbon profile and martensite distribution after quenching are shown in Figure 3. The
32、carburizedcasecontains mainly martensitewithabout10%retainedausteniteintheas-quenchedcondition.The retained austenite will transform to bainite during tempering process. The depth of the hardened caseincreases from the root to tip along the tooth face due higher cooling toward the tooth tip and the
33、deepermartensite distribution. The gear body microstructure is about 90% bainite and 10% martensite.a) b)Figure 3. a) Carbon distribution, and b) martensite distribution at the end of quenching6 12FTM21Figure 4 shows the phase distributions along line AB (Figure 1) at the end of the quenching proces
34、s. Withdecarburization, 100% martensite is formed right on the surface because the martensite transformation fin-ishing temperature (Mf) is higher than the room temperature for 0.4% carbon of this steel grade. Under thedecarburized layer, the carbon content is about 0.8%, and 12% retained austenitei
35、s predictedto bepresentafter quenching.For steel, the starting temperature (Ms) for martensite transformation decreases as the carbon levelincreases. Duringatraditionaloilquenchprocess,martensiteformationisdelayedinthecarburizedcaseduetoits highcarboncontent. Thereis avolume expansionwith martensite
36、formation. In acarburized gear, thesurface transforms to martensite after the case-core location has formed martensite during a traditional oilquench. The delayed surface expansion leads to residual compression in the carburized case. Ifdecarburization occurs before quenching, martensite formation s
37、tarts in the shallow decarburized layerearlier than in the subsurface layers, and the delayed volume expansion of the sub-surface under thedecarburized case will convert the stresses in the shallow decarburized case from compression to tension.Low temperature tempering does not have a significant ef
38、fect on the residual stress distribution. Both axialandtangentialresidualstressescontributiontothecrackingpossibilityduringgrinding,butaxialstressusuallydominates because of its higher magnitude. The contour plots of the axial residual stress are shown inFigure 5.Distortion analysis of large face ge
39、ar during press quenchLargegearsareoftenquenchedinapressinordertomeetthedimensionalrequirements. Thegeardistortioncanbesignificantly affectedby thequenchpress equipment, thediedesign, thequenchant flow and thepro-cesssequence. Inthissection,apressquenchingprocessforalargefacegearaboutonemeterindiame
40、terismodeled using DANTE. The causes of distortion are examined, and an improved process is suggested.a) b)Figure 4. Phase distributions at the end of quenching process: (a) with decarburization effect,(b) without decarburization effect7 12FTM21a) b)Figure 5. Contour plots of axial residual stresses
41、 at the end of quenching process(a) with decarburization effect, (b) without decarburization effectA simplified CAD model of the face gear is shown in Figure 6. The face gear has a large ratio of diameter tothickness. Thewebof this facegear is not planar, andit behaves likeawasher inside thedie duri
42、ngquench-ing. The main distortion mode is bow deflection in the axial direction, and a single tooth model with cyclicboundary conditions is effective for predicting this type of distortion. The quenching dies are assumed toberigidinthemodel. Thebottomdiesarefixedduringquenching. Thesameloadisapplied
43、tothehubdieandthetooth die. Both hub and tooth dies on the top can move freely in a vertical direction during the quenchingprocess. Quench oil is assumed to flow over the part on the top and bottom surfaces in a general inward tooutward direction. A friction coefficient of 0.15 is assumed between th
44、e dies and gear surfaces. The pressquenchmodel setupis showninFigure 6. ThereferencelineshowninFigure 6isusedtorepresent thegearshape during both heating and quenching processes.Thegear toothsectionis thicker thanthat of gear hub. Duringtheheatingprocess, thehubheats faster thanthetoothsection,andit
45、willreachthefurnacesettemperatureearlier. Thismeansthatthephasetransforma-tiontoaustenitestarts andfinishes earlier in thehub section. Material volume shrinks with transformationtoaustenite. Thetiming differencefor volumeshrinkage betweengear toothand hubcreates internal stressesandaxialdisplacement
46、. Assumingthatthegearsitsonasupportsurfacewiththebottomsurfacesofgearhubandtoothsectionscontactingthesupporter,theshapechangeofthegearduringheatingisshowninFigure 7.The reference line in Figure 6 is used to represent the gear shape. At different furnace heating times, themodel results show that it m
47、ay have only hub or tooth section contacting the support. Significant axial bowdeflectionwill occurduringafreefurnaceheatingprocess. However,thepredictedaxial deflectionat theendof heating is only 1 mm with the hub displaced upward.8 12FTM21Figure 6. Face gear CAD model and press quench model setupF
48、igure 7. Face gear axial deflection during furnace heatingDuringthepressquench,anexcessivepressloadcancausedistortionofthegeartoothduetoTRIPeffect,ormayevendamagethetoothprofilebyplasticdeformation. Ingeneral,anappliedloadisusedtoholdthegearinthedies insteadof totallyconstrainingthegear usingdisplac
49、ement control. Aminimum loadis preferredtoeffectively holdthegear inthedies. Aninsufficient loadmeans that thereactionforcefrom thegear exceedsthe applied press load, as shown in Figure 8, and a section of the part may separate from the die. At 16secondsduringquenching,martensiteformationinthehubandwebisalmostcompleted,sothegearwebhashigh strength. A press load of two tons is insufficient to hold the gear at this point. The top die is pushed upabout1.84mm,andthegeartoothsectionisseparatedfromthebottomdie. Withfurthermartensiteformationinthetoothsection, thegear toothmovesdowna
