1、12FTM23AGMA Technical PaperEnhancing Control ofDistortion ThroughOne Piece Flow HeatTreatmentBy V. Heuer, Lser, and T. Leist,ALD, and D. Bolton, ALD-TTEnhancing Control of Distortion Through One Piece Flow Heat TreatmentDr. Volker Heuer, Dr. Klaus Lser, and Dr. Thorsten Leist, ALD, and David Bolton,
2、ALD-TTThe 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.AbstractProper control of distortion has become even more important in new powertrain designs. To answer thedemandforf
3、uel-efficientvehicles,moderntransmissionsarebuiltmuchlighter,thereforethe componentsofthe transmission exhibit less wall thickness which makes them more sensitive to distortion. Distorted gearcomponentscancreatenoiseinthetransmission,requirepostheattreatmachiningprocessesandmayevencreate problems du
4、ring transmission assembly.By applying the technology of Low Pressure Carburizing (LPC) and High Pressure Gas Quenching (HPGQ),thedistortion causedby heat treatment canbesignificantly reduced. This technology has beensuccessfullyestablished in serial production for many different gear applications.W
5、ith the introduction of One Piece Flow - Heat Treatment, the distortion control can be further improved.This One piece Flow heat treatment allows for a rapid case hardening where the components are lowpressure carburized at high temperatures (1050C) followed by gas quenching. The components are nott
6、reatedinconventionalbigbatcheswithmultiplelayers,buttheyaretreatedinsmallbatchesconsistingofonelayer only. The quench intensity is controlled more precisely to allow for processes which are customizedindividually for each gear-component. The single-layer treatment providesS homogenous and rapid heat
7、ing of the components,S homogenous and rapid carburizing of the components,S homogenous and precisely controlled gas quenching.Allthevariationsfromlayertolayerareeliminated,whichleadstoreductionsindistortion-variationwithintheload.In addition, this new technology allows strong costs-savings for logi
8、stics. The manufacturing-line can becompletelyautomatedsincethepartsarefirst takenonebyonefromthesoftmachiningunit,thenheattreatedin time with the cycle-time of soft machining (“Synchronized heat treatment”) and then finally passed downone by one to the hard machining unit.The paper presents applica
9、tions for enhanced distortion control when using One Piece Flow - HeatTreatment.Copyright 2012American Gear Manufacturers Association1001 N. Fairfax Street, Suite 500Alexandria, Virginia 22314October 2012ISBN: 978-1-61481-054-43 12FTM23Enhancing Control of Distortion Through One Piece Flow Heat Trea
10、tmentDr. Volker Heuer, Dr. Klaus Loser, and Dr. Thorsten Leist, ALD,and David Bolton, ALD-TTIntroductionProperdistortioncontrolhasbecomeevenmoreimportantthaninpreviousdays. Distortedgearcomponentscausenoiseinthetransmissionandmayevencreateproblemsduringtransmissionassembly. Distortionhasa strong cos
11、t-impact, because distorted components often need to be hard-machined after heat treatment.Better control of distortion means:- less cycle time per part in hard-machining,- less hard-machining capacity needed and- less tooling cost for hard-machining.With an excellent control of distortion for some
12、applications hard machining can be completely eliminated.Distortion mechanisms and high pressure gas quenching, HPGQThe relevant mechanisms that cause distortion of components during heat treatment have been describedextensively in literature 1. Three different types of stress in the material contri
13、bute to distortion: residualstresses, thermal stresses and transformation stresses.These stresses are influenced by part-geometry, steel-grade, casting, forging, machining etc., and theydepend on the heat treatment. Ifthe totalstress inthe componentexceeds theyield stress,then distortionofthe compon
14、ent takes place. Figure 1 gives an overview over the potential factors influencing distortion.Walton 2 published the numerous potential factors that are influencing distortion in more detail.Figure 1. Potential factors influencing distortion4 12FTM23Byapplying thetechnology oflow pressurecarburizing
15、, LPC,and highpressure gasquenching, HPGQ,heattreat distortion can be significantly reduced. LPC is a case hardening process which is performed in a pres-sure of only a few millibar using acetylene as the carbon source in most cases. During HPGQ the load isquenchedusinganinertgas-streaminsteadofaliq
16、uidquenchingmedia. Usuallynitrogenorheliumareusedas quench gas 3 4.HPGQ offers a tremendous potential to reduce heat treat distortion. Conventional quenching-technologiessuch as oil- or polymer-quenching exhibit inhomogeneous cooling conditions. Three different mechanismsoccur during conventional li
17、quid quenching: film-boiling, bubble-boiling and convection. Resulting fromthesethreemechanismsthedistributionofthelocalheattransfercoefficientsonthesurfaceofthecomponentare very inhomogeneous, see Figure 2. These inhomogeneous cooling conditions cause tremendousthermal and transformation stresses i
18、n the component and subsequently distortion. During HPGQ onlyconvection takes place which results in much more homogenous cooling-conditions 5.Figure 2. Heat transfer coefficient and temperature-distribution in liquid- and gas-quenching 55 12FTM23Significant reductions of distortion by substituting
19、Oil-quench with HPGQ have been published 6. Anotheradvantage of HPGQ is the possibility to adjust the quench-intensity exactly to the needed severity bychoosing quench-pressure and quench-velocity. Typical quench pressures range from 2 bar to 20 bar. Thegas-velocity is controlled by a frequency conv
20、erter. Typical gas-velocities range from 2 m/s to 15 m/sdepending on the part-geometry and the steel-grade of the component. Figure 3 shows a typical industrialsystem for the HPGQ-process. Thebatches for such systems consist of several layers of productionparts,so called ”multiple layers”.One piece
21、flow - heat treatmentTodays production philosophy for gear components usually relies on the traditional separation between softmachining, heat treatment and hard machining. Heat treatment is performed in a central hardening shop.Thereisnocontinuousflowofproduction-partsbetweenthedifferentoperationss
22、uchassoftmachining,heattreatment, shot-peening and hard machining. Instead the parts are collected into batches and then movedfrom operation to operation. So large numbers of production-parts are stored in buffers or are in transitbetween the different operations.Inordertoestablishamoreeffective and
23、economic productionof gearcomponents, thegoal isto moveawayfrom batch type logistics and move towards a ”One Piece Flow” of production, see Figure 4.The goal is to move single parts from operation to operation instead of moving batches of parts. This onepieceflow,OPF,productionsystemwouldrealizeacon
24、tinuousflowofproductionparts andwouldavoidhugeeffortsforstorageandtransportationofpartsbetweenoperations7,8. Ifsuchatotalintegrationofalloper-ationscanbeestablished,thenthiswilloffernewpossibilitiesforautomation,whichagainleadstoareductionof costs. Additionally a higher level of automation will resu
25、lt in a reduction of defects in quality.Figure 3. ModulTherm heat treat system with gas quenching chamber (multiple layer treatment)6 12FTM23Figure 4. Gear manufacturing with central hardening shop and with ”One Piece Flow” integratedmanufacturing linesFigure 5 shows a new synchronized heat treatmen
26、t module for ”One Piece Flow” production which wasdeveloped by ALD Vacuum Technologies (Patent pending). This heat treatment module allows for totalintegration into the manufacturing line creating a synchronized production flow with gear machining.Following the philosophy of ”One Piece Flow” the par
27、ts are- taken one by one from the soft machining unit,- heat treated in time with the cycle time of soft machining (”Synchronized Heat Treatment”) and then- passed down one by one to the hard machining unit.Figure 5. SyncroTherm- heat treat system (single layer treatment; to allow for ”One PieceFlow
28、”-production, the parts are individually loaded on the tray and the treatment cycle of the trayis synchronized with the machining operations)7 12FTM23Although theparts arenot treatedindividually buttreated intrays, theparts areindividually loadedto theheattreat unit and individually unloaded from it
29、. So the continuous flow of single parts is established.In comparison to treatment of big batches in multiple layers, the single treatment provides- homogenous and rapid heating of the components,- homogenous and rapid carburizing of the components,- homogenous and precisely controlled gas quenching
30、.Allthevariationsfromlayertolayer areeliminated, whichleads toreductions indistortion-variation withintheload. Theconceptandthetechnologyof”OnePieceFlow”heattreatmenthavebeenpublishedearlierinmoredetail by the authors 9.Distortion study - comparison between multiple and single layer treatmentA disto
31、rtion study was initiated to quantify the improvement in distortion-control when switching frommultiple-layertosingle-layertreatment. AReactionInternalgearfroma6speedautomatictransmissionwaschosen as test-component. The Reaction Internal gear has an outer diameter of 152 mm, 103 internal teethand is
32、 made of 5130 material. A picture of the part is given in Figure 6. The case hardeningdepth CHDafterheat treat is specified as 0.3 . 0.6 mm and surface hardness is specified as 79 . 83 HRA.The tests with multiple layer treatment were performed in a ModulTherm-system (see Figure 3), while thetestswit
33、hsinglelayertreatmentwereperformedinaSyncroTherm-system(seeFigure 5). InbothcasestheLPC-process was applied at 900C using acetylene as carburizing source. Helium was used as quenchmedium for HPGQ in the ModulTherm system and the parts were quenched with an optimized dynamicquenching process as descr
34、ibed by the authors earlier 10. Nitrogen was used inthe SyncroTherm systemas quench medium for HPGQ.As of today the serial production of these Internal gears takes place in a ModulTherm systems with multiplelayer treatment. To analyze the distortion data from the ModulTherm system, a random load fro
35、m currentstandard production process was used and complete load-sizes were treated 10. 48 pre-measured partswere equally distributed into different layers of the load. Additionally to cover all ”extreme” positions in theload, it was made sure that parts from all 8 corners and parts from the middle o
36、f the load were geometricallyinspected. ApictureoftheloadisshowninFigure 7. IntheSyncroThermsystem,4testswereperformedwithsinglelayertreatmentwith8partsplacedeachtimeonthetray,seeFigure 8. Allfixturingwasmadeofcarbonreinforced carbon (CFC) material. Before the distortion-data was collected, it was m
37、ade sure that themetallurgical quality in terms of hardness profile, microstructure and core hardness was identical for bothpopulations of parts.Figure 6. Reaction Internal gear (d = 152 mm, 103 internal teeth)8 12FTM23Figure 7. Multiple layer - load of Internal ring gears (CFC fixturing)Figure 8. S
38、ingle layer - load of Internal ring gears (CFC fixturing)All measurements were performed with a CNC analytical gear-checker. Figure 9 shows the inspection of agear with the probe of the gear-checker moving along one tooth of the gear. Four teeth were inspected foreach gear and both left flank and ri
39、ght flank were examined per tooth.Figure 10 shows a comparison of the helix angle variation, Vbf, of the right flank measured for the multiplelayer treatment (ModulTherm) and for the single layer treatment (SyncroTherm). The already low values ofVbf from multiple layer treatment were further reduced
40、 when applying single layer treatment.Theaverageandthestandarddeviationofhelixanglevariation,Vbf,afterheattreatmentisgiveninFigure 11for both flanks. While the average variation of the left flank was only slightly reduced, the standard deviationwas reduced by 30% down to 7 microns when switching fro
41、m multiple to single layer treatment. For the rightflank the average of Vbf was reduced by 30% and the standard deviation of Vbf was reduced by 45%.9 12FTM23Figure 9. Geometrical inspection of a gear with a CNC analytical gear-checkerFigure 10. Helix angle variation Vbf after heat treatment (right f
42、lank); comparison betweenmultiple layer treatment (one test in ModulTherm) and single layer treatment (four tests inSyncroTherm)10 12FTM23Figure 11. Helix angle variation Vbf after heat treatment; comparison between multiple layertreatment (ModulTherm) and single layer treatment (SyncroTherm) (LF =
43、left flank; RF = rightflank; st. dev. = standard deviation)The lead taper of the Reaction Internal gears after heat treatment with multiple layers is shown in Figure 12andtheleadtaperaftertreatmentwithsinglelayertreatment(4tests)isshowninFigure 13. Forthetreatmentinmultiplelayers,the2layersfromthebo
44、ttomshowadifferentbehaviorthentheotherlayerswithachangeofsign. This means that the ”gradient of tooth-width” is different for the various layers. For the single layertreatmenttheleadtaperhasthesamedirectionforallparts. Thismeansthatinfutureoptimizations,thereisachance to reduce lead taper when treat
45、ing this component in a single layer.Figure 12. Lead taper after heat treatment; results from one test with multiple layer treatment(ModulTherm)11 12FTM23Figure 13. Lead taper after heat treatment; results from four tests with single layer treatment(SyncroTherm)When evaluating and comparing heat tre
46、at distortions, in most cases the change of geometry from green tothe treated component is considered. Figure 14 shows the standard deviation of the change of Helix angleFHbthroughheattreatment. ClearlythereislessvariationinFHbwhenswitchingfrommultipletosinglelayertreatment. For theleft flankthe sta
47、ndarddeviation wasreduced by43% andfor theright flankit wasreducedby 40% down to 3 microns.Figure 14. Standard deviation of the change of helix angle FHb through heat treatment;comparison between multiple layer treatment (ModulTherm) and single layer treatment(SyncroTherm) (LF = left flank; RF = rig
48、ht flank)12 12FTM23ThechangeofHelixvariationVbfthroughheattreatmentisshowninFigure 15. Whenswitchingfrommultiplelayer treatment to single layer treatment, the average change of the right flank was reduced by 64% and thestandard deviation of the change was reduced by 36%.Theloweramountofhelixanglevar
49、iationofthepartsfromsinglelayertreatmentindicates,thattheyareflatterafter heat treatment compared to the ones from multiple layer treatment.In this distortion study, the values from multipleand singlelayer treatmentwere comparedwhen applyingthesame carburizing temperature of 900C. In future works the results from multiple layer treatment andcarburizing at 900C will be compared with the single layer treatment and carburizing at 1050C.Integrated manufacturing lineAs de
