AGMA 11FTM23-2011 Integration of Case Hardening Into the Manufacturing-Line One Piece Flow.pdf

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1、11FTM23AGMA Technical PaperIntegration of CaseHardening Into theManufacturing-Line:“One Piece Flow”By V. Heuer, K. Loeser, G. Schmittand K. Ritter, ALD VacuumTechnologies GmbHIntegration of Case Hardening Into the Manufacturing-Line:“One Piece Flow”Dr. Volker Heuer, Dr. Klaus Loeser, Gunther Schmitt

2、 and Karl Ritter, ALD VacuumTechnologies GmbHThe 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.AbstractFor decades the gear industry has addressed the challenge to produce hi

3、gh performance components in acost-effective manner, maintaining lean inventories and highly reproducible quality.To meet design intent the components need to be heat treated, which traditionally takes place in a centralhardening shop. This separation between machining and heat treatment results in

4、additional costs fortransportation and logistics within the production-plant. The integration of heat treatment into themanufacturing line, has long been a goal of efficient production.Within the last 10 years it is possible to integrate heat treatment into the machining-facility by applying thetech

5、nology of Low Pressure Carburizing (LPC) and High Pressure Gas Quenching (HPGQ). Thecomponents are collected after soft-machining into larger batches and treated with LPC- andHPGQ-technology. This means however that the heat treatment is not synchronized with soft- andhard-machining since the compon

6、ents must be collected in buffers before heat treatment and must besingularized again after heat treatment.In order to totally integrate heat treatment into the manufacturing line and in order to synchronizeheat-treatment with machining, ALD Vacuum Technologies has developed a new heat treatment cel

7、l.Following the philosophy of “One Piece Flow” the parts 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.To allow for a rapid case hardening, t

8、he components are low pressure carburized at high temperatures(1050C) followed by gas quenching.In addition to the cost-savings gained from the improved logistics flow, the new concept in equipment offersthe following advantages:- individual processes customized for each gear-component- homogenous a

9、nd quick heating of the components and therefore low spread of distortion,- homogenous and controllable gas quenching and therefore low spread of distortion,- environmentally friendly carburizing and quenching and- compact and space-saving heat treat unit.Thepapershowsfirstresultsachievedwiththenewp

10、rocesstechnologyappliedinthenewheattreatmentcellconcept.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-023-03 11FTM24Integration of Case Hardening Into the Manufacturing-Line: “One Piece Flow”Dr. Volker Heu

11、er, Dr. Klaus Loeser, Gunther Schmitt and Karl Ritter, ALD VacuumTechnologies GmbHIntroductionTodays 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 sho

12、p.Thereisnocontinuousflowofproduction-partsbetweenthedifferentoperationssuchassoftmachining,heattreatment,shot-peeningandhardmachining. Insteadthepartsarebeingcollectedfirstintobatchesandthenmovedfromoperationtooperation. Solargenumbersofproduction-partsarebeingstoredinbuffersorareintransit between

13、the different operations. A continuous flow of production-parts between operations isrealized today only between some of the soft machining operations and between some hard machiningoperations.This discontinuous flow of production results in:- increased logistical efforts and efforts for documentati

14、on,- increased turn around times and ultimately in- increased production costs.New production philosophyInordertoestablishamoreeffectiveandeconomic productionof gearcomponents, thegoal istomoveawayfrom batch type logistics and move towards a “One Piece Flow” of production, see Figure 1.The goal is t

15、o move single parts from operation to operation instead of moving batches of parts. This onepieceflowproductionsystem,OPF,wouldrealizeacontinuousflowofproductionpartsandwouldavoidhugeeffortsforstorageandtransportationofpartsbetweenoperations1,2. Ifsuchatotal integrationof allopera-tions canbeestabli

16、shed, thenthis will offer new possibilities for automation, whichagainleads toareductionof costs. Additionally a higher level of automation will result in a reduction of defects in quality.Withinthelast10years,advancements intheheattreatment processhavemadeit possibletointegrateheattreatment at leas

17、t partly into the machining facility by applying the technology of low pressure carburizing,LPC, and high pressure gas quenching, HPGQ. But even with the introduction of LPC it was not possibletorealizeatotal integrationof all operations, becausethe equipment was basedon treatment of largebatches.Th

18、ese batches consist of several layers of production parts, so called “3D-batches”.Figure 1. Gear manufacturing with central hardening shop and with “One Piece Flow” integratedmanufacturing lines4 11FTM24Toallowforatotalintegrationofheattreatmentintothemanufacturingline,itwasnecessarytodevelopanewpro

19、cess technology and new equipment of case hardening. This new technology needs to provide thefollowing:- rapid case hardening to match the cycle time of soft machining,- low energy consumption and consumption of process gases,- low heat treat distortion,- integrated quality control,- compact designe

20、d piece of equipment and- an eco-friendly technology that can be integrated into a modern production environment.Development of “One Piece Flow”-case hardening technologyFor a rapid case hardening process that matches the cycle times of soft machining it is necessary toaccelerate the carburizing cyc

21、le drastically. Developments to accelerate the case hardening process havebeen in place for many years. The established process of low pressure carburizing, LPC, 3,4,5 was suc-cessfully elevated to higher temperatures to reduce process times 3,6. However this increase in processtemperature called fo

22、r the development of new micro-alloyed materials to avoid unwanted grain growth.Thereforethreefields wereidentified withthe needfor development: development of materials, process andequipment.Development of materialsConventional casehardeningsteels suchas 16MnCr5, 20MnCr5, 18CrNiMo7-6etc. candevelop

23、unwantedgraingrowthwhentreatedattemperaturesabove960C. Steelsuppliersstartedseveralyearsagotodevelopnew micro-alloyed steel grades to prevent unwanted grain growth 7. These micro-alloying elements formprecipitatesduringthesteelproductionprocesswhichactasgrainboundarypinningparticlesandthusinhibitabn

24、ormal grain growth during heat treatment at high temperatures.For hightemperaturecarburizingupto1050C suchsteels haveacontrolledamount of nitrogenN (120-170ppm)andaluminumAl(250-350ppm). FurtherasmallamountofniobiumNb(320400ppm)is beingaddedtothesteel. Thesteelproductionprocessrouteiscontrolledinawa

25、ythat anoptimum sizeanddistributionofthe Al(N)- and Nb(C,N)-precipitates are being secured. Several micro-alloyed steel-grades have beendeveloped and successfully tested jointly by steel suppliers and gear manufacturers 8,9 .Currently several gear manufacturers are in theprocess of adjusting their m

26、aterial specifications to allow forhightemperatureheat treatment. Anynegativeinfluenceof micro-alloyingonthegear machiningprocesseshave not been reported.Furthermore the material needs to have a sufficient and controlled hardenability to allow for quenching withmoderate quench-intensities. High quen

27、ch intensities should be avoided to facilitate a cost-effective gasquench process. Typical steel grades suited for that process are 20MnCr5-HH, 23MnCrMo5, 27MnCr5,18CrNiMo7-6 or 20NiMoCr6-5.Development of processLow pressure carburizing, LPC, is a case hardening process which is performed in a press

28、ure of only a fewmillibar in a protective atmosphere. In most applications acetylene is used as the carbon source. LPC is aclean and eco-friendly process that provides case hardened parts without any surface-oxidation 3,4,5.LPCisusuallycombinedwithhighpressuregasquenching,HPGQ. DuringHPGQtheloadisqu

29、enchedusinganinert gas-stream insteadof aliquid quenchingmedia. Usually nitrogenor helium are usedas thequenchgas. HPGQ provides clean components and offers significant potentials for lower distortion-valuescompared to oil-quenching 3.The case hardening process consists basically of the three steps

30、austenitizing, carburizing and quenching.Carburizingisadiffusioncontrolledprocess. Thediffusioncoefficientofcarboninsteelisstronglydependent5 11FTM24on temperature. This means that the carburizing step during LPC can be accelerated significantly if thecarburizingtakesplaceatelevatedtemperatures. For

31、examplewhencarburizingat1030Cinsteadof930Cthe total treatment timeof parts made of 18CrNiMo7-6 is reduced by 40% for a givencase hardeningdepthCHD = 1,5 mm 3.This high temperature low pressure carburizingprocess HT-LPC has recently been successfully developedand tested 3,6. Combined with gas quenchi

32、ng it offers a rapid case hardening process with low values ofheat treat distortion. Figure 2shows thecomparisonbetweenaconventional LPC process andtheHT-LPCprocess. For a given CHD of 0,65 mm the cycle time is reduced from 180 min to 40 min.Development of equipmentThe carburizing cycle can be accel

33、erated drastically by application of HT-LPC. However, even with theacceleratedprocess, thereducedcycletimewouldnot allowsynchronous processing of individual part com-ponentsmatchingthecycletimefromsoftmachining. Thereforeitwasdecidedtoaccumulateenoughpartsto assemble a single layer of parts for case

34、 hardening or so called “2D-batches”, see Figure 3.Figure 2. Process cycle of low pressure carburizing, LPC, and high temperature low pressurecarburizing, HT-LPC; CHD = 0,65 mm for both casesFigure 3. “2D-batch” for single layer heat treatment6 11FTM24These 2D-batches can be heated up to carburizing

35、 temperature much quicker then large 3D-batches con-sisting of several layers due to effective heat transfer by direct radiation in vacuum. Additionally the singlelayerconcept offersaveryhomogenous treatmentinallprocess stepssuchasaustenitizing, carburizingandquenching.Figure 4 and Figure 5 show the

36、 new synchronized heat treatment module which was developed by ALDVacuumTechnologies(Patentpending). Thisheattreatmentmoduleallowsfortotal integrationintotheman-ufacturing line creating a synchronized production flow with gear machining.Following the philosophy of “One Piece Flow” the parts are- tak

37、en 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.Althoughtheparts arenot treatedindividually buttreatedintrays, theparts areindividually loadedtotheheatt

38、reat unit and individually unloaded from it. So the continuous flow of single parts is established.A2D-batchwithuntreated“green”partsentersthesystemthroughthedoorofthequenchcell. Afterevacuat-ing, the tray is then brought with an elevator through the housing into one of the “hot zones”. A hot zone i

39、s asingle slot that can holdone tray of parts. Thereare 6hot zones vertically arrangedinside thehousing. Thepressureinall hot zones is identical, but eachhot zonehas its owntemperaturecontrol andits own supply ofprocess-gases. Once the tray has entered a hot zone, it is being heated up rapidly from

40、all sides and thencarburizedat1050Cusingacetylene. Afterthecarburizingiscompleted,thetrayisbroughtwiththeelevatorback to the quench cell. After closing the door between quench cell and housing, the quench cell is floodedwithnitrogen whichcirculates throughthe cell. A heat exchanger inside thequench

41、cell is usedto recool thecirculatinggas. Thesystemworkswithamaximumgaspressureof10barnitrogen. Buttokeeprunningcostslow,itisrecommendedtoquenchwithmax. 6barnitrogen. Oncethequenchingiscompleted,thequenchcellisventedwithair. Thenthepartsareremovedfromtheheattreatunitandtheyenterthetemperingunit. Afte

42、rtempering the parts are singularized again and passed down individually to the hard machining unit.Thenewheattreatsystemhasanintegratedqualitycontrol. Iftheprocessparametersareoutofspecificationfor a tray of parts, then this tray is automatically discharged from the manufacturing line and directed

43、to aquarantine area.Figure 4. SyncroTherm - heat treat module with the quench cell in the front and the attachedhousing with 6 vertically arranged hot-zones (LWH=6m2,8 m3,8 m)7 11FTM24Figure 5. SyncroTherm - heat treat systemResultsThecycletimesasgiveninFigure 2weretestedandverifiedsuccessfully. Fig

44、ure 6showstheheatingcurvesof different gear components treated in a single layer. The curves were recorded with a mobile temperaturelogging device.Figure 6. Temperature curves of different gear components heated by direct radiation in vacuumwith single layer treatment (gear wheel 1: 0,9 kg and D = 1

45、15 mm; gear wheel 2: 1,5 kg andD = 135 mm; final drive ring: 1,7 kg and D = 185 mm)8 11FTM24All threedifferent gear-wheels wereheatedwithin16 minup tothe carburizingtemperature of 1050C. Thisquick heating in combination with the HT-LPC process results in very short process cycles.Gears with differen

46、t geometries made of various steel-grades and other automotive components weretreatedinthenewheat treatunit. A trayof gearwheels madeof 18CrNiMo7-6 wascarburizedwithHT-LPCat1050C. Thetreatmenttimewas38min. Figure 7showsthequalityachievedonthesegears. ThetargetedCHD of 0,6 mm and all other specified

47、values were met successfully.Inanothertrial,variousgearwheelsmadeof27MnCr5-materialweretreated,seeFigure 8. Forthesegearsitis important to note that the CHD was not defined at 550HV but at 650HV. It took only 48min to reachCHD(650HV) = 0,59 mm. All other quality targets were met successfully as well

48、.Wheel-bearings made of 16MnCr5 were case hardened in the new heat treat system as well, see Figure 9.ThechallengeforthesebearingswastoachieveacasehardeningdepthofCHD(550HV)=0,7.1,0mminatreatment time of less then 70 min.Within a process cycle of only 68min the targeted case hardening depth of CHD (

49、550HV) = 0,83 mm wasreached. All other quality-requirements were fulfilled as well. Figure 10 shows the hardness-profile of thebearings and the carbon-profile of disc-samples made of the same material after HT-LPC at 1050C.Figure 7. Process results on gear wheels (18CrNiMo7-6) treated for 38min with HT-LPC in aSyncroTherm-unitFigure 8. Process results on gear wheels (27MnCr5) after HT-LPC process in aSyncroTherm-unit9 11FTM24Figure 9. Process results on wheel-bearings