1、10FTM04AGMA Technical PaperLow Distortion HeatTreatment ofTransmissionComponentsBy Dr. V. Heuer and Dr. K. Lser,ALD Vacuum TechnologiesGmbH, D.R. Faron, GeneralMotors, and D. Bolton, ALD-TTLow Distortion Heat Treatment of Transmission ComponentsDr. Volker Heuer and Dr. Klaus Lser, ALD Vacuum Technol
2、ogies GmbH,Donald R. Faron, General Motors, and David Bolton, 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.AbstractIn many applications the high demands regarding
3、service life of transmission components can be reachedonly by the application of a customized case hardening. This case hardening process results in a wearresistant surface-layer in combination with a tough core of the component.However as a side-effect the components get distorted during heat treat
4、ment. This distortion has a significantcost-impact, because distorted components often need to be hard-machined after heat treatment. Thereforethe proper control of distortion is an important measure to minimize production costs.By applying the technology of low pressure carburizing (LPC) and high p
5、ressure gas quenching (HPGQ) heattreat distortion can be significantly reduced.HPGQ provides a very uniform heat transfer coefficient. The predictability of movement during quenching ismore certain and uniform throughout the load. Further improvements can be achieved by “DynamicQuenching” processes
6、where the quenching severity is varied during the quench sequence by step control ofthe gas velocity. Proper fixturing is another factor for distortion control. Modern CFC-materials (carbonreinforced carbon) are well suited as fixture-material for gas quenching.The paper presents how LPC and HPGQ pr
7、ocesses are successfully applied on Internal ring gears for a 6speed automatic transmission. The specific challenge in the heat treat process was to reduce distortion insuch a way that subsequent machining operations are entirely eliminated. As a result of extensivedevelopment in the quenching proce
8、ss and the use of specialized CFC- fixtures it was possible to meet thedesign metrological requirements.The Internal ring gears addressed in this report have been in continuous production since 2006. Subsequenttesting and monitoring over a two year period progressively demonstrated that consistent m
9、etrology wasachieved and quality inspection was reduced accordingly.Copyright 2010American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October 2010ISBN: 978-1-55589-979-03Low Distortion Heat Treatment of Transmission ComponentsDr. Volker Heuer and Dr. Kla
10、us Lser, ALD Vacuum Technologies GmbH,Donald R. Faron, General Motors, and David Bolton, ALD-TTIntroductionProper distortion control has become even moreimportant than in previous days. To answer thedemand for fuel-efficient vehicles, moderntransmissions are built much lighter. Therefore thecomponen
11、ts of the transmission exhibit less wall-thickness which makes them more sensitive todistortion. Distorted gear components cause noisein the transmission, can require post heat treatmachining processes and may even create prob-lems during transmission-assembly. Thereforedistortion control has become
12、 more important thanever.By applying the technology of Low PressureCarburizing (LPC) and High Pressure Gas Quench-ing (HPGQ) heat treat distortion can be significantlyreduced. LPC is a case hardening process which isperformed in a pressure of only a few millibar usingacetylene as the carbon source i
13、n most cases.During HPGQ the load is quenched using an inertgas-stream instead of a liquid quenching media.Usually nitrogen or helium are used as quench gas.With an optimized distortion control it is possible tosimplify the process chain significantly. Figure 1shows how the process chain can be simp
14、lified, ifthe specified geometrical values of the componentscan be guaranteed after gas quenching.If the simplified process chain can be applied, thenthis will result in lower costs per part, lower through-put times and lower energy consumption duringproduction. Since there is no need to dispose any
15、oil after the quench and since cleaning operationsafter the quench are unneeded, the simplifiedprocess chain is much more environmentallyfriendly as well.For the Internal ring gears addressed in this report,the parts used to be heat treated with an inductionhardening-process. This process requires a
16、 50carbon and high alloy steel-grade, which is verychallenging for machining, or a non-ferritic grade ofcast iron, which is challenging for casting. So theintent was to change from induction hardening tocase hardening and to guarantee a low level ofdistortion after case hardening, to allow for direc
17、tassembly into transmission.Distortion mechanismsThe plastic deformation of metallic componentsduring heat treatment is referred to as distortion.Distortion occurs if the stress in the material ex-ceeds the yield stress of the material. During casehardening the components are exposed to hightemperat
18、ures in the range of 880C to 1050C andthe yield stress decreases strongly with increasingtemperature of a component. Three different typesof stress in the material need to be distinguished:- Residual stresses (they are induced before heattreatment by casting, forging, machining etc.).1Figure 1. Conv
19、entional and new process chain for the manufacturing of gear components4- Thermal stresses (they are caused by thetemperature gradient while heating andquenching).- Transformation stresses (they are caused bythe transformation from ferrite to austenite dur-ing heating and transformation from austeni
20、te tomartensite/bainite during quenching).These three types of stresses overlay with eachother and add up to the total stress in thecomponent. They are influenced by part-geometry,steel-grade, casting, forging, machining etc., andthey depend on the heat treatment. If the totalstress in the component
21、 exceeds the yield stress,then plastic deformation (distortion) of thecomponent takes place. The chronology and theheight of the three types of stresses leading to dis-tortion are dependent on numerous differentfactors, see Figure 2.High pressure gas quenching (HPGQ)The technology of high pressure g
22、as quenching(HPGQ) offers a tremendous potential to reduceheat treat distortions. Conventional quenching-technologies such as oil- or polymer-quenchingexhibit very inhomogeneous cooling conditions.Three different mechanisms occur duringconventional liquid quenching: film-boiling, bubble-boiling and
23、convection. Resulting from these threemechanisms the distribution of the local heattransfer coefficients on the surface of thecomponent are very inhomogeneous. Theseinhomogeneous cooling conditions cause tremend-ous thermal and transformation stresses in thecomponent and subsequently distortion. Dur
24、ingHPGQ only convection takes place which results inmuch more homogenous cooling-conditions, seeFigure 3.Significant reductions of distortion by substitutingOil-quench with HPGQ have been published 9.Another advantage of HPGQ is the possibility toadjust the quench-intensity exactly to the neededseve
25、rity by choosing quench-pressure andquench-velocity. Typical quench pressures rangefrom 2 bar to 20 bar. The gas velocity is controlledby a frequency converter and typical gas-velocitiesrange from 2 m/s to 15 m/s depending on the part-geometry and the steel-grade of the component.Figure 2. Potential
26、 factors influencing distortion of bearing-rings 25Figure 3. Heat transfer coefficient and temperature-distribution in liquid- and gas-quenching 3Equation 1 describes the heat transfer coefficient asa function of gas-velocity, gas density and the typeof gas. 4=Cw0,70,7d0,30,39c0,31p0,69(1)whereC con
27、stant factor (depending on quench cell);w gas velocity; gas density;d diameter of component; viscosity of the gas;cpspecific heat capacity of the gas; thermal conductivity of the gasTypical gases applied for HPGQ are nitrogen andhelium. 5. To achieve the required core hardnessin gears of low alloyed
28、 case hardening steels,helium as quenching medium and a gas pressure of20 bar is necessary for many applications. Theusage of this low density gas allows to quench withvery high gas velocity by using reasonable motorpower. In combination with an advancedgas-recovery technology, exhibiting a recovery
29、 rate 99.5 %, gas quenching is very economic in spite ofthe helium gas price. The positive experiences withgas quenching have induced gear suppliers to usecase hardening steels with better hardenability thusbeing able to quench bigger transmission-components as well.6For many applications it is not
30、the absolute height ofdistortion causing manufacturing problems but thespread of distortion. So for many applications thechallenge is to optimize the HPGQ in such way thatit provides a heat treatment process with very littlespread of distortion within a load and over time fromload to load.Furnace eq
31、uipment and fixturing fordistortion controlThe design of the gas quenching chamber is of keyimportance to minimize distortion. The chamberneeds to provide a high gas velocity to ensure thatthe core-hardness specification is met and thechamber needs to provide a very uniform distribu-tion of the gas-
32、velocity to minimize the spread ofdistortion within the load. Intensive numerical flowcalculation (CFD-studies) and experimentalstudies (with the Institute for industrial aerodynam-ics at Aachen-University) led to the design of thequenching chamber of the ModulTherm - system,see Figure 4. 6Two high-
33、powered gas circulators arranged to leftand right of the cylindrical housing accelerate thequenching gas to a high velocity in the chamber. Avery homogeneous flow through the charge isreached by means of several flow guides.The design of the chamber is modular and can beequipped with a gas flow reve
34、rse system. Thequench chamber is suitable for standard gasquenching processes with constant gas pressureand gas velocity as well as for new quenchingprocesses such as “Dynamic-Quenching”.As in the case of liquid quenching, proper fixturesand optimized loading of the parts is important forgas quenchi
35、ng, too. Alloy fixtures are widely used inheat treatment. However after long-term servicethe fixtures tend to deform due to high temperaturedeformation, which has a negative effect on thedistortion of the loaded parts. Moreover due to thepick up of carbon and subsequent formation ofcarbides the fixt
36、ures undergo dimensional growthcreating further problems during handling inautomated external transportation devices.Figure 4. ModulTherm heat treat system with gas quenching chamber7As an alternative, carbon composite materials, e.g.,CFC, were introduced for the use as fixtures in heattreating appl
37、ications. Low pressure carburizingfurnaces with high pressure gas quenching are per-fectly suited for the use of CFC-fixtures (Figure 5).Due to the usage of walking beam transportationany wear and overstressing of the fixtures isavoided. The use of oxygen-free hydrocarbons in avacuum environment and
38、 the inert quenching gasesduring the quenching process avoid any surfacereactions with the fixtures. In this service environ-ment one can take full advantage of the excellentmaterial properties of CFC, which has a very highdeformation resistance at high temperatures, lowthermal expansion coefficient
39、, and very low specificweight. Fixtures from CFC are designed to carrymore parts, exhibiting less gross weight therebyincreasing productivity and reducing energy costs.The major advantage however is that CFC-fixturesdo not show deformation during the heat treatmentprocess thereby assuring optimum po
40、sitioning ofthe parts. This has an significant, positive effect onpart distortion.Dynamic quenchingTo achieve optimum quenching results with respectto microstructure, hardness, and distortion, the gasquenching parameters need to be well adjusted.To further reduce distortion, a quenching processhas b
41、een developed where the quenching paramet-ers gas pressure and/or gas flow velocity arestepwise varied during quenching, see Figure 6.This process, so-called “Dynamic Quenching” istypically divided into three steps 7:- Step 1: high quenching severity until a certainpart-temperature is reached- Step
42、2: quenching severity is reduced for a settime to allow for temperature equalization in thepart.Figure 5. Load of Internal ring gears on CFCfixturingTime/sec.TemperatureFigure 6. Schematic illustration of dynamic quenching for specimen of different sizes8- Step 3: quenching severity is increased aga
43、inuntil the end of the quenching process. Thecontrol system in the quenching chamber allowsto control the different quenching steps of“Dynamic Quenching” in a very accurate waywith a good reproducibility. Optimum results areachieved when using helium. The light quench-ing gas helium can be decelerat
44、ed andaccelerated very precisely for optimumdistortion control.Distortion studyAn intensive process optimization program wasstarted before the start of production of the 6-speedautomatic transmission. The specific challengewas to optimize distortion control of the Internal ringgears. The goal was to
45、 eliminate hard machiningcompletely on these components thus simplifyingthe process chain as postulated in Figure 1.Furnace supplier and transmission manufacturerworked in close cooperation and successfullyimplemented a serial process duly for the start ofproduction in 6/2006. The process consists o
46、f LPCusing acetylene as carburizing source and HPGQusing helium as quench medium. Prior to carburiz-ing, the parts are heated under an atmosphere of1,2 bar nitrogen. This “convective heating” isapplied to achieve uniform temperature distributioninside the load while heating up. Once thecarburizing t
47、emperature is reached, the pressure islowered to a few millibars and the carburizing is initi-ated. The application of HPGQ with DynamicQuenching and the use of CFC-fixtures madesubsequent machining-operations unnecessary.The findings were published by the authors of thispaper in 2006. 8Initially al
48、l Internal ring gears from each load werechecked for excessive distortion with a so called“roll checker”. A “roll checker” is an automatedmeasurement system utilizing a rolling master heldby a pivoting yoke. The yoke enables the roll masterspindle to move in the lead and taper directions asthe rolli
49、ng master rotates in tight mesh with the testcomponent. Separate transducers located in thegimble head monitor lead and taper travel.To become more cost-efficient, the transmissionmanufacturer wanted to abandon this one hundredpercent “roll checker” inspection of all parts andchange to a spotwise control of distortion. The goalwas to inspect only two gears per load. Therefore itwas necessary to further reduce the amount ofdistortion and the quenching process was optimizedagain in 8/2008. Distortion stu
