1、11FTM24AGMA Technical PaperInduction Hardening ofGears with SuperiorQuality and FlexibilityUsing SimultaneousDual Frequency(SDF)By C. Krause, F. Biasutti, eldecSchwenk Induction GmbH, andM. Davis, eldec Induction U.S.A.Induction Hardening of Gears with Superior Quality andFlexibility Using Simultane
2、ous Dual Frequency (SDF)ChristianKrause,F.Biasutti,eldecSchwenkInductionGmbH,andMarkDavis,eldecInduction U.S.A.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.AbstractInduc
3、tion hardening of gear teeth is a well known for its challenges, but also for its potential for improvedquality and process control. For complex geometric parts like gears, the power density and inductionfrequencyneedtobeadjustedverypreciselytoachievetherequiredhardeningpattern. Sincethe1940s,itiswe
4、ll known that working with two simultaneous frequencies is the optimal way to heat a geared part up tohardening temperature. The key point in this process is that the medium frequency (about 10 kHz) affectsprimarily the tooth root and the high frequency affects first of all the tip of the tooth and
5、the flanks. The rightcombinationofthepowerdensitiesofmedium-andhighfrequencyenergyvaluesandtheheatingtimearethecrucial factors to reach a contour true heating pattern and, thereby, a contour true hardening pattern. Shortheatingtimesandhighpowerdensitiesarecriticalrelevantfactorsinordertoachieveconto
6、urtruehardenedgears.While the benefits are known, the practical combination of medium and high frequency induction areelectromechanically restricted and difficult to achieve. Today we are able to get benefits from this processusingSDFtechnologywithhighpowerdensity(upto8 kW/cm ). Abigbenefitistheflex
7、ibility. Theuserisable to manipulate the intensity of the heat sources by manipulating the power ratio of the two frequencies.Thisgivesusthepossibilitytoachieveaspecialhardeningpatternonaspecificgeargeometry. Ifwewanttoheat treat different gear geometry we can change power ratio and heating time to
8、reach the designatedhardeningpattern. Togetacontourtruehardeningpattern,ashortheatingtimeisnecessarytopreventheatconduction into the remainder of the part. Usually the heating times are between 100 and 500 milliseconds.TodaySDF-generatorsareabletoexactlycontroltheenergyvalueinthesetimeframes. Wecanr
9、educethedistortion that occurs from heat treatment because of the smaller volume of material (HAZ) affected byinduction heating with ultra-short short heating times.Theauthorswilldescribethestateoftheartofinductionhardeningofgearswithsimultaneousdualfrequencyusing case studies which show the possibi
10、lities to manipulate the hardening pattern in a positive way fordifferent desired gear geometries.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-024-73 11FTM25Induction Hardening of Gears with Superior Qual
11、ity and Flexibility UsingSimultaneous Dual Frequency (SDF)Christian Krause, F. Biasutti, eldec Schwenk Induction GmbH,and Mark Davis, eldec Induction U.S.A.IntroductionInduction hardening is a surface hardening process and belongs to those hardening methods where nochangeinthechemicalcompositionofth
12、esurfaceisnecessary. Theprocessofinductionhardeningconsistsof heating via electromagnetic induction and quenching normally by using a water-polymer solution.Electromagnetic induction is based on Faradays Law, which says that an electromotive force (EMF) isinduced in a material which has electrical c
13、onductivity be changing the magnetic flux in dependence upontime. Because of the “skin effect” the induced current flows in a layer of specific thickness. This thicknessdependsontheresistivityofthematerialtobeheated,onthemagneticpermeabilityofthematerialandonthefrequencyofcoilcurrent. Thehigherthefr
14、equencytheloweristheskindepth. Firstapplicationsforinductionsurface hardening were the heat treating of crankshafts and the hardening of railroad tracks 1. A few yearslatertheyevenstartedtohardengearsbyusingelectromagneticinduction. Inordertoreachgoodmechanicalpropertiesthebestwaytohardenagearistore
15、achacontourtruehardeningpattern2. Soonitwasclearthatthe optimal way to reach a contour true hardening pattern is by using two frequencies simultaneously 3.In 1952, Kegel 4, 5 describes an induction hardening process for gears by using 2 frequencies. The gearswere preheated with a medium frequency of
16、 10 kHz to a temperature of 500-600C. After that preheating asecond heating step with high frequency (150-250 kHz) followed to bring the gear surface to hardening tem-perature. Thus it is possible to reach a hardening pattern that nearly follows the gearing profile. He alsodescribed a process with s
17、horter heating times (500 ms-1500 ms) by increasing the power density up to 8kW/cm2. Itwasclearthatusingbothfrequenciesatthesametimewouldbringthemthebestresults. Thisideaexists nearly from the beginning of using induction technology for gear surface hardening 6.Wolfgang Schwenk (eldec Schwenk Induct
18、ion GmbH) started to build simultaneous dual frequency(SDF)-generatorsinthelate90s. Fromtheliterature,itisknownthathighpowerdensitiesarenecessaryforacontourtruehardeningprocess. Consequently,thecompanyeldecstartedtobuildhigh-powerSDF-gen-erators. In 2004, eldec installed the first 1000kW-SDF hardeni
19、ng system in the US. Since 2006, eldecSchwenk Induction is usinga3MWSDF-generator for many different process developments, especiallygears.SDF TechnologyBy using two frequencies simultaneously on a common inductor coil, a contour true hardening pattern isachievable. The hardening of steel consists o
20、f two major steps: Heating and Quenching. In this article theauthorswillhavefocusononlyontheheatingprocess. HeatingwithSDF-Technologyistobecharacterizedby some specific attributes:S high power densityS heating sections of the gear teeth depends upon the frequencies of both heat sourcesS time for aus
21、tenization from 100 to 500 msS high temperature gradient during austenizationS adjustability of the power for the two frequencies for each specific applicationS heating of smaller total volume leads to less distortionToreachacontourtrueheatingpatternandtherewithacontourtruehardeningpatternpowerdensi
22、tiesupto8kW /cm2are necessary. Thus it is possible to reach the hardening temperature in the surface in a very short4 11FTM25time. Asaresultwecanpreventtheheatconductionintotheteethrespectivelyintothepartwhich wouldleadto significant more hardness depth in the tip of the tooth as well as in the root
23、. Figure 1 shows the reachablehardening pattern for an exemplarily gear geometry (gear module 2.6, outer diameter 78 mm) by usingdifferent power densities.It is clear to see that with a higher power density a better hardening contour is achievable and the benefit ofashorter time necessary to reach h
24、ardening temperature is gained.ThesecondcharacteristicattributeduringheatingwithSDFisthatthesourcesofheatoccurindependenceof both frequencies simultaneous at different locations. The skin depth of the induced current dependsessential on the frequency. Equation 1 7 describesthe correlationbetween fre
25、quencyand skindepth oftheinduced current.(1)= 503m f ,mm =22 f momwhere is skin depth, mm; is resistivity, mm2/m;m0magnetic permeability of vacuum, H/m;m relative magnetic permeability;f frequency of the inductor current, Hz.Ifonlyhighfrequency(HF)isused(Figure2a)forheating,thesourcesofintenseheatis
26、mainlyinthetipofthetooth. This effect occurs because of the difference in coupling distance between coil and tip of the tooth andcoil and tooth root. The coupling distance in the tip is shorter and as a result this leads to a more efficientheating process at the tips.By only using medium frequency (
27、MF) (Figure 2b) for heating to hardening temperature, the frequency is toolow to affect the small tip volume of theteeth. Hardening temperaturefor steels,which areused forinductionhardening,ishigherthanCurietemperature(769C)8. AboveCurietemperaturethemagneticpropertiesofthematerialarechangingformfer
28、romagnetictoparamagnetic. That leadsto asignificant smallerpermeabil-ityandthustoahigherskindepthoftheinducedcurrent. Thenecessarychangeinmicrostructurefromferriteto austenite also direct to a higher specific electrical resistance of the material. As a result the skin depth isincreasing. Because of
29、the higher skin depth, the current is induced into the larger volume (body) of the partand flows on the surface, in the root of the tooth. Thus, the primary source of heat intensity is located in theroot with medium frequency.Power density 3 kW/cm2Power density 5 kW/cm2Power density 8 kW/cm2Figure 1
30、. Reachable hardening pattern in dependence of power density and heating time,samples were sand blasted and etched with 2 percent nitric acid5 11FTM25a) high frequency b) medium frequencyFigure 2. Heating pattern above Curie temperatureByusingSDF(HFandMFsimultaneously),thesourcesofheatintensityarelo
31、catedinthetipandintherootat the same time. By adjustment of the power ratio between MF and HF and the heating time it is possible tocontrol the hardening pattern very precisely.Contour true hardening of geared parts is a process were short austenization times are used. The heatingtimes are usually b
32、etween 100 500 ms. As described above, short heating times are necessary to get acontour true heating pattern. It is needed to limit the heat conduction from the heated surface into the mainbody of the part.A necessary requirement for a hardening process of steel is to heat the material to a tempera
33、ture whereausteniteisformedandtodissolvecarbonfromcompoundintoaustenite. Bothstepsarediffusionprocesses,whichmeanthattheyneedbothTimeandTemperaturetosucceed. Becausetheheatingtimesareveryshort,it is necessary to use higher austenization temperatures to reach a homogeneously formed austenite and,ther
34、eby, a homogeneous martensitic microstructure after the hardening process.One must also be on the lookout for faulty microstructure that can occur as the result of incomplete or badtransformation in some cases due to poor conditions, such as starting microstructure, chemical compositionof the steel,
35、heating rateand possiblythe endtemperature reached. Forshort austenizationtimes, astartingmicrostructure with well distributed carbon respectively carbides is preferable. As a result the carbon hasshort diffusion paths and just a small time is needed to distribute the carbon homogeneously into auste
36、nite.Thus a good starting microstructure is a quenched and tempered initial condition. A ferrite/pearlite structuremay also lead to good results if the amount of ferrite is small and most of the pattern is pearlite.If proper martensitic transformation does not occur, the following solutions can be a
37、pplied 9:S higher temperatures for austenization;S focus the sources of heat directly in the critical area;S preheating;S choice of material, thinking about the initial state (qt, normalized).During induction surface hardening with short heating times the core of the part is not influenced by the he
38、attreatment. That means that the necessary core strength and the core hardness has to be adjusted beforemachining and hardening.One of the big advantages of induction heating with SDF is that it is possible to independently adjust thepower and heating time of both frequency portions (HF and MF).6 11
39、FTM25Thus with one SDF-generator it is possible to adjust the desired hardening pattern for different parts anddifferent gear geometries by controlling heating time and power ratio of high and medium frequency.Figure 3 and Figure 4 from 10 are presenting exemplarily the influence of heating times an
40、d power ratios tothe achievable hardening pattern in radial and normal direction.A significant attribute of this kind of induction surface hardening is the short heating time to hardeningtemperature of a small material volume, which means that only that part of the work piece is heated(hardened) tha
41、t you want heat treated. Thus, only a small amount of heat flows (or “sinks”) from the surfaceintothepart. Heatingofasmallvolumeleadstosmalldistortion. Jonesetal.11comparedintheirarticlefrom2010 the distortion of AGMA quality class 10 bevel gears that have been carburized to those that have beenSDF-
42、 inductionhardened. TheAuthorsconcluded: “.wehavefoundthatinductionhardeningallowsmuchcloser control of surface distortion (less distortion) than conventional carburizing.”. Rodman et al. 12reported in their article about the distortion of a gear hardened with SDF: “.low and uniform dimensionalgeome
43、tric changes. “. They measured a radius deviation (used gear dimensions: 28 teeth, module of 2.6,material: 42CrMo4(1.7225)intherootof5mm-17mmandinthetip -13mm3mm. Furthermoretheynoticedapitch deviation interval of -0.5- 0.1 (root) and -0.6- 0.2 (tip).ApplicationsInductioncontourhardeningalwaysfindsm
44、oreapplicationinsubstitutionofthecarburizingprocess. Hereareexamplesofthestateoftheartofthistechnology. Adissertationprojectfordevelopmentofcontourhardenedwormgears(Figure 5)wasdevelopedtogetherwiththeAssociationofDrivelineTechnologyinGermany13.The goal of the project was to demonstrate capability o
45、f surface induction hardening and show the possibilityto replace conventional case hardening process.Induction contour hardening also represents the state of the art process for steering parts. Since these areknown to be relevant Safety components, the heat treatment process has to assure high quali
46、ty levels andrepeatability.Power density 5 kW/cm2Heating time 180 msPower ratio MF/HF 80%/20%Power density 8 kW/cm2Heating time 140 msPower ratio MF/HF 50%/50%Figure 3. Hardening pattern in normal direction for different power ratios 10, samples werepolished and etched with 2 percent nitric acidPowe
47、r density 5 kW/cm2Heating time 180 msPower ratio MF/HF 80%/20%Power density 5 kW/cm2Heating time 160 msPower ratio MF/HF 50%/50%Power density 5 kW/cm2Heating time 170 msPower ratio MF/HF 50%/50%Figure 4. Exemplary hardening patterns in radial direction for 3 different heat treatmentconditions 10, sa
48、mples were sand blasted and etched with 2 percent nitric acid7 11FTM25Figure 5. Hardening pattern of a SDF hardened worm gear 14, heating time 350 ms, powerapplied 400 kW MF + 166 kW HF, frequency: 11 kHz MF and 236 kHz HF, sample was sand blastedand etched with 2 percent nitric acidNowadays the cha
49、llenge for the manufacturers and designers is to maintain the mechanical properties of anassembly while reducing size andweight dimensionsof thecomponents. Hardening thegeared partcontourinstead of common through hardening process is fundamental to achieve these ambitious goals.A hardening process has being developed for different steering racks and steering pinions, see exemplaryFigure 6.Induction hardening of a ring gear (Figure 7) is another process that has been developed. The ring gearbelongs
