1、10FTM02AGMA Technical PaperImproving HeatTreating Flexibility forWind Turbine GearSystems ThroughCarburizing, Quenchingand Material HandlingAlternativesBy W. Titus, AFC-HolcroftImproving Heat Treating Flexibility for Wind Turbine GearSystems Through Carburizing, Quenching and MaterialHandling Altern
2、ativesWallace (Jack) Titus, AFC- HolcroftThe 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.AbstractPart handling and processes for heat treating large gears have created chal
3、lenges for decades. Growth inwind energy technology has focused more attention on this issue in recent years. The vast majority ofinstallations processing such large parts utilize conventional methods via pit furnace systems. Suchequipment has inherent limitations with respect to quench flow and par
4、t handling, making true improvementsin areas such as distortion control difficult due to physical limitations of this processing approach. Thispresentation will explain alternative methods for heat treating large components that allow part distortion to beminimized. Benefits will be quantified regar
5、ding cost savings to produce such gearing and quality.Copyright 2010American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October 2010ISBN: 978-1-55589-977-63Improving Heat Treating Flexibility for Wind Turbine Gear Systems ThroughCarburizing, Quenching an
6、d Material Handling AlternativesWallace (Jack) Titus, AFC-HolcroftIntroductionCommercial size wind turbines shown in Figure 1and their gearboxes Figure 2, are designed tosurvive in extreme environmental conditions, mostnotably high wind forces, desert heat and arcticcold. However robust the design a
7、nd use of materi-als, the typical gearbox struggles to meet theirdesign life. Studies by the National RenewableEnergy Laboratory (NREL) 1 have shown that onaverage a gearboxs time to repair can be as shortas five years give or take when the economic pay-back model requires 20 years. Many failures se
8、emto occur first in the bearings, also outlined in theNREL report. Studies are continuing to identify theroot causes for these and other failures. Otherpreliminary studies indicate that the gearbox and orplanetary carrier systems are subjected to high-er-than-anticipated stresses and that the materi
9、alsused may not have the strength to resist thestresses encountered. Contaminated lubricationhas also been targeted as a potential problem area.Since ferrous alloys are the most economical choicefor drive train components, heat treating is a majorconsideration when designing for material strengthand
10、 fatigue resistance.Historically pit furnaces and the pit quench havebeen employed to case harden these and otherlarge gears because for treaters there just has beenno other choice. They are used primarily becausethey exist and alternative equipment has not, andthere is the comfort factor weve alway
11、s done itthat way. There are some who will say that pitfurnaces offer a higher quality product but thats notso, as data presented later will show. In addition, pitfurnaces can be energy hogs especially whencarburizing cycles of 35 hours and longer arecommon.Figure 1. Commercial size wind turbine4Fig
12、ure 2. Heat treated parts of a typical wind turbine gearboxEasy site preparationCommon sense suggests that eliminating the needfor a pit is a very desirable concept, offering morefreedom for heat treat site selection. Often the deepexcavations needed for pit furnaces and quenchtanks pose environment
13、al issues in brown as wellas green field sites. Further, if expanding an exist-ing facility is contemplated, what facilities engineerhasnt experienced a flooded pit or power outagetrying to install a pit? Who wants to accommodatean extra-high bay and additional large crane?Lower installation costs a
14、re achieved by reducingthe overall furnace equipment height and using apitless quench tank. Emerging economic realitiesfor commercial as well as captive heat treatersdictate that the need for a more flexible and cost ef-fective method of carburizing and quenching isoverdue for large gears. Wind ener
15、gy has attractedthe attention of commercial heat treaters but theydont want to put all of their eggs in one basket, like apit furnace; they need equipment flexibility if thewind turbine market fails to live up to expectations.Cost effective materialCase hardening is necessary to achieve the toothstr
16、ength and wear resistance in ferrous alloys andthe process allowing the most economical use ofmaterials is carburizing. Adding carbon to the wearsurfaces of bearings and gear teeth can be morecost effective than induction hardening where theentire part contains the alloy, even areas that playno role
17、 in the stressed application. Both processesrequire quenching to achieve the hardnessnecessary and both result in distortion that must becorrected by grinding to bring the part to operatingdimension.Anyone associated with manufacturing and heattreating precision drive systems knows grinding is anece
18、ssary process required to obtain final size andsurface finish after quenching. In addition, grindingremedies minor machining errors resulting frommanufacturing very large gears. Grinding, althougha critical requirement, is very expensive and can beminimized if quench distortion can be controlled and
19、held to a minimum. If distortion can be predictedthen designers can plan for it, thereby providing onlythe amount of material to be removed. Helical bevelgears are especially troublesome because of theirtendency to “unwind”. The unwinding if predictablecan be accommodated in the design andmachining.
20、Quenching optionsObviously quenching is a very difficult process tocontrol and one that can ruin a perfectly successfulcarburizing process. Only recently with thedevelopment of sophisticated computerized fluiddynamics (CFD) has liquid quenching, specificallyoil quenching, been targeted for investiga
21、tion;maybe thats because most heat treaters wish itwould just go away no chance. Since heat treatingferrous material began eons ago, oil has no matchfor the range and control of quenching response.No other medium works better than oil when cor-rectly applied knowing its limitations.S Polymer has bee
22、n tried and in some applicationsit can provide acceptable results in smaller5gears but concentration maintenance is an issueand it too has disadvantages, creating a need formore investigation into controlling distortion.S Salt is a great quenching medium but has foundalmost no application with large
23、 gears. Historic-ally, however, salt has been used extensively toreduce distortion in smaller gears.S Helium high pressure quenching is very good atcontrolling distortion but cannot match thequench severity of liquid thats required for verymassive parts.S Press quenching with oil is the most accepte
24、dand practiced method for distortion control withgears and bearing cups & cones, but to date ithas seen little if any application for smaller windturbine gears.S IntensiQuench is a patented high intensitywater quench method designed to impartcompressive stresses onto the part surface inaddition to a
25、llowing the use of lower alloy steels.Proprietary software computes the quench timeto form martensite and the time required for thecore heat to temper the martensitic case whenthe part is raised from the water thenre-immersed into the water for final quenching.Although diverse in their markets and a
26、pplication,all of the above quenching options can be imple-mented by AFC-Holcrofts modular heat treatingsystem: EZ-Lynks.Unlike smaller automotive drive trains, large gearcomponents by necessity employ very highhardenability steels due to their size. However, highhardenability is a two-edged sword,
27、easier toharden but at greater risk of distortion in quenching.Again, a better solution would allow several quench-ing techniques as documented above. Open pit oilquenching and the required fixturing offer little in theway of flexibility for quenching options. Many gearscan be press quenched or othe
28、rwise individuallyquenched but not from pits where the awkwardhanging fixture makes it impossible to removeindividual parts.Large is a relative term when applied to gears andfor this paper is defined as 24” to 36” diameterplanetary gears and up to 65” to 84” diameter spurgears. Quenching sun gears,
29、shafts, pinions andplanetary gears by their vertical orientation can becontrolled somewhat easier but still requiresattention to flow uniformity and velocity. In manyapplications, fairly large gears can be pressquenched with oil or water but only if the furnacetype and material handling can provide
30、the accessfor removal.The IQ furnace has been the preferred furnace forcommercial and captive heat treaters alike, primar-ily due to the flexibility offered in process, load sizeand efficiency. With electric as well as gas firedradiant tubes, the IQ furnace has providedunmatched utility and quality
31、for all industrial,aerospace and military markets worldwide.AFC-Holcrofts UBQ series of IQ furnaces are thepreferred product for many of the worlds largestdrive train suppliers. Today AFC-Holcrofts batchfurnace as shown in Figure 3 is carburizing andquenching planet gears used by a major windturbine
32、 manufacturer. Flame Metals ProcessingCorp. in Rogers, MN is a certified heat treat suppliermeeting all of the AGMA and wind turbine qualityrequirements. Even though AFC-Holcroft has soldIQ furnace designed for very large loads, up to 72”(1829 mm) square and 12,000 lbs (5454 kg), theyhave not been a
33、pplied to large gears probably be-cause of rear handler (RH) capacity restrictions andthe quench tank design. EZ-Lynks, AFC-Holcroftsalternative heat treating system shown in Figure 4a,Figure 4b and Figure 4c eliminates the pit disad-vantage and overcomes the material handlingconcerns by separating
34、the material handling fromthe furnace proper allowing more flexibility in the hotzone and quench tank designs.Figure 3. UBQ integral quench batch furnace6Figure 4a. EZ-Lynks modular heat treatingsystemFigure 4b. EZ-Lynks modular hot zoneFigure 4c. EZ-Lynks modular system can belocated anywhere and e
35、asily expanded asneededThe EZ-Lynks cell illustrated in Figure 5a andFigure 5b consists of multiple hot zone modules,enclosed floor mounted oil quench tank, wash, tem-per and transfer shuttle. All equipment is rated to16,000 lbs (7,272 kg). A manufacturer or heat treat-er with an existing facility d
36、oes not have to invest in apit or extra high bay to accommodate EZ-Lynks.Doors can be designed to open sideways, radianttubes can be removed from the rear and quenchtanks can rest on the floor. Process flexibility is theresult of a traversing and rotating transfer car,Figure 6, capable transferring
37、loads automaticallythroughout the cell according to a preconfiguredrecipe.Figure 5a. EZ-Lynks modular heat treat floor plan7Figure 5b. EZ-Lynks modular heat treat elevationFigure 6. EZ-Lynks rotating transfer shuttleThe Achilles heel of pit quenching is the huge flamecreated when the load is immerse
38、d in the oil.Flames rage to engulf the crane, cables and truckand risk igniting the soot-covered ceiling. Figure 7and Figure 8 illustrates the typical pit quenching ofgears. Who hasnt been tempted to run for the exitswhen pit quenching a massive load in oil? A morecommon sense solution like that pro
39、vided by EZ-Lynks would protect the building from oil ignitionwithin an enclosure much like the IQ furnace. Apneumatic elevator can provide consistent motioninto the oil even during an electrical interruption.Figure 7. Typical pit stacked gear load8Figure 8. Resulting flame from pit oil quenchof sta
40、cked gearsRecipe driven quench distortion controlMartensite, the transformation product providingthe hardness and strength required in ferrousalloys, creates a volumetric expansion that contrib-utes to distortion especially when formed at unequalrates in the carburized case. A major key tocontrollin
41、g distortion is creating a heat transfer rateuniformly over the part surface, in this case over thegear teeth profile and root profile. In addition, andperhaps as important, is the heat transfer over thegear hub or side areas that can cause oil canningand bore tapering. Typically in pit quenching fi
42、x-tured gears are stacked with little or no spacebetween them, resulting in trapped oil vapor andcreating slower cooling than at the top or bottom ofthe gear stack, Figure 9.Figure 9. Vertical oil flow in typical pitquenchTraditional pit quench systems circulate oil frombottom to top leaving the oil
43、 between gears tostagnate, vaporize and reduce heat transfer. Atypical method to counteract or reduce the differen-tial cooling effect of vapor in pit quenching is toreduce the overall agitation which increases ratherthan reducing oil vapor over the entire gear or stackof gears in an attempt to redu
44、ce distortion byslowing the heat transfer of the entire gear. Onlydue to the high hardenability of expensive alloys canheat treaters even consider this approach. Unfortu-nately this technique fails to account for theexposed top of the part surface that is free torelease the oil vapor via gravity. CF
45、D modelingstudies by others have shown that with little or noagitation, the top surface of quenched parts coolsfaster due to the gravity effect of vapor bubblesreleasing. In addition, depending on the gear webdesign vapor can be trapped under the gear, furthercausing nonuniform cooling. A new approa
46、ch tomore uniform quenching is needed. Oil can becirculated from side-to-side, flushing the oil vaporfrom between gears or from under gears, Figure 10.Vertical oil flow works well for and is required forpinions, sun gears and stacked planetary gearswhere vertical straightness is critical. EZ-Lynksqu
47、ench system shown in Figure 11a and Figure 11b9can provide recipe-selected oil flow in either direc-tion, side-to-side or vertical, or a combination ac-cording to process requirements. For improved ODroundness control while employing side-to-sideflow, gears can be rotated 360 exposing the entireOD t
48、o very uniform and high velocity oil flow directlyat the gear teeth. This feature also assures themaximum quench penetration into the tooth profileand root. As a result, any problematic NMTP can beeliminated.Figure 10. EZ-Lynks side-to-side oil flow, oilquench optionsFigure 11a. EZ-Lynks tank design
49、 for verticaloil flow, oil quench optionFigure 11b. EZ-Lynks tank design forside-to-side oil flow, oil quench optionCarburizing, as stated above, is the mostcost-effective process for improving the wear resist-ance and strength of ferrous alloys, especiallygears. Atmosphere (endothermic) carburizing isthe most applied process throughout the worldsimply because it is so predictable even withoutadvanced simulation models. However, with off-lineand on-line case profile modeling the process isalmost foolproof. Still, due to the very high vestedcost of
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