1、11FTM27AGMA Technical PaperManufacturing andProcessing of a NewClass of Vacuum-Carburized Gear Steelswith Very HighHardenabilityBy C.P. Kern, Dr. J.A. Wright, Dr.J.T. Sebastian, J.L. Grabowski,QuesTek Innovations LLC, andD.F. Jordan, and T.M. Jones,Solar AtmospheresManufacturing and Processing of a
2、New Class of Vacuum-Carburized Gear Steels with Very High HardenabilityC.P. Kern, Dr. J.A. Wright, Dr. J.T. Sebastian, J.L. Grabowski, QuesTekInnovations LLC, and D.F. Jordan, and T.M. Jones, Solar AtmospheresThe statements and opinions contained herein are those of the author and should not be cons
3、trued as anofficial action or opinion of the American Gear Manufacturers Association.AbstractFerriumC61tandC64tarenewsecondary-hardeningsteelsthatprovidesuperiormechanicalpropertiesversus 9310, 8620, Pyrowear Alloy 53 and other steels typically used for power transmission, such assignificantly highe
4、r core tensile strength, fracture toughness, fatigue strength and thermal stability (i.e.tempering temperature). One recent example of their application is the 2010 Small Business InnovationResearch(SBIR)PhaseIIAwardQuesTekreceivedfromtheU.S.ArmytodemonstratetheapplicationofC61totheforwardrotorshaft
5、ofCH-47Chinookhelicopter(workinginconjunctionwithTheBoeingCorporation),inorder to reduce the weight of the shaft by 15-25% and provide other benefits.Thispaper will reviewthe significantmanufacturing andprocessing benefitsthatarisefrom thisnewclassofsecondary-hardening steels, and analyze the potent
6、ial implications and opportunities. C61 and C64 werecomputationally designed to take advantage of high-temperature, low-pressure (i.e. vacuum) carburizationtechnology, in part by combining carburizing and austenitizing steps as well as being designed to have veryhigh hardenability. The very high har
7、denability of these steels permits a mild gas quench subsequent tolow-pressurevacuumcarburizingandreducespartdistortion,thusreducinggrindstockremoval,simplifyingfinal machining and heat treat operations. A framework analysis will be used to compare totalmanufacturing/productioncostsandimpacts(includ
8、ingenvironmental)ofthesenewsteelsversustraditionalgear steels. Conclusions and recommendations will be drawn regarding best manufacturing practices andappropriate use of these new steels for product applications.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAle
9、xandria, Virginia 22314October 2011ISBN: 978-1-61481-027-83 11FTM28Manufacturing and Processing of a New Class of Vacuum-Carburized Gear Steelswith Very High HardenabilityC.P. Kern, Dr. J.A. Wright, Dr. J.T. Sebastian, J.L. Grabowski, QuesTekInnovations LLC, and D.F. Jordan, and T.M. Jones, Solar At
10、mospheresIntroductionCarburized steel gears are widely used for power transmission in rotorcraft, transportation vehicles,agricultural and off-road equipment, industrial rotating equipment, and thousands of other applications.Historically,alloysrequiringcarburizationwereputthroughanatmosphere(gas)pr
11、ocess. However,inrecentyears, the advancement of low-pressure (i.e., vacuum) carburizing has lead to certain applications to takeadvantage of reduction in process steps and improvements in case profile uniformity. A new class of gearsteels,FerriumC61andC64,werespecificallydesignedanddevelopedtomaxim
12、izethebenefit ofvacuumcarburization processes.FerriumC61andC64arehighlyhardenable, secondaryhardeningmartensiticsteels. Thehighhardenabilityof these alloys allows for a mild gas quench, which can used in vacuum carburizing, that promotes uniformmartensitictransformationthroughouttheentirecomponent a
13、llowingfor lessdistortionandthus reducingtheamount of grinding stock removal required. Thesealloys werealso designedwith agrain pinningdispersionparticlethatallowsfor theuseofhigher processingtemperatures availableinvacuumcarburizingtoincreasethe carbon diffusion and reduce cycle time. The grain pin
14、ning dispersion particle also allows for increasedforgingtemperatures ingear productionthat canextendthelifeof aforgingdie. Inaddition, thealloys useanefficientM2Ccarbidethatrequireslesscarboncontentthantraditional alloysthat achievehardeningusinganepsiloncarbide. Inadditiontothemanufacturingbenefit
15、soutlinedabove,thesealloysalsohavesignificantlyimproved core properties that lead to performance advantages compared to conventional gear steels.Acomparisonofatmosphereandvacuumcarburizingaddressessomeoftheadvantagesanddisadvantagesofeachwillbepresented. Withtheadvancementof vacuumcarburizingprocess
16、esinrecentyears, thereareagrowingnumber of applications that maketheuseof vacuumcarburizinganeffectiveprocessingselection.A framework comparison of a high-performance racing and high-performance aerospace application areused as two examples that can benefit from the use of these new alloys processed
17、 via vacuum carburizing.Design and overview of Ferrium gear steel alloysFerrium C61t and C64t are two new alloys being used or considered for power transmissionapplications.Both of these alloys utilize an efficient nanoscale M2C carbide strengthening dispersion within a Ni-Co lathmartensiticmatrix.
18、QuesTekdesignedthesealloysconsideringthecomplexinterplayofcriticaldesignfactorsincluding: martensitic matrix stability (Mstemperature); M2C carbide thermodynamic stability and formationkinetics;matrixcleavageresistance;andembrittlingphasethermodynamicstability withtheuseof theirsuiteof computational
19、 models 1, 2. See Figure 1.QuesTeks Ferrium C-series alloys are advanced new gear steels designed for significant manufacturingandperformanceadvantages over conventional aerospacegear steels that cansignificantly streamlinegearproduction, decreasingleadtimes and reducingcost. These steels take advan
20、tageof vacuum carburizationthermal processing and have high-hardenability that allows for mild-gas quenching, eliminating the need foroil-quenchdies,thusreducingtheamountofmachiningstockrequiredduetodistortion. Thealloyswerealsodesigned to use an efficient strengthening mechanism that requires up to
21、 50% less carbon compared tocurrentstate-of-the-artalloys,thereforereducingthecarburizationtimebyupto50%. Inadditiontothemanu-facturing benefits the alloys also have performance enhancement benefits that allow for increased powertransmission, reduced weight, and increased thermal stability. Computat
22、ionally designedand developedbyQuesTek Innovations, under Navy, Army, and internal funding, the alloys are commercially available fromLatrobe Specialty Steel Company located in Latrobe, PA. Commercial procurement and processingspecification documents, such as Aerospace Materials Specification (AMS),
23、 are currently in development.4 11FTM28Figure 1. The “Design Chart” used by QuesTek to design the Ferrium C64 alloy. The hierarchicalrelationships between processing, structure, properties, and performance are summarizedgraphically and serve as the template for alloy designHigh hardenability, design
24、ed to use high-temperature, low-pressure (i.e., vacuum) carburizationmethodsFerrium C61 and C64 were specifically designed to achieve high hardenability and use high-temperature,low-pressure (i.e., vacuum) carburization and gas quenching process methods, permitting significantreductions in manufactu
25、ring costs and schedules due to:S Shorter thermal processing times at higher carburizing temperatures (see Figure 2).S Elimination of separate hardening and oil quenching process steps after carburization (combination ofcarburizingandaustenitizingsteps;seeFigure 2);candoubletheefficiencyofacurrentfa
26、cilitybyelimina-tion of many copper plating and stripping operations associated with current thermal processing, andeliminate of the costs and setup time associated with custom quench press dies currently required.S Reductionofgrindingoperationsandcosts,smallerexcessstockremovalandwastebyreducingque
27、nchdistortion and avoidance of the intergranular oxide formation typical of in pre-oxidation steps of conven-tional alloys. Due to higher hardenability of Ferrium gear steels (see Figure 3), a slower gas quenchprocess resulting in uniform properties and very low distortion after heat treating can be
28、 achieved. Thiscanbeespeciallybeneficialforcomponentswithlargercross-sectionwherecoolingratesinthecoremaybe slower.Increased forgabilityThese alloys were designed to be worked at higher temperatures compared to the incumbent alloys. Thereason behind this is the increased thermal stability of the gra
29、inpinning dispersionused. Where alloys suchas X53 typically have grain pinning dispersion particles that go into solution around 1830F, the particlesemployedin Ferrium gear steels arestableto2250F. This allows anincreasedforgingrangeby over300F.This increase in temperature also allows increased thro
30、ughput and longer die life.5 11FTM28Figure 2. Comparison of thermal processing path associated with the carburization,austenitizing, and tempering of 9310 compared to Ferrium gear steelsNote: Elimination of three thermal processes and associated plating/striping with each processFigure 3. Jominy end
31、-quench comparison of 9310 and Ferrium C61 per ASTM A255Greater core strengthThesealloysexhibitcoresteeltensilestrengths(UTS)of229ksiormore,whichisa35+%increasecomparedto conventional gear steels and allows significant reductions in part size and weight, particularly wherestructural components are i
32、ntegrated with gearing into single components.Greater high temperature survivabilityFerrium C-series alloys exhibit increasedthermalstability comparedtoAISI9310orPyrowear X53,becausethey were designed to be tempered at 900F or 950F, and thus can withstand service temperatures up to500F hotter than A
33、ISI 9310 or Pyrowear X53. Increased thermal stability is expected to result in greaterability for a gearbox to survive “oil-out” or low lubricant situations, and endure other high-temperatureoperating conditions. See Figure 4.Additional information about the properties and development status of each
34、 alloy can befound in09FTM14,Design, Development and Application of New High-Performance Gear Steels.6 11FTM28Figure 4. Comparison of mechanical properties and thermal stability (via tempering temperature)Atmosphere vs. vacuum (low-pressure) carburizingThe history of case hardening via carburization
35、 has been well presented over the years. While atmospherecarburization still is the predominant processing method used, vacuum carburization has been starting tomake progress as the preferred process in certain applications. While some consider these competingtechnologies, they may be consideredcomp
36、lementary as each processes has benefits given theapplicationinvolved. Atmosphere carburizing is still cost effective for large batch production and extremely largecomponents,whilevacuumcarburizingiscost effectivefor lowerbatchproductionandprecisionapplications(wheremachiningtolerances arepertinent)
37、. Thereis stillabroadrangeofapplications inbetweentheaboveexampleswherebothprocesseswill workeffectively, buttypically isdecidedbyajudgmentcall orpreferenceonthepart of thematerial andprocessingor manufacturingengineerresponsibleforthethermalprocessing.A summary of the advantages and disadvantages o
38、f both atmosphere and vacuum carburizing are givenbelow 3, 4.Atmosphere carburizingAdvantagesS Low capital equipment costS High volume outputS Good experimental process controlDisadvantagesS Need to condition equipment or keep in constant operationS Large grind stock required on material to remove i
39、nter-granular oxide layerS Large case depth variations between flank and rootS Safety/fire prevention issuesS Environmental pollution due to CO and NOxemissionVacuum carburizingAdvantagesS Reduction in post grinding due to eliminating the presence of inter-granular oxide layerS Higher temperature ca
40、pabilities (increased carbon diffusion reduces processing time)S More uniform case depth between flank and root (also good blind hole penetration)7 11FTM28S Only need to operate equipment while processing parts (reduced energy consumption)S Can use both oil and gas quench mediumS Use of inert gasses
41、 reduces pollution during out-gasS Reduced distortion due to uniform case and the use of gas quench mediumDisadvantagesS Higher initial equipment costS Part cleanliness can affect diffusion of carbonS Smaller furnace load capabilityA typical gear manufacturingpathis outlinedinFigure 5. From this pro
42、cess youwill seethat theadvantagesofvacuumcarburizingarecontainedwithinboththehardeningandgrindingprocesses. Theamount ofstepscontainedwithineachofthesetwoprocessesisdifferentforvariousapplications. Twoexamplesintheracingand aerospace markets will be outlined below to describe the step reduction and
43、 potential time and costsavings.A more detailed breakdown of the hardening process comparison between atmosphere and vacuumcarburizingis showninFigure 6. As shown,theadvantagesof vacuumcarburizationallowfor acombinationof carburization and austenitizing steps, while eliminating the need for a stress
44、 relief in between theprocesses. In addition, the increased temperature capabilities of vacuum carburization allow for increaseddiffusion of carbon, therefore reducing total cycle time. The use of vacuum carburizing canreduce theneedfor quench dies due to the more uniform case profile, allowing for
45、more uniform martensitic transformation(less stress gradient associated with phase change from FCC to BCC) and reduce the amount of distortioncontainedwithinacomponent. Inaddition, theuseofhighhardenablealloys, FerriumC61andC64, allowformild gas quenching that even further reduces the distortion cau
46、sed by non-uniform conversion 5, 6(Figure 7).Processing of Ferrium C61 and C64 has also demonstrated that the need for a pre-oxidation step is notrequiredforactivationpriortocarburization;insteadhydrogencleaningcanbeusingduringtheheatingstageofcarburization/austenization. Theamountofmaskingoperation
47、scanalsobereducedbytheuseofvacuumcarburizing. Intraditional atmospherecarburizing, astop-off paint or copperplatewillneedtobeappliedandremovedtwicethroughoutthehardeningoperation. Thenumberofmaskingsteps canbereducedby 50per-centusingvacuumcarburizationduetotheintegrationofthecarburizingandausteniti
48、zingsteps. Thismeansthat agivenproductionlinecandoubleits through-put of the sameplating lineby usingvacuum comparedtoatmosphere carburizing.Figure 5. Process schematic of a typical case-hardened gear8 11FTM28Figure 6. Comparison of the reduction in processes required during hardening betweenatmosph
49、ere and vacuum carburizingFigure 7. Image showing that the amount of distortion is related to the quench rate for alow-alloy steel of 0.25” diameter. Milder quench rates promote uniform cooling, reducing thestress gradient and subsequently distortionThegear grindingprocess alsoreceives benefitfrom vacuumcarburizingcomparedtoatmospherecarburiz-ing. A comparison of the grinding operations for a high-performance racing and aerospace application areoutlined in Figure 8. After thermal processing thereare threemain reasons for material removal: removal ofan inter-gra
