1、08FTM18AGMA Technical PaperGear Corrosion Duringthe ManufacturingProcessBy O. El-Saeed and G. Sroka,REM Chemicals, Inc. andG. Blake, Rolls-RoyceCorporationGear Corrosion During the Manufacturing ProcessOmer El-Saeed and Gary Sroka, REM Chemicals, Inc. and Gregory Blake,Rolls-Royce CorporationThe sta
2、tements 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.AbstractNomatterhowwellgearsaredesignedandmanufactured,gearcorrosioncanoccurthatmayeasilyresultincatastrophicfailure. Sincecorrosio
3、nissporadicandarareeventandoftendifficulttoobserveintherootfilletregion or in finely pitched gears with normal visual inspection, it may easily go undetected. This paperpresentstheresultsofanincidentthatoccurredinagearmanufacturingfacilityseveralyearsagothatresultedin pitting corrosion and intergran
4、ular attack (IGA). It showed that superfinishing can mitigate the damagingeffectsofIGAandpittingcorrosion,andsuggeststhatthesuperfinishingprocessisasuperiorrepairmethodfor corrosion pitting versus the current practice of glass beading.Copyright 2008American Gear Manufacturers Association500 Montgome
5、ry Street, Suite 350Alexandria, Virginia, 22314October, 2008ISBN: 978-1-55589-948-63Gear Corrosion During the Manufacturing ProcessOmer El-Saeed and Gary Sroka, REM Chemicals, Inc. andGregory Blake, Rolls-Royce CorporationIntroductionPitting corrosionPitting is one of the most insidious forms ofcorr
6、osion; it can cause failure by perforation whileproducing only a small weight loss on the metal.Also, pits are generally small and often remainundetected. A small number of isolated pits on agenerally uncorroded surface are easily overlooke-d. A large number of very small pits on a generallyuncorrod
7、ed surface may not be detected by visualexamination, or their potential for damage may beunderestimated. When pits are accompanied byslight or moderate general corrosion, the corrosionproducts often mask them. 1Surfacepittingisoftenbarelyvisibleevenat1030X magnification, and can therefore often go u
8、nde-tected. The corroded region below the surface canbemuchlargerthanindicatedbythesurfaceareaofthe pit. ASTM G46-94, Standard Guide for Ex-amination and Evaluation of Pitting Corrosion,states: “Pitsmayhavevarioussizesandshapes. Avisualexaminationofthemetalsurfacemay showaround,elongated,orirregular
9、opening,butitseldomprovides an accurate indication of corrosion be-neath the surface. Thus, it is often necessary tocross section the pit to see its actual shape and todetermine its true depth.” 2 For example, the G46standard presents a chart of possible variations inthe cross-sectional shapes of co
10、rrosion pits. Seefigure 1.Consequently, only one insignificantly appearingnarrow pit could ultimately lead to bending fatiguefailure.Crevice corrosionCrevice corrosion is a localized form of corrosionthatoccursinnarrowopenings orspaces wherethelocalizedchemicalenvironmentisdifferentthanthatof its su
11、rroundings. The change in the crevicechemicalenvironmentcanbecausedbyadepletionof the inhibitor or the oxygen, a shift to acid condi-tions or a build up of aggressive ion species in thecrevice. Crevice corrosioncommonly occursunderwashers, seals, threads and surface deposits.When the chemical enviro
12、nment within the creviceis different than that of its surroundings, an electro-chemical cell is created resulting in corrosion thatcan be as damaging as pitting corrosion.Figure 1. Variations in cross-sectional shapes of corrosion pits 24Intergranular corrosionAnother type of corrosion attack is int
13、ergranular orintercrystalline corrosion, during which a small vol-ume of metal is preferentially removed from pathsthat follow the structural dissimilarities along grainboundaries to produce fissures or cracks. Thesame kind of subsurface fissures can be producedby transgranular or transcrystalline c
14、orrosion. Inthis a smallvolume ofmetal isremoved inpreferen-tialpathsthat proceedacross orthrough thegrains.Intergranular and transgranular corrosion some-times are accelerated by tensile stress. Inextremecase the cracks proceed entirely through themetal,causing rupture or perforation. This conditio
15、n isknown as stress corrosion cracking (SCC). 3Nguyen et al.4, in an earlier paper discussed whygears are very susceptible to corrosion during themanufacturing process. In order to protect workersand the environment, the use of oil-based rust pre-ventives and rust-inhibiting machining coolantshave b
16、een minimized. The gear manufacturingprocess is complex, and requires machining, plat-ing, carburization, grinding, plating removal, andnitaletchinspectionoftenfollowedbyglassbeadingor shot peening. During this entire process, gearsareoftenleftexposedtotheenvironmentforseveralweekswithouttheuseofrus
17、tpreventives. Theyarehandled by a number of personnel, and experiencemany back and forth trips between the shop floorand the metrology laboratory.Aerospace gears require state-of-the-art designand precision manufacturing to meet the needs oftodaysperformancedemands. Havingsaidthat,allof the efforts
18、can be for naught if pitting andintergranular corrosion occur. Such corrosion canlead to disastrous, premature failure. The severityof the problem will be illustrated with two actualexperiences described in detail in Part I and Part IIof this paper. Part I is a short experiment to answerthe question
19、 whether or not one drop of sweatinad-vertentlyfallingonanaerospacegearcouldresultinserious damage. Part II discusses a study of IGAand pitting corrosion that was detected on aero-space gears, and the ability of superfinishing toremove this damage.Part I: Unexpected low cycle bendingfatigue failureI
20、ntroductionRecently,theAerospaceResearchBlocattheGearResearch Institute of The Pennsylvania StateUniversity conducted a study of bending fatigueperformanceofAMS6308testgears.1)(AMS6308is commercially available as Carpenters Pyro-wear53 and Latrobes Lesco 53.) Several gearsexperienced unexpected low
21、cycle bending fatiguefailure, and the root cause was determined to becorrosionpitsinthe rootfillet region. Thedisturbingpartofthisfindingisthatthepittingwasnotvisibletothe naked eye, and could only be seen at 30X mag-nification. Consequently, these pits escaped themanufacturers as well as the testin
22、g laboratorysinspections.Since aerospace gears lackrust preventivesduringportions of the manufacturing cycle, one mightquestion whether or not one drop of sweatinadvertently falling on a gear could cause majorcorrosion problems-leading to premature bendingfatigue failure.Test procedureTestspecimens:
23、 Becauseoftheirreadyavailability,Falex AMS 6260 (E-9310) Steel V-Blocks (Part#000-502-024) having a 58-60 HRC were chosenastestspecimens. Thesewerecleanedoftheirrustpreventive using a non-chlorinated solvent(carburetor cleaner) followed by acetone as recom-mended by Falex. A drawing of the V-Block i
24、sshown in Figure 2.One of the V-Blocks was left in the ground (as-received) condition. See Table 1 for the surfaceroughness values of the ground V-Block, andFigure 3 for the surface profile._1)This was an unpublished study completed by the Aerospace Research Bloc whose sponsors include Avio Group(It
25、aly), Boeing (Mesa and Philadelphia) ,Curtiss Wright Controls, Honeywell International, Pratt the V-Blockwas polished with 1500-grit paper to remove thegreater part of the corrosion deposits. The surfacewas thencleaned with#0000 steelwool followedbyultrasonic cleaning in a mild caustic cleaning solu
26、-tion. Pitting corrosion, crevice corrosion, and IGAwere observed on both the superfinished surfaceand ground surface. See Figures 7 and 8.Conclusion1. Asingledropofsweathasthepotential tocauseserious corrosion damage on aerospace gears.2. Corrosion pits that are only visible under 30Xmagnification
27、can cause low cycle bending fa-tigue, as reported by the Aerospace Bloc.Therefore, one drop of sweat inadvertentlyfalling on a gear can result in premature failure.3. In this study, the superfinished and the finelygroundsurfaceswereequivalentwithregardstocorrosion resistance.4. Currently, production
28、 aerospace gears are notscrupulously examined in their root fillet areausing 30X magnification. It is suggested thataerospace gear corrosion warrants furtherinvestigation.7Superfinished GroundBaselineTime = 0.0 hrTime = 0.5 hrTime = 1.5 hrTime = 2.3 hrFigure 5. View 2 - Photographs of a superfinishe
29、d (L) and ground V-Block (R) with one drop ofartificial sweat deposited on the surfaceSuperfinished GroundTime = 127 hrFigure 6. View 2 - Photograph of the superfinished (L) and ground V-Block (R) Surface after 127hours8Superfinished GroundFigure 7. View 3 - Superfinished (L) and ground (R) after me
30、chanical cleaning,showing residual corrosionSuperfinished Ground100X500X1000XFigure 8. SEM images of test specimens after mechanical cleaning. Superfinished (L) andground (R). Deep pits are visible in the 100X images. The white deposits cover shallower pits.IGA cracks are visible in the 1000X images
31、.9Part II: IGA and pitting corrosion duringManufacturing.In 2000, Rolls-RoyceCorporation sentused/scrapcarburized AISI 9310 gasifier train gearshaftsto REM Chemicals, Inc. for edge radiusing. SeeFigure 9. Initial inspection revealed light contactdamage on the gear flanks.The gearshaft was superfinis
32、hed using chemicallyaccelerated vibratory finishing, as describedelsewhere.5 6 See Figure 10.The superfinished gears were subjected to a rigor-ousinspectionuponreturntoRolls-Royce. Initially,itwasassumedthatthedamage wascaused bythesuperfinishing process, since it is carried out in avery weak acidic
33、 medium. REM conducted theirowninspectionatametallurgicallaboratory(Ander-son the gear wassectioned, polished, and examined. IGA was de-tected, confirming Rolls-Royces initial findings.Light contact fatigue damage was also detected bythe laboratory. Figure 11 shows the sections andthe target inspect
34、ion areas. Photomicrographs ofthe various areas are shown in Figures 12-16.Figure 9. As received used/scrap carburizedAISI 9310 gasifier train gearshaftFigure 10. Superfinished used/scrapcarburized AISI 9310 gasifier traingearshaftFigure 11. Locations of IGA and contactdamage on gearshaft sections (
35、Top) takenfrom the gearshaft (Bottom)10Figure 12. Photomicrographs of Area 1 at500X magnification. Circles show several IGAcracks.Figure 13. Photomicrographs of Area 2 at 50Xmagnification showing residual machininglinesFigure 14. Photomicrograph of Area 3 at 500Xmagnification. Circles show visible I
36、GAcracks in the valleys of the machining linesFigure 15. Photomicrograph of Area 4 at 500Xmagnification showing contact surfacedamage11Figure 16. Photomicrograph of Area 5 at 500Xmagnification. Circle shows contact surfacedamageIn order to demonstrate that the superfinishingprocess did not cause IGA
37、, a Falex 9310 V-BlockwassuperfinishedunderthesameconditionsastheAISI 9310 gearshaft. The V-Block was sectioned,polishedandtheV-areawasexamined. Thephoto-micrographs showed no IGA or pitting. See Figure17. This definitively confirmed that superfinishingdoes not induce IGA.Onceitwasproventhesuperfini
38、shingprocessdoesnotinduceIGAorpittingonAISI9310,Rolls-RoycethenquestionedwhetheritwouldremovetheIGAorexacerbate the problem by deepening the cracks.The reason for the latter question stems from theacidicchemicalsusedinthesuperfinishingprocess.In this process, the refinement chemistry creates acoatin
39、g on the gears that is continuously wiped offby the media. However, the media only removesthe peaks leaving the valleys of the metal surfaceuntouched. Metallurgists often expressedconcerns that theacidic chemistryhad thepotentialto cause corrosion in the valleys of the metal sur-face. This was a rea
40、sonable concern, eight yearsago, when the superfinishing process was beingintroduced to the aerospace gear industry.To investigate this concern, Rolls-Royce providedREM Chemicals, Inc. with another gearshaft thathad IGA for further evaluation. See Figure 18.Figure 17. Photomicrograph of the V-Area o
41、fthe superfinished V-Block at 500Xmagnification. No pitting or IGA wasdetected.The gearshaft was sectioned, polished, and exami-ned. SEM images confirmed the presence of IGA.See Figure 19. The maximum nominal depth of theIGA was 0.0002”.The gearshaft was then superfinished such thatapproximately0.00
42、02”ofmetalstockwasremoved.It was then sectioned, polished, and examined forthe final inspection. The SEM image clearly showsthatthelayercontainingtheIGAwascompletelyre-moved. See Figure 20. The surface at 5000Xshows that it is extremely smooth. This proved thatthe superfinishing process is not only
43、metallurgical-ly safe, but is also capable of repairing damagedsurfaces. However, the depth of the damage mustnot exceed the metrological tolerance limits of thegear teeth.12Figure 18. Cross section of gearshaft studied to determine the effect of superfinishing on IGA.IGA was detected on areas A & B
44、Figure 19. SEM at 5000X magnification,showing the presence of IGA. Circle shows atypical IGA crack on areas A & B. Thenominal IGA depth is 0.0002”Conclusions:1. Gears are susceptible to IGA and corrosionduring the manufacturing and/or storageprocesses.2. Superfinishing using chemically acceleratedvi
45、bratory finishing does not exacerbate IGA.3. Superfinishing, in fact, can be used to removecorrosion, contact damage and the IGA layer.Figure 20. SEM images of Areas A and area Bshowing that superfinishing removed the IGA13References1. Metals Handbook Ninth Edition, FailureAnalysis and Prevention, A
46、merican Society forMetals, pages 176-179.2. ASTM G46-94, Standard Guide for Examina-tion and Evaluation of Pitting Corrosion.3. Materials Performance, February 2008.4. Nguyen, S.T., Manesh, A., Reeves, J., andMahan, D.M., Minimization of In-Process Cor-rosion of Aerospace Gears.5. Arvin, J., Manesh,
47、 A., Michaud, M., Sroka, G.,and Winkelmann, L., The Effect of ChemicallyAccelerated Vibratory Finishing on GearMetrology, 02FTM01, AGMA Fall TechnicalMeeting, St. Louis, MO, October 2002.6. Winkelmann,L.,Bell,M.,andElSaeed,O.,TheCapacityofSuperfinishedVehicleComponentsto Increase Fuel Economy, DETC2007-34860,ASME 2007 International Design EngineeringTechnical Conferences & Computers andInformation in Engineering Conference,September 4-7, 2007, Las Vegas, Nevada,USA.
