/" Methods forImproving YieldofLiquidSaltBathNitrocarburized 13-8PHSteelComponents IrwinHochler_ Abstract Liquidsaltbathnitrocarburizing (LSBN) isathermochemical diffusion surface treatment technique thatcan produce excellent casehardened stainless steelgearsandpinions. However, theprocess didnotyield consistent results fora13-8PHstainless steel,finetoothpitchpinion.Aeroflex Laboratories hadbeen experiencing spalling (flaking) problems withthecaseproduced onthispinion,buthasbeenabletoimprove yieldsfrom50%to95%ofeachprocessed lotbyworking inconjunction withKoleneCorporation, theU.S. manufacturer ofthesaltbaths,totightlycontroltheprocess parameters oftemperature, time,andsurface preparation. Thispaperwillpresent thetheoryandpractice ofLSBN,andoffersuggestions onhowtoimprove process yield.Aeroflex's casehistoryofa19-tooth, 4.7-mm (0.185in)diameter, 13-8PHshaftpinionforaspacequalitystepmotorwillbediscussed. Thepinioninquestion underwent LSBNbeforeleadplating. Theplatinglabobserved highlyreflective spots ontheshaft,indicating thatspalling hadoccurred. Discussions withtheoriginal nitriding vendor suggested thatthisisacommon situation withfinepitchgearsbecause thehardened layerbecomes excessively thick atthegearedges, causing embritUement. Inaddition, PHsteelsweresaidtoundergo astructural change afterLSBNthatincreases thepossibility ofdamage tothehardened surface. Crosssections ofunacceptable shaftsrevealed thatcasedepthswereinfactthick(approximately 0.01mm to0.02mm(0.0005 into0.001in)),butwithinthevendor's limits(approximately 0.005mmto0.02mm (0.0002 into0.001in)).Inaddition, visitstothevendor's facilities revealed thatdifficulty incontrolling the degree ofairagitation isaproblem associated withusinglargecommercial tanksforprocessing small, preciseparts.Poorairagitation resultsinnon-uniform casedepthsforthesesmallparts. Discussions withKoleneCorporation revealed thatthepartsshouldbetreated inalaboratory environment, andcasedepthcontrolled foruniformity byadjusting residence timeandbathtemperature. Inaddition, improved surface preparation ofthepartspriortoLSBNhelpedassuredepthsnogreaterthan0.0127mm (0.0005 in).Implementation ofthesemethods produced the95%acceptance rateforeverylotprocessed. Introduction Aeroflex Laboratories wascontracted todesignandbuildaspaceflightquality90degreestepper motorwith a19-tooth 4.7-mm (0.185-in) diameter, 13-8PHstainless steelpinionontheendoftherotorshaft.The customer specified theshafttobecasehardened withaliquidsaltbathnitrocarburizing (LSBN) process prior tofinalassembly inthemotor.Thecustomer specified allprocess parameters, including asuggested LSBN processor. Aftersending theshaftstothissourceforLSBNprocessing, theshaftswerethensenttothe customer forleadionplatingofthepinion.Theoperator noticed approximately 50%ofthepinions hadshiny spotsontheteeth(LSBNshouldproduce auniform flatblacktograyfinish),andreported toAeroflex that spalling (flaking) ofthecasehadoccurred. Figure1showsspalling atthetipofthepinion(AreaC). Subsequent investigations concerning waystoimprove theyieldledAeroflex tocontract KoleneCorporation, themanufacturer oftheLSBNsalts,totreatthepinions inalaboratory environment inordertobettercontrol thecriticalprocess parameters. Thiseffortresulted in95%ofnewlyprocessed lotsofpinions showing no spelling. *Aeroflex Laboratories, Farmingdale, NY Proceedings ofthe35thAerospace Mechanisms Symposium, AmesResearch Center,May9-11,2001 133