LOW-PRESSURE CARBURIZING PROCESS DEVELOPMENT OF …

1 With more aerospace customers looking for improved mechanicalproperties in bearing materials than offered instandard through hardened steel grades,New Hampshire Ball Bearings Inc. initiated aprogram to develop a LOW-PRESSURE carburizingprocess that could providethe required properties innewer carburizablegrades including M50 Carey*New Hampshire Ball Bearings , Dan Herring*The Herring GroupElmhurst, Ill.*Member of ASM International and member, ASM Heat Treating SocietyiTech Div., New HampshireBall Bearings Inc. (NHBB),manufactures precisionbearings for use in aero-space and industrial applications,including various ball bearing androller bearing configurations ( ).

2 Historically, a majority of the ma-terials used in aerospace bearing ap-plications have consisted ofthrough-hardened materials suchas AISI 52100 alloy steel and M50(intermediate high speed, molyb-denum type tool steel) alloy the past several years, bearingmanufacturers have seen an in-crease in the number of inquiriesfrom aerospace customers who arechallenged with bearing applica-tions that require superior mechan-ical properties compared with thoseprovided by standard through-hardened materials. To meet theseever-increasing requirements, car-burizable grades of steel such asM50 NiL(a nickel-low carbonvariant of M50 tool steel having thechemical composition C, Cr, Ni, Mo, V,balance Fe) are now being specifiedfor these applications.

3 In doing so,wear resistance and fatigue strengthproperties (Table 1) are achieved onthe case-hardened surfaces compa-rable to those of through-hardenedmaterials, but with a higher level offracture toughness in the core of thepart. NHBB developed low-pres-sure CARBURIZING technology tohandle these RequirementsThe first step in developing LPCtechnology at NHBB was to specifythe requirements for the furnace de-sign and operation. It was deter-mined that one key feature of such asystem would be its flexibility to per-form both CARBURIZING cycles andstandard vacuum heat required a furnace that wouldproduce a minimal amount ofsooting to avoid contaminating sur-faces of non-carburized parts.

4 Otherfeatures identified as being essentialincluded: Ability to perform both oil quench-ing and gas quenching Optimum case uniformity Operator-friendly controls andprogramming Data acquisition capability Must meet all Aerospace Mate-HEAT TREATING PROGRESS MAY/JUNE 2007 43 HLOW-PRESSURE CARBURIZING PROCESS DEVELOPMENT OF M50 NiLTable 1 Typical Mechanical Property Data for Various Bearing SteelsTempering temperature,Fracture toughness Fatigue life Alloy F ( C)Hardness, HRC(KIc)*, ksi /in. (MPa ) (L10dynamic life factor)440C350 (175) 58-62 19 ( ) 52100 375 (190) 60-64 18-20 ( ) 1-6 M50 1000 (540) 60-64 18-20 ( ) 10 M50 NiL975 (525) 47 max.

5 50-52 ( ) 12-16 * ASTM 1 Representative precision Specifications (AMS) pertainingto pyrometry (AMS 2750) and heattreatment of parts (AMS 2759)It was determined that a LPC fur-nace would be best-suited to meetthese requirements, and NHBB in-stalled in 2005 an Ipsen InternationalInc. (Cherry Valley, Ill.) AvaC (acety-lene vacuum CARBURIZING ) system(Fig. 2). PROCESS DevelopmentAfter an extensive literature reviewon LPC and gathering informationon LPC from various other resources,NHBB created a cause and effect di-agram outlining the potential sourcesof inherent variability in the LPCprocess (Fig. 3). At the same time, car-burizing simulation programs weredeveloped to model the case profilesof M50 NiL and other materials.

6 Sev-eral iterations of the M50 NiLsimu-lation programs were conductedbased on the results of the develop-ment CARBURIZING cycles performedon the furnace were designed to verifythe furnace would operate properlyand produce the required carburizedsurface. These baseline cycles werecompleted using standard carburizingsteels (AISI 1018 and AISI 9310). Car-burizing parameters for these initialcycles were selected using valuescommon to industry practices forthese grades of the required CARBURIZING re-sults were achieved in the furnace,results of initial test cycles indicatedthe need for modifications to the pro-gramming format for improvedrecipe flexibility ( , allow for addi-tional boost/diffuse segments andprovide the ability to program inminutes/seconds instead ofhours/minutes)

7 And modificationsto the furnace hardware, both ofwhich were handled the initial tests, addi-tional baseline test cycles on the stan-dard CARBURIZING steels were per-formed to verify the effects of severalkey PROCESS variables on resultingcase depth, microstructure, and near-surface carbon content. Variables in-cluded: CARBURIZING temperature Boost/diffuse times and respec-tive ratios44 HEAT TREATING PROGRESS MAY/JUNE 2007 Fig. 2 CARBURIZING chamber of the low-pres-sure CARBURIZING 3 Cause and effect diagram outlining potential sources of variation in thelow-pressure CARBURIZING 5 Photomicrograph showing veryhigh level of retained austenite in the chamferarea of an inner roller-bearing ring; 3% nitaletch.

8 200 Fig. 4 Case hardness profiles of two M50 NiL LPC DEVELOPMENT CoreDepth, 62 58 54 50 46 42 Hardness, HRCPart designOperatorMethodFurnaceMeasurementMa terialSurface finishGeometryCase depth surfacesMicroindentation hardnessRetained austeniteSurface carbonResidual stressMicrostructureTemperature uniformityGas dispersion systemDesignQuenchingVacuum leak rateExperienceTrainingControlsProgrammin gPM load transfer timeTypePart cleanlinessPrior microstructureRobust carburizingprocess (Y) CARBURIZING temperatureBoost timeGas typeGas flow ratePartial pressureMaskingNo.

9 Of boost/diffuse segments and ratiosLoad surface areaQuench methodHardening/temperingLoad configuration and fixturing Number of boost/diffuse seg-ments Relationship between boost/dif-fuse cycles and surface carbon Partial pressures during boostand diffuse segments Relationship between part sur-face area and gas flow rate Carbon flux as a function of tem-peratureIn all instances, resulting casedepths, microstructures, and surfacecarbon measurements were consis-tent with expected NiL CARBURIZING DevelopmentNHBB used the results of the ini-tial baseline tests to develop a robustcarburizing procedure for M50 NiL(AMS 6278).

10 M50 NiL is a carburiz-able grade of steel used in the aero-space industry in critical applicationsrequiring higher core fracture tough-ness properties compared with thethrough-hardened (non-carburiz-able) M50 grade of steel. The rela-tively high level of chromium in M50 NiL ( ) presents a challengewhen developing a carburizingprocess due to an increase in the ef-fective carbon absorption of the ma-terial[1].The first few M50 NiLtest cycleswere modeled to target an effectivecase depth of to in. ( mm). Effective case depth is de-fined as the perpendicular distancefrom the case hardened surface to adepth equal to a hardness of 58 CARBURIZING temperatures inthe range of 1600 to 1900 F (870-1040 C) were selected based on tem-peratures commonly used in the in-dustry for LPC.