1、05FTM02The Effects of Pre Rough MachineProcessing on Dimensional DistortionDuring Carburizingby: G. Blake, Rolls- Royce - Transmissions and StructuresTECHNICAL PAPERAmerican Gear Manufacturers AssociationThe Effects of Pre Rough Machine Processing onDimensional Distortion During CarburizingGregory B
2、lake, Rolls- Royce - Transmissions and StructuresThe 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.AbstractA study was conducted to isolate the influence of pre-rough machine
3、 processing on final dimensionaldistortion. Methods are discussed to aid process development and minimize dimensional change duringcarburizing. The study examined the distortion during carburizing between five possible raw material startingconditions. Coupons were used and manufactured from each pop
4、ulation of material processing. All couponswere carburized and hardened at the same time. Dimensions were made before and after carburizing using ascanning coordinate measurement machine. The results show that dimensional distortion during carburizingincreases with mechanical and thermal processing.
5、Copyright 2005American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2005ISBN: 1-55589-850-51The Effects of Pre Rough Machine Processing onDimensional Distortion During CarburizingGregory Blake, Rolls-Royce Transmissions and StructuresIntroductionT
6、his paper presents the methods and results of anempirical study that was conducted to aid processdevelopment of a carburized aerospace gear. Theobjective of the study was to determine the contribu-tion of premachining material processing on dimen-sional distortion during carburizing. Five possiblera
7、w material starting conditions were evaluated.The five prerough machining conditions studiedwere: (i) normalized AMS6265 barstock, (ii) hard-ened and tempered (core-treated) AMS6265 bar-stock at 1725_F, (iii) hardened and tempered (core-treated) AMS6265 barstock at 1550_F, (iv)normalized AMS6260 for
8、ging, and (v) hardenedand tempered (core-treated) AMS6260 forging at1725_F.Cost, time, and lurking variables were minimized byuse of a standard distortion coupon in place of actu-al aerospace gears. The coupon design is shown inFigure 2. French (1930) used this type of coupon tostudy dimensional dis
9、tortion during repeatedquenching. Frenchs coupon was scaled as neces-sary for use in this study. The diametrical changesof the coupon indicate the volume changes duringhardening. The width of the slot reflects the magni-tude of internal stresses set up by the volumetricchanges (French, 1930). French
10、 (1930) shows thatdimensional distortion increased as the number ofquench cycles increased. The distortion coupongap width increased with each quench cycle, thusindicating that residual stresses were increasingwith each thermal cycle.The hypothesis is that dimensional distortion in-creases as therma
11、l and mechanical processing ofthe raw material increases prior to machining. Theimplication is that dimensional distortion can be in-fluenced before the raw material enters the machin-ing process.Precarburizing process variables and their influ-ence on dimensional distortion were studied pre-viously
12、. The Instrumented Factory (INFAC) (1994)studied the effects of processing variables prior tocarburizing. The INFAC study evaluated residualstresses induced by turning and hobbing and theircontribution to dimensional distortion. Mechanicaland thermal processing of the raw material, howev-er, was not
13、 included in the study.Background and Literature ReviewA dedicated manufacturing cell to produce smallaerospace gears was designed and implemented.The design of the manufacturing cell and processwas to minimize lead time and cost. The shapingprocess was used to generate the spline and gearteeth. The
14、 resultant gear and spline surface integri-ty produced by the newly designed process wasdeemed unacceptable due to machining tears thatwould not clean up during gear grinding. An exam-ple of the post-shaped tooth surface is shown inFigure 1.Figure 1. Gear tooth surface, post shaping(Evard, 2005)Surf
15、ace integrity is the description and control of themany possible alterations produced in a surface lay-er during manufacturing. Surface integrity can beevaluated based on a minimum data set. The dataset is composed of surface texture, macrostructure,microstructure, and microhardness alterations(MRA,
16、1980). The data set of macrostructure will in-clude surface imperfections such as pits, tears, and/or laps.2The raw material selected for use in the newmanufacturing cell was normalized bar stock, whichwas within the engineering requirements of the fin-ished gear. The soft normalized bar stock wasvi
17、ewed as a good choice for machinability. Many lit-erature sources supported this conclusion. Mott(1985) defined machinability as being related to theease with which a material can be machined withreasonable tool life. Verzahntechnik Lorenz (1980)and Cluff (1992) indirectly used a similar definitions
18、tating that machinability (reasonable tool life) de-creases as material hardness increases. The twokey terms are “ease of material removal” and “rea-sonable tool life.” An indication of expected surfaceintegrity is not present using these definitions ofmachinability.Material hardness can be used as
19、a machinabilityindicator due to the close relationship betweenhardness and microstructure (Mullins, 1990). How-ever, hardness is an accurate representation of ma-chinability only for similar microstructures. Mullins(1990) states that a tempered martensite matrix willexhibit superior machinability to
20、 a pearlite matrix ofsimilar hardness. Woldman (1937) studied micro-structure and machinability and noted that a micro-structure selected for long tool life would not neces-sarily produce good surface integrity.Based on literature and experience, a temperedmartensitic microstructure was desired to p
21、roducethe required surface integrity. The addition of thehardening and tempering operation was viewed asa risk to changing the dimensional distortion duringcarburizing and hardening.A great amount of manufacturing development hadbeen done implementing the new cell. The dimen-sional distortion during
22、 carburizing and hardeninghad been established and had been determined ac-ceptable and manageable. The addition of a hard-ening and tempering operation prior to rough ma-chining was viewed as an addition to cost, lead time,and risk of increased dimensional distortion duringcarburizing. Increased dim
23、ensional distortion wouldthen require more process development time andcost.Problem StatementCommon ground: Aerospace power transmissioncomponents must be manufactured to the highestquality standard while minimizing cost of nonquality.Destabilizing condition: Gear tooth surfaces incon-sistently have
24、 poor surface integrity (“tears”) pres-ent after finish flank grinding. The surface defectsare produced during the semi-finishing, preharden-ing, operation. These gears are then deviated, re-worked, and/or scrapped.Contributing factors: Aerospace gears are expen-sive and have long lead times. A stud
25、y of many vari-ables is not always practical using actual gears.Problem: The shaping machine used in themanufacturing cell has limited cutting parameters.Literature suggests that hardening and temperingthe material prior to any machining will improve thesurface integrity during shaping. The material
26、structure is then martensitic and hardness rangesfrom Rc 25 to Rc 32. However, literature also sug-gests that this fix could negatively influence dimen-sional distortion during hardening.Solution: Material samples made of different micro-structure and hardness will be fabricated andtested. Paired da
27、ta studies statistically analyzingdimensional distortion will be performed on cou-pons of similar size and process.AssumptionsThe material samples are assumed to fully repre-sent their population. For example, a group ofnormalized material samples is assumed to repre-sent all normalized material.The
28、 change in coupon gap width is assumed to rep-resent the relative dimensional distortion of an actu-al gear.Methods & ProceduresThis section contains the details of couponmanufacturing and processing. Standard distortioncoupons were manufactured for each population asshown in Figure 2. The dimension
29、s of the couponare proportional to the gear being developed andare shown in Figure 3.3Figure 2. Distortion coupon, GR-0010Figure 3. Detailed Specimen Drawing (units = inches)The coupons from each population were machined,stress relieved, carburzied, and hardened together.The coupons were randomly lo
30、cated in the carbu-rization furnace and quench basket. A summary ofthe populations is shown in Table 1. A sample sizeof ten was used with one additional sample used formetallurgical evaluation. The letter designation willbe used from this point on to identify the population.The raw material requirin
31、g hardening and temper-ing was heat treated in-house as listed in Table 2and Table 3 prior to any machining. The normalizingprocess was done per the AMS specification prior toreceiving the material. The manufacturing and in-spection stages of the coupons are listed in Table 4.The coupons were carbur
32、ized and hardened using acycle common to the actual gear (see Table 5).4Table 1. Specimen populationsMaterial Form Pre-machining Heat Treatment Quantity Expected Structure IdentificationAMS6265 Barstock Normalized 10*PearliteinFerriteMatrixAAMS6265 Barstock Normalized + Harden 1725F 10*PearliteinFer
33、riteMatrixBAMS6260 Forging Normalized 10*PearliteinFerriteMatrixCAMS6260 Forging Normalized + Harden 1725F 10*Tempered Marten-siteDAMS6265 Barstock Normalized + Harden 1550F 10*Tempered Marten-siteE*One additional specimen was manufactured for metallurgical evaluationTable 2. Hardening and tempering
34、 process1725FOperation Operation Description10 Load20 Core HardenTemperature 1725 FTime Temp: 1 hour min.Atmosphere: EndogasQuench in 110 - 190 F oilto 200 F max. part temperature25 Wash30 Load40 Temper to BHN 258-301Temp. (ref.) 980FTime temp: 2 hours min.50 Unload60 Clean69 InspectTable 3. Hardeni
35、ng and tempering process1550FOperation Operation Description10 Load20 Core HardenTemperature 1550 FTime Temp: 1 hour min.Atmosphere: EndogasQuench in 110- 190 F oilTo 200 F max. part temperature25 Wash30 Load40 Temper to BHN 258-301Temp. (ref.) 980FTime temp: 2 hours min.50 Unload60 Clean69 Inspect5
36、Table 4. Manufacturing process of couponsOperation10 Heat treat as necessary20 Rough turn outer diameter leavinggrind stock30 Finish grind outer diameter40 Rough cut over all length leavinggrind stock50 Stamp ID60 Finish grind face70 Finish grind 2nd face80 EDM inner diameter and slot90 Stress relie
37、ve 300F min 1 hour100 CMM inspection110 Carburize (carb & hard process inseparate table)120 CMM inspect130 Harden & temper140 CMM inspectTable 5. Carburize and hardening processOperation Operation Description10 Carb:0.030”-0.035” cycle1700F, 1.5 hrs1700F, 1.15%C, 5 hrs1700F, 0.85%C, 2 hrsFurnace coo
38、l to 1000FAir cool to ambient20 Harden:1500F, 0.85%C, 2 hrsQuench in 110 to 190Foil for 10 min30 Temper:300F, 3 hrs40 Stabilize:-100F, 3 hrs50 Temper:300F, 3 hrs6Dimensional measurements A Zeiss Prismoscanning coordinate measuring machine (brass tag#253685) was used to perform all measurements.The o
39、utside diameter, inside diameter, and the gapwidth of every coupon was measured before carbu-rizing, after carburizing, and after hardening. Eachtime, the outside diameter and etched face werescanned and set as reference. The gap width wasmeasured at a constant radius of 0.7000 inchesfrom the refere
40、nce center. All measurements weretaken in a plane 0.1500 inches (half overall length)from the reference face using a 0.054 inch diameterprobe. The coupons were soaked in mineral spirits,wiped dry and rinsed with alcohol before each mea-surement. The cleaned coupons were placed in theCMM room twenty-
41、four hours before measurementto thermally soak and stabilize. The CMM roomtemperature is held at 69_F+/- 2_F. The actualmeasurements are contained in Appendix Athrough D. A sample inspection report is inAppendix D.FindingsThis section contains all of the data and findings col-lected during the study
42、. Data collected includescharacterization of the pre-carburizationmicrostructure and dimensional measurements.Pre-carburization microstructure - To documentthe pre-carburized material, an extra coupon wasmanufactured from each population for metallurgi-cal evaluation. The evaluation was performed af
43、terfinish machining and before carburizing. The chem-istry of Sample E is not reported. The sample waslost during the metallurgical evaluation process.The hardness (see Table 6), chemistry (see Table7), and microstructure (see Figures 5-13) wereevaluated on the etched face side of the coupon intwo r
44、adial locations, center and near the outerdiameter.Table 6. Pre- Carburization hardness (Weinrich, 2003)& (Ballard, 2003)Hardness BHN 3000kg loadFace location Sample A Sample B Sample C Sample D Sample ECenter 207 302 255 285 269Near O.D. 207 285 248 302 269Table 7. Pre- Carburization chemistry in w
45、eight % (Weinrich, 2003)Location C Mn Cr Ni Mo P S Si Al CuA Center 0.080 0.510 1.220 3.180 0.080 0.012 0.006 0.260 0.010 0.020A Near O.D. 0.080 0.510 1.230 3.170 0.080 0.013 0.006 0.280 0.010 0.020B Center 0.100 0.470 1.240 3.220 0.090 0.012 0.006 0.270 0.010 0.020B Near O.D. 0.090 0.620 1.250 3.20
46、0 0.080 0.012 0.006 0.270 0.010 0.020C Center 0.130 0.660 1.450 3.120 0.090 0.016 0.006 0.310 0.050 0.010C Near O.D. 0.130 0.640 1.430 3.120 0.080 0.014 0.006 0.270 0.040 .01D Center 0.080 0.630 1.300 3.050 0.110 0.014 0.018 0.230 0.010 0.150D Near O.D. 0.070 0.620 1.300 3.020 0.100 0.014 0.020 0.25
47、0 0.020 0.150E CenterSample LostE Near O.D.am os7Figure 5. Sample A Center Microstructure 100X5% Nital etch (Weinrich, 2003)Figure 6. Sample A near O.D. microstructure100X 5% Nital etch (Weinrich, 2003)Figure 7. Sample B center microstructure 100X5% Nital etch (Weinrich, 2003)Figure 8. Sample B near
48、 OD microstructure100X 5% Nital etch(Weinrich, 2003)Figure 9. Sample C center microstructure 100X5% Nital etch (Weinrich, 2003)Figure 10. Sample C near OD microstructure100X 5% Nital etch (Weinrich, 2003)8Figure 11. Sample D center microstructure 100X5% Nital etch (Weinrich, 2003)Figure 12. Sample D
49、 OD microstructure 100X5% Nital etch (Weinrich, 2003)Figure 13. Sample E center microstructure 100X5% Nital etch (Ballard, 2003)Dimensional measurements and descriptivestatistics Measurements were recorded beforecarburization, after carburization, and after harden-ing. Details of the measurement method are in theMethods and Procedures section. Serial numbersB5 and B6 were lost during carburization and serialnumber C10 was scrapped during manufacturing.Descriptive statistics of the pre-carburization, postcarburization, and post harden
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