High Performance Gears Using Powder Metallurgy (P/M ... · Powder metallurgy is also a proven...

HighPerformanceGears Using PowderMetallurgy (P/M)Technology

Richard Slattery, Francis G. Hanejko,Arthur J. Rawlings, Mike Marucci and

Kalathur Sim Narasimhan

AbstractTo close the gap between conventional P/M

and wrought steel, a process is needed thatenables P/M gear producers to manufacture heli-cal gears with sintered densities greater than95% pore free via single press/single sinter pro-cessing. This higher density provides enhancedmechanical properties with a higher elastic mod-ulus (Ref. 2). High density provides bettermechanical properties, such as tensile strength,fatigue, and impact toughness. Still, even minoramounts of porosity can have a significant nega-tive effect on certain characteristics. In particu-lar, high performance gears require full densityin the critical stress region to withstand the highHertzian contact stresses.

Secondary processing, such as surface densi-fication, enables the production of a highly engi-neered porous core component with full or nearfull density in the critical stress region of the gear(Ref. 3). What is needed is to merge the practiceof surface densification with higher core densitiesto expand the potential applications for P/M.

Described in this paper is a P/M parts-makingtechnology capable of producing single pressedand sintered helical gears with core densitiesapproaching 7.4 g/cm3. Description of a proto-type run will be presented with the resulting sin-tered part densities and part-to-part variability.To further enhance the performance and geome-try of these helical gears, they were subsequent-ly surface densified via rolling. Improvements inthe surface density and gear quality will bedescribed.

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Management SummaryPowder metallurgy (P/M) techniques have proven successful

in displacing many components within the automobile drivetrain, such as: connecting rods, carriers, main bearing caps, etc.(Ref. 1). The reason for P/M’s success is its ability to offer thedesign engineer the required mechanical properties withreduced component cost.

Powder metallurgy is also a proven technology to producehigh strength gears for the automotive market. P/M techniquescan manufacture spur gears with overall part densities up to 7.5g/cm3 via today’s powder production, compaction and sinteringprocesses—combined with double pressing. However, helicalgears are more difficult to produce to these same densitiesbecause the geometry does not lend itself to the doublepress/double sinter process.

But advances in powder production, compaction, and sinter-ing—combined with secondary operations—have enabled corepart densities up to 7.4 g/cm3 and fully dense tooth flanks in heli-cal gears.

Richard Slattery is vice president of engineering forCapstan Atlantic in Wrentham,Massachusetts. He has been in thepowder metallurgy industry for 16years, working in both powder andparts production. Most recently, hehas focused his research on devel-oping high performance, precisionhelical pinions.

Francis G. Hanejko works in the research & develop-ment department of HoeganaesCorp. as manager for electromag-netics and customer applications.He assists in research involvinghigh density processing of powdermetals. A materials scientist,Hanejko has worked for more than25 years for Hoeganaes, based inCinnaminson, New Jersey, in vari-ous aspects of powder metallurgy,including preproduction of gearapplications.

Arthur J. Rawlingsis a senior development engineerin Hoeganaes’ customer applica-tions department. He’s involved indeveloping new materials, includ-ing preparation of test partsthrough powder compaction. Hiswork experience also includes pro-duction, through development tech-niques, of helical gears using P/Mcompaction and subsequent pro-cessing.

Mike Marucci is a senior development engineerin Hoeganaes’ research & develop-ment department. His responsibili-ties include research into sinter-hardening materials and into highdensity processing of powder met-als, including high density process-ing of powder metal gears.

Dr. Kalathur SimNarasimhan is vice president and chief tech-nology officer for Hoeganaes.He oversees Hoeganaes’ prod-uct and process developmentsand has been in the powdermetal industry for 30 years.

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automatic transmissions. The requirements ofthese gears are high surface hardness, good coretoughness, fatigue strength for tooth bending,rolling contact fatigue resistance, and high densi-ty to resist pitting and sub-surface spalling duringservice (Ref. 4). Currently, these gears aremachined from either AISI 8620 steel or AISI5120 steel carburized and machined to meet thedimensional specifications.

Conventional press and sinter technologiesoffer low production cost to produce these parts;however, the corresponding mechanical proper-ties are inadequate. Powder forging a blank andthen machining it will give the requiredmechanical properties but at a cost that is pro-hibitive. It is speculated that a technologyenabling core densities ≥ 7.4 g/cm3 with a fullydense surface will meet the mechanical proper-ty requirements of this application, yet be eco-nomically competitive.

In an effort to satisfy the diverse mechanicalrequirements of these helical pinion gears,Capstan Atlantic and Hoeganaes Corp. collabo-rated on the implementation of a new compactiontechnology (AncorMax D™) that enables theattainment of high core densities without theneed to preheat the powder. The advantage of thisprocess is its greater flexibility in the manufac-ture of high-density P/M parts. It will be demon-strated that this process can produce helical gearswith sintered densities approaching 7.4 g/cm3.

Following compaction and sintering, Capstanemployed its proprietary surface densificationtechnology to densify the high stressed region ofthe gear. This technology, coupled with theattainment of the proper heat-treated microstruc-ture, can produce gears that approach the per-formance of wrought steel gears.

The AncorMax D ProcessThe AncorMax D process is a patent pending

premixing technology that optimizes both thelubricant and binder additions to achieve anincrease of 0.05–0.15 g/cm3 in density relative toa conventional premix (Refs. 5, 6).

Coupled with this increase in green and sin-tered densities, the AncorMax D process offersreduced alloy segregation, reduced dusting, supe-rior flow, and enhanced die fill (Ref. 7). Thesefactors give the opportunity for greater consisten-cy and higher quality P/M parts.

Attaining high-density, single press, singlesinter P/M parts has been an objective of the P/Mindustry for many years. ANCORDENSE™ pro-cessing was introduced approximately eight

High Performance Gears Using PowderMetallurgy (P/M)Technology

Figure 1—Comparison of the compressibility of a regular premix with AncorDense andAncorMax D.

Figure 2—Strength of FLN2-4405 sintered at 1,120°C (2,050°F).

Base Nickel, Graphite, Lube, PremixMaterial % by weight % by weight % by weight Technique

1 FL-4400 2.0 0.6 0.75 Standard Premix

2 FL-4400 2.0 0.6 0.55 AncorMax D

Gre

en D

ensi

ty (g

/cm

3 )

Compaction Pressure (MPa)

AncorMax D

Standard Acrawax BlendAncorDense

300 400 500 600 700 800 900

7.40

7.35

7.30

7.25

7.20

7.15

7.10

7.05

7.00

6.95

6.90

6.85

Stre

ngth

(MPa

)

300 400 500 600 700 800 900


UTS

YS

900

800

700

600

500

400

300

IntroductionAt the present time, P/M is successful in two

primary performance areas. The first is in densityranges of less than 7.1 g/cm3 with low- to medi-um-strength requirements. The second main appli-cation area is fully dense P/M via powder forging.Here a sintered preform is hot forged to near pore-free density, maximizing the mechanical proper-ties of the P/M component.

An example of a part family that offers signifi-cant volume opportunities for the P/M industry arehelical gears used in the planetary gear sets of

Table 1—Alloy Test Matrix (FLN2-4405).

AncorMax D0.75% (by weight) of Acrawax (add-mixedlubricant)




years ago; it is a technology that requires pre-heating the die, punches, and powder to approxi-mately 120–150°C (250–300°F) (Ref. 8).

The challenge of maintaining consistent tem-perature during the compaction process has pre-vented widespread acceptance of this processingtechnique. The AncorMax D system requiresonly die heating in the range of 60–70°C(140–160°F). The powder remains unheated untilit enters the die cavity.

The advantages of this system include theaforementioned density gain of 0.05–0.15 g/cm3,less ancillary equipment, and reduced powderwaste. Limitations include a maximum partlength of approximately 25 mm (1.0 in.) due tolimited heat transfer in the powder and the con-tinued need for a heated die. Also, compactionpressures greater than 550 MPa (40 tsi) are a pre-requisite to attain the improvement in density.

Shown in Figure 1 is a comparison of thegreen density vs. compaction pressure for anFLN2-4405 material made via regular premixing,ANCORDENSE premixing, and AncorMax Dpremixing. General observations from this chartare as follows: the ANCORDENSE premix givesthe highest green densities at the lowest com-paction pressures, and the maximum green densi-ty achieved for ANCORDENSE and AncorMaxD are nearly the same.

The benefit of the AncorMax D processing isobserved at compaction pressures greater than 40tsi (550 MPa). The maximum density achievedwith either the ANCORDENSE or AncorMax Dtechnology is dependent upon the pore-free den-sity of the premix with a limiting value of 98% ofthe pore-free density (PFD).

Properties of AncorMax D FLN2-4405To illustrate the potential benefits of using the

AncorMax D process, a comparison of themechanical properties of an FLN2-4405 premixprocessed via two routes was performed. Table 1shows the test alloy matrix.

Reference is made to Figure 1 for the com-pressibility curve for the two materials. It is notedthat compaction pressures greater than 550 MPa(40 tsi) show an increase in green density.However, with the compaction pressure at 415MPa (30 tsi), the increase in density is minimal.In addition to compressibility samples, standardMPIF dog bone tensile samples were compactedwith a die temperature of 63°C (140°F). Rotatingbending fatigue (rbf) samples were produced bymachining standard test samples from blanks alsocompacted at a die temperature of 63°C (140°F).

Figure 3—Strength of FLN2-4405 sintered at 1,260°C (2,300°F).


Stre

ngth

(MPa

)

300 400 500 600 700 800 900

900

800

700

600

500

400

300

UTS

Figure 4—Photograph of helical gear pressed in the collaborative effort. Note gear has a 22°helix angle with an OAL of 22.5 mm. The major diameter of the gear is 42.5 mm and the ID ofthe gear is 23.5 mm.

Figure 5—Porosity profiles of roll densified test samples.

Initial AttemptSecond Attempt

% P

oros

ity

Distance from Rolled Surface, mm0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

9 876543210

All samples were sintered under the followingconditions.

• Continuous Belt Furnace,• Standard Water Jacket Cooling,• Sintering Temperature: 1,120°C & 1,230°C(2,050°F & 2,300°F),• Time at Temperature: 25 min., and• Atmosphere: 75%N2, 25%H2 (by volume).

YS

AncorMax D0.75% (by weight) ofAcrawax (add-mixedlubricant)




Increasing the density of a P/M componentincreases the tensile and fatigue strength of thefinished part. Figures 2 and 3 demonstrate that the0.10 g/cm3 increase in density achieved with theAncorMax D processing results in a 10% increasein ultimate tensile strength (UTS) and yieldstrength (YS) at equivalent compaction pressures.Additional increases in strength can be achievedby increasing the sintering temperature from1,120°C to 1,260°C (2,050°F to 2,300°F) by com-paring the data presented in Figures 2 and 3.

The rbf data presented in Table 2 demon-strates the same beneficial effect of increasingdensity on the fatigue survival limits. The 0.10g/cm3 increase in density yields approximately a10% increase in fatigue life.

The rbf survival limits for the samples sin-tered at 1,230°C (2,300°F) are lower than thesamples sintered at 1,120°C (2,050°F). Thissuperior fatigue performance of the 1,120°C sin-tered material is related to the heterogeneousmicrostructure. At 1,120°C, the admixed nickeldoes not completely diffuse into the iron matrix,resulting in a nickel-rich second phase that actsto arrest crack propagation.

Conversely, the homogeneous microstructureof the high temperature sintered samples lackthese crack-arresting particles with a resultingdecrease in fatigue performance (Refs. 9, 10).

Prototype Production of a Helical GearTo verify the production capability of the

AncorMax D process, Capstan Atlantic andHoeganaes Corp. collaborated in the compactionand sintering of the helical gear shown in Figure 4.


During this effort, a standard premix materialconsisting of FLN2-4405 composition (Table 1)was compacted as a reference at pressures rang-ing from 550–830 MPa (40–60 tsi) in non-heatedtools at a press speed of ~8 strokes per minute.

Phase two of the trial involved compaction ofan AncorMax D premix of the same FLN2-4405composition (with the exception of the lubricantaddition and type) utilizing the same helical geartooling. Compaction pressures of 550–830 MPa(40–60 tsi) were utilized with a die temperatureof 65°C (150°F). In both compaction trials, thepart height was varied from 6 mm (0.25 in.) upto approximately 25 mm (1.0 in.).

Results from the compaction trial are sum-marized in Table 3. Similar to the results achievedwith laboratory test samples, the AncorMax Dgears exhibited an overall density increase ofapproximately 0.05–0.10 g/cm3 compared to thestandard premix at equivalent compaction pres-sures. One notable aspect of this work demon-strated that the AncorMax D with its reducedlubricant level gave satisfactory compaction andejection up to a part height of 22.5 mm.

Compacting gears with overall lengths (OAL)greater than 22.5 mm resulted in high die ejectionforces with corresponding poor surface finishes.

Part-to-part weight variability of the gearsproduced with the AncorMax D process showedreduced scatter relative to the standard material(Table 4). This reduced scatter will lead toreduced dimensional variation in the productionof the actual component.

After compaction, the gears were sintered at1,120°C (2,050°F) in a 90%N2, 10%H2 (by vol-ume) sintering atmosphere for approximately 30minutes at temperature. The sintered gears wereevaluated for sintered density; the results arepresented in Table 5. More than 3,000 gearswere compacted during this pre-productioneffort. Twelve sectional densities were evaluatedby sectioning a part into four quadrants aroundthe diameter and into three height regions. Theresult of this analysis is shown in Table 6.

It is worth noting that the density gradientfrom top to bottom and around the circumfer-ence is uniform within +/– 0.02 g/cm3. This uni-formity of density is significant because uniformdensity throughout the part minimizes distortionduring sintering. Methods to Improve the Physical Properties

of P/M ComponentsAchieving higher sintered density is perhaps

the primary method to improve the performance


Compaction Sintering 90% UTS, MPa % of UTS DensityPressure, Temp., °C Survival (90%) g/cm3

MPa Limit, MPa

690 1,120 266 759 34.4 7.34

830 1,120 292 745 38.2 7.41

690 1,230 252 759 32.5 7.38

830 1,230 273 841 31.8 7.44

Table 2—RBF Data for AncorMax D Processed FLN2-4405.


Premix Gear OAL, mm Green Density at Green Density at Green Density at550 MPa, g/cm3 690 MPa, g/cm3 830 MPa, g/cm3

Standard Premix 12 7.12 7.26 7.26

19 7.13 7.25 7.2825 7.03 7.25 7.28

AncorMax D 12 7.24 7.34 7.3819 7.25 7.32 7.34

22.5 7.05 7.28 7.34

Table 3—Green Density Results of Helical Gear Trial.




of a P/M part. However, recent experimentalwork has shown that heat treat practice and sec-ondary operations can also have a significanteffect on actual part performance. This sectionwill review recent advancements in theseprocesses and demonstrate how these advancescan lead to improved gear performance.

It is well known that carburizing producesfavorable compressive stresses on the surfacesof components. This phenomenon applies toP/M components as well. Several researchershave found a 15–20% improvement in rotatingbending fatigue properties (Refs. 11, 12). In addi-tion, the lower carbon core produces a materialwith greater impact toughness and core ductility.

Despite the positive effect of carburizing onmechanical properties, one key material charac-teristic in which P/M falls short of wrought steelsis in the area of rolling contact fatigue resistance.In rolling contact fatigue, the high subsurfacestresses resulting from the gear contact area andrelative slip have shown the need for full densityin the critical stress regions to withstand theHertzian contact stress associated with rollingcontact fatigue of high performance gears.

To improve the gear performance of P/Mcomponents, surface densification is sometimesused to densify the highly stressed region of thepart, without affecting the core characteristics.

Benefits of surface densification can include:

• A composite structure with a pore-free caseand a porous core,

• Potentially lower component cost because thehigh density region is only in the criticalstress region of the part,

• Improved gear geometry and tolerancebecause the P/M gear is rolled against a pre-cision roll die, and

• Potentially add the ability to crown the toothof the densified P/M gear.

Surface densification is a process that locallydensifies the surface of a pressed and sinteredP/M preform. The concept of densifying onlythe surface is that maximum Hertzian stress in agear is developed in the near surface region ofthe gear and diminishes with increasing distancefrom the surface. Thus, a gear with a surfacedensified case (to the appropriate depth) willgive rolling contact fatigue properties equivalentto a wrought steel gear. Process development atCapstan Atlantic showed that the depth of thepore-free densified layer can range from a mini-

Figure 6—Gear tooth lead error traces on rolled densifiedhelical gears taken at 90° intervals on the gears.

Four Teeth Evaluated @ 90°, Both Flanks

Left Flank

Face Width20 mm

0.01 mmCrown, CentralWithinFaceWidth

Right Flank

0.01 mmCrown, CentralWithinFaceWidth

Face Width


Process Var. 1σ gms Range gms.

Standard Premix 0.48 1.66AncorMax D 0.20 1.10

Table 4—Part-to-Part Weight Variability @ 830 MPa.

Premix Gear OAL, mm @550 MPa @690 MPa @830 MPa g/cm3 g/cm3 g/cm3

Standard Premix 12 7.12 7.24 7.28

19 7.12 7.25 7.29

25 6.99 7.18 7.23

AncorMax D 12 7.26 7.41 7.45

19 7.28 7.43 7.45

22.5 7.07 7.43 7.45

Position North (front of die) South East West

Top 7.38 7.40 7.37 7.41

Middle 7.37 7.39 7.40 7.39

Bottom 7.41 7.39 7.40 7.40

Table 6—Density Variations within a Sintered Helical Gear, g/cm3.

Table 5—Sintered Density of Helical Gears at 1,120°C (2,050°F).

Precision Helical Lead Traces

Results from Gear Rolling Operation

0.25 mm




mum of 0.38 mm to greater than 0.70 mm. Figure 5 presents metallographic analysis of

the porosity distribution for cylindrical test spec-imens that were densified via surface rolling totwo distinct depths. Near full density wasachieved at subsurface distances up to 0.75 mm(0.030 in.). Beyond the densified layer, theporosity level increases to the level of the as-sin-tered component.

The data shown in Figure 5 demonstrate theversatility of the rolling process in producingdensified layers as well as the ability to densifyto specific depths depending upon the require-ments of the final application. This densified layerproduces improvements in the rolling contactfatigue results as shown by Sanderow (Ref. 13).

In addition to the benefits of improved rollingcontact fatigue, the surface densification has theadded benefit of improved gear geometry and thepossibility of incorporating crowning on the sur-face of the gear.

Production of Surface Densified GearsThis section of the paper will describe the actualproduction of a surface densified helical gearshown in Figure 4. The intent of this section is tooutline the steps necessary to achieve surfacedensification and the resultant gear geometriesand densification profile.

Machining. P/M is often considered a netshape process that requires little or no machin-ing. However, machining is sometimes requiredto incorporate undercuts, cross-holes, etc.Machining is also used to improve the dimen-sional accuracy of some P/M components.Machining of P/M gears, unlike their wroughtsteel counterparts, is limited to the inside diame-ter. The benefits from this single machining oper-ation are: precise inside diameter tolerances andnear perfect alignment of the gear teeth to thecentral axis of the gear.

Gear rolling. As briefly described earlier,another potential operation for the enhancementof gear geometry, as well as tooth fatigueendurance is gear rolling. With the incorporationof a specifically engineered rolling process, onehas the ability to achieve optimum tooth align-ment while surface densifying the gear teeth todepths in excess of 0.7 mm. An additional bene-fit of this operation is a “mirror-like” surface fin-ish achieved on the tooth flanks, resulting in amuch quieter running gear, which is critical insatisfying ever-increasing NVH requirementsamong our customer base.


Figure 9—Density gradient of rolled densified helical gear tooth measured at crowned region.

Figure 7—Involute gear traces on rolled densified helicalgears taken at 90° intervals on the gears.

Figure 8—Photomicrograph of rolled densified helicalgear tooth taken through crowned section.

Distance across Tooth Center (mm)

Poro

sity

(vol

ume%

)

10

9

8

7

6

5

4

3

2

1

00 0.5 1 1.5 2 2.5 3

15° 21°

Four Teeth Evaluated @ 90°, Both Flanks

Left Flank

Pitch Diameter

0.015 mmInvolute Crownat PitchDiameter

End of Active Profile

Start ofActive Profile

15° 21°

Pitch Diameter

0.015 mmInvolute Crownat PitchDiameter

End of Active Profile

Start ofActive Profile

Precision Helical Pinion Involute Profile

Right Flank

0.25 mm




Shown as Figures 6 and 7 are the gear tracesevaluating gear tooth lead error and involuteprofile. Beyond the necessity to densify the sur-face and improve gear quality, another challengepresented by this application is the requirementof a “crown” central within the face width of thegear (shown in the lead traces) along with a peakinvolute at the pitch diameter of the gear (shownin the involute traces).

First, looking at the lead traces (Figure 6),notice that the start and stop points of the traceare at the same level, while there is a positivecrown in the middle of the trace. Typically withP/M gears, we would see a hollow in the centralregion of the face width of the tooth caused bythe low dense region at or about the middle ofthe gear’s face width.

We call this the “density dip” effect. Thecompaction of a high density gear preform withuniform density top to bottom, minimized this“density dip.” Thus in combination with aspecifically engineered rolling process, a 0.01mm crown was created on both gear flanks.

The involute profile of the gear, or the “toothshape,” is a function of not only the rolling oper-ation, but also the as-pressed tooth shape. Toyield a specific involute profile, one must utilizemore of a systems approach. Note on the invo-lute traces that the peak positive involute is atthe theoretical pitch line of the gear.

In addition to the dimensional and surfacefinish benefits realized by rolling, surface densi-fication can also occur. Typically, whererequired, gear tooth flanks are densified to adepth of 0.3–0.7 mm. The primary benefits ofthis surface densification are increased fatiguelife by inducing compressive stresses on thetooth surfaces and wear resistance.

Shown in Figure 8 is the photomicrograph ofthe surface densified helical pinion. The gearwas sectioned at a 22° angle to the vertical toshow the uniformity of densification on the lead-ing and trailing sides of the gear tooth.

Figure 9 shows the depth of densification onthe gear. This first attempt at densifying thehelical pinion produced a less dense case thanwhat was developed on the toroidal test sam-ples. The reasoning for the more porous caseis simple: The rolling conditions were notoptimized. Prior experience at Capstan Atlantichas produced densified layers in gears equiva-lent to the test samples (data shown in Fig. 5).What made this particular effort more challeng-

ing was incorporating the crowing on the gears.Capstan Atlantic had not attempted to roll a P/Mgear with a crown prior to this effort; thus thedeformation behavior is not completely under-stood.

Additional trials with rolling this gear formare certain to produce the desired densified layerand form detail.

SummaryThe sintered density distribution within a

P/M component is paramount to obtaining goodmechanical property performance. Described inthis paper is a new parts-making process utiliz-ing a unique binder/lubricant system (AncorMaxD) that enables the production of high-densityP/M parts through a combination of moderatedie temperature 65°C (145°F) plus higher com-paction pressures.

It was demonstrated that sintered densities upto 7.4 g/cm3 are possible using laboratory testsamples of an FLN2-4405 material. In addition tothe high density, it was demonstrated that the den-sity uniformity from top to bottom and around thecircumference is quite consistent.

The laboratory testing was extended to the pro-totype production of a helical gear with a 22° helixangle. As observed in the laboratory production oftest samples, the AncorMax D showed a similardensity improvement in the helical gear of0.05–0.10 g/cm3 greater compared to a standardpremix of FLN2-4405. The prototype productionof the AncorMax D premix had reduced part-to-part weight variability, resulting in reduced fin-ished part variability.

Beyond the development of the new binder/lubricant system, surface densification is a manu-facturing technology capable of producing fulldensity on the surface of the gear with full densi-fication ranging from a minimum of 0.38 mm(0.015 in.) to a maximum of 0.70 mm (0.028 in.).

This surface densification creates a compositecomponent giving the benefits of pore-free densi-ty in the critically stressed case region of the partwith ~95% density in the core to provide for therequired core properties. In addition to the densi-fied case structure, the surface finish of the rolledpart is better than that of a ground finish.

The processing route proposed for the highperformance helical gear is as follows:• Compact to 7.3+ g/cm3 density using the

AncorMax D processing,• Sinter,• Machine bore for concentricity (if necessary),




• Surface densify up to 0.70 mm (0.028 in.),• Carburize, and• Finish machine as required.

This processing offers the mechanical proper-ties of a high-density part, the surface fatigueresistance of a wrought part, and potentially thelow cost inherent to P/M processing.

Surface densification gives the followingadvantages:

• Pore-free tooth surface,• Excellent (“mirror-like”) surface finish, • Increased wear resistance,• Reduced noise, • Improved corrosion resistance,• Minimal tooth-to-tooth & total composite

error,• Redirects helix angle to improve gear tooth

lead while incorporating a tooth crown,• Customized tooth profile, and• Improved fatigue endurance.

It is worth noting that the carburizing stepdevelops a hard wear resistant surface layer plusproduces desirable surface compressive stressesthat give enhanced bending fatigue characteris-tics. Increases in the survival limit of rotatingbending fatigue samples are on the order of15–20%. Additionally, carburizing gives a desir-able acicular martensitic microstructure in thecase region (Ref. 13). The synergistic effect ofsurface densification and carburizing offers theend-user the potential for wrought steel gear per-formance with the low cost inherent with P/Mprocessing.

Finally, and perhaps most significantly, sur-face densification has the added benefit of pro-ducing a part with high dimensional precision.Lead and involute gear checking of the as-rolledgear shows that the gear error is equivalent to aconventionally machined wrought steel gear.

Additionally, the roll densification processwas able to produce a crown on the gear that fur-ther enhances the NVH performance of the gear.Gear crowning is impossible to achieve throughP/M part compaction, but this feature—and itsconsistency—can be produced as a direct conse-quence of the uniformity of sintered densitythroughout the part and of the expert engineeringof the rolling process.

Additional experimental work is required toproduce the helical gear with the depth of densi-

fication achieved in the test samples. However,significant progress was made in the potentialproduction of this component via P/M processing.The next steps include:

• Densifying the gears to 0.030 in. (0.70 mm),• Carburizing the part to measure the potential

distortion during heat treatment,• Simulated gear testing to verify the perform-

ance of the proposed processing, and• Actual gear testing of the finished gears to

qualify the process.r

References1. White, Don. “State of the North American P/M Industry,”Advances in Powder Metallurgy and Particulate Materials,Part 8, pp. 1-1–1-12, Metal Powder Industries Federation,Princeton, NJ, 2002.2. Material Standards for P/M Structural Parts, 2000Edition, MPIF Standard 35, Published by Metal PowderIndustries Federation.3. Shivanath, R., R. Peters, P. Jones and E. El-Sawaf.“Vacuum Carburization of High-Performance AutomotivePM Parts,” Industrial Heating, May 2001, pp. 37–39.4. Gear Design, Manufacturing and Inspection Manual,Published by the Society of Automotive Engineers,Warrendale, PA, 19096, 1990, p. 51.5. Donaldson, I.W., et al. “Processing of Hybrid Alloys toHigh Densities,” Advances in Powder Metallurgy andParticulate Materials, Part 8, pp. 8-170–8-185, MetalPowder Industries Federation, Princeton, NJ, 2002.6. Marucci, M., M. Baran and K. Narasimhan. “Propertiesof High Density Sinter-Hardening P/M Steels ProcessedUsing an Advanced Binder System,” Advances in PowderMetallurgy and Particulate Materials,Part 13, pp. 13-48–13-59, Metal Powder Industries Federation, Princeton,NJ, 2002. 7. Semel, F., and M. McDermott. “Recent Application ofBinder Treatment Technology,” Advances in PowderMetallurgy and Particulate Materials,Volume 1, pp. 2-23–2-49, Metal Powder Industries Federation, Princeton,NJ, 2002.8. Rutz, H.G., and F.G. Hanejko. “High Density Processingof High Performance Ferrous Materials,” Advances inPowder Metallurgy and Particulate Materials-1994,Vol. 5,Metal Powder Industries Federation, 1994, pp. 117–133.9. Rutz, H., T. Murphy and T. Cimino. “The Effect ofMicrostructure on Fatigue Properties of High DensityFerrous P/M Materials,” Advances in Powder Metallurgyand Particulate Materials-1996, Vol. 5, pp. 135–155, MetalPowder Industries Federation, 1996.10. Narasimhan, K., S. Polasik and N. Chawla. “SurfaceReplication as Means of Monitoring Fatigue CrackInitiation and Propagation in Ferrous P/M Alloys,”Advances in Powder Metallurgy and Particulate Materials,Part 12, pp. 12-48–12-59, Metal Powder IndustriesFederation, Princeton, NJ, 2001. 11. Saritas, S., W. James and A. Lawley. “Fatigue Propertiesof Sintered Steels: A Critical Review,” Presented at theEuropean Powder Metallurgy Association, October 2001. 12. Taylor, A., F. Hanejko and A. Rawlings. “AdvancedSinter-Hardening Materials and Practices,” Advances inPowder Metallurgy and Particulate Materials, Part 13, pp.13-48–13-59, Metal Powder Industries Federation,Princeton, NJ, 2002. 13. Sanderow, H. “Final Report, Rolling Contact Fatigue(RCF) Test Program,” Prepared by the Center for PowerMetallurgy Technology (CPMT), Princeton, NJ, September2001.14. Jandeska, W. 2002, General Motors Corp., private com-munication.


This paper was presented

under the title “Powder

Metallurgy of High Density

Helical Gears” at the 2003

International Conference on

Powder Metallurgy &

Particulate Materials, held

June 8–12, 2003, in Las

Vegas, Nevada, and was pub-

lished in the conference’s

proceedings, Advances in

Powder Metallurgy &

Particulate Materials. The

paper is republished here

with permission from the

conference’s sponsor, the

Metal Powder Industries

Federation.

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