May 2009 MPR 130026-0657/09 ©2009 Elsevier Ltd. All rights reserved.

special feature special feature

For years nickel steel powder sprockets have been in use in the SUV transfer case market. The heat-treated, nickel steel

powder provides a high strength and high wear resistant material for this very demanding application. The cost of nickel increased from $5 per lb in October 2005 to $24 per lb in May 2007. Because of this almost five fold increase in the commodity price a replacement material for the powder metal nickel steel was sought that would have compa-rable properties, comparable processing steps at a raw material cost that would be equal to the nickel steel powder metal prices of 2005. A replacement material

was found that has comparable proper-ties at a competitive price. Material com-position, testing data and microscopic analysis will be presented.

Powder metal sprockets were first designed into automotive transfer cases in 1994. These initial components were large chain system sprockets. Why use metal powder to make automotive trans-fer case parts? “Consumers want to get what they pay for but they don’t want to pay for more than they need. This forced design engineers to find the least expen-sive manufacturing processes to get the job done.”[1]

The original design requirements for ultimate strength were matched to the

MPIF Standard 35 designations and the FN-0205-105HT material was chosen. “Powder metal nickel steels are used typi-cally for heat treatable structural parts requiring the combination of strength, wear resistance and impact properties.”[2]

This material would allow the manu-facture of a single pressed – single sin-tered, heat treated part that would meet the engineering requirements yet offer significant cost savings over a machined wrought steel sprocket. At that time, ele-mental nickel used as a premix additive to iron based powder cost about $2.50 per lb.; thus making powdered metal the preferred production methodology over a machined part.

No substitute for testing in automotive marketWith a rise in the price of nickel seen in recent years, this paper from the Washington world congress investigates the use of alternative materials for the production of SUV transfer case sprockets and sun gears, and finds cooperation to be an essential component ...

Figure 1. Nickel and molybdenum prices vs. time [3] Figure 2. Nickel and molybdenum prices vs. time [3]

14 MPR May 2009 metal-powder.net

The sun gears used in the SUV transfer case market have an even more demanding level of design requirements than sprock-ets. In 2001, a powder metal replacement to the machined gear was sought that would duplicate the toughness, impact resistance and wear resistance of the wrought machined gear. An MPIF FLN2-4400 + 0.2 carbon alloy, at a typical tooth density of 7.3g/cc was selected. This dou-ble pressed, double sintered, heat treated material would have high impact resistance resulting from the combination of 2.0% elemental nickel and 0.2% core carbon. The pre-alloyed 0.85% molybdenum base iron provided additional toughness and hardenability to this material system. This material and processing almost bested the performance of wrought machined gears. Increasing the tooth density to 7.4g/cc minimum (7.45 typical) improved overall gear performance, enabling the sun gear to pass all testing requirements.

As with the sprockets, potential cost savings for the sun gears facilitated the conversion of these two parts to powdered metal. A key component to the reduced cost of powdered metal was the rela-

tively insignificant cost contribution of the nickel and molybdenum alloy additions to the powder metal raw material. As the raw material cost of nickel and molybde-num increased dramatically during 2005 and 2006, the cost of the powder metal sprockets and sun gears also increased dra-matically and alternative materials were sought. This paper discusses the path to a new material.

SprocketsIn an attempt to develop a material that would perform, equal to or better than the current material, several iterations were tested. Using production tools, sam-ples were manufactured and tested. The manufacturing process followed the pro-duction intent process, including compac-tion, sintering and heat treating.

The two main criteria a transfer case sprocket must pass are: wear from the chain impacting the sprocket teeth dur-ing continuous use, and ultimate strength testing. Dynamometer testing serves two purposes: one is to induce cyclic fatigue at various loads and speeds, and the other is

to create impact loads and contact fatigue. Contact stresses can create a ‘piano key’ effect on sprocket teeth from the chain loading. This causes partial densification as well as tooth wear. The wear can cause fine metal particles to contaminate the oil, which can lead to increased wear of other components. Ultimate strength testing consists of mounting the transfer case on a fatigue stand, holding the front output sta-tionary and applying a load until failure.

The current production design utilised MPIF: FN-0205 at 6.9g/cc minimum den-sity in the teeth. This design was able to withstand ultimate strength testing of the transfer case. The current design also completed the required high speed and low range dynamometer testing, without incident. The first iteration tested was MPIF: FL-4405 modified 0.3% molybde-num and 0.7% carbon at 6.9g/cc in the sprocket teeth and the same heat treat specification as the current production parts. These samples passed dynamom-eter testing with reduced tooth wear as compared to the current parts.

Compared to the original material, ultimate strength testing resulted in a 7% reduction in strength. This was not acceptable, so a slightly modified heat treat process was attempted. The ultimate strength did improve slightly but did not meet the current production parts. The next attempt was to increase the den-sity slightly while still retaining the single press single sinter processing.

The density was increased to 7.1 g/cc in the teeth on the one sprocket that failed during testing. The parts made to this change performed as well as the current production parts. Dynamometer testing proved to be successful while improved sprocket wear at the sprocket tooth to chain interface was observed.

In previous testing the reduction in molybdenum required an increase in carbon to improve the hardenability of the material. The improved hardenabil-ity allows the same heat treat cycle to be used, which reduces the in-plant complex-ity of having to run separate cycles for each family of parts.

Presented in Figure 3 is a photo-graph of the sprocket evaluated in this study. Shown in Figures 4 and 5 are the core photomicrographs of the original FN0205 sprocket and the FL 4405 (modi-fied 0.30% molybdenum) sprocket. The microstructure of the original FN0205 Figure 3. Photograph of sprockets.

Table 1. Chemical Analysis of Standard and Proposed Materials for Sprockets

Original New

Iron Balance Balance

Carbon 0.55 0.55

Nickel 2.00 0

Molybdenum 0 0.30

May 2009 MPR 15metal-powder.net

sprocket consists of ferrite, pearlite and nickel rich areas. The microstructure of the 0.30% molybdenum sprocket is fer-rite plus uniformly dispersed ‘divorced’ pearlite. This uniformity results from the pre-alloyed molybdenum. Micro-hardness values for the FN 0205 mate-rial ranged from 163 to 222 HV 100 gf. Micro-hardness values for the 0.30% molybdenum sprocket ranged from 160 to 225 HV 100 gf. Thus no differences in hardness values were observed.

Sun gearsThe sun gears examined in this study were originally produced using an FLN2-4405 material (0.85% prealloyed molybdenum with a 2.0% elemental nickel addition). These sun gears are typically produced to high density at two distinct density levels, one part is processed to achieve 7.4g/cc density in the teeth, and the second with a 7.3 g/cc tooth density. Samples were pro-duced utilising production tooling with the objective of eliminating the nickel and reducing the molybdenum content in the powder metal steel. It should be noted that because of the reduced nickel content in the experimental materials, the experi-mental parts showed dimensional differ-ences from normal production. However, for this test program, these differences were compensated for in the test set-up.

After prototype production utilising the no nickel experimental material, the first series of tests conducted was a single tooth bending in which the gear was set in a fixture on two teeth roughly 45º apart and then a load is applied to the top of the gear until one or both of the supporting

teeth fail. The results obtained were simi-lar to that of sprocket testing in which the single tooth strength was approximately 4.6% lower than production. At a 7.4 g/cc density there wasn’t any opportunity to increase density. So the next thing to do was reduce the amount of nickel in the original material by 50%. The single toothbending strength was within 0.25% of the production parts.

Shown in Figure 6 is a photograph of the sun gear. Figures 7 and 8 show the

case microstructure after carburising for the FLN2-4405 and the 0.30% pre-alloyed molybdenum with 0.80% elemental nick-el addition. Figure 9 displays the core microstructures for both materials.

Comparing the case microstructure of the standard FLN2-4405 to the 0.30% molybdenum with 0.80% nickel, the lower nickel material has less nickel rich regions but the overall martensitic case is the same. Differences in the core again reflect the lower nickel and

Figure 4. Photomicrograph of original FN0205, etched with 2% nital / 4% picral.

Figure 5. Photomicrograph of 0.30% molybdenum replacement material, etched with 2% nital / 4% picral.

Figure 6. Photograph of sun gears.

Table 2. Chemical Analysis of Standard and Proposed Materials for Sun Gears.

Original New

Iron (Fe) Balance Balance

Carbon (C) 0.20 0.35

Nickel (Ni) 2.00 0.80

Molybdenum (Mo) 0.85 0.30

16 MPR May 2009 metal-powder.net

subsequent lesser amount of nickel rich regions. Additionally, the core micro-structure of the 0.30% molybdenum with 0.80% nickel material has a higher carbon content, thus a greater amount of “divorced pearlite”. Core hardness values for the standard material averaged 293 HV 100 gf and 312 HV 100 gf for the 0.30% molybdenum with 0.80% nickel material.

The micro-indention hardness traverse of the 0.30% molybdenum with 0.80% nickel material is generally higher than the standard FLN2-4405. This difference can be accounted for by higher carbon potential during carburising. However, the reduced alloy content material can be successfully carburised to meet hardness requirements of the sun gear both for the case and core requirements.

The testing showed that the mate-rial for both the sprockets and the sun gears could be changed without com-promising performance, and the cost savings are significant. Seven sprock-ets were involved in the elimination of nickel activity. Two sun gears were evalu-ated. The average sprocket weight is 2 lbs. Based on 2% nickel and the large annual volumes, this equated to 22 000

lbs of nickel per year and cost savings of about $500 000 per year. The aver-age sun gear weight is 1.2 lbs. Based on nickel being reduced by 1.2% per part and molybdenum being reduced by 0.55%, this equated to annual savings of 5 500 lbs of nickel and 2 500 lbs of molybdenum and annual cost savings of about $180 000.

ConclusionThe change from the original powder metal materials for both the sun gears and the sprockets taught us two lessons.

The first lesson is that material standards can give us a quick path to material properties and selection, but a specific part geometry and application may lend itself to a mate-rial that on the surface appears inferior to another material. The best way to find out if a material can be substituted is to make parts and test the parts. In this way actual results can be obtained from parts produced using production tooling, production processes and a new material.

The second lesson is in cooperation and partnership between a material supplier, parts maker and the part end user (transfer case manufacturer). In the cases described here, the material manufacturer was facing price increases in raw material that was out of his control. In turn, the material sup-plier sought to pass these increases onto the parts manufacturer, who would pass the increases onto the part end user. Everyone in the supply chain was motivated to keep the costs down as much as possible. To this end, the part end user was open to making print changes and testing parts at their expense. The part manufacturer was

willing to sample different materials and manufacture the parts at their expense. The powder supplier was willing to provide technical support to suggest what material alternatives may work and then to provide sample materials to be tested. Only with the cooperation of all three parties was the change possible that would save money and business for everyone. In an increased glo-bal marketplace, cooperation in the supply chain is an essential component, in keeping US manufacturing competitive.

This paper is reproduced with permis-sion from the Metal Powder Industries Federation (MPIF).

Figure 7. Case microstructure of FLN2-4405, etched with 2% nital / 4% picral.

Figure 8. Case microstructure of 0.30% molybdenum with 0.8% nickel, etched with 2% nital / 4% picral.

Figure 9. Core microstructure of FLN2-4405, etched with 2% nital / 4% picral.

The AuthorThis article was derived from Material cost savings in powder metal transfer case sprockets and sun gears by Marc Legault, Borg Warner; Fran Hanejko, Hoeganaes; and David Pendrak, Capstan Atlantic, a paper that was given at the 2008 World Congress on Powder Metallurgy and Particulate Materials in Washington DC.

References1. Pease, L.F., West, W.G., 2002,

“Fundamentals of Powder Metallurgy”

2. MPIF Standard 35, “Materials Standards for P/M Structural Parts, 2007 edition” pp 14.

3. www.InfoMine.com/commodi-ties/12-80