Cost-Effective Material for Heat-Treated Gear Applications
Traditional PM materials intended for heat treatment often include copper, nickel or molybdenum in total quantities of 2-6%. In the light of recent cost increases for these alloying elements, these materials are becoming less competitive compared to solid-steel process routes. A new diffusion-alloyed steel powder having a lean alloy composition has been designed to perform well in different heat-treatment operations. This paper describes the performance of this new powder in commonly used heat-treatment operations. Static and dynamic properties achieved with both test specimens and components will be presented.
Alloying elements commonly used in materials intended for heat treatment are copper, nickel and molybdenum. The fluctuations in raw-material prices for these elements in recent years have been volatile, making material cost predictions difficult.
Distaloy AQ is a new, lean diffusion-alloyed powder intended for manufacturing of heat-treated PM components. The composition is Fe-0.5Ni-0.5Mo, where the base powder is pure iron to which Ni and Mo have been diffusion bonded. The low alloying content makes it less sensitive to future price fluctuations, and the diffusion-alloying concept ensures a consistent robust and economical mass production of components. The diffusion-alloying process also allows for maximum compressibility. The possibility to reach high densities is beneficial for both mechanical performance and heat-treatment processing because carbon penetration is easier to control. The low alloying content also results in good machinability and sizing properties in the as-sintered condition but still provides adequate hardenability to achieve high hardness and strength after heat treatment. This is favorable as some machining might be necessary already in the soft sintered state.
Previous work has demonstrated that the new Distaloy AQ material could achieve mechanical properties comparable to more highly diffusion- or pre-alloyed material systems after heat-treatment operations.[1-3] While tensile properties are important for design purposes, most applications are subjected to cyclical loading, which necessitates an understanding of the fatigue performance of a material. The current work focuses on understanding the complete mechanical performance of the new alloy over a variety of carbon contents. The fatigue performance of the new alloy was investigated using both test bars and prototype gears. The combination of tensile strength and fatigue performance in the heat-treated condition makes this new alloy interesting for gear applications.
Powders and Materials
Three different base powders have been used in the studies presented in this paper. In addition to the newly developed diffusion alloy, Distaloy AQ, Distaloy AB and Distaloy AE from Höganäs AB have been used. The compositions of the different powders are shown in Table 1.
To evaluate the influence of carbon content, density and heat-treatment processing conditions on the mechanical properties, eight different powder mixes with the compositions shown in Table 2 were prepared. Natural graphite, UF 4 and Lube E from Höganäs AB were used in the manufacturing of the mixes.
For mix 1, 2 and 3, tensile specimens were compacted to achieve sintered densities of 6.80, 7.00, 7.10 and 7.25 g/cm3 on a 60-ton hydraulic compaction press. Warm-die compaction and a tool temperature of 60°C were used to compact the specimens for the 7.25 g/cm3 density level. Plane bending fatigue specimens were compacted from mix 2 and 4 to green densities of 7.10 g/cm3.
In addition to the test specimens, gears were compacted from mix 2 and mixes 4 to 8 to a green density of 7.10 g/cm3. The gear parameters are shown in Table 3.
Sintering was carried out on a mesh-belt furnace at 1120°C (2048˚F) for 30 minutes in a 90/10 N2/H2 atmosphere with a cooling rate of 0.5°C/s.
For heat treatments, the specimens were austenitized at 920°C (1688˚F) for 20 minutes at different carbon potentials to create both through hardening and case hardening. Oil quench was applied with an oil temperature of 60°C. The specimens were tempered at 200°C (392˚F) for one hour in air.
Carbon content of the as-heat-treated parts was determined using infrared combustion techniques according to ASTM E 1019. The apparent hardness was measured on the impact specimens according to MPIF Standard 43, and sintered density was determined using MPIF Standard 42. Tensile strength was evaluated according to ASTM E8-09.
Plane bending fatigue was tested on ISO 3928 test bars with R = -1 and a run-out limit of 2 x 106 cycles. Evaluation of the fatigue data was performed according to MPIF Standard 56.
Tooth root bending fatigue was performed on gears with stresses evaluated according to ISO 6336/DIN 3990. A picture of the gear and schematic of the testing is shown in Figure 1.
The microstructures obtained after heat treatment of material A, B and C at a density of 7.00 g/cm3 are shown in the micro photographs in Figure 2. A completely martensitic microstructure was obtained for all three materials (i.e. for carbon contents of 0.5-0.7%). Micro-indentation hardness measurements were also performed. The microhardness (HV 0.1) values obtained – 653, 692 and 749 for material A, B and C respectively – are typical for martensitic microstructures. As expected, increased carbon content increases the microhardness. One item of note is the lack of large nickel-rich austenite. Typical diffusion-alloyed materials with higher Ni content will still contain large islands of nickel-stabilized austenite in the heat-treated condition.
The apparent hardness measurements of material A, B and C are shown in Figure 3. As expected, increasing carbon content and density resulted in an increased hardness. Hardness levels greater than 40 HRC were achieved for material C with the highest carbon content at densities greater than 7.00 g/cm3.
The tensile strength obtained after quench and tempering of material A, B and C at different density levels is shown in Figure 4. As can be seen, the tensile strength increases with increasing density for all three materials. Depending on density level, tensile strengths in the range of 800-1,300 MPa could be achieved at carbon content of 0.5-0.6%. For material C, with the highest carbon-content level of 0.70%, the tensile strength drops off, especially for the two higher-density levels. This is due to an increased brittleness of the material at this carbon level.
Plane Bending Fatigue Strength
The fatigue strength of material B in the quench and tempered condition is presented in Table 4. Four reference test bars of material D were processed and tested under the same condition as material B. The density of the fatigue specimens was around 7.15 g/cm3.
Material B obtained about 10% higher fatigue performance compared to material D. This difference is partly due to the slightly higher density of material B.
Gear-Tooth Root Bending Results
Quench and Temper
Tooth-root bending fatigue tests were performed on the quenched and tempered prototype gears described in Table 3.
The endurance limit was evaluated in 10-12 points using the staircase method by Dixon and Mood. The SN curve slope in the limited fatigue-life region was evaluated by testing at two load levels where 10%, 50% and 90% survival probability was estimated according to lognormal statistic distribution. The obtained data is listed in Table 5.
The gear-tooth root bending fatigue results show that the lean diffusion alloy (material B) is comparable to the more highly alloyed material E and very close to material D. For material B, a gear-tooth root bending fatigue limit higher than 550 MPa (s 90%) was obtained. The SN curve for material B is shown in Figure 5.
Tooth root bending fatigue tests were also performed on the case-hardened prototype gears of material F, G and H. The purpose of case hardening is to take advantage of the alloying to achieve a high surface hardness and ductile core. The endurance limit was evaluated in the same way as described in the previous paragraph. The obtained data is listed in Table 6.
The results show that the lean diffusion alloy material B obtained a higher fatigue strength compared to the much more highly alloyed material H – 695 MPa versus 658 MPa (s50%). Gear-tooth root bending fatigue limits of close to 700 MPa (s50%) were achieved for both material F and G. The SN curve for material F is shown in Figure 6.
The conclusions that can be drawn from this study are the following:
• The highest tensile-strength levels for the new Distaloy AQ material were achieved at carbon content in the range of 0.5-0.6%.
• Depending on density level, tensile strengths of 800-1,300 MPa could be achieved.
• Plane bending fatigue performance of Distaloy AQ was slightly higher compared to the more highly alloyed Distaloy AB material. Plane bending fatigue limits of 381 MPa (s50%) and 358 MPa (s90%) were achieved at a density of 7.16 g/cm3.
• Root bending fatigue limit of quenched and tempered gears made of Distaloy AQ was slightly lower than for the Distaloy AB-based material – 554 MPa versus 587 MPa (s90%). However, a higher tooth root bending fatigue limit was obtained compared to the Distaloy AE material – 554 MPa versus 503 MPa (s90%).
• Root bending fatigue limit of case-hardened gears was slightly lower for Distaloy AQ material compared to the Distaloy AB-based material – 695 MPa versus 710 MPa (s50%) – but higher compared to the Distaloy AE material – 695 MPa versus 658 MPa (s50%). IH
This paper was presented at the EPMA conference, Euro PM 2011, in Barcelona, SPAIN.
For more information: Contact Ulf Eng-ström, manager technical sales, Höganäs Sweden AB, 26383 Höganäs, Sweden; tel: +46 42 338 338; fax: +46 42 338 188; e-mail: firstname.lastname@example.org; web: www.hoganas.com
References available online
1. H. Sanderow and T. Prucher, “Mechanical Properties of Diffusion Alloyed Steels: Effect of Material and Processing Variable,” Advances in Powder Metallurgy and Particulate Materials,compiled by C. Lall and A. Neupaver, Metal Powder Industries Federation, Princeton, NJ, 1994, Vol. 7, pp. 341-354.
2. U. Engström, B. Johansson, et al, “Properties of Diffusion Bonded Alloys Processed to High Densities,” Advances in Powder Metallurgy and Particulate Materials, compiled by M. Phillips and J. Porter, Metal Powder Industries Federation, Princeton, NJ, 1995, Part 10, pp. 77-95.
3. R. Warzel III, R. Frykholm, et al, “Lean Diffusion Alloyed Steel for Heat Treat Application,” Advances in Powder Metallurgy and Particulate Materials, compiled by M. Bulger and B. Stebick, Metal Powder Industries Federation, Princeton, NJ, 2010, Part 7, pp. 30-43.
4. J. Dixon and A.M. Mood, “A Method for Obtaining and Analyzing Sensitive Data,” Journal of the American Statistical Association, 1943, pp. 109-126.