You may ask why we advocate the use of Ultra High Temperature Sintering (UHTS) for powder metal parts – as previously mentioned, this revolutionary sintering process offers performance-enhancing benefits such as lighter weight, smaller size and higher performance). fatigue strength) to various applications (reducer sets, transmission components, ceiling fans, mobile applications, parking claws, etc.).
Here we summarize the experimental work performed to hone optimal alloying and sintering practices. We then demonstrate how this optimization results in mechanical properties superior to any current powder metallurgy structural steel grade. We support our conclusions with mechanical property data and microstructural analysis.
Ultra-high temperature sintering - an alternative that traditional sintering cannot achieve
Conventional sintering of compacted iron powder metal parts often requires compromises between equipment availability, equipment capabilities and productivity, while also needing to be cost competitive.
As mentioned previously, conventional sintering is typically performed at 2050°F (1120°C) in a hydrogen/nitrogen atmosphere. Most components produced today are made from iron alloys with copper, nickel and molybdenum as the main alloying elements. These alloying elements are very effective at 2050°F; however, they do not exploit the full potential of black powder metals in terms of inherent formability and mechanical properties comparable to forged steel parts.
Alloying elements such as silicon, chromium and vanadium are widely used in forged steel metallurgy to provide excellent mechanical properties while reducing the total alloy content. These three alloying elements are not typically associated with conventional pressed and sintered black powder metals. Therefore, the ability to use these in conjunction with powder metal processing will bring a greater degree of comfort to the current part design community.
Powder metal can utilize fully pre-alloyed materials with added elements in the liquid metal, or powder grades pre-mixed with various elemental additions can be used to create custom pre-mixes. The rationale for not using fully pre-alloyed steel grade powders is that the compressibility of steel powders is reduced, resulting in relatively low density of compacted powder metal parts and corresponding reduction in mechanical properties.
UHTS is able to use these alloying elements as custom pre-mixed additives, maintaining a high ingredient density while providing a completely uniform alloy distribution that takes full advantage of their strength, toughness and wear resistance.
Research and development
As part of an extensive research campaign, a range of alloy compositions were studied with the aim of optimizing new powder metal materials with good compressibility and high mechanical properties through the use of customized premixes and UHTS. Rather than boring you with the nuances of our R&D work, we would like to simply present you the results of this work and discuss sintering practices and mechanical properties.
The alloy composition, plus UHTS, is an iron alloy consisting of molybdenum, vanadium, chromium, silicon and nickel with a sintered carbon content of 0.5%. The composition is a mixture of pre-alloyed iron powder and an iron alloy containing appropriate alloying elements. A carbon content of 0.5% was chosen because it represents a good balance between sintering hardening potential and potential carburizing potential to increase surface hardness and wear resistance. Such compositions are readily available through conventional powder metal premixing techniques or advanced premixing techniques.
Next step: sintering
Sintering is accomplished in a uniquely designed furnace capable of achieving temperatures in excess of 2500 °F (1370 °C) in a variety of sintering atmospheres. We recognize that "good" sintering practices (2050°F (1120 °C)) do not provide the alloy uniformity required for high performance targets. High temperature sintering at 2300 °F (1260 °C) (better) was studied to evaluate the performance of the alloy at this sintering condition. Finally, we evaluated UHTS to determine what benefits were achieved.
"Good" sintering (2050°F) is the mainstay of the powder metal industry, but does not fully diffuse elemental alloy additions. Even though copper melts at about 1983 °F, it will not be completely homogenized during a "good" sintering process. While copper significantly increases strength relative to iron-carbon materials, copper effectively creates a halo of copper-rich regions in the iron powder, which often makes heat treatment a thorny issue.
Better sintering methods (2300°F) will fully diffuse the copper, but is questionable for such advanced alloy materials. In this study, it was observed that sintering at 2300 °F leaves islands of elemental alloy additions, preventing full utilization of these additions.
The proposed best sintering practices produce completely homogeneous materials at temperatures in excess of 2500 °F. Another advantage of optimal sintering is significant pore rounding and elimination of large pores. Pore rounding and pore elimination are critical to produce superior mechanical properties.
Before discussing the achieved mechanical properties, please note the sintering atmosphere. A typical sintering atmosphere is a mixture of hydrogen and nitrogen (usually 10% hydrogen or less). Unfortunately, alloying elements such as chromium and vanadium can easily "absorb" nitrogen, especially at such high sintering temperatures. If chromium or vanadium forms nitrides during sintering, the effectiveness of these elements is reduced or even completely lost. Therefore, not only is the sintering temperature critical for good performance, but the atmosphere must also prevent the formation of diffusion barriers that would hinder full utilization of the added alloy.
Mechanical Properties: What are the benefits of ultra-high temperature sintering?
As mentioned previously, the sintered carbon content of 0.5% was chosen for reasons of potential sinter hardening and carburization (if required). Table 1 summarizes the tensile properties of optimized alloys that were sinter hardened at 2300°F, >2500°F, and finally sintered at >2500°F, followed by quenching and tempering to fully optimize mechanical properties.
Table 1: Tensile properties of optimized alloys
| Sintering Temp | Condition | Yield Strength, psi | Tensile Strength, psi | Tensile Elongation, % |
|---|---|---|---|---|
| 2300°F (better) | Sinter Hardened | 125,000 | 162,000 | 1.5 |
| >2500°F (best) | Sinter Hardened | 133,000 | 187,000 | 1.7 |
| >2500°F (best) | Quench & Temper | 183,000 | 213,000 | 1.7 |
You now have a material that is superior to any standard sinter-hardened grade.
If additional heat treatment (quenching and tempering) is performed, the strength increases again without any loss in elongation.
It should be noted that in MPIF standard 35 the best strength achieved is 0.85% molybdenum and 4% nickel. Material sintered at 2300°F has a tensile strength of about 200,000 psi and almost no tensile elongation. Therefore, the above materials far exceed the performance of any standard powder metallurgy ferrous material. If higher surface hardness is required, carburizing heat treatment can be performed. Expect a slight increase in strength but still maintain the same elongation. Properties of sinter porosity after sintering at 2300 °F and >2500 °F. The inherent PM porosity at >2500 °F is smaller and more dispersed throughout the part; this translates into superior mechanical properties as shown in Table 1.
Summary: What does this mean for the potential of PM in more demanding applications?
The potential for ultra-high temperature sintering (>2500°F) offers the possibility of unique alloys with superior mechanical properties, including strength, wear resistance, and toughness.
The optimized alloys discussed take advantage of the advantages of powdered metal materials and forged steel metallurgy. The addition of alloying elements unrelated to powder metal opens the door to new, more demanding applications, further enhancing the role of powder metallurgy as a cost-competitive, material-efficient process that now also has mechanical properties approaching those of forged steel. Additional advantages.
If you choose to use this material as a sinter hardened, quenched and tempered, or carburized material, the mechanical properties can be determined based on your specific application.
If you need a sintering process, choosing us at H&Z will ensure that you get high-quality, reliable, customized solutions to meet your needs. We have rich experience and advanced technology to provide you with professional sintering services. We will spare no effort in material selection, process optimization, or quality control. When you choose H&Z, you'll work with a passionate and talented team to bring success and outstanding results to your projects.
