Improving Part Performance with Ultra High-Temperature Sintering.

You may be asking why we’re advocating for the use of Ultra High-Temperature Sintering (UHTS) for powder metal components – As discussed previously, this revolutionary sintering process offers performance-enhancing benefits (such as lighter weight, reduced size, and higher fatigue strength) to a wide variety of applications (speed reducer sets, transmission components, ceiling fans, mobility applications, parking pawls, and more). 

Here, we summarize the experimental work that was performed to hone in on the optimal alloy(s) and sintering practice. Then we demonstrate how this optimization produced mechanical properties superior to any current PM structural steel grade. We support our conclusions with mechanical property data and microstructural analysis.    

Ultra High-Temperature Sintering - The Alternative That Conventional Sintering Can’t Do

Conventional sintering of compacted ferrous powder metal components was often the compromise between equipment availability, equipment capabilities, and productivity with the need to be cost-competitive.  

As noted previously, conventional sintering is done at nominally 2050°F (1120°C) in hydrogen/nitrogen atmospheres. Most components produced today utilize ferrous alloys with copper, nickel, and molybdenum being the primary alloying elements. These alloying elements are quite effective at 2050°F; however, they do not fully exploit the full potential of ferrous powder metal in terms of the inherent shape-making with mechanical performance that can rival wrought steel components.  

Alloying elements such as silicon, chromium, and vanadium, which are extensively used in wrought steel metallurgy, offer superior mechanical performance at reduced total alloying contents. This trio of alloying elements is not normally associated with conventional pressed and sintered ferrous powder metal. Thus, the ability to use these in conjunction with powder metal processing will give a greater degree of comfort to the current part design community. 

Powder metal can utilize either a fully prealloy material with elements added to the liquid metal or create a custom premix using powder grades premixed with various elemental additions. The rationale for not using a fully prealloyed steel grade powder is a degradation of the compressibility of the steel powder, giving a compacted powder metal part with relatively low density and corresponding reduced mechanical property performance.  

UHTS enables the ability to use these alloying elements as customized premix additives that maintain the high component density, while providing a fully homogeneous alloy distribution – thus fully maximizing their benefits for strength, toughness, and wear resistance.  

Research & Development

As part of an extensive research activity, a series of alloy compositions were investigated with the aim of optimizing a new powder metal material with good compressibility and high mechanical properties through the use of customized premixes and UHTS. Rather than bore you with the nuances of our research and development efforts, we’d like to simply show you the results of this work and discuss the sintering practice and mechanical property performance.   

The alloy composition, coupled with UHTS, was a ferrous alloy consisting of molybdenum, vanadium, chromium, silicon, and nickel with a sintered carbon content of 0.5%. This composition was a mix of prealloyed iron powder and ferroalloys containing the appropriate alloying elements. A carbon content of 0.5% was chosen because it represents a good balance between the potential for sinter hardening and potentially carburizing for greater surface hardness and wear resistance. This composition is readily achieved through either conventional powder metal premixing techniques or advanced premixing technologies. 

The Next Step: Sintering

Sintering was accomplished in a uniquely designed furnace capable of achieving in excess of 2500 °F (1370 °C) with a variety of sintering atmospheres. We recognized that ‘good’ sintering practice, 2050°F (1120 °C), was not going to give the required alloy homogeneity required for the high-performance target. An investigation of high-temperature sintering, 2300 °F (1260 °C), (better) was conducted to evaluate the performance of this alloy under this sintering condition. Lastly, 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 copper that melts at ~1983 °F does not fully homogenize during ‘good’ sintering practices. Although copper significantly improves the strength relative to an iron-carbon material, the copper effectively creates a halo of copper-rich regions in the iron powder often making heat treatment a tricky proposition.  

Better sintering practices, 2300 °F, will fully diffuse the copper but is questionable with this advanced alloy material. In this study, it was observed that sintering at 2300 °F left islands of the elemental alloy additions, thus preventing the full utilization of these additions.  

The best sintering practice proposed, in excess of 2500 °F, created a fully homogeneous material. The other advantage of the best sintering is significant pore rounding and large pore elimination. Pore rounding and pore elimination are essential for generating exceptional mechanical property performance.   

Before discussing the mechanical properties achieved, note the sintering atmosphere.  The typical sintering atmosphere is a blend of hydrogen and nitrogen (typically 10% hydrogen or less). Unfortunately, alloying elements such as chromium and vanadium are prone to ‘pick up’ nitrogen, especially at these high sintering temperatures.  If either the chromium or vanadium forms a nitride during sintering, the effectiveness of these elements is diminished or even totally lost. Thus not only is sintering temperature a key to good performance, but the atmosphere must prevent the formation of a diffusion barrier blocking the full utilization of the added alloys. 

Mechanical Property Performance:  What Are the Benefits of Ultra High-Temperature Sintering?

As noted earlier, a sintered carbon content of 0.5% was chosen for reasons of potential sintering hardening and carburizing, if required. Shown in Table 1 is a summary of the tensile properties of the optimized alloy sintered-hardened at 2300°F, >2500°F and lastly sintered at >2500 °F, followed by quenching and tempering to fully optimize mechanical property performance. The sintered density of the samples evaluated in Table 2 was in the range of 7.15 to 7.2 g/cm³.

Table 1:  Tensile properties of Optimized Alloy 

Advantages of UHTS for potential sintering hardening are increased yield strength, tensile strength, and tensile elongation. These are exceptional strengths for a sintered hardened material. Couple that with some measure of elongation you now have a material that outperforms any of the standard sinter hardening grades. 

If additional heat treating (quench & tempered) is performed, the strengths increase again without any loss in the elongation. 

It should be noted that in Standard 35 from MPIF, the very best strength achieved was 0.85% molybdenum with 4% nickel. Material sintered at 2300°F was about 200,000 psi tensile strength with almost no tensile elongation. Thus, the above material far exceeds the performance of any standard PM ferrous materials. A carburizing heat treatment can be performed if greater surface hardness is required. It’s expected the strength will increase slightly but still retain the same elongation.  

Below are two photo-micrographs illustrating the nature of the sintered porosity after sintering at 2300 °F and >2500 °F.  The inherent PM porosity at >2500 °F is smaller and more diffuse throughout the part; the implications of this are the superior mechanical properties shown in Table 1. 

Summary:  What Does this Mean for the Potential of PM in More Demanding Applications?

The potential of ultra high-temperature sintering (>2500°F) offers the possibility of unique alloys that promise exceptional mechanical properties including strength, wear resistance, and toughness.  

The optimized alloy discussed utilizes the best of both powder metal materials and wrought steel metallurgy. Incorporating alloying elements not associated with powder metal has opened the door to new more demanding applications that further enhance powder metallurgy's role as a cost-competitive, material-efficient process, now with the added benefit of mechanical properties that approach those of wrought steels.  

If you choose to use this material as a sintered hardened, quench & tempered, or carburized material, the mechanical properties can be targeted to your exact application.