Advances in Powder Metallurgy: Ultra-High-Temperature Sintering.

Posted by Horizon Technology - August 15, 2019

Sintering is a powder metallurgy processing step in which particles fuse into either a solid or porous body (sintered neck formation) at a temperature below the melting point of the major element in the powder. 

Think of ice cubes melting together in a glass of water. The longer the ice sits in the water, the harder it is to break those cubes up. That’s a good analogy of sintered neck formation.

We’ve previously discussed how high-temperature sintering can take your component’s properties to a level conventional sintering can’t match. But through unique, recent advances in powder metallurgy, you can take your component yet another step further -- through a process we like to call ultra-high-temp sintering.

Before High-Temperature Sintering, There Was Conventional Sintering

Of course, we can’t tout the greatness of ultra-high-temperature sintering without first explaining conventional sintering at standard temperatures. If you’re already familiar with sintering, skip to the next section!

An advantage of sintering is that it enables alloying additions to either completely or partially diffused into the base material and enhance strength or other key requirements of a sintered metal.

 

(Video courtesy Harb Nayar of TAT Technology)


Let’s focus on sintering of iron powder materials. The objectives of sintering are:

  1. Remove the lubricant from the powder compact
  2. Reduce the oxygen content on the surface of the powder.
  3. Create sintered necks between adjacent powder particles
  4. Fully or partially diffuse added elements such as graphite, nickel, copper, etc.

The typical sintering temperature for iron powder is 2050° F for 10-60 minutes. Because sintering bonds the particles via atomic diffusion, it’s both temperature- and time-at-temperature-dependent.  High-temperature sintering of ferrous material is performed in the range of 2125°F to 2300°F.

The added graphite is a good example of the need to diffuse and homogenize an additive to the powder. Most commonly, die-pressed ferrous powders are produced with almost no combined carbon; the reason is that alloyed carbon reduces compressibility, limiting the ability to achieve high density. But a carbon-free part will not have the strength required for many applications. So, graphite is added to the powder mix -- it diffuses completely into the iron powder during sintering, making steel.

Copper is another elemental alloy used in many PM parts, for much of the same reason as graphite. Once initial diffusion happens, the subsequent diffusion of the copper into the iron slows down. Thus, the copper is not completely homogenized in the part. Other elemental elements (such as nickel) will only partially diffuse because they have a higher melting point. Yes, you can prealloy these elements but there are certain trade-offs to consider, including ability to achieve high density and somewhat limited alloy selection.  

Advances in Powder Metallurgy & Sintering

OK; enough already of conventional sintering. What about high-temperature and ultra-high-temperature sintering? These types of sintering processes are all about furthering your part’s potential:

High-Temperature Sintering

More than 90% of all powder metal parts are sintered at 2050 F°.  Raising the sintering temperatures above the conventional range accelerates the diffusion of atoms across the iron particle interfaces. This results in greater sinter neck formation and more pore rounding. Additionally, the homogenization of the elemental alloy additions is enhanced, giving the potential for higher performance. 

Both of these concepts imply greater mechanical or DC magnetic properties, which means a stronger or simply better PM part. High-temperature sintering also allows for the use of more innovative powder metal materials that can give:

  • Better hardness
  • Potentially lower raw material cost
  • … at the same (or even higher) level of mechanical properties!  


Ultra-High-Temperature Sintering

Ultra-high-temperature sintering (UHTS) is defined as sintering ferrous powder at temperatures approaching 2500° F

The benefits of high-temperature sintering are augmented with UHTS. Sintered neck formation and alloy homogenization get a big boost from this new processing technique. Also, use of powder additions is expanded -- even ones not normally associated with conventional ferrous PM -- for even greater mechanical property improvements.   

Recently, Horizon Technology has collaboratively developed the technology to sinter iron powder parts at temperatures up to 2500°. What does this do that conventional high-temperature sintering does not?  Ultra-high-temperature sintering gives even more flexibility in alloy design, optimizing mechanical properties for unique project requirements.

Not only can you have a combo of prealloyed materials with elemental additions, you also get significantly enhanced atomic diffusion and homogenization. With greater pore rounding, this technology promises greater static and dynamic properties such as:

  • Tensile strength
  • Fatigue resistance
  • Impact strength

advances in powder metallurgy high-temperature sintering chartLet’s take a look at the pore-rounding effect of UHTS. Above are unetched photomicrographs of a proprietary alloy developed by Horizon, sintered at 2300° F (left) and 2500° F (right). The sintered densities of both samples were approximately the same (7.1 g/cm³). However, on the left you see a large number of fine pores more or less outlining the prior particle boundaries. On the right, the outlining of the prior particle boundaries is reduced and the appearance of the porosity is more rounded and the number of fine pores is reduced.

What does this mean for the mechanical properties of UHTS vs. conventional high-temperature sintering?

 

Mechanical Property Comparison: 2300°F vs. UHTS 

Material

Density, g/cm³

Transverse Rupture Strength, psi

Yield strength, psi

Tensile Strength, psi

Elongation,%

FC-0208

7.2

155,000

65,000

75,000

<1

FC-0208 heat-treated

7.2

150,000-

No defined yield point

105,000

<1

FN-0205 heat-treated

7.1

185,000

130,000

145,000

<1

2300 F

7.1

305,000

125,000

162,000

1.5

2500 F

7.1

350,000

185,000

215,000

1.7

 

The ultimate tensile strength increased from 162,000 psi to ~215,000 psi. Equally important is the improved elongation characteristics in the UHTS samples. Elongation is a good measure of damage tolerance. Both samples showed metallographic analysis of nearly 100% tempered martensite.

If we refer to MPIF Standard 35 for powder material standards, this combination of properties is not available in any of the listed materials (at a sintered density of 7.1 g/cm³).

Yes, a 4% nickel steel with 0.5% graphite and 0.85% molybdenum will give higher strength. But this material was also sintered at 2300° and evaluated at a minimum density of 7.3 g/cm³. It showed elongation of 1% or less.

The damage tolerance of this material will be less than the UHTS material.

More to Come ...

We stress that the results in the table above are based on preliminary testing. Additional tweaks are coming:

  • Advanced compaction technology (a Horizon forte)
  • New alloy compositions
  • Sintering process improvements

These adjustments will improve sintered density and mechanical properties even more.

We also need to investigate the effects of specialized heat treating, such as carburizing and carbo-nitriding, on these alloy systems. Expect these specialized processes to upgrade the overall performance possible with ultra-high-temp sintering.

These developments are ongoing and will be documented in future blogs. We expect great things from this research and look forward to working with powder metal parts users to exploit this technology!

Learn how to IMPROVE My Product >

 

(Thanks to Hoeganaes Corp. for its assistance in the physical testing and metallographic analysis)

Topics: Properties, Processes


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