Sintering in Powder Metallurgy: Sinter Hardening & Heat Treatment.

Posted by Horizon Technology - July 18, 2019

A discussion about the efficiency and performance possibilities of modern metal manufacturing is incomplete without the concept of sintering in powder metallurgy.

Sintering is an inherent step in powder metallurgy. This processing step heats the compacted part to fuse the loosely bonded particles, lending strength and hardness. After sintering, the PM part is often heat treated by reheating to permit quenching and tempering to further increase the hardness and strength of the compacted component.

One advantage of powder metallurgy is the ability to sinter the part and then, thanks to controlled cooling, create a wide variety of strength and hardness combinations. At one end of this range is very slow cooling to create a soft magnetic part; the other extreme is effectively atmosphere quenching the part to form a martensitic structure with the same hardness as quenching -- but without the additional processing.

Out of several metal hardening process types, sinter hardening stands out. Why? Sinter hardening combines the benefits of sintering and hardening into one operation. You minimize secondary processing steps and enjoy greater dimensional precision.

For the metallurgically savvy: Conventional heat treatment (reheating and quenching) forms martensite within the part that has high hardness but is quite brittle. Tempering lowers the hardness slightly but dramatically increases the strength. This has been the tried-and-true method for hardening in both PM and conventional parts manufacturing.

However, using special sintering furnaces coupled with engineered powder materials creates the opportunity to eliminate the reheating step and merge it into the sintering step. You get the same transformation along with the benefits of strength and hardness.

Let’s go deeper into what makes sinter hardening and sintered metal parts stand out from other heat treatment services and competing manufacturing processes.

Defining Sinter Hardening & Sintering in Powder Metallurgy

sintering in powder metallurgy graphic

During this particular form of heat treatment, heat is applied under a controlled atmosphere so the temperature peaks below the melting point of the primary metal in the mix. Today, special sintering furnaces eliminate the need for expensive post-sinter heat treating. The end result is a part that’s either finished or semi-finished, requiring no or very little secondary machining.

Sinter hardening is an accelerated cooling process under the heat treatment umbrella. Where it differs is that it increases the hardness of the metal, making it more resistant to deformation when placed under stress.

Sinter hardening eliminates the need for a secondary quench hardening treatment  and provides better dimensional control. An accelerated process, of course, can keep down production and labor costs.

The obvious reaction is that this is a potential cost savings. But we’ll also discuss the environmental-friendly on this nature. Not only are we saving the planet, if you look at the tolerances and stackups, we can give you a benefit there as well. More bang for less buck.

Sinter Hardening Vs. Other Heat Treatment Processes

We have all these heat treating processes in metal manufacturing. With sinter hardening, your manufacturer can heat treat the part during the sintering process.

Imagine all the energy that goes into sintering a part. Then you have to re-establish all that energy to (conventionally) heat treat the part. In the era of energy efficiency, sinter hardening can save 60% of the energy cost required to heat treat a PM part. This is a great way to advance of the idea of powder metal parts being “green.”

Energy use affects your production cost, so the comparison below may interest you: Think of the numbers as a ratio of time and effort it takes to complete each process:

Conventional sintering,
energy consumption

Sinter hardening,
energy consumption

Conventional sintering: 1.0

Conventional sintering: 1.0

Heat treatment: 0.6 (could be higher depending on the operation)

Heat reheating: 0

Tempering 0.1

Tempering 0.1

Total: 1.7

Total: 1.1


Proper heat treating controls the hardness and strength of the final part. Your friendly powder metallurgy company can achieve some performance properties through careful sintering alone. But compared with traditional heat treatment, sinter hardening can give components comparable qualities and better dimensional precision.

Other heat treatment processes include:

  • Quenching and tempering
  • Carburizing
  • Precipitation hardening
  • Tempering (also used with sinter hardening)

Tempering may increase toughness, but it can also result in a loss of hardness.

(Related article: Toughness Vs. Hardness)

Sintering Cost Vs. Other Heat Treating Processes (don’t get into the various other types of heat treating too much)

Powder metal is literally a crucial component of low-cost manufacturing for the automotive industry (and potentially others). The many advantages of powder metal eliminate the need for higher-cost forming technologies associated with other means of fabrication.

Sintered metal products have great net-shaping capability and eliminate or reduce the need to include costly metal removal processes into the production line. This also leads to a great reduction in waste and scrap byproduct, which improves profitability and meets the standards of eco-conscious companies (and states and nations!).

OK, But What About Performance?

With higher strength and hardness, sinter hardened parts offer excellent performance for a variety of industries. Their durability and adaptability have made them proven successes in:

  • Gearing
  • Lawn and garden equipment
  • Firearms
  • Aerospace applications
  • Military applications
  • Automotive production

Engineers in these industries who seek out one of the few powder metallurgy suppliers with sinter hardening capabilities will hold a competitive edge.

Beyond the Typical

Remember: No additional thermal processing necessary!

If you think sinter hardening makes sense for you, discuss your end use and performance needs with your powder metal manufacturer. Chances are, powder metallurgy is a cost-efficient alternative to your current way of doing things.

Not every manufacturer has these capabilities. Even rarer is the PM manufacturer that can simultaneously sinter harden while sintering at high and ultra-high temperatures. Inquire with your manufacturer before coming up with a plan.

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Topics: Properties, Processes

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