Originally shared April 26, 2023

In our third letter, we’ll take a “walk” through a sintering furnace to demonstrate how it works. We’ll start by sharing a video created by Kyle Pond, TAT’s IT Marketing Manager, which shows what happens to a PM part as it moves through a typical sintering cycle as a function of temperature:

In the above video, we walk with a Fe-Cu-C-lube green part using just 5 round iron particles with admixed lubricant, graphite, and copper as it travels from the front to the back end of a continuous conveyor furnace that is 50 to 100 feet long over a period of 2 to 3 hours. The path this part must take can be seen on this schematic:

The video and schematic together show the critical milestones a green part must pass through before exiting as a well-sintered part. The critical milestones are briefly described below:

  • (A) Delubing: Completed by 1200-1500°F under an OXIDIZING atmosphere.

  • (B) Oxide Reduction: Completed by 1600-1800°F under a REDUCING atmosphere.

  • (C) Carbon Diffusion: Completed by 1950°F under a Carbon NEUTRAL atmosphere.

Milestones A, B, and C MUST be fully completed in that sequence for a part to be considered metallurgically clean. At this point it is steel and not Iron. This must happen before admixed Copper in the green parts begins to melt. TAT Technologies president Harb Nayar refers to this as preparation of the part for to give it optimum strength and dimensional control during the following sintering steps:

  • (D) Transient Liquid Phase: this involves both copper melting and its diffusion into the Fe-C matrix. During this part of the walk, two things are observed: (1) Fe-C now has become Fe-Cu-C alloy, making the part stronger. (2) Neck diameter between the particles grows quickly to increase their bond strength.The temperature is 2000+ °F and the required atmosphere composition is reducing with low dew point

  • (E) Solid State Sintering: The part is held at the peak temperature for 10-20 minutes. Neck diameter grows further and pores between the particles become rounded to further increase bond strength and provide some toughness to the sintered part. At this milestone, the peak temperature is 2050-2100°F and the atmosphere composition is reducing with low dew point. Observing the microstructure at this milestone, one can say the part is well-sintered. It is a homogeneous alloy of Fe-Cu-C and all particles well-bonded together.

  • (F) Initial Cooling: This is where the cooling rate can be controlled from relatively slow to much faster. Depending upon the cooling rate, the microstructure can be varied between low to high hardness.

  • (G) Final Cooling: This relatively slower end of the walk takes 20 to 40 minutes before the parts exit the furnace. Atmospheres at this stage are only a little higher than the ambient temperature in the room.

By the end of this process, if you have a smooth-running sintering operation going through all the above milestones in sequence, you will have a high-quality, metallurgically clean, and soot-free sintered part.

If you found this information useful and you would like to learn more, TAT Technologies has designed two four day long classes periodically given at our facility in St. Mary’s, PA. The first class covers above milestones A to C and the second class covers D to G. Many critical experiments are carried out by the students during the each class to ensure full understanding of these milestones and how they lead to ideal sintered parts. All experiments are hands-on and conducted with an eight zone sintering furnace. Please contact me for more details if you are interested in our next round of classes. At time of writing, our next classes will begin July 18, 2023.