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My sintered parts have a frosty, dull, matte finish. What causes this and how can I prevent it?

Tom Philips
Sr. Principal Applications Engineer

Sintered parts should exit the furnace with a shiny, bright finish. If they don’t, that is a sign of a problem in your process. Oxygen or air may be infiltrating the furnace at the front entrance. Also, if the oxidizing potential in the preheat zone is too high, it can cause oxidation on the powder metal part surface. This oxidized surface reduces as the part travels through the highly reducing atmosphere in the hot zone, causing it to lose its shiny finish and appear dull and matte. In addition to a dulled finish, you may notice lower surface hardness due to the surface decarburization that resulted from the oxidation.

To help solve this problem, you can add a flame curtain at the front end of the furnace. The curtain should be attached to the door to provide full coverage of the front entrance, plus the flame should be directed downwards. You can also control the dew point in the preheat zone so it’s oxidizing enough to facilitate de-lubrication, but does not oxidize the metal.

For more tips and techniques to help you control furnace atmospheres, get in touch with an Air Products applications engineer by contacting us online or calling us at 800-654-4567 (mention code 749).
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