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Ask the Experts Metals

My sintered PM parts come out of the furnace "sooty." How do I prevent this?

Mark Wells
Metals Industry Engineer

To resolve a sooting problem, you must first identify the type of soot. There are three main types: adherent soot; loose, granular soot; and shiny or oily soot. All are associated with hydrocarbons from either lubricants or enriching hydrocarbon gas. Adherent soot looks like a stain and is difficult to remove. It is generally produced by the pyrolysis of lubricant in the preheat zone. Loose, granular soot appears as a black snow on the top of the parts and is produced from lubricant vapors in the hot zone. Shiny soot appears as a uniform black coating on exposed surfaces. The catalytic cracking of natural gas on the parts produces this type of soot.

Once the type of soot is known, the problem can be resolved by evaluating factors such as atmosphere flow, flow balance, preheat dew point, belt speed, belt loading, temperature profile, part density, percent lubricant, and furnace condition.
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