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Topic: CO production in pack case hardening  (Read 1871 times)

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Offline jak321

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CO production in pack case hardening
« on: August 01, 2016, 02:49:14 AM »
Hi folks, not sure if this is the correct place for this question.
I'm wanting to case harden a couple of small pieces of mild steel.
I'm placing them in small lengths of stainless tube and heating to red heat for over an hour (900 degrees +).
I'm packing the tubes with powdered graphite and 2% sodium carbonate. I've read so many recipes for this packing material that I am starting look for a shop that sells newt eyes and parts of toads :)
It is claimed that with just carbon in the material, the process goes too slowly, and so various carbonates are recommended such as Na, Ca, and Ba. Ba seems to be the preferred accelerator, and looking up the heating of these it appears that barium carbonate releases its CO2 at a higher temperature than the others. However it's been phased out due to its toxicity. But I'm curious why carbon dioxide is required here, unless it has a higher affinity to carbon to make CO.
But, apparently, the CO acts as a carrier gas, taking C to the surface of the steel, depositing it to slowly diffuse below the surface for later quenching in cold water. Thus this leaves oxygen to return to the red hot carbon to become CO and so the cycle goes. My question is, why is carbon dioxide needed here, and how might it speed up this carbon transfer reaction?
Thanks for any help here, this is only a retirement project, but I like to know why I am doing things, cheers, jak

Offline Enthalpy

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Re: CO production in pack case hardening
« Reply #1 on: August 01, 2016, 05:43:30 PM »
Hi jak321, welcome!

I haven't done it myself, so I can't give very practical advices.

Carburizing has been done by direct contact with carbon-rich solids and liquids like tar. A carrier gas brings the carbon from graphite to the target, changing its composition (for instance between CO2 and CO) at the source and destination but keeping gaseous. I suspect that a gas' main advantage is to leave the parts cleaner, and also to make the contact more repeatable. Graphite powder alone couldn't make a good contact with the target parts.
https://en.wikipedia.org/wiki/Carburizing
900-950°C is already very bright red to orange.

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