oxidation vs decarburization

[SaMaJoEni]

Hallo,

 

I am currently assisting on a project which deals with the decarburization of steel alloys (Fe-C-Cr) and a repeatedly occuring problem is the oxidation of our spezimen during the actual decarburization treatment.

 

Initially, carbon dioxide was used as decarburization gas (at 1000°C) but it lead to the formation of an oxidation shell that interfered with our measurements (via CO sensor) to such a degree that the collected data became useless. The system was purged with argon before the actual treatment started so that the oxygen solely originated from the reaction gas itself. We replaced the carbon dioxide with hydrogen in the next step and the parasitic oxidation effect was sucessfully reduced to an acceptable level that way but the gas reacted at a very slow rate with our material which lead to the decision to wash the gas and thus accelerate the decarburization reaction. Unfortunately, due to the washing of the hydrogen the oxidation of the specimens increased strongly once again so that we are considering to use either a combination of washed argon and hydrogen or carbon dioxide and hydrogen instead of washed hydrogen alone. In this case, hydrogen should prevent the formation of an oxidation shell while carbon monoxide or washed argon is decarburizing the specimen.

 

I am not really sure if this can work that way and now I come to my actual question, could anybody give me some word of advice how I can calculate in any gas mixture whether the decarburization or the oxidation is the quicker reaction during the treatment, depending on paramters like temperature, flow-rate, etc.?

 

Thanks a lot, I would appreciate any helpful advice.

[Enthalpy]

Hi SaMaJoEin, welcome here!

 

I'm no expert in the field. Wiki suggests

http://en.wikipedia.org/wiki/Decarburization

O₂, CO₂, H₂O to transform C into CO - I expect them to form iron oxides necessarily -

or H₂ to transform C into CH₄. Citing them:

 

"Stainless steel... can only be decarburized by reacting with dry hydrogen, which has no water content"

because chromium (which your have) oxidizes too readily.

 

You should remove the hydrogen atmosphere before the part is completely cold, or in a subsequent step, bake the part (like 15min at 200°C) to remove the absorbed hydrogen which embrittles steel.