Carbon Pickup in heat treatment

[mahadhatu]

I have discovered that low carbon steels pickup a lot of carbon some times in the range of 100% while heat treating these steels in a natural gas fired furnaces. I noticed this carbon pickup in steels containing less than 0.03% carbon. It appears that the part less than 3" thick gets fully carburized in about 10 hours of heat treating time.

Has any one else faced a similar problem? I know that diffusion of carbon is very quick when the difference in carbon concentration is high but was surprised to find my low carbon steels approaching 0.05% carbon even in the center of cross section.

Please share your experiance in this matter.

Thank you.

Mahadhatu

 

[stanweld]

Carburizing atmospheres can be obtained in natural gas fired furnaces. From your description, the burners in your furnace are severly out of adjustment and need to be adjusted to produce a neutral to slightly oxidizing flame.

 

[metman]

mahadhatu,
3" thick carburized to center in 10 hrs does not seem right. I don't have the formula handy but as I recall, depth of carburizing is inversely proportional to the square root of time. Are you certain that the original chemistry was correct or maybe the material mislabled?

Jesus is THE life,
Leonard

 

[mahadhatu]

Yes Sir I ran this experiment on a 3" diameter runner bar taken from our regular CA6NM heats which had carbon at 0.02%. The carbon at the end of this experiment was 0.043% in the runner bar on the surface as well as cross section.

Stanweld, I will post my findings after checking the flames on the burners. Thank you for the suggestion.

 

[metengr]

The CA6NM is not low carbon steel, as implied in your original post. CA6NM is a nickel-bearing martensitic stainless steel. At solution treatment temperatures for this alloy, the austenitic structure has high solubility for carbon in a reducing atmosphere. If the carbon concentration is high enough, and for the time duration of 10 hours at temperature, I can believe your results for a CA6NM material.

The heat treatment of CA6NM must be performed in a neutral atmosphere furnace to avoid carbon pick-up.

 

[arunmrao]

metengr is right in saying that the scenario changes when one talks of CA6NM alloy.Also why is the soaking time 10 hours (for 3 inch dia bar.)? The burners will have to be checked to maintain a neutral atmosphere. If possible please repeat in a electrically heated furnace for confirmation of results.