What's decarb?

[Dancing Blade]

Hi guys,

one of the things I'm not quite sure about is decarb. What is it exactly, I assume is the blackening of the blade, and I hear is not good, but can someone explain what is it exactly? and how to avoid it? thank you!

Dario

 

[Russ Andrews]

Decarb is short for Decarburization.
When steel is exposed to high heat, and oxygen,
the carbon combines with available oxygen, and
forms scale.

Are you using stock removal, or forging..?

 

[LRB]

And this carbon is lost from the steel.

 

[Dancing Blade]

oooh I see so decarb doesn't allow the steel to be as hard as it could be because it loses the carbon content... oh yeah it's bad then ah ah. I am using stock removal. How can I avoid decarb? tool wrap?

 

[Will Leavitt]

You'll avoid decarb by using a quality heat treater.

Decarb is the reason for the folk lore of old pocket knives getting better after you sharpened them a few times. You'd work the decarb layer out and have the "good" steel exposed after a few sharpenings.

I avoid (or attempt to avoid) decarb by forging my blades a little thicker for "sacrificial" steel and keeping a close eye on my forge atmosphere.

 

[mete]

Don't confuse 'scale' [iron oxide] with decarburization [loss of carbon from the steel] .If you forge or HT you will get decarb !! You can minimise it but not eliminate it ! A reducing [carbon rich] atmosphere in the forge or furnace will help .Nature wants to balance things out .So if the furnace atmosphere has lower carbon than the steel , carbon from the steel will go into the atmosphere.

 

[Dancing Blade]

what's a quality heat treater?

 

[sunshadow]

Decarb is short for Decarburization.
When steel is exposed to high heat, and oxygen,
the carbon combines with available oxygen, and
forms scale.

not exactly. Scale is iron oxide Fe2O3, Fe3O4, etc. Decarb is alpha iron, Fe that is missing carbon. Carbon is a mobile element that will diffuse from high concentration to low concentration. Heated oxygen will remove carbon at the surface, forming CO and CO2 leaving behind Fe, carbon will diffuse toward this low concentration along grain boundaries

These micrographs are carbon diffusion along a weld zone between 1095 and wrought iron. the light areas are basically just iron without carbon. the dark areas are pearlite. You can see that carbon has left the 1095 following the grain boundaries and diffused into the wrough iron along the weld zone. Assume that oxidizing slowly is going to create the same kind of carbon poor region at the surface as the wrought iron is in these pictures. Carbon will migrate along the grain boundaries to try to fill the vacancies, and as it is exposed to oxygen it to will be removed.
Scale is when the metal itself is oxidized. It will outpace decarb if you run hot enough in a slightly oxidizing atmosphere, if you don't get freaked out by leaving something behind on the anvil each time you forge. I don't have any micros yet for a comparison set, maybe soon.

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