Is this what decarb looks like?

J. Keeton

J. Keeton

Joined
Nov 15, 2005
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I've seen these circles from time to time post heat treat of 1084 and file steel. Is this decarb? What causes them?

Thanks,
JK

 
That type of blistering happens with over heating or heating too long at temperature in an oxygen rich environment.

Decarb is part of the surface of the steel where the carbon burns off and becomes a low/lower carbon area. Scale is separate/different from decarb. Decarb is not usually very deep, maybe .002-.003 deep. Decarb is mostly ferrite and does not take on more carbon preventing it from going too deep.

Forging, rolling, heat treating etc. all produce decarb unless done in an oxygen free environment.

Hoss
 
I call that blistering. It is caused by overheating (burning) the steel in HT. It may grind off, but you will have to remove more than you would for decarb - probably .005"or more each side. The circles may stay in the finished blade to some extent.

Tell us about your HT and the method you used to heat the blade.
 
I use my forge and a magnet. Guess I need to check more frequently with the magnet on the way up. I quench in heated vegetable oil.

With much grinding/sanding it came out of the blade in question.

Thanks for the classification of what I'm seeing. I didn't know about blistering. I'll have to be more careful on the heat.
 
If you ever pull your steel out of the forge and it’s throwing off sparks, you probably got it too hot.
 
Generally if you see that kind of blistering you have over shot the temp by quite a bit. This also usually means you have blown out your grain size. This leads to a brittle blade.
 
I'm still having some issues with blistering 1084. Do yall have any strategies for forge heat treating 1084 with a magnet. Muffle tube? I'm running my blown forge at 0.5psi (trying to get it as low as possible). Also keeping a little "dragons breath" rolling out the front (maybe this is my issue? too much oxygen?).

My tips are getting hot significantly faster than the rest of the blade.

Does the blistering completely ruin the knife? I've been able to sand past it in most cases.

Thanks,
JK
 
Turn down the forge. HT requires far less heat and flame than forging. Lower the gas pressure to the lowest it will still hold a flame. If you have a choke plate on the burner, close it more.

Many venturi forges won't throttle back enough, and are still too hot for HT. In those cases, keep the blade moving. Roll it over, move it in and out, and place the tip in the coldest part of the forge … or even out the back port part of the time.

Coat the blade tip with satanite or APT-641 or similar product. This will slow down the tip overheating.

A muffle is very helpful.

Blistered steel is ruined. You can cut the tip off and make a shorter knife sometimes.
 
Thanks for the answers Stacy.

Do you think there can be varying degrees of "blistering compromise"?

I'm working on a 3-1/2'' blade right now with a few "quench circles" on it. Just finished tempering it. I've dropped it from over head onto concrete four times, tip dropped it onto concrete once, 10 degree flex tested it with 1'' of the tip in the vise and smacked it around with a hammer while vised up. Any other test I can do?
 
Last edited:
Stacy E. Apelt - Bladesmith said:
Turn down the forge. HT requires far less heat and flame than forging. Lower the gas pressure to the lowest it will still hold a flame. If you have a choke plate on the burner, close it more.

Many venturi forges won't throttle back enough, and are still too hot for HT. In those cases, keep the blade moving. Roll it over, move it in and out, and place the tip in the coldest part of the forge … or even out the back port part of the time.

Coat the blade tip with satanite or APT-641 or similar product. This will slow down the tip overheating.

A muffle is very helpful.

Blistered steel is ruined. You can cut the tip off and make a shorter knife sometimes.
Click to expand...

You can substitute one of the small Venturi burners that Atlas Forge sells. It’s hot enough to get a propane tank forge to heat treat temps, but not hot enough to get that size forge to welding temps.
 
As a follow-up, I've been able to fix this issue by A.) opening the back of my forge B.) Dialing down to ~ 1/2 psi on my regulator C.) Constantly moving the blade and being a magnet Nazi D.) Using a "dark" trashcan to get a good view on the steel color while walking up to non-magnetic.

Thanks again for the help!
 
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