Burning off O2 in a kiln?

Brian Avila

Brian Avila

Joined
Jan 2, 2011
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I decided that I am going to make a couple kitchen knives out of AEB-L for my brother for his wedding present.

I have heard many different ideas on how you can heat treat AEB-L. I am not saying this is the right way or that I am going to do it this way. But I "COULD" use no foil soak for 3-5 minutes and then quench in Parks 50. And go to town. This lead me to thinking about burning off the O2 in my kiln to help with the atmosphere in the kiln. I could put coal into the back of the kiln and it would burn off the O2.

I was wondering if anyone has done this? I also am curious about if there was any problems. IF you burn coal and stuff catches on fire how would you put it out. There are coal fires under ground all the time and they can't put them out.

Just curious....

NOTE: I will be using foil and some Al plates to quench.
 
I very little about forging, however I do know adding oxygen is of no value whatsoever. ABE-L is a great steal that will give you a fine cutting knife, but sticking it into a forge to harden it can just give you zilch in results. Frank
 
Yeah, I know about the gas injection. I am not willing to do that. I use an EvenHeat anyways.

The direction I am going with is just use the foil, but I have to figure that part out.
 
I use charcoal to produce a reducing atmosphere in my kiln. Stacy warned about reducing atmosphere ruining Kanthal kiln elements in a matter of a few heats, (and a quick google seemed to have a plausible explanation for how that would work) but my kiln has not been bothered, probably because it is a laboratory kiln and the elements are sealed in a refractory muffle unlike the kilns most people use for knives which have exposed elements. If you have an evenheat I would not risk it, their exposed elements are too likely to get ruined (but if you do try, a couple of lumps of hardwood charcoal works really well, and forget the fire risk, when the kiln is closed it is sealed, not really enough oxygen to support combustion) YMMV, the kiln I am building for bigger blades will also have bedded sealed elements so I don't have to worry about the problems of low budget commercial kilns like the Evenheat and the Paragon

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Stick with the foil: it just works.

Charcoal in the oven does seem to help reduce scaling on Carbon steel IME, but it is not as effective as anti scale paint. My very limited understanding of the chemistry leads me to think the charcoal approach would be even less effective on stainless than on Carbon steels.

Sunshadow, what steel were you treating?

The charcoal will give Carbon Monoxide when it burns with limited Oxygen. If you try it, be aware of this.

My oven is self built with exposed elements and using the charcoal had no noticeable effect.
 
I know it sounds good, but there are variables tat are usually missed in deciding if this will work:
1) The amount of carbon in a chunk of coal/charcoal/wood put in the kiln is pretty small compared to the amount of oxygen in the air. It will be depleted long before a HT is run.
2) The soot that will form in the oven is pure carbon. It is also conductive. I have no idea what amount it would take to be an issue, but there could be some effect on the current flow in the coils as well as shorting risks until it all combines with oxygen and becomes CO2/CO. It isn't doing any good once the carbon is gone. As the oxygen level drops, the gas formed becomes CO.
3) The gasses created are reactive. CO and CO2 both will combine with the hot kanthal elements and shorten the life. While the risk and quantity aren't that great, CO is not a good thing to put in the shop air you breathe. They also can combine with the blade metal and have some effect on carbon depletion..... How much...I don't know. But CO and CO2 are both sources of oxygen at the high temperatures in a kiln. For those who don't know it...CO is a flammable gas... and it combines with carbon. You can weld with CO and acetylene. One of the properties of CO is that it is 500% as active as O2 in combining with iron. That is why it combines so well with the hemoglobin in your blood and won't let go easily. I think the decarb could be worse in a pure CO atmosphere in the kiln. Luckily, there would be a mix of CO and CO2.....again, what ratio...I don't know.
4) Unless you seal the oven up tightly, there will be new oxygen leaking in as the chamber oxygen gets consumed. How much and how fast would be hard to determine.
5) If you don't want oxygen in the kiln, the better way to eliminate it is to replace it with something that does not combine with the carbon, the steel, or the elements....argon. Adding an argon infuser and the argon are not particularly expensive.
6) Even simpler is to remove the blade from the oxygen. On blades heated to below 1600F,m a simple blade coating will keep most of the O2 from the blade. There are several commercial HT coatings as well as clay/satanite. For blades heated to higher temps. a simple foil packet will solve the problem.

My take on this is that there is little gain and too many risk variables than it is worth for using this method. A roll of stainless HT foil will take care of the problem. Cost is about $2 per blade.
 
I am treating 1084, 1095, 15n20, O-1 and various Damascus mixes, I wish I still had access to the met lab I used to work at, as I never bothered to do a comparative decarb thickness measurement between no charcoal and with charcoal. With the hardwood lump charcoal there is no soot, and a one inch chunk lasts a few hours as it only gets fresh oxygen when blades go in or come out. The mechanism by which the CO destroys kanthal elements is that the CO being an extremely reactive reducing agent (which is how ironsmelting works, you generate CO by incomplete combustion and it strips the oxygens off of FE2O3 or FE3O4 which is the iron oxide component of most ores) Kanthal forms a protective aluminum oxide coat the first time it is heated, and the CO strips the oxygen from that. The short of it is (like with high temp salts) if you don't know what you are doing, don't try it, if you don't have a sealed element kiln, don't try it. It has cut my decarb to virtually indetectable without a microscope and metallurgical cross sectioning, but without access to a metallurgical microscope I cannot measure the difference between with or without

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Some makers put a slip of paper in the packet to "eat up" the trapped oxygen. In a properly folded and sealed foil packet, there is such a small amount of oxygen that it really isn't worth worrying about. It won't hurt, but I find no issue without it. I understand that the pro HT shops never do this.
 
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