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Hearth Operation for High Carbon Steel


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Hello Everyone,

 

I'm been very excited to read a lot of what has been written about smelting, and remenlting on here, and I want to begin someplace. But after hearing Mark Green and a few others talk about the much more controlable results in carbon control with hearth refining, I think that would be a better rounte, my goal is to melt scrap mild, old nails and things down into orishigane for use in traditional style tantos and kitchen knives. I'd like the ability to influence the carbon so I can produce jacket steel or core steel. It's my understanding that the folding and welding will burn some carbon, so starting with something like C 1.5% would result in a forged bar of .6-.8%?

 

After reading about the hearth method I think I've got some understanding, can anyone check/verify what I think I've read?

 

Size in terms of floor area only seems to be important for size of material firings, and I'm assuming charcoal size (I seem to recall a figure of approx 8-10% of diameter), Height above tuyere I'm not sure about, The photos, show only a few inches?

 

Tuyere angle in "Ancient Carburizing of Iron into steel" by Wagner siting Evenstad's research has 0 degrees from horizontal, for refining bloom into iron (slag removal and I'm guessing some decarburizing) the bottom is packed at 1 inch from the bottom of the tuyere. For the production of steel, it's packed at 2" from the tuyere.

 

The deeper the floor under the tuyere, the more carbon uptake, up to a point where temperature drops below a certain amount? But that would start to result in cast iron rather than the desired outcome, right? There are of course a lot of variables other than that, but if air volume/pressure, charcoal size remained the same would that generally be what happens?

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Hi Trevor,
Hearth-refining/ re-melting takes some practice, just like anything.

 

When I started, I just went straight off of the Evenstad papers. It all seemed to make sense to me, and I have been very lucky in my success.
But, I was mostly working with slaggy bloom pieces, and once processed bloom iron. This material is IMHO, easier the using modern iron and steel.

I have tried the shallow hearth methods using modern/old mild steel, and found that it worked better if I added some sand/slag to the process.

I use the flat tuyere angle, and the floor height adjustments. I do not pile the charcoal much over the bar/billet/bloom I'm trying to melt, as I like to keep track of it, and turn it in the air flow to melt better.

3-4 inch drop is the max I use. 2 in. was never deep enough for me. If it needs more carbon, or further refinement, I will just run it through again.

I use a air mattress blower for my air, with a variac device, usually set at about 45-50%.
1 in. of smaller charcoal is best.
I assume you have seen all my Picasa pages on this??
If not: https://picasaweb.google.com/106800196895572422821

Have big fun!!


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Hi Mark, Just who I was hoping to hear from. Yes I've looked through your Picasa pages, the photo essay's were a great overview of what Evenstad was talking about, but there were a few more details that your response helps point me to the right direction.

 

I don't have any bloom to work with, what about wrought? I have some of the old Globe elevator. But maybe that is best saved for some san mai or something of that nature.

 

Time to get on with the clay digging!

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This is a great thread. I've also looked through Mark's picasa page and dimensions like depth while discussed haven't really been defined other than those similar to the evenstad pages. I've been considering using scrap in a hearth for adding some extra dimension to my work. Not that I've actually forged a knife in almost 10 years. It's now back on the list of upcoming projects.

 

Thank you to Trevor for asking the question, and Mark for your ongoing willingness to share your experience.

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Wrought will work just fine. The process will separate most of the excess slag out of the wrought if done right. And, carburize your steel, if you want it to.

 

It will take practice. Just like anything. :)

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You got lots of good advice here. I would add that the major controls for carbon content, beyond hearth depth, are overall temperature, and most importantly, the length of time you you cook the ingot after you've melted everything.
What I do is after I've added all my charge, I feel the top of the "bloom" with a rod. It will have all kinds of peaks and lumps on it. Then all the sudden it will flatten out- the carbon content has matched the temperature so that the outside starts to melt. That's when I pull it, or you could give it a litte tine if you want higher carbon. But once the surface goes liquid, the carbon moves around really fast- if you wait too long you'll have a puddle of cast iron.

Unlike Mark, I like a little bit of angle in my tuyere, just enough to even out the heat across the hearth. I find with a flat tuyere, I end up with the hot spot really close to the tuyere, and it chokes up too soon. Hearth shape probably affects this.

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Great advice Lee!! That IS very important.

The air flow time, is one of those "practice things" :) It will take a few tries to figure that part out.

 



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What if you are starting with cast iron?

I am going to try my hand at this soon. I got bit by the steel making bug at Shards.

I plan on posting pics of my process so feel free to tell me what I am doing wrong! :lol:

-Gabriel

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Gabriel, Cast to steel is tricky at best. You may need to try that a few (dozen) times before you get it right. Or not.

 

It can be done. Have some fine sand on hand to help flux it.

For myself, and others, this operation was quite a challenge. But we were using cast made from smelting ore. mostly. :)

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if I am understanding correctly... and please excuse my ignorance in advance...

Keep the tuyere close to the floor because I need to burn off the extra carbon? pile my cast iron on top and feed charcoal till it is sticking but not melted?

-Gabriel

 

again... please excuse my ignorance. I learn best by doing it and figuring it out as I go.

Edited by grpaavola
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When melting in the hearth, cast can be very tricky.

 

Right before it melts, it becomes much like cottage cheese. Then it will suddenly melt. I sometime used crushed fine slag as a flux for this. That seems to help add some slag back to it, making it easier to work, but have done it both ways. Too much slag or sand, and it will protect the melt too much from the wind.
I have always kept the floor high for this, (more like 2in.) and let the air stay on the melt for a few. The air on the melt, should blow out some of the carbon. If it was sparking good when it was melting, that was likely the carbon burning off as well.
It's not easy. At least for me it wasn't. I think I tried 5-6 times before I got it to work right. Then, it's just remembering what you did. :)
Your just going to have to play. It is the only way. I was just guessing at it most of the time.
There are other ways to forge decarb cast, but I have only tried a few times, and haven't had great luck. It seemed a lot harder then the melt. I've been meaning to try that some more, but haven't made any cast iron in the smelter for some time now. But that is OK on it's own account. :)

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Unlike Mark, I like a little bit of angle in my tuyere, just enough to even out the heat across the hearth. I find with a flat tuyere, I end up with the hot spot really close to the tuyere, and it chokes up too soon. Hearth shape probably affects this.

 

If you have angle to the tuyere, that would mean that the depth of the melt has to increase a little bit to be out of the oxidizing zone right?

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For the cast iron, I have had poor luck relying on the blast only. By adding iron oxide, in the form of bloomery slag, or iron ore, or hammerscale, to the melted cast, you can decarburize.
FeC + FeO > 2 Fe + CO

When it turns from a liquid puddle to a solidish bloom, you're there.

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  • 4 weeks later...

I've got a clay mix sitting to hydrate, but have one more question, what diameter do you have on the inside? I scratched out plans for a 12" diameter x 5" deep hearth, maybe a little deeper for more ash bed, does that seem right?

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Yeah, that's about right. I built mine a 10" x 12" oval, the tuyere going in along the long axis. But i just did that because that's what the Evenstad measurements seemed to convert to, I doubt the oval probably matters too

much.

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