Building a coal forge - material question

Hello all,
While my gas forge is very nice, and does the job well, I am discovering that using it is just way, way too hot for the summertime,
even with vent fans right above it- it's like pointing a jet heater at myself!
As I recently joined the local blacksmithing club, I now have a source for good coal, and I'm thinking that this may be the way to go for summertime use- from what I've observed, they tend to be quite a lot cooler than a forced-sir propane rig unless you're right next to them.
So here's the deal- I can make just about anything out of sheet goods, bar stock or tubing. I don't have a cupola (yet), so casting iron is out of the question for now. I have a very lenient employer that allows us to use materials, within reason, for personal projects- and I also have a bit of stainless steel that I bought a while back availible for whatever I need. I've got access to all sorts of industrial-grade toys for building this sucker, so that is what I'm going to do.
But, since I enjoy having the flexibility to make things that I need, I try to make sure that I use as little time and material as possible when doing government work. That's the point of this post- I already know how to overbuild just fine, but I need to figure out what I can get away with without sacrificing quality.
I will be fabricating the firepot, grate, clinker breaker, and table from stainless steel, unless there is a good reason not to do so. Commonly availible sizes range from 16ga to 7ga (.063"-.188" nom.) While thicker may be better, it represents a signifigant increase in difficulty and cut time on the laser to use a thicker material. One of the guys I work with has a bit of stainless tubing that he estimates is about as thick as 12 ga sheet metal (.105") that is about the size of a coffee can that he is willing to give me, as he has no real use for it. I am considering using this for a firepot, if it is thick enough.
But there's the trouble- there is plenty of information regarding cast iron coal forges, but I have seen very little about any made from stainless steel. What I need to figure out is what material I need to have to make decent parts that will hold up to the application.
What I have in mind, if it will work, is 12 ga tubing for the firepot, a laser-cut 7ga grate, a 12-10ga table with a 1-2" lip around the edge, and an 11 ga clinker breaker. The blower piping will most likely be black iron pipe from the hardware store, though it may be stainless or 4140 depending on what we have floating around in the back shop where we keep tubing and bar stock. Any comments on this would be welcome- I have worked with stainless steel quite a bit, but that has never included working with it at a high temperature. If I use mild steel, I can build up to .25" thick, or thicker if I make a square firepot rather than a round one (though that would require a little shopping, and would delay the project.) If the 12ga firepot is way too thin, there is a possibility that I can get it step bent from thicker material in two half circles and weld it together, but that is a reasonably signifigant investment of the brake operator's time, and I'd like to avoid it if the thinner firepot will work. In a worst-case senario, I'll track down some cast iron pipe, but I'd just as soon use the stainless. While we have some other grades, I will be using 304- that's the common stuff, and it's less expensive than some of the other materials.
We're kind of in a lull at work right now, so this is the perfect time to do this- when there is no chance that it will interfere in any way with running "real" parts.
For what it's worth, the fixation on stainless here is a result of a few different factors- first, I am fairly certain that the melting point of stainless steel is higher than that of carbon steel, though I have not looked it up. Second, my gas forge, fabricated from cold-rolled carbon steel, and painted with hi-temp barbeque paint has developed some surface rust after less than six months of use. And third, I just want to use it because it's cheap and easy for me to get (There has to be *some* compensation for living on the edge of outright poverty in a small job shop, after all...)
The hood will likely just be made from whatever galvanized sheet we've got tucked away. It doesn't seem necessary to go all-out with the stainless for that, considering the price of the stuff these days.
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Prometheus wrote:

Stainless turns to garbage under sustained high heat conditions in the presence of oxygen. If the pot is well laid out and used, it should stay fairly cool anyways.
I'd be none too thrilled about the chrome and nickel burning off into the area either. I find that I taste the stuff when polishing, grinding, or welding stainless.
My prefs would run towards mild steel plate, say 1/2", and a coat or three of a high heat ceramic paint.
Rust and corrossion are just a reality. <shrug> You can accept that it happens and get on with things, or you can do battle with it forever, and it gets in the way of getting on with things. It still rusts, though.
I like the idea of stainless for the hood better than for the pot. Lot's of stainless sheet goods ripped out of resaraunt installations and commercial kitchens, that could be drafted into the duty.
If I had to PAY for the stuff.... Gavanised is good! :-)
Cheers Trevor Jones
The forge pots I have seen were of cast iron. Cheap and durable
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After going through many branding-irons and running-irons made of plain mild steel... what was discovered was low alloy or carbon steel is the best.
Used to we'd weld on old lock shakles to make a running iron that would last a while.
Then one of the guys bought a truck load of wornout "sugar beet conveyor belt" which is made of some sort of low alloy medium carbon steel. A guy taking blacksmithing classes looking for stuff to do flatten one out for me. He said it was "tough to work". <shrug>
Good heat transfer and longer lasting by oh... 10 times over any mild steel running irons we ever used. When they get all rusted up and rough they don't work worth anything and the mild steel would get rusted-up and rough real quick.
I don't know why that is tho. :/ (I've never been into studying corrosion resistance)
The only positive experience with stainless steel has been the fact I can take a stainless steel rod (like a hotdog on a coat hanger;) and heattreat my pocket knife blades and springs with that sucker and it don't sag at all. :) As if it hadn't got yellow-orange with a weight hangin off the end of it. :)
The one I fixed up years ago, is still straight as anything. :)
Stainless steel had to to be good for something, huh? :)
Pro, as far as melting points, without looking it up, I'd bet they are so close to the same you wouldn't know the difference, since they are both based on iron. If someone can prove me wrong on that, it'd be cool to understand why they are different. ;)
"temperature specific" yeild points is way different tho. :)
Alvin in AZ
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iforgeiron.com has blueprints for building forges and a forum section if you have questions on any procedure. Instead of trying to come up with a plan from scratch why not check out what is already out there.
ron
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wrote:

I did poke through there a bit, but all I saw were two plans for brake drum forges- but that isn't to say they aren't there. There's a lot of stuff on that site. Any chance you've got a link?
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Prometheus wrote:

From a semi 5th wheel plate BP0115 Using a 55 gal drum is BP0238 and BP0133 A portable forge BP0232 two wheel rim forges BP0463 and BP0462
Those are what I found from a once over.
ron
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On Thu, 31 May 2007 06:49:05 -0500, Prometheus

One point that may, or may not, apply. At high temperatures stainless absorbs carbon, if it is available, and tends to crack. Stainless exhaust system on aircraft, for example, cracked continuously (back when I had to fix um) although it is probably that most of your forge will not get that hot.
Bruce in Bangkok (brucepaigeatgmaildotcom)
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I don't doubt they were breaking but have a real hard time believing it was from absorbing of carbon.
Stainless steel tends to expell carbon, see?
Got anything for me to read on that subject?
Alvin in AZ
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On Sun, 3 Jun 2007 04:16:45 +0000 (UTC), snipped-for-privacy@Example.com wrote:

Well, I don't want to get into seeing who can spit further but when I was in the Air Force working on reciprocating engine aircraft it was the rule that you never marked on a stainless exhaust with a lead pencil "because the metal would absorb carbon from the mark and crack". Like you, I was a non believer so I made a nice "U" shaped mark on the edge of a B-29 turbo stack. After a 10 hour flight there was a "U" shaped piece missing -- right along the line of my mark.
Magic?
Bruce in Bangkok (brucepaigeatgmaildotcom)
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Hmmm... :)
Another reason for me to not like stainless steel? :)
Alvin in AZ
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