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Guest Tai

forge insulation

33 posts in this topic

Hi!

I've started building a small gas forge using ceramic fiber for insulation.

Ceramic fiber is quite fragile so many people recommend coating it with ITC100 to protect it and improved insulation.

I was wondering if you could coat the ceramic with a layer of refractory cement ( let say 1/2 inch) to make it solid, then with itc100? Or even just with cement?

 

Would this cause problems to reach welding temperature? Do you think the cement would stick well to the fiber?

Thank's!

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Part of the benefit from any ceramic fiber is the "loft" in the insulation.  The dead air space inside the wool is what gives it the insulating ability.  Putting something heavy, as in refractory castible or rammable, together with the wool will collapse the air space and negate the insulating quality of the fiber material.  

 

It's even a risk to put the ITC100 or similar compounds directly on the wool for the same reason.  Although that would be a lighter coating and less prone to air cell collapse, I'd think the effect would be the same.  Talking to one of the local refractory experts, the infrared compounds work a little better, in her opinion, when painted on hard refractory.  I think if I was going to use soft walls, I'd coat one layer of kaowool with ITC or Macote and have one full lofted thickness layer behind that.  

 

Up to this point in my shop I've used only hard cast refractory insulators and hot face.  I think the ideal forge (of horizontal tube type) would have a hard castible floor and insulating wool upper.  It would be more complex in assembly that way than casting tubes.  

 

For simplicity, the next one I put together will likely be a castible tube, inside a light steel shell with the wool blanket draped over the top.  I think that combination is the ultimate in simplicity minimizing potential damage to the expensive wool.  But, exposed wool loft also degrades over time due to the accumulation of shop dirt.  It seems less so than burning it up in the forge.  

 

The times when other smiths have brought their new baby forges to my shop, or to local guild meetings, for show and tell, the majority of the time they don't get to welding heat is because of the lack of effective insulation.  The radiant loss through the side walls exceeds the forge's ability to retain heat.

 

Although I've read about Don's vertical forge, I am not qualified to comment on anyone's experience with that design.  Their comments would add much to this discussion.

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Mike,

 

Don't put the wool outside of the metal. It traps the heat and can melt the can.

 

Are you only using hard castible for your forges, no insulation?

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I think it works best to saturate the wool rather than coating it. I make a thin slip and work the wool through it, then ring out the excess. The inside surfaces automatically glaze over when fired, creating a thin hard inner shell that won't sag at welding heat.

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Thanks for the advices!

 

The hard floor is a good idea,Thinking of casting a piece of refractory cement that follows the curve of the forge trapping the flux at it's bottom. The rest being wool.

 

Tai, your post made me think ot high temperature clay. I have some grey/blue clay around where I live. Do you know what type of clay resist the best real high temps? I've heard of kaolin, but can't remember if it was in relation to high firing clay, any idea??

 

Thank's for the help!

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Don,

 

Oh no, I'm a very big fan of insulation.  Howard helped me build the big burner I've used since 1988 or so.  It's based on a Pendray style blown noisy dragon's breath type.  14 inch diameter 3/16 inch thick cold water casing with 3 inches of 2700 degree castible insulator next to the metal jacket and 3 inches of castible hot face on the inside.  About 8 inches of usable tube, but over the past six months things are sagging and bubbling a bit.  I'm getting my mind around building something new with venturis.

 

Still, it's been a workhorse.  It takes about 30 minutes to get to welding heat.  The thermal mass is what I like.  I can throw a cold ten-fifteen pound billet into the fire whole and not pull the heat down very much at all.  

 

I used to have some old kaowool blanket laying over the top to trap more heat.  I haven't noticed any change in the outer casing.  If I take it apart, I'll look for degradation.

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Pendray wrapped one of his melting furnaces with wool and he said it melted the can and it ran out on the floor.

 

I think perhaps he had a hot spot on the can and covered it up. With the cast insulation that you have on your forge the outside temps should never get that hot.

 

I have gone to a flux and slag resistant ramable from Harbison-Walker called Ruby AMC. They patch the ladles with it in the steel industry. You cut off slices from the clay block and fashion them into position. There is no drying time, fire it and you have a super hard, flux resistant container.

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Mike, could you give some details on how you did the double lining for your forge? I was thinking of relining my bucket forge, or making a larger, more evenly heating one for heat treating.  I'm using homemade castable, 1 part portland cement, one part sand, one part horticultural perlite, basically perlite concrete, and it's pretty tough stuff.  My thought was that I could adjust the quantity of perlite; higher for a weaker but better insulating mix, lower for a tough inner layer.

 

Cheers,

 

David

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The ramable is premixed and comes wrapped in a plastic bag, but once you open the bag it can dry out. The best way to use it is to share it with someone. One block is enough to do several forges.

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The hard iron shell is outside.  Purchase some high alumina refractory insulator 2700 degrees and hot face.  

 

Then we took some cheap tin air duct tubing, cut off the funky sheet metal lock, adjusted it to the right diameter for a three inch wall thickness and fixed it to itself with pop rivets, set it in place and slopped in the refractory.  Vibrated the wet material to get out the most air bubbles.  The repeated the process for the hot face lining (smaller diameter).  We used styrofoam sheet to keep the right air holes where they should be.  

 

Once the castible was hard we burned out the styrofoam with a torch.  It melts right away.  The AP Green stuff we used required two heats to "set" the material.  It was a long time ago and Howard keep this sort of trivia in his head.  I think the first heat was just to warm it up to about 400 degrees.  The second heat took it up to about 1000 degrees.  Once things cooled down and the burner hardware was put together she fired up.

 

I use firebrick to adjust the size of the front opening.  Plus I can move the brick to form a slot forge for general iron work on small spots.  There is a small hole in the back for long stuff.

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Yeah, but does it get hot? :o

It looks like you could melt fire in that thing! The firebrick to make an adjustable front is a great idea. Time to build another forge...I'm jealous!

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:D

 

It gets hot enough to melt the firebrick doors and the refractory inside gets very soft and runny sometimes when I'm working on wrought iron/nickel welding.  

 

The burner is a stainless nozzle.  It's oxidized over the years and the oxides have drooped over the burner opening to interfere with the fire.  

 

This picture shows that the burner chamber is a trapezoid.  There is a one inch thick space in the center that spreads the fire out like the paint burner flares used on propane torches.  The fire comes in on a tangent to the circumference.  

 

My first real shop had a wood floor.  One of the first times I had this thing up to welding heat I kept smelling wood burning.  I'm pretty careful.  I looked all around for tell tale smoke and couldn't see anything.  The smell kept persisting.  

 

What happened was the borax flux was running out the small back hole and solidifying into glass.  Like a hot stalagtite it kept getting longer and longer until it finally reached the floor and warmed up the wood.  I quit working to look for the source of the heat, the glass cools, no more burn.  I'd spray the floor with water to make sure nothing was on fire.  I'd go back to work, more glass, more heat etc.  I chased this thing for two days until I finally looked behind the fire and saw the glass thread.  Strange things happen to smiths.  

 

Things do get hot enough to run out of this firepot.  It uses propane fairly cheaply, but I'm sure its not as cost effective as the venturi type forges.  Those seem downright miserly.  

 

For all the work I've got done with this forge, I'm ever grateful to Alfred and Howard for their ideas about how a forge should work.  

 

And, it's a very cool fire to just sit and stare into....

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hey, this is just a thing i have done to put ITC100 or stainite in a forge.. i make a thin mix and spray in in the forge with cheap sand blaster..just blast it on the kowool,,

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I bought a used NC low boy which had board for insulation with a cast floor.The board in the top had cracked badly and flux had all but gotten the floor.I completely gutted the shell and relined it with a rammable .It took about 80 lb. of the 100lb. box of clay. It's a bit slower to get to temp but has no problem getting welding heat.I actually can back off the heat when it's up to full heat.The outer shell gets hotter than before but not red hot.the only problem this creates is the outer shell is rusting bad after it cools.I have noticed the rammable I use is a bit corrosive.Not sure why.Rammably will trap moisture between the wall and refractory when curing and needs to be fired slowly to prevent a steam explosion.I built a wood fire to cure mine.before firing .After it had cured good I took some more of the clay and made a thinner slurry and painted the inner wall .it exposed some of the gravel and seems to reflect more heat now.I love it ,It's tough and the only drawback I see is it's heavy making a portable a little more weighty.My next shell whether a box or pipe will be stainless.Don,do you pack the clay any ?

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Thanks for the details Mike.  I'm unclear on how the burner/nozzle is constructed, i.e., what is inside the sticky-out trapezoid bit?  Any details would be welcome.

 

thanks,

 

David.

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The trapezoid appears to be a "pre-burner" to insure complete mixing and combustion.

 

Are you piping in propane through a needle valve or a standard gate valve?

 

On castable refractories, I have been told that the higher the temp. rating the more durable the material, but the lower the insulating capacity. IOW, the high alumina is rated to 3,000 degrees and is nearly buttet proof, but it sucks up a lot of heat and gives very little back. A standard castable rated to 2,300 degrees insulates much better, but it is somewhat fragile (not like kaowool, but relative to the high alumina). 2,500-2,700 is a compromise in terms of durability and insulation--Does this concur with other peoples' understanding of castable refractories?

 

Thanks,

 

John

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EVERYBODY READ THIS PART:  I wear a No 5 welding spectacle to see into the fire when at welding heat.  The spectacles also allow me to see the flux transformations and flow along the surfaces of the billets.  But that's not the main reason.  Safety.  I highly recommend welding glasses of some type for all who work in the welding fire.  UV light, especially that concentrated type, will induce and accelerate the formation of cataracts in the eye.  And, I've had the fire spit loose pieces of flux/scale out.  Everytime one of those little meteors hits my hair or beard, I say a prayer of thanks that I adopted this practice early and don't violate my rules about it very often.  One of those in the eye, and...the eye doctor makes several boat payments.  

 

The picture contains all the hardware parts.  The blower attaches to 2 inch galv pipe up to a 90 angle.  The gas is piped in through a needle valve to 1/4 inch galv.  The LP flows straight in with the air mix hitting the corner creating turbulence.  

 

I had a 0-15 lb redhead regulator attached directly to the tank and ran it that way for at least 14 years.  After playing with some Randal Graham-designed venturis, I switched to a 0-50 lb redhead at tank pressure.  It required a new learning curve for gas flow.  I don't have to crack the needle valve very much to get way more gas than I was used to.  The higher pressure available makes the venturis much more efficient.  The big blown forge had enough static air pressure that it didn't require as much gas pressure.  

 

The stainless nozzle is, if memory serves, a 1.5 inch 300 series stainless ID tube forged down to 7/8 inch.  Just a hole, nothing special, but the narrowing increases the exit speed of the gas/air mix.  

 

The trapezoid is a "fan" that creates a spread in the gas flow and is exactly that, a pre-burner or flame diffuser.  The trapezoid is castible hot face-refractory lined.  We left a one inch styrofoam board in the middle when casting so the burner chamber is flat.  It's one inch thick and spreads in a fan shape from the mouth of the gas nozzle (narrower) to the point where it meets the tangent hole (wider) into the main body of the refractory tube that is the forge itself.  During run up, the flame front is across the entire mouth of the tanget.  When at heat, the only flame is from the mouth of the forge as you see in the picture, inside is just plasma.  

 

As to the temp range of refractory, it's what was available given our knowledge at the time.  I've been satisfied with the performance of the forge to this point, and still use it.  It's warming up even as I type this.  However, I'm no refractory expert.  If somebody can come up with improved refractory performance data, I'm all ears.  The next generation of forge I build will have some 3000 degree insulator and 2800 degree hot face.  I'd like to move into crucible casting like Poindexter so I can experiment with specific alloy mixtures of steel.  

 

I make no claim about perfect insulation.  After a day's work, the forge casing is plenty hot.  It's not red, but i don't leave anything on top except stuff that will take heat.  I do use the case to prewarm billets before their turn in the real fire.  After I shut down, it will still be warm to the touch the next morning.

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Thanks Mike.  I've just finished pouring a new forge (on a smaller scale) inspired by yours.  I'm using a side-arm venturi burner, with a Y-shaped flame distributor, in the hope of getting an even heat over a twelve inch forge length.  I went with a Y to break up the flame and protect the metal from the direct heat of the burner.  Who knows, it might even work...

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With a cylindrical forge and a burner entering on a tangent, is uneven heat a problem? Tim Zowada's design has a single burner going straight in and the vortex created seems to take care of heat distribution automatically.

 

John

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Tim Zowada's forges are the first I saw that used a tangential burn. It does burn more evenly and efficiently than a straight in burner.

 

Some commercial horizontal forges with the burners coming straight down from the top the flame will not reach complete combustion until the flame has bounced up off the floor. It will leave a dark spot as it is heating up. Working in that type of fire is like working inside the cone on a torch, it is not the hottest nor the cleanest part of the flame.

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I think that this may depend on the type of burner.  The venturi I am using produces a tight, contained flame, wheras I get the impression that blown burners produce a wider, 'softer' flame, although I've never seen a blown burner working outside a forge.  If the gasses are exiting a burner at a lower velocity, my guess would be that they have more time to spread out and billow about before loosing some of their energy to the general forge environment.  Also, there is a lower limit to the space you can heat with a single burner when using venturis- if you want to heat a longer length, you have to reduce the diameter of the chamber, which will bring the burner closer to metal being heated, again increasing the difference between the heat close the burner, and the ends of the forge.  I figured that by using a tangental flame distributor type arrangement I can keep the overall volume of the forge small, and therefore more efficent and economical.  I'll let you know how it works.

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I have an old two burner venturi type that I use for general iron work.  It works like two propane torches.  The result is two hot spots in the bottom and inefficient gas burnage.    

 

The tubular forge still has hot spots, but they tend to be the top of the forge and radiate heat downward to the floor.  The steel sits on the bottom while warming.  I tend to "cook" while working the steel, eg. I move and turn the billet onto a hotter area of the floor.  There is a flame shadow if the billet is big enough.  The swirl is blocked by a thick billet and the lee side is darker.  

 

I agree with David and cannot recommend leaving any knife quality steel in direct flame.  I've smelted billets or parts thereof doing that.  I'm sure that the localized heating also cooks carbon away faster and grain growth problems abound.  

 

Ron Reil and colleagues started really improving designs.  There were reports that his burners are capable of melting refractory.  This was alleged to be true if direct flame from these things was allowed to hit the wall of the forge.  The flame entry angle, length of flame front and flame flare or concentration all have to be considered in forge design.  

 

I hate to think I'm using a functional antique.

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Following a few false starts, I fired up my burner tonight. It is based on Tim Zowada's design as presented many years ago in "The Blade Magazine."

 

It was a bit weird trying to "tune" it in the open air. I blew it out a couple times, sort of like an oxy-acetylene torch (the design is essentially an oxy-propane torch). I can't help but think it will be a bit easier to control inside a forge, and with a lot of practice.

 

The gate valves for air and propane are also new to me. The adjustment for air seems much more precise than a sheet metal flap, while the one for propane seems a bit less precise than a needle valve.

 

Now, to make a forge body....

 

John

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