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What happens to refractory if it gets too Hot?


tlreif

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In conversing on another thread it was pointed out that refractory board was not rated for high enough temerature. My question is how hot is hot enough,(Technically 2300 degrees is forge welding heat. and I have seen it done colder than that) and what happens to the refractory if the rated temps are exeeded? Do we have a nuclear meltdown that goes to China or what? Seriously though, what does happen? I know the refractory board i use in my forges has seen 2500 degrees and I have not seen any difference in it.

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For the fiber board and wool, it'll shrink and lose some insulating ability. If you push it far enough, eventually it'll melt. But yes, refractories do have built in safety margins. They don't fail catastrophically the moment you exceed the recommended operating temp by a few degrees.

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I use 2 off 1" layers of fibre board. It's a 3 off 3/4" burner forge and the unsupported area is about 9" by 24". The board is rated at 2300 deg. It 's stood up pretty well over a few years. The inner layer has a few cracks in it running through the holes for the burners but it's still holding together. The top layer is just fine. I don't do much if any forge welding but I do run it very hot at times.

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Come on guys, If it's rated at 2300 Degrees do ya realy think it's worldsend
At 2310 degees? D__m Son If your car goes over 55Mph are ya gonna explode?
lETS GET REAL HERE. If ya have a 1 burner forge that allways gets to welding heat
Let us know.A 2 BURNER REACHING WELDING TEMPS LET US KNOW. I/WE NEEd TO KNOW.
forgeing temp is good welding is great.
Ken.

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In case my first post wasn't clear, let me make it so: exceeding the recommended temp a bit will make wool and board shrink a little and lose a little insulating value. If you push them very far, they can melt. I know this has happened to some people who tried to use unprotected wool for crucible steel furnaces. But they were dealing with temps significantly hotter than welding heat. It's not likely to be a big problem in a normal forge.

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Since propane burns at about 3100F over firing the insulation is easy. I have melted a hole in my firebrick floor. I'll replace the brick before it goes all the way though. The wool seems to have shrunk some, but not enough to affect its position. I put it in rather tight to start with, and the wrinkles are gone now.

Phil

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In conversing on another thread it was pointed out that refractory board was not rated for high enough temerature. My question is how hot is hot enough,(Technically 2300 degrees is forge welding heat. and I have seen it done colder than that) and what happens to the refractory if the rated temps are exeeded? Do we have a nuclear meltdown that goes to China or what? Seriously though, what does happen? I know the refractory board i use in my forges has seen 2500 degrees and I have not seen any difference in it.


I use a very small forge and run it at welding heat all the time. Over time, I've trashed a lot of refractory materials.

Refractory temperature ratings are primarily based on their mechanical properties. When shrinkage, slump, distortion etc exceed acceptable limits, usually a few percent, then its above its max working temp. Often these small changes in shape, cause cracking when it cools. If you continue to heat it past that point things get progressively worse and finaly it will melt. 3100F is the theoretical combustion temperature for propane under ideal conditions. You are unlikely to get that in a forge but at welding heat, there will be regions above 2400.

Temperature alone, is not the only issue. Some refractories become increasing vulnerable to oxidizing or reducing atmospheres as the temperature rises.

Kaowool is very vulnerable to oxygen at high temps. If you dont coat it, or if the combustion gases find their way into the lining this will trash it quickly. But if its protected it seems to do fine even at welding heat. I have some Inswool rated 3000F. If the combustion gas gets to it, it dies just as fast as the regular stuff. One reason to use rigidizer is that makes the material less permeable and helps to prevent this.

I bought a kiln shelf yesterday in the pottery supply store. These are used all the time as forge floors and are exposed to welding heat. I was surprised to learn it was only rated "cone 10" just below 2400F. Its essentially mullite which holds together well past 3000F. Then I realized that potters make stacks of these shelves in their kilns supporting them with small blocks, so slumping is an issue. In a forge the whole shelf is supported and it can be used at higher temps.
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Refractory is typically rated for it's "working temp" not it's max temp. Hard refractories tend to heat check from thermal shock.

I've had less than happy results using the Kaowool board but it worked. Kiln shelf is a LOT more temp durable.

Any base at near welding temps will dissolve silicate refractories. alumina refractories are a lot more resistant and typically higher temp. Phosphate or phosphate bonded refractories tend to be VERY high temp and hot caustics have no effect.

Frosty the Lucky.

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