Insulation Cracking

[Belz]

Hi All,

After trawling through many a post on these forums, I came to the conclusion that what I would like to know may be so deep, that I may not find it.
Hence, my first post.

A little background on the forge I created. Much of which, I took from these forums.
It is made from 10mm thick steel pipe, 10" I.D. I used a refractory glue to adhere the 1400^oC fibre blanket to the inside of the pipe, and a second layer of insulation bonded to the first. Following this, I washed the blanket with Kaowool hardener (I used quite a bit to get some strength into the blanket), and once it was fully dried, I applied a Blakite (http://www.morganthermalceramics.com/sites/default/files/datasheets/1_jm2600blakiteblakitevjm3300.pdf) wash over this, and gradually built up the thickness, layer by layer once each previous one had dried. I left the final coat to dry for a good few weeks (in Aus weather at that). 
I have some ITC-100 analogue material to wash over the top of that, but, as it always seems to happen... I fired up the forge prior to applying it, to get an idea of the improvement in heating times once I actually did apply it.
After the first firing, I noticed some cracking in my blakite layer, very straight, running perpendicular to the curvature of the blanket. Second firing, it has worsened. Not significantly, but nonetheless, I want to reduce/eliminate it.

My question for anyone with the knowledge (I know some of you will have the knowledge), is firstly, how should I address the cracking in this current forge? What material and how should I apply it, to eliminate it? 
I am always wary of those nasty fibre blanket fibres, and want to reduce the possibility of them being airborne as much as possible. (I know that the hardener should have done that, but I tend to be over careful when my lungs are concerned)

Also, this current forge heats well even without the ITC-100 equivalent, admittedly. However, I believe it is likely on the higher volume side of a 3/4" T-Rex burner, at ~500 cubic In. for forge welding (I haven't cranked the PSI above 10 yet, mostly running at about 5, with an idle circuit dropping to who knows what -> pressure gauge post idle needed).
As such, I will be gathering some more pipe and likely be making a smaller volume forge, at about 300 cubic In. (designed in Sketchup). Given this, can anyone suggest / help me out with a similar layering to that which I did previously, with tips to eliminate the cracking etc?

I can follow up with pictures if necessary.

Thank you,

D.

[Michael Cochran]

I think pictures could definitely be helpful. I haven't had any cracking in my homemade gas forge (a one gallon paint can) until I dropped it one day after I got done with it. The creaking I have would be fixed easily enough with a couple more layers of the satanite coating I put on it. Mine are just hairline cracks but if the cracks you have are bigger I don't know.

[Michael Cochran]

I think pictures could definitely be helpful. I haven't had any cracking in my homemade gas forge (a one gallon paint can) until I dropped it one day after I got done with it. The creaking I have would be fixed easily enough with a couple more layers of the satanite coating I put on it. Mine are just hairline cracks but if the cracks you have are bigger I don't know.

[Belz]

I will follow up with pictures later, but as you said, the cracks which have formed are also only hairline as well. 

So, it is inevitable for hairline cracks to form, and just to apply coats over them? This is the first I have made, so I didn't/don't know what to expect from the shell.

[Frosty]

Welcome aboard, glad to have you. I also commend you for reading about the subject before building and asking for help. Pictures would help a lot.

However, your description is pretty thorough and as is common for newcomers I feel you're overthought your forge build considerably. The shell doesn't need to do anything but hold the ceramic blanket in position, keep fibers from escaping to the air and support your work. 10mm. steel pipe is WAY overkill unless you're heating dozer axles and such. Common over build, my first pipe forge is made from the same weight pipe, 3/8" wall x 10". WAY too heavy.

I really like stainless steel stove pipe. Stainless reflects heat much better than most metal pipes, it can be worked with hand tools and joinery with sheet metal screws or pop rivets is perfect.

Cementing the ceramic blanket to the steel shell is not only unnecessary it leads to more work later. ceramic blanket IS a wear item, it IS going to degrade and need replacement. You aren't going to be able to wash it or plaster it and prevent degradation.

If you cut the blanket just a bit larger than necessary to fit the ID of the shell same for the second layer to the outer layer compression will hold them in place. The inside layer needs a hard floor or just placing work on it will tear it to shreds very quickly. Good floor materials are: high temperature split fire brick or ceramic kiln shelf. they are reasonably robust and can take a beating from every day use at high temperatures.

Now I get to the heart of your question, cracking. What you over did this time is trying to plaster the inside of your forge. The contact layer, (Fire contact) is going to degrade the best you can do is slow it, you can't make in invulnerable. Well, not at a price we can afford anyway. The thicker the you plaster the inside the more inevitable heat checking (cracking from heat cycling) is. Even fire brick heat checks over time.

You can make it last longer with a kiln wash. A stiffener or hardener for the ceramic blanket will help it survive being poked scraped and gouged by the work. A high infra red albedo wash will increase it's heat reflectivity for in some cases a hotter interior in most cases better fuel economy.

If you forge weld the popular fluxes are caustic at welding temperatures and WILL dissolve silicates. Unprotected ceramic blankets will dissolve when touched by molten flux like hot water on cotton candy. Hot flux will dissolve holes in fire brick, hard or soft as it will in kiln shelf. Unprotected kiln shelf will last longer as it is a high alumina refractory. A good kiln wash will help protect the forge interior from flux but nothing will be 100%.

The key to minimizing frustration is to accept the fire contact surfaces in your forge WILL wear out and make them easy to replace. That's why I recommend simple compression to hold the contact layer in place against the hard refractory floor. Split hard fire brick last quite a while even with hot flux on it and you can just cut the blanket inner liner so the brick rests on the outer layer and tuck the contact liner in. Kiln shelf can lay directly on the inner liner as it's pretty thin.

After you've fitted the inner contact liner wash it all. First with hardener/stiffener and let it dry. Read the directions and wet the liner and shelf BEFORE applying the stiffener. The masons here will know what "Buttering" means but I'll explain for those who aren't. If you apply a cement, plaster, hardener, stiffener, etc. to a DRY surface the surface will draw the moisture from the wash where they contact. This dry layer prevents it from bonding as there's a dry layer of wash acting like a release agent. Wetting the surface first allows full contact and the wash will soak into the surface or fibers and will bond as it dries.

ITC-100 is a high zirconium silicate kiln wash, has a high IR reflectivity and is pretty robust where flux is concerned. Unfortunately our forges do NOT get hot enough to fire the kaolin matrix component let alone the zirconia.

I used a phosphate bonded 4,000f rammable refractory that's no longer available in my first pipe forge. the stuff is designed for a working temperature of 4,000f in an ammonia atmosphere. I believe 4,000f ammonia vapor would go through normal refractory like a Star Trek Phaser through Klingon toilet paper. Regardless that forge heat checked even though it's still pretty impervious to molten flux after more than 30 years.

I hope that long winded post is more help than confusing.

Frosty The Lucky.

 

[DanielC]

I get a 3500F castable refractory that has a high resistence to flux. I lined my new forge in it, but haven't made plates out of the same material yet to test it out some molten flux.

One of my close friends gets it straight from the ceramic company who makes it, and they don't usually sell to the public that I know of. My friend just got lucky.

 

Wit rather high temps it is shrunk in a few spots. But only where I places new refractory on top of or blended into already day dried refractory. Haven't had it crack anywhere else yet.

[Latticino]

Please be aware that most high alumina castable refractory (that is resistant to flux) is very dense and does not work well as an insulator.  So you will certainly need a  good insulating layer behind the refractory if you want to have any level of efficiency in firing your forge.  As an interior liner, like used by DanielC it should work well.

It is also somewhat subject to thermal shock , which it will experience given rapid thermal cycling.  This propensity to cracking under thermal stress gets worse the thicker the layer of refractory you install.  Note also that the coefficents of thermal expansion between the castable refractory and insulation tend to vary, so a layer of high temperature glass fiber insulation between the two is a good idea (unless your insulation is the glass fiber blanket...).  I have in the past added stainless steel needles to the refractory mix to help with this, though any exposed to the surface burn out and it is a real bear to mix up the refractory with those needles.  As far as I know these high temperature refractories were designed for forges, kilns and furnaces that operate continuously for long periods of time and users slowly heat them up and cool them down.

 

 

 

[Belz]

Thank you all for the replies, it is much appreciated.

As for the 10mm walled pipe, yes, definitely heavy and definitely overkill. From that I have learnt not to force buy pipe, especially after using a metal hole saw to painstakingly cut out the burner inlet hole (10 minutes later). It is one of the reasons for contemplating another build to be honest (I would say its about 20kg). That, and to save fuel.
Stainless sounds like the way to go, given the pipe I have stays quite hot a LONG time after shutdown (again, likely due to the wall thickness retaining heat).
What wall thickness would you recommend for the stainless?

A kiln shelf would help, as at the moment I have the piece laying mid air in the forge, which does give even heating, but once I get to forge welding (eventually) I will need some floor protection. Luckily in QLD there are plenty of pottery supply stores, and that isn't a problem.
I'm starting to think this current build will be reserved for larger pieces, not needing to be forge welded. Again, beginners big eyes got the better of me.

I didn't wet the surface before applying the hardener, which I will do next time. The explanation was great too, people say that you should wet the surface prior, but not why. Give people the details, let them learn.
You said that the kiln shelf can sit on the inner liner. Should this be pushed down flat against the inner liner to form it somewhat, then remove it, wet and use the hardener on the flattened inner liner, then put the kiln shelf back in and flatten it again to form fit it, while it dries?
After the hardener has completely dried, should the Blakite be used thinly, as a kiln wash? Do I need to re-wet the surface prior to applying it? 
Also, prior to applying the ITC-100, do I need to re-wet the surface?

The Castable Refractory sounds interesting, but if not dried correctly that could also crack, yes? Does a monolithic castable refractory, when dried correctly, have thermal checking as well, seeing as it would only be a single material with the same thermal expansion/contraction properties?
I understand that hobby forges are put under a lot more thermal stress than commercial, due to the ability for them to reduce the temperature gradually. Unfortunately a downfall of these forges huh.

Okay, now for a left hand turn. 
What do people think of soft firebrick lined forges, such as the design attached? Soft firebricks cut at the correct angles, blakite mortar and IR coating? I've seen the whole, "brick pile forge" thing, but I understand that circular forges heat better than square due to the vortex effect. Just wondering about the lifespan of such a forge, and the heating qualities when compared to blanket lined forges. 
Has there been any comparison of maximum heat capacity/heating times between two forges of same internal volume, lined with brick vs kaowool?
It would be interesting to do such an experiment (Sorry, scientist coming out in me there).

I'm glad I posted in this forum, you guys are very helpful.

Thank you,

D.

[Latticino]

Did a softbrick forge just like you have pictured for my first construction.  It lasted through only 3 firings, but that was probably because I used older softbrick that may not have been rated for the elevated temperatures (I'm not completely certain that any softbrick is available that is rated for over 2,300 deg F).  It started melting opposite the burner flare.  Softbrick also has a bit more thermal mass than the insulation blanket, so will take longer to run up to heat, but will hold that heat a little better.  As far as I know, softbrick is also prone to damage from contact with hot flux, though not as much as the glass blanket.

If you are a scientist it is pretty easy to calculate the relative heat flux out the sides of your forge with a variety of insulating methods/materials.  All you need to know is the U value for the material, the dimensions of the forge and the heating capacity of the burner.  To get the actual heating characteristic is a bit more complicated due to door openings and flue losses, but the relative proportion between two insulators can be addressed without too much trouble.

Monolithic refractory castings are pretty strong, when cast properly and not subject to  extreme thermal cycling.  As I mentioned before the refractory is a lousy insulator, and thickness matters a lot.  There are also castable insulation materials, but they don't expand and contract the same as the refractory typically.That is why I recommended the intervening layer of glass fiber if you go that route.  You would also be able to use a cheaper castable insulation.

IF I were making an industrial forge for major production of pattern welded billets I would look into getting a section of high alumina precast tubing for an inner liner.  Then cast a larger ring out of insulating castable with a 3" gap on the interior of this new ring from the outside of the precast tubing.  I would pack the interstitial space with high temperature blanket and go to town.  The only difficulty would be to make a burner port in the precast, but that could be drilled out with either a diamond hole cutter or the old silicon carbide slurry and rotating steel tube method.  Something like that should last for years if properly used.

Otherwise realize that a forge interior is also a consumable and run with it.  Small cracks in your inner liner are not a bid deal provided molten flux doesn't find a way to enter same.

Does a monolithic castable refractory, when dried correctly, have thermal checking as well, seeing as it would only be a single material with the same thermal expansion/contraction properties?

Yes it can, depending on the thickness of the material.  Even though the castable refractory is not a good insulator, on thicker sections it is possible to have a significant difference between the temperature at the interior of the material and on the outer walls.  Depending on the material, the thickness the configuration of the casting and the heating/cooling rate some pretty significant stresses can be evident.  Check with the material supplier for recommended firing/cooling rates.

[Belz]

Yep, that was what I was thinking would happen with the softbrick. Again, back to the "consumable" material thing huh.

That being estimations of course, based on approximations. Real data would be great to have, although, time/money consuming I know. One day I will compare the two.

Your idea with the two castables and glass middle sounds interesting. I wonder how thick the interstitial glass layer actually needs to be, given the thermal expansion/contraction of the two wouldn't be more than 10mm at maximum surely?  I'll have to do some more research.

Yeah, small cracks can be repaired once they become more of an issue.
Always good to get into the nitty gritty details of this stuff.

D.

[Johan]

Hey guys I'm new here and wasn't sure on which thread to post in, sorry of its the wrong one. I'm in the middle of building my own coal forge and I just had a couple questions. The main part of the forge is made out of an old tire rim, I'm thinking of making a fire pot out of 1/4 steel plate and insulating the space between the firepot and rim. Is this steel thick enough to resist the heat? I would like to know if I should even bother making a firepot or if I should just line the rim with Clay. If so whats a good mixture for that? And if I make the firepot could I use the same mixture for insulation?

 

Thanks, Johan

[ThomasPowers]

For that set up I would skip adding a firepot. As for clay I used what I dug out of the creek back where I used to live and mixed it with wood ashes.  Solid fuel forges usually do not use insulation. If you need to drop the temperatures down air gaps are better.

[Johan]

Alright thanks, would it last longer if I made a firepot? It's not too much work so in a way I wouldn't mind it being overkill. Do you have any other tips?

Here is a few pictures, let me know what you guys think.

[ThomasPowers]

Yes, No or Maybe depending on a hundred little details you haven't provided.  If you don't use it; anything will last till the building collapses on it.

[Johan]

Alright then......

[Johan]

I came here for help BTW, not sarcasm.

[ThomasPowers]

And I offer both; you are free to ignore both as well or to go through and read the massive amounts of information already posted on this set of forums.  I would be surprised if most questions have not been answered many times before. 

[Johan]

I agree that's why I mentioned I didn't know whether this was the right thread, I wanted to stay away from searching for 3 hours lol. All Id like to know is what's better in the end game, regardless of how much work it will be? Assuming I'll be using it once or twice a week.

Thanks

[ThomasPowers]

Small work/Large work?  Forge Welding? How brutal are you with forge linings? Possible exposure to water?  (You have mentioned the fuel, so that's to the good)

When you get right down to it EVERYTHING in a smithy is a consumable---including the smith.  Increasing the time between rework/replacement generally ups the cost and complication.  For example buying a rammable refractory to line a forge with vs using creek clay and ashes.  Using stainless for everything instead of mild steel . Protecting your vision and your hearing.... There is no one RIGHT way to do things and most of the differences are dependent on details related to the person using it. 

My first forge was made from a 1930's era dry sink.  I lined it with creek clay and wood ashes in a linear trough design with an adjustable tuyere length. I used it for several years using charcoal as the fuel.  When I was done with it I would slide a piece of sheet metal over it and put a weight on top.  This was for OKC conditions/materials.

[Latticino]

I'm more expert regarding gas forges, so take this for what it is worth.  As Thomas mentioned you might be better served by looking at some of the already worked out forge designs illustrated elsewhere on this site.  There is tons of great info in the section on solid fuel forges.  Not sure why your time searching is more valuable than mine writing, but I'm feeling generous today.

I've never used a coal forge that didn't have a firepot for concentrating the coke produced.  Looks to me like your configuration could use the liner recommended by Thomas to, at minimum, level the bottom of the forge and close up the holes left by the drum configuration (I'm a fan of rammable refractory, but it does cost...).  Personally I would probably include some level of Duck's nest at the air exit point.

Clearly you will need some kind of blower and connection to same from your Tee fitting.  The necked down size and gate valve are a bit overkill as regards flow metering, so hopefully you have a blower that will push around 140 CFM if you plan on getting a larger fire up to welding temps.

You also will probably want to put in some kind of grating at the floor of the forge to keep the fuel from dropping down the air blast pipe.  A clinker breaker is nice, but I've heard good things about a simple grid of 1/2" bar.

If you are going to use the forge inside, and possibly outside as well, you will probably need to make a hood to direct the heat, smoke and fumes away.  The side draft type work a treat and aren't that hard to build.

Finally you will need to make an ash dump gate for the bottom, so you can clean out the dirt leg and keep the air going the right way.

[Johan]

Hey guys thanks for the generous responses.

This is gunna be my first time using a forge and doing any kind of smithing, I mainly plan on using it for blades, forming and quenching. I would like to do some forge welding with it. I am leaning towards putting in a firepot made out of 1/4 mild steel plate and insulating the space between that and the tire rim. As for the floor of the forge I'm thinking of taking a 2" pipe endcap and drilling some 1/4" holes in it. The ash dump will just be a end cap screwed on the bottom. My air will either be coming from a 6hp shop vac or an inflated unit for a greenhouse, I'll take a look at the specs tonight to see if its sufficient. I can make  a quick drawing tonight and I'll post it so you guys can take a final look and make some suggestions if need be. 

Thanks again, Johan

[ThomasPowers]

If you insulate your firepot it will burn out faster.  WAY too much air; I've done tons of forge welding using fractional horsepower blowers and you want to go 6HP!!!!

[Johan]

Hense the control valve lol. So do you think it would be wise to build a firepot but don't insulate around it? That way some air can come up through the rim and cool it down a little? Also do you think the 2 inch end cap would be sufficient for the duck or do I need something heavier?

[ThomasPowers]

No; I think what I have posted here several times before.

[Johan]

Ok, thanks