A thin hotface medium/low mass beer keg (drum size) furnace for melting Iron

Photos from tonight, with the bowl removed, and the pattern made flat, with a 1" downward lip, and a 2" upward lip at the lid vent opening.


The refractory mixing went pretty well.
I mixed it in two batches to make it easier on the mixer.
I used about 4% stainless needles for the lid. The furnace shell used about 3% needles.
A mixer takes a lot of grunt work out of mixing refractory, not to mention mixing in the needles, and it allows using the exact amount of water, which was about 5 lbs of water per 55 lbs of refractory.
Hand mixing the exact amount of water can be a challenge (that is how I mixed my last furnace refractory, and ended up adding a lot of extra water to be able to hand-mix it).

And I did not run out of refractory, so that is good.
With the little refractory I have remaining, I will add a collar around the burner tube at the tuyere, to give a little more positive seal between the burner tube and the furnace shell.

 

I love that Hobart mixer.
It makes mixing resin or SS sand a breeze.
I hope it never breaks, as I will be singing the blues, and will have to manufacture something to replace it (I bought it used, and even then it was pricey. There is no chance of me ever buying another one).

Those are handles in the hot face at the thickest part which is the outer lip.
In the interest of simplicity and minimalism, I am going to forgo a lid lifter and just lift this off by hand.
It weighs about 30 lbs. so should not be a problem, and with the lid-within-a-lid design, I should be able to add metal and skim all without opening the lid, and so would only remove the lid when it is time to pour.

I will probably just cut two layers of ceramic blanket and place them on top the lid, and rigidize them with furnace cement thinned with water and sprayed on.

I am still contemplating how to close the top of the beer keg shell, and may just spray rigidizer on the top of the two layers of ceramic blanket that will be around the hot face.

As you can see, I am half-way designing and half-way making up design as I go.
The only real absolutes are:

1. The furnace have as low a mass as possible/practical using Mizzou refractory.
2. The interior shell/hot face must be completely insulated/isolated from anything that conducts heat such as the metal shell.
3. The furnace must be capable of repeated iron melts. The main intent of this furnace is to melt iron; lots of iron like in the 1,000 (+) lbs range over its lifetime.
4. The interior shell/hot face must be easily replaceable.
5. The furnace must sit as close to the ground as possible to minimize the lift height, since I will probably hand-lift the crucibles out using standard lifting tongs.
6. The entire furnace with all enclosing metal, frame, etc. must be light enough to easily move indoors, and also light enough to transport.

So far the shell is 72 lbs, and the lid is about 30 lbs. The shell/frame will probably add another 40 lbs., so overall perhaps 150 lbs for the complete furnace.

The visual looks of the furnace are strictly secondary to its function.
This is very much a function over form design, although I am trying to avoid the "Franken-furnace" look of my last furnace.
I am trying to keep things looking as tidy as possible since everyone here seems to build spectacular looking (and excellent functioning) furnaces, so I have to give some attention to keeping up with the Joneses of the casting world.

I am going to go out and see if the refractory has set, and if so start stripping off forms.

Below are today's dimensions (the design is morphing as we go).
I changed the height of the upper lip on the lid from 1" to 2" last night, so a slight difference from the drawings below.
This furnace began using a standard beer keg, and the idea was to keep everything within the beer keg shell in one neat and tidy package, however......(things never seem to work out that simple for me).......I could not resist making a modular design that would work with a wide range of crucible sizes, and so I ended up cutting up the beer keg to enlarge the diameter of the stainless steel shell, and I let the refractory shell dimensions be dictated by the dimensions of the largest crucible (perhaps an A60 at the extreme, using a shell extension), while maintaining a little less than 2" clear around the largest part of the crucible.

There will be two layers of 1" ceramic blanket around the shell, two layers of 1" ceramic blanket on top the lid, one layer of insulating fire brick below the furnace, and those bricks will have a 1" layer of ceramic blanket above and below them.
A rigid insulating refractory board would be better below the shell, but I don't have any of that material.

For a #30 crucible, I would add an extension ring (made from 1" refractory with needles) to the top of the shell to raise the lid a few inches, to avoid any impingement of flame on the side of the crucible.
From a circumference standpoint, I think it may be possible to use up to a A50 or perhaps an A60 (10.25" and 11" maximum diameters respectively), using extension rings on top the shell; not that I intend to cast anything that large, but the possibility is there.
In reviewing the engine designs I want to cast, I am hoping that I can cast the parts I want using a #20 maximum crucible, hand lifting out the crucible, but using a pouring cart to prevent breaking my back.

The metal beer keg shell shown in the drawing is not tall enough for this design, so I will have to add a stainless extension ring at the top to match the height of the ceramic blanket.
Probably will just rigidize the top of the blankets, or I may add a metal cover to keep the junk out of the ceramic blanket, but the cover will not touch the refractory shell (so as to prevent a heat-sink effect).

The 1.25" x 9.0" object at the bottom of the second drawing is a hard fire brick, and I will probably use one or two to close off the lid opening a bit. If I had a little extra refractory I would ram a small lid, and I may buy a small bag of refractory and make the inner lid next year if all goes well with this furnace.

I will have multiple plinths (probably three), with the idea being to keep the top of the crucible near the bottom of the lid (for easy skimming) with any one of the crucible sizes shown.
The plinths are rammed from Mizzou and also use the stainless needles.
I can fine-tune the exact height of the plinths using half a fire brick on top the plinth, if necessary.

Edit:
I ended up making a domed lit, and used one 1" layer of ceramic blanket and one layer of insulating fire brick (detailed later in this thread).

 

That hasn't been my experience. My hot face is a hair under 1" and I do have a couple of cracks, but I don't have needles within it, poured it fairly wet, and I've transported it in the back of a trailer over a fairly rough ride, (after which the cracks appeared). Not sure what kind of physical abuse you think your hot face will endure beyond flame/heat.

 

I used packing tape to seal up the top of the gap and did a refractory cap. While the pour was still workable I placed the finished lid on top with plastic in between and let the 'cap' take the form of the lid mating surface. I've had my burner and bouncy castle blower maxed out and have not noticed any blow out of flame between the lid/base. I'm confident it's because of this added step.

 

That is good information to know, and sort of verifies what I am guessing.

I suspect that the Mizzou with needles is going to be a very tough and durable hot-face shell, and it should be easily repairable with something like ITC200 if it does develop any problem spots.

I have found with my heavy furnace that it is best to patch the cracks as they appear, and don't wait until they get large, since a large crack seems to propgate the heat and make the cracks worse.
My first furnace did not have needles, and it cracked very significantly, to the point where I had to add a steel band around the top of it to keep it from opening up 1/4" cracks.
But the cracked refractory in my heavy furnace is still perfectly usable, since a crack does not mean the refractory is crumbling.
My cracked refractory is still extremely solid, as witnessed when I cut the second tuyere in it (it was difficult even using an air chisel), and perfectly usable.
On my heavy furnace, I have used a small amount of ITC 200 (cheaper than ITC100) to fill in the cracks as they appear. It only takes a very small amount of ITC200 to do this every three or four pours.

Scavenger mentioned some minor cracking with needles in the refractory, but apparently nothing that opened up into large gaps like I experienced without the needles.

One concern I have is that my shell will not be secured to anything; it will be sitting on ceramic blanket and insulating fire brick, so transporting in a car or trailer may jolt the shell out of place.
What I may do is install three stainless all-thread rods with a T-cap around the lower end of the stainless outer shell, and when transporting or moving the furnace, the rods could be turned in to give a light contact with the shell.
I guess I would also need three rods around the top of the shell to get it totally secured, but I don't want to operate the furnace with any metal touching the shell (other than the burner tube), to prevent a heat-sink effect.

 

That is not a bad idea, but I am out of refractory, and I am trying really hard to adhere to the "less-is-more" philosophy for this furnace, and so will probably just rigidize the top of the ceramic blanket, with perhaps a metal cap if I can figure out how to make one.
My last furnace insulation shell was made from aluminum roof flashing, and while that makes for a quick and easy insulation cover, it also melts easily when any significant heat leak contacts it, and so it has melted around the top near the lid, and also melted at the tuyere from leaks at both locations.

I think the top cover needs to be stainless steel, but I don't have any flat stainless sheet that I can think of.
I am thinking of my wife's large stainless cook pots; I wonder what would happen to me if one of those vanished mysteriously one day.

 

I pulled the lid out of the form a minute ago since it had set, but it would have been better to leave it in the form and add water to get to full cure.
I had a little crumbling along the bottom lip, but no big deal.

The lid turned out a bit notchy, but I will touch up the exposed surfaces with a little ITC200 to make it look pretty for the photo ops.

The lifting mechanism is already complete (my two arms), so no work required there.
The 30 lb lid weight makes it easy to lift and handle.

Both the lid and the shell are considered expendable items with a finite life, but if I can get 100 (+) iron pours out of them I will be more than happy.

If this furnace works out well, I will probably make some permanent forms so I can quickly ram up a new liner and lid.
For the shell forms, I have an old hot water heater that would make a good metal outer form, and would just use a sonotube again for the inner form.

For the lid forms, would probably glue a couple of sheets of 3/4" plywood together, and use a router to to cut the groove for the lower lip, and the recess for the 1" thick lid.
I can make several stepped cuts for the lid area to get a bit of a dome shape.
I would cut the plywood lid form into perhaps four pie-shaped pieces and waterproof them; taping them together with aluminum tape for use.
I would make some split metal bands for the collar on top the lid in order to get that shape a little more uniform.

I put the lid in a plastic bag and added water to get the refractory wet and allow a full cure.
It will stay in the bag for two weeks.

Note that the wide-angle lens distorts the shape a bit, making it look domed, but it is actually flat. The handles don't really lean outwards either, they are basically vertical.

It is also interesting to note that a top form was not required.
The refractory is stiff enough not to sag without a top form.

I suspect that the top edge of the furnace shell will see the most wear and tear, and so as a preemptive move, I may make some curved inserts from hard fire brick and cement them to the top edge of the shell using ITC200, in order to make the top edge renewable.
I have a 4" diamond blade that goes in an angle grinder, and I would use that to cut out some curved hard fire brick inserts.

 

I put in a new driveway at my house about 25 years ago, and I consulted with a structural engineer that I worked with, before I had the driveway poured.
He recommended using 4,000 psi concrete, and he said to cover the concrete with plastic as soon as it had set, and keep the concrete wet for two weeks, watering it at least once a day under the plastic.

He was correct; 25 years later there is not a single crack in the entire driveway, and no spalling on the surface either.

So that is what keyed me into putting my furnace refractory in a plastic bag and wetting it right after it set, and leaving it wet for two weeks, to achieve a full and uniform cure.
The structural engineer said concrete driveways spall on top because the top dries out too fast and never reaches full cure, so it breaks off.
I suspect the same is true with refractory, and I think the instructions actually mention keeping the refractory wet until it reaches full cure (I will look up the instructions I found).

 

Here are the instructions for Mizzou from the site where I purchased the material:

Mixing & Using Instructions
MIZZOU CASTABLE

--------- GENERAL INSTRUCTIONS --------
Material should be stored in a dry place.
Porous back-up materials or wood forms should be waterproofed.
Absorption of water can result in reduced flow for the product.
Forms must be stout and water tight.
This product is designed to be mixed with water and then poured/handcast into place.
For best results, water should be maintained at 50-70F.

--------- MIXING REQUIREMENTS ---------
Approximate Water For Installation: 55 lbs. to 5 pints of water.

Mix for at least three minutes.
For best results, wet mix temperature should be maintained at 60-75F.
Minor adjustments to the amount of water are permissible to achieve desired flow.
Do not exceed 11.0% water under any circumstances.

--------- INSTALLATION ---------
Place material promptly.
Do not trowel to slick finish.
At temperatures above 60F, air cure, keeping surfaces damp and/or covered, for 16-24 hours typically or until a hard set has developed.
Lower temperatures will increase the time before a hard set develops.
The best results are achieved at curing temperatures of 90-110F.
Keep material from freezing during air cure and preferably until a dryout can be initiated.
Freezing of this product prior to water removal can cause structural damage.

-------- HEAT UP SCHEDULE --------
Never enclose a castable in a vapor-tight encasement as a dangerous steam explosion may result.

Typical dryout schedule for a single layer, 9" thick or less:

Ambient to 250F at 75F per hour, hold at 250F 1/2 hour per inch thickness.

250F to 500F at 75F per hour, hold at 500F 1/2 hour per inch thickness.

500F to 1000F at 75F per hour, hold at 1000F 1/2 hour per inch thickness.

1000F to use temperature 75F per hour

And some info on the Mizzou refractory product:

Mizzou Castable Refractory:

This is a 3000 degree F, High Alumina Castable Refractory for making forges.
You can cast the entire refractory liner for a forge or just the floor using this material!
This material has a very high resistance to flux, if you're doing a lot of forge welding and have a dedicated forge for forge welding in, this is a great material to use!
The only trade-off is that it isn't as good of an insulator as Inswool, but as with all things in life there are trade-offs, and you should consider the benefits of the durability of this material.

- 3000 degree temperature rating
- Highly flux resistant
- Density = 141 lb/ft^3
- Mix with water at a ratio of 5.2# or 0.6 gallons of water per 55#

- This material has a thermal conductivity of 7.4 btu-in/hr-F-ft^2 at 2000 degrees F

 

Quite the leap from "16-24 hrs" to 2 wks......lol

Nothing wrong with airing on the side of caution, but 14 times the suggested high end might be considered a tad overkill.

 

For dryout, I don't use the lightbulb stuff, but rather just put a small propane burner in the tuyere, and a pyrometer in the lid opening, and turn the burner on very low to begin with.

I slowly increase the burner pressure over time to adhere to the dryout schedule above.
It would seem like a lot of propane used, but the burner operates on a very low pressure for most of the time, so actually little propane used during the dryout process.

It looks like for my dryout, it will be as follows:

1. Ambient to 250 F @ 75 F per hour (about 3 hours), then hold at 250 F (for 1/2 hour).
2. 250 F to 500 F @ 75 F per hour (about 3 hours), then hold at 500 F (for 1/2 hour).
3. 500 F to 1,000 F @ 75 F per hour (about 7 hours), then hold at 1,000 F (for 1/2 hour).
4. 1,000 F to use temperature (assume 2,500 F) @ 75 F per hour (about 20 hours?).

The first three steps will require 14.5 hours, which seems a bit excessive in my opinion, especially for such a low and thin mass.

I will not be able to do the last step at 20 hours.
That would be a lot of propane.
Hopefully the idea is to drive out the water that is inside the refractory, and nothing else, since I will not be able to do the 20 hours thing.
I am assuming that due to the density of this material, it has a low permeability when it sets, and thus driving out the water from the interior is a slow process.


I will probably do steps 1-3 in about 12 hours, and then do step 4 in perhaps 2 hours right before an iron melt.
It would be wise to season a new crucible during the dryout process to save on propane.
The seasoning instructions for a Morgan clay-graphite crucible are as follows:

The PREHEATING / FIRST USE instructions for fuel-fired furnaces is as follows (per Morgan):
A new crucible shold be preheated empty in order to minimize the termperature gradient across the crucible wall. If there is a risk of the crucible having absorbed some moisture, then the furnace should be initially heated slowly up to approximately 392 F (200 C) and held at this temperature until all moisture has been driven off.
A new crucible should initially be heated slowly and evenly to 1112 F (600 C) on low power, avoiding local impingement of flame on the curcible. Subsequently the full heat input rate should be utilized to achieve a uniform bright red condition over the whole crucible at approximately 1650 F (900 C) at which point the crucible should be charged immediately, taking care not to pack metal tightly or bridge ingots across the crucible.

The speed of the dryout depends on how much water comes out of the refractory in each step.

If the refractory begins to steam excessively, then the heating process needs to slow down, since otherwise you will probably have steam explosions that will ruin the refractory.

 

Yes, I noticed that when I went back and dug out the instructions.
That is a good thing since I would really like to begin the dryout a lot sooner than two weeks from now, and before any the ice and snow hit may hit (typically in January/February round here).
December weather here is generally mild, and that has held true this year.
But no sense removing the shell and lid from their bags until it is time to begin dryout.

If everything goes really well, perhaps an iron test in January.

Now I need to start working on the shell and base, cutting some ceramic blanket, and I will be close to calling this build complete.

 

I did the garbage bag for a few days, then a small light bulb for a few more days.. Then a bigger light bulb for another few days. Not a single crack. Nice and slow wins this race. Screw that goofy schedule. That's for users that need this stuff cured in the shortest time possible and that's how to do it. First time I fired was low heat and raised it slowly over a few fires. I was in no rush.

 

I started "Operation Dryout" today, and dried the furnace for about 3 hours on propane before the tank ran out (the reason I hate using propane; the tank always runs out when you really need more).

I think I got it dry enough to start it on the siphon nozzle running diesel, with no combustion air (you can dial a siphon-nozzle way down by running without the combustion air, and letting it naturally aspirate).

A lighter weight furnace does not take as long to dry out as a 3" thick one.
It does take quite a while for the water to work its way through the refractory, I guess due to the density of the material.

I did not use a light bulb; I don't see that really doing anything.
I used a small propane burner, and began on low, then ramped it up slightly after about an hour, being careful to avoid getting too much steam coming out of the refractory.
The refractory has to approach something along the order of 150 F or more before it really starts giving up much moisture, so unless you have a seriously large light bulb, I don't see it doing anything significant.

I really like the large opening in the lid, and I think that it will make skimming a breeze, especially iron skimming which tends to be gummy and difficult.
(Edit: The large lid opening turned out to be a bad idea with iron, as we found out on the first iron melt.)
I will start with hard fire bricks as my small lid, to close the opening down a bit, but I need to order a small bag of Mizzou and make a more durable inner lid.

 

After I rammed my furnace and letting it set up for a week or so, don’t remember exactly,
I placed a wooden circle which had a slot cut in it over the exhaust hole and spaced the lid from the main body with a 1/8” spacer.
I hung a 500 watt heating bulb in it for a few? days until I no longer observed any moisture beads on the aluminum spacers I used to elevate the lid.
At that point I resorted to fire curing.

I’ld have to go dig up the old thread but I seem to remember the chamber temps hitting close to 300* just from the heating bulb.

Slow and steady for the backyard caster is the ideal process.

 

I used one of those yellow portable halogen spot lights. They get stupid hot! The only propane i use these days is for dewax and wax work. Anyone know how to refill those little green camping bottles from a bbq tank? I know I've seen it done, but slept since then...

 

They make an adapter for filling the green camp bottles, but you have to know what you are doing.
I was going to start doing that when we use to camp out a lot, and my buddy said he knew of a guy who was doing that and blew his arm off when one exploded.
That sort of sapped my enthusiasm for refilling them.

 

thanks pat. Screw it. I'll just pay the money and toss them in the trash. It's best to pick your battles wisely.

 

Oh come on Jason, you can video and get 5 million views while you arm is floating out in space somewhere.

10 million views if you strap a GoPro to your arm and live feed it back to earth.