Cast-Iron Furnace Build Using Partially 3D-Printed Molds

Because of the advice I've received (and the fact I had to re-print the tuyere mold anyway due to the error in measurement), I decided to increase the overall height of the furnace to 18" which makes the internal chamber 16.5" tall. The mold is now ready for casting the refractory. I'm planning to use stainless steel needles mixed in as well to provide some extra strength.

 

Steel needles may actually promote cracking due to expansion co-efficient differences of steel and refractory. The refractory I use is plenty strong enough: 60-80 MPa compressive strength so about 3-4 time stronger than normal concrete. The main reason to have a steel jacket to cast the refractory into, is that it confines any cracked refractory in the outward direction and due to the circular geometry any cracked pieces are wedged inwards similar to a key stone in an stone arch. The steel jacket also prevents jets of flame exiting the cracks further damaging things or the dreaded white vapours of diesel oil ready to escape via cracks and ignite with a powerful whoomp!.

Something like a beer keg (or 1.5 kegs joined) or an old gas tank or compressor tank can be used. I have a bottom hatch secured with wingnuts to allow a damaged base and plinth to be removed and a new precast base and plinth to be placed at the bottom and grouted in. The base and plinth cops a lot of abuse compared to the walls and spillage of metal and slag too which is hard on it.

 

It looks like you're about ready. A couple comments about the concrete forms: If they are not coated in some fashion they will absorb and rob the refractory of water - possibly to the detriment of the refractory and the integrity of the form. The interior of the tubes is probably coated with some sort of poly film already but the outside is probably is not. That means your outer tube is probably OK as is but your inner one should be treated on its exterior surface somehow. If you're in fact using a vibrator to set the refractory you should be able to wrap a couple turns of Saran Wrap. That should do it since it doesn't have to withstand ramming. Incidental moisture probably won't be a big deal, but it needs be minimized.
Considering what those tubes are designed for the inner tube could potentially collapse if it's not reinforced somehow, especially if there is moisture leakage. Again vibrating vs ramming minimizes the issue but I should still be considered. One method is to fit 3 or 4 plywood disks inside the tube. A 3" hole in the center of the disks is helpful for removal after the pour.
Lastly, the inner form will float or move off-center if it is not anchored. If you have read over Pat J's build thread I think he mentioned having that issue. Maybe it was in another build thread by Pat, but at any rate it's a real concern.
I've built a number different furnaces and forges of various shapes and sizes and have had all of these issues in one form or another, sometimes as a very unpleasent surprise.

Pete

 

I'm going to encourage you as strongly as I can, a Furnace needs a metal shell
as you heat and cool it , it WILL crack , those cracks open, and the pieces of your furnace naturally push outward, as you patch and fill the cracks, it exasperates the expansion outward , my furnace has cracked so bad, it split open the metal shell
( Yes it is only galvanized roof flashing, and it is 8ish years old )
now my furnace shell is literally held together with hose clamps so the shell will not allow the refractory shell to expand anymore
your first major crack will probably cause your furnace to crumble on the next heat,
any abuse wether neglegent or porposeful(cleaning) will probably destroy all your hard work




V/r HT1

P.S. i agree with all the other advice you have previously gotten , save I would use the stainless needles, and intend to on my next furnace

 

To add to the above comments:
I am not aware of anyone demonstrating actual benefit from stainless needles. They sound (sort of) good in theory.
Refractory will crack. So, why allow the cracking to occur randomly? Commercial furnace makers cast their hot faces
in segments—-pre-made cracks.
An area especially likely to crack is the tuyere itself and its junction to the body of the furnace. The abrupt transitions
from tuyere to hot face will involve large temperature gradients causing the refractory to break. Making the tuyere as a
separate entity and itself in two halves bound together with steel hose clamps would accommodate the differential
heating in this area and reduce uncontrolled cracking. The tuyere can just be floated into the hot face and, with
minimal support it will stay where you want it. Stuffing a bit of wool between the tuyere and hot face will limit leaking
of vaporized oil. Mine leaks a little vapor all the time. No biggie. Some time’s there is a little flame licking around the
tuyere and sometimes not. No biggie.
Many metal bands or a sheet metal shell will help hold the pieces together.

There is nothing like trying to digest a lot of opinions and then making a furnace and running it for a while to improve on the design for the next furnace. At every foundry there seems always to be a next furnace. ;-)

Denis

Added, the gentle wafting of vapor around the tuyere is not the same as a jet of hot gas shooting ot a hole in the refractory. As Pete mentioned above, jets of hot gas are destructive and will eat steel fairly rapidly and chew away at refractory as well. It is kind of the difference between a candle flame and an Oxy/Acetylene torch.

 

I only read the Abstract and the Summary ... but very interesting results
https://conservancy.umn.edu/bitstream/handle/11299/163062/Peterson_umn_0130M_14742.pdf?sequence=1

V/r HT1

 

Tell me about it, every time I post there seems to be a new set of really good comments bringing up previously unconsidered points.

In any case, my thought with the needles is that it may be able to hold together the refractory after a crack has formed, like rebar in concrete to some extent. Therefore, even if more cracks form due to differing thermal expansion, the overall integrity of the wall is maintained. As explained in the linked paper, 3% needles seems to be optimal.

As for casting the wall in multiple pieces/changing the tuyere it is a bit late for a total redesign. Maybe in the next furnace. Same goes for a full metal shell. In the case of the metal shell, wouldn't this reduce the insulation properties of the furnace by wicking heat away more quickly to cold spots? I guess it wouldn't matter if it is fully surrounded by wool. I'd also be concerned about corrosion when the metal is placed so close to the hotface. Anyway, I do plan to use kanthal or stainless wire to wrap around the refractory and wool insulation for a little extra support. I'm thinking for my next furnace I'm going to spend a little more time on the mold and make it both segmented and reusable so that I can easily make replacement refractory panels.

On the other hand, for this build I will follow the excellent advice that is posted and coat the inside of the mold with a waterproof film and fill the inner form with sand for support. I had tried to cover any exposed/cut edges of the tube with tape but didn't realize the outside wasn't waterpoof.

 

Indeed, needles might help. How a Taconite furnace experience translates to a home foundry is a good question. Likely the taconite furnace is heated to operating temperature and remains at that temp for days, weeks, or months. Most of us home foundry types run ours in 1 to 2 hour cycles which is much more destructive from an expansion and contractoioin point of view.

You will not need to use any special strapping around the wool. It will likely only get to 500 degrees or so. At least that is my experience having heat gunned mine on many occasions. It cetainly should not approach 1000 degrees. Even a 1000 degrees will not casue significant deterioration of ordinary steel wire or bands.

Corrosion or scaling of your shell should not be a problem. I have used (and so have a whole lot of other folks here) ordinary steel barrels for shells---20 ga low carbon steel---which has never shown any significant corrosion even after more than a year and many melts of service. My furnace lives outdoors in the rainy PNW with a removable steel lid to fend off most of the rain.

I am more or less alone, so far as I know, in making a segmental furnace, so, caution on that advice. But, my segmental Blu-ram furnace has held up FAR better than my prior cast "uni-body" attempts. They held up fairly well. But it is nice to not have to worry (much) about the hot face gradually crazing and cracking until it may collapse.

Have fun with this. I think it is good to just, at some point, go for it. You'll have plenty of time for second-guessing while you tend you furnace with future melts.

Denis

Added: I see Kelly's keyboard was clickety clacking at the same time mine was. Similar message, too, I see.

 

The refractory is in the mold. Decided to not use the stainless needles even though I had them because I didn't feel like it would be possible to fill in the tuyere mold. It was already very difficult to get the refractory under the inner part of it and I'm not 100% sure I actually did fill it completely. Might need to do some patching up after the fact.

For vibration, I flipped my vibratory polisher from harbor freight upside down and secured it to the plywood under the mold by drilling a hole and using the screw on the top to hold it on. It clearly did not transfer enough vibration using this method, but I think it might have helped a little. I packed the refractory into the mold and also tapped the side of the sonotube very thoroughly during filling which seemed to really help to compact it. We'll soon see how it turned out. Best I've seen I'm supposed to let it cure for at least 2 days, maybe a bit longer if the nights get too cold (I'm in California so it isn't going to freeze).

I used just about as much as I calculated I'd need: about 40 pounds (4/5 of a bag). This means that the whole thing including the lid could probably be done with only one bag of Kastolite if you were to shorten the height a little bit. I bought two bags, so I'll have plenty of leftover.

 

Regarding mild steel shells and rust, I'd had a conversation with an old engineer who started in a foundry in the 1930's and went on to run an iron foundry locally. Laurie mentioned that blackboard paint was used to paint the hot steel shell of small furnaces. He said you made blackboard paint by mixing normal paint with lime powder... this makes sense from a corrosion viewpoint as steel can't rust if it's kept alkaline above a PH of 8 I think (like water based machining flood coolants). His furnace got to 68o deg F /360 deg C on the outside so the paint is going to burn off and maybe leave a lime based skin.

 

Check out 2000 f engine spray paint sold at automotive stores. It seems to work decently for rust prevention on furnaces. Though the color changes as you use the furnace more.

 

Thanks. I'd like to see the video when you're ready. No rush as it takes a little time to tweak in everything. It took me more than a dozen times lighting my furnace to feel somewhat comfortable. The way Keath shows it, he got it his first time.

The assessment it was based off of watching hundreds of his videos of him using using an operating machines and watching his creative editing. He is a YouTuber. His message is always delivered with great intentions, but I can't believe he's very rarely makes mistakes on video. I watched a video of his once where he was machining on a horizontal mill and just edited out the movement of the work piece or vice like it didn't happen. Sometimes I appreciate less than perfect but real! We are not all perfect, why portray?

The other thing I don't like on his videos is when he uses "we" (did this a lot... ).My understanding of that is, he did not do it,( and he has no real clue of how it really went together,)just that he supervised or directed it to be done.

This is like the last time I had a filling by my dentist. The insurance company said we did this work. I had to correct them, I finance them, they finance the dentist, and the dentist did the work. They actually did nothing!

@k.kuhn your furnace build is looking pretty good in its early stages. There is a good group here that provide a lot of unsolicited, nonmonetary advice. Hopefully you will one day soon provide you're stumbling stones to prevent onlookers from making the same mistakes like many of us have had.

We're not all perfect, why portray that we are? Sometimes learning from others mistakes is beneficial.

 

My understanding of Keith Rucker is he develops and test crop products for Bayer as an agronomist for his day job and volunteers at the Georgia Museum of Agriculture where a lot of his videos get shot. With YouTube there's a fine line between too much and too little content with an average viewer attention span measured in seconds. I've got no issues with how his videos are edited but then I can't watch a detailed four part Keith Fenner video on making something. Each to his own...

 

I appreciate your measured response.
PM sent.
Pete

 

Also p.m. sent.

 

The furnace looks good so far after taking off the outer mold. I let the refractory cure for about 3 days, keeping it moist under a lid to allow it to fully cure. There is quite a bit of porosity, probably due to inadequate vibration, but I managed to completely fill the tuyere and it looks like everything is pretty structurally sound. I'm planning to remove the sand from the inner mold and break that out today.

Starting to remove the sonotube:

Removed more:

Removing the 3D printed tuyere mold (hitting it with a blowtorch for a few seconds really did the trick to soften it up since it was only PLA).

The tuyere actually looks really good despite it being difficult to pack:



Remains of the tuyere mold:

Different angles of the furnace body: