The Shop > Metal Stuff
Foundry Furnace for the Tiny Shop
awemawson:
In one word : Temperature
Hard to get furnace hot enough
Hard to get suitable crucibles
I've melted mild steel many a time inadvertently in my ceramic chip forge, that is propane and blown air powered, but just the ends of bars left in too long for forging. Different matter containing the stuff at a suitable heat for pouring.
Mild steel melts nicely in a coke fired cupola furnace, but although you may feed it with steel, it's cast iron that comes out due to the carbon in the coke.
vtsteam:
Ironman has a video of converting steel into gray cast iron.
vtsteam:
Today it was gusty out, but I decided to test the tiny furnace out with propane instead of charcoal.
I also wanted to re-melt the aluminum from the last casting test to clean it up and get it back into ingots. The lost foam process necessarily gets loose sand mixed into the head discards of a pour, since the sand isn't moist and bonded with clay. Also the failed pour left some of the pattern plaster coat mixed with the metal, and the escaped aluminum during the ingot spill was also dirty. I had nothing to lose remelting these remainders in a test, and I wanted to see how well they'd clean up -- or if they would melt at all with my propane burner and the new furnace.
The burner I had on hand was pretty small and the result of an atmospheric torch project that I detailed here:
http://madmodder.net/index.php/topic,8633.0.html
I wasn't sure this would do the job, and I do plan to build the larger fan pressurized propane burner ironman uses for his small furnace. But the little burner was available, already made up, and a propane tank nearby, so it was worth a try to see what would happen.
vtsteam:
Well, it was a success all around. The small burner took some adjusting -- I found 8 psi was about right. The gusty wind did put it out a few times while I was trying to find the right settings of pressure, throttle valve, and flare position.
This is one clear drawback of an atmospheric torch -- it's definitely subject to wind gusts.
Eventually I found that if the furnace heated up sufficiently, it was less affected -- that took maybe the first five or ten minutess of heating -- it is still a small torch and there isn't the power to make the walls glow right away.
I also found that using my piece of stovepipe as a chimney on top of the furnace lessened the effect of the wind changes, and probably drew air around the torch, as well as keeping the torch drawing better.
Once the torch was drawing well and the walls starting to glow around the crucible, and I could see the crucible bottom starting to look dark red, I inched both the pressure and throttle up until I was at 10 psi and an open throttle. This sounded a little more ragged with the wind as it was. It almost blew out at one point, but reducing the throttle got it back on track.
When the aluminum appeared to start shifting and crumbling in the pot, I lifted the stovepipe off to give it a stir, but doing this allowed a gust to snuff out the flame. I closed the throttle, re lit the torch and inserted it back in the tuyere. The stovepipe stayed on after this. And the furnace was restored in a couple minutes to full throttle near 10 psi.
When the aluminum had melted and the pot and walls were glowing orange, I shut off the gas and skimmed the dross -- there was very little, and then poured small size muffin ingots. without incident.
Hard to say what the exact time to melt was because of the flame outs, re-starts and pressure adjustments, but it was roughly 15 to 20 minutes with this burner and furnace. As a comparison, the store bought charcoal took 12 minutes to melt the same amount aluminum in the same furnace with the same crucible. That was after it was lit, which takes maybe ten minutes, or so. So roughly similar total time.
One thing was obvious -- the furnace was unecessarily tall for propane melting -- charcoal takes up space, so needs a taller bore. I'd say a 20 lb cylinder would be plenty for a strictly propane furnace, though i still like an 8" dia. bore.
The ingots poured very clean, and look good. Since they are small, I'll probably use one at the bottom of other melts to start the molten pool in the crucible.
Here's what the setup looked like after the pour:
vtsteam:
Since this is also a test of the plaster/sand furnace lining, here are photos of the furnace and lid after two pours and the original bake for a day.
There is some hairline cracking in the lid near the exhaust opening, and the plaster there is softer -- I dug at it in a couple spots and it will crumble away if you do. However it isn't loose or flaking, and appears to be stable so far.
It's not dissimilar to insulating firebrick, which is also soft and can be dug away, and I have to say that my lids made of fireclay and sand also cracked and flaked eventually. Repairing them was part of maintenance over the years. Lids take a beating near the exhaust port. So far the plaster sand has not flaked, so there's no need to repair. We'll see how well it does over time.
The furnace body looks great. I do see a few small hairline cracks at the hottest part, near the tuyere but again, no actual damage at all.
The lid:
The furnace body:
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