Interesting experience this week...
- I built a dual burner hybrid crucible/reverb furnace over the past
few months. Basically, it consists of an old acetyline tank cored
out,upside down inside two 20lb propane tanks welded together inside
a 120 gal. water heater using perlite as insulation.
Since the acetyline tank was upside down, I was using a .75" black
iron pipe as a stopper in a Bessemer-style bottom pour arrangement.
No I've read many posts about iron and steel crucibles having a
shortened life, where others have used the same crucible for years.
I think I can now explain the difference, first hand..
Generally speaking, molten aluminum at 1250 deg. F is caustic to
iron based metals, and will readily dissolve cast iron, and more
readily dissolve carbon steel. Zinc alloys are inherently
more "acidic". Raise the temp. and the alloy will dissolve more
Now I had about 200lbs of miscelaneous scrap to be pigged out.
Some marine alloys, some extrusions, some castings. I know there
was some Magnesium alloy there due to the flare ups, but for the
most part let's assume that it was mostly Al, or Al-Zn, Al-Si
alloys. Preheating each piece in the reverb section of the furnace
until the "cottage cheese" state where each piece starts to crumble.
I sorted my scrap by appearance, selecting the heavier castings
later. The first day of mostly triple-track and other extrusions,
along with a block from a moped. By the end of the day, I saw the
molten aluminum dribbling out cooling and forming strange crystals
on the surface. The ingots seemed "tinny" sounding when popped out
of the mold, and over-pour was brittle. When cleaning out
the "crucible", I pulled the "stopper" to find the last 1/2" below
the actual plug very corroded and missing the rest of the pipe!
The next heat, after replacing the stopper, I used crushed glass
to "coat" the bottom portion of the stopper. Bringing it to a red
heat, I plunged it into a coffee-can full of the crushed glass (not
quite as fine as sand, but finer than crushed blue-stone). I
repeated this process 7-8 times untill I was confident that some of
the glass had fused to the pipe.
Now I know I had been running the lower burner way too hot the
previous day, since I was scooping out dross that was glowing cherry
red for some time after being removed from the furnace... Whoops.
The second heat, after getting the crucible to a red heat, I turned
the lower burner down to about 2psi. from 5psi., manifold pressure,
or about 20psi with the valve closed..
The second heat was much better. Since the previous day's
activities dissolved the junction between the crucible and drain, I
later (when I was cleaning out) found that the bottom third of the
lower propane tank was full of about 20lbs of molten aluminum!
Ok, so back to the crucible debate. After 2 days and about 200lbs
of scrap I have two thoughts:
1: Watch your heat. I had my first batch up to a glowing red
aluminum puddle, (very red crucible!) I'm guessing that since the
pinholes in the ingots and the color of the crucible, that the first
heat was around 1500 deg. F. Just a bit over the top..
2: Watch your alloys. Higher zinc alloys will definitly are more
caustic to iron, and even more towards steel.
Combine these two factors, and you have some serious problems. I
think that's the main reason for the iron corrosion I saw.
All in all it was a productive week. I proved the design could
soften iron, insulate well and filter out dross, efficiently
nonetheless. From a 100lb propane tank, running two burners, I
think I used about 50lbs over the three days I was melting. That's
my 2 cents on my week, How was your Thanksgiving?
- T. Tspooon,
Our Thanksgiving was great! Have you considered painting the inside of your crucible with a ladle coat?
Regards, Ray in FLA
> The second heat was much better. Since the previous day's
> activities dissolved the junction between the crucible and drain, I
> later (when I was cleaning out) found that the bottom third of the
> lower propane tank was full of about 20lbs of molten aluminum!
- --- In hobbicast@y..., Ray Brandes <rvb@r...> wrote:
> T. Tspooon,inside of your crucible with a ladle coat?
> Our Thanksgiving was great! Have you considered painting the
> Regards, Ray in FLAdrain, I
> ttspooon wrote:
> > The second heat was much better. Since the previous day's
> > activities dissolved the junction between the crucible and
> > later (when I was cleaning out) found that the bottom third ofthe
> > lower propane tank was full of about 20lbs of molten aluminum!Well, now that you mention it.. I'm planning on using a mix of clay,
boric acid and powdered glass (and water) as a wash in my crucible
furnace. I just thought the info might be usefull for those having
As the hybrid furnace has served it's purpose, It may be destined
for disassembly, that is, unless I find a house to buy!