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Interesting experience this week...

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  • ttspooon
    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
    Message 1 of 3 , Nov 30, 2002
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      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
      rapidly.

      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?
    • Ray Brandes
      T. Tspooon, Our Thanksgiving was great! Have you considered painting the inside of your crucible with a ladle coat? Regards, Ray in FLA
      Message 2 of 3 , Dec 1, 2002
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        T. Tspooon,
        Our Thanksgiving was great! Have you considered painting the inside of your crucible with a ladle coat?
        Regards, Ray in FLA

        ttspooon wrote:

        > 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!
      • ttspooon
        ... inside of your crucible with a ladle coat? ... drain, I ... the ... Well, now that you mention it.. I m planning on using a mix of clay, boric acid and
        Message 3 of 3 , Dec 1, 2002
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          --- In hobbicast@y..., Ray Brandes <rvb@r...> wrote:
          > T. Tspooon,
          > Our Thanksgiving was great! Have you considered painting the
          inside of your crucible with a ladle coat?
          > Regards, Ray in FLA
          >
          > ttspooon wrote:
          >
          > > 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!


          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
          crucible problems.

          As the hybrid furnace has served it's purpose, It may be destined
          for disassembly, that is, unless I find a house to buy!
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