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Thread: Arc melting in a crucible: how to make contact with crucible ?

  1. #1

    Arc melting in a crucible: how to make contact with crucible ?

    I have seen some videos on Youtube depicting a crucible in which the user sticks a single carbon electrode to melt the contents of the crucible which was cast iron.

    I tried it by making a steel bar with a ring in it in which the crucible fits and shaved the glazing off the crucible somewhat to enable proper contact. Admitted, it is an old one used earlier for melting cast iron in a propane furnace.

    I connected the cathode (the clamp) of the DC welder (max 200 A) and a carbon rod to the anode (where the welding stick normally has to be attached). After some fizzling around, shaving crucible, I managed to melt a tiny amount (10-20g) of cast iron in a few seconds, but later the crucible started arcing outside between the steel ring and the crucible, even almost melting a piece out of the ring. Moving and tightening did not help: it persisted arcing on the outside. The current was set to 100 A.

    Here a very short 4 seconds clip what happened :

    Does somebody have experience with this ?

  2. #2
    Senior Member
    Join Date
    Nov 2012
    Edmonton, Alberta
    So if I understand this right, you are inserting a carbon electrode into the crucible where there is some metal, the current arcs to the metal/ crucible, the current then flows through the crucible into that big rod and back to the welder. What is the crucible made of? Maybe you could make some sort of sleeve to sit around the crucible and increase the contact area/ decrease the opportunity for the current to arc at the places where there is a small air gap along the radius of the rod.

  3. #3
    Big steel arc furnaces have 2 carbon electrodes. One is neutral, one is the hot. Both get plunged into the metal. I'm pretty sure that the goal is an arc so the electrodes don't touch the metal just get really close. The arcing between the crucible and the ground ring in your setup is increasing the resistance thus lower the "heat" of the arc inside that's why the iron chills on you. The arcing isn't there at the beginning because the scrap iron has a small surface area in contact with the crucible. As it liquefies the surface area goes up the resistance goes down more current is passed through the ring till it's arcing at the ring. Heating metal also reduces the resistance of the metal. Another option maybe to start at 100amps once you get a puddle back it down to say 80... this is going to be a trial and error kind of thing till you find the power to keep melting the iron, keep it all molten but not cause arcing outside the crucible.

  4. #4
    The main problem (compounded by those listed by CBB) is the contact resistance between the crucible and the cathode.

    The contact zone will be a line in the best case. More then likely an interrupted line and possibly even as little as 3 dots.
    When all that current has to pass through a tiny zone the carbon in the crucible will start to heat up FAST and evaporate, the iron will start to oxidise and sparks start flying.

    A solution would be a copper band around the crucible.
    One kind of material that I would be tempted to try would be this:

    Strapped, bolted or possible even spring loaded around the crucible to maintain a proper contact zone with the crucible.
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  5. #5

    The crucible is a graphite-clay one which I also use in propane furnace.
    Yesterday I put two carbon rods inside the crucible which worked better (like a large scale commercial steel meltshop EAF which is however powered by three carbon rods as it is three phase AC). I then hover the rods over the metal puddle and don't dip them.

    And I tried a steel rod (6mm) on the anode which also works, as it melts down itself.

    The only other problem I have when the metal is not molten yet, the arc is not sustainable as there are almost no vapors. When arcing CaO + charcoal for making CaC2 the arc is sustainable as there are CaO vapors. I can use slag (like a commercial EAF) but that eats the crucible.

    I ordered a new pure carbon cilindrical crucible on ebay for trying with a single carbon rod inside it.

  6. #6
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