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Oil fired crucible furnace

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awemawson:

--- Quote from: awemawson on July 16, 2013, 02:27:15 AM ---Steve

I used to sprinkle something on the plinth brick to stop it sticking to the crucible but for the life of me I cannot remember what. Can't have been sand as that would fuse at those temperatures

--- End quote ---

Ha ha - I've remembered ! Not sprinkled but put - Morgan Crucible recommend placing a piece of cardboard between the plinth brick and the crucible to stop sticking and that's what I did. Gosh the old grey cells must be dying faster than I thought  :scratch:

vtsteam:
Thank you lads!   :beer: Yes, two layers of corrugated cardboard had been placed under the crucible, as well as a dusting of plumbago on the plinth. The problem seems to come when spilling a bit of slag when adding metal during the melt. Very fluid in the highest heat of the furnace It works down the outside of the crucible and slides into the space between the two. The cardboard carbon blows away near the glue line allowing it to take effect. Luckily it is spotty so tilting the crucible when skimming pulls it free.

The whole thing feels stiff taffy like when ready to pour -- including the crucible this time. Plinth sagged slightly and the crucible glued itself to the furnace wall and had to be pulled free as gently as I could manage, so I could get the tongs around it. Crucible is slightly distorted in shape today therefore. All very exciting when ready to pour!

vtsteam:

--- Quote from: NeoTech on July 16, 2013, 04:12:23 AM ---Would be awsome to see a cross cut or a surface cut out of that square block.

--- End quote ---

Here you are. The casting had a shrink cavity on the top face, and a couple of serious bubbles. These last two were hidden until the face was milled. The shrink cavity would probably not be a problem, since this piece was intended to be bored out fairly heavily, but the bubbles may have spoiled it. We'll see.

This casting was not as soft as the last one -- it made life a little difficult for the HSS mills, but was machinable. Plenty of graphite came out.


Clean face:





Shrink cavity and bubbles ( upper face)





How it is supposed to go -- both get bored out:

NeoTech:
That looks really nice and solid, no pours.. =)

I imagine you didnt do "crack" test wedges like ironman does in his vids?  Still very nice looking.

could that shrinkage been avoided if you had used some form of reservoir between the molding cavity and the sprue?

vtsteam:
Thnks NeoTech.

I'm trying to figure out how to avoid the shrink depression and bubbles next time. Iron is a little different than aluminum, but similar.

As a problem for me, shrinkage is less bad -- I can often plan a casting or work around it.

Big bubbles like those (6-8mm deep) are a serious problem. While I had shrink depressions sometimes in aluminum, I didn't have anything like those bubbles. Small ones once in  awhile maybe, and sometimes very fine gas porosity if the melt was stirred too much and it was a humid day and older scrap, but nothing like like these big bubbles.

Possibilities:

1.) Maybe the sand was rammed too hard (I did vent it with a wire, but I could try less ramming pressure)

2.) Maybe the sand was too moist (hard for me to judge -- it's new sand, Bentonite instead of fire clay, and this is iron instead of aluminum, so it's very different than what I'm used to. I could try molding drier.

3.) Maybe air got in while pouring. I used two arc shaped gates (instead of my usual one) and they were pretty deep this time compared to last when I had the misrun. So they channeled the iron very fast. The sprue was 1.25" dia and dowel shaped, not tapered. When I poured I tried to pour fast, but it really drained fast so the sprue did not look choked during the pour. It probably should have.

I could make up a tapered metal sprue cutter -- like the real molders use, and/or reduce the size of the sprue, reduce the number of gates, or reduce the depth of the gates in order to keep the sprue more easily choked with iron during the pour.

4.) Shrinkage depression --- the cope was 3.5" deep, and the pattern was entirely in the drag, so there was 3.5" of head pressure. I could build up above that to increase head pressure.

Or I could add a riser to feed hot metal as the casting shrinks. Although with such a small casting and a 1.25" dia sprue, I thought the sprue would feed it.

One problem with a riser is that it takes even more metal in the melt, and I'm starting to approach the maximum comfortable capacity of my crucible already at 7 pounds. I had to add metal twice during the melt. A sprue or riser would add another pound and a half or so.

Anyway those are the ideas I've had so far.

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