The Shop > Metal Stuff
Oil fired crucible furnace
vtsteam:
Checked out the crucible and furnace more closely this morning. The crucible really took a beating last time when it got stuck to the furnace wall, and is no longer usable. Here's a picture of the used one and a new one.
Besides the chips cracks and shape distortion, the walls had thinned quite a bit. I sure hope I'm not going to get only 3-4 melts out of a crucible or this is going to get expensive fast. I don't think the 3 hour marathon the other day did it any good. Also I really had the blast up at the last 20 minutes of the last pour -- maybe that was too much. Could also be to much air early on, or maybe even the soda ash flux. I could try a slightly more reducing flame.
Anyway, hope to get more pours out of the next one.
vtsteam:
The furnace lining on the other hand is doing great. It is just somewhat more glazed than it was. Starting to look like pottery.
NeoTech:
Hmm, if the extra volume of metall is a problem, i would prob. just cast the piece a bit thicker so not that much extra added material was needed and then just cut it down to size and hopefully get the imperfections out that way. =)
Isnt there someting like a flux for cast iron? Wouldnt that solve the gas inclusions problems?
awemawson:
You always get the crud and voids at the top of a shallowish casting unless you make gating to avoid it. I'd be tempted to make up the mould vertically with an extra sacrificial bit to machine off, and also increase the head of the pouring basin
MetalCaster:
A couple of suggestions from the things I have learned in the past:
1. All crucibles are not created equal, some are rated for ferrous metal, and some are not. Crucibles have a maximum temperature rating, and for cast iron work, you should be using a crucible that is documented for operation at around 3000F, ferrous-metal-rated (generally the clay type, but not all clay types).
2. According to the crucible literature I have seen, the burner should enter the furnace wall at the centerline where the crucible sits on the plinth, and the burner should not impinge on the crucible, but rather the burner flame should enter to one side of the crucible. This entry point is also important to get the flame to swirl around the crucible and distribute the heat evenly without overheating any one spot on the crucible.
3. The crucible literature mentions that the crucibles should be carefully lifted from the lower part of the crucible, with full support around the crucible.
4. There is a book called "Basic Principles of Gating and Risering" by the American Foundrymen's Society, Inc. that is very helpful in designing sprue systems that do not aspirate air, and gate and runner systems that fill the mold evenly while skimming off slag, and risers where needed for shrinkage, as well as venting to relieve gassing from the cores and mould itself. Venting is extemely critical if you are using water-based sand and core mixes. Oil-based sands do not work very well with cast iron since the metal is so hot it tends to wash and errode the sand. I have seen some use oil-based sand with cast iron with some success though.
I have heard reports of people using quality cast-iron-rated crucibles for 100 pours or morre. There is no reason for a good quality crucible to fail prematurely unless you use it in excess of its temperature or metal-type rating.
The soft fire bricks that have a lot of air entrained in them will not hold up to cast iron temperatures. The hard fire bricks apparently will hold up to cast iron temps as you demonstrate, but the hard bricks may need a little more insulation behind them since they will not insulate as well as air-entrained bricks.
Good luck.
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