machine#construction: casting#materials: iron

Iron Casting

Cast iron is usually melted in a cupola in a commercial setting but hobbyists also use gas fired crucible furnaces and cokeless cupolas.

The biggest problem with melting iron is indeed the container. If you have a suitable crucible, a crucible type furnace fired by gas, coke or charcoal can be used. This is a forced-induction type furnace with a blower to drive additional oxygen in with the fuel. The crucible is "crucial" in this technique. If it is dropped, banged, or just gets in a bad mood, it can break and spill 2300 degree molten iron all over everything... not generally a good thing for carbon-based life forms.

The cupola is the other type furnace most commonly used to melt iron. Instead of trying to contain the iron in the crucible, the cupola itself is the container. The cupola is just a straight chimney, lined with refractory and having a set of clamshell doors on the bottom.

Let's start a melt. The doors are closed, a thick layer of sand is tamped in to form the actual container, and then alternating layers of iron and coke are loaded into the top of the cupola. The fire is started in the cupola, the blast turned on, and the coke/iron burns downward, replenished through the top as required.
A few inches from the bottom (or feet depending on the size of the cupola) there is a tap hole (plugged with clay at the start) and a slag hole a little further up. Liquid iron puddles in the bottom and slag begins to finally run out the upper hole. Once the proper amount of iron has collected, a clay plug is knocked out of the tap hole and molten iron runs out into molds or a ladle (used to carry molten iron to the molds). The ladle is safer than the crucible, as it usually is a steel ladle with long handles and a refractory lining. You might drop it, but it's very hard to break.

If we want to keep melting, just pack another ball of clay (called a bod) in the tap hole, start the blower back up, and keep adding layers of iron and coke. If we want to wait a while, just leave everything open and don't start the blower. If we are finished, we open the doors, let everything fall out under the cupola and let it cool off. *1

modern iron alloys and common products that contain them/testing techniques,
explain what these mean:
grey iron
nodular iron
iron carbide (white iron)
wrought iron (technically not a cast material but here for the sake of clarity)

designing for use with cast iron,
casting techniques,
what does ferrosilicon do?,
heat treating techniques (explain what these mean: normalizing, hardening, ageing and artificial ageing)
wedge test, troubleshooting

*1 Mike Callahan

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Last-modified: Thu, 14 Dec 2006 19:07:51 GMT (1489d)
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