in a foundry, why does the molten metal not melt the vat in which it’s being heated?

Question

I mean, obviously the vat must be made of a metal with a higher melting point. But then how did they cast the metal to make that vat?

In: Chemistry

Answer 1

The Crucibles where I work are ceramic, a mixture of special clay (similar to what’s in a dinner plate) and graphite, like in a pencil- about 90% graphite. They’re molded like clay, pounded by hand with mallets in some cases, and slowly dried for days, even weeks until they harden. They have to be absolutely dry before use, any water in them will cause them to quite literally explode at molten metal temperatures.

The melting point of graphite is 3600 degrees C. Iron is molten above 1600C, and copper around 1200C, so graphite crucibles are quite capable of not melting under molten metal temperatures.

Instead, what gets them, is either thermal expansion causing cracking over time (The crucible gets larger, then smaller each time it is heated then cooled) or the reactivity of the metals in the melt reacting with the graphite and generating slag. Aluminium is our least favorite, if the glassy surface of the crucible is damaged, aluminium in an alloy can get in and eat a graphite crucible from the inside out.

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Then there are some other tricks, like an insulating sand liner behind the crucible, and water cooling behind that to keep the heat of the melt from cooking the heating coils, the hydraulics and the other stuff that surrounds the crucible.