Sometimes the difference between a crucible that consistently performs well and one that fails to meet even minimal standards may have nothing to do with the crucible itself. Rather, the performance difference may have everything to do with how the crucible is handled, operated and maintained at the foundry. For example, if some crucibles provide a long service life while other identical crucibles fail after a couple of heats, operating conditions at the foundry typically causes those premature failures. Often, following some basic operational procedures will eliminate early crucible failures. Here is a countdown list of the top ten ways to help you maximize your crucibles’ service life.
10.Check crucible for cracks and damage when received. Be sure to handle and store it properly. Improper handling easily damages crucibles. When a new crucible arrives, inspect it carefully for chips, cracks and abrasions and never put a damaged crucible into service. Also, when moving crucibles to a furnace or into storage, never roll them: this damages the protective glaze. And, never stack crucibles one inside the other because they will crack. Finally, never sit them directly on a concrete surface because they will absorb water on the bottom.
9. Install crucibles according to the manufacturer’s instructions. While installing a crucible is quick and easy, care must be taken to follow the instructions regarding base size, clearances between the crucible and the furnace and the initial preheating of the crucible. Too small a base may fail to provide sufficient support, and the wrong base height in a fuel-fired furnace may cause the burner flame to create an overheated and oxidized area on the crucible. Insufficient clearances between the crucible and the furnace sides and top may cause cracking when the heated crucible expands. Also, insufficient initial heating may result in crucible failure on first use.
8.Prevent thermal shock with proper preheating. While some types of crucibles are designed to resist damage from rapid heating, all crucibles require careful preheating if allowed to cool between melts. Thermal shock causes cracking in crucibles heated too quickly. Follow specified preheating recommendations.
7. Never exceed the crucible's maximum temperature limits and never let metal freeze in the crucible. Every crucible has a maximum temperature limit. If you exceed that limit the crucible will be damaged and may fail. Keep close control of crucible temperatures. Allowing molten metal to solidify in the crucible also may damage the crucible when that metal is reheated for pouring.
6. Avoid physical damage by charging correctly and carefully. Dropping a heavy casting or ingot into a furnace crucible may chip or crack the crucible, leading to failure. Follow proper charging practice of first loading small charge materials and then carefully lowering heavier materials on top of cushioning base. It is important also not to pack charge materials tightly into the furnace. When wedged material is heated, it expands and can crack the crucible.