GRAPHITE BREAKDOWN: FLUXING CRUCIBLE

Fluxing of graphite crucibles helps increase the reusability and stalls the breakdown of graphite particles. The use of flux, however, becomes a double-edged sword. The use of flux in high fluidity silicon alloys such as United's ULTRALOY and 200 Series causes the flux to adhere to the fluid metal. This flux will now contain graphite and silicon oxide. If the metal is hand poured and the stream is clear, no problems should arise. if the stream is cloudy, the flux containing oxide will be poured into the metal, with a possibility of freezing internally. In automatic machines such as the Inresa, flux never should be used, due to the clogging of crucible. Whenever possible a gas cover should be used. Forming gas, Argon or Nitrogen are all helpful in retarding breakdown of the crucible as well as preventing oxidation of any alloy.

RECOMMENDATIONS

In summary, we would recommend the following:

1. If possible use ceramic crucible, or clay-graphite.
    
2. Ask suppliers to obtain higher grade graphite. Increased cost should be offset by increased crucible life.
    
3. Fluxing is recommended for hand pouring operations with clay-graphite crucibles.
    
4. Use a gas cover under all heating, melting and pouring conditions.
    
5. Re-shot used metal if black particles appear in trees. Flux and remove the oxide build-up before pouring
    
6. Always watch trees for signs of build-up. Elimination of these contaminants will greatly reduce unnecessary rejects.
    
7. Never use borax in silicon carbide crucibles.