GRAPHITE BREAKDOWN IN CRUCIBLES

Pure graphite crucibles are the most common form of crucible used by casters who melt with electric induction heating. Serious problems can occur if the graphite breaks down to powder form and is poured with the cast. The main problem is graphite inclusions. These inclusions begin as small black deposits that form at the tree base. Usually the tree base will show small cracks. These cracks when magnified will show a jet black deposit. As the accumulation of graphite increases, the tree will become darker with a dendritic type structure (dark crisscross pattern) creeping up the tree. When the accumulation becomes large enough, cracking with black deposits will appear.

Graphite which is a man made material, is available in three quality grades. The raw material used to manufacture graphite is susceptible to quality differences similar to raw material changes in investment powder. The main difference between graphite quality is the pore size or porosity rate. The higher grade of graphite has a 10 times lower porosity rate.