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www.moderncasting.com for info on extending crucible life.
directionally oriented graphite structures provide excellent impact resistance. A durable protective glaze also can prevent damage
from rough handling, which leads to oxidation damage. Extruded aluminum ingots often have sharp edges that cut deeply into the crucible body, leading to damaging cracks.
Fluxes & Additives All crucibles offer some level of resistance to corrosion
and chemical attack. Most fluxes and other metal treatments used in melting aluminum and other nonferrous metals are highly corrosive and require a crucible with a high level of resistance to chemical attack. Tis resistance is best imparted by a consistently dense crucible material structure and a durable protective glaze. If the melting application involves use of corrosive metal treatments, the appropriate level of protection against these agents is necessary.
Degassing & Refining Degassing aluminum and aluminum alloys typically
involves bubbling inert gas, usually nitrogen, through the molten bath while agitating the bath with a rotor designed to break apart and disperse the gas bubbles. Tese small bubbles then pull the undesirable hydrogen and oxides out of the bath and carry it, along with dross and inclusions, to the surface, where the gas escapes into the air and the solid material can be removed. Tis process, often used along with fluxing agents, physically erodes the crucible and attacks it chemi- cally. Terefore, a dense and mechanically strong crucible, highly resistant to chemical attack, is required. Silicon carbide crucibles provide excellent resistance to elevated temperature erosion and chemical corrosion. Also, isostatically pressing a crucible produces a random arrangement of the graphite structure. Tis contributes to creating denser products that can effectively survive erosive and corrosive conditions. Many refining and metal treatment processes used with other nonferrous metals also call for a crucible that is mechanically strong and chemically resistant. In refining and melting precious metals, the crucible must
provide clean metal by incorporating non-wetting proper- ties. Tat means the crucible must be well sealed against metal penetration and imparted with a dense crucible mate- rial structure and a durable protective glaze. A dense, non- wetting crucible will help reduce slag and dross accumula- tion to make cleaning the crucible easier when it is empty.
Emptying the Furnace
Crucibles for melting and holding molten metal that is dipped out of the furnace need to be designed for easy access to the metal and with high thermal efficiency. Tis allows the furnace to hold the metal at the proper temperature with minimal fuel or power use. Crucibles for furnaces that are tilted for pouring often
require integral pouring spouts that provide the reach and accuracy needed for the pour.
38 | MODERN CASTING May 2012
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