example, a crucible’s ability to melt metals from iron to zinc may not be as important to aluminum alloy melt- ing operations as a crucible’s ability to resist corrosion damage from metal treatment fluxes.
Melting & Holding Temperatures Te metals and alloys melted or
held generally determine the tempera- ture range in which the crucible must operate. Crucibles must never be heated above their maximum temperature. Tis can lead to dangerous failure. Operat- ing below the lower temperature limit also can cause problems. For example, crucibles designed for the high tempera- ture melting of copper-based alloys will oxidize if used at the low temperatures required for zinc melting. Melting and holding practices
involving metal temperatures also must be taken into consideration when selecting a crucible. If operations involve superheating, the higher metal tempera- tures should be taken into account.
Crucibles that provide impact resistance are often selected for furnaces that are manually charged with ingots and other solid materials.
Rate of Temperature Change Te ability of a crucible to
handle rate of temperature change is as important as its minimum and maximum temperature limits. If operational practices lead to frequent heating and cooling cycles or are oth- erwise subject to rapid temperature changes, a crucible that is resistant to thermal shock should be selected. Some crucible types are better for
handling rapid temperature change than others. For example, high carbon content of the graphite in a crucible imparts high thermal conductiv- ity and non-wetability. When that graphite forms a directionally ori- ented matrix, the crucible provides high thermal shock resistance. This is critical to metalcasting facility applications where temperatures can change by several hundred degrees in seconds.
How the Crucible Is Charged
If a furnace is continuously charged with molten metal, it may not require a crucible designed to be highly resistant to physical damage. However, if metal ingots or other heavy materi- als make up the bulk of the charge and are not carefully lowered into the furnace with an automatic loading system, it may be necessary to select a mechanically strong crucible able to survive physical shock. Crucibles featuring high carbon content and
May 2012 MODERN CASTING | 37
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