• Method of charging. • Fluxes or additions used. • Degassing or refining processes. • Method of slag or dross removal. • Method of emptying the crucible. Any additional processes or
requirements that may be operation- specific also should be considered. Tis includes tolerating/avoiding alloy cross-contamination. In addition to detailed operation
information, the metalcaster must obtain information about available crucible materials, characteristics and performance. On a practical level, a single crucible type may not offer the highest level of every desir- able characteristic for each applica- tion. Crucible performance charac- teristics often involve trade-offs. For example, the crucible with the best thermal conductivity may not offer the best protection against thermal shock. Prioritize a list of crucible properties most important for the application and review those priori- ties with the crucible supplier.
Furnace Capacity, Dimensions & Type The furnace metal capacity will
dictate the needed capacity of the crucible. Similarly, the available space in the furnace will dictate the dimensions and space available for the crucible. This also will help determine whether the crucible should include a pouring spout. Other, less obvious factors must be considered in choosing a crucible to match the furnace type, as well.
Fuel-Fired Furnaces Fuel-fired furnaces include furnaces
powered by gas, oil, propane or coke. Each of the fuels directly exposes the crucible to the heating source and provides a different level of heat, nor- mally measured in BTUs. Te selected crucible must be able to withstand the maximum BTUs the furnace fuel can apply. In gas, oil and propane furnaces, the crucible must be able to tolerate the effects of the burner flame at the base and be tapered to allow the flame to circulate around the crucible from bottom to top for even heating. Te material also must resist oxidation damage from the flame and accom-
modate the rate of thermal change the crucible will experience. Good thermal conductivity and
even heating are important proper- ties in transferring heat from the interior of the furnace through the crucible to the metal charge. Crucibles with high graphite content in the carbon binder offer high thermal conductivity for fast melting in gas-fired furnaces.
Electric Resistance Furnaces Electric resistance furnaces provide
even, all-around heating of a crucible and are ideally suited to precise tem- perature control in metal holding appli- cations. However, they are slower than fuel-fired furnaces in melting applica- tions. Consequently, energy efficient crucibles with high graphite content in the carbon binder are often selected to provide high thermal conductivity for faster melting in these furnaces.
Multiple considerations must be taken into account in selecting the right crucible for a specific application.
Electric resistance furnaces are
typically basin-shaped and provide a uniform distance between the crucible and the furnace heating elements.
Induction Furnaces Selecting a crucible for an induc-
tion furnace is more complex. In some applications, such as refining precious metals, crucibles designed to be heated in the furnace’s inductive fields are used to melt the charge. In other applications, crucibles allowing the inductive field to pass through and heat the metal charge directly are used. Terefore, the electrical characteristics of the crucible should be matched to both the operating frequency of the furnace and the melting application. In some designs, lower frequency induction furnaces require crucibles with high silicon carbide content, and in other applications, higher frequency induction furnaces require crucibles with high clay content. Matching the crucible’s electrical resistance to the induction furnace is key in preventing crucible overheating. Most crucibles designed for induc-
tion furnaces are cylindrical to provide uniform distance between the crucible and furnace coil. Some small furnaces designed for removable crucibles fea- ture a tapered coil to match the profile of bilge-shaped crucibles.
Removable Crucible Furnaces All of the previously mentioned
furnace types can be designed for removable crucibles. Tese can be charged while outside the furnace or after installation and are removed from the furnace for pouring. Like crucibles used only for metal transfer, they are bilge or A-shaped to allow for lifting with tongs designed to properly sup- port the crucible.
Furnace Power Limitations
A cylindrical crucible is installed in an induction furnace.
A final factor to consider when documenting crucible requirements based on furnace specifications is the availability of power. In many loca- tions, power for melting or holding
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