cycle specified by the crucible manu- facturer should be followed exactly and never rushed. Te proper preheating cycle depends on the crucible material, metal, furnace type and whether it is used for melting or holding. For induction furnaces, the preheating
cycle takes furnace frequency, coil dimen- sions and resistivity of the metal into account. In all cases, preheating should be done gradually in its early stages to prevent thermal shock damage (Fig. 5).
Crucible Care During Normal Operations
Fig. 3-5. Clockwise from right: Flame directed to one spot on a crucible produced this damage. Lifting shanks must be properly sized for the crucible and provide adequate support. Thermal stress caused this crack extending across the crucible’s bottom.
spout tray for crucible expansion.
• Use a ceramic fiber blanket to seal the space between the furnace top and the top edge of the crucible.
• In a tilting furnace, use locating bricks designed to hold the crucible in position while allowing for crucible expansion (Fig. 2). Allow a 0.25 to 0.375-in. (6 to 10-mm) expansion gap between the crucible and the grip bricks. (Use a combus- tible spacer.)
• In gas or oil-fired furnaces, ensure the furnace flame travels in a tangential pattern around the crucible and does not over- heat one spot on the surface (Fig. 3). Even heating is essential to achieving a long service life.
• To prevent damage to the crucible, properly seal the drain holes, which allow molten metal to flow out of the furnace shell into a contain- ment area in the event a crucible fails during melting or holding.
In general, the drain hole is sealed using a thin (0.04-in. [1-mm] or less) zinc plate in electric-fired fur- nace applications. A drain hole gate is used in gas or oil-fired furnaces. Te main installation consideration
for a removable crucible furnace is to be sure the crucible is undamaged and properly sized for the furnace. Te lifting shanks also should be properly sized to adequately support the entire crucible (Fig. 4).
Preheating Your Crucible Before First Use
All new crucibles
and completely cooled used crucibles must be preheated empty before they are placed into service for melting or holding operations or to transfer molten metal. Te preheating
Most damage that reduces a
crucible’s service life occurs amid the rush to meet production schedules.
44 | MODERN CASTING September 2011
A new crucible is easier to handle and install with care than a cru- cible already in use. Most damage that reduces a crucible’s service life occurs amid the rush to meet pro- duction schedules. Managers who want to maximize service life and prevent in-service crucible failure must emphasize proper crucible care in their operational and safety train- ing programs. Every furnace operator should
adhere to the following practices for various melting and holding operations: Charging Your Crucible—
Fig. 6. The impact of charge material dropped into the crucible produced this damage.
Improper charging practices are crucible killers. Dropping heavy charge materials into an empty crucible causes physical damage that reduces crucible service life and can cause sudden crucible failure (Fig. 6). When charging an empty crucible, small scrap materials and returns should be carefully placed in the bottom of the crucible first to protect the interior surface. Ten ingots can be care- fully added. If you are only charging ingots, slowly lower them vertically into the furnace. Use tongs if the furnace is deep. Do not pack metal tightly; allow room for the metal to expand to reduce the likelihood of bridging. Some crucible materi- als are more resistant to physical damage
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