Maximize Your Crucible’s Service Life
Crucibles with a long service life allow managers to plan longer production campaigns, schedule crucible replacement with minimal downtime and reduce replacement frequency. Further, mindful crucible care avoids dangerous incidents of failure occurring during use. While service life depends on the manufacturer and product quality, metalcasters also can extend service life though proper crucible care.
For the best results, your crucible should be the right one for your application. Crucible choice depends on:
- specific alloy or range of alloys;
- type of furnace;
- melting and holding temperatures;
- rate of temperature change;
- how the crucible is charged;
- how slag or dross is removed;
- how the crucible is poured.
Once the crucible is purchased and delivered, it’s up to you to properly care for and use it to optimize service life.
Handle Your Crucible With Care
When your crucible arrives on your receiving dock, perform an inspection to verify it did not suffer shipping damage. First, make sure it was well protected with packing materials during shipment and the materials are intact. Next, clear away enough of the packing to gain a clear view of the entire crucible. Examine the crucible as you would a new car delivered to you off the showroom floor. Look for chips, visible cracks, abrasions or other surface damage. If you see or suspect damage, contact your crucible supplier. Never place a damaged crucible in service. It is likely to fail and is a serious safety hazard.
If the new crucible passes your inspection, move it carefully to storage or the furnace. Ideally, crucibles should be moved on the pallet or in the shipping carton. Certain actions must never be performed when moving a crucible:
- Never roll the crucible. Rolling it on its side, base or top will crush the protective glaze and possibly damage the underlying material. Damage to the glaze will lead to oxidation of the crucible.
- Never stack one crucible inside another. The weight of the top crucible can fracture the lower one, while abrasion where the inner and outer surfaces make contact will damage the protective glaze. When stacking crucibles, separate them with hardboard or similar material.
- Never set crucibles directly on the floor. The hard surface may damage the bottom. More importantly, the crucible may absorb humidity coming from the floor and create thermal shock when heated. Set the crucibles on pallets.
- Never use banding directly on the crucible to secure it during movement. Add padding under the band to protect the surface.
Crucibles should be stored at temperatures above 32F (0C) in a dry area. Cold or wet crucibles should not be placed into service because they could cause dangerous water or molten metal explosions.
Installing Your Crucible
Furnace crucible installation processes differ based on furnace type, size and design, but several standard practices should be followed:
- Inspect the crucible immediately before installation to be certain it is not damaged from handling and storage and its temperature is above the freezing point of water. If the crucible was subjected to high humidity, it could be damaged by thermal expansion in the walls if the temperature drops below the freezing point. A damaged crucible must not be installed.
- Make sure the base block is level, properly centered in the furnace, at the correct height and larger than the crucible bottom.
- For larger crucibles, use secure straps or a lifting mechanism designed for the specific crucible to lift it into the furnace. Center it on the base block and ensure it is level.
- Provide at least a 0.33-in. (8-mm) gap between the crucible and the top and sides of the furnace to allow for crucible expansion. An insufficient expansion gap will result in cracking.
- If the crucible has a spout, be sure at least a 1.5-in. (38-mm) gap exists between the spout and the furnace 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. Allow a 0.25 to 0.375-in. (6 to 10-mm) expansion gap between the crucible and the grip bricks. (Use a combustible spacer.)
- In gas or oil-fired furnaces, ensure the furnace flame travels in a tangential pattern around the crucible and does not overheat one spot on the surface. 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 containment 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 furnace applications. A drain hole gate is used in gas or oil-fired furnaces.
The main installation consideration for a removable crucible furnace is to be sure the crucible is undamaged and properly sized for the furnace. The lifting shanks also should be properly sized to adequately support the entire crucible.
Preheating Your Crucible Before
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. The preheating cycle specified by the crucible manufacturer should be followed exactly and never rushed. The 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 dimensions and resistivity of the metal into account. In all cases, preheating should be done gradually in its early stages to prevent thermal shock damage.
Crucible Care During Normal Operations
A new crucible is easier to handle and install with care than a crucible already in use. Most damage that reduces a crucible’s service life occurs amid the rush to meet production schedules. Managers who want to maximize service life and prevent in-service crucible failure must emphasize proper crucible care in their operational and safety training programs.
Every furnace operator should adhere to the following practices for various melting and holding operations:
Charging Your Crucible—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. 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. Then ingots can be carefully 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 materials are more resistant to physical damage than others. Know how resistant your crucible is and respect its limitations.
Heating Your Crucible—Even a warm, recently emptied crucible requires gradual and controlled heating before full melting heat is applied. Proper heating will prevent damage from thermal shock. Some crucible materials are more sensitive to rapid temperature changes than others, so different crucible types require different heating cycles. Know and follow the heating guidelines provided by your crucible manufacturer.
Protecting Your Crucible From Chemical Attack—If your melting or holding application requires the heavy use of fluxes or produces large amounts of slag or dross, you must use a crucible type that resists chemical attack and erosion. You can help any crucible type’s chemical resistance by using the minimum quantity of flux required and only adding it after the metal is fully molten. This will help reduce slag and dross, as well. Dross has a high expansion rate and causes internal pressure that can damage the crucible and accelerate chemical attack. Carefully clean your crucible every day while it is still red-hot.
Never Overheat Your Crucible—Every crucible has a maximum temperature limit. Exceeding it will damage the crucible, reduce its service life and possibly lead to crucible failure. The best defense against overheating is to take frequent temperature readings of the molten bath with a portable or fixed pyrometer. Remove from service any crucible that was subjected to temperatures above its maximum limit.
~Didier Finck, Dieter Heumannsckaemper, Brett Fiddes and Ramdas Chitalker, Morgan Molten Metal Systems, Wallingford, Connecticut