Tips for maximizing crucible life in your aluminum foundry.
The crucible holds a very prominent place in metal-casting's history. Early on, man learned the power of the process when he discovered a hollowed-out stone could be used as a container for melting metal.
By the time of the industrial revolution, clay and graphite crucibles were being produced in Europe, and the crucible, the primary tool of melting and holding metal, has been refined many times since. The last few years have brought newer technology to the market in isostatic pressing and special glazes.
Isostatic pressed crucibles are more dense and thus conduct heat faster, but may be less resistant to thermal shock. They can be helpful in certain applications.
The growing popularity of electric resistance melting led to the development of low temperature glazes and specialty glazes, which were developed to help deal with other unique applications, such as the sealed systems of low-pressure casting.
While technology has advanced the crucible, remembering the basics is where foundrymen can truly extend the life of the crucible. Crucible life is determined primarily by foundry procedures. The good manager knows that all shop procedures and equipment must be constantly reviewed and maintained.
It is estimated that the U.S. aluminum foundry industry consumes $20 million in crucibles a year. Taking simple steps to optimize crucible campaigns can save aluminum foundries as much as 50% in those costs alone.
In addition to enlisting the expertise and the resources of crucible suppliers (who will help fine-tune your operation), aluminum metalcasters can discover greater crucible productivity by remembering some basic tips for proper crucible care.
Oxidation of a crucible occurs as the carbon bond is slowly burned out and the physical properties of the crucible change. The most significant of these changes are the reduction of conductivity (which increases the melt time and energy consumption), and the decrease in mechanical strength.
Oxidation is a natural part of the aging process of all crucibles. Certain foundry conditions, however, can speed the process toward premature failure.
Oxidation around the top rim of the crucible is a major cause of early crucible failure in aluminum melting operations, especially in electric resistance furnaces. The telltale sign of top rim oxidation is a series of small cracks running vertically around the top of the crucible.
1) Maintain proper air/fuel mixture. Fuel-fired furnace mixtures should produce a slightly oxidizing flame (a flame with slightly green tips). Too much oxygen in the mix will produce a flame with a high oxidizing environment (deep green flame) and contribute to crucible oxidation. Regular maintenance (in conjunction with your local utility company, if necessary) will keep your burners "in-tune."
2) Ask your supplier about the availability of special glazes. Some crucible manufacturers offer an additional glaze on the top portion of the crucible to help delay the onset of oxidation. If top rim oxidation has been a problem in your operation, then this extra glaze is probably worth any additional cost.
3) Use proper installation procedures. Top rim oxidation often occurs when the top of the crucible doesn't receive enough heat to activate the protective glaze. Foundries often put ceramic fiber on top of the crucible to allow for expansion. But if the fiber is packed into the combustion chamber between the crucible and furnace wall, the top portion of the crucible is actually insulated and receives little or no heat. This accelerates oxidation.
4) Keep the crucible full of metal when possible. When the crucible is only half full, the difference in the heat ratio between the upper and the lower halves can cause cracking and the empty inside wall is more exposed to oxidizing conditions.
Most aluminum fluxes contain alkaline salts (chlorides and fluorides). Particularly detrimental is sodium fluoride. Crucibles are porous at operating temperatures and the flux can actually travel through the crucible wall toward the hottest point - the furnace combustion chamber.
Flux attack on crucibles can be easy to diagnose. The classic sign is a distorted outside wall and a foamy-looking glaze that is sometimes greenish in color [ILLUSTRATION FOR FIGURE 1 OMITTED]. Recommendations:
1) Use the minimum amount of flux needed to clean the melt. Overfluxing is probably the major cause of flux attack on crucibles. Closely follow the recommendations of the flux manufacturer.
2) Never put flux in the bottom of an empty crucible. The alkaline fluxes melt at a lower temperature than the aluminum and when no molten aluminum is present, the flux will react directly with the crucible wall.
3) Do not superheat the melt. Since the flux is activated by heat and seeks the hottest point, overheating the melt produces a much higher furnace temperature and accelerates the migration of the flux through the crucible wall.
4) Keep the sidewalls of the crucible free of slag buildup. This slag contains flux material that is released when heated, and with no melt to go into, quickly penetrates the crucible wall.
In fuel-fired furnaces, perhaps the biggest cause of premature failure of crucibles is poor burner maintenance or flame impingement. Crucibles work best in an environment that provides even, consistent heating. The flame needs to be unrestricted and swirl freely around the crucible.
Burner impingement can cause a variety of problems, and the visual signs are often difficult to interpret. Horizontal cracks are common, but a clear sign of uneven heating is a crucible that has different colors and glaze conditions on its outside [ILLUSTRATION FOR FIGURE 2 OMITTED], which indicates hot and cold regions.
1) Maintain proper burner alignment. Many flame impingement problems are caused simply by poor burner adjustment. The old rules of thumb still apply - the flame should fire directly into the combustion chamber and swirl around the crucible to the top. It should never hit directly onto the crucible or baseblock. The center-line of the burner port should be at or below the top of the baseblock.
2) Keep furnace refractory walls and floors free of debris. Many crucible failures happen due to flame impingement caused by debris on the furnace floor or wall that diverts the flame directly onto the crucible and causes overheating. A small piece of refractory or slag on the floor, or an uneven furnace wall can divert the flame from its natural swirl and create a hot spot, or disturb the flame pattern so much that a portion of the crucible receives little or no heat.
3) Keep the drain hole covered. An open drain hole can cause a funnel effect and draw in cold air when the burners are turned off. The rapid cooling can lead to thermal cracks.
Crucible installation varies from shop to shop. Installing a crucible may not seem to require high technological expertise, but in the rush to minimize the downtime of crucible installation, basic principles are often forgotten.
1) Always use the proper size crucible stand (baseblock). The baseblock must be: made of the same material as the crucible to ensure that they both expand and contract at the same rate; the correct height to provide proper burner alignment; and be 2 in. larger in diameter than the crucible base to give a margin for error on installation. (You will never hit it exactly on center when installing the crucible.)
You can reuse a baseblock, but it isn't recommended. Stands also oxidize and become fragile with use, and since the cost of a new baseblock is minimal compared to the cost of the crucible and the installation labor, it's wise to use a new stand with each new crucible installation.
Crucibles used in electric resistance furnaces should also be set on crucible stands.
2) Don't bind the crucible to the furnace. Crucibles expand considerably during heating and must have room for expansion. Furnace designs that cement the crucible to the furnace or bind it with heavy lids will lead to expansion cracks. Most manufacturers recommend layering a thermal blanket on top on the crucible to act as an expansion cushion [ILLUSTRATION FOR FIGURE 3 OMITTED].
The oxides in aluminum cause the accumulation of slag deposits on the inside of the crucible wall, and a buildup of slag is quite harmful to crucible life. When heated, this slag layer expands much faster than the crucible. At 1292F (700C), the expansion of the slag is 10 times that of the crucible, and a 3/8-1/2 in. layer can crack a crucible.
This slag also contains fluxing material and increases the danger of flux attack.
Clean the walls of the crucible daily. This slag is soft and pliable when hot and is easily removed by scraping the wall vertically with a tool that has rounded edges and a curvature similar to the inner wall of the crucible [ILLUSTRATION FOR FIGURE 3 OMITTED].
The competitive nature of the foundry market has caused most foundries to tighten all shop procedures, and crucibles broken by dropped or wedged ingot and scrap are less common today. There are, however, practices that are sometimes overlooked.
1) Don't load tilt furnaces in the tilt position. Although this seems to make it easier to be careful loading ingots, in fuel-fired furnaces the flame pattern can be disrupted in the tilt position leading to hot spots on the crucible.
2) Don't leave a heel of metal in the crucible. While leaving a heel of molten metal between shifts speeds the breakdown of the next heat, a heel should never be allowed to harden in the crucible. Heels encourage expansion cracks, and the metal may still contain fluxing material that will be released into the hot crucible.
Shipping and Handling
No matter how careful the shippers and your shop personnel are, handling damage can occasionally happen.
"Ring-test" all crucibles prior to installation. Small hairline cracks are undetectable to the eye, but are easily detectable to the ear. Tap the inside rim of the crucible with a metallic object. The crucible should have a bell-like ring. A deadened sound ("thud") may indicate the presence of a crack.
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|Author:||Robison, Stephen T.|
|Date:||Nov 1, 1996|
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