BASICS OF CRUCIBLE MELTING Most crucibles used in the metals industry for melting, holding and/or transferring operations are preformed vessels manufactured from carbon–bonded silicon carbide or ceramic–bonded clay graphite. Crucibles may be as small as teacups or may hold several tons of metal. They may be fixed in place within a furnace structure or may be designed to be removed from the furnace for pouring at the end of each melt. Crucibles are used in gas and oil–fired furnaces, in electric resistance furnaces and in induction furnaces. They come with or without pouring spouts and in a wide variety of traditional and specialized shapes.
CRUCIBLE ADVANTAGES With their unique ability to melt, hold and transfer metal using a single vessel and to allow even incompatible alloy changes to be made simply by switching vessels, the operational flexibility of crucibles is unchallenged. However, even when fixed within the furnace structure, crucibles offer important advantages when compared to directly heated fuel–fired furnaces and to electric resistance or induction furnaces with rammed refractory linings: •
Lower Metal Loss / Cleaner Metal — In fuel-fired furnaces where the metal is heated directly by a flame produced by gas or oil burners, there are both high levels of metal loss and increased oxidation due to the combustion products generated. In fuel–fired crucibles furnaces, however, the flame is isolated from the metal and the metal is heated indirectly with heat conducted through the crucible. This indirect heating greatly reduces metal loss and metal oxidation, resulting in lower metal purchase costs, lower energy costs per pound of metal poured and less slag or dross formation.
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Alloy Flexibility — Even when fixed-in-place in a furnace, a crucible’s high density interior surface reduces the likelihood of cross contamination when alloy changes are made.
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Quick Replacement — Removing a worn or damaged crucible from a furnace and installing a new crucible is a faster, simpler and less technically demanding process than removing and replacing a worn or damaged rammed refractory furnace lining. Also, because crucibles are preformed to exact material and dimensional specifications, there are few concerns about improperly compacted refractory or incomplete sintering.
CRUCIBLES THEN AND NOW Crucibles, in the form of well-fired clay pots, were present at the dawn of the metals industry, some 6,000 years ago, and gave early metal workers the ability to melt metal in a durable container that could then be used to pour the molten material into a mold. At their most basic level of form and function, crucibles today are little changed from their early prototypes. But in their materials, construction, thermal efficiency and service life, For additional information on Morgan Molten Metal Systems’ products or to find a location nearest to you, please visit:
www.morganmms.com Submitted for publication by the AFS/ASM Association in the upcoming Metals Handbook
BASICS OF CRUCIBLE MELTING CRUCIBLES THEN AND NOW
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modern crucibles are vastly superior to those in use just a short time ago. Designed for a wide range of traditional applications and new processes, today’s crucibles are formed using a wide variety of materials and processes that impart specific characteristics, capabilities and limitations to each of the many types of crucibles available from crucible manufacturers. Determining what properties your application requires of a crucible is the most important step in selecting the proper crucible to meet your needs.
MATCHING CRUCIBLE AND APPLICATION In the metals industry, most crucibles are used to melt, hold and/or transfer nonferrous metals — commonly aluminum, zinc and their alloys and copper and copper-based alloys, as well as for precious metals. They can also be used for higher temperature alloys, such as, nickel bronze and cupronickel and, to a lesser extent, for melting ferrous metals, such as gray iron. Knowing the metal you want to melt, hold and/or transfer, however, is just the start in selecting the right crucible type for your application. You also must know the specific crucible properties that are important for your particular application. These properties include: •
Maximum temperature limits — It is important that the highest temperature reached by your application does not exceed the maximum temperature rating of the crucible.
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Tolerance of thermal shock — If your application results in rapid heating or cooling of the crucible, you will need to select a crucible type that resists thermal shock.
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Impact resistance and mechanical strength — If the crucible in your application is used to transfer metal or if it is subject to the impact of dropped charge materials, you will want a crucible with the strength and durability to survive to these activities.
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Erosion/corrosion resistance — Does your application produce slag, dross or other byproducts or use fluxes that chemically attack the inside surface of the crucible? If so, you will want a crucible that resists such attack.
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Oxidation resistance — When exposed to air at high temperatures, graphite in a crucible will oxidize. Crucible glazes are designed to afford protection to the crucible material and different glaze compositions are specified depending on the intended operating temperature.
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Non-wettability — If it is important in your application that the inside surface of the crucible resist the intrusion of molten metal into pores in the crucible material, you will need a crucible that minimizes wetting.
For additional information on Morgan Molten Metal Systems’ products or to find a location nearest to you, please visit:
www.morganmms.com
BASICS OF CRUCIBLE MELTING MATCHING CRUCIBLE AND APPLICATION
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Electrical properties — If your application requires a crucible that heats inductively, you will need a crucible designed for use in an induction furnace and with a specific electrical resistivity that optimizes the coupling of the crucible with the induction coil.
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Thermal conductivity — Not all crucibles offer the same level of heat transfer through the crucible and into the metal. To achieve the lowest energy costs, you should select the crucible with the highest level of thermal conductivity that meets all of the other requirements of your application.
Crucibles have been called the first composites and modern crucibles use a carefully balanced combination of materials to achieve the important characteristics listed here. •
Graphite — A foundry crucible usually contains 30% to 50% by weight of flake graphite, depending on the properties required. The carbon content of graphite imparts high thermal conductivity and non-wettability and, coupled with its platelike layered matrix structure, provides high thermal shock resistance. This is critical to foundry applications where temperatures can change by several hundred degrees in seconds.
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Silicon Carbide — A foundry crucible typically will contain 10% to 50% of silicon carbide, based on the desired characteristics. Silicon carbide provides excellent resistance to high temperature erosion and corrosion and imparts a high level of thermal conductivity to the crucible.
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Silicon Metal — Fine powders of silicon metal or alloys are used to increase strength and erosion resistance, to protect the graphite and carbon bonding matrix from oxygen and to help ensure an even glaze coating.
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Glass — Glass is used to provide a semi-permeable barrier to gases, controlling the ingress of oxygen and egress of combustion products.
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Aluminosilicates — Aluminosilicates are used to modify the refractory properties of the crucible or internal glass composition.
There is not a single crucible material recipe or manufacturing process that offers the highest level of every desirable characteristic for every application. Therefore, your best course will be to prioritize the list of crucible properties most important for your application and review those requirements with your crucible provider to find the crucible product best suited to your needs.
For additional information on Morgan Molten Metal Systems’ products or to find a location nearest to you, please visit:
www.morganmms.com
BASICS OF CRUCIBLE MELTING CRUCIBLE CARE With proper care, crucibles will provide long service life, but like every system used in the processing of molten metal, crucibles must be handled properly and must be operated within their design parameters. All crucibles may be damaged by: •
Excessive Temperatures — Crucibles must not be heated beyond their specified maximum temperature capability. Excessive temperatures will cause crucible failure.
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Thermal Shock — Structural damage may be caused by too rapid heating or cooling of the crucible. Uneven heating or frequent thermal cycling between cold and hot can also result in crucible damage.
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Mechanical Shock — Damage caused to the crucible by the impact of charge materials and/or, in the case of removable crucibles, by impacts during movement.
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Damage to Crucible Glazes — Crucible glazes serve as a barrier against oxygen attack of the carbon material making up the crucible. These glazes may be damaged by any of the previously noted hazards and such damage will lead to a reduction in crucible life.
Crucibles must be thoroughly inspected for shipping damage when received. If cracked, chipped or otherwise damaged, they must not be installed. Crucibles should be stored inside in a warm and dry environment and must be completely dry before use. All claygraphite crucibles must be properly annealed before being subjected to the first full heat. This will help the crucibles withstand thermal shock throughout their service life. All crucibles should be preheated using the recommended procedure whenever entering use from a cold, i.e., room temperature, condition. During operations, the crucible should be inspected for damage after each heat and slag or dross buildup inside the crucible must be removed. Damaged or worn crucibles must be replaced immediately. Select the right crucible for your application, install it properly, operate it within its design parameters, protect it from physical and thermal damage and replace it immediately when damaged or worn. Follow these rules and your crucibles will provide excellent service, operational efficiencies and significant economies.
For additional information on Morgan Molten Metal Systems’ products or to find a location nearest to you, please visit:
www.morganmms.com