This article is designed to serve as a guide for selecting the optimal crucible for your operation. It explains the relationship between metal melting/holding operations and specific crucible characteristics. It provides support for but does not replace the need for metal melters and crucible suppliers to work closely together in the crucible selection process.

If you melt metal or hold a molten bath, chances are your operation is unique. Your particular combination of furnaces, alloys, working practices, metallurgical treatments, pouring arrangements and end products are not likely to be duplicated at any other facility. So choosing a crucible that will provide maximum performance for your operation is an individualised and complex task.

This article is designed to serve as a guide for selecting the optimal crucible for your operation. It explains the relationship between metal melting/holding operations and specific crucible characteristics. It provides support for but does not replace the need for metal melters and crucible suppliers to work closely together in the crucible selection process.

The modern crucible is a highly heterogeneous, graphite-based composite material, which relies on its material composition and control of the graphite&#;s structural alignment to achieve the performance required. 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 fuel&#;fired furnaces, in electric resistance furnaces, in induction furnaces or simply to transfer molten metal. They come with or without pouring spouts and in a wide variety of traditional and specialized shapes.

They also offer many different performance characteristics since each application presents a complex set of temperature, chemical and physical parameters which define the technical boundaries within which the crucible has to be designed to operate.

So how do you select the right crucible for your operation from the extensive range of crucible types and materials available to you?

The best approach is to begin with your own detailed assessment of your operations. You need to fully document and, where possible, quantify all aspects of your melting, holding and metal handling processes. These include:

• The capacity, dimensions and type of your furnace
• The specific alloy or range of alloys you melt
• The melting and/or holding temperatures you maintain
• The temperature change rate the crucible will experience
• How the crucible is charged
• The fluxes or additions used
• Degassing or refining processes
• How slag or dross is removed
• How the crucible is emptied.

These nine categories reflect the more common factors you must take into account when selecting a crucible to match your specific requirements. You also should consider any additional processes or requirements that might be specific to your operations. An example might be your ability to tolerate or your need to avoid alloy cross-contamination.

While you bring the detailed information on your own operations to the crucible selection process, your crucible supplier must contribute a high level of expertise on crucible materials, characteristics and performance. For the greatest selection, look for a crucible supplier able to offer overlapping crucible product lines suitable for each specific metal but offering different operational characteristics. Then, working together, you will be able to closely match a specific crucible to your specific requirements. Achieving this match is the key to crucible safety, performance and maximum service life.

Be aware, however, that on a practical level, there may not be a single crucible type that offers the highest level of every desirable characteristic for your application. Crucible performance characteristics often involve trade-offs. For example, the crucible with the best thermal conductivity may not also offer the best protection against thermal shock. Therefore, you should prioritise the list of crucible properties most important for your application and review those priorities with your crucible supplier.

Furnace Capacity, Dimensions and Type

The capacity, dimensions and type of furnace you use will establish most of the observable details about your crucible. For example, when you know the metal capacity your furnace was designed for, you will know what capacity your crucible should provide. Similarly, the dimensions of the space for the crucible in your furnace will dictate the dimensions and shape of your crucible. This also will determine if your crucible must include a pouring spout. But choosing a crucible to match your furnace type will give you many other less obvious factors to consider.

Fuel-fired furnaces

Fuel-fired furnaces include furnaces powered by gas, oil, propane or coke. Each of these fuels directly exposes the crucible to the heating source and each provides a different level of heat, normally measured in BTUs. Any crucible selected must be able to withstand the maximum BTUs the furnace fuel is able to apply to the crucible. In gas, oil and propane furnaces, the crucible must be able to withstand the effects of the burner flame at the base of the crucible and the crucible must be tapered to allow the flame to circulate around the crucible from bottom to top. This allows even heating of the crucible. The crucible material also must be able to resist oxidation damage from the flame and accommodate the rate of thermal change the crucible will experience.

Good thermal conductivity and even heating are important crucible properties in transferring the 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 to a crucible and are ideally suited to precise temperature control in metal holding application. But they are slower than fuel-fired furnaces in melting applications. 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.

Crucibles designed for electric resistance furnaces are normally basin shaped and provide a uniform distance between the crucible and the furnace heating elements.

Induction furnaces

Selecting crucibles for induction furnaces is a more complex task. In some applications, such as refining precious metals, crucibles designed to heat in the furnace&#;s inductive fields are used to melt the charge. In other applications, crucibles that allow the inductive field to pass through them and heat the metal charge directly are used. Therefore, it is important to match the electrical characteristics of the crucible to the operating frequency of the furnace and to the melting application. For example, 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 a crucible&#;s electrical resistivity to the induction furnace is key to preventing crucible overheating.

Most crucibles designed for induction furnaces are cylindrical to provide a uniform distance between the crucible and the furnace coil. However, some small furnaces designed for removable crucibles feature a tapered coil to match the profile of bilge-shaped crucibles.

Removable crucible furnaces

All of the above furnace types can be designed to use removable crucibles. These crucibles can be charged while outside or when installed in the furnace, but they are removed from the furnace for pouring. Like crucibles used only for metal transfer, they are bilge-shaped or A-shaped to allow them to be lifted with tongs designed to properly support the crucible.

Furnace power limitations

A final factor to consider when documenting your crucible requirements based on your furnace&#;s specifications is power availability. In many locations, power for melting or holding might not be available at all times or might be prohibitively expensive at certain
times or at certain levels. If this is the case at your facility, it may be particularly important to select an energy efficient crucible.

Metals You Melt and/or Hold

Knowing what metals and alloys you melt or hold will tell you a lot about what characteristics you need in a crucible. Your detailed catalogue of the metals you intend to melt will help to establish the maximum temperature the crucible must support for melting and holding, will define how the metal will interact with the crucible material both chemically and physically and it will be a key factor in determining what characteristics your optimal crucible should offer. A case in point, in melting copper-based alloys in fuel-fired furnaces, roller formed silicon carbide crucibles perform better due to higher thermal shock resistance. In other types of furnaces, crucibles are often selected because of their high density. Less dense and more porous crucibles may allow erosion.

Carbon-bonded and ceramic-bonded clay graphite and silicon carbide crucibles are widely use in melting and holding aluminum and aluminum alloys, aluminum-bronze, copper and copper-based alloys, cupro-nickel and nickel-bronze alloys, precious metals, zinc and zinc oxide. Crucibles also are used in melting cast iron. Taken together as a group, these metals represent a temperature range from 400°C/750°F to °C/°F.

While some crucible types support metal temperatures encompassing a broad spectrum of metals, it often is necessary to select crucibles targeted to specific metals or alloys and with more limited operating temperature ranges. Selecting such crucibles is often more advantageous because they offer performance characteristics important to your operations. For example, using a crucible able to melt metals from iron to zinc may not be as important to your aluminum alloy melting operation as having a crucible limited to the temperature range you need but able to resist corrosion damage from your metal treatment fluxes.

Melting and Holding Temperatures

Generally speaking, the metals and alloys you melt or hold will determine the temperature range within which your crucible must be able to operate. Crucibles must never be heated above their maximum temperature. This can lead to dangerous crucible failure. However, operating below the crucible&#;s lower temperature limit can also cause problems. For example, crucibles designed for the high temperature melting of copper-based alloys will oxidize if used at low temperatures for zinc melting.

Melting and holding practices involving metal temperatures also need to be taken into consideration in selecting crucibles. If your operations involve superheating, you will need to take the higher metal temperatures reached into account.

Rate of Temperature Change

The ability of a crucible to handle the rate of temperature change is as important as its minimum and maximum temperature limits. If your operational practices lead to frequent heating and cooling cycles for the crucible or otherwise subject it to rapid temperature changes, you will need to select a crucible that is resistant to thermal shock. Some crucible types are much better at handling rapid temperature change than others. For example, high carbon content of the graphite in a crucible imparts high thermal conductivity and non-wetability. And when that graphite forms a directionally oriented matrix, the crucible also provides high thermal shock resistance. This is critical to foundry applications where temperatures can change by several hundred degrees in seconds. Your crucible supplier can advise which crucibles provide the best resistance to thermal shock for your application.

How the Crucible Is Charged

If your furnace is always charged with molten metal, it probably does not need a crucible designed to be highly resistant to physical damage. However, if metal ingots or other heavy materials make up the bulk of your charge and they are not carefully lowered into the furnace via an automatic loading system, you may want to select a crucible that is mechanically strong and able to survive physical shocks. Crucibles featuring high carbon content and a directionally oriented graphite structure provide excellent impact resistance.

You also will want a crucible with a durable protective glaze. Damage to the glaze from rough handling can lead to oxidation damage to the crucible. Extruded aluminum ingots often have sharp edges that cut deeply into a crucible&#;s body leading to damaging cracks.

Fluxes and Additives

All crucibles offer some level of resistance to corrosion and chemical attack. But most fluxes and other metal treatments used in melting aluminum and other nonferrous metals are highly corrosive and require a crucible that offers a high level of resistance to chemical attack. This resistance is best imparted by both a consistently dense crucible material structure and a durable protective glaze. If your melting application involves the use of corrosive metal treatments, you certainly will want a crucible offering the appropriate level of protection against these agents.

Degassing and Refining

Degassing aluminum and aluminum alloys typically involves bubbling inert gas, usually nitrogen, through the molten bath while agitating the bath with a rotor designed to break apart and disperse the gas bubbles. These small bubbles then pull the undesirable hydrogen and oxides out of the bath and carry it, along with dross and inclusions to the surface where the gas escapes into the air and the solid material can be removed. This process, often used along with fluxing agents, physically erodes the crucible and attacks it chemically as well. Therefore, a dense and mechanically strong crucible that is highly resistant to chemical attack is required. Silicon carbide crucibles provide excellent resistance to elevated temperature erosion and to chemical corrosion. Also, when isostatically pressed, crucibles form a random arrangement of the graphite in their structure. This contributes to creating denser products that can survive erosive and corrosive conditions more effectively.

Many refining and metal treatment processes used with other nonferrous metals also call for a mechanically strong and chemically resistant crucible.

In refining and melting precious metals, it is particularly important that the crucible you use provide clean metal by incorporating non-wetting properties. That means that the crucible must be well sealed against metal penetration. This characteristic is imparted by having a dense crucible material structure and a durable protective glaze.

Slag and Dross Removal

A dense, non-wetting crucible also will help reduce slag and dross accumulation and will make it easier to clean the crucible when it is empty.

Emptying the Furnace

Crucibles for melting and holding molten metal that is dipped out of the furnace need to be designed for easy access to the metal and with high thermal efficiency. This allows the furnace to hold the metal at the proper temperature with minimal fuel or power use.

Crucibles for furnaces that are tilted for pouring often require integral pouring spouts that provide the reach and accuracy needed for the pour.

Conclusion

With a full and detailed understanding of all aspects of your metal melting and/or holding operations, you and your crucible supplier will be well positioned to select a crucible product that meets your specific operational requirements and provides a consistently longer service life.

Graphite Crucibles

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Introduction

Here is everything on the internet you need to know about graphite crucibles.

You will learn:

• What is a Graphite Crucible?What is a Graphite Crucible?
• How Graphite Crucibles are Used
• How a Graphite Crucible is Made
• Maintaining a Graphite Crucible
• And much more &#;

Chapter One &#; What is a Graphite Crucible?

A graphite crucible is a container used for melting and casting non-ferrous, non-iron metals such as gold, silver, aluminum, and brass. Their thermal conductivity, high temperature resistance, small thermal expansion coefficient for high temperature applications, and anti strain properties to rapid heating and cooling make graphite crucibles an ideal metal casting tool.. They are resistant to the effects of acids and alkaline solutions and have excellent chemical stability.

Graphite is produced from natural graphite, a naturally occurring crystalline form of carbon and is manufactured by combining graphite with fire resistant clay or carbon dioxide.

Synthetic graphite is made by processing petroleum pitch and petroleum coke, which are byproducts of the oil refining process. It has a purer high fixed carbon content with very few impurities and a low sulfur content.

Graphite crucibles do not contaminate molten metals because the graphite material is fused and does not loosen. The quality of a graphite crucible is determined by how it is manufactured, which influences its structure, density, porosity, and strength

Chapter Two &#; How Graphite Crucibles are Used

The non-reactive nature of graphite crucibles makes them ideal for use in the casting process. Their excellent heat performance helps in melting metals quickly for faster production cycles. Since graphite crucibles are resistant to chemicals and corrosion, they are not affected by workshop conditions, characteristics that make them durable and long lasting.

During casting, temperatures are increased to decrease the tensile and yield strength of the metals alloys being cast. The temperature at which metals melt varies depending on the type of metal. Factors that influence casting are the temperature of the alloy being cast and the temperature of the crucible. Graphite crucibles are exceptionally capable of providing the proper vessel for casting due to their high resistance to the effects of increases in temperature, regardless of the type of metal alloy.

The many hundreds of shapes of graphite crucibles are categorized by letters, which begin with A. Each form is divided into subcategories that are determined by the crucibles inside diameter (d or ID), outer diameter (D or OD), and height (H) and its shape. The crucible pictured below is cylindrical with a flat bottom and no spout or lid.

The different forms of graphite crucibles also refer to their shapes, which vary as widely as the different dimensional forms. They can be cylindrical with or without a spout, shaped like a cup, or include a top edge and lid, to name a few.

Graphite crucibles have slowly developed into an essential part of metal forming. They can be as small as teacups or large enough to hold several tons of molten metal and be permanent parts of furnaces.

Graphite crucibles are used in fuel fired, electric, and induction furnaces or as a method for transferring and moving molten metals. They have to be designed to fit the temperature, chemical, and physical requirements of the specific operation.

Fuel Fired Furnace

A fuel fired furnace is powered by gas, oil, propane, or coke and requires a graphite crucible capable of withstanding the maximum amount of energy or BTUs from the furnace. Gas, oil, and propane-fueled furnaces use crucibles designed to withstand the burner flame around the tapered shape of the crucible, which allows for the even distribution of heat.

Electric Resistance Furnace

Graphite crucibles for electric resistance furnaces must be specially designed since electric furnaces heat up much slower than fuel fired furnaces. Crucibles have to have a high graphite content in the carbon binder for energy savings and high thermal conductivity. They are basin shaped and are placed at equal distance from the heating elements.

Induction Furnace

The selection for fuel fired and electric furnaces graphite crucibles is much easier than selecting one for an induction furnace. In one type of induction furnace, crucibles are used to melt the charge, while in other types, the inductive field passes through them. The crucible must match the operating frequency of the furnace and the specific application. In low frequency furnaces, the crucible is made with high silicon and carbide content. In high frequency furnaces, they are made of clay. Correct matching prevents overheating the crucible.

Removable Crucible Furnaces

Furnace crucibles are "A" shaped so that they can be lifted with tongs to be removed from the furnace to pour out the molten metal. They can be charged inside or outside of the furnace and allow for pouring their contents.

Tilting Furnace

A graphite crucible for a tilting furnace remains stationary as the furnace tilts to pour the molten metal. Tilting furnaces can be either induction or electrical and are capable of melting steel, iron, copper, brass, gold, platinum, silver, nickel, palladium, and their alloys.

Pit Furnace

A pit furnace is located below ground level. The crucible is lowered into the furnace and has the metal to be melted placed in it. Coke is packed around the crucible in the heating chamber. Once the metal is melted, the crucible is lifted out.

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Chapter Three &#; Metals Melted in Graphite Crucibles

The type of metal to be processed determines the type of crucible that will be required. The structure and design of the crucible must be able to support the maximum melting temperature of the metal and hold it. This is further determined by how the metal and the crucible interact, chemically and physically.

Graphite Crucible Metals

Copper

Copper based alloys that are melted in a fuel fired furnace are processed using a silicon carbide graphite crucible due for thermal shock resistance.

Aluminum

Crucibles for the processing of aluminum and aluminum alloys are carbon or ceramic bonded clay graphite and silicon carbide since these metals melt at 400°C or 750°F to °C or °F.

Gold

Graphite crucibles used for melting gold are made of a superior grade graphite and have thermal shock resistance, thermal stability, oxidation resistance, and excellent mechanical strength. They are designed to withstand temperatures of over ° C or ° F.

Silver

Graphite crucibles for melting silver are similar to those used to melt gold and capable of withstanding temperatures over °C or °F. The body of the crucible is made of natural graphite and keeps its chemical and physical properties. When melting at a high temperature, the thermal coefficient is small but has strain resistance to rapid heating or cooling.

Brass

Brass has a low melting point and must be heated rapidly before the component metals oxidize. For working with brass, a graphite crucible is ideal due to its durability and ability to heat up quickly.

Chapter Four &#; How is a Graphite Crucible Made?

Graphite crucibles are made from natural or synthetic graphite. The difference in production methods is due to the unique characteristics of each material. The manufacture of natural graphite crucibles involves the use of clay graphite ceramic bonded or silicon carbide carbon bonded graphite that use the refractory properties of silicon and graphite to conduct heat but still maintains its structural strength.

The production of synthetic graphite involves the processing of petroleum coke, pitch coke, and carbon black. The steps of the process include preparation of the powder, shape forming, baking, pitch impregnation or densification, and graphitization.

Production of Synthetic Graphite

Powder Preparation

Prior to beginning production the raw materials are changed into a powder by crushers and ball mills. The powder is prepared in accordance with the required particle size distribution and blended into a paste using coal tar pitch or petroleum pitch as a binder.

Shape Forming

There are three methods for shape forming, which are extrusion, vibromolding, and isostitcal pressing.

• Extrusion: The extrusion method involves forcing the pasty mixture through a die to form rods, bars, plates, or tubes, which are cut to required lengths. Extruded graphite is isotropic, or uniform.
• Vibration: The vibration method includes pouring the pasty material into a mold, which is sealed with a metal plate. As the mold vibrates, the pasty material is compacted.
• Isostatic: Isostatic pressing involves applying pressure to the mixture by a liquid medium that surrounds the material. The flexible mold is submerged in the pressurized liquid for forming.

Baking

During the baking process, parts are heat treated at a temperatures between 900° C and ° C or 650° F and ° F, which results in thermal decompositioning of the binder into carbon and other components. The carbonization process binds the powder particles. Since the volume of the binder has higher volume than the carbon, pores are formed whose size is determined by the amount of binder.

Pitch Impregnation

The impregnation process is designed to reduce the porosity of the carbon parts and includes the use of material that is lower in viscosity than the original binder. The low viscosity allows the impregnated material to fill the gaps left by the removal of the binder.

Graphitization

Graphitization is another heating process where the parts are heat treated at extreme temperatures that range between ° C to °C or ° F to ° F. The result of the process is the changing of the carbon in the part to crystalline graphite, which changes the physical properties of the material. A further outcome of the heating is the vaporization of impurities such as binder residue, gases, oxides, and sulfur.

Production with Natural Graphite

Silicon carbide is made using the Acheson process, where silica sand and carbon are heated in a furnace that produces a power or large mass changed to a powder.

Graphite

Graphite is mined in open pit mines or underground depending on the location of the graphite deposits.

Mixing and Bonding

Silicone carbide and graphite are blended with additives such as ferro silicon or ferro manganese and mixed with bonding materials, which is completed in a kneading mill.

Molding

Graphite crucibles can be formed using hand, rolling, rotary, or compression molding. The forming method determines the structure, density, porosity, and strength of the crucible.

• Hand: Hand molding is used for specially designed crucibles that are necessary for specific and unique operations.
• Rotary: Rotary molding uses a rotation machine that performs the molding operation and has a knife that squeezes the material into a completed mold.
• Compression: The compression method uses a form of roller forming, where compression forces the graphite material against the sides of the mold.

Coking

In coking, the shaped crucibles are moved through an oven that reaches ° C or ° F.

Impregnation

Glazing protects the exterior and inner surfaces of the crucible from oxidation. The purpose of impregnation is to protect the internal structure of the crucible, which increases the crucible's lifespan. The impregnation chamber is a vacuum and pressure chamber. Once the completed forms are loaded, the chamber is filled with the impregnation chemicals and heated filling the pores of the crucibles.

Glazing

The carbon binders and graphite in crucibles would burn when exposed to heat. To prevent this, glass like glazes are applied to the exterior and interior of the finished crucible to seal it from oxygen. The glaze is designed for resistance to chemicals and thermal shock as well as damage from use.

Kiln

The glazed graphite crucibles are passed through large kilns for firing. Crucibles are fired on all sides by gases that reach temperatures specifically set for the type of crucible and glaze, which is between ° C and ° C or ° F and ° F.

Testing

The final step in the manufacturing process is testing to ensure the graphite crucible meets the needs of customers. Things that are tested include quality, durability, measuring, and temperature.

• Quality: Quality testing is completed to check for any impurities and the grade of graphite. This is done to prevent any contaminants or impurities from entering the melting process. The quality of a crucible determines the number of heatings it can endure.
• Durability: The durability test determines the crucible's maximum melting point.
• Temperature: The temperature test puts the crucible through rapid heating and cooling to match the casting process.
• Measuring: A trained specialist measures the height and width of the crucible to ensure it meets manufacturing standards.

Final Finishing

Before or after testing, crucibles are painted for identification and finishing purposes before being shipped.

The Manufacture of Graphite Crucibles

Methods for manufacturing graphite crucibles are vibration molding, isostatic pressing, and compression molding. The quality of a graphite crucible is determined by the method that is used to manufacture it, which determines its structure, density, porosity, and its mechanical strength.

Isostatic Pressing

The molding process forms graphite crucibles by isostatic pressure using powder metallurgy. Equal pressure is applied to the powder to uniformly compact it to the proper density and microstructure. The process can be performed cold or hot. Graphite crucibles formed by this method have excellent properties that are uniformly distributed throughout the entire mass without a grain direction, or are anisotropic.

The high density and small particle size of this type of crucible creates a very strong machinable graphite tool with resistance to high temperatures in controlled environments, electro-conductivity, and self lubricating properties.

Compression Molding

Compression molding follows the same principles as isostatic molding where a fine powder is placed under great pressure. To form the crucible, hydraulic pressure is applied to graphite powder in a steel mold. The advantages of compression molding are its simple process, short production cycle, high efficiency, low labor costs, less shrinkage, and high product quality.

Graphite crucibles produced by compression molding have a fine grain structure that can be used to replace more expensive isostatically pressed graphite crucibles. The limitation to the process is the restriction on the dimensions of the crucibles.

Vibration Molding

Vibration molding is used to produce large crucibles and includes the use of a pasty mixture of graphite. The pasty mixture is placed in the mold and a metal plate is placed over it. The mixture is compacted by vibrating the mold. After compacting, the molded crucible is baked for two or three months at temperatures close to o C. In order to avoid cracks or defects, the temperature is precision controlled. At the end of the baking process, the crucible will have achieved its desired hardness.

Chapter Five &#; Care of Graphite Crucibles

The handling and care of a graphite crucible determines how well it will perform and last. Though the failure of a crucible may seem to be related to its use, in many instances, it is from how the crucible is handled, operated, and maintained that determines its length of usefulness. Basic operational practices and procedures can prevent the early demise of a crucible.

Inspection

The first step in crucible handling begins when it arrives. Newly received crucibles should be inspected for chips, cracks, or abrasions.

Stacking

Stacking of crucibles inside each other leads to cracking and should be avoided.

Moisture

An enemy to graphite crucibles is moisture. They have to be stored in ventilated and dry areas to avoid any contact with moisture.

Thermal Shock

To avoid thermal shock to a crucible, it should be preheated especially if it is allowed to cool between uses. Thermal shock cracks the crucible if it is heated too quickly.

Charging

To properly charge a crucible, it should first be loaded with small charge materials and then loaded with larger ones. Materials to be processed should not be packed tightly since they will expand and crack the crucible.

Flux

Though crucibles are designed to resist chemicals, they can be damaged by flux, which should be added after the materials are fully molten. When flux is added and the worked material is solid, the flux attacks the surface of the crucible.

Direct Flame

Fuel fired furnaces have a direct flame burner that may have excess air. The excess air and direct flame causes oxidation damage to the surface of the crucible. Oxidation can also occur if the melted metal is held at a minimal temperature for an extended period of time.

Dross

Dross or slag buildup has a low thermal conductivity, which requires the furnace to burn hotter. The buildup absorbs flux that increases the chemical attack on the crucible&#;s surface. This can be prevented by regular removal of dross.

Cleaning

Cleaning a crucible involves the removal of chemicals from processing, which involves the use of hydrochloric acid that dissolves most compounds except for carbon ones. To remove carbon compounds, nitric acid is used. Once the acids have done their work, they can be removed with potassium pyrosulfate, sodium carbonate, or borax to melt and remove cleaning agents.

Temperature Limit

Crucibles are designed to endure a specific temperature, which differs according to the type of material being worked. Exceeding the temperature limit can seriously damage or destroy the crucible. This is prevented by carefully monitoring the crucible during its use.

Source: (from almathcrucibles.com) Crucible Maximum Temperature Limit (G) Graphite Carbon °C or °F

Preheating

Prior to using a crucible, it should be preheated at 500oF or 260oC for two hours and allowed to cool slowly. This process removes any residual moisture and prevents cracking.

Crucible Tongs

Tongs should match the shape and design of the crucible and should not place any pressure on the sides of the crucible.

Chapter Six &#; The Two Forms of Graphite

Graphite can be mined or synthetically produced from petroleum byproducts from the oil refining process. Mined or natural graphite is known as plumbago, black lead, and mineral carbon and is found in layers in a lamellar shape with a grey to black luster, a greasy feel, and in flaky, crystalline, and amorphous forms. Its quality depends on its physical properties.

Synthetic graphite is made by high temperature treatment of amorphous carbon materials, which include calcined petroleum coke and coal tar pitch that are composed of graphitizable carbon. Its porosity plays a large role in controlling its thermal expansivity with a temperature that depends on the strength of its polygranular structure

Synthetic Graphite

Synthetic graphite is not as crystalline as natural graphite but has a higher purity carbon content. The two types of synthetic graphite are electrographite and graphite blocks. Electrographite is produced in electric ovens, while graphic blocks, or isotropic graphite, is made from coke that has a different structure than that used to produce electrographite.

Synthetic or artificial graphite has superior properties compared to natural graphite. Its excellent purity allows it to be more predictable and controllable making it the perfect option for specialized industries. The manufacturing process that is used to produce synthetic graphite determines its physical and chemical properties.

Synthetic graphite powder is made by heating petroleum coke or petroleum pitch above their graphitization temperature. In some instances, it is collected by screening lathe turnings of electrodes and nipples.

Synthetic graphite is used in several industries that include electronics, the military, aerospace, defense, and nuclear power.

Graphite electrodes are used in the steel making process to melt scrap iron and steel.

Nuclear Graphite

Special grades of synthetic graphite are used as matrix and neutron moderators in nuclear and fusion reactors.

Commercial Products

Several commercial products are made from synthetic graphite due to its durability and long life, which include fishing rods, golf club shafts, bicycle frames, sports car body panels, the fuselage of the Boeing 787, and pool sticks.

Natural Graphite

Natural graphite is like mica and consists of sheets of flat molecules held together by Van der Waals forces, a dependent weak interaction between atoms and molecules. These weak forces make graphite soft enough to erode by friction.

The two forms of graphite are hexagonal and rhombohedral that have similar properties but different graphene layers. Each type can be converted and processed into the other form.

Graphite&#;s thermal stability and electrical and thermal conductivity makes it ideal for use as electrodes and high temperature refractory production. The one drawback to graphite uses is its ability to oxidize at temperatures over 700°C.

The forming of graphite is the reaction of carbon compounds with hydrothermal solutions, magmatic fluids, or the crystallization of magmatic carbon.

Graphite is used for refractory production, batteries, steel, brake linings, foundry facings, and lubricants.

Pencil Lead

Though the material in pencils has been referred to as lead for many years, in actuality, it is a form of clay graphite.

Refractories

Crucible production began with clay graphite but has progressed to alumina graphite and silicon carbon graphite. Graphite is also used in bricks as a lining for steel blast furnaces.

Batteries

With the rise in portable electronics, graphite has gained greater use in the production and fabrication of batteries. It is used twice as much as lithium carbonate.

Steelmaking

Graphite is used to raise the carbon content of molten steel and as a lubricant for dies.

Brake Linings

In the production of brake linings, graphite has become a replacement for asbestos.

Foundry

A graphite coating is used for the lining of molds making it easier to remove cast parts. Its high temperature resistance makes it easier to separate parts after cooling.

Conclusion

• A graphite crucible is a container used for melting and casting non-ferrous, non-iron, metals such as gold, silver, aluminum, and brass.
• The quality of a graphite crucible is determined by how it is manufactured, which influences its structure, density, porosity, and strength.
• Graphite crucibles are ideal for melted metal processes because they are non-reactive and able to survive extremely high temperatures.
• The type of metal to be processed determines the type of graphite crucible that will be used.
• The manufacturing of graphite crucibles involves the use of clay-graphite-ceramic bonded or silicon-carbide-carbon bonded materials that use the refractory properties of silicon and graphite to conduct heat but add structural strength.
• There are two forms of graphite used in the production of graphite crucibles - synthetic and natural.

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Additional reading:
Common Issues And Maintenance Tips For Graphite Electrodes

If you melt metal or hold a molten bath, chances are your operation is unique. Your particular combination of furnaces, alloys, working practices, metallurgical treatments, pouring arrangements and end products are not likely to be duplicated at any other facility. So choosing a crucible that will provide maximum performance for your operation is an individualised and complex task.

This article is designed to serve as a guide for selecting the optimal crucible for your operation. It explains the relationship between metal melting/holding operations and specific crucible characteristics. It provides support for but does not replace the need for metal melters and crucible suppliers to work closely together in the crucible selection process.

The modern crucible is a highly heterogeneous, graphite-based composite material, which relies on its material composition and control of the graphite&#;s structural alignment to achieve the performance required. 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 fuel&#;fired furnaces, in electric resistance furnaces, in induction furnaces or simply to transfer molten metal. They come with or without pouring spouts and in a wide variety of traditional and specialized shapes.

They also offer many different performance characteristics since each application presents a complex set of temperature, chemical and physical parameters which define the technical boundaries within which the crucible has to be designed to operate.

So how do you select the right crucible for your operation from the extensive range of crucible types and materials available to you?

The best approach is to begin with your own detailed assessment of your operations. You need to fully document and, where possible, quantify all aspects of your melting, holding and metal handling processes. These include:

• The capacity, dimensions and type of your furnace
• The specific alloy or range of alloys you melt
• The melting and/or holding temperatures you maintain
• The temperature change rate the crucible will experience
• How the crucible is charged
• The fluxes or additions used
• Degassing or refining processes
• How slag or dross is removed
• How the crucible is emptied.

These nine categories reflect the more common factors you must take into account when selecting a crucible to match your specific requirements. You also should consider any additional processes or requirements that might be specific to your operations. An example might be your ability to tolerate or your need to avoid alloy cross-contamination.

While you bring the detailed information on your own operations to the crucible selection process, your crucible supplier must contribute a high level of expertise on crucible materials, characteristics and performance. For the greatest selection, look for a crucible supplier able to offer overlapping crucible product lines suitable for each specific metal but offering different operational characteristics. Then, working together, you will be able to closely match a specific crucible to your specific requirements. Achieving this match is the key to crucible safety, performance and maximum service life.

Be aware, however, that on a practical level, there may not be a single crucible type that offers the highest level of every desirable characteristic for your application. Crucible performance characteristics often involve trade-offs. For example, the crucible with the best thermal conductivity may not also offer the best protection against thermal shock. Therefore, you should prioritise the list of crucible properties most important for your application and review those priorities with your crucible supplier.

Furnace Capacity, Dimensions and Type

The capacity, dimensions and type of furnace you use will establish most of the observable details about your crucible. For example, when you know the metal capacity your furnace was designed for, you will know what capacity your crucible should provide. Similarly, the dimensions of the space for the crucible in your furnace will dictate the dimensions and shape of your crucible. This also will determine if your crucible must include a pouring spout. But choosing a crucible to match your furnace type will give you many other less obvious factors to consider.

Fuel-fired furnaces

Fuel-fired furnaces include furnaces powered by gas, oil, propane or coke. Each of these fuels directly exposes the crucible to the heating source and each provides a different level of heat, normally measured in BTUs. Any crucible selected must be able to withstand the maximum BTUs the furnace fuel is able to apply to the crucible. In gas, oil and propane furnaces, the crucible must be able to withstand the effects of the burner flame at the base of the crucible and the crucible must be tapered to allow the flame to circulate around the crucible from bottom to top. This allows even heating of the crucible. The crucible material also must be able to resist oxidation damage from the flame and accommodate the rate of thermal change the crucible will experience.

Good thermal conductivity and even heating are important crucible properties in transferring the 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 to a crucible and are ideally suited to precise temperature control in metal holding application. But they are slower than fuel-fired furnaces in melting applications. 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.

Crucibles designed for electric resistance furnaces are normally basin shaped and provide a uniform distance between the crucible and the furnace heating elements.

Induction furnaces

Selecting crucibles for induction furnaces is a more complex task. In some applications, such as refining precious metals, crucibles designed to heat in the furnace&#;s inductive fields are used to melt the charge. In other applications, crucibles that allow the inductive field to pass through them and heat the metal charge directly are used. Therefore, it is important to match the electrical characteristics of the crucible to the operating frequency of the furnace and to the melting application. For example, 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 a crucible&#;s electrical resistivity to the induction furnace is key to preventing crucible overheating.

Most crucibles designed for induction furnaces are cylindrical to provide a uniform distance between the crucible and the furnace coil. However, some small furnaces designed for removable crucibles feature a tapered coil to match the profile of bilge-shaped crucibles.

Removable crucible furnaces

All of the above furnace types can be designed to use removable crucibles. These crucibles can be charged while outside or when installed in the furnace, but they are removed from the furnace for pouring. Like crucibles used only for metal transfer, they are bilge-shaped or A-shaped to allow them to be lifted with tongs designed to properly support the crucible.

Furnace power limitations

A final factor to consider when documenting your crucible requirements based on your furnace&#;s specifications is power availability. In many locations, power for melting or holding might not be available at all times or might be prohibitively expensive at certain
times or at certain levels. If this is the case at your facility, it may be particularly important to select an energy efficient crucible.

Metals You Melt and/or Hold

Knowing what metals and alloys you melt or hold will tell you a lot about what characteristics you need in a crucible. Your detailed catalogue of the metals you intend to melt will help to establish the maximum temperature the crucible must support for melting and holding, will define how the metal will interact with the crucible material both chemically and physically and it will be a key factor in determining what characteristics your optimal crucible should offer. A case in point, in melting copper-based alloys in fuel-fired furnaces, roller formed silicon carbide crucibles perform better due to higher thermal shock resistance. In other types of furnaces, crucibles are often selected because of their high density. Less dense and more porous crucibles may allow erosion.

Carbon-bonded and ceramic-bonded clay graphite and silicon carbide crucibles are widely use in melting and holding aluminum and aluminum alloys, aluminum-bronze, copper and copper-based alloys, cupro-nickel and nickel-bronze alloys, precious metals, zinc and zinc oxide. Crucibles also are used in melting cast iron. Taken together as a group, these metals represent a temperature range from 400°C/750°F to °C/°F.

While some crucible types support metal temperatures encompassing a broad spectrum of metals, it often is necessary to select crucibles targeted to specific metals or alloys and with more limited operating temperature ranges. Selecting such crucibles is often more advantageous because they offer performance characteristics important to your operations. For example, using a crucible able to melt metals from iron to zinc may not be as important to your aluminum alloy melting operation as having a crucible limited to the temperature range you need but able to resist corrosion damage from your metal treatment fluxes.

Melting and Holding Temperatures

Generally speaking, the metals and alloys you melt or hold will determine the temperature range within which your crucible must be able to operate. Crucibles must never be heated above their maximum temperature. This can lead to dangerous crucible failure. However, operating below the crucible&#;s lower temperature limit can also cause problems. For example, crucibles designed for the high temperature melting of copper-based alloys will oxidize if used at low temperatures for zinc melting.

Melting and holding practices involving metal temperatures also need to be taken into consideration in selecting crucibles. If your operations involve superheating, you will need to take the higher metal temperatures reached into account.

Rate of Temperature Change

The ability of a crucible to handle the rate of temperature change is as important as its minimum and maximum temperature limits. If your operational practices lead to frequent heating and cooling cycles for the crucible or otherwise subject it to rapid temperature changes, you will need to select a crucible that is resistant to thermal shock. Some crucible types are much better at handling rapid temperature change than others. For example, high carbon content of the graphite in a crucible imparts high thermal conductivity and non-wetability. And when that graphite forms a directionally oriented matrix, the crucible also provides high thermal shock resistance. This is critical to foundry applications where temperatures can change by several hundred degrees in seconds. Your crucible supplier can advise which crucibles provide the best resistance to thermal shock for your application.

How the Crucible Is Charged

If your furnace is always charged with molten metal, it probably does not need a crucible designed to be highly resistant to physical damage. However, if metal ingots or other heavy materials make up the bulk of your charge and they are not carefully lowered into the furnace via an automatic loading system, you may want to select a crucible that is mechanically strong and able to survive physical shocks. Crucibles featuring high carbon content and a directionally oriented graphite structure provide excellent impact resistance.

You also will want a crucible with a durable protective glaze. Damage to the glaze from rough handling can lead to oxidation damage to the crucible. Extruded aluminum ingots often have sharp edges that cut deeply into a crucible&#;s body leading to damaging cracks.

Fluxes and Additives

All crucibles offer some level of resistance to corrosion and chemical attack. But most fluxes and other metal treatments used in melting aluminum and other nonferrous metals are highly corrosive and require a crucible that offers a high level of resistance to chemical attack. This resistance is best imparted by both a consistently dense crucible material structure and a durable protective glaze. If your melting application involves the use of corrosive metal treatments, you certainly will want a crucible offering the appropriate level of protection against these agents.

Degassing and Refining

Degassing aluminum and aluminum alloys typically involves bubbling inert gas, usually nitrogen, through the molten bath while agitating the bath with a rotor designed to break apart and disperse the gas bubbles. These small bubbles then pull the undesirable hydrogen and oxides out of the bath and carry it, along with dross and inclusions to the surface where the gas escapes into the air and the solid material can be removed. This process, often used along with fluxing agents, physically erodes the crucible and attacks it chemically as well. Therefore, a dense and mechanically strong crucible that is highly resistant to chemical attack is required. Silicon carbide crucibles provide excellent resistance to elevated temperature erosion and to chemical corrosion. Also, when isostatically pressed, crucibles form a random arrangement of the graphite in their structure. This contributes to creating denser products that can survive erosive and corrosive conditions more effectively.

Many refining and metal treatment processes used with other nonferrous metals also call for a mechanically strong and chemically resistant crucible.

In refining and melting precious metals, it is particularly important that the crucible you use provide clean metal by incorporating non-wetting properties. That means that the crucible must be well sealed against metal penetration. This characteristic is imparted by having a dense crucible material structure and a durable protective glaze.

Slag and Dross Removal

A dense, non-wetting crucible also will help reduce slag and dross accumulation and will make it easier to clean the crucible when it is empty.

Emptying the Furnace

Crucibles for melting and holding molten metal that is dipped out of the furnace need to be designed for easy access to the metal and with high thermal efficiency. This allows the furnace to hold the metal at the proper temperature with minimal fuel or power use.

Crucibles for furnaces that are tilted for pouring often require integral pouring spouts that provide the reach and accuracy needed for the pour.

Conclusion

With a full and detailed understanding of all aspects of your metal melting and/or holding operations, you and your crucible supplier will be well positioned to select a crucible product that meets your specific operational requirements and provides a consistently longer service life.

Graphite Crucibles

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Introduction

Here is everything on the internet you need to know about graphite crucibles.

You will learn:

• What is a Graphite Crucible?
• How Graphite Crucibles are Used
• How a Graphite Crucible is Made
• Maintaining a Graphite Crucible
• And much more &#;

Chapter One &#; What is a Graphite Crucible?

A graphite crucible is a container used for melting and casting non-ferrous, non-iron metals such as gold, silver, aluminum, and brass. Their thermal conductivity, high temperature resistance, small thermal expansion coefficient for high temperature applications, and anti strain properties to rapid heating and cooling make graphite crucibles an ideal metal casting tool.. They are resistant to the effects of acids and alkaline solutions and have excellent chemical stability.

Graphite is produced from natural graphite, a naturally occurring crystalline form of carbon and is manufactured by combining graphite with fire resistant clay or carbon dioxide.

Synthetic graphite is made by processing petroleum pitch and petroleum coke, which are byproducts of the oil refining process. It has a purer high fixed carbon content with very few impurities and a low sulfur content.

Graphite crucibles do not contaminate molten metals because the graphite material is fused and does not loosen. The quality of a graphite crucible is determined by how it is manufactured, which influences its structure, density, porosity, and strength

Chapter Two &#; How Graphite Crucibles are Used

The non-reactive nature of graphite crucibles makes them ideal for use in the casting process. Their excellent heat performance helps in melting metals quickly for faster production cycles. Since graphite crucibles are resistant to chemicals and corrosion, they are not affected by workshop conditions, characteristics that make them durable and long lasting.

During casting, temperatures are increased to decrease the tensile and yield strength of the metals alloys being cast. The temperature at which metals melt varies depending on the type of metal. Factors that influence casting are the temperature of the alloy being cast and the temperature of the crucible. Graphite crucibles are exceptionally capable of providing the proper vessel for casting due to their high resistance to the effects of increases in temperature, regardless of the type of metal alloy.

The many hundreds of shapes of graphite crucibles are categorized by letters, which begin with A. Each form is divided into subcategories that are determined by the crucibles inside diameter (d or ID), outer diameter (D or OD), and height (H) and its shape. The crucible pictured below is cylindrical with a flat bottom and no spout or lid.

The different forms of graphite crucibles also refer to their shapes, which vary as widely as the different dimensional forms. They can be cylindrical with or without a spout, shaped like a cup, or include a top edge and lid, to name a few.

Graphite crucibles have slowly developed into an essential part of metal forming. They can be as small as teacups or large enough to hold several tons of molten metal and be permanent parts of furnaces.

Graphite crucibles are used in fuel fired, electric, and induction furnaces or as a method for transferring and moving molten metals. They have to be designed to fit the temperature, chemical, and physical requirements of the specific operation.

Fuel Fired Furnace

A fuel fired furnace is powered by gas, oil, propane, or coke and requires a graphite crucible capable of withstanding the maximum amount of energy or BTUs from the furnace. Gas, oil, and propane-fueled furnaces use crucibles designed to withstand the burner flame around the tapered shape of the crucible, which allows for the even distribution of heat.

Electric Resistance Furnace

Graphite crucibles for electric resistance furnaces must be specially designed since electric furnaces heat up much slower than fuel fired furnaces. Crucibles have to have a high graphite content in the carbon binder for energy savings and high thermal conductivity. They are basin shaped and are placed at equal distance from the heating elements.

Induction Furnace

The selection for fuel fired and electric furnaces graphite crucibles is much easier than selecting one for an induction furnace. In one type of induction furnace, crucibles are used to melt the charge, while in other types, the inductive field passes through them. The crucible must match the operating frequency of the furnace and the specific application. In low frequency furnaces, the crucible is made with high silicon and carbide content. In high frequency furnaces, they are made of clay. Correct matching prevents overheating the crucible.

Removable Crucible Furnaces

Furnace crucibles are "A" shaped so that they can be lifted with tongs to be removed from the furnace to pour out the molten metal. They can be charged inside or outside of the furnace and allow for pouring their contents.

Tilting Furnace

A graphite crucible for a tilting furnace remains stationary as the furnace tilts to pour the molten metal. Tilting furnaces can be either induction or electrical and are capable of melting steel, iron, copper, brass, gold, platinum, silver, nickel, palladium, and their alloys.

Pit Furnace

A pit furnace is located below ground level. The crucible is lowered into the furnace and has the metal to be melted placed in it. Coke is packed around the crucible in the heating chamber. Once the metal is melted, the crucible is lifted out.

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Chapter Three &#; Metals Melted in Graphite Crucibles

The type of metal to be processed determines the type of crucible that will be required. The structure and design of the crucible must be able to support the maximum melting temperature of the metal and hold it. This is further determined by how the metal and the crucible interact, chemically and physically.

Graphite Crucible Metals

Copper

Copper based alloys that are melted in a fuel fired furnace are processed using a silicon carbide graphite crucible due for thermal shock resistance.

Aluminum

Crucibles for the processing of aluminum and aluminum alloys are carbon or ceramic bonded clay graphite and silicon carbide since these metals melt at 400°C or 750°F to °C or °F.

Contact us to discuss your requirements of Graphite continuous casting mold. Our experienced sales team can help you identify the options that best suit your needs.

Gold

Graphite crucibles used for melting gold are made of a superior grade graphite and have thermal shock resistance, thermal stability, oxidation resistance, and excellent mechanical strength. They are designed to withstand temperatures of over ° C or ° F.

Silver

Graphite crucibles for melting silver are similar to those used to melt gold and capable of withstanding temperatures over °C or °F. The body of the crucible is made of natural graphite and keeps its chemical and physical properties. When melting at a high temperature, the thermal coefficient is small but has strain resistance to rapid heating or cooling.

Brass

Brass has a low melting point and must be heated rapidly before the component metals oxidize. For working with brass, a graphite crucible is ideal due to its durability and ability to heat up quickly.

Chapter Four &#; How is a Graphite Crucible Made?

Graphite crucibles are made from natural or synthetic graphite. The difference in production methods is due to the unique characteristics of each material. The manufacture of natural graphite crucibles involves the use of clay graphite ceramic bonded or silicon carbide carbon bonded graphite that use the refractory properties of silicon and graphite to conduct heat but still maintains its structural strength.

The production of synthetic graphite involves the processing of petroleum coke, pitch coke, and carbon black. The steps of the process include preparation of the powder, shape forming, baking, pitch impregnation or densification, and graphitization.

Production of Synthetic Graphite

Powder Preparation

Prior to beginning production the raw materials are changed into a powder by crushers and ball mills. The powder is prepared in accordance with the required particle size distribution and blended into a paste using coal tar pitch or petroleum pitch as a binder.

Shape Forming

There are three methods for shape forming, which are extrusion, vibromolding, and isostitcal pressing.

• Extrusion: The extrusion method involves forcing the pasty mixture through a die to form rods, bars, plates, or tubes, which are cut to required lengths. Extruded graphite is isotropic, or uniform.
• Vibration: The vibration method includes pouring the pasty material into a mold, which is sealed with a metal plate. As the mold vibrates, the pasty material is compacted.
• Isostatic: Isostatic pressing involves applying pressure to the mixture by a liquid medium that surrounds the material. The flexible mold is submerged in the pressurized liquid for forming.

Baking

During the baking process, parts are heat treated at a temperatures between 900° C and ° C or 650° F and ° F, which results in thermal decompositioning of the binder into carbon and other components. The carbonization process binds the powder particles. Since the volume of the binder has higher volume than the carbon, pores are formed whose size is determined by the amount of binder.

Pitch Impregnation

The impregnation process is designed to reduce the porosity of the carbon parts and includes the use of material that is lower in viscosity than the original binder. The low viscosity allows the impregnated material to fill the gaps left by the removal of the binder.

Graphitization

Graphitization is another heating process where the parts are heat treated at extreme temperatures that range between ° C to °C or ° F to ° F. The result of the process is the changing of the carbon in the part to crystalline graphite, which changes the physical properties of the material. A further outcome of the heating is the vaporization of impurities such as binder residue, gases, oxides, and sulfur.

Production with Natural Graphite

Silicon carbide is made using the Acheson process, where silica sand and carbon are heated in a furnace that produces a power or large mass changed to a powder.

Graphite

Graphite is mined in open pit mines or underground depending on the location of the graphite deposits.

Mixing and Bonding

Silicone carbide and graphite are blended with additives such as ferro silicon or ferro manganese and mixed with bonding materials, which is completed in a kneading mill.

Molding

Graphite crucibles can be formed using hand, rolling, rotary, or compression molding. The forming method determines the structure, density, porosity, and strength of the crucible.

• Hand: Hand molding is used for specially designed crucibles that are necessary for specific and unique operations.
• Rotary: Rotary molding uses a rotation machine that performs the molding operation and has a knife that squeezes the material into a completed mold.
• Compression: The compression method uses a form of roller forming, where compression forces the graphite material against the sides of the mold.

Coking

In coking, the shaped crucibles are moved through an oven that reaches ° C or ° F.

Impregnation

Glazing protects the exterior and inner surfaces of the crucible from oxidation. The purpose of impregnation is to protect the internal structure of the crucible, which increases the crucible's lifespan. The impregnation chamber is a vacuum and pressure chamber. Once the completed forms are loaded, the chamber is filled with the impregnation chemicals and heated filling the pores of the crucibles.

Glazing

The carbon binders and graphite in crucibles would burn when exposed to heat. To prevent this, glass like glazes are applied to the exterior and interior of the finished crucible to seal it from oxygen. The glaze is designed for resistance to chemicals and thermal shock as well as damage from use.

Kiln

The glazed graphite crucibles are passed through large kilns for firing. Crucibles are fired on all sides by gases that reach temperatures specifically set for the type of crucible and glaze, which is between ° C and ° C or ° F and ° F.

Testing

The final step in the manufacturing process is testing to ensure the graphite crucible meets the needs of customers. Things that are tested include quality, durability, measuring, and temperature.

• Quality: Quality testing is completed to check for any impurities and the grade of graphite. This is done to prevent any contaminants or impurities from entering the melting process. The quality of a crucible determines the number of heatings it can endure.
• Durability: The durability test determines the crucible's maximum melting point.
• Temperature: The temperature test puts the crucible through rapid heating and cooling to match the casting process.
• Measuring: A trained specialist measures the height and width of the crucible to ensure it meets manufacturing standards.

Final Finishing

Before or after testing, crucibles are painted for identification and finishing purposes before being shipped.

The Manufacture of Graphite Crucibles

Methods for manufacturing graphite crucibles are vibration molding, isostatic pressing, and compression molding. The quality of a graphite crucible is determined by the method that is used to manufacture it, which determines its structure, density, porosity, and its mechanical strength.

Isostatic Pressing

The molding process forms graphite crucibles by isostatic pressure using powder metallurgy. Equal pressure is applied to the powder to uniformly compact it to the proper density and microstructure. The process can be performed cold or hot. Graphite crucibles formed by this method have excellent properties that are uniformly distributed throughout the entire mass without a grain direction, or are anisotropic.

The high density and small particle size of this type of crucible creates a very strong machinable graphite tool with resistance to high temperatures in controlled environments, electro-conductivity, and self lubricating properties.

Compression Molding

Compression molding follows the same principles as isostatic molding where a fine powder is placed under great pressure. To form the crucible, hydraulic pressure is applied to graphite powder in a steel mold. The advantages of compression molding are its simple process, short production cycle, high efficiency, low labor costs, less shrinkage, and high product quality.

Graphite crucibles produced by compression molding have a fine grain structure that can be used to replace more expensive isostatically pressed graphite crucibles. The limitation to the process is the restriction on the dimensions of the crucibles.

Vibration Molding

Vibration molding is used to produce large crucibles and includes the use of a pasty mixture of graphite. The pasty mixture is placed in the mold and a metal plate is placed over it. The mixture is compacted by vibrating the mold. After compacting, the molded crucible is baked for two or three months at temperatures close to o C. In order to avoid cracks or defects, the temperature is precision controlled. At the end of the baking process, the crucible will have achieved its desired hardness.

Chapter Five &#; Care of Graphite Crucibles

The handling and care of a graphite crucible determines how well it will perform and last. Though the failure of a crucible may seem to be related to its use, in many instances, it is from how the crucible is handled, operated, and maintained that determines its length of usefulness. Basic operational practices and procedures can prevent the early demise of a crucible.

Inspection

The first step in crucible handling begins when it arrives. Newly received crucibles should be inspected for chips, cracks, or abrasions.

Stacking

Stacking of crucibles inside each other leads to cracking and should be avoided.

Moisture

An enemy to graphite crucibles is moisture. They have to be stored in ventilated and dry areas to avoid any contact with moisture.

Thermal Shock

To avoid thermal shock to a crucible, it should be preheated especially if it is allowed to cool between uses. Thermal shock cracks the crucible if it is heated too quickly.

Charging

To properly charge a crucible, it should first be loaded with small charge materials and then loaded with larger ones. Materials to be processed should not be packed tightly since they will expand and crack the crucible.

Flux

Though crucibles are designed to resist chemicals, they can be damaged by flux, which should be added after the materials are fully molten. When flux is added and the worked material is solid, the flux attacks the surface of the crucible.

Direct Flame

Fuel fired furnaces have a direct flame burner that may have excess air. The excess air and direct flame causes oxidation damage to the surface of the crucible. Oxidation can also occur if the melted metal is held at a minimal temperature for an extended period of time.

Dross

Dross or slag buildup has a low thermal conductivity, which requires the furnace to burn hotter. The buildup absorbs flux that increases the chemical attack on the crucible&#;s surface. This can be prevented by regular removal of dross.

Cleaning

Cleaning a crucible involves the removal of chemicals from processing, which involves the use of hydrochloric acid that dissolves most compounds except for carbon ones. To remove carbon compounds, nitric acid is used. Once the acids have done their work, they can be removed with potassium pyrosulfate, sodium carbonate, or borax to melt and remove cleaning agents.

Temperature Limit

Crucibles are designed to endure a specific temperature, which differs according to the type of material being worked. Exceeding the temperature limit can seriously damage or destroy the crucible. This is prevented by carefully monitoring the crucible during its use.

Source: (from almathcrucibles.com) Crucible Maximum Temperature Limit (G) Graphite Carbon °C or °F

Preheating

Prior to using a crucible, it should be preheated at 500oF or 260oC for two hours and allowed to cool slowly. This process removes any residual moisture and prevents cracking.

Crucible Tongs

Tongs should match the shape and design of the crucible and should not place any pressure on the sides of the crucible.

Chapter Six &#; The Two Forms of Graphite

Graphite can be mined or synthetically produced from petroleum byproducts from the oil refining process. Mined or natural graphite is known as plumbago, black lead, and mineral carbon and is found in layers in a lamellar shape with a grey to black luster, a greasy feel, and in flaky, crystalline, and amorphous forms. Its quality depends on its physical properties.

Synthetic graphite is made by high temperature treatment of amorphous carbon materials, which include calcined petroleum coke and coal tar pitch that are composed of graphitizable carbon. Its porosity plays a large role in controlling its thermal expansivity with a temperature that depends on the strength of its polygranular structure

Synthetic Graphite

Synthetic graphite is not as crystalline as natural graphite but has a higher purity carbon content. The two types of synthetic graphite are electrographite and graphite blocks. Electrographite is produced in electric ovens, while graphic blocks, or isotropic graphite, is made from coke that has a different structure than that used to produce electrographite.

Synthetic or artificial graphite has superior properties compared to natural graphite. Its excellent purity allows it to be more predictable and controllable making it the perfect option for specialized industries. The manufacturing process that is used to produce synthetic graphite determines its physical and chemical properties.

Synthetic graphite powder is made by heating petroleum coke or petroleum pitch above their graphitization temperature. In some instances, it is collected by screening lathe turnings of electrodes and nipples.

Synthetic graphite is used in several industries that include electronics, the military, aerospace, defense, and nuclear power.

Graphite electrodes are used in the steel making process to melt scrap iron and steel.

Nuclear Graphite

Special grades of synthetic graphite are used as matrix and neutron moderators in nuclear and fusion reactors.

Commercial Products

Several commercial products are made from synthetic graphite due to its durability and long life, which include fishing rods, golf club shafts, bicycle frames, sports car body panels, the fuselage of the Boeing 787, and pool sticks.

Natural Graphite

Natural graphite is like mica and consists of sheets of flat molecules held together by Van der Waals forces, a dependent weak interaction between atoms and molecules. These weak forces make graphite soft enough to erode by friction.

The two forms of graphite are hexagonal and rhombohedral that have similar properties but different graphene layers. Each type can be converted and processed into the other form.

Graphite&#;s thermal stability and electrical and thermal conductivity makes it ideal for use as electrodes and high temperature refractory production. The one drawback to graphite uses is its ability to oxidize at temperatures over 700°C.

The forming of graphite is the reaction of carbon compounds with hydrothermal solutions, magmatic fluids, or the crystallization of magmatic carbon.

Graphite is used for refractory production, batteries, steel, brake linings, foundry facings, and lubricants.

Pencil Lead

Though the material in pencils has been referred to as lead for many years, in actuality, it is a form of clay graphite.

Refractories

Crucible production began with clay graphite but has progressed to alumina graphite and silicon carbon graphite. Graphite is also used in bricks as a lining for steel blast furnaces.

Batteries

With the rise in portable electronics, graphite has gained greater use in the production and fabrication of batteries. It is used twice as much as lithium carbonate.

Steelmaking

Graphite is used to raise the carbon content of molten steel and as a lubricant for dies.

Brake Linings

In the production of brake linings, graphite has become a replacement for asbestos.

Foundry

A graphite coating is used for the lining of molds making it easier to remove cast parts. Its high temperature resistance makes it easier to separate parts after cooling.

Conclusion

• A graphite crucible is a container used for melting and casting non-ferrous, non-iron, metals such as gold, silver, aluminum, and brass.
• The quality of a graphite crucible is determined by how it is manufactured, which influences its structure, density, porosity, and strength.
• Graphite crucibles are ideal for melted metal processes because they are non-reactive and able to survive extremely high temperatures.
• The type of metal to be processed determines the type of graphite crucible that will be used.
• The manufacturing of graphite crucibles involves the use of clay-graphite-ceramic bonded or silicon-carbide-carbon bonded materials that use the refractory properties of silicon and graphite to conduct heat but add structural strength.
• There are two forms of graphite used in the production of graphite crucibles - synthetic and natural.

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