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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.
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.
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.
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.
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 based alloys that are melted in a fuel fired furnace are processed using a silicon carbide graphite crucible due for thermal shock resistance.
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 1600°C or 2912°F.
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 2000° C or 3632° F.
Graphite crucibles for melting silver are similar to those used to melt gold and capable of withstanding temperatures over 2000° C or 3632° 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 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
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.
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.
During the baking process, parts are heat treated at a temperatures between 900° C and 1200° C or 650° F and 2200° 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.
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 is another heating process where the parts are heat treated at extreme temperatures that range between 2700° C to 3000°C or 4900° F to 5450° 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 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.
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.
In coking, the shaped crucibles are moved through an oven that reaches 1000° C or 1800° F.
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.
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.
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 1000° C and 1350° C or 1800° F and 2450° F.
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.
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.
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 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 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 1000o 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.
The first step in crucible handling begins when it arrives. Newly received crucibles should be inspected for chips, cracks, or abrasions.
Stacking of crucibles inside each other leads to cracking and should be avoided.
An enemy to graphite crucibles is moisture. They have to be stored in ventilated and dry areas to avoid any contact with moisture.
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.
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.
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.
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 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 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.
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)
Maximum Temperature Limit
(G) Graphite Carbon
3000°C or 5432°F
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.
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 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.
Special grades of synthetic graphite are used as matrix and neutron moderators in nuclear and fusion reactors.
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 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.
Though the material in pencils has been referred to as lead for many years, in actuality, it is a form of clay graphite.
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.
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.
Graphite is used to raise the carbon content of molten steel and as a lubricant for dies.
In the production of brake linings, graphite has become a replacement for asbestos.
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.
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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