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Graphite is widely used as a material for Continuous casting molds. Continuous casting is a casting method in which the steps of pouring, solidification and withdrawal (extraction) of the casting from an open end mold are carried out continuously. Graphite molds (dies) may be used in both types of the process: vertical continuous casting and horizontal continuous casting . Most molds are fabricated from isostatically pressed graphite, however extruded graphite is also used for vertical casting of large ingots. The following alloys are manufactured by continuous casting in graphite molds:
Low wettability
In order to prevent sticking and breaking a casting extracted from the mold its material should possess poor adhesion to the solidifying metal. It is achieved by low wetting of the mold by the liquid metal. Graphite is not wettable by most molten non-ferrous metals and alloys. Ferrous alloys (eg. Cast irons) wet graphite due to the reaction with carbon, which results in formation of carbides Fe3C. Graphite is also wetted by alloys containing nickel (more than 6% of nickel), zinc or tin.
Heat liberated during cooling down and solidification of the cast metal should be removed from the mold. The value of the heat transferred through the mold in a time unit determines the solidification rate and the pulling (extraction) speed of the casting. Thermal conductivity of the mold material should be high enough to provide fast heat removal. Thermal conductivity of graphite used for continuous casting molds is in the range 560-960 BTU*in/hr*ft²*ºF (80-140 W/m*K), which is comparable to thermal conductivity of metals: aluminum 1540 BTU*in/hr*ft²*ºF (222 W/m*K); steel 450 BTU*in/hr*ft²*ºF (65 W/m*K). Thermal conductivity of graphite depend on temperature: it decreases to a half as the temperature rises to 1300°F (704°C).
Casting mold is heated inside by the entering molten metal and cooled down from the outside by the water-cooled copper jacket (or directly by water). The resulting temperature distribution throughout the mold is extremely non-uniform. Low thermal expansion of the mold material helps to prevent distortion the mold shape and secures the casting geometry. Thermal expansion of graphite is 2-2.8*10-6 °F-1 (3.5-5*10-6 °C-1), which is considerably lower than that of metals: aluminum 13*10-6 °F-1 (23*10-6 °C-1); steel 7*10-6 °F-1 (13*10-6 °C-1).
Thermal shock resistance of a material may be estimated in accordance to the formula:
Rs = (λ*σF)/(α*E)
Where: Rs– thermal shock resistance; λ- thermal conductivity; σF– flexural strength; α- coefficient of thermal expansion (CTE); E– modulus of elasticity.
Graphite has excellent thermal shock resistance due to favorable combination of high thermal conductivity, low coefficient of thermal expansion, low modulus of elasticity (1500 ksi / 10.3 GPa) and relatively high flexural strength (7500 psi / 52 MPa).
Self-lubrication
Graphite is a solid lubricant. Low friction between the mold surface and the solidifying metal ensures smooth withdrawl (extraction) of the casting without cracks and minimizes thickness of the defective periferial skin. Specific layered crystal structure of graphite determines its self-lubricating properties providing low friction without additional oil lubrication.
Good mechanical strength
In contrast to other materials mechanical strength (tensile strength, compressive strength, flexural strength) of graphite increases with a temperature rise.
Graphite is easily machined. Molds of complicated shapes with close tolerances may be obtained by milling, turning, sawing, grinding and surface finishing. Fine finish of the inner mold surface (ground or polished) is important for decrease of the material wettability and friction. Good quality of the outer surface ensures minimal air gap between the mold and the water-cooled jacket, which results in better heat transfer.
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With the development of science and technology, graphite with its good physical and chemical properties has gradually become the preferred material for mold production, graphite mold is widely used in various fields.
1. Graphite mold for non-ferrous metal continuous casting and semi-continuous casting
In recent years, advanced production methods such as direct continuous (or semi-continuous) manufacturing of rods or pipes from molten metal are being promoted at home and abroad. Domestically, this method has been adopted in copper, copper alloy, aluminum, aluminum alloy, etc. Artificial graphite is considered to be a more suitable material as a mold for continuous casting or semi-continuous casting of non-ferrous metals. Production practice proves that as the use of graphite mold, due to its good thermal conductivity (the thermal conductivity determines the solidification speed of the metal or alloy), the mold's self-lubricating performance and other factors not only increase the mold speed, but also due to the size of the ingot accurate, smooth surface, uniform crystal structure, can be directly processed in the next process. This not only greatly improves the rate of finished products, reduces the loss of waste products, but also greatly improves the quality of products.
2. Mould for pressure casting
Artificial graphite has been successfully used in pressure casting of nonferrous metals. For example, zinc alloy and copper alloy castings produced by pressure casting mould made of artificial graphite material have been used in automobile parts and so on.
3. Graphite mould for centrifugal casting
Graphite mould has been successfully used in centrifugal casting. The United States has adopted artificial graphite mold with wall thickness of more than 25 mm to centrifugal cast bronze sleeve. In order to prevent the burning loss of artificial graphite mould, some anti-oxidation measures can be taken. After casting a certain number of castings, if it is found that the inner surface of the mold is burnt, the size of the inner hole of the mold can be enlarged to cast large-size casing.
4. Hot pressing die
The artificial graphite hot-pressing die used in the pressure sintering of cemented carbide has the following characteristics: First, if the pressing temperature is increased to 1350-1450 degrees, the required unit pressure can be reduced to 67-100 kgf/cm2 (That is, 1/10 of the cold pressure); Second, pressurization and heating are performed in the same process, and a compact sintered body can be obtained after a short time of sintering.
5. Graphite mold for glass forming
Due to the chemical stability of graphite material, it is not easy to be infiltrated by molten glass, and the composition of glass will not be changed. Graphite material has good heat shock resistance and small size change with temperature. Therefore, it has become an indispensable mold material in glass manufacturing in recent years. It can be used to make mold of glass tube, elbow, funnel and other special-shaped glass bottles.
6. Sintered graphite mold and others
Taking advantage of the extremely small thermal deformation of artificial graphite materials, sintering molds and brackets for transistors can be manufactured, which are now widely used, and it has become an indispensable material for the development of the semiconductor industry. In addition, graphite molds are also used in molds for cast iron, durable molds for various non-ferrous metals, molds for cast steel, molds for heat-resistant metals (titanium, zirconium, molybdenum, etc.) and aluminium heat welding mold for welding rails, etc.
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