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.
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.