Grey, or gray, iron castings are iron alloyed parts produced in foundries, not to be confused with forges, where they are formed via closed mold operations for applications in a wide variety of industries. Industries that grey iron castings serve include agriculture, architecture, automotive, construction, electronics, irrigation, machinery construction, transportation, and ventilation. Grey iron castings lend durability and strength to any operation for which they are used.
To acquire grey iron, metalworkers must first mine iron ores or iron oxides and then process it in a special type of blast furnace, called an electric induction furnace or a cupola. The generally accepted composition of cast grey iron is 95% iron by weight, plus 1 to 3% silicone and 2.1 to 4% carbon. The high amount of silicone, which produces graphite when heated, is the element responsible for grey iron’s coloring. It manifests itself when fractures form, revealing its graphitic microstructure.
The exact appearance of individual grey iron depends on both temperature and timing. Other than appearance, variable properties of castings include thermal conductivity, energy dissipation, resistance to wear and deformation, and melting point. Additional elements that may appear in grey iron castings, whether intentionally, to alter specific properties, or unintentionally, include manganese and sulfur; the latter is often added into molten iron to increase its hardness. When metalworkers have reached the grey iron mixture they want, they proceed to casting.
Iron foundry workers have a variety of processes at their disposal with which to produce grey iron castings, though they all more or less follow the same basic procedure. First, the grey iron is liquefied, then it is poured into a mold, where it is allowed to cool then ejected.
Different grey iron casting processes include centrifugal casting, die casting, and sand casting.
Centrifugal casting is a method used to cast cylindrical parts and components that produces fine-grained outer surfaces with even dispersion. To work, liquefied grey iron is poured into a mold that rotates at high speeds as it is introduced. Centrifugal casting stands apart from many other types of casting because it is mainly used to create stock materials for further machining, as opposed to parts shaped for specific applications. Grey iron castings made via this process can also be almost any diameter, length, and thickness.
Next, die casting is popular for continuous cycle or the fast production of castings. It uses reusable molds that are made in the shape of the desired finished product. These molds, or dies, are formed out of two hardened tool steel dies.
Sand molding is a low-cost and low-maintenance casting option that uses a sandy mixture as a mold. The four main mixtures from which manufacturers may choose are called no bake, dry sand, skin-dried, and green sand.
No bake molds are disposable molds, made of sand, a quick-setting resin, and a catalyst. They are formed with the help of a casting flask, which is usually made out of plastic, wood or metal. No bake molds employ a heat-free, cold-setting casting process.
During dry sand molding, unbonded sand is mixed with an adhesive or a chemical binder, shaped to form a mold and then baked a certain temperature to finish. To make castings, molten grey iron is poured into the mold, taking its shape.
To create a mold using the skin-dried molding method, also known as air-dried molding, operators mix water, sand, a binder like clay and bonding additives and then dry it with a torch or heating lamp.
Green sand molds are made the same way, minus the bonding additives and the heat drying.
No matter the method chosen, it is many times the case after casting that the newly formed castings need to undergo secondary processes to finish them and to remove imperfections like burrs and scale. These processes include deburring, annealing, machining, galvanizing, painting, and more. To create more finished products, most iron foundries have the equipment and staff available to perform such processes in-shop.
The applications of grey iron castings are many. As non-flammable building components, they provide superior fire protection to wooden support structures. Also, their graphite content is useful in energy dissipation of parts like engine blocks and crankshafts, which depend on the dampening of mechanical vibrations. Grey iron castings are also frequently used to make cases, dies, pipes, and decorative embellishments.