Cast Iron Grades and Chemical Composition
Cast iron is an alloy of iron that contains from two to four percent of carbon, as well as varying amounts of silicon, manganese, and other impurities. Different grades of cast iron are available depending on the chemical composition of the alloy. These different grades are most commonly referred to by their American Iron and Steel Institute code numbers, but the specifications for each grade are also commonly referenced by their ASTM (American Society for Testing and Materials) numbers.
The most common types of cast iron used in industrial applications are gray cast iron (usually referred to as Grade ASTM A48) and ductile cast iron (usually referred to as Grade ASTM A536). Meehanite cast iron is an especially strong type of cast iron and is classified by the ASTM as Grade ASTM A875.
Gray Cast Iron
Gray cast iron, also known as “chilled” iron, is the most common form of cast iron available. As the name implies, the surface of cast parts made from gray iron are typically gray. Gray iron is characteristically brittle and is known for its ability to “ring” when struck. Gray cast iron is generally composed of 2.5–4.3% carbon, 1.0–3.0% silicon, 2.0–4.0% manganese, and 0–1.0% magnesium. Sulfur and phosphorus, present as impurities in the raw materials used to make cast iron, can also be found in gray iron.
Gray iron is an ideal material for casting, as the graphitic carbon present in the alloy acts as a lubricant during the casting process, reducing friction and thus allowing for the production of high-quality castings. Gray iron is also very thermally conductive, making it ideal for applications that require high heat conductivity. Due to its brittle, non-malleable nature, gray iron is not used extensively in structural applications.
Ductile Cast Iron
Ductile cast iron is a type of cast iron characterized by its malleable, elastic, good fatigue strength, and impact resistance properties. Ductile iron is composed of melted iron that has had graphite particles seated into it in the form of nodules or spheroids. Ductile iron contains more carbon and silicon than gray iron, usually ranging from 2.8-4.9% carbon and 1.4-3.8% silicon. It also contains 1.5-3.5% magnesium (Mg) and 0.15-0.5% cerium (Ce).
The addition of magnesium to the iron provides ductile iron with its malleability, good fatigue strength, and impact resistance properties. The presence of cerium helps to prevent the graphite particles from bonding together resulting in better impact and fatigue strength.
Meehanite Cast Iron
Meehanite cast iron is a specialized, high-strength form of cast iron. It is named after the Meehanite Metal Corporation, which pioneered the alloy in the early 1920s. It contains 5.5-7.5% percent carbon and 1.4-3.4% silicon, and is different from gray or ductile iron in that it also contains a higher percentage of manganese and nickel (2.0-4.2% and 1.3-3.3%, respectively).
The high carbon and manganese content of Meehanite cast iron imparts extremely high strength and hardness values to the material. Its strength and hardness is rivaled only by steel — but unlike steel, Meehanite cast iron has good machinability and thermal properties, making it ideal for structural uses.
Conclusion
Cast iron is a very versatile alloy used in a wide range of industrial applications. The specific grade of cast iron used in an application will depend on the desired properties of the material — such as tensile strength, thermal conductivity, and machinability. Gray cast iron is the most common type of cast iron, and is often used in casting applications due to its lubricating properties. Ductile cast iron is known for its good fatigue strength, while Meehanite cast iron is an extremely strong, high-strength variant of cast iron.
