Cast iron is not pure iron. It is an alloy, a mixture of iron with other elements deliberately added to change the material’s properties. This material has been used for centuries in applications ranging from cookware to industrial machinery. Understanding its composition requires establishing a baseline for what “pure” iron means in a metallurgical context.
What is Metallurgically Pure Iron?
Metallurgically pure iron is defined by an extreme lack of other elements, typically achieving a purity level greater than 99.8% iron (Fe). This refinement is rarely seen outside of specialized laboratory settings or specific commercial grades used for magnetic applications. The most telling characteristic of pure iron is its carbon content, which must be incredibly low.
The maximum accepted carbon content for true pure iron is often cited as less than 0.008% by weight. Even commercial soft irons maintain carbon levels at or below 0.04%. Trace amounts of other elements like silicon, sulfur, or phosphorus may be present, but they are considered impurities rather than alloying agents. This baseline of near-total iron content serves as the standard against which all other iron-based materials are measured.
The Exact Composition of Cast Iron
Cast iron fundamentally separates itself from pure iron because it is defined by an extremely high carbon content, which acts as the primary alloying agent. To be classified as cast iron, the material must contain more than 2% carbon. Most standard grades fall in the range of 2.5% to 4.5% carbon by weight, confirming its status as an iron-carbon alloy and not a pure metal.
The carbon in cast iron is often present as free graphite flakes or nodules, creating the material’s distinct microstructure. Another element is silicon, typically added at a concentration of 1% to 3%. Silicon aids significantly in the casting process by promoting the formation of graphite rather than iron carbide. This combination of high carbon and moderate silicon content distinguishes cast iron from steel, which is limited to less than 2% carbon.
Physical Differences Between Pure Iron and Cast Iron
The difference in composition translates into a divergence in physical properties, making pure iron and cast iron suitable for entirely different uses. Pure iron is characterized by high ductility, meaning it is soft and highly malleable, capable of being hammered into sheets or drawn into thin wires. Conversely, the high carbon content in cast iron makes it hard but extremely brittle.
This brittleness is due to the structure of the internal carbon, which limits the material’s ability to deform under stress. While pure iron has a melting point around 1535°C, the addition of carbon and silicon significantly lowers the melting point of cast iron, making it ideal for pouring into complex molds. The additives in cast iron also give it an inherent resistance to oxidation and corrosion, which is superior to pure iron’s vulnerability to rust.