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Influence of Alloys on the Properties of Cast Iron

Alloys are different metals mixed with each other. This gives the resulting metal different properties from the two materials than formed it. Cast iron is itself an alloy of iron and carbon, but adding other materials can change the cast iron. Even changing the amount of carbon, or the way the carbon molecules are arranged, gives cast iron other properties and makes it more useful.

Carbon Levels and Shapes
- Iron is soft on its own, but carbon gives it strength. A tiny bit of carbon in otherwise pure iron -- less than 0.09 percent carbon -- is called ferrite. Cast iron has between 2 percent and 4 percent carbon. The shape of the carbon is determined by how the iron cools after it is cast and the other elements alloyed with it. It can form into hard iron carbide, or more often a softer graphite. All carbon gives iron strength; higher amounts in certain shapes make it easier to work in some ways.
Types of Graphite Iron
- Hard, brittle iron with little graphite is called white iron. Ductile iron is formed when there is more graphite that forms spherical shapes. Its name come from its ability to be drawn into wire -- its ductility. Flaked iron occurs when the graphite forms into flakes instead. This iron is good for machine working because of its strength and wear resistance. Compacted graphite iron, or CGI, has rounded flakes of graphite. Its properties are between flaked and ductile iron. Malleable iron has graphite in irregular nodes and is malleable -- able to be beaten into shapes.
Alloyed Iron
- The term "alloyed iron" refers to cast iron with other metals mixed in, often in addition to the carbon. The changes to iron's properties depend on the amounts of metal and which metals are added. Other metals are often mixed only in fractions of percents, since too much will make them a different type of steel instead of cast iron; even a tiny bit can change iron's properties.
Other Metals and Their Effects
- Chromium is used both inside steel and as a plating because of its corrosion resistance.
  
  Aluminum is added to iron in order to keep the iron from oxidizing, or corroding in contact with oxygen. Malleable iron often has cerium, because this element helps it form nodes. Chromium adds hardness and resistance to corrosion. In large amounts, chromium makes stainless steel, while small amounts make cast iron more durable. Copper improves iron's resistance to corrosion from the atmosphere. Manganese is a useful alloy because it is a deoxidizing agent, and improves hardness. Another deoxidizing material is silicon. Silicon also encourages the formation of graphite for machine working, as in flaked iron.

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