Properties of Steel

The physical properties of different types of steel and steel alloys depend primarily on the amount of carbon and its distribution within the iron. Before the heat treatment most steels are a mixture of three substances: ferrite, pearlite and cementite. Ferrite or a-iron also contains small amounts of carbon and other elements; it is soft and pliable.

Cementite is iron carbide Fe₃C. This iron compound is hard and brittle because it contains about seven percent carbon.

Pearlite is a mixture of ferrite and cementite which has a specific composition and structure. The physical properties of pearlite are between those of its two components. The toughness and hardness of non-heat treated steel depend on the proportions of these three components.

If the amount of carbon in steel increases, the amount of ferrite decreases and the amount of perlite increases until the steel consists entirely of pearlite. This state occurs when the steel contains approximately 0.8% carbon. Steel with a higher carbon concentration is a mixture of pearlite and cementite.

The heat treatment converts ferrite and pearlite into an allotrope iron-carbon alloy which is called austenite. Austenite (g-iron) dissolves all free carbon in the metal. When the steel is cooled slowly austenite converts back to ferrite and pearlite. If the cooling is rapid however, part of the austenite solidifies while the rest is converted to martensite which is an unstable form of the ferrite-cementite system.

The name martensite is also used as a term for other steel alloys with similar composition.