Steel | Composition, Properties, Types, Grades, & Facts

The base metal:

iron

Study the production and structural forms of iron from ferrite and austenite to the alloy steel

Iron ore is one of the most abundant elements on Earth, and one of its primary uses is in the production of steel. When combined with carbon, iron changes character completely and becomes the alloy steel.

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The major component of steel is iron, a metal that in its pure state is not much harder than copper. Omitting very extreme cases, iron in its solid state is, like all other metals, polycrystalline—that is, it consists of many crystals that join one another on their boundaries. A crystal is a well-ordered arrangement of atoms that can best be pictured as spheres touching one another. They are ordered in planes, called lattices, which penetrate one another in specific ways. For iron, the lattice arrangement can best be visualized by a unit cube with eight iron atoms at its corners. Important for the uniqueness of steel is the allotropy of iron—that is, its existence in two crystalline forms. In the body-centred cubic (bcc) arrangement, there is an additional iron atom in the centre of each cube. In the face-centred cubic (fcc) arrangement, there is one additional iron atom at the centre of each of the six faces of the unit cube. It is significant that the sides of the face-centred cube, or the distances between neighbouring lattices in the fcc arrangement, are about 25 percent larger than in the bcc arrangement; this means that there is more space in the fcc than in the bcc structure to keep foreign (i.e., alloying) atoms in solid solution.

Iron has its bcc allotropy below 912° C (1,674° F) and from 1,394° C (2,541° F) up to its melting point of 1,538° C (2,800° F). Referred to as ferrite, iron in its bcc formation is also called alpha iron in the lower temperature range and delta iron in the higher temperature zone. Between 912° and 1,394° C iron is in its fcc order, which is called austenite or gamma iron. The allotropic behaviour of iron is retained with few exceptions in steel, even when the alloy contains considerable amounts of other elements.

There is also the term beta iron, which refers not to mechanical properties but rather to the strong magnetic characteristics of iron. Below 770° C (1,420° F), iron is ferromagnetic; the temperature above which it loses this property is often called the Curie point.

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