Steel is an alloy that is produced by adding carbon to iron. Steel is much tougher and harder than pure Iron, but different amounts of carbon produce different types of steel. The introduction of carbon atoms in the ferrite material produces distortions in the crystal structure of the material, and the sheets of metal cannot slip across each other as easily. Therefore, the addition of carbon increases the yield strength of the material.

The way by which the alloy is processed can manipulate many of the material's properties. Following are several of the different types of steel that can be produced using different mechanical and thermal techniques during production.

When the percentage of carbon in the ferrite material exceeds 0.01%, all of the carbon cannot stay dissolved in the iron. A new material called iron carbide (Fe₃C) appears in steal. Iron carbide is a very hard and brittle crystalline material, also called cementite. The presence of any cementite crystals dispersed throughout the steel reduces slipping and, therefore, increases yield strength.

Pearlite is alternating layers of ferrite and cementite and has a carbon content of about 0.8% by weight. Pearlite results from the act of cooling steel slowly from high temperatures. If there is less than 0.8% carbon in the steel, extra ferrite particles form and are interspersed in the pearlite. If there is more than 0.8% carbon, extra cementite is formed and interspersed in the pearlite. Pearlite will be discussed in more detail lower, when we discuss fine and course pearlite.

Austentite crystals form when steel is heated to above 723° C. In this solid to solid phase transition, the ferrite, pearlite, and cementite particles in the steel all convert to austentite. This is a nonmagnetic material in which the iron atoms are arranged in a face-centered cubic lattice. Although pure austentite only exists at high temperatures, it is significant because it can dissolve more carbon than ferrite. It can dissolve 2.06% at 1148° C. When the steel is cooled back down, the reverse transformation occurs, but rapid cooling to certain temperatures will produce important new materials. If the austentite is quickly cooled to 600-650° , fine pearlite is formed. If austentite is quickly cooled to 260-400° , bainite is formed. If cooled below 200° C, the austentite forms a new material called martensite.

Fine pearlite is thinly layered pearlite. It forms because the carbon cannot travel the long distances in order to diffuse to form the thickly layered course pearlite. Fine pearlite allows less slipping than course pearlite, therefore, it has a higher yield strength.

Because steel is cooled to a lower temperature, the carbon diffuses an even shorter distance and forms tiny nodules of cementite within the steel. These nodules are situated between sheets of ferrite. This property produces the layered metal we call bainite. Bainite has a higher yield strength than fine pearlite.

Martensite

Austentite must be cooled very quickly to obtain martensite. Martensite forms because at the low temperatures there is not enough thermal energy for the carbon to diffuse at all. Instead, the austentite attempts to turn into ferrite without getting rid of the excess carbon. The result is basically distorted ferrite that is stretched in one direction. Because the ferrite contains more than the normal 0.1% carbon, it is called supersaturated. Martensite has a high yield strength, but its resistance to plastic deformation cause metals containing martensite to be very brittle.

Rapid cooling (quenching) of austentite is necessary to form martensite, but quenching causes the steel to contract, and stresses are trapped in the metal, which make it weaker. In order to relieve the stresses, the metal is often tempered. Though tempering reduces stress, it also softens the metal, so to obtain different qualities in the metal different amounts of quenching and tempering are utilized.