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( Steel) Ferrite (a-iron, d-iron; soft)
Austenite (?-iron; harder)
Spheroidite
Pearlite (88% ferrite, 12% cementite)
Bainite
Martensite
Ledeburite (ferrite-cementite eutectic, 4.3% carbon)
Cementite (iron carbide, Fe₃C; hardest)

Though steel had been produced by various inefficient methods long before the Renaissance, its use became more common after more efficient production methods were devised in the 17th century. With the invention of the Bessemer process in the mid-19th century, steel became a relatively inexpensive mass-produced good. Further refinements in the process, such as basic oxygen steelmaking, further lowered the cost of production while increasing the quality of the metal. Today, steel is one of the most common materials in the world and is a major component in buildings, tools, automobiles, and appliances. Modern steel is generally identified by various grades of steel defined by various standards organizations.

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, with very different properties; understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure ferrite or a-iron, a fairly soft metallic material that can dissolve only a small concentration of carbon (no more than 0.021&_160;wt% at 910&_160;°C, C1024Fe). Above 910&_160;°C ferrite undergoes a phase transition from body-centered cubic to a face-centered cubic (FCC) structure, called austenite or ?-iron, which is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.03 wt% carbon at 1154&_160;°C, C10.4Fe).^[5] As carbon-rich austenite cools, the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, resulting in a cementite-ferrite mixture. Cementite is a stoichiometric phase with the chemical formula of Fe₃C. Cementite forms in regions of higher carbon content while other areas revert to ferrite around it. Self-reinforcing patterns often emerge during this process, leading to a patterned layering known as pearlite (Fe₃C6.33Fe) due to its pearl-like appearance, or the similar but less beautiful bainite.

Perhaps the most important polymorphic form is martensite, a chemically-metastable substance with about four to five times the strength of ferrite. A minimum of 0.4&_160;wt% of carbon (C54Fe) is needed to form martensite. When austenite is quenched to form martensite, the carbon is "frozen" in place when the cell structure changes from FCC to BCC. The carbon atoms are much too large to fit in the interstitial vacancies and thus distort the cell structure into a body-centered tetragonal (BCT) structure. Martensite and austenite have an identical chemical composition. As such, it requires extremely little thermal activation energy to form.

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