Bainite

v • d • e Iron alloy phases
Austenite (γ-iron; hard) Bainite Martensite Cementite (iron carbide; Fe3C) Ledeburite (ferrite - cementite eutectic, 4.3% carbon) Ferrite (α-iron, δ-iron; soft) Pearlite (88% ferrite, 12% cementite) Spheroidite
Types of Steel
Plain-carbon steel (up to 2.1% carbon) Stainless steel (alloy with chromium) HSLA steel (high strength low alloy) Tool steel (very hard; heat-treated)
Other Iron-based materials
Cast iron (>2.1% carbon) Wrought iron (almost no carbon) Ductile iron

Iron-carbon phase diagram, showing the eutectoid temperature and composition, at which bainite can form.

Bainite is a mostly metallic substance that exists in steel after certain heat treatments. First described by Davenport E. S. and Edgar Bain, it forms when austenite (a solution of carbon in iron) is rapidly cooled past a critical temperature of 723°C (about 1333°F).

A fine non-lamellar structure, bainite commonly consists of ferrite and cementite. It is similar in constitution to pearlite, but with the ferrite forming by a displacive mechanism similar to martensite formation, usually followed by precipitation of carbides from the supersaturated ferrite or austenite.

When formed during continuous cooling, the cooling rate to form bainite is higher than that required to form pearlite, but lower than that to form martensite, in steel of the same composition.

Bainite is generally stronger but less ductile than pearlite.

External links

• Online textbook devoted to bainite, from Cambridge University Press and the Institute of Materials

• The Alloying Elements in Steel, by Edgar C. Bain

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