Pearlite

Encyclopedia : P : PE : PEA : Pearlite

Iron alloy phases
Austenite (γ-iron; hard) Bainite Martensite Cementite (iron carbide; Fe3C) Ferrite (α-iron; soft) Pearlite (88% ferrite, 12% cementite)
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

Pearlite occurs at the eutectoid of the iron-carbon phase diagram (near the lower left).

Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite (88 wt%) and cementite (12%) that occurs in steel. It forms by a eutectoid reaction as austenite is slowly cooled below 723°C. The eutectoid composition of Austenite is approximately 0.77% carbon [link]; steel with less carbon content will contain a corresponding proportion of relatively pure ferrite crystallites that do not participate in the eutectoid reaction and cannot transform into pearlite.

The appearance of pearlite under the microscope resembles mother of pearl (also a lamellar structure), from which it takes its name.

A similar structure with lamellae much smaller than the wavelength of visible light lacks this pearlescent appearance. Called bainite, it is prepared by more rapid cooling. Unlike pearlite, whose formation involves the diffusion of all atoms, bainite grows by a displacive transformation mechanism.

Reference

1. [Thorough discussion of pearlite formation.]

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