Pearlite

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For the amorphous volcanic glass see perlite.

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
Carbon steel (≤2.1% carbon; low alloy) Stainless steel (steel with chromium) HSLA steel (high strength low alloy) Tool steel (very hard)
Other iron-based materials
Cast iron (>2.1% carbon) Wrought iron (contains slag) Ductile iron

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

Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of alpha-ferrite (88 wt%) and cementite (12%) that occurs in some steels and cast irons. It forms by a eutectoid reaction as austenite is slowly cooled below 727°C. The eutectoid composition of Austenite is approximately 0.8% carbon [1]; 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 lamelle 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.