Pearlite
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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 727°C. The eutectoid composition of Austenite is approximately 0.77% carbon [1] (http://www.ariuseickert.com/ScissorTechnicalArticles/EutectoidRegionOfSteel.htm); 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.
Reference
- Thorough discussion of pearlite formation. (http://www.ariuseickert.com/ScissorTechnicalArticles/EutectoidRegionOfSteel.htm)ja:パーライト