Pearlite

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. During slow cooling pearlite forms by a eutectoid reaction as austenite is below 727°C (the eutectoid temperature).

The eutectoid composition of austenite is approximately 0.8% carbon; 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. Likewise steels with higher carbon contents will form cementite before reaching the eutectoid point. The proportion of ferrite and cementite forming above the eutectoid point can be calculated from the iron/iron—carbide equilibrium phase diagram using the lever rule.

Pearlite was first identified by Henry Clifton Sorby and initially named sorbite, however the similarity of microstructure to nacre and especially the optical effect caused by the scale of the structure made the alternative name more popular.

Pearlite is a common microstructure occurring in many grades of steels.

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

Properties

It has very high strength, hardness and toughness. It has excellent wear resistance because of a strong lamellar network of ferrite and cementite.

Application

Because of its good strength and hardness, it is used for making cutting tool materials, High strength wires, Knives, chisels, punch nails etc.

References

Comprehensive information on pearlite

Introduction to Physical metallurgy by Sidney.H.Avner, second edition, McGraw hill publications.

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