Pearlite

pearlite [′pər‚līt]

(geology)

perlite

(metallurgy)

A lamellar aggregate of ferrite (almost pure iron) and cementite (Fe₃C) often occurring in carbon steels and in cast iron.

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perlite

A siliceous volcanic rock; under heat it expands to 15 to 20 times its original volume, forming an excellent lightweight aggregate; used in plaster or gypsum wallboard, as loose-fill thermal insulation, and as an aggregate in concrete.

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Pearlite 

(in metallurgy), a structural component of iron-carbon alloys (steels and cast irons) that consists of a eutectic mixture of two phases, ferrite and cementite. In alloy steels the two phases that combine to form pearlite are different carbides.

Pearlite is a product of the eutectic decomposition of austenite upon relatively slow cooling of iron-carbon alloys below 723°C. In this process, γ-iron is transformed into α-iron, in which the solubility of carbon is only about 0.02 percent. The excess carbon precipitates as cementite or carbides.

A distinction is made between lamellar and granular pearlite. In the former, which is more common, both phases are present as lamellas; in the latter, rounded granules, or globules, of cementite are arranged against a background of ferrite globules. As supercooling proceeds, the number of pearlite colonies increases; that is, the number of sections of the piece of metal in which ferrite and cementite or carbide lamellas are uniformly oriented increases. Furthermore, the lamellas of pearlite become thinner as the temperature drops.

The mechanical properties of pearlite depend primarily on the interlamellar distance, that is, the total thickness of the two adjacent lamellas. Decreasing this distance increases ultimate strength and yield strength and decreases the ductile-to-brittle transition critical temperature. The structure of pearlite facilitates mechanical working of steel. Certain dispersed varieties of pearlite are sometimes called sorbite or troostite.

REFERENCE

Bunin, K. P., and A. A. Baranov. Metallografiia. Moscow, 1970.

R. I. ENTIN