Pearlite

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pearlite

[′pər‚līt]
(geology)
(metallurgy)
A lamellar aggregate of ferrite (almost pure iron) and cementite (Fe3C) often occurring in carbon steels and in cast iron.

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

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.
References in periodicals archive ?
The analysis of crank shaft failure reveals a crack at surface originated due to sharp boundaries of primary type I graphite nucleated in pearlitic matrix may be the root cause of fatigue failure based on the fractography analysis [2].
sigma]] = -1, and for constant amplitude loading for ferritic and pearlitic materials.
Hong MH, Reynolds WD, Tarui T, Hono K (1999) Atom Probe and Transmission Electron Mcroscopy investigations on heavily drawn pearlitic steel wire.
1996) does not regard the white etching layers to be 'traditional' martensite either, in reference to Liebelt and Knotthe (1997) who suggests that the maximum temperature limitation of the processes occurring in the wheel-rail contact is temperature increment by 400 K; this value is not sufficient for austenitisation of pearlitic material and for its subsequent quenching into the martensite region.
Fatigue fracture through the fully pearlitic alloy in the hydrogen environment appeared to be transgranular, and light etching with 5 % nital revealed evidence of the lamellar structure that was not evident on the as-fractured surface.
The Effect of Molybdenum, Cooper and Nickel on the Pearlitic and Martensitic Hardenability of Ductile Cast Iron, Trans.
In testing against another CBN material where pearlitic cast iron brake discs were finished, the CBN200 machined 12,000 parts per edge vs.
The results of laboratory and industrial corrosion tests of various boiler steels and pearlitic steel 12X1M[PHI] with coating containing 73% Ni and 16% Cr have shown that the resistance of superheater tubes to chlorine corrosion depends on the content of alloying elements as well as on the particular element content ratio.
Modified nodular iron materials with a pearlitic or acicular matrix (such as Walzen Irle KST-P and KST-A) are available.
PHOTO : Superplastic steels are made by converting the pearlitic structure of an ultrahigh carbon steel (top photo) to a superplastic structure (bottom photo) with a combined heat treatment and deformation process.