Pearlite


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Related to Pearlite: Bainite, cementite

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 goal of the metallographic analysis was to find the pearlite occurrence and thus the minimum cooling rate to avoid the formation of pearlite as a function of the alloy composition.
3, a) indicated that the microstructure of given location mainly contained of ferrite (sample 1), and the structure was changing into pearlite + ferrite (sample 3 in Fig.
For materials with low bulk density, for example, mixtures having pearlite [[rho].
The first stage is the pearlite dissolution and the second stage is the ferrite to austenite transformation, which finishes at [Ac.
This transformation occurs in the cementitic lamellas in the pearlite of the original rail or wheel into a mixture of coarse deformed particles which the effects of enormous deformation further divide into ultra-fine grain structures.
Image analysis of pearlite spheroidization based on the morphological charact-erization of cementite particles.
Moving towards the inner diameter, the microstructure shifts to an equilibrium grey iron microstructure (also known as grey cast iron), comprised of flake graphite particles in a pearlite matrix.
Expanding the temperature interval for deformation from 900[degrees]C down to 650 and 600[degrees]C caused pearlite to form in the structure.
The microstructures of these steels range from 100 % ferrite to nearly 100 % pearlite (AISI 1080), with the X42 steel being intermediate, with less than 50 % pearlite in a highly banded structure oriented along the crack propagation direction.
The gray cast iron HT200 in this test is widely used in as met al matrix, Its microscopic structure is flake graphite + pearlite (Figure 1), depending on the molding process divided bionic coupling unit into laser surface melting unit (LMU).
These crystals are characterised by a troostite structure, with individual areas of plate-shaped pearlite.