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 ?
It can be concluded from these tests that all gray iron showing improved machinability in the aged condition contained some amount of free ferrite, while gray iron showing increased cutting forces after aging had no free ferrite but was entirely pearlitic with cementite/ steadite phases.
These findings have established that experimental processes may lead to generation of structures analogous to the discussed WEL only by mechanical spreading of metallic dust from pearlitic steel under high pressure at temperatures lower than the transformation temperature required for generation of austenite (Djahanbakhsh et al.
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 system features one-micron Heidenhain glass scales and the thermo-balanced, artificially-aged, pearlitic cast-iron structure that ensures the equipment is fully isostatic and will not deform or distort over time or temperature change.
A thermobalanced and artificially aged pearlitic cast iron structure makes the STP-35 EzVision isostatic and unable to deform or distort over time or during temperature changes.
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.
Tenders are invited for Automatic line shot blasting transmission type for straight and curved pipe fittings from pearlitic steels.
The fracture capability and effect of microstructure after austempering, optimizing the fracture parameters and comparing the conventional pearlitic C70S6 (produced in compliance with DIN 17100 and inspected according to EN 10204) and bainitic C70S6 have been investigated in this study.
The original structure of measured sample was a pearlitic one, respectively ferrite-pearlitic, with hardness ranging from 170 HV to 180 HV.