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Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite forms by a eutectoid reaction as austenite cools below 723 °C (1,333 °F) (the eutectoid temperature).
As a result of the transformation, the microconstituents, pearlite and bainite, form; pearlite forms at higher temperatures and bainite at lower. TTT diagram of isothermal transformations of a hypoeutectoid carbon steel; showing the main components obtained when cooling the steel and its relation with the Fe-C phase diagram of carbon steels.
At lower coiling temperatures, cementite forms fine pearlitic colonies, whereas at higher temperatures, it precipitates as coarse particles at grain boundaries. This morphological difference influences the rate of austenite formation and decomposition, with fine cementite promoting faster transformations due to its increased surface area and ...
When austenite is cooled extremely slowly, it will form large ferrite crystals filled with spherical inclusions of cementite. This microstructure is referred to as "sphereoidite". If cooled a little faster, then coarse pearlite will form. Even faster, and fine pearlite will form.
In materials science, lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. They are often observed in cases where a phase transition front moves quickly, leaving behind two solid products, as in rapid cooling of eutectic (such as solder) or eutectoid (such as pearlite ...
Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). [1] First described by E. S. Davenport and Edgar Bain, [2] [3] it is one of the products that may form when austenite (the face-centered cubic crystal structure of iron) is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite ...
The specific cooling rate that is necessary to avoid the formation of pearlite is a product of the chemistry of the austenite phase and thus the alloy being processed. The actual cooling rate is a product of both the quench severity, which is influenced by quench media, agitation, load (quenchant ratio, etc.), and the thickness and geometry of ...
The needles will usually form when the brass cools from the recrystallization temperature, and will become very coarse if the brass is annealed to 1,112 °F (600 °C) for long periods. [16] Telluric iron, which is an iron-nickel alloy very similar to meteorites, also displays very coarse Widmanstätten structures. Telluric iron is metallic iron ...