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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).
Below 912 °C (1,674 °F), iron has a body-centered cubic (bcc) crystal structure and is known as α-iron or ferrite.It is thermodynamically stable and a fairly soft metal. α-Fe can be subjected to pressures up to ca. 15 GPa before transforming into a high-pressure form termed ε-Fe discussed below.
In carbon steel, cementite precipitates from austenite as austenite transforms to ferrite on slow cooling, or from martensite during tempering. An intimate mixture with ferrite, the other product of austenite, forms a lamellar structure called pearlite. The iron-carbon phase diagram
Pearlite, a lamellar structure consisting of alternating layers of ferrite and cementite, forms through cooperative nucleation and growth processes from austenite. The thickness of these layers has a direct impact on the mechanical properties of the steel. [10]
It is mostly found with cementite or pearlite in a range of cast irons. It is named after the metallurgist Karl Heinrich Adolf Ledebur (1837–1906). He was the first professor of metallurgy at the Bergakademie Freiberg and discovered ledeburite in 1882. Ledeburite arises when the carbon content is between 2.06% and 6.67%.
This increases the brittleness of the steel in a way that can only be relieved by recrystallizing. Widmanstätten structures made from ferrite sometimes occur in carbon steel, if the carbon content is below but near the eutectoid composition (~ 0.8% carbon). This occurs as long needles of ferrite within the pearlite. [16]
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Upon cooling slowly, the solution of iron and carbon (a single phase called austenite) will separate into platelets of the phases ferrite and cementite. This forms a layered microstructure called pearlite. Since pearlite is harder than iron, the degree of softness achievable is typically limited to that produced by the pearlite.
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