pearlite
/ PURL-ite /
Pearlite is a microstructure, not a single phase: a fine, layered sandwich of two phases, soft ferrite and hard cementite, stacked in alternating sheets. Under the microscope the layers diffract light like the inside of a seashell, which is where the name comes from. It is what a medium-carbon steel becomes on ordinary cooling, blending softness and hardness into a strong, tough material.
More precisely, pearlite forms by the eutectoid reaction: austenite of 0.76 wt% C, cooled through 727 degrees C, transforms into alternating lamellae of ferrite (0.022% C) and cementite (6.7% C). The overall carbon balances by the lever rule, giving about 88 percent ferrite and 12 percent cementite by weight. The layer spacing depends on cooling rate: slower cooling gives coarse pearlite (softer), faster cooling gives fine pearlite (harder and stronger), simply because less time means shorter diffusion distances.
A fully eutectoid (0.76% C) steel is 100 percent pearlite. Below that, a hypoeutectoid steel gives proeutectoid ferrite plus pearlite; above it, a hypereutectoid steel gives proeutectoid cementite plus pearlite, and the amounts follow the lever rule. Pearlite is the equilibrium product; cool too fast and the eutectoid reaction is bypassed for bainite or martensite, which is the whole basis of hardening heat treatment.
A 1080 steel cooled in air becomes nearly 100 percent pearlite; rail steel is largely pearlitic for wear resistance.
About 88% ferrite and 12% cementite by weight, set by the lever rule.
Pearlite is a two-phase microstructure (ferrite plus cementite), not a phase itself, so it never appears as a field on the phase diagram; fine and coarse pearlite differ only in lamellar spacing, set by cooling rate.