How does pearlite form when steel cools?

When steel cools, the formation of pearlite occurs through a process known as the eutectoidic transformation.1

As steel cools to approximately 727° C (1,341° F), the austenite crystals—which hold carbon in a solid solution—undergo a structural change. They transform into a fine, lamellar microstructure consisting of alternating platelets of ferrite and cementite (iron carbide).1

The characteristics of this resulting microstructure depend on the cooling rate:
* Slow cooling: Allows more time for carbon atoms to migrate through the crystal lattices, resulting in a coarser pearlite structure.2
* Faster cooling: Limits the time available for carbon diffusion, which produces a finer pearlite microstructure and increases the hardness of the steel.2

If the cooling rate is accelerated significantly, such as through quenching, the formation of pearlite can be suppressed entirely, leading instead to the creation of martensite, a much harder and more brittle phase.2