Why does rapid cooling produce finer pearlite and ferrite structures?

Rapid cooling produces finer pearlite and ferrite structures because it limits the time available for carbon atoms to diffuse through the metal lattice.1

In the context of steel metallurgy, the formation of pearlite—a lamellar microstructure of ferrite and iron carbide—requires carbon atoms to migrate and organize into these alternating layers. When steel is cooled slowly, carbon atoms have sufficient time to travel longer distances, allowing the carbide platelets to grow larger and the resulting microstructure to become coarser.213

Conversely, when the cooling rate is increased, the reduced time window prevents carbon atoms from migrating far, forcing the formation of a much finer, more densely packed microstructure. This refinement of the internal structure effectively increases the hardness of the steel. If the cooling rate is accelerated even further, such as through quenching, the transformation can be suppressed entirely, leading to the formation of martensite instead of pearlite.13