pearlite

Pearlite is a two-phase, lamellar microstructure found in steel and cast iron. It consists of alternating plates of ferrite (α-Fe) and cementite (Fe3C) formed by the eutectoid transformation of austenite. The eutectoid composition is about 0.76 weight percent carbon, and the transformation occurs at approximately 727°C.

During cooling through 727°C, austenite decomposes into ferrite and cementite, creating the characteristic alternating lamellae. The spacing of the lamellae depends on the cooling rate: slow cooling produces coarse pearlite with widely spaced lamellae, while rapid cooling yields fine pearlite with closely spaced lamellae. This microstructure gives intermediate mechanical properties between ferrite and cementite.

In steels with carbon content below the eutectoid, proeutectoid ferrite forms before pearlite; in steels above the eutectoid, proeutectoid cementite forms before pearlite. At the eutectoid composition, pearlite forms from austenite below 727°C without a preceding proeutectoid phase.

Pearlite’s properties reflect its lamellar structure: it is harder and stronger than pure ferrite but less brittle than cementite, with the exact behavior depending on lamellar spacing and carbon content. Finer pearlite typically increases strength and hardness while reducing ductility.

Pearlite is a key concept in metallography and heat treatment, used to assess and tailor the mechanical properties of steels. Its presence and morphology influence hardness, toughness, and machinability in practical applications.