pearlitestruktuuri

Pearlite structure, or pearlitestruktuuri, is a two-phase microstructure found in steel that forms when austenite transforms eutectoidally into ferrite and cementite. It consists of alternating lamellae or plates of ferrite (α-Fe) and cementite (Fe3C), giving a characteristic pearly appearance under optical microscopy.

The transformation occurs as steel cools through the eutectoid temperature, about 727°C in the iron–carbon system, at a carbon content near 0.76 wt%. The reaction is diffusion-controlled: carbon diffuses to form cementite while ferrite forms from the remaining iron. The resulting lamellar spacing depends on cooling rate and alloy composition. Slow cooling yields coarse pearlite with wider spacing, while faster cooling produces fine, closely spaced lamellae.

Pearlite morphology can be described as coarse or fine and, at certain conditions, as upper or lower pearlite, reflecting different cementite thickness and spacing formed during the transformation. The presence and spacing of the lamellae influence mechanical properties: pearlite generally increases strength and hardness compared with pure ferrite, while toughness and ductility depend on lamellar spacing and carbon content. Higher carbon content leads to more cementite and higher hardness but reduced ductility.

Observation and characterization are typically done by metallographic etching and light microscopy to measure interlamellar spacing. Pearlite structure is a key feature in heat treatment design, as processes such as normalizing, annealing, or spheroidizing manipulate the pearlite fraction and spacing to tailor strength, hardness, and toughness in steel.