Perliitti

Perliitti, or pearlite in Finnish, is a two-phase, lamellar microstructure of ferrite and cementite that forms in iron–carbon steels when austenite transforms at the eutectoid temperature around 727°C. It develops from austenite with approximately 0.76 weight percent carbon through the eutectoid reaction: austenite → ferrite plus cementite.

Microstructure and morphology: Pearlite consists of alternating plates or lamellae of ferrite (soft, largely pure iron) and cementite (Fe3C). The spacing and thickness of the lamellae depend on the cooling rate and the prior austenite grain size; slower cooling yields coarse pearlite, faster cooling yields fine pearlite. In micrographs, fine pearlite shows closely spaced lamellae, while coarse pearlite displays wider spacing.

Formation and composition: In hypo-eutectoid steels (carbon content less than 0.76%), pearlite forms after proeutectoid ferrite develops during cooling. In hyper-eutectoid steels (carbon content greater than 0.76%), pearlite forms after proeutectoid cementite. At near-eutectoid composition, pearlite is the dominant phase. Alloying elements such as manganese, silicon, chromium and nickel influence the temperature, kinetics and morphology of pearlite formation.

Properties and significance: Pearlite increases strength and hardness relative to pure ferrite but reduces ductility. The mechanical properties correlate with the lamellar spacing: finer pearlite generally yields higher strength and hardness. Pearlite is a common constituent in many carbon steels and can be promoted or refined by heat treatments such as normalization or annealing to tailor performance for specific applications.

See also: ferrite, cementite, bainite, martensite.