46MSE 2090: Introduction to Materials ScienceChapter 9, Phase DiagramsDevelopment of Microstructure in Iron - Carbon alloysMicrostructure depends on composition (carbon content) and heat treatment. In the discussion below we consider slow cooling in which equilibrium is maintained.Microstructure of eutectoid steel (I)
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47MSE 2090: Introduction to Materials ScienceChapter 9, Phase DiagramsWhen alloy of eutectoid composition (0.76 wt % C) is cooled slowly it forms perlite, a lamellar or layered structure of two phases: α-ferrite and cementite (Fe3C)The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%) by atomic diffusion. Mechanically, pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite.Microstructure of eutectoid steel (II)