Phase Transformations of Fe-C

Phase transformations involve some alteration of microstructure. Phase transformations can be divided into three categories:

• Diffusion-dependent with no change in phase composition or number of phases present (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.)
• Diffusion-dependent with changes in phase compositions and/or number of phases (e.g. eutectoid transformations)
• Diffusionless phase transformation - produces a metastable phase by cooperative small displacements of all atoms in structure (e.g. martensitic transformation).

Phase transformations do not occur instantaneously. Diffusion-dependent phase transformations can be rather slow and the final structure often depend on the rate of cooling/heating. Phase transformations dependent on temperature, time and composition.

Formation of Perlite

The family of S-shaped curves at different T are used to construct the TTT diagrams. The TTT diagrams are for the isothermal (constant T) transformations (material is cooled quickly to a given temperature before the transformation occurs, and then keep it at that temperature).Generally at low temperatures, the transformation occurs sooner (it is controlled by the rate of nucleation) and grain growth (that is controlled by diffusion) is reduced. Slow diffusion at low temperatures leads to fine-grained microstructure with thin-layered structure of pearlite (fine pearlite). At higher temperatures, high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite (coarse pearlite). At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexist with pearlite. Additional curves for proeutectoid transformation must be included on TTT diagrams (Fig 1-2)

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