Austenite (0.76 wt %C) à Ferrite (a) + Fe3C (0.02 wt%C)
If this phase transformation takes place as we are slow cooling, a microstructural component called pearlite is formed (Fig. 2). Pearlite consists of plates of Fe3C in a matrix of ferrite (a). Pearlite is formed from austenite containing the eutectoid composition of 0.76 wt%C. This microstructure develops as a result of the decomposition of austenite by nucleating carbide plates first and then the ferrite on the sides of the carbide plates.
Steels containing more than 0.76 wt%C are called hypereutectoid steels. For example, the phase diagram tells us that when a hypereutectoid steel containing 0.90 wt%C is heated to 1650°F (900°C), an austenite solid solution forms. Upon slow cooling, the first (or primary) phase formed is iron carbide (Fe3C). Upon cooling below 1333°F (723°C), austenite, which now contains 0.76 wt%C, transforms to pearlite. The microstructure of this alloy will consist of approximately 2% primary carbide and 98% pearlite.
Part 4: Martensite formation and tempering
Part 5: Time-temperature-transformation (TTT) Diagrams
Glossary of TermsAustenite: Austenite is the name given any solid solution in which gamma (γ) iron is the solvent. Austenite is the structure from which all quenching heat treatments must start.
Cementite: The common name for iron carbide, Fe3C, the chemical combination of iron and carbon. The stoichiometric phase.
Eutectoid: A eutectoid system is one in which a single-phase solid transforms directly to a two-phase solid.
Ferrite: Ferrite is the name given any solid solution in which alpha (α) iron is the solvent. The α-phase has a body centered cubic lattice. If you want to be precise, you call it α- ferrite.
Hypereutectoid steels: Hypereutectoid steels are those whose carbon content greater than 0.76 wt%.
Hypoeutectoid steels: Hypoeutectoid steels are those whose carbon content less than 0.76 wt%.
Pearlite: The two-phase mixture obtained right below the eutectoid point at 0.76 % carbon concentration.