In the Fe–C phase diagram near the eutectoid composition (~0.8 wt% C) at room temperature, what phase microstructure is common?

• A. Martensite
• B. Bainite
• C. Cementite
• D. Pearlite

Answer: (D)

Near the eutectoid composition, the austenite that exists at high temperature transforms at the eutectoid temperature into a lamellar mixture of ferrite and cementite. In the Fe–C system, this eutectoid point is about 0.76 wt% C and 727°C. When cooled slowly to room temperature, this γ → α + Fe3C transformation produces pearlite, a characteristic alternating layers of ferrite and cementite. Pearlite is the common microstructure at room temperature for steels near the eutectoid composition because the transformation proceeds through this equilibrium pathway, giving the familiar lamellar structure. If you quench rapidly, you can bypass pearlite and form martensite; bainite appears at different cooling rates and isn’t the typical near-eutectoid structure under slow cooling. Cementite is part of pearlite, not a separate equilibrium microstructure on its own.