When up to 0.8% carbon is added to iron, the resulting steel under equilibrium conditions will have which structure?

• A. Austenite
• B. Bainite
• C. Ferrite and Pearlite
• D. Martensite

Answer: (C)

The main idea is how carbon content determines the phases that form when steel is cooled slowly to equilibrium. For carbon levels up to about the eutectoid composition, the austenite that exists at high temperature will transform into a mixture of ferrite and pearlite as it cools. Pearlite is the lamellar combination of ferrite and cementite, so the overall structure you end up with at room temperature is a blend of ferrite and pearlite. This contrasts with non-equilibrium cooling, which can produce bainite or martensite, layers of which require faster cooling. So, ferrite and pearlite best describes the equilibrium microstructure for steel with up to roughly 0.8% carbon.