What type of grain structure is present on materials with less than 0.83% carbon?

What type of grain structure is present on materials with less than 0.83% carbon?

• A. ferrite and pearlite
• B. martensite
• C. austenite
• D. cementite

Answer: (A)

The key idea is how carbon content sets which phases appear in steel after cooling. At the eutectoid composition (about 0.77% C), austenite transforms into pearlite. When carbon is below that, the steel cools first to form ferrite (proeutectoid ferrite forms), and the remaining austenite then transforms at the eutectoid temperature into pearlite. The resulting microstructure at room temperature is a mix of ferrite and pearlite, which is the typical structure for low-carbon steels. Martensite would require very rapid quenching to trap austenite as a hard, brittle phase and is not the usual equilibrium structure for steels with less than about 0.8% C. Austenite is the high-temperature phase and is not present as a stable room-temperature structure in these steels. Cementite is a constituent within pearlite, not the overall grain structure for hypo-eutectoid steels. So, steels with less than about 0.83% carbon typically exhibit ferrite and pearlite at room temperature.

The key idea is how carbon content sets which phases appear in steel after cooling. At the eutectoid composition (about 0.77% C), austenite transforms into pearlite. When carbon is below that, the steel cools first to form ferrite (proeutectoid ferrite forms), and the remaining austenite then transforms at the eutectoid temperature into pearlite. The resulting microstructure at room temperature is a mix of ferrite and pearlite, which is the typical structure for low-carbon steels.

Martensite would require very rapid quenching to trap austenite as a hard, brittle phase and is not the usual equilibrium structure for steels with less than about 0.8% C. Austenite is the high-temperature phase and is not present as a stable room-temperature structure in these steels. Cementite is a constituent within pearlite, not the overall grain structure for hypo-eutectoid steels.

So, steels with less than about 0.83% carbon typically exhibit ferrite and pearlite at room temperature.