What is the resulting structure of a 0.6% carbon steel slowly cooled from 900°C to room temperature?

What is the resulting structure of a 0.6% carbon steel slowly cooled from 900°C to room temperature?

• A. Mostly pearlite with a small amount of cementite
• B. About 25% proeutectoid ferrite and 75% pearlite
• C. 100% pearlite
• D. About 50% proeutectoid ferrite and 50% pearlite

Answer: (B)

When carbon content is below the eutectoid composition (0.76% C), slow cooling causes proeutectoid ferrite to form before the eutectoid transformation. The remaining austenite then transforms at the eutectoid temperature into pearlite. The fraction of proeutectoid ferrite can be estimated with the lever rule at the eutectoid point: with C_eutectoid ≈ 0.76% and C0 ≈ 0.60%, the proeutectoid ferrite fraction is about (0.76 − 0.60) / 0.76 ≈ 0.21, i.e., around 20% ferrite and 80% pearlite. In practice this is typically rounded to about 25% proeutectoid ferrite and 75% pearlite, which matches the given answer. The proeutectoid ferrite contains very little carbon, while the pearlite portion forms from the remaining austenite and has the eutectoid composition (lamellae of ferrite and cementite). This explains why the final structure is not 100% pearlite and not a 50/50 mix.

When carbon content is below the eutectoid composition (0.76% C), slow cooling causes proeutectoid ferrite to form before the eutectoid transformation. The remaining austenite then transforms at the eutectoid temperature into pearlite. The fraction of proeutectoid ferrite can be estimated with the lever rule at the eutectoid point: with C_eutectoid ≈ 0.76% and C0 ≈ 0.60%, the proeutectoid ferrite fraction is about (0.76 − 0.60) / 0.76 ≈ 0.21, i.e., around 20% ferrite and 80% pearlite. In practice this is typically rounded to about 25% proeutectoid ferrite and 75% pearlite, which matches the given answer.

The proeutectoid ferrite contains very little carbon, while the pearlite portion forms from the remaining austenite and has the eutectoid composition (lamellae of ferrite and cementite). This explains why the final structure is not 100% pearlite and not a 50/50 mix.