Iron–carbon diagram fundamentals At what temperature does the eutectoid reaction occur in plain-carbon steel (austenite transforming to ferrite plus cementite)?

Introduction / Context:
The eutectoid temperature is the cornerstone of heat treatment in steels. It marks the transformation of austenite into the lamellar pearlite (ferrite plus cementite) in plain-carbon steels, controlling processes such as normalising, annealing, and hardening strategies.

Given Data / Assumptions:

• Plain-carbon Fe–C equilibrium diagram is considered.
• Eutectoid composition is around 0.76–0.8% C.
• Temperature values are for equilibrium conditions.

Concept / Approach:
The eutectoid reaction is written as: austenite → ferrite + cementite at approximately 723°C. This is the lower critical temperature (A1) for hypo- and hyper-eutectoid steels under equilibrium. Above this, austenite can exist; below, it decomposes to ferrite and cementite mixtures.

Step-by-Step Solution:
Locate eutectoid point on Fe–C diagram near 0.8% C.Read corresponding temperature: about 723°C for the eutectoid reaction.Confirm role in heat treatment: subcritical vs. supercritical heating referenced to 723°C.Therefore, the correct temperature is 723°C.

Verification / Alternative check:
Standard heat-treatment texts consistently cite 723°C as the A1 temperature in the equilibrium diagram for plain-carbon steels.

Why Other Options Are Wrong:
600°C is too low; 1147°C corresponds to eutectic reaction in Fe–C; 1493°C approaches iron’s melting point; 910°C relates to ferrite-to-austenite transition (A3) for pure iron, not the eutectoid in steels.

Common Pitfalls:
Confusing eutectic (liquid to two solids) with eutectoid (solid to two solids); misreading 723°C as approximate and picking nearby values without reason.

Final Answer:
723°C