Question

In a thermal equilibrium (phase) diagram of iron-carbon system, eutectoid point occurs at:

• A. 0.8% C and $723^{\circ}C$
• B. 0.2% C and $910^{\circ}C$
• C. 2.1% C and $1147^{\circ}C$
• D. 4.3% C and $1147^{\circ}C$

Hint:

Logic Tip: The suffix "-oid" means "resembling." Eutectic is liquid-to-solid. Eutectoid resembles a eutectic reaction but happens entirely in the solid phase! Steel is heat-treated entirely in the solid phase, so the crucial steel temperature must be the lower one ($723^\circ$C).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Iron-Carbon phase diagram shows the phases present in iron-carbon alloys at different temperatures and compositions. The eutectoid point is a specific invariant point where one solid phase (Austenite) transforms into two other solid phases (Ferrite and Cementite) simultaneously upon cooling.
Step 2: Key Formula or Approach:
Identify the three major invariant reactions in the Fe-C diagram:
Peritectic (1495°C)
Eutectic (4.3% C, 1147°C)
Eutectoid (0.76%–0.8% C, 723°C–727°C)

Step 3: Detailed Explanation:
The eutectoid reaction occurs at a temperature of approximately 723°C (often cited as 727°C in modern texts) and a carbon concentration of 0.8%. At this point, Austenite ($\gamma$) transforms into Pearlite, which is a lamellar mixture of Alpha-ferrite ($\alpha$) and Cementite ($Fe_3C$). Alloys with exactly this composition are called eutectoid steels.
Step 4: Final Answer:
The eutectoid point occurs at 0.8% C and 723°C.