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

Concept:
The Iron-Carbon (Fe-C) phase diagram is the foundational map for steel metallurgy. It features crucial "invariant points" where three phases exist in perfect equilibrium. The most important for heat-treating steel is the solid-state "eutectoid" reaction.

Step 1:
An eutectoid reaction occurs when a single solid phase transforms isothermally into two different solid phases upon cooling. In the Fe-C system, this is the transformation of solid Austenite ($\gamma$-iron) into a lamellar mixture of solid Ferrite ($\alpha$-iron) and solid Cementite ($Fe_3C$), collectively known as Pearlite.

Step 2:
Do not confuse eutectoid with eutectic! The *eutectic* point is where a liquid transforms into two solids. In the Fe-C diagram, the eutectic point occurs at exactly 4.3% Carbon and $1147^{\circ}C$ (matching Option D, which defines cast irons).

Step 3:
The maximum solid solubility of carbon in austenite is 2.14% C at $1147^{\circ}C$ (matching Option C). This composition physically marks the boundary line between steels and cast irons.

Step 4:
The "V-shaped" dip in the solid-state region of the diagram defines the eutectoid composition. This point lies squarely in the steel region at exactly 0.76% Carbon by weight (which is universally approximated as 0.8% C in engineering textbooks).

Step 5:
The horizontal tie-line marking the lower critical temperature ($A_1$ line) where this Austenite-to-Pearlite transformation occurs is fixed at $727^{\circ}C$ (historically referenced in older texts as $723^{\circ}C$). Matching the coordinates 0.8% C and $723^{\circ}C$ isolates Option (A) as the correct answer.