Eutectoid composition identification: A plain carbon steel containing 0.8% carbon and having a 100% pearlitic microstructure is termed as:

Introduction / Context:
The eutectoid composition in the Fe–C system is a cornerstone concept in steel metallurgy. At this point, austenite transforms completely to pearlite upon slow cooling, delivering a characteristic lamellar ferrite–cementite structure and a useful balance of strength and ductility.

Given Data / Assumptions:

• Carbon content: approximately 0.8% (textbook eutectoid for plain carbon steel).
• Microstructure after slow cooling: 100% pearlite.
• No alloying elements altering the eutectoid composition.

Concept / Approach:
At the eutectoid composition, austenite decomposes isothermally at the eutectoid temperature to form pearlite entirely. Steels below 0.8% C are termed hypo-eutectoid and show ferrite + pearlite; steels above 0.8% C are hyper-eutectoid and show pearlite + proeutectoid cementite. Therefore, a 0.8% C steel with 100% pearlite is properly called eutectoid steel.

Step-by-Step Solution:
Confirm composition at the eutectoid point (~0.8% C).Note complete transformation of austenite to pearlite on slow cooling.Classify steel accordingly as “eutectoid”.Select “eutectoid steel”.

Verification / Alternative check:
Fe–C phase diagram and metallography texts consistently describe 0.8% C steel as eutectoid with fully pearlitic structure at equilibrium.

Why Other Options Are Wrong:
Hypo-eutectoid indicates C < 0.8% with ferrite + pearlite.

Hyper-eutectoid indicates C > 0.8% with pearlite + cementite network.

“None of these” and “martensitic steel” do not match equilibrium slow-cooling behaviour.

Common Pitfalls:
Confusing eutectic (liquid reaction) with eutectoid (solid-state reaction); assuming heat treatment other than slow cooling.

Final Answer:
eutectoid steel