Understanding Eutectoid Composition in Carbon Steel

The question asks about the specific constituent formed when carbon steel with the eutectoid composition is at room temperature. This relates to the phase transformations that occur in steel based on carbon content and temperature, often represented by the Iron-Carbon phase diagram.

What is Eutectoid Composition?

In the context of steel, the eutectoid composition refers to a specific percentage of carbon in iron where a single solid phase (austenite) transforms into two different solid phases upon cooling below a certain temperature (the eutectoid temperature, approximately $727^\circ C$ or $1341^\circ F$). This transformation is called the eutectoid reaction.

For carbon steel, the eutectoid composition is approximately 0.76% to 0.8% carbon by weight.

Phase Transformation at Eutectoid Composition

When carbon steel with this eutectoid composition is cooled slowly from the austenite phase above $727^\circ C$ to room temperature, the austenite ($\gamma$-Fe, FCC structure) transforms completely at $727^\circ C$ into a specific two-phase microstructure. This microstructure is a fine, lamellar (layered) mixture of:

• Ferrite ($\alpha$-Fe): A soft, ductile phase with a body-centered cubic (BCC) structure, containing very little dissolved carbon.
• Cementite ($\text{Fe}_3\text{C}$): A very hard and brittle intermetallic compound containing 6.67% carbon by weight.

This lamellar aggregate structure formed by the eutectoid decomposition of austenite is known as pearlite.

Eutectoid Composition at Room Temperature

Therefore, for carbon steel, the microstructure present at room temperature, when the steel has the eutectoid composition (around 0.76-0.8% carbon) and has been cooled under equilibrium or near-equilibrium conditions (slow cooling), is pearlite.

Analyzing the Options

• pearlite: This is the lamellar mixture of ferrite and cementite that forms from austenite at the eutectoid temperature upon slow cooling. This corresponds exactly to the description of the structure at the eutectoid composition at room temperature under equilibrium conditions.
• ferrite: Ferrite is a phase in steel (BCC iron with interstitial carbon) but it is not the *entire* eutectoid constituent. Hypoeutectoid steels (less than ~0.76% C) will have proeutectoid ferrite plus pearlite at room temperature, but the eutectoid composition forms *only* pearlite.
• cementite: Cementite is also a phase in steel (iron carbide, $\text{Fe}_3\text{C}$) and is a component of pearlite. Hypereutectoid steels (more than ~0.76% C) will have proeutectoid cementite plus pearlite at room temperature, but the eutectoid composition forms *only* pearlite.
• martensite: Martensite is a non-equilibrium phase formed by rapidly quenching austenite. While it can form from austenite of eutectoid composition, it is not the structure present at room temperature under typical cooling conditions implied by the question's context of "eutectoid composition" without specifying rapid quenching. Martensite is a body-centered tetragonal (BCT) structure.

Based on the phase diagram and the definition of the eutectoid reaction and constituent, the eutectoid composition of carbon steel at room temperature (assuming slow cooling) is known as pearlite.