Eutectoid Steel Composition Explained

Eutectoid steel refers to a specific composition of steel, typically containing around 0.76% to 0.80% carbon. Understanding the structure of eutectoid steel is fundamental in metallurgy, especially when discussing heat treatments like annealing or normalizing.

The Eutectoid Transformation in Steel

The defining characteristic of eutectoid steel lies in its transformation behaviour upon cooling, as described by the Iron-Carbon phase diagram. When a steel with the eutectoid composition is heated above the eutectoid temperature (approximately 727°C or 1340°F), its structure consists of austenite ($\gamma$). Austenite is a high-temperature phase, an interstitial solid solution of carbon in face-centered cubic (FCC) iron.

Upon slow cooling from the austenite phase region, eutectoid steel undergoes a unique transformation at the eutectoid temperature. Unlike hypo-eutectoid steels (less than 0.8% C) which form pro-eutectoid ferrite first, or hyper-eutectoid steels (more than 0.8% C) which form pro-eutectoid cementite first, the eutectoid steel transforms entirely.

The eutectoid reaction is represented as:

$$ \gamma \rightarrow \alpha + Fe_3C $$

This reaction signifies that the austenite ($\gamma$) phase transforms directly into a mixture of ferrite ($\alpha$) and cementite ($Fe_3C$) phases upon crossing the eutectoid temperature.

Defining the Pearlite Microstructure

The structure resulting from the eutectoid transformation is called pearlite. Pearlite is not a single phase but a microconstituent, characterized by a lamellar (layered) structure consisting of alternating plates of ferrite and cementite.

Here's a breakdown of pearlite:

The layers of ferrite and cementite in pearlite are typically very fine, especially in rapidly cooled eutectoid steels, influencing the steel's mechanical properties.

Evaluating the Options for Eutectoid Steel

Based on the transformation principles:

• Option 1: Wholly pearlite - This accurately describes the microstructure of eutectoid steel after slow cooling below the eutectoid temperature. The entire austenite phase transforms into pearlite.
• Option 2: Wholly austenite - This is incorrect. Austenite ($\gamma$) is the phase present *above* the eutectoid temperature (727°C). Upon cooling, it transforms.
• Option 3: Pearlite and ferrite - This is incorrect. While ferrite is a component *within* pearlite, a eutectoid steel structure consists solely of pearlite. Ferrite would be present as a separate phase only in hypo-eutectoid steels (alongside pearlite).
• Option 4: Pearlite and cementite - Similar to option 3, this is incorrect. Cementite is also a component *within* pearlite. The eutectoid steel structure is entirely pearlite, not separate pearlite regions mixed with separate cementite regions (unless it's a hyper-eutectoid steel).

Therefore, the correct description of a eutectoid steel's microstructure after slow cooling is that it consists of wholly pearlite.