Question

Pearlite in a steel is formed from

• A. Ferrite
• B. Martensite
• C. Cementite
• D. Austenite

Hint:

Remember the phase transformations on the Iron-Carbon diagram. Austenite is the parent phase at high temperatures. Upon cooling, it can transform into:

• Pearlite (Ferrite + Cementite) on slow cooling.

• Bainite on moderate cooling.

• Martensite on very fast cooling (quenching).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Pearlite is a two-phase, lamellar (layered) microconstituent found in steels, composed of alternating layers of ferrite (\(\alpha\)-iron) and cementite (Fe\(_3\)C). It is formed through a eutectoid reaction.
Austenite (\(\gamma\)-iron) is a single-phase solid solution of carbon in face-centered cubic (FCC) iron, which is stable at high temperatures.

Step 2: Detailed Explanation:
The formation of pearlite is a key phase transformation in the iron-carbon system, best understood by looking at the Iron-Carbon equilibrium diagram.

• Steel is first heated into the austenite region (above 727°C for eutectoid steel), where the entire structure becomes a single-phase solid solution, Austenite.

• The steel is then cooled slowly. As it cools below the eutectoid temperature (727°C), the austenite phase becomes unstable and transforms.

• This transformation is a eutectoid reaction:
\[ \text{Austenite (\(\gamma\))} \xrightarrow{\text{Slow Cooling}} \text{Ferrite (\(\alpha\))} + \text{Cementite (Fe}_3\text{C)} \]

• This mixture of ferrite and cementite grows in a cooperative manner, forming the characteristic layered structure known as Pearlite. The transformation involves the diffusion of carbon atoms. In regions that become ferrite, carbon diffuses out, and in regions that become cementite, carbon diffuses in.

Therefore, pearlite is a direct decomposition product of austenite upon slow cooling. The other options are incorrect as they are either products themselves (Martensite) or components of pearlite (Ferrite, Cementite).

Step 3: Final Answer:
Pearlite is formed from the decomposition of Austenite during slow cooling through the eutectoid temperature. The correct answer is Austenite.