Fe–C phase understanding — microstructure when carbon is below the eutectoid When a steel containing less than 0.8% carbon is cooled slowly below the lower critical temperature, which phases are present in the final microstructure?

Introduction / Context:
Recognizing which phases appear in slowly cooled steels of various carbon contents is fundamental to heat treatment and mechanical property prediction. The eutectoid composition in the iron–carbon system is about 0.8% carbon; compositions on either side are called hypoeutectoid or hypereutectoid, with distinct transformation products.

Given Data / Assumptions:

• Equilibrium (slow) cooling below the lower critical temperature (A1).
• Plain carbon steels without strong alloying shifts.
• Carbon content compared to the eutectoid value ~0.8%.

Concept / Approach:
For hypoeutectoid steels (C < 0.8%), proeutectoid ferrite forms first as the austenite cools towards A1. At A1, the remaining austenite transforms into pearlite (alternating ferrite and cementite lamellae). Therefore the final mixture is ferrite plus pearlite. At exactly 0.8% C, the entire austenite becomes pearlite. For hypereutectoid (C > 0.8%), proeutectoid cementite forms prior to pearlite, giving cementite plus pearlite.

Step-by-Step Solution:

Verification / Alternative check:
Continuous cooling transformation diagrams (for slow rates) and the Fe–Fe3C equilibrium diagram both show ferrite plus pearlite below A1 for hypoeutectoid steels.

Why Other Options Are Wrong:

• Equal to 0.8%: yields pearlite only.
• More than 0.8%: produces pearlite plus proeutectoid cementite.
• Austenite only: stable only above critical temperatures.
• Martensite on slow cooling: martensite requires rapid quenching.

Common Pitfalls:
Confusing proeutectoid constituents; assuming pearlite is a separate phase rather than a lamellar mixture.

Final Answer: