Question

Which one of the following sets of constituents is expected in equilibrium cooling of a hyper-eutectoid steel from the austenitic state?

• A. Cementite and pearlite
• B. Ferrite and pearlite
• C. Ferrite and bainite
• D. Cementite and martensite

Hint:

Hyper-eutectoid steels (> 0.8% C) form cementite along grain boundaries before transforming the remaining austenite into pearlite.

Answer 1

The Correct Option is A

Hyper-eutectoid steel contains more than 0.8% carbon and follows a transformation sequence during equilibrium cooling. Step 1: Understanding the Cooling Process 1. At high temperatures, the steel is in the austenitic state (gamma phase). 2. Upon slow cooling below the eutectoid temperature (727°C), the excess carbon precipitates as cementite (Fe3C) along the grain boundaries. 3. The remaining austenite transforms into pearlite, which consists of alternating layers of ferrite and cementite.

Step 2: Identifying the Constituents - In hyper-eutectoid steel, the final microstructure after equilibrium cooling consists of cementite and pearlite. - Ferrite is found in hypo-eutectoid steel, making option (B) incorrect. - Bainite forms under non-equilibrium conditions, ruling out option (C). - Martensite forms only under rapid cooling (quenching), eliminating option (D).

Step 3: Conclusion The correct answer is (A) Cementite and pearlite, which are the equilibrium phases expected in a hyper-eutectoid steel after slow cooling.