Question

Austempering produces

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

Hint:

Lower bainite, formed at lower austempering temperatures (closer to \( M_s \)), has a finer needle-like structure and exhibits superior yield strength and toughness compared to upper bainite.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the microstructural constituent produced in steel by the austempering heat treatment process.

Step 2: Detailed Explanation:

• Definition of Austempering: Austempering is an isothermal heat treatment process designed to produce a fully bainitic microstructure in carbon and low-alloy steels.
Bainite is a microstructural phase consisting of a fine, plate-like mixture of ferrite and cementite that offers an excellent combination of high strength, toughness, and impact resistance.

• Process Steps:
1. The steel is heated into the single-phase austenite region to achieve complete austenitization.
2. It is then rapidly quenched in a hot bath (usually molten salt) maintained at a temperature below the pearlite-forming range but above the martensite start temperature (\( M_s \)), typically between \( 250^\circ\text{C} \text{ and } 450^\circ\text{C} \).
3. The steel is held isothermally at this temperature long enough for the austenite to transform completely into bainite (either upper or lower bainite, depending on the temperature).
4. Finally, it is cooled to room temperature.

• Comparison with Martensite: Unlike conventional quenching which produces brittle martensite (Option A) that requires tempering, austempering directly produces tough bainite. This eliminates the need for a separate tempering step and reduces the risk of thermal cracking and distortion.


Step 3: Final Answer:
Thus, austempering produces bainite, which corresponds to Option (B).