Question

The TTT diagram for eutectoid steel is shown below. The steel after complete austenitization at 1073 K is rapidly cooled to different temperatures and held for varying times (as indicated in Column I) followed by quenching to 300 K. Assuming isothermal transformation, match the heat treatment conditions in Column I with the corresponding microstructure in Column II.

• A. P-3; Q-2; R-1; S-4
• B. P-2; Q-3; R-1; S-4
• C. P-2; Q-1; R-3; S-4
• D. P-4; Q-3; R-1; S-2

Hint:

Use the TTT diagram systematically: - High T + long time → Pearlite. - Medium T + moderate time → Bainite. - Low T + quench → Martensite. - Short holding in bainite region + quench → Bainite + Martensite.

Answer 1

The Correct Option is B

Step 1: Understanding the TTT diagram.
The TTT (Time-Temperature-Transformation) diagram describes the isothermal transformation of austenite into pearlite, bainite, or martensite depending on temperature and holding time. - High temperature and slow cooling → Pearlite. - Intermediate temperature and moderate time → Bainite. - Very rapid cooling to low temperature → Martensite. - Interrupted cooling can give Bainite + Martensite.

Step 2: Case (P) – Held at 300 K indefinitely.
- If steel is quenched to a very low temperature (300 K, below martensite start temperature), the transformation is diffusionless. - The microstructure formed is entirely Martensite. \[ P \Rightarrow (2) \, \text{Martensite} \]

Step 3: Case (Q) – Held at 873 K for 4 minutes.
- 873 K lies just below the eutectoid temperature (around 1000 K) in the pearlite transformation zone. - The nose of the C-curve at ~1–10 s indicates pearlite forms in this region. - Holding for 4 minutes (longer than critical time) ensures pearlite formation. \[ Q \Rightarrow (3) \, \text{Pearlite} \]

Step 4: Case (R) – Held at 673 K for 20 minutes.
- 673 K is in the intermediate temperature range (around 600–700 K), which lies in the bainite transformation zone. - Holding long enough (20 min) allows austenite to fully transform into bainite. \[ R \Rightarrow (1) \, \text{Bainite} \]



Step 5: Case (S) – Held at 623 K for 2 minutes.
- 623 K lies in the bainite region, but 2 minutes is too short for full transformation. - After partial transformation to bainite, quenching to 300 K converts the remaining austenite to martensite. - Hence, a mixed microstructure forms. \[ S \Rightarrow (4) \, \text{Bainite + Martensite} \]

Step 6: Final Matching.
\[ (P, Q, R, S) = (2, 3, 1, 4) \] Thus, the correct option is: \[ \boxed{\text{(B) P-2; Q-3; R-1; S-4}} \]