Question

For the given schematic TTT diagram of an eutectoid steel, the following statement(s) is/are true for the heat treatment schedules HT-1, HT-2, and HT-3. 

• A. HT-3 leads to the formation of a pearlite microstructure
• B. HT-1 leads to a predominantly martensite microstructure
• C. HT-2 leads to a bainite microstructure
• D. HT-3 leads to a mixture of pearlite and bainite microstructure

Hint:

To read a TTT diagram, trace the cooling path. If the path misses the "nose" of the C-curves and drops below M\(_s\), you get martensite. If you cool fast past the pearlite nose and then hold in the lower temperature region, you get bainite. If you cool and hold in the upper temperature region (just below A\(_c1\)), you get pearlite.

Answer 1

The Correct Option is A, B, C

Step 1: Understanding the Concept:
A Time-Temperature-Transformation (TTT) diagram, also known as an isothermal transformation diagram, shows the microstructural changes that occur in a steel when it is held at a constant temperature after being austenitized. To interpret the diagram for a given heat treatment schedule, we trace the path of temperature versus time and observe which transformation regions the path crosses.
Step 2: Detailed Analysis of Each Heat Treatment Schedule:
- Heat Treatment HT-1:
1. The steel is austenitized (held above A\(_c1\)).
2. It is then rapidly cooled (quenched) to a temperature below the M\(_{start}\) (martensite start) line.
3. The cooling path does not intersect the "nose" of the pearlite or bainite transformation curves.
4. By crossing the M\(_{start}\), M\(_{50}\), and M\(_{90}\) lines, the austenite transforms into martensite.
Since the path goes below M\(_{90}\), more than 90% of the austenite transforms.
5. Therefore, HT-1 leads to a predominantly martensite microstructure. Statement (B) is correct.
- Heat Treatment HT-2:
1. The steel is austenitized.
2. It is rapidly cooled to a temperature between the "noses" of the pearlite and bainite curves, but above M\(_{start}\).
3. It is then held at this constant temperature.
4. The path enters the bainite transformation region (labeled "Bainite"). It crosses the start and finish lines for the bainite transformation.
5. The austenite completely transforms into bainite.
6. Therefore, HT-2 leads to a bainite microstructure. Statement (C) is correct.
- Heat Treatment HT-3:
1. The steel is austenitized.
2. It is rapidly cooled to a temperature just below the pearlite "nose".
3. It is then held at this constant temperature.
4. The path enters the pearlite transformation region (labeled "Pearlite"). It crosses the start and finish lines for the pearlite transformation.
5. The austenite completely transforms into pearlite.
6. Therefore, HT-3 leads to the formation of a pearlite microstructure. Statement (A) is correct. Statement (D) is incorrect because the path does not enter the bainite region.
Step 3: Why This is Correct:
By tracing each heat treatment path on the TTT diagram, we can determine the final microstructure. The analysis shows that HT-1 produces martensite, HT-2 produces bainite, and HT-3 produces pearlite. Therefore, statements (A), (B), and (C) are all true.