Question:medium
A plain carbon steel sample of eutectoid composition is heat treated from austenizing temperature to achieve a microstructure consisting of ONLY bainite.
Which ONE of the following is the corresponding heat treatment method?
A plain carbon steel sample of eutectoid composition is heat treated from austenizing temperature to achieve a microstructure consisting of ONLY bainite.
Which ONE of the following is the corresponding heat treatment method?
Which ONE of the following is the corresponding heat treatment method?
Show Hint
Compare the four heat treatments by where the isothermal hold sits on the TTT diagram.
Updated On: Aug 5, 2026
- Normalizing
- Annealing
- Martempering
- Austempering
Show Solution
The Correct Option is D
Solution and Explanation
Step 1: Set up a quick comparison in your head:
Four heat treatments are on offer, and each one has one signature product: normalizing gives fine pearlite, annealing gives coarse pearlite, martempering gives martensite, austempering gives bainite.
Since the question asks for ONLY bainite, we just need to match the product to the process.
Step 2: Think about the cooling path on a TTT diagram:
To get bainite, the cooling curve has to dodge the "nose" of the C-curve fast enough to avoid pearlite, then flatten out (isothermal hold) somewhere in the middle temperature band where bainite forms.
Normalizing and annealing both cool continuously without a hold, so they land in the pearlite region, not bainite.
Martempering also uses an isothermal hold, but that hold sits close to $M_s$ (martensite start), so the steel forms martensite once air cooling resumes, not bainite.
Step 3: Locate austempering on the diagram:
Austempering's hold sits lower than the pearlite nose but above $M_s$, right inside the bainite bay of the TTT diagram, and the hold continues until the transformation is complete.
Because the steel never drops below $M_s$ while austenite still remains, no martensite forms at all, the whole structure converts to bainite.
\[ M_s < T_{hold} < T_{nose}, \quad \text{held until transformation is 100 percent complete} \]
Final Answer:
The isothermal hold placed inside the bainite bay is what makes austempering the right choice. \[ \boxed{\text{Austempering}} \]
Four heat treatments are on offer, and each one has one signature product: normalizing gives fine pearlite, annealing gives coarse pearlite, martempering gives martensite, austempering gives bainite.
Since the question asks for ONLY bainite, we just need to match the product to the process.
Step 2: Think about the cooling path on a TTT diagram:
To get bainite, the cooling curve has to dodge the "nose" of the C-curve fast enough to avoid pearlite, then flatten out (isothermal hold) somewhere in the middle temperature band where bainite forms.
Normalizing and annealing both cool continuously without a hold, so they land in the pearlite region, not bainite.
Martempering also uses an isothermal hold, but that hold sits close to $M_s$ (martensite start), so the steel forms martensite once air cooling resumes, not bainite.
Step 3: Locate austempering on the diagram:
Austempering's hold sits lower than the pearlite nose but above $M_s$, right inside the bainite bay of the TTT diagram, and the hold continues until the transformation is complete.
Because the steel never drops below $M_s$ while austenite still remains, no martensite forms at all, the whole structure converts to bainite.
\[ M_s < T_{hold} < T_{nose}, \quad \text{held until transformation is 100 percent complete} \]
Final Answer:
The isothermal hold placed inside the bainite bay is what makes austempering the right choice. \[ \boxed{\text{Austempering}} \]
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