Question

Martensite in a mild steel is obtained by

• A. Annealing
• B. Normalising
• C. Hardening
• D. Martensite can not be formed

Hint:

Hardenability is not the same as hardness. Hardenability is the *ability to form martensite* (and thus harden). Low carbon steel can't be hardened significantly by quenching because its hardenability is too low. High carbon steel has higher hardenability, and alloy steels have the highest hardenability.

Answer 1

The Correct Option is D

Step 1: Recall what it takes to form martensite.
To get martensite you must cool austenite faster than the steel's critical cooling rate, fast enough that the TTT curve's "nose" is missed entirely and no pearlite or ferrite has time to form before the temperature drops below $M_s$.
Step 2: See what low carbon does to that nose.
Mild steel has very little carbon, generally under 0.25%. Low carbon content pushes the TTT diagram's nose very far to the left, meaning the ferrite and pearlite reactions start in a fraction of a second at almost any cooling condition.
Step 3: Compare with what quenching can realistically achieve.
Even the most severe quench available, such as iced brine, cannot pull heat out of a real component fast enough to beat that razor thin time window, especially away from the immediate surface. So diffusional ferrite and pearlite form before martensite ever gets a chance.
Step 4: Conclusion.
Because the required critical cooling rate for mild steel is higher than anything practically achievable, hardening, annealing, or normalizing mild steel will never yield a fully martensitic structure; for all practical engineering purposes martensite simply cannot form in it.
\[ \boxed{\text{Martensite can not be formed}} \]