Question

Strong hardener of ferrite is

• A. Chromium
• B. Molybdenum
• C. Nickel
• D. Silicon

Hint:

Remember that solid solution strengthening is maximized by a large difference in atomic radii between the solute and solvent atoms. For ferrite (iron), elements like Silicon and Phosphorus are very effective hardeners due to this principle.

Answer 1

The Correct Option is D

Step 1: Recall the mechanism behind solid solution hardening.
When a foreign atom substitutes into iron's BCC lattice, it locally distorts the surrounding lattice because its size does not perfectly match an iron atom. This local strain field resists the passage of dislocations, and the bigger the size mismatch, the stronger this resistance becomes.
Step 2: Compare atomic sizes with iron.
Chromium, molybdenum, and nickel all have atomic radii fairly close to that of iron, so they distort the ferrite lattice only mildly and are known more for other roles, chromium for corrosion resistance and hardenability, nickel for toughness, molybdenum for high temperature strength and hardenability.
Step 3: Where silicon stands out.
Silicon, by contrast, has a noticeably smaller atomic radius than iron. This larger mismatch creates much greater local lattice strain per atom added, which is exactly why silicon is famous in metallurgy for its strong solid solution strengthening effect on ferrite, it is even used deliberately in electrical and spring steels for this reason.
Step 4: Conclusion.
Comparing strengthening effect per percent addition, silicon produces the largest hardness increase in ferrite among the four elements listed.
\[ \boxed{\text{Silicon}} \]