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: Understanding the Concept:
The question asks to identify the strongest hardener of ferrite among the given options. Hardening of ferrite, in this context, refers to solid solution strengthening. Ferrite is the body-centered cubic (BCC) crystal structure of iron. Solid solution strengthening is a mechanism where an alloying element (solute) is added to a metal (solvent), causing lattice distortions that impede the movement of dislocations, thereby increasing the strength and hardness of the material.

Step 3: Detailed Explanation:
The effectiveness of an element in solid solution strengthening depends largely on the difference in atomic size between the solute and solvent atoms. A greater size difference leads to more significant lattice strain, which more effectively hinders dislocation motion.
Let's analyze the options:

• Chromium (Cr), Molybdenum (Mo), and Nickel (Ni) are common alloying elements in steel. They do contribute to solid solution strengthening in ferrite, but their primary roles are often different (e.g., Cr for corrosion resistance and hardenability, Mo for high-temperature strength, Ni for toughness and being an austenite stabilizer).

• Silicon (Si) is a substitutional solute in iron. The atomic radius of silicon is significantly smaller than that of iron. This large size mismatch creates substantial localized lattice distortion in the ferrite matrix. These distortions act as strong barriers to dislocation movement, making silicon a very potent and effective solid solution hardener for ferrite.
Comparing the strengthening effect per weight percent of the alloying element, silicon has one of the most significant impacts on the hardness of ferrite among common, non-interstitial alloying elements.

Step 4: Final Answer:
Based on the principle of solid solution strengthening and the atomic size difference with iron, silicon is the strongest hardener of ferrite among the choices provided.