ANS:A - Cobalt: It contributes to rerl hardness by hardening ferrite.

Cobalt is often used as an alloying element in steel, primarily in the production of high-speed steels and certain tool steels. The statement "Cobalt: It contributes to real hardness by hardening ferrite" suggests that cobalt contributes to the hardness of steel by hardening ferrite, a phase in the steel microstructure. Explanation:

1. Effect of Cobalt:
   • Cobalt, when added to steel, forms a solid solution with iron, thereby affecting the microstructure and properties of the steel.
   • Cobalt has a strong affinity for carbon, and it stabilizes carbides, leading to an increase in hardness.
   • Cobalt also promotes the formation of hard carbides such as M23C6, which are dispersed within the steel matrix.
   • These carbides contribute significantly to the hardness and wear resistance of the steel.
2. Contribution to Hardness:
   • By stabilizing carbides and forming hard phases within the steel matrix, cobalt enhances the hardness of the steel.
   • The presence of cobalt can increase the hardness of ferrite, which is one of the primary phases in steel microstructures.
   • Hardened ferrite, along with dispersed carbides, contributes to the overall hardness and wear resistance of the steel.
3. Applications:
   • Cobalt-containing steels are widely used in applications where high wear resistance, hardness, and hot hardness are required.
   • Examples include cutting tools, drills, taps, and other tools subjected to high temperatures and abrasive wear.

In summary, cobalt contributes to the hardness of steel by stabilizing carbides and promoting the formation of hard phases such as M23C6 carbides. This enhances the wear resistance and hardness of the steel, making it suitable for various high-performance applications. Therefore, the statement "Cobalt: It contributes to real hardness by hardening ferrite" is correct.