What is meant by case hardening?

• A. The interior is hardened; the surface remains soft.
• B. The surface is polished and hardened.
• C. The surface of the metal is hardened by diffusion of carbon or aluminum nitride. The interior remains strong and tough.
• D. Only the core is hardened; surface unaffected.

Answer: (C)

Case hardening means making a hard outer surface by diffusing alloying elements into the surface while keeping the interior relatively soft and tough. In steels, this is usually done by diffusing carbon (carburizing) or nitrogen (nitriding) into the surface, forming a hard, wear-resistant case, with the core remaining strong and able to absorb shocks. The statement reflects that idea—the surface is hardened by diffusion of carbon or nitrogen (the wording sometimes adds aluminum nitride as a diffusion element, but the key point is diffusion into the surface), and the interior stays strong and tough. The other ideas describe either hardening the core, finishing the surface without diffusion, or leaving the surface unaffected, which do not describe case hardening.