Which statement best describes case hardening?

Which statement best describes case hardening?

• A. The surface is hardened by diffusion of carbon or aluminum nitride, while the interior remains strong and tough.
• B. Only the interior is hardened.
• C. The surface is hardened by diffusion of carbon or aluminum nitride; the interior remains strong.
• D. The entire piece is uniformly hardened.

Answer: (C)

Case hardening is about hardening only the outer layer by diffusing alloying elements into the surface, typically carbon (carburizing) or nitrogen (nitriding). This creates a hard, wear-resistant surface while the inner material remains relatively softer and tougher to absorb impact. The statement that best matches this describes surface hardening by diffusion of carbon or nitrogen and leaves the interior strong. It’s about a hard exterior with a strong, still-yielding core, not about hardening the whole piece or only the interior.

Case hardening is about hardening only the outer layer by diffusing alloying elements into the surface, typically carbon (carburizing) or nitrogen (nitriding). This creates a hard, wear-resistant surface while the inner material remains relatively softer and tougher to absorb impact. The statement that best matches this describes surface hardening by diffusion of carbon or nitrogen and leaves the interior strong. It’s about a hard exterior with a strong, still-yielding core, not about hardening the whole piece or only the interior.