Which process diffuses carbon into the surface to harden it?

• A. Carburizing.
• B. Nitriding.
• C. Tempering.
• D. Annealing.

Answer: (A)

Carburizing diffuses carbon into the surface by heating steel in a carbon-rich environment at high temperature, allowing carbon atoms to move into the outer layers. After this diffusion, the piece is usually quenched to form a hard surface layer (often martensitic) while the core remains tougher and more ductile. This combination gives good wear resistance and fatigue life for components like gears and shafts.

Nitriding also hardens the surface but does so by diffusing nitrogen, forming hard nitrides, and it typically occurs at lower temperatures. Tempering relieves stresses and reduces brittleness after hardening, not by adding carbon to the surface. Annealing softens and increases ductility, also without diffusing carbon into the surface.