Which process describes heating metal to a high temperature, introducing carburising powder, soaking, reheating, and quenching to harden the surface?

• A. Case hardening
• B. Annealing
• C. Normalising
• D. Nitriding

Answer: (A)

This process is about hardening the surface by adding carbon to it. Heating the metal in a carbon-rich environment allows carbon atoms to diffuse into the outer layers, creating a high-carbon surface layer called the case. After soaking, the piece is heated again and then quenched so that the carbon-rich surface transforms to a hard, martensitic structure, while the inner core stays relatively soft and tough because its carbon content remains low. The result is a hard-wearing surface with a resilient core, ideal for parts like gears or shafts that need both hardness and toughness. Other processes don’t fit this description: annealing softens and relieves stress; normalising refines grain with air cooling; nitriding adds nitrogen instead of carbon to form a hardened surface.