Which heat-treatment process hardens the surface of a metal while leaving the inside softer and tougher?

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

Answer: (A)

Surface hardening focuses on creating a hard outer layer while keeping the inside softer and more tough. This gives the part excellent wear resistance at the surface without making the whole component brittle. Case hardening achieves this by diffusing alloying elements, usually carbon, into the surface at elevated temperature. The surface becomes enriched and, after quenching, forms a hard martensitic layer. The interior remains largely unchanged, so its microstructure stays softer and tougher, able to absorb shocks. In contrast, annealing softens the entire piece, and normalising refines the grain but doesn’t produce a distinct hard surface layer. Nitriding can also produce a hard surface, but case hardening is the classic method described when the goal is a hardened surface with a softer, tougher interior.