Which process specifically uses ammonia to introduce nitrogen into the surface to increase hardness?

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

Answer: (A)

Nitrogen diffusion from ammonia gas is used in nitriding. In gas nitriding, ammonia decomposes at elevated temperatures and releases nitrogen, which diffuses into the steel surface to form hard nitride compounds. This creates a very hard, wear-resistant outer layer without requiring high overall heat treatment of the bulk metal. The other options don’t rely on nitrogen: case hardening introduces carbon into the surface to harden it, annealing softens and relieves stresses, and normalising refines grain structure through air cooling. So the ammonia-based nitrogen diffusion described is characteristic of nitriding.