How is steel nitrided?

• A. The steel part is heated in retort in which there is an atmosphere of ammonia; Aluminum combines with the nitrogen to form aluminum nitride on the surface.
• B. The part is quenched in water after heating.
• C. Surface is coated with zinc nitride.
• D. The surface is carburized with carbon.

Answer: (A)

Nitriding is a surface hardening process where nitrogen is introduced into the steel surface from a nitrogen-rich environment. The part is heated in a retort with ammonia gas; ammonia decomposes at the surface, releasing nitrogen that diffuses into the steel to form nitrides, creating a hard, wear-resistant layer. If aluminum is present, it can react with the nitrogen to form aluminum nitride on the surface, which can enhance the protective nitride layer. This is different from quenching in water after heating (which is about rapid cooling, not nitrogen diffusion), coating the surface with zinc nitride (not a standard nitriding practice), or carburizing (which introduces carbon instead of nitrogen).