Which heat-treating operation would be performed when the surface of the metal is changed chemically by introducing a high carbide or nitride content?

• A. Case hardening
• B. Tempering
• C. Normalizing
• D. Annealing

Answer: (A)

Surface hardening by diffusion of alloying elements into the surface is being described. When the surface is enriched with carbon or nitrogen, hard compounds known as carbides or nitrides form in the outer layer, creating a very hard, wear-resistant case while the interior remains tougher and more ductile. This is achieved through case hardening methods such as carburizing or nitriding, which deliberately introduce carbon or nitrogen at elevated temperatures to create that hard surface layer with a gradual hardness transition into the core.

Other heat treatments change properties without adding new elements to the surface: tempering relieves brittleness after hardening, normalizing refines grain through air cooling after heating to a high temperature, and annealing softens and relieves stresses by heating and slow cooling. Hence, the described operation is case hardening.