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

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. Tempering
• B. Normalizing
• C. Case hardening
• D. Quenching

Answer: (D)

This question is about diffusion-based surface hardening. The process described changes the surface chemistry by introducing a high carbide or nitride content, typically through carburizing (carbon diffusion) or nitriding (nitrogen diffusion). The surface forms hard carbides or nitrides, giving a wear-resistant outer layer while the core remains tougher and more ductile. Quenching, by contrast, is rapid cooling to alter the whole piece’s microstructure; tempering relieves stresses after hardening; normalizing refines the grain by heating and air cooling. Therefore, the operation described is case hardening.

This question is about diffusion-based surface hardening. The process described changes the surface chemistry by introducing a high carbide or nitride content, typically through carburizing (carbon diffusion) or nitriding (nitrogen diffusion). The surface forms hard carbides or nitrides, giving a wear-resistant outer layer while the core remains tougher and more ductile. Quenching, by contrast, is rapid cooling to alter the whole piece’s microstructure; tempering relieves stresses after hardening; normalizing refines the grain by heating and air cooling. Therefore, the operation described is case hardening.