Which heat-treating operation creates a hard surface by introducing carbide or nitride into the surface layer?

Which heat-treating operation creates a hard surface by introducing carbide or nitride into the surface layer?

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

Answer: (A)

Hardening the surface by diffusion of carbide- or nitride-forming elements into the outer layer matches case hardening. In this process, steel is exposed to a carbon-rich or nitrogen-rich environment at high temperature, allowing carbon or nitrogen to diffuse into the surface and form hard carbides or nitrides. The resulting surface layer becomes very hard and wear-resistant, while the interior remains comparatively tougher, preserving overall toughness after quenching and sometimes tempering. Tempering softens a previously hardened material to reduce brittleness, not to create a surface diffusion layer. Normalizing refines the grain structure and yields uniform properties without forming a surface carbide or nitride diffusion layer. Annealing softens and ductilizes steel, also without generating the diffusion-based hard surface characteristic described here.

Hardening the surface by diffusion of carbide- or nitride-forming elements into the outer layer matches case hardening. In this process, steel is exposed to a carbon-rich or nitrogen-rich environment at high temperature, allowing carbon or nitrogen to diffuse into the surface and form hard carbides or nitrides. The resulting surface layer becomes very hard and wear-resistant, while the interior remains comparatively tougher, preserving overall toughness after quenching and sometimes tempering.

Tempering softens a previously hardened material to reduce brittleness, not to create a surface diffusion layer. Normalizing refines the grain structure and yields uniform properties without forming a surface carbide or nitride diffusion layer. Annealing softens and ductilizes steel, also without generating the diffusion-based hard surface characteristic described here.