What are two methods of case hardening?

• A. Carburizing.
• B. Nitriding.
• C. Quenching and tempering.
• D. Carburizing and nitriding.

Answer: (D)

Case hardening aims to create a hard surface while keeping the interior tougher. Two classic methods are carburizing and nitriding. Carburizing introduces carbon into the surface by exposing steel to a carbon-rich environment at high temperature, allowing carbon to diffuse into the outer layers. After diffusion, the part is treated (often quenched and sometimes tempered) to form a hard martensitic surface while the core remains softer. Nitriding introduces nitrogen into the surface at relatively low temperatures, typically using ammonia or a plasma, creating hard nitrides that yield a very wear-resistant surface with minimal distortion and little effect on the core properties. Quenching and tempering, on the other hand, hardens the entire part rather than just the surface, so it isn’t considered case hardening. Thus, carburizing and nitriding are the two methods used for case hardening.