In case hardening, what happens during the quenching stage?

• A. Metal is cooled in air
• B. Quenched in oil at 200 C
• C. After carburising, the metal is quenched by cooling to 760 C and dipped in cold water
• D. Quenched in liquid nitrogen

Answer: (C)

Quenching after carburising is about rapidly cooling the surface layer that has become rich in carbon to lock in a hard microstructure, while leaving the core comparatively softer. In this step, the part is first cooled to a temperature around the upper part of the transformation range (about 760 C) to control stress and distortion, and then dipped in cold water to achieve a rapid quench that forms a hard martensitic case on the surface. This combination gives a hard, wear-resistant outer layer without making the entire piece brittle or excessively distorted. Cooling in air would be too slow to harden the surface effectively, quenching in oil at a high temperature isn’t standard for this process, and liquid nitrogen provides an impractically extreme and uncontrolled rapid cooling.