The rapid cooling of heated metal for the purpose of hardening the metal is called __________.

• A. Carburizing
• B. Spheroidizing
• C. Annealing
• D. Quenching

Answer: (D)

Quenching is the heat-treatment step in which a heated metal is cooled rapidly to increase hardness. After heating the metal into the appropriate austenitic region, the quick cool-down—often in water, oil, or another quenching medium—prevents carbon diffusion and forces the crystal structure to transform, typically forming martensite in steel. Martensite is very hard, which is why quenching hardens the metal, though it can also make it more brittle if not tempered later.

Carburizing, by contrast, aims to add carbon to the surface at high temperature to create a hard outer layer without rapid cooling. Annealing involves heating and then slowly cooling to soften the metal and relieve stresses. Spheroidizing changes carbide shapes into spheres to improve ductility and machinability, not to rapidly harden the metal.