During which metal processing method is rapid cooling employed?

Rapid cooling is primarily associated with the quenching process in metal processing. Quenching involves the abrupt cooling of metals, typically by immersing them in water, oil, or other cooling mediums right after they've been heated to a specific temperature, often during processes like hardening. This rapid decrease in temperature can significantly alter the metal's microstructure, enhancing its hardness and strength.

In the context of metal processing, tempering, aging, and annealing are other treatment processes that involve heating and controlled cooling but do not focus on the rapid cooling aspect. Tempering, for instance, is done after quenching to relieve stresses and improve toughness, typically at a lower temperature. Aging is related to the heat treatment of certain alloys to improve strength, while annealing is a slow cooling process used to relieve internal stresses and improve ductility. These processes do not utilize the rapid cooling characteristic of quenching, which is fundamental in achieving the desired properties in metals resulting from that sudden temperature drop.