What hardening treatment involves heating cast metal to a high temperature and then rapidly cooling it?

The correct answer is quenching, as this hardening treatment specifically involves heating cast metal to a high temperature and then rapidly cooling it, typically using water, oil, or air. This rapid cooling transforms the structure of the metal, increasing its hardness and strength.

During the process of quenching, the elevated temperature allows the metal to reach a phase known as austenite, which is a high-temperature allotrope of iron. When the material is rapidly cooled, a hard structure called martensite is formed. The resulting material exhibits improved mechanical properties, making it suitable for applications that require high strength and wear resistance.

The other treatments listed, such as annealing, normalizing, and tempering, serve different purposes in the metalworking process. Annealing involves heating and then slowly cooling to relieve internal stresses and improve ductility, while normalizing also involves heating and air cooling to achieve a uniform grain structure without the specific aim of increasing hardness as through quenching. Tempering, on the other hand, is a post-quenching process where the metal is reheated to reduce brittleness and improve toughness, not the initial stage of hardening itself. Thus, quenching is uniquely defined by its combination of heating and rapid cooling, leading to the