Which cooling method forms small grains in metals?

Rapid cooling, often referred to as quenching in metallurgical processes, is the method that leads to the formation of small grains in metals. This occurs because when metals are cooled quickly from a molten or high-temperature state, they do not have sufficient time to form larger crystalline structures. Instead, the atoms are forced into a disordered arrangement, resulting in a finer grain structure.

A fine grain structure can enhance the mechanical properties of metals, such as strength and toughness, due to a phenomenon known as the Hall-Petch relationship, which states that smaller grains can impede dislocation movement, making the material stronger. Rapid cooling is often achieved through processes like immersion in water or oil after heating.

Other cooling methods, such as slow cooling and moderate cooling, allow more time for larger grains to form, while heating is not associated directly with cooling processes but rather with initial material preparation.