Which factor determines the grain size in metals?

• A. Ductility
• B. Heat treatment
• C. Hardness
• D. Color

Answer: (B)

Grain size in metals is determined by how the metal is heat treated. The heat treatment history sets how many grains nucleate and how far they can grow. At high temperatures, atoms move more easily, allowing grain boundaries to migrate and grains to become larger if the metal is held long enough. Conversely, rapid cooling or cycles that promote many nucleation events can produce finer grains. So the temperature, time at temperature, and cooling rate chosen in heat treatment are what establish the final grain size. Ductility, hardness, and color are properties that result from the microstructure, including grain size, but they don’t fix the grain size themselves.