What happens to the metallurgy of a metal tool when heat is applied?

When heat is applied to a metal tool, it typically becomes softer. This process, known as annealing, involves heating the metal to a certain temperature and then allowing it to cool. The heat helps to relieve internal stresses, reduce hardness, and improve workability.

During heating, the crystal structure of the metal changes, leading to a reduction in dislocations and defects within the metal's microstructure. As a result, the metal's ability to deform and be shaped increases, making it easier to work with in subsequent processes such as forging or machining.

The other options do not accurately reflect the typical response of metals to heat. While certain metals may become brittle under specific conditions or stresses, heat generally serves to soften them rather than harden them or leave them unchanged. This understanding is crucial in materials science and applied metallurgy, particularly in contexts where heat treatment processes are employed to manipulate the physical properties of metals for various applications.