How can the hardness of certain metals be increased?

Quenching significantly increases the hardness of certain metals, particularly steels. This process involves heating the metal to a high temperature and then rapidly cooling it, typically using water or oil. The rapid cooling transforms the metal's crystalline structure, enhancing its hardness and strength due to the formation of martensite, a very hard microstructure.

In contrast to quenching, heating alone does not necessarily increase hardness and can sometimes reduce it if not followed by appropriate cooling or other processes. Alloying can also improve hardness, but this process involves mixing different metals to create a new material with enhanced properties, rather than a direct change in the existing metal's microstructure through heat treatment. Polishing primarily focuses on aesthetics and surface finish rather than hardness. Hence, the correct choice is quenching, as it directly affects the internal structure of specific metals to increase their hardness effectively.