Air hardening is a type of heat treatment process used in metallurgy to improve the strength and hardness of certain types of metals. It involves heating the metal to a high temperature and then allowing it to cool slowly in still air. The cooling rate is slower than that of quenching, but faster than in normalizing. During the cooling process, the metal undergoes a phase change, which results in a hardened structure with improved mechanical properties. This process is often used in the production of high-strength steel alloys, as well as some types of tool steels and other metal products that require increased hardness and durability.