Editing Heat treating (section)

===Aging===
{{main|Precipitation hardening}}
Some metals are classified as ''precipitation hardening metals''. When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of the solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age " naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, maybe easier with a softer part.

Examples of precipitation hardening alloys include 2000 series, 6000 series, and 7000 series [[aluminium alloy]], as well as some superalloys and some [[stainless steel]]s. Steels that harden by aging are typically referred to as [[maraging steel]]s, from a combination of the term "martensite aging".<ref name="Dossett, 2006, 2-6" />