Age hardening, also called precipitation hardening, is a secondary heat treatment technique used to increase the strength of some alloys. The first step of aging is to fully anneal the material, then a second lower temperature heat treatment is performed to age harden the material. The aging process is similar to how salt or sugar will dissolve in hot water but when the solution is cooled down the salt or sugar particles come out of solution (i.e. precipitate) and will end up at the bottom of the glass. The age hardening process works in the reverse manner.
After the annealing operation, all of the particles are in solution in the material and the material is in a low strength condition with low hardness. When the material is heated to a predefined temperature and held for an extended period of time, the particles that are in solution in the material will begin to come of out of solution (i.e. precipitate) and group together. These groups of particles act to strengthen the material and increase the hardness.
Typical applications of Age Hardening – Aluminum, Magnesium, Nickel, Titanium, some stainless steels and some superalloys