Precipitation hardening stainless steels are a family of corrosion resistant alloys some of which can be heat treated to provide tensile strengths some three or four times that of an austenitic stainless steel such as grade 304 or grade 316. Therefore, precipitation hardening steels are characterized by aging heat treatment which leads to a precipitation of intermetallic compounds in the crystalline lattice of the steel which  improve  the mechanical characteristics, in particular toughness.

Hardening is achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum.

Precipitation hardening stainless steels are characterised into one of three groups based on their final microstructures after heat treatment. The three types are:  martensitic (e.g. 17-4 PH), semi-austenitic (e.g. 17-7 PH) and austenitic (e.g. X5NiCrTiMoVB25-15-2).