Precipitation-hardening (PH) stainless steels are chromium-nickel alloys containing precipitation-hardening elements such as copper, aluminum, or titanium. Precipitation-hardening stainless steels may be either austenitic or martensitic in the annealed condition. Those that are austenitic in the annealed condition are frequently transformable to martensite through conditioning heat treatments, sometimes with a subzero treatment. In most cases, these stainless steels attain high strength by precipitation hardening of the martensitic structure.
These steels can develop very high strength through the precipitation-aging heat treatment, which very fine particles form in the matrix of the steel to impart additional strength. High tensile strengths, in excess of 220 ksi (1520 MPa), with good ductility and toughness, are achievable in some grades. Corrosion resistance is comparable to austenitic steels. Common applications include:
Because these alloys are martensitic and precipitation strengthened, they are more difficult to fabricate than other stainless steels and usually require special heat treatments. Care must be taken when welding to follow proper pre- and post-thermal treatment procedures to ensure the desired microstructure and mechanical properties are obtained. Because of their processing requirements and low-volume use, these alloys are generally more expensive to apply than other grades of stainless steels.