Precipitation Hardening (PH) stainless steels are classified as martensitic or semi-austenitic. They develop their high strength and hardness through a variety of heat treatments resulting in a very high strength-to-weight ratio. Semi-austenitic grades are 17-7 PH® and PH 15-7 Mo®. They are austenitic in the annealed state, and martensitic in the hardened condition. Martensitic grades include 17-4 PH® and 15-5 PH®. The PH grades achieve high tensile properties in heat treated conditions.
Applications for PH steels include aerospace components, flat springs, and retaining rings.
Corrosion Resistance
Corrosion resistance of precipitation hardening stainless steels is generally superior to that of the standard hardenable martensitic stainless steels, but is not quite as good as chromium-nickel Type 304. Corrosion resistance of the PH stainless steels depends, to some extent, on the heat treated condition.
Accelerated laboratory corrosion tests have been conducted on the precipitation hardening stainless steels since their development, as well as extensive in-service use in a variety of corrosive conditions.
Formability
The semi-austenitic precipitation hardening stainless steels in Condition A can be readily formed prior to heat treatment. Work hardening and springback in these steels is similar to that of Type 301. Semi-austenitic PH stainless steels in Condition C and the martensitic PH stainless steels are extremely hard and strong and as such are limited to mild forming operations.
Heat Treatment
Standard heat treatments have been developed to achieve a variety of strength levels in the PH stainless steels. Heat treatment for the martensitic PH grades (17-4 PH® and 15-5 PH) are based on one- or two-step aging treatments. The semi-austenitic PH grades require a multi-step heat treatment to achieve their full strength potential.
Weldability
The precipitation hardening class of stainless steels is generally considered to be weldable by common fusion and resistance techniques. Special consideration is required to achieve optimum mechanical properties by considering the best heat-treated conditions in which to weld and which heat treatments should follow welding.
