15-5PH stainless steel (S15500) is a variant of the older 17-4PH (S17400) chromium-nickel-copper precipitation hardening stainless steel.

Both alloys exhibit high strength and moderate corrosion resistance. High strength is maintained to approximately 600°F (316°C). This alloy is designated Grade or Type XM-12 in several specifications.

15-5PH Stainless Steel was designed to have greater toughness than 17-4PH, especially in the through-thickness (short transverse) direction. This improved toughness is achieved by reduced delta ferrite content and control of inclusion size and shape.

The composition and processing of 15-5PH is carefully controlled to minimize its content of delta ferrite, which is present in 17-4PH material. Inclusion control is done by consumable electrode remelting using the electro-slag remelting (ESR) process.

15-5PH is martensitic in structure in the annealed condition and is further strengthened by a relatively low temperature heat treatment, which precipitates a copper containing phase in the alloy. Like  17-4PH, 15-5PH Stainless requires only a simple heat treatment; a one-step process conducted at a temperature in the range 900°F (482°C) to 1150°F (621°C) depending on the combination of strength and toughness desired. A wide range of properties can be produced by this one-step heat treatment.