ATI 15-5™ precipitation hardening stainless steel (S15500) is a variant of the older ATI 17-4™ (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.

The ATI 15-5™ alloy was designed to have greater toughness than S17400, 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 ATI 15-5™ alloy is carefully controlled to minimize its content of delta ferrite, which is present in the S17400 material. Inclusion control is done by consumable electrode remelting using the electro-slag remelting (ESR) process.

The S15500 alloy 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 the S17400 alloy, the S15500 alloy 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. Heat treatment in the 900°F (482°C) range produces the highest strength, although it is slightly less than those of semi-austenitic alloys like S17700 or S15700. The latter precipitation hardening stainless steels generally require more steps to complete heat treatment. The ATI 15-5™ alloy is generally better suited for plate applications than are the semiaustenitic alloys.