15-5 stainless steel is a martensitic, precipitation-hardening material with chromium, nickel and copper. It is often a first choice in the aerospace industry for fasteners and structural components. Its unique structure provides increased toughness and better corrosion resistance than its predecessor, 17-4 PH. Both inclusion control and a minimized amount of delta ferrite as compared to 17-4 stainless steel contribute to the greater toughness of 15-5. The alloy is further strengthened by a low temperature heat treatment which precipitates a copper containing phase in the alloy. 15-5 PH is able to meet the stringent mechanical properties required in the aerospace and nuclear industries.