17-4 stainless steel for 17-4 include components which require high hardness and/or corrosion resistance at temperatures of up to . Specific applications for 17-4 include the petroleum and chemical industries, as well as use in aircraft parts. 17-4 stainless steel SAE type 630 stainless steel (more commonly known as 17-4; also known as UNS 17400) is a grade of martensitic precipitation-hardened stainless steel. It contains approximately 15–17.5% (17) chromium and 3–5% (4) nickel, as well as 3–5% copper. 17-4 can be heat treated to high levels of strength and hardness, and features corrosion resistance and machinability comparable to austenitic 304 stainless. Being martensitic,
Martensitic stainless steel and internal clamps). Untempered martensite is low in toughness and therefore brittle. The characteristic body-centered tetragonal martensite microstructure was first observed by German microscopist Adolf Martens around 1890. In 1912, Elwood Haynes applied for a U.S. patent on a martensitic stainless steel alloy. This patent was not granted until 1919. Martensitic stainless steel can be nondestructively tested using the magnetic particle inspection method, unlike austenitic stainless steel. Also in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was