The American Iron and Steel Institute (AISI) defines stainless steel as steel that contains 10% or more chromium alone or with other alloying elements. Chromium increases hardness, strength, and corrosion resistance. Nickel gives similar benefits but adds hardness without sacrificing ductility and toughness. It also reduces thermal expansion for better dimensional stability. The earliest record of corrosion-resistant steel was the Iron Pillar of Delhi, India, circa 400 AD. It is high in phosphorus, which worked with weather conditions to create a protective passivation layer of iron oxides and phosphates. French metallurgist Pierre Berthier engineered the first documented corrosion-resistant material in 1821 for a cutlery application. Between 1904 and 1911, French researcher Leon Guillet developed alloys that today would be classified as stainless steel. In 1911, German Philip Monnartz documented the connection between chromium content and corrosion resistance. By 1913, Harry Brearly, a metallurgist in Sheffield, England, working on a corrosion-resistant alloy for gun barrels, was dubbed by some the inventor of stainless steel. The alloy that resulted from his work is today called martensitic stainless steel. At the same time at Krupp Iron Works in Germany, Eduard Maurer and Benno Strauss created an austenitic alloy. In the United States, Christian Dantsizen and Frederick Becket invented ferritic stainless steel. Stainless steels’ main categories are austenitic (200 and 300 Series), ferritic (400 Series), martensitic (400 and 500 Series), precipitation-hardening (PH), and duplex alloys. Austenitic stainless steels have the best corrosion resistance of all stainless steels because they contain at least 16% chromium. Added nickel and manganese hold the metal in an austenitic microstructure. AISI 304 stainless is a common alloy containing 18% chromium and 8% nickel. These alloys are usually characterized as ductile, weldable, and hardenable by cold forming. Ferritic stainless steels have 10.5 to 27% chromium and no significant nickel content, lowering their corrosion resistance. They are considered best for high-temperature instead of high-strength applications. Martensitic alloys contain 12 to 14% chromium, 0.2 to 1% molybdenum, and no significant amount of nickel. They have lower corrosion resistance than austenitic or ferritic alloys, but are considered hard, strong, slightly brittle, and hardenable by heat treatment. A common aircraft-grade martensitic stainless is AISI 440C, which contains 16 to 18% chromium and 0.95 to 1.2% carbon. PH stainless steels contain around 17% chromium and 4% nickel. This makes them as corrosion resistant as austenitic grades. Unlike austenitic alloys, however, heat treatment strengthens PH steels to levels higher than martensitic alloys. Duplex stainless steels, as their name indicates, are a combination of two of the main alloy types. The alloys’ mixture of 19 to 28% chromium, 0 to 5% molybdenum, and 5 to 7% nickel results in a mixed austenitic and ferritic microstructure. They are stronger than either austenitic and ferritic alloys and have better localized corrosion resistance. | 
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