Stainless steels are classified according to their metallurgical structure: austenitic, ferritic, martensitic, precipitation hardening, and duplex. Austenitic stainless steels contain chromium and nickel. They are hardened only through cold working and are non-magnetic although some may become magnetic by cold working. Austenitic stainless steels are identified as T300 series stainless steels (typical alloys T304, T316). Alloys containing chromium, nickel, and manganese are identified as T200 series (T201, T202). Ferritic stainless steels are straight chromium. They are magnetic, have good ductility, and resist corrosion and oxidation. Ferritic stainless steels cannot be hardened by heat treatment and are identified as T400 series (T409, T430). Martensitic stainless steels are also straight chromium but can be hardened by heat treatment. They are magnetic and resist corrosion in mild environments. They have fairly good ductility and some can be heat treated to tensile strengths in excess of 200,000 psi. Martensitic stainless steels are identified as T400 series (T410, T416). Precipitation-hardening stainless steels are chromium-nickel types identified as alloys T17-4 or T17-7 PH. They can be hardened by solution treating and aging. Duplex stainless steels have an annealed structure which is typically equal parts of austenite and ferrite. The duplex grades are highly resistant to corrosion and pitting. They have about twice the yield strength of conventional grades. T2205 is the standard alloy for duplex grade.