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Austenitic stainless steels containing chromium and nickel are identified as 300 Series types. Alloys containing chromium, nickel and manganese are identified as 200 Series types. The stainless steels in the austenitic group have different compositions and properties, but many common characteristics. They can be hardened by cold working, but not by heat treatment. in the annealed condition, all are essentially nonmagnetic, although some may become slightly magnetic by cold working. They have excellent corrosion resistance, unusually good formability, and increase in strength as a result of cold work.
Type 304 (frequently referred to as 18-8 stainless) is the most widely used alloy of the austenitic group. it has a nominal composition of 18% chromium and 8% nickel.
Ferritic stainless steels are straight-chromium 400 Series types that cannot be hardened by heat treatment, and only moderately hardened by cold working. they are magnetic, have good ductility and resistance to corrosion and oxidation. Type 430 is the general-purpose stainless of the ferritic group.
Martensitic stainless steels are straight-chromium 400 Series types that are hardenable by heat treatment. They are magnetic. They resist corrosion in mild environments. They have fairly good ductility, and some can be heat treated to tensile strengths exceeding 200,000 psi (1379 MPa).
Type 410 is the general purpose alloy of the martensitic group.
Precipitation-hardening stainless steels are chromium-nickel types. some containing other alloying elements, such as copper or aluminum. They can be hardened by solution treating and aging to high strength.
Duplex stainless steels have an annealed structure which is typically about equal parts of austenite and ferrite. Although not formally defined, it is generally accepted that the lesser phase will be at least 30% by volume.
Duplex stainless steels offer several advantages over the common austenitic stainless steels. The duplex grades are highly resistant to chloride stress corrosion cracking, have excellent pitting and crevice corrosion resistance and exhibit about twice the yield strength as conventional grades. Type 329 and 2205 are typical alloys.
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