Stainless steel is a generic name commonly used for that entire group of iron-based metal containing at least 10% chromium as an alloy metal. The name stainless was derived from the fact that the material does not stain, rust or corrode easily.
Over 150 stainless steel grades are produced commercially with about fifteen stainless steel grades being the most common. Alloy elements, notably nickel (Ni) and molybdenum (Mb) are sometimes added to the basic stainless composition to produce both variety and an improvement in properties.
There are five main classes of stainless steel:
Austenitic class stainless steel, a chromium-nickel-iron alloy, sequenced primarily, but not limited to, the 100, 200 and 300 numeric series, which comprises over 70% of all current stainless steel production. The 300 stainless steel grades, including Type 304, the most common and Type 316 the second most common and are seemly the most widely used. Other stainless steel grades in this stainless steel class are Types 301, 302, 302B, 303, 304, 304L, 304N, 305, 308, 309, 310, 316, 316L, 316F, 316N, 317, 317L, 321, 330, 347, 348, 384, AL-6XNTM, 904LTM, XM-19, nitronic 40, nitronic 50 and nitronic 60.
Martensitic class stainless steel, a chromium-iron alloy containing 10.5 to 17% chromium and controlled amounts of carbon. This type is extremely strong and tough, but not as corrosion resistant as the other two classes. Highly machineable and hardenable by heat treatment this stainless steel class contains Types 410, 416, 420, 420F, 431, 440A, 440B and 440C.
Ferritic class stainless steel, a chromium-iron alloy containing 17 to 27% chromium, a somewhat low carbon content and tends to be very corrosion resistant. Types 405, 409, 429, 430, 430F, and 446 are grouped in this stainless steel class.
Duplex stainless steel is approximately 50% ferritic and 50% austenitic resulting in a higher strength than either ferritic or austenitic steels. They are resistant to stress, corrosion, cracking and are weldable, but need care in the selection of welding consumables and heat input. Duplex stainless steels types 326, 329, 2304, 2205 and 2507 commonly available have moderate formability, are magnetic, but not as much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
Precipitation hardening (PH) stainless steels develop very high strength by adding elements such as Copper, Niobium and Aluminum to the steel. During heat treatment, very fine particles form in the matrix of the steel imparting additional strength. These stainless steels, types 13-8, 15-5, 15-7, 17-4 and 17-7 can be machined to quite intricate shapes and close tolerances before the final heat treating as there is minimal distortion occurring.
