Stainless steel is essentially a low carbon steel which contains chromium at 10% or more by weight. It is this addition of chromium that gives the steel its unique stainless, corrosion resisting properties. The chromium content of the steel allows the formation of a rough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen. There are more than 60 grades of stainless steel. However, the entire group can be divided into five classes. Each is identified by the alloying elements which affect their microstructure and for which each is named.
CATEGORIES OF STAINLESS STEEL
Austenitic - A family of alloys containing chromium and nickel (and manganese and nitrogen when nickel levels are reduced), generally built around the type 302 chemistry of 18% Cr, 8% Ni, and balance mostly Fe. These alloys are not hardenable by heat treatment.
Ferritic - This group of alloys generally containing only chromium, with the balance mostly Fe, are based upon the type 430 composition of 17% Cr. These alloys are somewhat less ductile than the austenitic types and again are not hardenable by heat treatment.
Martensitic - The members of this family of stainless steels may be hardened and tempered just like alloy steels. Their basic building block is type 410 which consists of 12% Cr, 0.12% C, and balance mostly Fe.
IV. Precipitation-Hardening - These alloys generally contain Cr and less than 8% Ni, with other elements in small amounts. As the name implies, they are hardenable by heat treatment.
Duplex - This is a stainless steel alloy group, or family, with two distinct microstructure phases -- ferrite and austenite. The Duplex alloys have greater resistance to chloride stress corrosion cracking and higher strength than the other austenitic or ferritic grades.
Cast - The cast stainless steels, in general, are similar to the equivalent wrought alloys. Most of the cast alloys are direct derivatives of one of the wrought grades, as C-8 is the cast equivalent of wrought type 304. The C preceding a designation means that the alloy is primarily used for resistance to liquid corrosion. An H designation indicates high temperature applications.
BENEFITS OF STAINLESS STEEL
Corrosion Resistance - Lower alloyed grades resist corrosion in atmospheric and pure water environment, while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, properties which are utilized in process plants.
Fire and Heat Resistance- Special high chromium and nickel-alloyed grades resist scaling and retain strength at high temperatures.
Hygiene - The easy cleaning ability of stainless steel make it the first choice for strict hygiene conditions, such as hospitals, kitchens, abattoirs, and other food processing plants.
Aesthetic Appearance - The bright, easily maintained surface of stainless steel provides a modern and attractive appearance.
Strength-to-Weight Advantage - The work-hardening property of austenitic grades, that results in a significant strengthening of material from cold-working alone, and the high strength duplex grades, allow reduced material thickness over conventional grades, therefore cost savings.
Ease of Fabrication - Modern steel-making techniques mean that stainless can be cut, welded, formed, machined, and fabricated as readily as traditional steels.
Impact Resistance - The austenitic microstructure of the 300 series provides high toughness, from elevated temperatures to far below freezing, making these steels particularly suited to cryogenic applications.
Long Term Value - When the total life cycle costs are considered, stainless is often the least expensive material option.
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