STAINLESS STEELS are iron-base alloys that contain a minimum of about 12% Cr, the amount needed to prevent the formation of rust in unpolluted atmospheres (hence the designation stainless). Few stainless steels contain more than 30% Cr or less than 50% iron. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide film. This oxide forms and heals itself in the presence of oxygen. Other elements added to improve particular characteristics include nickel, manganese, molybdenum, copper, titanium, silicon, niobium, aluminum, sulfur, and selenium. Carbon is normally present in amounts ranging from less than 0.03% to over 1.0% in certain grades. Figure 1 provides a useful summary of some of the compositional and property linkages in the stainless steel family.
- Production of Stainless Steels
With specific restrictions in certain types, the stainless steels can be shaped and fabricated in conventional ways. They are produced in cast, powder metallurgy (P/M), and wrought forms. Available wrought product forms include plate, sheet, strip, foil, bar, wire, semifinished products (blooms, billets, and slabs), and pipe and tubing. Coldrolled flat products (sheet, strip, and plate) account for more than 60% of stainless steel product forms. Figure 2 illustrates the most commonly employed mill processes for making various wrought stainless steel products. Production of stainless steels is a two-stage process involving the melting of scrap and ferroalloys in an electric-arc furnace(EAF) followed by refining by argon oxygen decarburization (AOD) to adjust the carbon content and remove impurities. Alternative, melting and refining steps include vacuum induction melting, vacuum arc remelting, electroslag remelting, and electron beam melting. Melting and refining of stainless steels is, however, most frequently accomplished by the EAF/AOD processing route. In fact, about 90% of all stainless steel produced in the United States is processed by EAF melting followed by AOD. During the final stages of producing basic mill forms—sheet, strip, plate and bar—and bringing these forms to specific size and tolerances, the materials are subjected to hot reduction with or without subsequent cold rolling operations, annealing, and cleaning. Further steps are required to produce other mill forms, such as wire and tube.
- Applications for Stainless Steels
Stainless steels are used in a wide variety of applications. Most of the structural applications occur in the chemical and power engineering industries, which account for more than a third of the market for stainless steel products (see the following table). These applications include an extremely diversified range of uses, including nuclear reactor vessels, heat exchangers, oil industry tubulars, components for chemical processing and pulp and paper industries, furnace parts, and boilers used in fossil fuel electric power plants.
- Shape of Stainless Steels
Tubular stainless steel: tubes, pipes
Fittings: elbows, tees, unions, couplings, reducers
Flanges, forgings, etc