The commercial alloys also contain a certain amount of carbon, silicon, manganese, sulfur, and so on. Īustenitic stainless steels can be best described in terms of the iron-chromium-nickel ternary alloy system. Further, manganese promotes the solubility of nitrogen in steel (as does chromium), making possible a low-nickel family of austenitic stainless steels that are high in manganese and nitrogen.įigure-1: the iron-chromium-nickel ternary diagram for austenitic stainless steels at 1100☌. In addition, although manganese does not seem to promote transformation of ferrite to austenite at high temperatures, it clearly does tend to stabilize austenite with respect to transformation to martensite at low temperatures. The prototype austenitizing element is nickel, but carbon, nitrogen, and copper all promote transformation of ferrite to austenite at high temperatures. The prototype ferritizing element is chromium, but molybdenum, niobium, titanium, aluminum, tungsten, and vanadium also promote ferrite. Wrought austenitic stainless steel compositions are based on a balance between alloy elements that promote ferrite formation and those that promote austenite formation. Basic Metallurgy & Ternary Phase Diamgram Type 316 is the modification by adding molybdenum for pitting resistance More molybdenum addition forms Type 317 Type 309 and Type 310 are the modifications by adding more chromium and nickel for strength and oxidation resistance Titanium, or niobium are added to Type 321/ Type 316Ti, or Type 347 respectively to stabilize the stainless steel against sensitization More nickel, molybdenum, and nitrogen are added to attain super austenitic stainless steels or Ni-Cr-Fe alloys such as S31254, 904L, N08367, N08800, N08810, N08020, N08811, N08925, and N08926, which have better corrosion resistance even in high-temperature environments. The whole austenitic stainless steel family can be viewed as originating from the standard austenitic stainless steel 304 (18Cr-8Ni or “18-8”). Wrought austenitic stainless steels can be subdivided into several categories: chromium-nickel alloys and chromium-manganese-nitrogen alloys. Moreover, they possess excellent ductility, formability, and toughness. They usually possess excellent cryogenic properties and good high-temperature strength. These steels are essentially nonmagnetic in the annealed condition and can be hardened only by cold working. This structure is attained through the liberal use of austenitizing elements such as nickel, manganese, and nitrogen. Sulfur or selenium may be added to certain grades to improve machinability.Īustenitic stainless steels have a face-centered cubic (fcc) structure.
Molybdenum, copper, silicon, aluminum, titanium, and niobium may be added to confer certain characteristics such as halide pitting resistance or oxidation resistance.
The nickel content is up to about 35%, and manganese content may be up to 15%. The oxide forms and heals itself in the presence of oxygen. They achieve stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film. The fibers impart greater strength to the wrought products in the longitudinal direction of grain flow.Ĭhromium content of wrought austenitic stainless steels generally varies from 16% to 26%. After the wrought process, austenitic stainless steels develop and attain a fiber like grain structure aligned in the principal direction of working. Wrought austenitic stainless steels refer to austenitic stainless steels which have been processed by one or a combination of two or more wrought processes such as forging, rolling, upsetting, bending, extruding, pressing and drawing. Definition, Composition, Characteristics & Classifications
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