In metallurgy, stainless steel, also known as inox steel or inox from French inoxydable (inoxidizable), is a steel alloy with a minimum of 10.5% chromium content by mass.
Stainless steel is a family of steels with varying chromium contents ranging as high as 28%. Chromium is the most important alloying element in stainless steels. It is the element that gives the stainless steels their basic corrosion resistance. Due to the presence of chromium, stainless steels do not corrode in contact with water as ordinary steel does. Elevated chromium contents and the addition of other alloying elements, in particular molybdenum, produce grades of stainless steel that are resistant to more corrosive environments, such as seawater and concentrated acids. Thus stainless steels are used where both the strength of steel and corrosion resistance are required.
Stainless steels differ from carbon steel by the amount of chromium present. Uncoated carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is nonprotective and allows corrosion to continue beneath the rust film. Since iron oxide possesses a greater volume than steel, the rust film expands and tends to flake and fall away exposing the underlying steel to the corrosive environment allowing additional corrosion to occur. In comparison, stainless steels contain sufficient chromium to undergo passivation, forming an inert film of chromium oxide on the surface. This corrosion resistant film forms spontaneously in the presence of oxygen, even in the low oxygen content in water. This film prevents corrosion by blocking oxygen diffusion to the steel surface. The film is self-healing and repairs itself if the film is disturbed, preventing corrosion from spreading into the bulk of the metal. The aforementioned 10.5% chromium is the minimum content required to form and maintain this passive film in the environment in which we live our daily lives. Higher chromium contents are required for more corrosive environments.
Stainless steel's resistance to corrosion and staining, low maintenance, and familiar lustre make it an ideal material for many applications. The alloy is milled into coils, sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, household hardware, surgical instruments, major appliances, industrial equipment (for example, in sugar refineries) and as an automotive and aerospace structural alloy and construction material in large buildings. Storage tanks and tankers used to transport orange juice and other food are often made of stainless steel, because of its corrosion resistance. This also influences its use in commercial kitchens and food processing plants, as it can be steam-cleaned and sterilized and does not need paint or other surface finishes.
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Properties
Oxidation
High oxidation resistance in air at ambient temperature is normally achieved with addition of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments. The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, and the metal remains lustrous and smooth. However, the layer does become visible when the temperatures are increased, first as colorful rainbow hues (depending on alloying elements, temperature, time, and atmosphere), later as a rough, dull, mostly brown surface due to the increased size of the metal oxide crystals. The layer is impervious to water and air, protecting the metal beneath, and this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. Corrosion resistance can be adversely affected if the component is used in a non-oxygenated environment, a typical example being underwater keel bolts buried in timber.
When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off, allowing the parts to weld together. When forcibly disassembled, the welded material may be torn and pitted, a destructive effect known as galling. Galling can be avoided by the use of dissimilar materials for the parts forced together, for example bronze and stainless steel, or even different types of stainless steels (martensitic against austenitic). However, two different alloys electrically connected in a humid, even mildly acidic environment may act as a voltaic pile and corrode faster. Nitronic alloys, made by selective alloying with manganese and nitrogen, may have a reduced tendency to gall. Additionally, threaded joints may be lubricated to provide a film between the two parts and prevent galling. Low-temperature carburizing is another option that virtually eliminates galling and allows the use of similar materials without the risk of corrosion and the need for lubrication.
Acids
Stainless steels are generally highly resistant to attack by acids, but this resistance depends on the kind and concentration of the acid, the surrounding temperature, and the grade of stainless steel. Type 904L is resistant to sulfuric acid at room temperature, even in high concentrations; Types 316L and 317L are resistant below 10%; and Type 304L should not be used in the presence of sulfuric acid at any concentration. All types of stainless steel resist attack by phosphoric acid, Types 316L and 317L more so than 304L. Types 304L and 430 have been successfully used with nitric acid. Hydrochloric acid will damage any kind of stainless steel, and should be avoided.
Bases
The 300 series of stainless steel grades is unaffected by any of the weak bases such as ammonium hydroxide, even in high concentrations and at high temperatures. The same grades of stainless exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some etching and cracking.
Organics
Types 316 and 317 are both useful for storing and handling acetic acid, especially in solutions where it is combined with formic acid and when aeration is not present (oxygen helps protect stainless steel under such conditions), though 317 provides the greatest level of resistance to corrosion. Type 304 is also commonly used with formic acid though it will tend to discolor the solution. All grades resist damage from aldehydes and amines, though in the latter case grade 316 is preferable to 304; cellulose acetate will damage 304 unless the temperature is kept low. Fats and fatty acids only affect grade 304 at temperatures above 150 °C (302 °F), and grade 316 above 260 °C (500 °F), while 317 is unaffected at all temperatures. Type 316L is required for processing of urea.
Electricity and magnetism
Like steel, stainless steel is a relatively poor conductor of electricity, with significantly lower electrical conductivity than copper. Other metals in contact with stainless steel, particularly in a damp or acidic environment, may suffer galvanic corrosion even though the stainless metal may be unaffected.
Ferritic and martensitic stainless steels are magnetic. Annealed austenitic stainless steels are non-magnetic. Work hardening can make austenitic stainless steels slightly magnetic.
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History
The corrosion resistance of iron-chromium alloys was first recognized in 1821 by French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. Metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be practical.
In 1872, the Englishmen Clark and Woods patented an alloy that would today be considered a stainless steel.
In the late 1890s Hans Goldschmidt of Germany developed an aluminothermic (thermite) process for producing carbon-free chromium. Between 1904 and 1911 several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.
Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull in Germany in 1908. In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On 17 October 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel as Nirosta.
Similar developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless steel. In 1912, Elwood Haynes applied for a US patent on a martensitic stainless steel alloy, which was not granted until 1919.
In 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times. The metal was later marketed under the "Staybrite" brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929. Brearley applied for a US patent during 1915 only to find that Haynes had already registered a patent. Brearley and Haynes pooled their funding and with a group of investors formed the American Stainless Steel Corporation, with headquarters in Pittsburgh, Pennsylvania.
In the beginning stainless steel was sold in the US under different brand names like "Allegheny metal" and "Nirosta steel". Even within the metallurgy industry the eventual name remained unsettled; in 1921 one trade journal was calling it "unstainable steel". In 1929, before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.
Effect of Alloying on Structure and Properties
Certain alloying elements promote the formation of a ferrite microstructure and are classified as ferrite stabilizers, while others are classified as austenite stabilizers because they promote the formation of an austenite microstructure. This is significant in the identification of the various families of stainless steel and their properties.
Chromium is the most important alloying element in stainless steel production. A minimum of 10.5% chromium is required for the formation of a protective layer of chromium oxide on the steel surface. Corrosion resistance increases with increasing chromium content. It also increases resistance to oxidation at high temperatures. Chromium is a ferrite stabilizer.
Nickel is a strong austenite stabilizer and the main reason for a nickel addition is to create an austenite microstructure. It also reduces the corrosion rate and is thus advantageous in acid environments. Stainless steels with 8% nickel and 18% chromium have a fully austenitic microstructure and exhibit superior welding and working characteristics in comparison to ferritic stainless steels.
Molybdenum increases resistance to both general corrosion (particularly acidic conditions) and localized corrosion (pitting, crevice corrosion, etc). Molybdenum is a ferrite stabilizer, which when used in austenitic alloys, must be balanced with austenite stabilizers in order to maintain an austenitic structure. Molybdenum is added to martensitic stainless steels to improve high temperature strength.
Carbon is a strong austenite stabilizer. It also substantially increases the mechanical strength. Carbon reduces the resistance to intergranular corrosion, if chromium carbides have been formed by exposure of the steel to high temperature, such as in the heat affected zone from welding. In ferritic stainless steels carbon will strongly reduce both toughness and corrosion resistance. In the martensitic stainless steels carbon increases hardness and strength, but is generally accompanied by a decrease in toughness.
Manganese is generally used in stainless steels in order to improve hot ductility. It is an austenite stabilizer and also increases the solubility of nitrogen, which is also an austenite stabilizer. Thus in some stainless steels the combination of manganese and nitrogen is used to replace some of the nickel to produce higher strength nitrogen-bearing austenitic stainless steels.
Nitrogen is a very strong austenite stabilizer. It also substantially increases the mechanical strength in the same manner as carbon. Nitrogen increases the resistance to localized corrosion, especially in combination with molybdenum. In ferritic stainless steels nitrogen will strongly reduce toughness and corrosion resistance.
Copper enhances corrosion resistance in certain acids (e.g. sulfuric acid) and promotes an austenitic microstructure. In precipitation hardening steels copper is used to form the intermetallic compounds that are used to increase the strength.
Titanium is both a strong ferrite and carbide former. In austenitic stainless steels it is added to increase the resistance to intergranular corrosion but it also increases the mechanical properties at high temperatures. In ferritic stainless steels titanium is added to improve toughness and corrosion resistance by lowering the amount of interstitials in solid solution. In martensitic steels titanium lowers the martensite hardness and increases the tempering resistance. In precipitation hardening steels titanium is used to form the intermetallic compounds that are used to increase the strength.
Like titanium, niobium is both a strong ferrite and carbide former. In austenitic stainless steels it is added to increase the resistance to intergranular corrosion but it also increases the mechanical properties at high temperatures.
Sulfur is added to certain stainless steels, the free-machining grades, in order to increase machinability. At the levels present in these grades sulfur will substantially reduce corrosion resistance, ductility and fabrication properties, such as weldability and formability.
Silicon improves resistance to oxidation at high temperatures and increases corrosion resistance in strongly oxidizing solutions, such as nitric acid or concentrated sulfuric acid. Silicon is a ferrite stabilizer.
Stainless steel families
Within stainless steels, there are four families
- Austenitic stainless steel (include the so called super-austenitics)
- Ferritic stainless steel
- Martensitic stainless steel
- Duplex stainless steel (also called austenitic- ferritic stainless steels)
Stainless steels are also classified by their crystalline structure:
- Austenitic, or 200 and 300 series, stainless steels have an austenitic crystalline structure, which is a face-centered cubic crystal structure. Austenite steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium, and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.
- Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, because of the lower chromium and nickel content. They are also usually less expensive. Ferritic stainless steels have a body-centered cubic crystal system and contain between 10.5% and 27% chromium with very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. These alloys can be degraded by the presence of sigma chromium, an intermetallic phase which can precipitate upon welding.
- Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machinable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (less than 2%), and carbon (about 0.1-1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic.
- Duplex steel stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Duplex stainless steels have roughly twice the strength compared to austenitic stainless steels and also improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19-32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.
- Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel.
The designation "CRES" is used in various industries to refer to corrosion-resistant steel. Most mentions of CRES refer to stainless steel, although the correspondence is not absolute, because there are other materials that are corrosion-resistant but not stainless steel.
Grades
There are over 150 grades of stainless steel, of which 15 are most commonly used. There are a number of systems for grading stainless and other steels, including US SAE steel grades.
Comparison of standardized steels
Standard finishes
Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (mill scale) is removed by pickling, and a passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.
- No. 0: Hot rolled, annealed, thicker plates
- No. 1: Hot rolled, annealed and passivated
- No. 2D: Cold rolled, annealed, pickled and passivated
- No. 2B: Same as above with additional pass through highly polished rollers
- No. 2BA: Bright annealed (BA or 2R) same as above then bright annealed under oxygen-free atmospheric condition
- No. 3: Coarse abrasive finish applied mechanically
- No. 4: Brushed finish
- No. 5: Satin finish
- No. 6: Matte finish (brushed but smoother than #4)
- No. 7: Reflective finish
- No. 8: Mirror finish
- No. 9: Bead blast finish
- No. 10: Heat colored finish--offering a wide range of electropolished and heat colored surfaces
Applications
Architecture
Stainless steel is used for buildings for both practical and aesthetic reasons. Stainless steel was in vogue during the art deco period. The most famous example of this is the upper portion of the Chrysler Building (pictured). Some diners and fast-food restaurants use large ornamental panels and stainless fixtures and furniture. Because of the durability of the material, many of these buildings still retain their original appearance. Stainless steel is used today in building construction because of its durability and because it is a weldable building metal that can be made into aesthetically pleasing shapes. An example of a building in which these properties are exploited is the Art Gallery of Alberta in Edmonton, which is wrapped in stainless steel.
Type 316 stainless is used on the exterior of both the Petronas Twin Towers and the Jin Mao Building, two of the world's tallest skyscrapers.
The Parliament House of Australia in Canberra has a stainless steel flagpole weighing over 220 metric tons (240 short tons).
The aeration building in the Edmonton Composting Facility, the size of 14 hockey rinks, is the largest stainless steel building in North America.
The Helix Bridge is a pedestrian bridge linking Marina Centre with Marina South in the Marina Bay area in Singapore.
- Cala Galdana Bridge in Menorca (Spain) was the first stainless steel road bridge.
- Sant Fruitos Pedestrian Bridge (Catalonia, Spain), arch pedestrian bridge.
- Padre Arrupe Bridge (Bilbao, Spain) links the Guggenheim museum to the University of Deusto.
- Unisphere, constructed as the theme symbol of the 1964 New York World's Fair, is constructed of Type 304L stainless steel as a spherical framework with a diameter of 120 feet (37 m) (New York City)
- Gateway Arch (pictured) is clad entirely in stainless steel: 886 tons (804 metric tons) of 0.25 in (6.4 mm) plate, #3 finish, type 304 stainless steel. (St. Louis, Missouri)
- United States Air Force Memorial has an austenitic stainless steel structural skin (Arlington, Virginia)
- Atomium was renovated with stainless-steel cladding in a renovation completed in 2006; previously the spheres and tubes of the structure were clad in aluminium (Brussels, Belgium)
- Cloud Gate sculpture by Anish Kapoor (Chicago, Illinois)
- Sibelius Monument is made entirely of stainless steel tubes (Helsinki, Finland)
- The Kelpies (Falkirk, Scotland)
- Man of Steel (sculpture) under construction (Rotherham, England)
- Juraj Jáno?ík monument (Terchova, Slovakia)
Stainless steel is a modern trend for roofing material for airports due to its low glare reflectance to keep pilots from being blinded, also for its properties that allow thermal reflectance in order to keep the surface of the roof close to ambient temperature. The Hamad International Airport in Qatar was built with all stainless steel roofing for these reasons, as well as the Sacramento International Airport in California.
Locomotion
The Allegheny Ludlum Corporation worked with Ford on various concept cars with stainless steel bodies from the 1930s through the 1970s to demonstrate the material's potential. The 1957 and 1958 Cadillac Eldorado Brougham had a stainless steel roof. In 1981 and 1982, the DeLorean DMC-12 production automobile used Type-304 stainless steel body panels over a glass-reinforced plastic monocoque. Intercity buses made by Motor Coach Industries are partially made of stainless steel. The aft body panel of the Porsche Cayman model (2-door coupe hatchback) is made of stainless steel. It was discovered during early body prototyping that conventional steel could not be formed without cracking (due to the many curves and angles in that automobile). Thus, Porsche was forced to use stainless steel on the Cayman.
Some automotive manufacturers use stainless steel as decorative highlights in their vehicles.
Rail cars have commonly been manufactured using corrugated stainless steel panels (for additional structural strength). This was particularly popular during the 1960s and 1970s, but has since declined. One notable example was the early Pioneer Zephyr. Notable former manufacturers of stainless steel rolling stock included the Budd Company (USA), which has been licensed to Japan's Tokyu Car Corporation, and the Portuguese company Sorefame. Many railcars in the United States are still manufactured with stainless steel, unlike other countries who have shifted away.
Budd also built two airplanes, the Budd BB-1 Pioneer and the Budd RB-1 Conestoga, of stainless steel tube and sheet. The first, which had fabric wing coverings, is on display at the Franklin Institute, being the longest continuous display of an aircraft ever, since 1934. The RB-2 Was almost all stainless steel, save for the control surfaces. One survives at the Pima Air & Space Museum, adjacent to Davis-Monthan Air Force Base.
The American Fleetwings Sea Bird amphibious aircraft of 1936 was also built using a spot-welded stainless steel hull.
Due to its thermal stability, the Bristol Aeroplane Company built the all-stainless steel Bristol 188 high-speed research aircraft, which first flew in 1963. However, the practical problems encountered meant that Concorde employed aluminium alloys.
The use of stainless steel in mainstream aircraft is hindered by its excessive weight compared to other materials, such as aluminium.
Medicine
Surgical tools and medical equipment are usually made of stainless steel, because of its durability and ability to be sterilized in an autoclave. In addition, surgical implants such as bone reinforcements and replacements (e.g. hip sockets and cranial plates) are made with special alloys formulated to resist corrosion, mechanical wear, and biological reactions in vivo.
Stainless steel is used in a variety of applications in dentistry. It is common to use stainless steel in many instruments that need to be sterilized, such as needles, endodontic files in root canal therapy, metal posts in root canal-treated teeth, temporary crowns and crowns for deciduous teeth, and arch wires and brackets in orthodontics. The surgical stainless steel alloys (e.g., 316 low-carbon steel) have also been used in some of the early dental implants.
Culinary use
Stainless steel is often preferred for kitchen sinks because of its ruggedness, durability, heat resistance, and ease of cleaning. In better models, acoustic noise is controlled by applying resilient undercoating to dampen vibrations. The material is also used for cladding of surfaces such as appliances and backsplashes.
Cookware and bakeware may be clad in stainless steels, to enhance their cleanability and durability, and to permit their use in induction cooking (this requires a magnetic grade of stainless steel, such as 432). Because stainless steel is a poor conductor of heat, it is often used as a thin surface cladding over a core of copper or aluminium, which conduct heat more readily.
Cutlery is normally stainless steel, for low corrosion, ease of cleaning, negligible toxicity, as well as not flavoring the food by electrolytic activity.
Jewelry
Stainless steel is used for jewelry and watches, with 316L being the type commonly used for such applications. It can be re-finished by any jeweler and will not oxidize or turn black.
Valadium, a stainless steel and 12% nickel alloy is used to make class and military rings. Valadium is usually silver-toned, but can be electro-plated to give it a gold tone. The gold tone variety is known as Sun-lite Valadium. Other "Valadium" types of alloy are trade-named differently, with such names as "Siladium" and "White Lazon".
Firearms
Some firearms incorporate stainless steel components as an alternative to blued or parkerized steel. Some handgun models, such as the Smith & Wesson Model 60 and the Colt M1911 pistol, can be made entirely from stainless steel. This gives a high-luster finish similar in appearance to nickel plating. Unlike plating, the finish is not subject to flaking, peeling, wear-off from rubbing (as when repeatedly removed from a holster), or rust when scratched.
3D printing
Some 3D printing providers have developed proprietary stainless steel sintering blends for use in rapid prototyping. One of the more popular stainless steel grades used in 3D printing is 316L stainless steel. Due to the high temperature gradient and fast rate of solidification, stainless steel products manufactured via 3D printing tend to have a more refined microstructure; this in turn results in better mechanical properties. However, stainless steel is not used as much as materials like Ti6Al4V in the 3D printing industry; this is because manufacturing stainless steel products via traditional methods is currently much more economically competitive.
Recycling and reusing
Stainless steel is 100% recyclable. An average stainless steel object is composed of about 60% recycled material of which approximately 40% originates from end-of-life products and about 60% comes from manufacturing processes. According to the International Resource Panel's Metal Stocks in Society report, the per capita stock of stainless steel in use in society is 80-180 kg in more developed countries and 15 kg in less-developed countries.
There is a secondary market that recycles usable scrap for many stainless steel markets. The product is mostly coil, sheet, and blanks. This material is purchased at a less-than-prime price and sold to commercial quality stampers and sheet metal houses. The material may have scratches, pits, and dents but is made to the current specifications.
Nanoscale stainless steel
Stainless steel nanoparticles have been produced in the laboratory. This synthesis uses oxidative Kirkendall Diffusion to build a thin protective barrier which prevent further oxidation. These may have applications as additives for high performance applications. For examples, sulfurization, phosphorization and nitridation treatments to produce nanoscale stainless steel based catalysts could enhance the electrocatalytic performance of stainless steel for water splitting.
Health effects
Stainless steel is generally considered to be biologically inert, but some sensitive individuals develop a skin irritation due to a nickel allergy caused by certain alloys.
Source of the article : Wikipedia
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