Stainless Steel

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defines Saint Louis, Missouri skyline.

In metallurgy, stainless steel is defined as an iron-carbon alloy with a minimum of 10.5% chromium content. The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel (note: it "stains less", but is not actually "stainless"). This material is also called corrosion resistant steel when it is not detailed exactly to its alloy type and grade, particularly in the aviation industry. As such, there are now different and easily accessible grades and surface finishes of stainless steel, to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are everyday cutlery and watch straps.

Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made environments; however, it is important to select the correct type and grade of stainless steel for the particular application.

High oxidation resistance in air at ambient temperature is normally achieved with additions 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, which means that the metal remains lustrous. It is, however, impervious to water and air, protecting the metal beneath. Also, 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. When stainless steel parts such as Nut (hardware)s and Screw#bolts are forced together, the oxide layer can be scraped off causing the parts to welding together. When disassembled, the welded material may be torn and pitted, an effect that is known as galling.

Nickel also contributes to passivation, as do other less commonly used ingredients such as molybdenum and vanadium.

Commercial value of stainless steel {| align="right"| is clad with type 302 stainless steel.|-| sculpture on the Niagara-Mohawk Power building in Syracuse, New York and staining, low maintenance, relative inexpense, and familiar luster make it an ideal base material for a host of commercial applications. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is [Steel mill into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, a structural alloy in automotive and aerospace assembly and building material in skyscrapers and other large buildings.

Stainless steel is also used for jewelry and watches. The most common stainless steel alloy used for jewelry is 316L. It can be re-finished by any jeweler and unlike silver will not oxidize and turn black.

Stainless steel is 100% recyclable. In fact, an average stainless steel object is composed of about 60% recycled material, 25% originating from end-of-life products and 35% coming from manufacturing processes.{{cite web| url = http://www.worldstainless.org/ISSF/Files/Recycling/Flash.html| title = The Recyling of Stainless Steel ("Recycled Content" and "Input Composition" slides)| accessdate = 2006-11-19| year = 2006| format = Flash| publisher = International Stainless Steel Forum-->

Corrosion Even a high-quality alloy can corrode under certain conditions. Because these modes of corrosion are more exotic and their immediate results are less visible than rust, they often escape notice and cause problems among those who are not familiar with them.

Pitting corrosion Passivation relies upon the tough layer of oxide described above. When deprived of oxygen (or when a salt such as chloride competes as an ion), stainless steel lacks the ability to re-form a passivating film. In the worst case, almost all of the surface will be protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen. In extreme cases, the sharp tips of extremely long and narrow pits can cause stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in stainless alloys, but it can be prevented by ensuring that the material is exposed to oxygen (for example, by eliminating crevices) and protected from chlorides wherever possible.

Pitting corrosion can occur when stainless steel is subjected to high concentration of Chloride ions (for example, sea water) and moderately high temperatures. A textbook example for this was a replica of the Jet d'Eau fountain in Geneva, ordered by an Arab Sheikh for installation in the Red Sea – King Fahd's Fountain. The difference between the freshwater of Lake Geneva and the saltwater of the sea called for much greater specialisation of the engineering processes and materials involved, as a straight duplicate of the Geneva fountain would not have survived long in the saltwater environment.

Rouging Rouging is a very peculiar phenomenon, which occurs only on polished stainless steel surfaces with very low surface roughness in a pure water environment. This effect is most common in pharmaceutical industries. The very pure water used in nuclear reactors can also cause rouging.http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/2006/in200627.pdf It occurs because pure water is lacking any ions and pulls the metal ions of the passive stainless steel surface into solution. Iron ions do not dissolve at neutral pH and will precipitate as an iron hydroxide film, which has a reddish colour, hence the name rouging.

Intergranular corrosion Some compositions of stainless steel are prone to intergranular corrosion when exposed to certain environments. When heated to around 700 °C, chromium carbide forms at the intergranular boundaries, depleting the grain edges of chromium, impairing their corrosion resistance. Steel in such condition is called sensitized. Steels with carbon content 0.06% undergo sensitization in about 2 minutes, while steels with carbon content under 0.02% are not sensitive to it.A special case of intergranular corrosion is called "weld decay" or "knifeline attack" (KLA). Due to the elevated temperatures of welding the stainless steel can be sensitized very locally along the weld. The chromium depletion creates a electrochemistry with the well-protected alloy nearby in highly corrosive environments. As the name "knifeline attack" implies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable.Denny A. Jones, Principles and Prevention of Corrosion, 2nd edition, 1996, Prentice Hall, Upper Saddle River, NJ. ISBN 0-13-359993-0

It is possible to reclaim sensitized steel by heating it to above 1000 °C and holding at this temperature for a given period of time dependent on the mass of the piece, followed by quenching it in water. This process dissolves the carbide particles, then keeps them in solution.

It is also possible to stabilize the steel to avoid this effect and make it welding-friendly. Addition of titanium, niobium and/or tantalum serves this purpose; titanium carbide, niobium carbide and tantalum carbide form preferentially to chromium carbide, protecting the grains from chromium depletion. Use of extra-low carbon steels is another method and modern steel production usually ensures a carbon content of 6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series. The higher alloy content of "Superaustenitic" steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications.

Ferrite (iron) stainless steels are highly corrosion resistant, but less durable than austenitic grades. They contain between 10.5% and 27% chromium and very little nickel, if any. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.

Martensite stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly Machining, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12–14%), molybdenum (0.2–1%), zero to less than 2% nickel, and about 0.1–1% carbon (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic. It is also known as "series-00" steel.

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. There is a rising trend in defence budgets to opt for an ultra-high-strength stainless steel if possible in new projects as it is estimated that 2% of the U.S. GDP is spent dealing with corrosion. The Lockheed-Martin JSF is the first aircraft to use a precipitation hardenable stainless steel – Carpenter Custom 465 – in its airframe.

Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50:50 mix although in commercial alloys the mix may be 40:60 respectively. Duplex steel have improved strength over austenitic stainless steels and also improved resistance to localised corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.

Comparison of standardized steels {|class="wikitable"! EN-standardSteel no.DIN! EN-standardSteel name! ASTM/AISISteel type! Unified numbering system|---| || ||440A || S44002|---| 1.4112 || ||440B || S44004|---| 1.4125 || ||440C || S44003|---| || ||440F || S44020|---| 1.4016 || X6Cr17 ||430 || S43000|---| 1.4512 || X6CrTi12 ||409 || S40900|---| 1.4310 || X10CrNi18-8 ||301 || S30100|---| 1.4318 || X2CrNiN18-7 || 301LN || N/A|---| 1.4307 || X2CrNi18-9 ||304L || S30403|---|1.4306 || X2CrNi19-11 ||304L || S30403|---|1.4311 || X2CrNiN18-10 ||304LN || S30453|---|1.4301 || X5CrNi18-10 ||304 || S30400|---| 1.4948 || X6CrNi18-11 ||304H || S30409|---|1.4303 || X5CrNi18 12 ||305 || S30500|---| 1.4541 || X6CrNiTi18-10 ||321 || S32100|---| 1.4878 || X12CrNiTi18-9 ||321H || S32109|---| 1.4404 || X2CrNiMo17-12-2 ||316L || S31603|---| 1.4401 || X5CrNiMo17-12-2 ||316 || S31600|---| 1.4406 || X2CrNiMoN17-12-2 ||316LN || S31653|---| 1.4432 || X2CrNiMo17-12-3 ||316L || S31603|---| 1.4435 || X2CrNiMo18-14-3 ||316L || S31603|---| 1.4436 || X3CrNiMo17-13-3 ||316 || S31600|---| 1.4571 || X6CrNiMoTi17-12-2 ||316Ti || S31635|---|1.4429 || X2CrNiMoN17-13-3 ||316LN || S31653|---|1.4438 || X2CrNiMo18-15-4 ||317L || S31703|---| 1.4539 || X1NiCrMoCu25-20-5 ||904L || N08904|---| 1.4547 || X1CrNiMoCuN20-18-7 || N/A || S31254|---|}

Stainless steel Grades is not exhaustive

200 Series—austenitic chromium-nickel-manganese alloys
300 Series—austenitic chromium-nickel alloys

Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
Type 303—easier machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with International Organization for Standardization ISO 3506.
Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with International Organization for Standardization ISO 3506.
Type 309— better temperature resistance than 304
Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. 316 steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination. It is also known as "marine grade" stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most watches that are made of stainless steel are made of Type 316L; Rolex is an exception in that they use Type 904L. Also referred to as "A4" in accordance with International Organization for Standardization ISO 3506.
Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.

400 Series—ferritic and martensitic chromium alloys

Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409—cheapest type; used for automobile Exhaust pipe; ferritic (iron/chromium only).
Type 410—martensitic (high-strength iron/chromium). Wear resistant, but less corrosion resistant.
Type 416— easy to machine due to additional sulfur
Type 420—"Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel". Also known as "surgical steel". Excellent polishability.
Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
Type 440—a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly. It can be hardened to around Rockwell_scale hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as "razor blade steel". Available in four grades 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered a more desirable choice in knifemaking than 440A except for diving or other salt-water applications.

500 Series—heat resisting chromium alloys
600 Series—martensitic precipitation hardening alloys

Type 630—most common PH stainless, better known as 17-4; 17% chromium, 4% nickel

Stainless steel 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 Annealing (metallurgy) to change the properties of the final material. Any oxidation that forms on the surface (scale) is removed by Pickling (metal), and the 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 polished rollers
No, 2BA - Bright Anealed (BA) same as above with highly polished rollers
No. 3 - coarse abrasive finish applied mechanically
No. 4 - brushed finish
No. 6 - matte finish
No. 7 - reflective finish
No. 8 - mirror finish
No. _ - bead blast finish

History A few corrosion-resistant iron artifacts survive from antiquity. A famous (and very large) example is the Iron pillar, erected by order of Kumara Gupta I around the year 400. However, unlike stainless steel, these artifacts owe their durability not to chromium, but to their high phosphorus content, which together with favorable local weather conditions promotes the formation of a solid protective passivation layer of iron oxides and phosphates, rather than the non-protective, cracked rust layer that develops on most ironwork.

The corrosion resistance of iron-chromium alloys was first recognized in 1821 by the France metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. However, the 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 of practical interest.

This situation changed in the late 1890s, when Hans Goldschmidt of Germany developed an aluminothermic (thermite) process for producing carbon-free chromium. In the years 1904–1911, several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.

In Germany, Friedrich Krupp Germaniawerft built the 366-ton sailing-yacht "Germania" featuring a chrome-nickel steel hull in 1908. In 1911, Philip Monnartz reported on the relationship between the chromium content and corrosion resistance. On October 17 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel.

Similar industrial developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless.

However Harry Brearley of the Firth-Brown research laboratory in Sheffield, England is most commonly credited as the "inventor" of stainless steel, but many historians feel this is disputable. In 1913, while seeking an erosion-resistant alloy for gun barrels, he discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was popularized a few years later, in a January 1915 newspaper article in the New York Times.

Uses in sculpture, building facades and building structures

Stainless steel was particularly in vogue during the art deco period. The most famous example of this is the upper portion of the Chrysler Building (illustrated above). Diners and fast food restaurants feature large ornamental panels, stainless fixtures and furniture. Owing to the durability of the material, many of these buildings still retain their original and spectacular appearance.
In recent years the forging of stainless steel has given rise to a fresh approach to architectural blacksmithing. The work of Giusseppe Lund illustrates this well.
Also pictured above, the Jefferson National Expansion Memorial is clad entirely in stainless steel: 886 Tons (804 metric tonnes) of 1/4" (6.3 mm) plate, #3 Finish, Type 304.
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, Australia, has a stainless steel flagpole weighing over 220 tonnes.
Stainless Steel is the fourth common material used in metal wall tiles, and is used for its corrosion resistance properties in kitchens and bathrooms.
The aeration building in the Edmonton, Alberta Composting Facility, the size of 14 NHL hockey rinks, is the largest stainless steel building in North America.
The United States Air Force Memorial has an austenitic stainless steel structural skin.