Stainless steel

In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is a steel alloy with a minimum of 10.5%[1] chromium content by mass.

Stainless steel does not readily corrode, rust or stain with water as ordinary steel does. However, it is not fully stain-proof in low-oxygen, high-salinity, or poor air-circulation environments.[2] There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and corrosion resistance are required.

Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide; and, because of the greater volume of the iron oxide, this tends to flake and fall away. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure, and, due to the similar size of the steel and oxide ions, they bond very strongly and remain attached to the surface.[3]

Passivation occurs only if the proportion of chromium is high enough and oxygen is present.

History

An announcement, as it appeared in the 1915 New York Times, of the development of stainless steel[4]

A few corrosion-resistant iron artifacts survive from antiquity. A famous example is the Iron Pillar of Delhi, erected by order of Kumara Gupta I around AD 400. Unlike stainless steel, however, 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 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 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.[5] 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.[6][7][8]

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.[9]

Also 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.[4] 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.[10] 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.[11]

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".[12] In 1929, before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.[13]

Properties

Oxidation

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.[14] 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. 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, an effect known as galling. This destructive 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 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 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 steel is generally highly resistant to attack from acids, but this quality depends on the kind and concentration of the acid, the surrounding temperature, and the type of steel. Type 904 is resistant to sulfuric acid at room temperature, even in high concentrations; type 316 and 317 are resistant below 10%, and 304 should not be used in the presence of sulfuric acid at any concentration. All types of stainless steel resist attack from phosphoric acid, 316 and 317 more so than 304; Types 304L and 430 have been successfully used with nitric acid. Hydrochloric acid will damage any kind of stainless steel, and should be avoided.[15]

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, especially with solutions containing chlorides such as sodium hypochlorite.[15]

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.[15]

Electricity and magnetism

Like steel, stainless steel is a relatively poor conductor of electricity, with lower electrical conductivity than copper.

Ferritic and martensitic stainless steels are magnetic. Annealed austenitic stainless steels are non-magnetic. Work hardening can make austenitic stainless steels slightly magnetic.

Applications

The arch rises from the bottom left of the picture and is shown against a featureless clear sky
The 630-foot-high (190 m), stainless-clad (type 304) Gateway Arch defines St. Louis's skyline
The pinnacle of New York's Chrysler Building is clad with Nirosta stainless steel, a form of Type 302[16][17]
A stylized figure of a male human with outstretched arms and head tilted slightly forward, wearing a winged and crested helmet, mounted on the facade of a building
An art deco sculpture on the Niagara-Mohawk Power building in Syracuse, New York

Stainless steel’s resistance to corrosion and staining, low maintenance and familiar lustre make it an ideal material for many applications. There are over 150 grades of stainless steel, of which fifteen are most commonly used. 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.

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.

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.

Some automotive manufacturers use stainless steel as decorative highlights in their vehicles.

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 retain their original appearance.

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.[17]

The Parliament House of Australia in Canberra has a stainless steel flagpole weighing over 220 tonnes (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.

Bridges

Monuments and sculptures

Other

Automotive bodies

The Allegheny Ludlum Corporation worked with Ford on various concept cars with stainless steel bodies from the 1930s through the 1970s, as demonstrations of 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 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.

Passenger rail cars

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.

Aircraft

Budd also built an airplane, the Budd BB-1 Pioneer, of stainless steel tube and sheet, which is on display at the Franklin Institute.

The American Fleetwings Sea Bird amphibious aircraft of 1936 was also built using a spot-welded stainless steel hull.

The Bristol Aeroplane Company built the all-stainless steel Bristol 188 high-speed research aircraft, which first flew in 1963.

The use of stainless steel in mainstream aircraft is hindered by its excessive weight compared to other materials, such as aluminum.

Jewelry

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.[20] Other "Valadium" types of alloy are trade-named differently, with such names as "Siladium" and "White Lazon" .

Dentistry

Stainless steel is used in a variety of applications in dentistry. It is common to use stainless steel in many of the sterilized instruments such as needles,[21] 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.[22] The surgical stainless steel alloys (e.g., 316 low-carbon steel) have also been used in some of the early dental implants.[23]

Maintenance of stainless steel

If treated or stored incorrectly, any grade of stainless steel may discolor or stain. To maintain optimum appearance, the surface should be cared for regularly.

Maintenance during installation

The quality of installation affects the durability and lifespan of stainless steel.[24] Therefore it is important to make sure stainless steel is in good condition before installation. Normally, giving it a quick clean is enough prior to installation. However, if surface contamination is present, more attention is required. In fields such as aerospace, pharmaceuticals and food handling, an extremely high standard of cleanliness may be required so extra care should be taken.

Routine maintenance

Maintenance is required to maintain the quality and appearance of steel. Depending on the environment, it is carried out between one and ten times per year. A proper maintenance routine significantly prolongs the life of stainless steel.[25]

Tools used for maintenance

Recycling and reuse

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.[26] 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.

Types of stainless steel

Main article: SAE steel grades
Several heavy pieces of bent pipe with flange connections, strapped down to a wooden pallet
Pipes and fittings made of stainless steel

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels virtually non-magnetic and less brittle at low temperatures. For greater hardness and strength, more carbon is added. With proper heat treatment, these steels are used for such products as razor blades, cutlery, and tools.

Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel, similar to nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:

  • 200 Series—austenitic chromium-nickel-manganese alloys. Type 201 is hardenable through cold working; Type 202 is a general purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.[27]
  • 300 Series—The most widely used austenite steel is the 304, also known as 18/8 for its composition of 18% chromium and 8% nickel.[28] 304 may be referred to as A2 stainless (not to be confused with AISI grade A2 air hardening alloy tool steel containing about 5% chromium). The second most common austenite steel is the 316 grade, also called marine grade stainless, used primarily for its increased resistance to corrosion. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless, is often used in cutlery and high-quality cookware. 18/0 is also available.
Superaustenitic stainless steels, such as Allegheny Ludlum alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion because of high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus 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, for example ASTM A387 is used in pressure vessels but is a low-alloy carbon steel with a chromium content of 0.5% to 9%.[29] Low-carbon versions, for example 316L or 304L, are used to avoid corrosion problems caused by welding. Grade 316LVM is preferred where biocompatibility is required (such as body implants and piercings).[30] The "L" means that the carbon content of the alloy is below 0.03%, which reduces the sensitization effect (precipitation of chromium carbides at grain boundaries) caused by the high temperatures involved in welding.
Swiss Army knives are made of martensitic stainless steel.
The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications. Duplex grades are characterized into groups based on their alloy content and corrosion resistance.
  • Lean duplex refers to grades such as UNS S32101 (LDX 2101), S32304, and S32003.
  • Standard duplex is 22% chromium with UNS S31803/S32205 known as 2205 being the most widely used.
  • Super duplex is by definition a duplex stainless steel with a Pitting Resistance Equivalent Number (PREN) > 40, where PREN = %Cr + 3.3x(%Mo + 0.5x%W) + 16x%N. Usually super duplex grades have 25% chromium or more and some common examples are S32760 (Zeron 100 via Rolled Alloys), S32750 (2507) and S32550 (Ferralium),.
  • Hyper duplex refers to duplex grades with a PRE > 48 and at the moment only UNS S32707 and S33207 are available on the market.

Comparison of standardized steels

EN-standard

Steel no. k.h.s DIN

 EN-standard

Steel name

 SAE grade UNS
1.4109 X65CrMo14 440A S44002
1.4112 X90CrMoV18 440B S44003
1.4125 X105CrMo17 440C S44004
440F S44020
1.4016 X6Cr17 430 S43000
1.4408 G-X 6 CrNiMo 18-10 316
1.4512 X6CrTi12 409 S40900
410 S41000
1.4310 X10CrNi18-8 301 S30100
1.4318 X2CrNiN18-7 301LN
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
X5CrNi30-9 312
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
1.4362 X2CrNi23-4 2304 S32304
1.4462 X2CrNiMoN22-5-3 2205 S31803/S32205
1.4539 X1NiCrMoCu25-20-5 904L N08904
1.4529 X1NiCrMoCuN25-20-7 N08926
1.4547 X1CrNiMoCuN20-18-7 254SMO S31254

Stainless steel grades

There are a number of systems for grading stainless and other steels. The article on US SAE steel grades details a large number of grades with their properties.

Stainless steel in 3D printing

Some 3D printing providers have developed proprietary stainless steel sintering blends for use in rapid prototyping. Currently available grades do not vary significantly in their properties.

Stainless steel finishes

Main article: Brushed metal
Matte surface of pipe, with a few horizontal scratches
316L stainless steel, with an unpolished, mill finish

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.

Producers

Some major producers of stainless steel

See also

References

  1. ↑ "The Stainless Steel Family" (PDF). Retrieved 8 December 2012.
  2. ↑ "Why is Stainless Steel Stainless?". Retrieved 6 September 2013.
  3. ↑ "Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail". Corrosionclinic.com. Retrieved on 29 June 2012.
  4. ↑ 4.0 4.1 "A non-rusting steel". New York Times. 31 January 1915.
  5. ↑ "A Proposal to Establish the Shipwreck Half Moon as a State Underwater Archaeological Preserve" (PDF). Bureau of Archaeological Research, Division of Historical Resources, Florida Department of State. May 2000.
  6. ↑ "ThyssenKrupp Nirosta: History". Retrieved 13 August 2007.
  7. ↑ German Patent Office: Deutsches Reichspatent Nr. 304126, erteilt am 18. Oktober 1912
  8. ↑ German Patent Office: Deutsches Reichspatent Nr. 304159, erteilt am 21. Dezember 1912
  9. ↑ Carlisle, Rodney P. (2004) Scientific American Inventions and Discoveries, p. 380, John Wiley and Sons, ISBN 0-471-24410-4
  10. ↑ Howse, Geoffrey (2011) A Photographic History of Sheffield Steel, History Press, ISBN 0752459856
  11. ↑ Cobb, Harold M. (2010). The History of Stainless Steel. ASM International. p. 360. ISBN 1-61503-010-7.
  12. ↑ Moneypenny, J.H.G. (1921). "Unstainable Steel". Mining and Scientific Press. Retrieved 17 February 2013.
  13. ↑ Bonnier Corporation (1930). Popular Science. Bonnier Corporation. pp. 31–. ISSN 0161-7370.
  14. ↑ Ashby, Michael F.; David R. H. Jones (1992) [1986]. "Chapter 12". Engineering Materials 2 (with corrections ed.). Oxford: Pergamon Press. p. 119. ISBN 0-08-032532-7.
  15. ↑ 15.0 15.1 15.2 Davis (1994), Stainless Steels, Joseph R., ASM International, p. 118, ISBN 978-0-87170-503-7
  16. ↑ Start of production: First coil on new mill at the Wayback Machine (archived May 30, 2013). thyssenkrupp-nirosta.de
  17. ↑ 17.0 17.1 What is Stainless Steel? at the Wayback Machine (archived September 24, 2006) nickelinstitute.org
  18. ↑ "Stainless steel bridge" "Stainless Steel Bridge in Bilbao". Outokumpu.
  19. ↑ Gateway Arch Fact Sheet. Nps.gov. Retrieved on 29 June 2012.
  20. ↑ "What is Valadium?".
  21. ↑ Malamed, Stanley (2004). Handbook of Local Anesthesia, 5th Edition. Mosby. ISBN 0323024491. p. 99
  22. ↑ Anusavice, Kenneth J. (2003) Phillips' Science of Dental Materials, 11th Edition. W.B. Saunders Company. ISBN 0721693873. p. 639
  23. ↑ Misch, Carl E. (2008) Contemporary Implant Dentistry. Mosby. ISBN 0323043739. pp. 277–278
  24. ↑ "Maintenance of Stainless Steel Products". 2013. Retrieved 11 October 2013.
  25. ↑ Kopecki, E. (1937). Cleaning Stainless Steel. ASTM International.
  26. ↑ "The Recycling of Stainless Steel ("Recycled Content" and "Input Composition" slides)" (FLASH). International Stainless Steel Forum. 2006. Retrieved 19 November 2006.
  27. ↑ Habara, Yasuhiro. Stainless Steel 200 Series: An Opportunity for Mn. Technical Development Dept., Nippon Metal Industry, Co., Ltd.
  28. ↑ Stainless Steel – Grade 304 (UNS S30400). azom.com
  29. ↑ ASTM A 387/ A387M – 06a Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum
  30. ↑ Material Properties Data: Marine Grade Stainless Steel. Makeitfrom.com. Retrieved on 29 June 2012.

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