Stainless steel

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Stainless steel cladding is used on the Walt Disney Concert Hall

In metallurgy, stainless steel,[1][2] 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 and a maximum of 1.2% carbon by mass.[3][4]

Stainless steels are most notable for their corrosion resistance, which increases with increasing chromium content. Additions of molybdenum increases corrosion resistance in reducing acids and against pitting attack in chloride solutions. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure. Resistance to corrosion and staining, low maintenance, and familiar luster make stainless steel an ideal material for many applications where both the strength of steel and corrosion resistance are required.

Stainless steel is rolled into sheets, plates, bars, wire, and tubing to be used in: cookware, cutlery, surgical instruments, major appliances; construction material in large buildings, such as the Chrysler Building; industrial equipment (for example, in paper mills, chemical plants, water treatment); and storage tanks and tankers for chemicals and food products (for example, chemical tankers and road tankers). Corrosion resistance, the ease with which it can be steam cleaned and sterilized, and unnecessary need for surface coatings has also influenced the use of stainless steel in commercial kitchens and food processing plants.

Corrosion resistance[edit]

Stainless steel (bottom row) resists salt-water corrosion better than aluminium-bronze (top row) or copper-nickel alloys (middle row)

Stainless steels do not suffer uniform corrosion, like carbon steel, when exposed to wet environments. Unprotected carbon steel rusts readily when exposed to the combination of air and moisture. The resulting iron oxide surface layer (the rust) is porous and fragile. Since iron oxide occupies a larger volume than the original steel this layer expands and tends to flake and fall away exposing the underlying steel to further attack. In comparison, stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in air and even the small amount of dissolved oxygen in water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal.[3] This film is self-repairing if it is scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade.[5][6]

The resistance of this film to corrosion depends upon the chemical composition of the stainless steel, chiefly the chromium content.

Corrosion of stainless steels can occur when the grade is not suited for the working environment.

It is customary to distinguish between four forms of corrosion: uniform, localized (pitting), galvanic and SCC (stress corrosion cracking).

Uniform corrosion[edit]

Uniform corrosion takes place in very aggressive environments, typically chemical production or use, pulp and paper industries, etc. The whole surface of the steel is attacked and the corrosion is expressed as corrosion rate in mm/year (usually less than 0.1 mm/year is acceptable for such cases). Corrosion tables provide guidelines.[7]

This is typically the case when stainless steels are exposed to acidic or basic solutions. Whether a stainless steel corrodes depends on the kind and concentration of acid or base, and the solution temperature. Uniform corrosion is typically easy to avoid because of extensive published corrosion data or easy to perform laboratory corrosion testing.

However, stainless steels are susceptible to localized corrosion under certain conditions, which need to be recognized and avoided. Such localized corrosion is problematic for stainless steels because it is unexpected and difficult to predict.

Stainless steel is not completely immune to corrosion as shown in this desalination equipment.

Acids[edit]

Acidic solutions can be put into two general categories: reducing acids, such as hydrochloric acid and dilute sulfuric acid, and oxidizing acids, such as nitric acid and concentrated sulfuric acid. Increasing chromium and molybdenum content provides increased resistance to reducing acids, while increasing chromium and silicon content provides increased resistance to oxidizing acids.

Sulfuric acid is one of the largest tonnage industrial chemicals manufactured. At room temperature Type 304 is only resistant to 3% acid while Type 316 is resistant to 3% acid up to 50 °C and 20% acid at room temperature. Thus Type 304 is rarely used in contact with sulfuric acid. Type 904L and Alloy 20 are resistant to sulfuric acid at even higher concentrations above room temperature.[8][9]

Concentrated sulfuric acid possesses oxidizing characteristics like nitric acid and thus silicon bearing stainless steels also find application.

Hydrochloric acid will damage any kind of stainless steel, and should be avoided.[10][11]

All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentration and elevated temperature attack will occur and higher alloy stainless steels are required.[12][13]

In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid. As the molecular weight of organic acids increase their corrosivity decreases. Formic acid has the lowest molecular weight and is a weak acid. Type 304 can be used with formic acid though it will tend to discolor the solution. Acetic acid is probably the most commercially important of the organic acids and Type 316 is commonly used for storing and handling acetic acid.[14]

Bases[edit]

Stainless steels Type 304 and 316 are 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.[15]

Increasing chromium and nickel contents provide increasing resistance.

Organics[edit]

All grades resist damage from aldehydes and amines, though in the latter case Type 316 is preferable to 304; cellulose acetate will damage 304 unless the temperature is kept low. Fats and fatty acids only affect Type 304 at temperatures above 150 °C (302 °F), and Type 316 above 260 °C (500 °F), while Type 317 is unaffected at all temperatures. Type 316L is required for processing of urea.[10]

Localized corrosion[edit]

Localized corrosion can occur in a number of ways, e.g. pitting corrosion and crevice corrosion. Such localized attack is most common in the presence of chloride ions. Increasing chloride levels require more highly alloyed stainless steels.

Localized corrosion can be difficult to predict because it is dependent on many factors including:

  • Chloride ion concentration (However, even when the chloride solution concentration is known, it is still possible for chloride ions to concentrate, such as in crevices (e.g. under gaskets) or on surfaces in vapor spaces due to evaporation and condensation.)
  • Increasing temperature increases susceptibility
  • Increasing acidity increases susceptibility
  • Stagnant conditions increase susceptibility
  • The presence of oxidizing species, such as ferric and cupric ions

Pitting corrosion resistance[edit]

This is probably the most frequent form of corrosion. The corrosion resistance of stainless steels to pitting corrosion is often expressed by the PREN (Pitting Resistance Equivalent Number) obtained through the formula:

PREN = %Cr+3.3%Mo+16%N where the terms correspond to the contents by weight % of Chromium, Molybdenum and Nitrogen respectively in the steel.

The higher the PREN, the higher the pitting corrosion resistance. Increasing chromium, molybdenum and nitrogen contents provide increasing resistance to pitting corrosion.

Crevice corrosion[edit]

While the PREN is a property of the stainless steel, crevice corrosion occurs when poor design has created confined areas (overlapping plates, washer-plate interfaces, etc.) and when the PREN is not high enough for the service conditions. Design and good fabrication techniques combined with correct alloy selection can prevent such corrosion.[16]

Stress corrosion cracking[edit]

Stress corrosion cracking (SCC) is a sudden cracking and failure of a component without deformation.

It may occur when three conditions are met:

  • The part is stressed (by an applied load or by a residual stress )
  • The environment is aggressive (high chloride level, temperature above 50 °C, presence of H2S)
  • The stainless steel is not sufficiently SCC resistant

The SCC mechanism results from the following sequence of events:

  1. Pitting occurs
  2. Cracks start from a pit initiation site
  3. Cracks then propagate through the metal in a transgranular or intergranular mode.
  4. Failure occurs

Whereas pitting leads in most cases to unsightly surfaces and in a worst case to perforation of the stainless sheet, failure by SCC can lead to very damaging consequences. It is therefore considered as a special form of corrosion.

As SCC requires several conditions to be met, it is relatively easy to avoid it:

  • reduce the stress level (the oil and gas specs provide requirements for max stress level in H2S containing environments)
  • assess the aggressiveness of the environment (high chloride content, temperature above 50 °C, etc.)
  • select the right type of stainless steel: super austenitics such as grade 904L or super duplex (ferritic stainless steels and duplex stainless steels are very resistant to SCC)

Galvanic corrosion [17][edit]

Galvanic corrosion (also called 'dissimilar metal corrosion') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. The most common electrolyte is water, ranging from fresh water to seawater. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. Stainless steel, due to its superior corrosion resistance relative to most other metals, including steel and aluminum, becomes the cathode accelerating the corrosion of the anodic metal. An example is the corrosion of aluminum rivets fastening stainless steel sheets in contact with water.[18] The relative surface areas of the anode and the cathode are important. In the above example, the surface of the rivets will be small compared to that of the stainless steel sheet. However if stainless steel fasteners are used to assemble aluminum sheets, galvanic corrosion will be much slower because the galvanic current density on the aluminum surface will be order of magnitude smaller. A similar, but frequent mistake, is to assemble stainless steel with carbon steel fasteners; whereas using stainless steel to fasten carbon steel plates is usually okay. Providing electrical insulation between the dissimilar metals, where possible, is effective at preventing this type of corrosion.

High temperature corrosion (scaling)[edit]

At elevated temperatures all metals react with hot gases. The most common high temperature gaseous mixture is air, and oxygen is the most reactive component of air. Carbon steel is limited to ~900 °F (480 °C) in air. Chromium in stainless steel reacts with oxygen to form a chromium oxide scale which reduces oxygen diffusion into the material. The minimum 10.5% chromium in stainless steels provides resistance to ~1,300 °F (700 °C), while 16% chromium provides resistance up to ~2,200 °F (1,200 °C). Type 304, the most common grade of stainless steel with 18% chromium, is resistant to ~1,600 °F (870 °C). Other gases such as sulfur dioxide, hydrogen sulfide, carbon monoxide, chlorine, etc. also attack stainless steel. Resistance to other gases is dependent on the type of gas, the temperature and the alloying content of the stainless steel.[19][20]

Oxidation resistance increases with Cr content, as well as Si and Al. Small additions of cerium and yttrium increase the adhesion of the oxide layer on the surface.[21]

Fe Cr Al ferritic stainless steels with Al up to 5% are used for electrical resistance alloys in the form of wire or ribbons.[22]

Properties[edit]

Physical properties[edit]

Properties of a few common grades are listed below.

Physical Properties of Stainless steels
Designations Density Modulus of
elasticity
Mean coefficient of
thermal expansion
[10−6·K−1]
Thermal
Conductivity
Specific
Heat
Electrical
resistivity
EN
[]
AISI/ASTM at 20 °C
[kg/dm3]
at 20 °C
[GPa]
20 °C
200 °C
20 °C
400 °C
at 20 °C
[W/(m·K)]
at 20 °C
[J/(kg·K)]
at 20 °C
[(Ω·mm2)/m]
Austenitic stainless steels
1.4301 304 7,9 200 16,5 17,5 15 500 0,73
1.4401 316 8,0 200 16,5 17,5 15 500 0,75
Duplex stainless steels
1.4462 2205 7,8 200 13,5 14,0 (g) 15 500 0,80
1.4362 2304 7,8 200 13,5 14,0 (n) 15 500 0,80
1.4501 7,8 200 13,5 (n.r.) 15 500 0,80
Ferritic stainless steels
1.4512 409 7,7 220 11,0 12,0 25 460 0,60
1.4016 430 7,7 220 10,0 10,5 25 460 0,60
Martensitic stainless steels
1.4021 420 7,7 215 11,0 12,0 30 460 0,60
1.4418 7,7 200 10,8 11,6 15 430 0,80
Precipitation hardening stainless steels
1.4542 630 7,8 200 10,8 11,6 16 500 0,71

Electricity and magnetism[edit]

Left nut is not stainless and is rusty

Like steel, stainless steels are relatively poor conductors of electricity, with significantly lower electrical conductivity than copper.

Ferritic and martensitic stainless steels are magnetic.

Annealed austenitic stainless steels are non-magnetic. Work hardening can make cold-formed austenitic stainless steels slightly magnetic.

Galling[edit]

Galling, sometimes called cold welding, is a form of severe adhesive wear which can occur when two metal surfaces are in relative motion to each other and under heavy pressure. Austenitic stainless steel fasteners are particularly susceptible to thread galling, although it also occurs in other alloys that self-generate a protective oxide surface film, such as aluminum and titanium. Under high contact-force sliding this oxide can be deformed, broken and removed from parts of the component, exposing bare reactive metal. When the two surfaces are the same material, these exposed surfaces can easily fuse together. Separation of the two surfaces can result in surface tearing and even complete seizure of metal components or fasteners.[23][24]

Galling can be mitigated by the use of dissimilar materials (bronze against stainless steel), or using different stainless steels (martensitic against austenitic). Additionally, threaded joints may be lubricated to provide a film between the two parts and prevent galling. Also, Nitronic 60, made by selective alloying with manganese, silicon and nitrogen, has demonstrated a reduced tendency to gall.

History[edit]

An announcement, as it appeared in the 1915 New York Times, of the development of stainless steel in Sheffield, England.[25]

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 John T. Woods and John Clark patented a "Water Resistant" alloy in Britain, that would today be considered a stainless steel.[26][27]:11

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.[28]

In 1908, Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull in Germany. 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.[29][30][31]

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.[32]

Monument to Harry Brearley at the former Brown Firth Research Laboratory in Sheffield, England

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.[25]

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.[33] 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.[34]

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".[35] In 1929, before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.[36]

Stainless steel families[edit]

There are four main families, which are primarily classified by their crystalline structure: austenitic, ferritic, martensitic and duplex.

Austenitic stainless steel[edit]

Austenitic stainless steel is the largest family of stainless steels, making up about two-thirds of all stainless steel production. They possess an austenitic microstructure, which is a face-centered cubic crystal structure. This microstructure is achieved by alloying with sufficient nickel and/or manganese and nitrogen to maintain an austenitic microstructure at all temperatures from the cryogenic region to the melting point. Thus austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures.

Their yield strength is low (200 to 300MPa), which limits their use for structural and other load bearing components. Duplex stainless steels tend to be preferred in such situations because of their high strength and corrosion resistance.

Their elongation is high, which allows very important deformation in fabrication processes (such as deep drawing of kitchen sinks)

They are weldable by all processes. The most frequently used is electric arc welding.

Thin sheets and small diameter bars can be strengthened by cold working, with an associated reduction of elongation. However, if they are welded, the welded area will return to the low strength level of the steel before cold working. This limits the use of cold-worked austenitic stainless steels.

They are essentially non-magnetic and maintain their ductility at cryogenic temperatures.

They can be further subdivided into two sub-groups, 200 series and 300 series:

  • 200 Series are chromium-manganese-nickel alloys, which maximize the use of manganese and nitrogen to minimize the use of nickel. Due to their nitrogen addition they possess approximately 50% higher yield strength than 300 series stainless steels. 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.[37]
  • 300 Series are chromium-nickel alloys, which achieve their austenitic microstructure almost exclusively by nickel alloying; some very highly alloyed grades include some nitrogen to reduce nickel requirements. 300 series is the largest group and the most widely used. The best known grade is Type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8%/10% nickel, respectively. The second most common austenitic stainless steel is Type 316. The addition of 2% molybdenum provides greater resistance to acids and to localized corrosion caused by chloride ions.

Low-carbon versions, for example 316L or 304L, are used to avoid corrosion problems caused by welding. The "L" means that the carbon content of the alloy is below 0.03%.

Ferritic stainless steels [38][edit]

Ferritic stainless steels possess a ferrite microstructure like carbon steel, which is a body-centered cubic crystal structure, and contain between 10.5% and 27% chromium with very little or no nickel . This microstructure is present at all temperatures, due to the chromium addition, and like austenitic stainless steels these are not hardenable by heat treatment. They cannot be strengthened by cold work to the same degree as austenitic stainless steels. They are magnetic like carbon steel.

As they do not contain Nickel, they cost less than austenitic grades and are now present in a wide range of industries.

Common grades are 409 and 409Cb (Cb is the US name for Nb) with about 10.5%Cr, the latter being used mostly for automobile exhaust pipes in North America, 430 (17%Cr) for architectural applications, for kitchenware, sinks, slate hooks, roofing, etc...). Additions of Nb, Ti, Zr to grade 430 allow a good weldability and such grades are used for automotive exhaust pipes, for white goods (dishwashers, refrigerator doors, chimney ducts, solar water heaters etc..

Higher Cr ferritics (22%Cr) are now used for power plates for Solid Oxide Fuel Cells (SOFC) operating at temperatures around 700°C.

Electrical resistance ferritic grades Fr-Cr-Al are not included in these groups, as they are designed for oxidation resistance at elevated temperatures. [39]

Martensitic stainless steels[edit]

Martensitic stainless steels offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and creep resisting steels. They fall into 4 categories (with some overlap):[40]

  1. Fe-Cr-C grades: They were the first grades used and they are still widely used in engineering and wear-resistant applications.
  2. Fe-Cr-Ni-C grades: In these grades, some of the carbon is replaced by nickel. They offer a higher toughness and a higher corrosion resistance. Grade EN 1.4303 (Casting grade CA6NM) with 13%Cr and 4%Ni is used for most Pelton, Kaplan and Francis turbines in hydroelectric power plants [41] because it has good casting properties, a good weldability and a good resistance to cavitation erosion.
  3. Precipitation hardening grades: Grade EN 1.4542 (a.k.a. 17/4PH), the best known grade, combines martensitic hardening and precipitation hardening. It achieves high strength and good toughness and is used in aerospace among other applications.
  4. Creep-resisting grades: Small additions of Nb, V, B, Co increase the strength and creep resistance up to about 650 °C.

Heat treatment of martensitic stainless steels[edit]

Martensitic stainless steels form a family of stainless steels that can be heat treated to provide the adequate level of mechanical properties.

The heat treatment typically involves three steps:[42]

  • Austenitizing, in which the steel is heated to a temperature in the range 980 - 1050 °C depending on the grades. The austenite is a face centered cubic phase.
  • Quenching (a rapid cooling in air, oil or water). The austenite is transformed into martensite, a hard body-centered tetragonal crystal structure. The as-quenched martensite is very hard and too brittle for most applications. Some residual austenite may remain.
  • Tempering, i.e. heating to around 500 °C, holding at temperature, then air cooling. Increasing the tempering temperature decreases the yield strength and ultimate tensile strength but increases the elongation and the impact resistance.

Nitrogen-alloyed martensitic stainless steels[edit]

Replacing some of the carbon in martensitic stainless steels by nitrogen is a fairly recent development. The limited solubility of nitrogen has been increased by the pressure electroslag refining (PESR) process in which melting is carried out under a high nitrogen pressure. Up to 0.4% nitrogen contents have been achieved leading to higher hardness/strength and higher corrosion resistance. As the PESR is expensive, lower but significant nitrogen contents have been achieved using the standard argon oxygen decarburization (AOD) process.[43][44][45][46][47]

They are magnetic. They are not as corrosion resistant as the common ferritic and austenitic stainless steels due to their low chromium content.

Duplex stainless steel[edit]

Duplex 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. They are characterized by high chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the yield strength of austenitic stainless steels. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steels Types 304 and 316.

Duplex grades usually divided into three sub-groups based on their corrosion resistance: lean duplex, standard duplex and super duplex.

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.The pulp and paper industry was one of the first ones to use extensively duplex stainless steel. Today, the oil and gas industry is the largest user and has pushed for more corrosion resistant grades, leading to the development of super duplex and even so-called hyper duplex grades. More recently, the less expensive (and slightly less corrosion-resistant) lean duplex has been developed, chiefly for structural applications in building and construction (concrete reinforcing bars, plates for bridges, coastal works) and for the water industry.

Precipitation hardening stainless steels[edit]

Precipitation hardening stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades.

There are based on 3 types:[48]

- Martensitic 17-4 PH, [49](AISI 630 EN 1.4542) contains about 17% Cr, 4%Ni, 4%Cu and 0.3% Nb.

Solution treatment at about 1040°C followed by quenching results in a relatively ductile martensitic structure. Subsequent ageing treatment at 475°C precipitates Nb and Cu-rich phases that increase the strength up to above 1000 MPa yield strength. This outstanding strength level finds uses in high tech applications such as aerospace (usually after remelting to eliminate non-metallic inclusions and thereby to increase fatigue life). Another major advantage of this steel is that ageing, unlike tempering treatments, is carried out at a temperature that can be applied to (nearly) finished parts without distortion and discoloration.

- Semi Austenitic 17-7PH [49] (AISI 631 EN 1.4568) contains about 17%Cr, 7.2% Ni and 1.2%Al.

Typical heat treatment involves first solution treatment and quenching. At this point, the structure remains austenitic. Martensitic transformation is then obtained either by a cryogenic treatment at -75°C or by severe cold work (over 70% deformation, usually by cold rolling or wire drawing). Ageing at 510°C, which precipitates the Ni3Al intermetallic phase, is carried out as above on nearly finished parts. Yield stress levels above 1400 MPa are then reached.

- A286 [50](ASTM 660 EN 1.4980) has the following typical analysis Cr 15%, Ni 25% Ti 2.1% Mo 1.2% V 1.3% and B 0.005%

The structure remains austenitic at all temperatures.

Typical heat treatment involves solution treatment and quenching, followed by ageing at 715°C. Ageing forms Ni3Ti precipitates and increase the yield strength to about 650MPa at room temperature. Unlike the above grades, the mechanical properties and creep resistance of this PH steel remain very good at temperatures up to 700°C.

A286 is in fact a Fe-based superalloy, used in jet engines and gas turbines, turbo parts, etc.

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.[51]

Grades[edit]

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.

Standard finishes[edit]

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.

  • 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

Production process and figures[edit]

Production process[edit]

Most of the world stainless steel production is produced by the following processes.

  • EAF (Electric Arc Furnace) in which stainless steel scrap, other ferrous scrap and ferro alloys (Fe Cr,Fe-Ni, Fe Mo, Fe Si ...) are melted. The molten metal is then poured into a ladle and transferred into the AOD
  • AOD (Argon Oxygen Decarburization) allows the removal of carbon in the molten steel and other composition adjustments to achieve the desired chemical composition of the steel
  • CC (Continuous Casting) in which the molten metal is solidified into slabs (typical section is 20 cm thick and 2 m wide) for flat products or blooms (sections vary widely but 25cmx25cm is about the average).
  • HR (Hot Rolling): The slabs and blooms are reheated in a furnace and then hot rolled. Hot rolling reduces the thickness of the slabs to produce about 3mm thick coils. Blooms on the other hand are hot rolled into bars (that are cut into lengths at the exit of the rolling mill) or wire rod which is coiled.
  • CF (Cold finishing): This is a very simplified overview.

Hot rolled coils are pickled in acid solutions to remove the oxide scale on the surface, then subsequently cold rolled (Sendzimir rolling mills), annealed in a protective atmosphere, until the desired thickness and surface finish is obtained. Further operations such as slitting, tube forming, etc. can be carried out in downstream facilities.

Hot rolled bars are straightened, then machined to the required tolerance and finish.

Wire rod coils are subsequently processed to produce

  • cold finished bars on drawing benches
  • fasteners on boltmaking machines
  • wire on single or multipass drawing machines

Further information can be obtained on the websites of most producers. An example is provided here.[52]

Production figures[edit]

World stainless steel production figures re published every year by ISSF.[53]

Overall stainless steel production (flat and long products):

Stainless steel meltshop production in 1000 metric Tons
Year
European Union
Americas
China
Asia without China
Other countries
World
2018 7386 2808 26706 8195 5635 50729
2017 7377 2754 25774 8030 4146 48081
2016 7280 2931 24938 9956 672 45778
2015 7169 2747 21562 9462 609 41548
2014 7252 2813 21692 9333 595 41686
2013 7147 2454 18984 9276 644 38506

The stainless steel production in China accounted for more than 50% of the world production in 2017.

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

Breakdown of production by families of stainless steels in 2017:

  • Austenitic stainless steels Cr - Ni (also-called 300*-series): 54%
  • Austenitic stainless steels Cr - Mn (also called 200*-series): 21%
  • Ferritic and martensitic stainless steels (also called 400*-series): 23%

This breakdown is quite stable over the years.

  • 300, 200 and 400 refer to the ASTM/AISI grade numbering system for stainless steels

Applications[edit]

The pinnacle of New York's Chrysler Building is clad with Nirosta stainless steel, a form of Type 302[54][55]
Stainless steel is used for industrial equipment when durability and cleanability are important

Architecture[edit]

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.[55]

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.

La Geode in Paris is a nearly spherical building. The dome is composed of 6433 polished stainless steel equilateral triangles that form the sphere that reflects the sky.

Bridges

Stainless steel is quite frequently used for pedestrian and for road bridges. Product forms are tubes (Helix bridge), plates (Cala Galdana bridge), or reinforcing bar [56](Champlain Bridge).

  • Cala Galdana Bridge in Menorca (Spain) was the first stainless steel road bridge.
  • Champlain Bridge, Montreal, Canada[56]
  • Oudesluijs bridge in Amsterdam, a 3D printed stainless steel bridge using Construction 3D printing[57]
  • Padre Arrupe Bridge (Bilbao, Spain) links the Guggenheim museum to the University of Deusto.[58]
  • Sant Fruitos Pedestrian Bridge (Catalonia, Spain), arch pedestrian bridge.
  • Stonecutter's bridge, Hong Kong, China[56]
  • The Helix Bridge is a pedestrian bridge linking Marina Centre with Marina South in the Marina Bay area in Singapore.
Art, monuments and sculptures
  • 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)
  • Blossom pavilion (Shanghai, China) by Zhan Wang, created in 2015
  • Cloud Gate sculpture by Anish Kapoor (Chicago, Illinois)
  • Cristo de Chiapas (Tuxla Guttierez, Mexico) by Jaime Latapí López. Created in 2007
  • 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.[59] (St. Louis, Missouri)
  • Juraj Jánošík monument (Terchova, Slovakia)
  • La danse de la fontaine émergente (Paris, France) by Chen Zhen. Created in 2008
  • Man of Steel (sculpture) under construction (Rotherham, England)
  • Metamorphosis (Charlotte, NC, USA) by David Černỳ. Created in 2011
  • Sibelius Monument is made entirely of stainless steel tubes (Helsinki, Finland)
  • Sun Voyager (Reykjavik, Iceland) by Jon Gunnar Arnason 9mx18mx7m. Created in 1990
  • The Big Elk (Stor-Eldval, Norway) by Linda Bakke. Created in 2015
  • The Kelpies (Falkirk, Scotland)
  • 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)
  • United States Air Force Memorial has an austenitic stainless steel structural skin (Arlington, Virginia)
Airports

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.

Water[edit]

Stainless steels have a long history of application in contact with water[60] due to their excellent corrosion resistance. Applications include a range of conditions from plumbing,[61] potable[62] and waste water treatment[63] to desalination.[64] Types 304 and 316 stainless steels are standard materials of construction in contact with water. However, with increasing chloride contents higher alloyed stainless steels such as Type 2205 and super austenitic and super duplex stainless steels are utilized.[65]

Important considerations to achieve optimum corrosion performance are:[66]

  • choose the correct grade for the chloride content of the water;
  • avoid crevices when possible by good design;
  • follow good fabrication practices, particularly removing weld heat tint;
  • drain promptly after hydrotesting.

Pulp, paper and biomass conversion[edit]

Stainless steels are used extensively in the Pulp and Paper industry for two primary reasons, to avoid iron contamination of the product and their corrosion resistance to the various chemicals used in the paper making process.[67][68]

A wide range stainless steels are used throughout the paper making process. For example, duplex stainless steels are being used in digesters to convert wood chips into wood pulp. 6% Mo superaustenitics are used in the bleach plant and Type 316 is used extensively in the paper machine.

Chemical processing and petrochemical[edit]

Stainless steels are used extensively in these industries for their corrosion resistance to both aqueous, gaseous and high temperature environments, their mechanical properties at all temperatures from cryogenic to the very high, and occasionally for other special physical properties.[69][70][71][72]

Food and beverage[edit]

Austenitic (300 series) stainless steel, in particular Type 304 and 316 or sometimes 400 series is used, is the material of choice for the food and beverage industry. Stainless steels do not affect the taste of the product, they are easily cleaned and sterilized to prevent bacterial contamination of the food, and they are durable.

Stainless steels are used extensively in cookware, commercial food processing, commercial kitchens, brewing beer, wine making, and meat processing.[73]

Acidic foods with high salt additions, such as tomato sauce, and highly salted condiments, such as soya sauce may require higher alloyed stainless steels such as 6% Mo superaustenitics to prevent pitting corrosion by chloride.

Locomotion[edit]

Automobiles

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 DMC DeLorean 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 largest use of stainless steel in cars is the exhaust line. Environment protection requirements of reducing pollution and noise for whole life of cars led to the use of ferritic grades typically AISI409/409Cb in North America, EN 1.4511 and 1.4512 in Europe. They are used for collector, tubing, muffler, catalytic converter, tailpipe. Heat resisting grades typically EN1.4913 or 1.4923 are used in parts of turbochargers, other heat resisting grades for EGR (Exhaust gas recirculation) and for inlet and exhaust valves. In addition, common rail injection systems and particularly the injectors rely on stainless steels.

Stainless steel has proved to be the best choice for miscellaneous applications, such as stiffeners for windshiel wiper blades, balls for seat belt operation device in case of accident, springs, fasteners, etc.

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.

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. India is developing its rail infrastructure and has started to put new stainless steel coaches in service.[74] South Africa is also commissioning stainless steel coaches.[75]

Aircraft

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. Similarly the experimental mach 3 American bomber, the XB70 Valkyrie, made extensive use of stainless steel in its external structure due to the extreme heat encountered at those high speeds.

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

Spacecraft
ISS module Harmony with outer stainless steel panels

Stainless steel also has an application in spaceflight. The early Atlas rockets have used stainless steel (this formed their fuel tanks). The outer cladding of the modules and the Integrated Truss Structure of the International Space Station use stainless steel alloys. Components of the future Space Launch System, and the structural shell of the SpaceX BFR will be the second and third rockets respectively to use stainless steel.

Medicine[edit]

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.[citation needed]

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,[76] 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.[77] The surgical stainless steel alloys (e.g., 316 low-carbon steel) have also been used in some of the early dental implants.[78]

Energy[edit]

Stainless steels are extensively used in all manner of power stations, from nuclear[79] to solar.[80] Furthermore, stainless steels are ideally suited as mechanical supports for power generation units when the permeation of gases or liquids are required, such as filters in cooling water or hot gas clean up[81] or as structural supports in electrolytic power generation.[82]

Hydrogen is on its way to be a major energy carrier in the coming years.

Stainless steel is used in electrolysers (PEM -Proton exchange Memebranesand SOEL -Solid Oxide Electrolysers being the most common) that convert electrical energy into hydrogen gas by water electrolysis: water filtration, power plates, gas dryer ....

Conversely, Stainless steel is used in Fuel cells that do the opposite, i;e. react Hydrogen with oxygen to produce water and electrical energy: Power plates, tubing, fittings, etc.....

Culinary[edit]

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.[citation needed]

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,[83] for low corrosion, ease of cleaning, negligible toxicity, as well as not flavoring the food by[84] electrolytic activity.

Jewelry[edit]

Stainless steel is used for jewelry and watches, with 316L being the type commonly used for such applications. Oxidizing stainless steel briefly gives it radiant colors that can also be used for coloration effects.[85] 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.[86] Other "Valadium" types of alloy are trade-named differently, with such names as "Siladium" and "White Lazon".

Firearms[edit]

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[edit]

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.

Sustainability - Recycling and reusing[edit]

The average carbon footprint of stainless steel (all grades, all countries) has been estimated to be 2.90Kg CO2 per Kg of stainless steel [87] of which 1.92 are emissions from raw materials (Cr, Ni, Mo...); 0.54 from electricity and steam, and 0.44 are direct emissions (i.e; by the stainless steel plant). As this is an average, countries that have low CO2 emissions in their electricity production (such as France with nuclear energy) will have lower figures. Ferritics without Ni will have a lower CO2 footprint than asutenitics wit 8%Ni or more.

Stainless steel is 100% recyclable.[88][89][90] 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.[91]. What prevents a higher recycling content is the availability of stainless steel scrap, in spite of a very high recycling rate. 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.

Carbon footprint must not be the only factor for deciding the choice of materials:

- over any product life, maintenance, repairs (when possible) or early end of life (planned obsolescence) can increase the overall footprint way beyond the difference between materials only. In addition loss of service (typically for bridges) may induce large hidden costs (queues, wasted fuel, loss of man-hours;..)

- how much material is used to provide a given service varies with the performance, particularly the strength level, which allows lighter structures and components.

Nanoscale stainless steel[edit]

Stainless steel nanoparticles have been produced in the laboratory.[92][93] 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.[94]

Health effects[edit]

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.[citation needed] Stainless steel leaches small amounts of nickel and chromium during cooking.[95]

See also[edit]

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