From Wikipedia, the free encyclopedia
In metallurgy, stainless steel, also known as inox steel or inox from French “inoxydable”, is a steel
alloy with a minimum of 10.5% chromium content by mass.
Stainless steel does not readily corrode, rust or stain with water as ordinary steel does, but despite
the name it is not fully stain-proof, most notably under low-oxygen, high-salinity, or poor-circulation
environments.
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 resistance to corrosion 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 due to 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.
Passivation only occurs if the proportion of chromium is high enough and oxygen is present.
History
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. 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 ThyssenKrupp Nirosta.
Similar developments were taking place contemporaneously in the United States, where Christian Dantsizen
and Frederick Becket were industrializing ferritic stainless steel. In 1912, Elwood Haynes applied for
a US patent on a martensitic stainless steel alloy, which was not granted until 1919.
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. The metal was later marketed under the ‘Staybrite’ brand by Firth Vickers in England
and was used for the new entrance canopy for the Savoy Hotel in London in 1929. Brearley applied for a
US patent during 1915 only to find that Haynes had already registered a patent. Brearley and Haynes pooled
their funding and with a group of investors formed the American Stainless Steel Corporation, with
headquarters in Pittsburgh, Pennsylvania.
In the beginning stainless steel was sold in the US under different brand names like ‘Allegheny metal’ and
‘Nirosta steel’. Even within the metallurgy industry the eventual name remained unsettled; in 1921 one
trade journal was calling it “unstainable steel”. In 1929, before the Great Depression hit, over 25,000
tons of stainless steel were manufactured and sold in the US.
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. 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 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.
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 304 is resistant to
sulfuric acid at room temperature, even in high concentrations, but type 316 and 317 are only resistant at
low concentrations. All types of stainless steel resist attack from phosphoric acid, 316 and 317 more so
than 304; and Types 304L and 430 have been successfully used with nitric acid. Hydrochloric acid will
damage any kind of stainless steel, and should be avoided.
Bases
The 300 series of stainless steel grades is unaffected by any of the weak bases such as ammonium hydroxide,
even in high concentrations and at high temperatures. The same grades of stainless exposed to stronger
bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some
etching and cracking, especially with solutions containing chlorides.
Organics
Types 316 and 317 are both useful for storing and handling acetic acid, especially in solutions where it
is combined with formic acid and when aeration is not present (oxygen helps protect stainless steel under
such conditions), though 317 provides the greatest level of resistance to corrosion. Type 304 is also
commonly used with formic acid though it will tend to discolor the solution. All grades resist damage
from aldehydes and amines, though in the latter case grade 316 is preferable to 304; cellulose acetate
will damage 304 unless the temperature is kept low. Fats and fatty acids only affect grade 304 at
temperatures above 150 °C (302 °F), and grade 316 above 260 °C (500 °F), while 317 is unaffected at all
temperatures. Type 316L is required for processing of urea.
Electricity and magnetism
Similarly to steel, stainless steel is a relatively poor conductor of electricity, with a few percent of
the lectrical conductivity of copper.
Ferritic and martensitic stainless steels are magnetic. Austenitic stainless steels are non-magnetic.
Applications
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.
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
Cala Galdana Bridge in Minorca (Spain) is the first stainless steel road bridge.
Sant Fruitos Pedestrian Bridge (Catalonia, Spain), arch pedestrian bridge.
Padre Arrupe Bridge (Bilbao, Spain) links the Guggenheim museum to the University of Deusto.
Monuments and sculptures
The Unisphere, constructed as the theme symbol of the 1964-5 World’s Fair in New York City,
is constructed of Type 304L stainless steel as a sphere with a diameter of 120 feet, or 36.57 meters.
The Gateway Arch (pictured) is clad entirely in stainless steel: 886 tons (804 metric tonnes) of 0.25
in (6.4 mm) plate, #3 finish, type 304 stainless steel.
The United States Air Force Memorial has an austenitic stainless steel structural skin.
The Atomium in Brussels, Belgium was renovated with stainless-steel cladding in a renovation completed
in 2006; previously the spheres and tubes of the structure were clad in aluminium.
The Cloud Gate sculpture by Anish Kapoor, in Chicago US.
The Sibelius monument in Helsinki, Finland, is made entirely of stainless steel tubes.
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.
Maintenance of stainless steel
If treated or stored incorrectly, all grades of stainless steel may discolor or stain. In order to maintain
optimum appearance, the surface of stainless steel must be cared for regularly.
Maintenance during installation
The quality of installation affects the durability and life-time of stainless steel. Therefore it is
important to make sure stainless steel is in good condition before installation. Normally, a quick
cleaning is enough for installation in most cases. However, if surface contamination is present, more
attention is required. In addition, an extremely high standard of cleanliness may be required for stainless
steel applied used in the fields of aerospace, pharmaceuticals, and food handling.
Routine maintenance
Maintenance is required to maintain the quality 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-time
of stainless steel.
Tools used for maintenance
Soft cloth and water: suitable for cosmetic issues and general cleaning
Mild detergent: needed if stains cannot be easily lifted with water
Glass cleaner: useful for removing fingerprints and similar stains
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. 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
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:
Austenitic, or 200 and 300 series, stainless steels have an austenitic crystalline structure, which is a
face-centered cubic crystal structure. Austenite steels make up over 70% of total stainless steel
production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or
manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting
point of the alloy.
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.
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. 304 may be referred to as A2 stainless (not to be confused with A2 grade
steel, also named Tool steel, a steel). 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 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%. 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). 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.
Ferritic stainless steels generally have better engineering properties than austenitic grades, but have
reduced corrosion resistance, because of the lower chromium and nickel content. They are also usually less
expensive. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can
contain lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades
include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. These alloys can be degraded by the presence
of chromium, an intermetallic phase which can precipitate upon welding.
Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely
strong and tough, as well as highly machinable, and can be hardened by heat treatment. Martensitic
stainless steel contains chromium (12–14%), molybdenum (0.2–1%), nickel (less than 2%), and carbon
(about 0.1–1%) (giving it more hardness but making the material a bit more brittle). It is quenched and
magnetic.
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.
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. Duplex stainless steels have
roughly twice the strength compared to austenitic stainless steels and also improved resistance to localized
corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by
high chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic
stainless steels.
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.
Stainless steel grades
There are a number of different 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
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-wide range of electropolished and heat colored surfaces
