Stainless steel

In metallurgy, stainless steel is defined as an iron-carbon alloy with a minimum of 10.5% chromium content.^[1] The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel (it "stains less", but is not actually stain proof). This material is also called corrosion resistant steel when it is not detailed exactly to its alloy type and grade, particularly in the aviation industry. As such, there are now different and easily accessible grades and surface finishes of stainless steel, to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are everyday cutlery and watch straps.

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

High oxidation resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments.^[2] The chromium forms a passivation layer of chromium(III) oxide (Cr₂O₃) when exposed to oxygen. The layer is too thin to be visible, which means that the metal remains lustrous. It is, however, impervious to water and air, protecting the metal beneath. Also, this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off causing the parts to weld together. When disassembled, the welded material may be torn and pitted, an effect that is known as galling.

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

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Commercial value of stainless steel

Stainless steel's resistance to corrosion and staining, low maintenance, relative inexpense, and familiar luster make it an ideal base material for a host of commercial applications. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is milled into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, a structural alloy in automotive and aerospace assembly and building material in skyscrapers and other large buildings.

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

Stainless steel is 100% recyclable. In fact, an average stainless steel object is composed of about 60% recycled material, 25% originating from end-of-life products and 35% coming from manufacturing processes.^[4]

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 non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are used as razor blades, cutlery, tools etc.

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

Stainless steels are also classified by their crystalline structure:

• Austenitic, or 300 series, stainless steels comprise 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. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless is often used in flatware. Similarly 18/0 and 18/8 is also available. “Superaustenitic” stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series. The higher alloy content of "Superaustenitic" steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications.^{[citation needed]}

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

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

• Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel. There is a rising trend in defense budgets to opt for an ultra-high-strength stainless steel if possible in new projects as it is estimated that 2% of the U.S. GDP is spent dealing with corrosion. The Lockheed-Martin JSF is the first aircraft to use a precipitation hardenable stainless steel – Carpenter Custom 465 – in its airframe.

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

Comparison of standardized steels

Stainless steel grades

• 200 Series—austenitic chromium-nickel-manganese alloys
  • Type 201—austenitic that is hardenable through cold working
  • Type 202—austenitic general purpose stainless steel
• 300 Series—austenitic chromium-nickel alloys
  • Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
  • Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
  • Type 303—easier machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with International Organization for Standardization ISO 3506.^[5]
  • Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with International Organization for Standardization ISO 3506.^[5]
  • Type 304L—the 304 grade but specially modified for welding.
  • Type 309— better temperature resistance than 304
  • Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. 316 steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination. It is also known as "marine grade" stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most watches that are made of stainless steel are made of Type 316L; Rolex is an exception in that they use Type 904L. Also referred to as "A4" in accordance with International Organization for Standardization ISO 3506.^[5]
  • Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.
• 400 Series—ferritic and martensitic chromium alloys
  • Type 405—a ferritic expecially made for welding applications
  • Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
  • Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
  • Type 410—martensitic (high-strength iron/chromium). Wear resistant, but less corrosion resistant.
  • Type 416— easy to machine due to additional sulfur
  • Type 420—"Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel". Excellent polishability.
  • Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
  • Type 440—a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly. It can be hardened to around Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as "razor blade steel". Available in four grades 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered a more desirable choice in knifemaking than 440A except for diving or other salt-water applications.
  • Type 446—For elevated temperature service
• 500 Series—heat resisting chromium alloys

• 600 Series—martensitic precipitation hardening alloys
  • Type 630—most common PH stainless, better known as 17-4; 17% chromium, 4% nickel

Stainless steel finishes

  Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (scale) is removed by pickling, and the passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.

• No. 0 - Hot Rolled Annealed, thicker plates
• No. 1 - Hot rolled, annealed and passivated
• No, 2D - cold rolled, annealed, pickled and passivated
• No, 2B - same as above with additional pass through polished rollers
• No, 2BA - Bright Anealed (BA) same as above with highly polished rollers
• No. 3 - coarse abrasive finish applied mechanically
• No. 4 - brushed finish
• No. 6 - matte finish
• No. 7 - reflective finish
• No. 8 - mirror finish
• No. _ - bead blast finish

History

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

The corrosion resistance of iron-chromium alloys was first recognized in 1821 by the French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. However, the metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be of practical interest.

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

In Germany, Friedrich Krupp Germaniawerft built the 366-ton sailing-yacht "Germania" featuring a chrome-nickel steel hull in 1908.^[7] In 1911, Philip Monnartz reported on the relationship between the chromium content and corrosion resistance. On October 17 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel.^[8]

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

However Harry Brearley of the Firth-Brown research laboratory in Sheffield, England is most commonly credited as the "inventor" of stainless steel, but many historians feel this is disputable.^{[citation needed]} In 1913, while seeking an erosion-resistant alloy for gun barrels, he discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was popularized a few years later, in a January 1915 newspaper article in the New York Times.^[6]

Uses in sculpture, building facades and building structures

• Stainless steel was particularly in vogue during the art deco period. The most famous example of this is the upper portion of the Chrysler Building (illustrated above). Diners and fast food restaurants feature large ornamental panels, stainless fixtures and furniture. Owing to the durability of the material, many of these buildings still retain their original and spectacular appearance.
• In recent years the forging of stainless steel has given rise to a fresh approach to architectural blacksmithing. The work of Giusseppe Lund illustrates this well. [1]
• Also pictured above, the Gateway Arch is clad entirely in stainless steel: 886 Tons (804 metric tonnes) of 1/4" (6.3 mm) plate, #3 Finish, Type 304. [2]
• 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. [3]
• The Parliament House of Australia, in Canberra, Australia, has a stainless steel flagpole weighing over 220 tonnes.
• Stainless Steel is the fourth common material used in metal wall tiles, and is used for its corrosion resistance properties in kitchens and bathrooms. [4]
• The aeration building in the Edmonton Composting Facility, the size of 14 NHL hockey rinks, is the largest stainless steel building in North America. [5]
• The United States Air Force Memorial has an austenitic stainless steel structural skin.

See also

• AISI steel grades
• Budd Company – Historically notable user of stainless steel
• Surface finishing
• Architectural steel

References