Everything about Stainless Steel totally explained
In
metallurgy,
stainless steel is defined as an iron-carbon
alloy with a minimum of 11.5 wt%
chromium content. Stainless steel doesn't stain, corrode or rust as easily as ordinary steel (it "stains less"), but it isn't stain-proof. It is also called
corrosion resistant steel when the alloy type and grade are not detailed, particularly in the aviation industry. There are different 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
cutlery and
watch straps.
Stainless steel differs from carbon steel by amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amount of chromium present so that a passive film of chromium oxide forms which prevents further corrosion.
High oxidation-resistance in
air at ambient
temperature are 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 (Cr
2O
3) 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's known as
galling. This destructive galling can be best avoided by the use of dissimilar materials, for example bronze to stainless steel, or even different types of stainless steels (
martensitic against
austenitic, etc.), when metal-to-metal wear is a concern. In addition, Nitronic alloys (trademark of Armco, Inc.) reduce the tendency to gall through selective alloying with manganese and nitrogen.
Nickel also contributes to passivation, as do other less commonly used ingredients such as
molybdenum and
vanadium.
Commercial value of stainless steel
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 coils, 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.
Orange juice tankers (sometimes also other chemical tankers) often have their tanks made of stainless steel.
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, won't 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.
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 are 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.
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, 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.
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%), nickel (0-<2%), and carbon (about 0.1-1%) (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's estimated that 2% of the U.S. GDP is spent dealing with corrosion. The Lockheed-Martin Joint Strike Fighter 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 steels 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
| EN-standardSteel no. k.h.s
DIN
|
EN-standardSteel name
|
ASTM/AISISteel type
|
UNS |
| |
|
440A |
S44002 |
| 1.4112 |
|
440B |
S44004 |
| 1.4125 |
|
440C |
S44003 |
| |
|
440F |
S44020 |
| 1.4016 |
X6Cr17 |
430 |
S43000 |
| 1.4512 |
X6CrTi12 |
409 |
S40900 |
| |
|
410 |
S41000 |
| 1.4310 |
X10CrNi18-8 |
301 |
S30100 |
| 1.4318 |
X2CrNiN18-7 |
301LN |
N/A |
| 1.4307 |
X2CrNi18-9 |
304L |
S30403 |
| 1.4306 |
X2CrNi19-11 |
304L |
S30403 |
| 1.4311 |
X2CrNiN18-10 |
304LN |
S30453 |
| 1.4301 |
X5CrNi18-10 |
304 |
S30400 |
| 1.4948 |
X6CrNi18-11 |
304H |
S30409 |
| 1.4303 |
X5CrNi18 12 |
305 |
S30500 |
| 1.4541 |
X6CrNiTi18-10 |
321 |
S32100 |
| 1.4878 |
X12CrNiTi18-9 |
321H |
S32109 |
| 1.4404 |
X2CrNiMo17-12-2 |
316L |
S31603 |
| 1.4401 |
X5CrNiMo17-12-2 |
316 |
S31600 |
| 1.4406 |
X2CrNiMoN17-12-2 |
316LN |
S31653 |
| 1.4432 |
X2CrNiMo17-12-3 |
316L |
S31603 |
| 1.4435 |
X2CrNiMo18-14-3 |
316L |
S31603 |
| 1.4436 |
X3CrNiMo17-13-3 |
316 |
S31600 |
| 1.4571 |
X6CrNiMoTi17-12-2 |
316Ti |
S31635 |
| 1.4429 |
X2CrNiMoN17-13-3 |
316LN |
S31653 |
| 1.4438 |
X2CrNiMo18-15-4 |
317L |
S31703 |
| 1.4539 |
X1NiCrMoCu25-20-5 |
904L |
N08904 |
| 1.4547 |
X1CrNiMoCuN20-18-7 |
N/A |
S31254 |
Stainless steel grades
100 Series—austenitic chromium-nickel-manganese alloys
- Type 101—austenitic that's hardenable through cold working for furniture
- Type 102—austenitic general purpose stainless steel working for furniture
200 Series—austenitic chromium-nickel-manganese alloys
- Type 201—austenitic that's 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—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.
- Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO 3506.]]
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. In 1911, Philip Monnartz reported on the relationship between the chromium content and corrosion resistance. On October 17, 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel.
Similar industrial developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless.
Harry Brearley of the Firth-Brown research laboratory in Sheffield, England is commonly credited as the "inventor" of stainless steel. In 1913, while seeking an erosion-resistant alloy for gun barrels, he 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. 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.
- Also pictured above, the Gateway Arch is clad entirely in stainless steel: 886 tons (804 metric tonnes) of ¼″ (6.3 mm) plate, #3 Finish, Type 304. (External Link
)
- 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. (External Link)
- The Parliament House of Australia in Canberra has a stainless steel flagpole weighing over 220 tonnes.
- The aeration building in the Edmonton Composting Facility, the size of 14 NHL hockey rinks, is the largest stainless steel building in North America. (External Link)
- The United States Air Force Memorial has an austenitic stainless steel structural skin.
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