What is stainless steel?
‘Stainless’ is a term coined early in the development of these steels for cutlery applications. It was adopted as a generic name for these steels and now covers a wide range of steel types and grades for corrosion or oxidation resistant applications.
Stainless steels are iron alloys with a minimum of 10.5% chromium. Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness.
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.
The main elements in stainless steel
Stainless steel or corrosion-resistant steel is a kind of metallic alloy that is found in a variety of forms. It serves our practical needs so well that it is difficult to find any sphere of our life, where we do not use this type of steel. The major components of stainless steel are: iron, chromium, carbon, nickel, molybdenum and small quantities of other metals.
These include metals such as:
- Nickel
- Molybdenum
- Titanium
- Copper
Non-metal additions are also made, the main ones being:
- Carbon
- Nitrogen
CHROMIUM AND NICKEL:
Chromium is the element that makes stainless steel stainless. It is essential in forming the passive film. Other elements can influence the effectiveness of chromium in forming or maintaining the film, but no other element by itself can create the properties of stainless steel.
At about 10.5% chromium, a weak film is formed and will provide mild atmospheric protection. By increasing the chromium to 17-20%, which is typical in the type-300 series of austenitic stainless steels, the stability of the passive film is increased. Further increases in the chromium content will provide additional protection.
|
Symbol |
Element |
| Al | Aluminum |
| C | Carbon |
| Cr | Chromium |
| Cu | Copper |
| Fe | Iron |
| Mo | Molybdenum |
| Mn | Manganese |
| N | Nitrogen |
| Ni | Nickel |
| P | Phosphorous |
| S | Sulfur |
| Se | Selenium |
| Ta | Tantalum |
| Ti | Titanium |
Nickel will stabilize the austenitic structure (the grain or crystal structure) of the stainless steel and enhance the mechanical properties and fabrication characteristics. A nickel content of 8-10% and above will decrease the tendency of the metal to crack due to stress corrosion. Nickel also promotes repassivation in case the film is damaged.
MANGANESE:
Manganese, in association with nickel, performs many of the functions attributed to nickel. It will also interact with the sulfur in stainless steel to form manganese sulfites, which increases the resistance to pitting corrosion. By substituting manganese for nickel, and then combining it with nitrogen, strength is also increased.
MOLYBDENUM:
Molybdenum, in combination with chromium, is very effective in stabilizing the passive film in the presence of chlorides. It is effective in preventing crevice or pitting corrosion. Molybdenum, next to chromium, provides the largest increase in corrosion resistance in stainless steel. Edstrom Industries uses 316 stainless because it contains 2-3% molybdenum, which gives protection when chlorine is added to the water.
CARBON:
Carbon is used to increase strength. In the martensitic grade, the addition of carbon facilitates hardening through heat-treating.
NITROGEN:
Nitrogen is used to stabilize the austenitic structure of stainless steel, which enhances its resistance to pitting corrosion and strengthens the steel. Using nitrogen makes it possible to increase the molybdenum content up to 6%, which improves corrosion resistance in chloride environments.
TITANIUM AND MIOBIUM:
Titanium and Miobium are used to reduce the sensitization of stainless steel. When stainless steel is sensitized, intergranular corrosion can occur. This is caused by the precipitation of chrome carbides during the cooling phase when parts are welded. This depletes the weld area of chromium. Without the chromium, the passive film cannot form. Titanium and Niobium interact with carbon to form carbides, leaving the chromium in solution so a passive film can form.
COPPER AND ALUMINUM:
Copper and Aluminum, along with Titanium, can be added to stainless steel to precipitate its hardening. Hardening is achieved by soaking at a temperature of 900 to 1150F. These elements form a hard intermetallic microstructure during the soaking process at the elevated temperature.
SULFUR AND SELENIUM:
Sulfur and Selenium are added to 304 stainless to make it machine freely. This becomes 303 or 303SE stainless steel, which is used by Edstrom Industries to make hog valves, nuts, and parts that are not exposed to drinking water.
Types of stainless steel
THE AISI DEFINES THE FOLLOWING GRADES AMONG OTHERS:
Also known as “marine grade” stainless steel due to its increased ability to resist saltwater corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants.
304/304L STAINLESS STEEL
Type 304 has slightly lower strength than 302 due to its lower carbon content.
316/316L STAINLESS STEEL
Type 316/316L Stainless Steel is a molybdenum steel possessing improved resistance to pitting by solutions containing chlorides and other halides.
310S STAINLESS STEEL
310S Stainless Steel has excellent resistance to oxidation under constant temperatures to 2000°F.
317L STAINLESS STEEL
317L is a molybdenum bearing austenitic chromium nickel steel similar to type 316, except the alloy content in 317L is somewhat higher.
321/321H STAINLESS STEEL
Type 321 is basic type 304 modified by adding titanium in an amount at least 5 times the carbon plus nitrogen contents.
410 STAINLESS STEEL
Type 410 is a martensitic stainless steel which is magnetic, resists corrosion in mild environents and has fairly good ductility.
DUPLEX 2205 (UNS S31803)
Duplex 2205 (UNS S31803), or Avesta Sheffield 2205 is a ferritic-austenitic stainless steel.
STAINLESS STEELS ARE ALSO CLASSIFIED BY THEIR CRYSTALLINE STRUCTURE:
- Austenitic 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 is 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 means they are fearsomely expensive and similar performance can usually be achieved using duplex steels at much lower cost.
- Ferritic stainless steels are highly corrosion resistant, but far lessdurable than austenitic grades and cannot be hardened by heat treatment. 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%), no 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.
- 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 60:40. 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 and lower nickel contents than austenitic stainless steels.
History of Stainless Steel
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 19041911, several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel. In 1911, Philip Monnartz of Germany reported on the relationship between the chromium content and corrosion resistance of these alloys.
Harry Brearley of the Brown-Firth research laboratory in Sheffield, England is most 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. However, similar industrial developments were taking place contemporaneously at the Krupp Iron Works in Germany, where Eduard Maurer and Benno Strauss were developing an austenitic alloy (21% chromium, 7% nickel), and in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless.
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Post time: Jun-16-2022