What is Stainless Steel?
First and foremost stainless steel's great claim to fame is its ability to resist corrosion.
Stainless Steel is an alloy of iron with at least 10.5% chromium by weight. It can have a lot more, as much as 30%.
Information about Chromium from the US Geological Survey.
In order to modify various properties such as ability to machine, brittleness, strength, resistance to corrosion, other substances are added to the alloy. Varying quantities of Nickel, manganese and carbon are often present. Specialty alloys also contain molybdenum, tungsten, nitrogen, titanium, niobium, zirconium, cerium, sulphur, silicon and copper.
Each of these other substances bring qualities to the stainless steel at the expense of other characteristics.
We think of Stainless Steel as being able to resist rusting and tarnishing, but many alloys of SS have been developed to resist highly corrosive environments such as combustion chambers or chemical holding containers, handling equipment and pipes, often in high temperature environment.
How does Stainless steel resist corrosion?
When chromium is exposed to oxygen, an invisible layer of chromium oxide (Cr2O3) is formed on the surface of the metal. This is known as the "oxide skin". This oxide skin protects the metal against further rusting/oxidation. If oxide skin gets damaged, a new layer of oxide gets formed when the chromium is exposed to air (which contains oxygen).
What do the colours often painted on the end of metals stock including Stainless steel mean?
Nothing looks more like a piece of stainless steel than another piece of stainless steel. In order to be able to identify visually a specific alloy, most metal suppliers will paint the end of the stock and sometimes also the middle with a distinctive colour. Since there is no standard for colour coding, if you are looking to identify a specific alloy you need to know who the supplier is and look up the colour in their supplied charts.
What are the numbers used after the name STAINLESS STEEL
These numbers indicate specific alloys which are classified according to their crystal structure, and which share similar characteristics. More further down.
The development of Stainless steel is a relatively recent event. It was recognized at the beginning of the 1800 and was subject to a great deal of research and development particularly in the early 1900's. By the 1930's it was being manufactured in relatively large amounts.
In the early 1900's the research and development was fast and furious and there was quite a lot of competition. There are many claims of who "discovered" stainless steel first. It looks more like it was developped and improved gradually by several researchers, in several countries notably France, England and Germany, then in the US, over a period of about 100 years.
- 1820- Stoddard and Farraday, 2 English scientists noted the resistance of iron-chromium alloys to some acids.
- 1821- Pierre Berthier, a French metallurgist noted SS's. acid resistance and suggested a use in cutlery.
- 1872- Clark and Woods, 2 English researchers patented a weather and acid resistant SS. alloy.
- 1830- Brustlein, a French researcher noted the importance of low carbon content chromium.
- 1895- Goldschmidt, a German developer this time, was able to produce carbon free chromium
- 1904- Leon Guillet did extensive research on iron-chromium alloys and published research on what we would consider the 400 series today.
- 1908- Friedrich Krupp Germaniawerft built a yacht Germania with a chrome-nickel steel hull
- 1909- Giesen, in England published research results on what we would consider the 300 series.
- 1911- Monnartz and Borchers, Germans, noted that superior corrosion characteristics occured when the chromium content was at least 10.5%.
- 1912- Harry Brearley produced an allow which is considered by some to be the first real SS. He was doing research on ways of avoiding the erosion of gun barrels.
- 1912- Benno Strauss and Eduard Maurer patented stainless steel Nirosta
- 1912- Christian Dantsizen and Frederick Becket, Americans, were producing SS in industrial quantity
- 1919 Elwood Haynes was granted a patent for a martensitic alloy.
- 1929- SS was being produced in relatively large quantities, notably by John Brown & Co and Thomas Firth & Sons. In the late '20's 25,000 tons were produced, in the US
What are Martensitic, Ferritic and Austenitic Stainless Steel Alloys?
Besides the basic Iron-Chromium, there are different elements added Stainless steel and each bring specific properties. The Crystal Structure is another factor that helps determine the properties of the specific alloy.
Stainless steels can be classified into 3 main types of crystal structures. These are Austenitic, Ferritic and Martensitic.
Austenitic Stainless Steel
These are the most common types of stainless that are generally available.
- Non Magnetic. If it has been work hardened then it can be very slightly magnetic. If annealed then it can revert back to being non magnetic.
- Contain high levels of Chromium and Nickel and low levels of Carbon
- Highest levels of resistance to corrosion
- Easily cleaned
- Can be formed. This means it can be drawn into sheets and wire.It can also be welded and shows toughness, particularly at low temperatures. Low carbon varieties are used when welding is required.
- Have a face-centered cubic crystal structure. In this type of structure, a cube shaped crystal is formed which has an atom at each corner of the cube and another atom in the center of the faces.
- Contain no more than .08% of carbon and cannot be heat treated. Some low grades contain no more than .03% carbon. These low grades are labeled L as in 304L.
- If worked cold, the metal can become harder, stronger and more resistant to stress.
- Heating to 1045 Celcius then quenching will re-soften the metal.
- Low Yield stress and high tensile strength.
- More expensive than ferritic SS.
- Widely known as the 300 series, most common alloys are the iron-chromium-nickel steels.
- 200 series are characterized by low nickel content. They were developed in periods of high priced nickel and are cheaper to produce than 300 series. Nickel prices fluctuate and have gone through periods where the price is quite high, or nickel was not available, and alternative alloys were developed. Lower nickel means lower formability including ductility. Called Chrome-manganese stainless steel. The 200 series show lower corrosion resistance and are not used for difficult environments but find lots of uses where conditions are not hugely difficult.
Uses for Austenitic Steel
There is some overlap for use between the different alloys. These are only some of the uses.
Ferritic Stainless Steel
- These are plain chromium stainless steels usually with a low nickel and carbon content (often no more than .05 carbon and some less.)
- They are magnetic
- Have good mechanical properties. Positioned between other stainless steel families. They are stronger than Austenitic alloys. Elongation and other forming properties are similar to high strength carbon steels.
- Good resistance to corrosion particularly stress corrosion cracking
- Although more difficult to weld, some alloys can be welded.
- Because they contain no or little nickel, they are not subject to the price fluctuations which influence the higher nicklel alloys.
Uses for Ferritic Stainless Steel
- 409: Tubing for auto exhaust and catalytic converter casing
- 430: Kitchen sinks, cutlery and kitchen equipment.
- 441: Specialty alloy with superior mechanical strength at high temperatures. Used in the hot areas of exhaust systems.
- AISI 444: High pitting resistance which makes it suitable for aggressive outdoor environment such as marine coast. Can be welded and bent. Contains Molibdinum and Nitrogen.
- 3CR12: A less expensive weldable, utility SS. Used in mining, material handling and sugar industry. Resistant to atmospheric corrosion and wet abrasive corrosion.
Martensitic Stainless Steel
Martensitic (named for a german scientist, Adolf Martens) is one of the crystalline structures of Stainless Steel. Because it contains more carbon than other grades, .1% to as much as 1% it can have a wide range of properties. Although it has a lower chromium content than most other Stainless alloys (at least 11.5% Chromium), and shows only moderate corrosion resistance, it is a true Stainless Steel.
It has a body centered tetragonal structure. Martensitic Stainless steels share properties with carbon steels. They can be hardened and strengthened by heat treatment. They are "HARD" ferro-magnetic alloys.
When annealed, they can be machined and are ductile. After heat treatment, they become harder but more brittle and loose ductility and toughness.
- Corrosion resistance of martensitic SS is lower than other varieties. They are much more corrosion resistant than just steel though. This makes them perfectly suitable for moderate conditions.
- They are strong and tough and can be machined.
- Because of the carbon content, they can be heat treated, hardened and tempered. In fact, like regular steel, after hardening they are usually tempered otherwise they are too brittle.
- Because of the higher carbon content, they don't weld as well as some other SS alloys.
- Martensitic stainless steel are not produced in as large quantities as the austenitic and ferritic grades. The products they are used for are usually not consumer products. Knives, razor strips and cutting tools are exception
- Alloys are usually classified in the 400 series.
Uses for Martensitic Steel
- Razors, blades and cutting tools including knives
- Surgical Instruments
- Gears, valves, bearings, turbine parts
- Aerospace parts.
- Offshore oil and gas equipment, pumps, shafts
- Mixers and stirrers.
Duplex Stainless Steel
It is possible to produce stainless steel by controlling the cooling, that has a combination of 2 crystal structures. In Duplex Stainless steel both ferritic and austenitic grains can be found.
- About twice as strong as regular austenitic or ferritic alloys. This means that parts can be thinner leading to saved weight and cost.
- Tougher and more ductile than ferritic alloys but not as strong as austenitic Stainless Steel
- Corrosion resistance similar to austenitic alloys. This depends on the composition of the duplex alloy.
- Shows better stress corrosion cracking resistance than the austenitic alloys.
- Stronger than austenitic alloys
- Because of reduced nickel and clybdenum content it is less expensive than similar austenitic alloys with similar properties.
Link to a guide to high-performance alloys published by the Moly Review.
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