Austenitic stainless steel is one of the five families of stainless steel (along with ferritic, martensitic, duplex and precipitation hardened).[1] Its primary crystalline structure is austenite (face-centered cubic). Such steels are not hardenable by heat treatment and are essentially non-magnetic.[2] This structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen.[citation needed] The Incoloy family of alloys belong to the category of super austenitic stainless steels.[3]
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During World War II, the Schaeffler diagram was invented by Anton, who was then a budding metallurgist in the employ of two American welding electrode manufacturers, Harnischfeger Company and A.O. Smith Corporation.[4]
Austenitic stainless steels are divided into 300-series and 200-series subgroups. In 300 series stainless steels the austenitic structure obtained primarily by adding nickel. In 200 series stainless steels the structure is obtained by adding manganese and nitrogen, with a small amount of nickel content, making 200 series a cost-effective nickel-chromium austenitic type stainless steel.
300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type 304, also known as 18/8 or A2. Type 304 is extensively used in such items as cookware, cutlery, and kitchen equipment. Type 316, also known as A4, is the next most common austenitic stainless steel. Some 300 series, such as Type 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attack (e.g. pitting and crevice corrosion).
Average content by weight (%) of the major alloying elements of most common Cr-Ni austenitic stainless steel grades[5] Euronorm (EN) number[6] EN designation AISI grade[7] C Cr Mo Ni Others Melts at[8] Remark 1. X10CrNi18-8 301 0.10 17.5 NS 8 NS For springs 1. X5CrNi18-10 304 < 0.07 18.5 NS 9 NS A very common austenitic stainless steel grade 1. X2CrNi18-9 304L < 0.030 18.5 NS 9 NS Similar to the above but not susceptible to intergranular corrosion thanks to a lower C content. 1. X8CrNiS18-9 e 303 < 0.10 18 NS 9 0.3 Sulphur is added to improve machinability. 1. X6CrNiTi18-10 321 < 0.08 18 NS 10.5 Ti: 5×C ≤ 0.70 Same as grade 1. but not susceptible to intergranular corrosion thanks to Ti which "traps" C. 1. X5CrNiMo17-12-2 316 < 0.07 17.5 2.2 11.5 NS Second best known austenitic grade. Mo increases the corrosion resistance. 1. X2CrNiMo17-12-2 316L < 0.030 17.5 2.25 11.5 NS Same as above but not susceptible to intergranular corrosion thanks to a lower C content. 1. X6CrNiMoTi17-12-2 316Ti < 0.08 17.5 2.25 12 Ti: 5×C ≤ 0.70The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series.[9]
Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless. This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid. Alloy 20 has excellent mechanical properties and the presence of niobium in the alloy minimizes the precipitation of carbides during welding.
Heat resisting grades can be used at elevated temperatures, usually above 600 °C (1,100 °F).[10][11]
They must resist corrosion (usually oxidation) and retain mechanical properties, mostly strength (yield stress) and creep resistance.
Corrosion resistance is mostly provided by chromium, with additions of silicon and aluminium. Nickel does not resist well in sulphur containing environments. This is usually taken care of by adding more Si and Al which form very stable oxides. Rare earth elements such as cerium increase the stability of the oxide film.
Typical composition of the major grades EN EN designation AISI/ASTM UNS C Cr Ni Si Mn Others 1. X8CrNiTi18-10 321H S < 0.1 18 10.5 - - Ti: ≤ 5×C 1. X6CrNiSiNCe19-10 - S 0.06 19 10 - - N: 0.16; Ce: 0.05. 1. X15CrNiSi20-12 309 - < 0.2 20 12 2.0 - - 1. X12CrNi23-13 309S S < 0.08 23 13 < 0.75 - - 1. X25CrMnNiN25-9-7 - - 0.25 25 7 - 9 - 1. X15CrNi25-21 310S S < 0.1 25 20 - - - 1. X15CrNiSi25-21 314 S < 0.15 25 20 1.8 - - 1. X10NiCrAITi32-20 "Alloy 800" N < 0.12 21 32 - - Al: 0.4; Ti: 0.4 1. X6NiCrSiNCe35-25 "Alloy 353MA" S 0.06 25 35 - - N: 0.15; Ce: 0.06. 1. X12NiCrSi35-16 330 N < 0.15 18.5 35 - -Type 309 and 310[12] are used in high temperature applications greater than 800 °C (1,500 °F).
Note: ferritic stainless steels do not retain strength at elevated temperatures and are not used when strength is required.
Austenitic stainless steel can be tested by nondestructive testing using the dye penetrant inspection method but not the magnetic particle inspection method. Eddy-current testing may also be used.
Grade EN 1. (also known as A286) is not considered strictly as a heat resisting steel in standards, but this is popular grade for its combination of strength and corrosion resistance.[13][14][15]
Typical composition EN No. EN designation AISI/ASTM UNS C Cr Ni Mo Others 1. X6NiCrTiMoVB25-15-2 660 S 0.05 15 25 1.25 V: 0.3; Ti: 2.0; B: 0.006. Minimum mechanical properties Condition Yield stress, min Ultimate tensile strength, min Elongation, min (%) Solution treated and aged 590 MPa (86 ksi) 900 MPa (130 ksi) 13It is used for service temperatures up to 700 °C (1,300 °F) in applications such as:
Stainless steel is commonly found in many industries and environments; from the workstations and utensils of industrial kitchens to medical devices and automotive applications.
But like all metal alloys, there are multiple types and grades of stainless steel. Stainless steel can be graded, above the series of alloys e.g. 300, 400, etc., by its chemical composition. These overarching groups are known as Austenitic, Martensitic and Ferritic. In this article, we’ll explore Austenitic Stainless Steel in more detail.
Because of the alloying elements, Austenitic stainless steels are Face-Centered-Cubic in atom structure, meaning they are very tough, with plenty of ductility to be bent and formed. Austenitic stainless steels are high in chromium and nickel.
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With a Body-Centered-Cubic atomic structure, Martensitic stainless steels are much harder, with much less malleability. This makes them ideal for utensils such as knives and scrapers, where a sharp edge is required.
Thanks to its crystal structure, Austenitic stainless steel is relatively malleable and ductile. This means it is suited well to make production manufacturing techniques, such as bending, shearing, stamping and punching.
Whilst stainless steel is rather more expensive than mild steel, the ease of working, coupled with the fact it does not need to be protected from corrosion, means it can be cheaper than some alternatives.
To find out how much the stainless steel will be for your project, check out the grades we stock here.
316 grade stainless steel has a tensile strength of around 550 MPa, this is approximately equivalent to that of S355 grade mild steel, a medium strength steel used in structural applications.
This means stainless steel can be used for structures and other high stress components in place of steel, but with better corrosion resistance.
As with most modern materials, there are many different grades of stainless steel, termed series’, these range from 200 up to 400, with many sub-grades in-between such as 202, 304, 430, etc.
The grades that are Austenitic are those in the 200 and 300 series, this includes the most commonly used 304, and 316 grades.
Thanks to its great workability, excellent corrosion resistance and ease of manufacture, Austenitic stainless steel is found in all walks of life.
In medical settings, it is used to make trays, counter tops and trolleys, as well as components in medical devices such as staple guns and other hand tools.
There are many uses in the automotive world too, with many small components such as hose clamps and safety critical components such as housings for airbag mechanisms and such. In the performance world stainless steel exhausts are commonly made from 316 grade stainless steel.
There are hundreds of applications for stainless steel in aerospace too, thanks to its resistance to corrosion and low temperatures, it is often used for pipework, bracketry and many other applications.
Stainless steel is relatively easy to manufacture but needs some extra considerations when compared to mild steel
Stainless steels in general are made in similar ways to mild steel, with electric arc furnaces providing the heat to melt the raw materials, but stainless steel does differ in that a process called Argon Oxygen Decarburization is performed after the refining process.
AOD can be split into three steps, Recarburization, reduction Desulfurization. These three processes remove carbon, retrieve alloying elements such as chromium, and then remove sulfur, respectively.
After the molten stainless steel is produced with the correct alloying elements, the material is transported to different production processes depending on the end material shape; pressure or continuous casting is used to produce sheet and strip, and a slabbing mill is used to produce bars, sections and structural shapes.
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