Austenitic stainless steel
Austenitic stainless steel is a specific type of stainless steel alloy. Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic and duplex. Austenitic stainless steels possess austenite as their primary crystalline structure (face-centered cubic). This austenite crystalline structure is achieved by sufficient additions of the austenite stabilizing elements nickel, manganese and nitrogen. Their crystalline structure prevents austenitic steels from being hardenable by heat treatment and makes them essentially non-magnetic.
200 and 300 seriesEdit
There are two subgroups of austenitic stainless steel. 300 series stainless steels achieve their austenitic structure primarily by a nickel addition while 200 series stainless steels substitute manganese and nitrogen for nickel, though there is still a small nickel content.
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 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).
|EN* number||EN designation||AISI** grade||C||Cr||Mo||Ni||Others||Remark|
|1.4301||X5CrNi18-10||304||< 0.07||18.5||ns||9||ns||A very common austenitic stainless steel grade|
|1.4307||X2CrNi18-9||304L||< 0.030||18.5||ns||9||ns||Same as above but not susceptible to intergranular corrosion thanks to a lower C content.|
|1.4305||X8CrNiS18-9 e||303||< 0.10||18||ns||9||0.3||Sulphur is added to improve machinability.|
|1.4541||X6CrNiTi18-10||321||< 0.08||18||ns||10.5||Ti: 5×C ≤ 0.70||Same as grade 1.4301 but not susceptible to intergranular corrosion thanks to Ti which "traps" C.|
|1.4401||X5CrNiMo17-12-2||316||< 0.07||17.5||2.2||11.5||ns||Second best known austenitic grade. Mo increases the corrosion resistance.|
|1.4404||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.|
|1.4571||X6CrNiMoTi17-12-2||316Ti||< 0.08||17.5||2.25||12||Ti: 5×C ≤ 0.70|
**AISI: American Iron and Steel Institute
ns: not specified
The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series.
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 austenitic stainless steelsEdit
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/or 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.
|1.4878||321H||S32109||< 0.1||18||10.5||-||-||Ti: ≤ 5×C|
|1.4818||-||S30415||0.06||19||10||-||-||N: 0.16; Ce: 0.05.|
|1.4833||309S||S30908||< 0.08||23||13||< 0.75||-||-|
|1.4876||"Alloy 800"||N08800||< 0.12||21||32||-||-||Al: 0.4; Ti: 0.4|
|1.4854||"Alloy 353MA"||S35315||0.06||25||35||-||-||N: 0.15; Ce: 0.06.|
Note: ferritic stainless steels do not retain strength at elevated temperatures and are not used when strength is required.
Precipitation Hardening grade EN 1.4980Edit
|1.4980||660||S66286||0.05||15||25||1.25||V: 0.3; Ti: 2.0; B: 0.006.|
|Condition||Yield stress, min. (MPa)||Ultimate tensile strength, min. (MPa)||Elongation, min. (%)|
|Solution treated and aged||590||900||13|
It is used for service up to temperatures up to 700 °C in:
- aerospace (standardized in AMS 5731, AMS 5732, AMS 5737 and AMS 5525 standards),
- industrial gas turbines,
- automotive (turbo parts), etc.
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