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.[1] 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.[2]

Convergent beam electron diffraction (CBED) transmission electron micrograph of a [111] zone axis of austenitic stainless steel

200 and 300 series

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).

Average content of the major alloying elements of most common Cr-Ni austenitic stainless steel grades[3]
EN* number EN designation AISI** grade C Cr Mo Ni Others Remark
1.4310 X10CrNi18-8 301 0.10 17.5 ns 8 ns For springs
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

*EN: Euro Norm (European Standards)[4]

**AISI: American Iron and Steel Institute[5]

ns: not specified

The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series.[6]

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 steels

Heat resisting grades can be used at elevated temperatures, usually above 600 °C.[7][8]

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.

Typical composition of the major grades
EN № AISI/ASTM UNS C Cr Ni Si Mn Others
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.
1.4828 - - < 0.2 20 12 2.0 - -
1.4833 309S S30908 < 0.08 23 13 < 0.75 - -
1.4872 - - 0.25 25 7 - 9 -
1.4845 310 S31008 < 0.1 25 20 - - -
1.4841 314 S31400 < 0.15 25 20 1.8 - -
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.
1.4886 330 N08330 < 0.15 18.5 35 - -

Type 309 and 310,[9] are utilized in high temperature applications greater than 800 °C.

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.

Precipitation Hardening grade EN 1.4980

Grade EN 1.4980 (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.[10][11][12]

Typical composition
EN № AISI/ASTM UNS C Cr Ni Mo Others
1.4980 660 S66286 0.05 15 25 1.25 V: 0.3; Ti: 2.0; B: 0.006.
Minimum mechanical properties
Condition Yield stress, min. (MPa) Ultimate tensile strength, min. (MPa) Elongation, min. (%)
Solution treated and aged 590 900 13

It is used for service temperatures up to 700 °C in applications such as:

  • aerospace (standardized in AMS 5731, AMS 5732, AMS 5737 and AMS 5525 standards),
  • industrial gas turbines,
  • automotive (turbo parts), etc.

See also

References
  1. The International Nickel Company (1974). "Standard Wrought Austenitic Stainless Steels". Nickel Institute.
  2. "Stainless Steel". Encyclopaedia Britannica.
  3. "EN Standard: Stainless Steels -List of stainless steels".
  4. European Committee for Standardization -. "Materials".
  5. "American Iron and Steel Institute".
  6. American Iron and Steel Institute. "Design Guidelines for the Selection and Use of Stainless Steels". Nickel Institute.
  7. M, Rouby (1990). Lacombe, P (ed.). Les aciers inoxydables. Les Editions de Physique. pp. Chapter 26. ISBN 2-86883-142-7.
  8. "EN 10088-1 Standard : Stainless steels Part1: List of stainless steels".
  9. "310 310S Stainless Steel". TubingChina.com Stainless Steel Directory. Retrieved 2015-09-18.
  10. "Matweb data base".
  11. "Aubert&Duval Datasheet" (PDF).
  12. "Aircraftmaterials Datasheet".

Austenitic stainless steels Jean H. Decroix et al.

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