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Austenitic stainless steel

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

Austenitic stainless steel is one of the five classes of stainless steel by crystalline structure (along with ferritic, martensitic, duplex and precipitation hardened[1]). Its primary crystalline structure is austenite (face-centered cubic) and it prevents steels from being hardenable by heat treatment and makes them 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]

History

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Schaeffler diagram

During World War 2 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]

AISI 200 and 300 series

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ASSs 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
1.4310 X10CrNi18-8 301 0.10 17.5 NS 8 NS 1420 For springs
1.4301 X5CrNi18-10 304 < 0.07 18.5 NS 9 NS 1450 A very common austenitic stainless steel grade
1.4307 X2CrNi18-9 304L < 0.030 18.5 NS 9 NS 1450 Similar to the 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 1420 Sulphur is added to improve machinability.
1.4541 X6CrNiTi18-10 321 < 0.08 18 NS 10.5 Ti: 5×C ≤ 0.70 1425 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 1400 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 1400 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

The 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 austenitic stainless steels

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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
1.4878 X8CrNiTi18-10 321H S32109 < 0.1 18 10.5 - - Ti: ≤ 5×C
1.4818 X6CrNiSiNCe19-10 - S30415 0.06 19 10 - - N: 0.16; Ce: 0.05.
1.4828 X15CrNiSi20-12 309 - < 0.2 20 12 2.0 - -
1.4833 X12CrNi23-13 309S S30908 < 0.08 23 13 < 0.75 - -
1.4872 X25CrMnNiN25-9-7 - - 0.25 25 7 - 9 -
1.4845 X15CrNi25-21 310S S31008 < 0.1 25 20 - - -
1.4841 X15CrNiSi25-21 314 S31400 < 0.15 25 20 1.8 - -
1.4876 X10NiCrAITi32-20 "Alloy 800" N08800 < 0.12 21 32 - - Al: 0.4; Ti: 0.4
1.4854 X6NiCrSiNCe35-25 "Alloy 353MA" S35315 0.06 25 35 - - N: 0.15; Ce: 0.06.
1.4886 X12NiCrSi35-16 330 N08330 < 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.

Precipitation Hardening grade EN 1.4980

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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.[13][14][15]

Typical composition
EN No. EN designation AISI/ASTM UNS C Cr Ni Mo Others
1.4980 X6NiCrTiMoVB25-15-2 660 S66286 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) 13

It is used for service temperatures up to 700 °C (1,300 °F) 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

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References

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  1. ^ The International Nickel Company (1974). "Standard Wrought Austenitic Stainless Steels". Nickel Institute. Archived from the original on 2018-01-09. Retrieved 2018-01-09.
  2. ^ "Stainless Steel". Encyclopaedia Britannica. 21 June 2024.
  3. ^ "Super Alloy INCOLOY Alloy 800 (UNS N08800)". AZoNetwork. 3 July 2013.
  4. ^ Guiraldenq, Pierre; Hardouin Duparc, Olivier (2017). "The genesis of the Schaeffler diagram in the history of stainless steel". Metallurgical Research & Technology. 114 (6): 613. Bibcode:2017MetRT.114..613G. doi:10.1051/metal/2017059.
  5. ^ "EN Standard: Stainless Steels -List of stainless steels".
  6. ^ European Committee for Standardization -. "Materials".
  7. ^ "American Iron and Steel Institute".
  8. ^ "Stainless steel melting points". Thyssenkrupp Materials (UK) Ltd. Retrieved 28 May 2022.
  9. ^ American Iron and Steel Institute. "Design Guidelines for the Selection and Use of Stainless Steels". Nickel Institute. Retrieved 2018-01-09.
  10. ^ M, Rouby (1990). Lacombe, P (ed.). Les aciers inoxydables. Les Editions de Physique. pp. Chapter 26. ISBN 2-86883-142-7.
  11. ^ "EN 10088-1 Standard : Stainless steels Part1: List of stainless steels".
  12. ^ "310 310S Stainless Steel". TubingChina.com Stainless Steel Directory. Retrieved 2015-09-18.
  13. ^ "Matweb data base".
  14. ^ "Aubert&Duval Datasheet" (PDF).
  15. ^ "Aircraftmaterials Datasheet".
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