Austenitic stainless steel

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Convergent beam electron diffraction (CBED) transmission electron micrograph of a [111] zone axis of austenitic stainless steel Deus Ex Ferro.tif
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]

Contents

AISI 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, 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 [4]
Euronorm (EN) number [5] EN designation AISI grade [6] CCrMoNiOthersMelts at [7] Remark
1.4310X10CrNi18-83010.1017.5NS8NS1420For springs
1.4301X5CrNi18-10304< 0.0718.5NS9NS1450A very common austenitic stainless steel grade
1.4307X2CrNi18-9304L< 0.03018.5NS9NS1450Similar to the above but not susceptible to intergranular corrosion thanks to a lower C content.
1.4305X8CrNiS18-9 e303< 0.1018NS90.31420Sulphur is added to improve machinability.
1.4541X6CrNiTi18-10321< 0.0818NS10.5Ti: 5×C ≤ 0.701425Same as grade 1.4301 but not susceptible to intergranular corrosion thanks to Ti which "traps" C.
1.4401X5CrNiMo17-12-2316< 0.0717.52.211.5NS1400Second best known austenitic grade. Mo increases the corrosion resistance.
1.4404X2CrNiMo17-12-2316L< 0.03017.52.2511.5NS1400Same as above but not susceptible to intergranular corrosion thanks to a lower C content.
1.4571X6CrNiMoTi17-12-2316Ti< 0.0817.52.2512Ti: 5×C ≤ 0.70

The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series. [8]

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 (1,100 °F). [9] [10]

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
ENEN designationAISI/ASTMUNSCCrNiSiMnOthers
1.4878X8CrNiTi18-10321HS32109< 0.11810.5--Ti: ≤ 5×C
1.4818X6CrNiSiNCe19-10-S304150.061910--N: 0.16; Ce: 0.05.
1.4828X15CrNiSi20-12309-< 0.220122.0--
1.4833X12CrNi23-13309SS30908< 0.082313< 0.75--
1.4872X25CrMnNiN25-9-7--0.25257-9-
1.4845X15CrNi25-21310SS31008< 0.12520---
1.4841X15CrNiSi25-21314S31400< 0.1525201.8--
1.4876X10NiCrAITi32-20"Alloy 800"N08800< 0.122132--Al: 0.4; Ti: 0.4
1.4854X6NiCrSiNCe35-25"Alloy 353MA"S353150.062535--N: 0.15; Ce: 0.06.
1.4886X12NiCrSi35-16330N08330< 0.1518.535--

Type 309 and 310 [11] 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

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. [12] [13] [14]

Typical composition
EN No.EN designationAISI/ASTMUNSCCrNiMoOthers
1.4980X6NiCrTiMoVB25-15-2660S662860.0515251.25V: 0.3; Ti: 2.0; B: 0.006.
Minimum mechanical properties
ConditionYield stress, minUltimate tensile strength, minElongation, min (%)
Solution treated and aged590 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:

See also

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Duplex stainless steels are a family of alloys with a two-phase microstructure consisting of both austenitic and ferritic phases. They offer excellent mechanical properties, corrosion resistance, and toughness compared to other types of stainless steel. However, duplex stainless steel can be susceptible to a phenomenon known as 475 °C embrittlement or duplex stainless steel age hardening, which is a type of aging process that causes loss of plasticity in duplex stainless steel when it is heated in the range of 250 to 550 °C. At this temperature range, spontaneous phase separation of the ferrite phase into iron-rich and chromium-rich nanophases occurs, with no change in the mechanical properties of the austenite phase. This type of embrittlement is due to precipitation hardening, which makes the material become brittle and prone to cracking.

References

  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.
  3. ↑ "Super Alloy INCOLOY Alloy 800 (UNS N08800)". AZoNetwork. 3 July 2013.
  4. ↑ "EN Standard: Stainless Steels -List of stainless steels".
  5. ↑ European Committee for Standardization -. "Materials".
  6. ↑ "American Iron and Steel Institute".
  7. ↑ "Stainless steel melting points". Thyssenkrupp Materials (UK) Ltd. Retrieved 28 May 2022.
  8. ↑ American Iron and Steel Institute. "Design Guidelines for the Selection and Use of Stainless Steels". Nickel Institute. Retrieved 2018-01-09.
  9. ↑ M, Rouby (1990). Lacombe, P (ed.). Les aciers inoxydables. Les Editions de Physique. pp. Chapter 26. ISBN   2-86883-142-7.
  10. ↑ "EN 10088-1 Standard : Stainless steels Part1: List of stainless steels".
  11. ↑ "310 310S Stainless Steel". TubingChina.com Stainless Steel Directory. Retrieved 2015-09-18.
  12. ↑ "Matweb data base".
  13. ↑ "Aubert&Duval Datasheet" (PDF).
  14. ↑ "Aircraftmaterials Datasheet".
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