Ferritic stainless steel

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Ferritic stainless steel (SUS445J2) is used for the roof exterior of the Kyocera Dome Osaka, Japan. Kyocera Dome and Osaka Loop Line 2016-07-02.jpg
Ferritic stainless steel (SUS445J2) is used for the roof exterior of the Kyocera Dome Osaka, Japan.

Ferritic stainless steel [2] [3] forms one of the five stainless steel families, the other four being austenitic, martensitic, duplex stainless steels, and precipitation hardened . [4] For example, many of AISI 400-series of stainless steels are ferritic steels. By comparison with austenitic types, these are less hardenable by cold working, less weldable, and should not be used at cryogenic temperatures. Some types, like the 430, have excellent corrosion resistance and are very heat tolerant. [5]

Contents

History

Canadian-born engineer Frederick Mark Becket (1875-1942) at Union Carbide industrialised ferritic stainless steel around 1912, on the basis of "using silicon instead of carbon as a reducing agent in metal production, thus making low-carbon ferroalloys and certain steels practical". [6] He discovered a ferrous alloy with 25-27% Chromium that "was the first of the high-chromium alloys that became known as heat-resisting stainless steel." [7]

Ferritic stainless steels were discovered early but it was only in the 1980s that the conditions were met for their growth:

Metallurgy

Fe - Cr Phase diagram Diagramme phase Fe Cr.svg
Fe – Cr Phase diagram

To qualify as stainless steel, Fe-base alloys must contain at least 10.5%Cr.

The iron-chromium phase diagram shows that up to about 13%Cr, the steel undergoes successive transformations upon cooling from the liquid phase from ferritic α phase to austenitic γ phase and back to α. When some carbon is present, and if cooling occurs quickly, some of the austenite will transform into martensite. Tempering/annealing will transform the martensitic structure into ferrite and carbides.

Above about 17%Cr the steel will have a ferritic structure at all temperatures.

Above 25%Cr the sigma phase may appear for relatively long times at temperature and induce room temperature embrittlement.

Chemical composition

Chemical composition of a few grades of stainless steel.
Main alloying elements only: chromium (Cr) along with:
Ni, Mo, Nb, Ti, and C, N; Balance: Fe
AISI / ASTMENWeight %
CrOther elementsMelts at
4051.400012.0 – 14.0—
409L1.451210.5 – 12.56(C+N)<Ti<0.65
410L1.400310.5 – 12.50.3<Ni<1.0
4301.401616.0 – 18.0—1510 [10]
4391.451016.0 – 18.00.15+4(C+N)<Ti<0.8
430Ti1.451116.0 – 18.0Ti: 0.6
4411.450917.5 – 18.50.1<Ti<0.6

0.3+3C<Nb<1.0

4341.411316.0 – 18.00.9<Mo<1.4
4361.451316.0 – 18.00.9<Mo<1.4

0.3<Ti<0.6

4441.452117.0 – 20.01.8<Mo<2.5

0.15+4(C+N)<Ti+Nb<0.8

4471.459228 – 30.03.5<Mo<4.5

0.15+4(C+N)<Ti<0.8

Corrosion resistance

The pitting corrosion resistance of stainless steels is estimated by the pitting resistance equivalent number (PREN).

PREN = %Cr + 3.3%Mo + 16%N

Where the Cr, Mo, and N, terms correspond to the contents by weight % of chromium, molybdenum and nitrogen respectively in the steel.

Nickel (Ni) has no role in the pitting corrosion resistance, so ferritic stainless steels can be as resistant to this form of corrosion as austenitic grades.

In addition, ferritic grades are very resistant to stress corrosion cracking (SCC).

Physical properties

Ferritic stainless steels are magnetic. Some of their important physical, electrical, thermal and mechanical properties are given in the table here below.

Physical properties of the most common ferritic stainless steels
AISI / ASTMDensity
(g/cm3)		Electrical
resistance
(μΩ·m)		Thermal
conductivity
at 20 °C
(W/(m·K))		Specific
heat
0...100 °C
(J/(kg·K))		Thermal
expansion
0...600 °C
(10−6/K)		Young's
modulus
(GPa)
409 / 4107.70.582546012220
4307.70.602546011.5220
430Ti / 439 / 4417.70.602546011.5220
434 / 436 / 4447.70.602346011.5220
4477.70.621746011220

Compared to austenitic stainless steels, they offer a better thermal conductivity, a plus for applications such as heat exchangers. The thermal expansion coefficient, close to that of carbon steel, facilitates the welding to carbon steels.

Mechanical properties

Mechanical properties (cold rolled)
ASTM A240EN 10088-2
—UTS

(MPa, min)

0.2% yield stress

(MPa, min)

Elongation

(%, min)

—UTS

(MPa)

0.2% yield stress

(MPa, min)

Elongation

(%, min)

409390170201.4512380 – 56022025
410415205201.4003450 – 65032020
430450205221.4016450 – 60028018
439415205221.4510420 – 60024023
441415205221.4509430 – 63025018
434450240221.4113450 – 63028018
436450240221.4526480 – 56030025
444415275201.4521420 – 64032020

Applications

Related Research Articles

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<span class="mw-page-title-main">Austenite</span> Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

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<span class="mw-page-title-main">Martensitic stainless steel</span> One of the 5 crystalline structures of stainless steel

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<span class="mw-page-title-main">Carbon steel</span> Steel in which the main interstitial alloying constituent is carbon

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<span class="mw-page-title-main">Superalloy</span> Alloy with higher durability than normal metals

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<span class="mw-page-title-main">Duplex stainless steel</span> Stainless steel that has both austenitic and ferritic phases

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<span class="mw-page-title-main">475 °C embrittlement</span> Loss of plasticity in ferritic stainless steel

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. ↑ "屋根:大阪ドーム" (in Japanese). Japan Stainless Steel Association. Retrieved 12 October 2023.
  2. ↑ Lacombe, P.; Baroux, B.; Beranger, G., eds. (1990). Les Aciers Inoxydables. Les éditions de Physique. pp. Chapters 14 and 15. ISBN   2-86883-142-7.
  3. ↑ The ferritic solution. 2007. ISBN   978-2-930069-51-7. Archived from the original on 21 December 2019. Retrieved 14 July 2019.
  4. ↑ The International Nickel Company (1974). "Standard Wrought Austenitic Stainless Steels". Nickel Institute. Archived from the original on 9 January 2018. Retrieved 9 January 2018.
  5. ↑ "304 vs 430 stainless steel". Reliance Foundry Co. Ltd. Retrieved 28 May 2022.
  6. ↑ "Frederick Mark Becket American metallurgist". Encyclopaedia Britannica. 7 January 2021.
  7. ↑ Cobb, Harold M. (2012). Dictionary of Metals. ASM International. p. 307. ISBN   9781615039920.
  8. ↑ Charles, J.; Mithieux, J.D.; Santacreu, P.; Peguet, L. (2009). "The ferritic family: The appropriate answer to nickel volatility?". Revue de Métallurgie. 106: 124–139. doi:10.1051/metal/2009024.
  9. ↑ Ronchi, Gaetano (2012). "Stainless Steel for House-ware". Metal Bulletin.
  10. ↑ "Stainless steel melting points". Thyssenkrupp Materials (UK) Ltd. Retrieved 28 May 2022.
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