Ferritic stainless steel explained

Ferritic stainless steel[1] [2] forms one of the five stainless steel families, the other four being austenitic, martensitic, duplex stainless steels, and precipitation hardened.[3] 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.[4]

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".[5] 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."[6]

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

Metallurgy

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

AISI / ASTM! rowspan="2"
ENWeight %
CrOther elementsMelts at
4051.400012.0 – 14.0
409L1.451210.5 – 12.56(C+N)
410L1.400310.5 – 12.50.3
4301.401616.0 – 18.01510[9]
4391.451016.0 – 18.00.15+4(C+N)
430Ti1.451116.0 – 18.0 Ti: 0.6
4411.450917.5 – 18.50.1
4341.411316.0 – 18.00.9
4361.451316.0 – 18.00.9
4441.452117.0 – 20.01.8
4471.459228 – 30.03.5

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 / ASTM!Density
(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

ASTM A240 ! colspan="4"
EN 10088-2
UTS(MPa, min)0.2% yield stress
(MPa, min)
Elongation(%, min)UTS(MPa)0.2% yield stress
(MPa, min)
Elongation(%, min)
409 390170201.4512380 – 56022025
410415205201.4003450 – 65032020
430450205221.4016450 – 60028018
439 415205221.4510420 – 60024023
441415205221.4509430 – 63025018
434 450240221.4113450 – 63028018
436450240221.4526480 – 56030025
444415275201.4521420 – 64032020

Applications

Notes and References

  1. Book: Les Aciers Inoxydables. Lacombe. P.. Baroux. B.. Beranger. G.. Les éditions de Physique. 1990. 2-86883-142-7. Chapters 14 and 15.
  2. Book: The ferritic solution. 2007. 978-2-930069-51-7. 14 July 2019. 21 December 2019. https://web.archive.org/web/20191221173515/http://www.worldstainless.org/publications/brochures_and_posters. dead.
  3. Web site: The International Nickel Company. 1974. Standard Wrought Austenitic Stainless Steels. Nickel Institute. 2018-01-09. 2018-01-09. https://web.archive.org/web/20180109181144/https://nickelinstitute.org/~/media/Files/TechnicalLiterature/StandardWroughtAusteniticStainlessSteels_1229_.ashx. dead.
  4. News: 304 vs 430 stainless steel . 28 May 2022 . Reliance Foundry Co. Ltd. .
  5. News: Frederick Mark Becket American metallurgist . Encyclopaedia Britannica . 7 January 2021.
  6. Book: Cobb . Harold M. . Dictionary of Metals . 2012 . ASM International . 307 . 9781615039920 .
  7. 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. 10.1051/metal/2009024.
  8. Web site: Stainless Steel for House-ware. Ronchi. Gaetano. 2012. Metal Bulletin.
  9. News: Stainless steel melting points . 28 May 2022 . Thyssenkrupp Materials (UK) Ltd.