Ferritic stainless steel

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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[edit]

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:

  • It was possible to obtain very low carbon levels at the steelmaking stage.
  • Weldable grades were developed.
  • Thermomechanical processing solved the problems of "roping" and "ridging" that led to inhomogenous deformation during deep drawing and to textured surfaces.
  • End-user markets (such as that of domestic appliances) demanded less expensive grades with a more stable price at a time when there were large variations of the price of nickel.[7] Ferritic stainless steel grades became attractive for some applications such as houseware.[8]

Metallurgy[edit]

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[edit]

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 / ASTM EN Weight %
Cr Other elements Melts at
405 1.4000 12.0 – 14.0
409L 1.4512 10.5 – 12.5 6(C+N)<Ti<0.65
410L 1.4003 10.5 – 12.5 0.3<Ni<1.0
430 1.4016 16.0 – 18.0 1510[9]
439 1.4510 16.0 – 18.0 0.15+4(C+N)<Ti<0.8
430Ti 1.4511 16.0 – 18.0 Ti: 0.6
441 1.4509 17.5 – 18.5 0.1<Ti<0.6

0.3+3C<Nb<1.0

434 1.4113 16.0 – 18.0 0.9<Mo<1.4
436 1.4513 16.0 – 18.0 0.9<Mo<1.4

0.3<Ti<0.6

444 1.4521 17.0 – 20.0 1.8<Mo<2.5

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

447 1.4592 28 – 30.0 3.5<Mo<4.5

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

Corrosion resistance[edit]

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[edit]

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 / 410 7.7 0.58 25 460 12 220
430 7.7 0.60 25 460 11.5 220
430Ti / 439 / 441 7.7 0.60 25 460 11.5 220
434 / 436 / 444 7.7 0.60 23 460 11.5 220
447 7.7 0.62 17 460 11 220

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[edit]

Mechanical properties (cold rolled)
ASTM A240 EN 10088-2
UTS

(MPa, min)

0.2% yield stress

(MPa, min)

Elongation

(%, min)

UTS

(MPa)

0.2% yield stress

(MPa, min)

Elongation

(%, min)

409 390 170 20 1.4512 380 – 560 220 25
410 415 205 20 1.4003 450 – 650 320 20
430 450 205 22 1.4016 450 – 600 280 18
439 415 205 22 1.4510 420 – 600 240 23
441 415 205 22 1.4509 430 – 630 250 18
434 450 240 22 1.4113 450 – 630 280 18
436 450 240 22 1.4526 480 – 560 300 25
444 415 275 20 1.4521 420 – 640 320 20

Applications[edit]

References[edit]

  1. ^ 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.
  2. ^ The ferritic solution. 2007. ISBN 978-2-930069-51-7.
  3. ^ The International Nickel Company (1974). "Standard Wrought Austenitic Stainless Steels". Nickel Institute. Archived from the original on 2018-01-09. Retrieved 2018-01-09.
  4. ^ "304 vs 430 stainless steel". Reliance Foundry Co. Ltd. Retrieved 28 May 2022.
  5. ^ "Frederick Mark Becket American metallurgist". Encyclopaedia Britannica. 7 January 2021.
  6. ^ Cobb, Harold M. (2012). Dictionary of Metals. ASM International. p. 307. ISBN 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. doi:10.1051/metal/2009024.
  8. ^ Ronchi, Gaetano (2012). "Stainless Steel for House-ware". Metal Bulletin.
  9. ^ "Stainless steel melting points". Thyssenkrupp Materials (UK) Ltd. Retrieved 28 May 2022.