Ferritic stainless steel

Ferritic stainless steelforms one of the five stainless steel families, the other four beingaustenitic,martensitic,duplex stainless steels,andprecipitation hardened.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.

Ferritic stainless steel (SUS445J2) is used for the roof exterior of theKyocera Dome Osaka,Japan.

History

Canadian-born engineer Frederick Mark Becket (1875-1942) atUnion Carbideindustrialised 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".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."

Ferriticstainless steelswere 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 thesteelmakingstage.
• Weldablegrades were developed.
• Thermomechanical processingsolved the problems of "roping" and "ridging" that led to inhomogenous deformation during deepdrawingand to textured surfaces.
• End-user markets (such as that ofdomestic appliances) demanded less expensive grades with a more stable price at a time when there were large variations of the price ofnickel.Ferritic stainless steel grades became attractive for some applications such as houseware.

Metallurgy

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 theaustenitewill transform intomartensite.Tempering/annealingwill transform the martensitic structure intoferriteandcarbides.

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 temperatureembrittlement.

Chemical composition

Chemical composition of a few grades of stainless steel.
Main alloying elements only:chromium(Cr) along with:
Ni,Mo,Nb,Ti,andC,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
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

Thepitting corrosionresistance 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 byweight %ofchromium,molybdenumandnitrogenrespectively 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 asausteniticgrades.

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

Physical properties

Ferritic stainless steels aremagnetic.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 toaustenitic stainless steels,they offer a betterthermal conductivity,a plus for applications such asheat exchangers. Thethermal expansion coefficient,close to that ofcarbon steel,facilitates theweldingto carbon steels.

Mechanical properties

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

• Lower-cost of recent-productionkitchenware
• White goods
• Solar heaters
• Slate hooks
• Coins