Difference between Ferrite and Austenite Steel

What is the difference between ferrite and austenite steel?

The classification of stainless steel is made possible on account of the different kinds of microstructure. Out of these four classes, the two most popular classes are ferritic and austenitic stainless steels. The microstructure of these alloys is the internal arrangement of the crystal. The arrangement of the crystal is what gives the alloy its mechanical and it’s chemical characteristics. The microstructure of Ferritic stainless steel consists of ferrite crystals. Ferrite crystals are a kind of iron that contains trace quantities of carbon, which amounts up to 0.025%. The Ferrite crystals tend to absorb a limited amount of carbon. This is because of its body centered cubic crystal structure. The arrangement is such that there is one iron atom at each corner, in addition to one in the center. This central ferrous atom is what gives the ferritic class of stainless steel its magnetic properties. On the other hand, austenitic stainless steel, which is a gamma-phase iron, which is an allotrope of iron. At an elevated temperature range of 1,674 to 2,541 °F, the alpha iron undergoes a phase transition. Therefore, the alpha iron, which was a body-centered cubic or BCC structure gets converted to the face-centered cubic or FCC configuration of gamma iron. This changed configuration of a gamma iron is referred to as austenite.

Ferrite and austenite microstructure

Ferritic steels have a body centered cubic crystal structure. This means there is one ferrous atom present at every one of the eight corners, and one atom at the core. In this setting, each of the eight corners is also the corner of another cube. Hence, the corner ferrous or iron atoms will be shared equally among eight unit cells. On the other hand, austenite, which has a face-centered cubic crystal structure has atoms at the corners. As the name suggests, the atoms are present at the center of the faces of its cellular unit. Atoms in an FCC or face-centered cubic arrangement are packed together in a very snug manner. Hence, the atoms in the microstructure will occupy about 74% of its volume. Since they are packed snugly, this kind of structure is also referred to as cubic closest packing or CCP.

Carbon solubility in ferrite and austenite

In comparison to austenite, the carbon solubility of ferrite is low. Being a solid solution of carbon and iron, a percentage of about 0.025%, which means that the solubility of carbon in ferrous is 0.02%. Since pure iron is already a structure at room temperature, the interatomic spaces are small. Therefore, sphere shaped carbon atoms cannot accommodate the ferrous atoms. This is what makes the solubility of carbon is low in ferrite. Moreover, the carbon atom is small, which makes it impossible for it to act as a substitute, yet, it is too large for an interstitial solid solution.  On the other hand, the carbon solubility in iron in an austenite region is about is 2.11%, which is significantly higher than in ferrite regions. This is because austenite has an fcc structure.  Due to this structure, the interatomic spacing of austenite larger than ferrite. Having a larger spacing makes it easy for austenite to accommodate carbon atoms in their spaces.

The density of ferrite and austenite

BCC is heavier than FCC, which means that Ferrite has a higher density in comparison to austenite. The reason for FCC being lighter is that their symmetry or arrangement is what offers closely packed planes in various directions. This is why a face-centered cubic or FCC crystal structure will manifest more ductility. And so the chances of deformation for austenite are greater under load before breaking, especially if compared to a body-centered cubic structure. The lattice in a  body-centered cubic, although cubic, is not closely packed like the FCC type. Hence, BCC or ferrite tend to be strong metals.

The hardness of ferrite and austenite

Ferrite is known to be harder than austenite. Usually, elements such as chromium, molybdenum, silicon, and niobium foster ferrite. Most ferritic steels contain chromium content at the 13.5% range, which means they are capable of undergoing successive transformations from the alpha to gamma and back to the alpha phase during the formation of ferrite. Along with being magnetic, ferrite crystals are known to be harder and brittle, as compared to the soft and ductile crystals of austenite.

Ferrite stainless steel composition

Typical composition (%)

AISI

C

Cr

Mo

Other

410S

0.08

12

    

409

0.03

11

  

0.5 Ti

430

0.08

17

    

430Nb

0.05

17

  

0.6 Nb

430Ti

0.05

17

  

0.6 Ti

434

0.08

17

1

  

444

0.02

18

2

0.4 (Ti+Nb)

446

0.15

24

 

 

447

0.01

29

3.8

0.1Cu, 0.1Ni

Ferrite stainless steel grades at a glance

  • Type 409 stainless steel
  • 430 stainless steel
  • 430LI stainless steel
  • 434 stainless steel
  • 439 stainless steel
  • Type 442 stainless steel
  • 444 stainless steel
  • 446 stainless steel

Mechanical properties of ferrite stainless steel

Common name

Yield MPa

Tensile

MPa

Elongation at break %

Modulus

GPa

409

170

380

20

220

4003, 3/5Cr12

L:320

T:360

480

18

220

430

205

450

22

220

444

275

415

20

220

304

270

650

57

200

Carbon steel

300

430

25

215

Ferrite stainless steel Physical properties

Property

Ferritic

Density Value (kg/m3)

7700

Thermal conductivity

(20°C, W/m.°C

25

Thermal expansion

(0-100°C μm/m/°C)

10.5

Electrical resisivity

(nΩ.m)

600

Specific heat range

(0-100°C, J/kg.°C

430-460

Mechanical properties of austenite steel

  Tensile Strength Yield Strength

Austenitic

600

250

Duplex

700

450

Ferritic

500

280

Martensitic

650

350

Precipitation Hardening

1100

1000

Austenitic stainless steel chemical composition chart

SS Grade Composition wt% Microstructure
C (max) Si (max) Mn (max) Cr Ni Mo Others Austenite - A
Ferrite - F
304 0.08 0.75 2.0 18/20 8/11 - - A+2/8%F
304L 0.035 0.75 2.0 18/20 8/11 - - A + 2/8%F
304H 0.04 - 0.10 0.75 2.0 18/20 8/11 - - A + 2/8%F
304N 0.08 0.75 2.0 18/20 8/11 - 0.1/0.16N A + 2/8%F
316 0.08 0.75 2.0 16/18 11/14 2/3 - A + 3/10%F
347 0.08 0.75 2.0 17/20 9/13 - Nb : 10xC A + 4/12%F
321 0.08 0.75 2.0 17/19 9/12 - Ti: 5xC A + 4/12%F
310 0.15 0.75 2.0 24/26 19/22 - - 100% A
309 0.08 1.0 2.0 22/24 12/15 - - A + 8/15%F
308L (generally filler metal only) 0.03 1.0 2.0 19/21 10/12     A + 4/12%F

Austenitic stainless steel grades list

  • 304
  • 304L
  • 304H
  • 304N
  • 316
  • 347
  • 321
  • 310
  • 309
  • 308L (generally filler metal only)

Effect of retained austenite steel

The effect of retained austenite steel depends on the acting stresses on the impact-fatigue strength. Retained austenite help to increase the impact-fatigue resistance at a high-stress level, but under low stress, it decreases.

Characteristics of austenite steel

  • Strength at Temperature up to approximately 1900F
  • Cold Workability
  • Low Thermal Conductivity
  • High Formability

Austenite steel equivalent grades

Austenite steel

UNS No

BS

Euronorm No.

301

S30100

301S21

1.4310

302

S30200

302S25

1.4319

303

S30300

303S31

1.4305

304

S30400

304S31

1.4301

304L

S30403

304S11

1.4306

304H

S30409

-

1.4948

(302HQ)

S30430

394S17

1.4567

305

S30500

305S19

1.4303

309S

S30908

309S24

1.4833

310

S31000

310S24

1.4840

310S

S31008

310S16

1.4845

314

S31400

314S25

1.4841

316

S31600

316S31

1.4401

316L

S31603

316S11

1.4404

316H

S31609

316S51

-

316Ti

S31635

320S31

1.4571

321

S32100

321S31

1.4541

347

S34700

347S31

1.4550

403

S40300

403S17

1.4000

405

S40500

405S17

1.4002

409

S40900

409S19

1.4512

410

S41000

410S21

1.4006

416

S41600

416S21

1.4005

420

S42000

420S37

1.4021

430

S43000

430S17

1.4016

440C

S44004

-

1.4125

444

S44400

-

1.4521

630

S17400

-

1.4542

(904L)

N08904

904S13

1.4539

(253MA)

S30815

-

1.4835

(2205)

S31803

318S13

1.4462

(3CR12)

S41003

-

1.4003

(4565S)

S34565

-

1.4565

(Zeron100)

S32760

-

1.4501

(UR52N+)

S32520

-

1.4507

Also View