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Metallurgy and Materials Engineering Department

University of Indonesia

2007

Karakteristik Dan Pemilihan Material

Duplex Stainless Steel

Dr.-Ing. Bambang Suharno

Dr. Ir. Sri Harjanto


Department of Metallurgy and Materials Engineering UI

History of Duplex Stainless Steel

1929 : Avesta Jernverke (25% Cr, 5% Ni)

1933 : J-Holtzer Company, France,

salah komposisi dari 18%Cr-9%Ni-2.5%Mo (AustenitikSS) menjadi 20%Cr-8%ni-2.5%Mo (Duplex SS)

dimana terdapat fasa ferrite dalam matriks austenite,ketika di heat treatment ternyata tidak sensitif terhadapintergranular corrosion

1970 : Duplex di kembangkan di Germany danSwedia (sejalan dengan ditemukannya AOD process)

1980 : Dikembangkan Super Duplex SS, ditandaidengan nilai PREN > 40 (= %Cr + 3.3% Mo + 16% N)



Phase Diagram of Duplex SSUniversity of

Indonesia


Metallurgy of Duplex SS

Selama solidification, duplex pertama kali

membentuk ferrite

Pada penurununan temperatur austenite terbentuk

Pada cast duplex

a structure of austenite islands in a ferrite matrix (dark)

Pada wrought alloys the microstructure has a morphology of laths of

austenite in ferrite matrix (dark)





Typical Microstructure of Duplex SS

(wrought and cast)

wrought

cast



Microstructure of Duplex SS

Duplex stainless steels have microstructures

a mixture of austenite and ferrite

The microstructure shows an

image of a duplex 3RE60 grade

ferrite



Ferrite Phase

Keberadaan ferrite dalam austenite menyebabkan:

better intergranular corrosion resistance and

stress corrosion cracking resistance

Dibandingkan dengan “austenitic stainless steel”

Tetapi keberadaan ferrite dalam austenite juga:

Menyebabkan terjadinya reaksi pembentukan berbagaivariety of secondary phases,

Berakibat terhadap corrosion resistance

mechanical properties (khususnya impact toughness)



Karakteristik Duplex Stainless

Steels

Austenitic stainless steel:

good weldability and low-temperature toughness,

their chloride SCC resistance and strength are comparatively

poor.

Ferritic stainless steel:

good resistance to chloride SCC but have poor toughness,

especially in the welded condition. A duplex microstructure with

high ferrite content can therefore have poor low-temperature

notch toughness,

whereas a structure with high austenite content can possess

low strength and reduced resistance to chloride SCC.





Karakteristik Duplex Stainless Steel

Memiliki mikrostruktur a mixture of austenite and ferrite.

resistant to stress corrosion cracking, meski tidak sebaikferritic stainless steel

Ketangguhannya (toughness)

Lebih tinggi dibanding ferritic stainless steel

Tetapi lebih rendah dibanding austenitic stainlesssteel

Their strength is 2 X greater than austenitic SS (annealed)

Have general corrosion resistances = or >> than 304 and 316

in general their pitting corrosion resistances are >> than 316.

They suffer reduced toughness below –50°C and after exposureabove 300°C

so are only used between these temperatures

are also magneticÆeasily differentiate them from common

austenitic SS (non magnetic)



Stess-Strain Curves for Duplex SS



Sifat Mekanik Duplex SSUniversity of

Indonesia


Application of Duplex SS

Chemical processing, transport and storage

Oil and gas exploration, offshore rigs and refining

Marine environments

Pollution control equipment

Pulp & paper industry

Desalination plants and seawater systems

Pressure vessels, reactor tanks and heat exchangers

Rotors, impellers and shafts in industrial equipment

Digesters, storage and clarifier tanks, stock washersfor the pulp and paper industry

Absorber towers, FGD systems in pollution control





Duplex Stainless Steels

Duplex stainless steel:

usually have a low carbon

additions of molybdenum, nitrogen, tungsten, andcopper.

Typical Cr contents are 20 to 30% and

Ni contents are 5 to 10 %.

The specific advantages offered by duplex stainlesssteels over conventional austenitic stainless steelsare

strength,

chloride stress-corrosion cracking resistance,

and pitting corrosion resistance.



Heat treatment

UR 35N (2304) = 980°C

UR 45N (2205) = 1050°C

UR 47/52N+ (2507) = 1120°C

Temperature andcooling rate (water cooling) essential toproduce a sound

structure.

Fixes the phase balance and the partitionof elements between phases.

Dissolves intermetallic phases.

Heat treatmenttemperature

PREN values

UR 35N (2304) = 26%

UR 45N (2205) = 33/36%

UR 47/52N+ (2507) = 42%

High Cr+Mo+W

Increase drastically therisk of brittle intermetallicphases precipitations.

Strong alpha forming effect.

Improve general and localized corrosionresistance.

‘’PREN=Cr+3,3(Mo+0,5W)+16N’’

Chromium

Molybdenum

(Tungsten)

UR 35N (2304) = 4%

UR 45N (2205) = 5,5%

UR 47/52N+ (2507) = 7%

Cost is high !Strong gammaforming effect .

Improves low temperature impactstrength.

Nickel

UR 35N (2304) = 0,12%

UR 45N (2205) = 0,17%

UR 47/52N+ (2507) = 0,25%

(Be careful with low N2 contents !)

Risk of porosities

(welding) .

Chromium nitrides

precipitates.

Strong gamma forming effect. Efficient

even for rapid thermal cycles (HAZ).

Positive effect on localized corrosion at

ambient improves mechanical propertiesNitrogen

COMMENTSCOMMENTS

OTHER EFFECTSOTHER EFFECTS

TO CONSIDERTO CONSIDERAIMAIM

ELEMENTS ANDELEMENTS AND

PROCESSPROCESSVARIABLESVARIABLES

METALLURGY OF DUPLEX STAINLESS STEELSMETALLURGY OF DUPLEX STAINLESS STEELS



Group of Duplex Stainless Steel

1. Lean duplex,

2304 (S32304), which contain little or free of Mo

Fe-23Cr-4Ni-0.1N

2. Standard 2205 (S32205)

for more than 80% of duplex use

Fe-23Cr-5.5Ni-3Mo-0.15N

PREN = 30 -36 (Pitting Resistant Index)

3. High alloy 25 %Cr duplex such as 255 (S32550) and S31260

Fe-25Cr-5Ni-2.5Mo-0.17N-Cu

PREN = 32 - 40

4. Superduplex, with 25-26 %Cr and increased Mo and N

such as 2507 (S32750)

Fe-25Cr-7Ni-3.5Mo-0.25N-W-Cu

PREN > 40



Pitting Resistance Index

Composition also plays a major

role in the corrosion resistance of

duplex stainless steels.





Pitting Resistance Index

Seri 2507 PREN = [Cr%] + 3.3 [Mo%] + 16 [N%] ≥ 40

S=0.001.274.07250.020

OthersNMoNiCr C

Seri 2304 PREN (Cr%) + 3.3 (Mo%) = 16 (N%) ≥ 24

S = 0.0010.10.24230.020

OthersNMoNiCr C

Seri 2205 PREN = [Cr%] = 3.3 [Mo%] = 16 [N%] ≥ 34

S=0.0010.183.15.622.10.020

OthersNMoNiCr C



Komposisi Duplex SS

UNSNumber

Type C Mn P S Si Cr Ni Mo N Cu Other

S31200 ... 0 .03 2. 00 0 .04

5

0.03

0

1.0

0

24.0-

26.0

5.5-6.5 1.20-

2.00

0.14-

0.20

... ...

S31260 ... 0 .03 1. 00 0 .03

0

0.03

0

0.7

5

24.0-

26.0

5.5-7.5 2.5-3.5 0.10-

0.20

0.20-

0.80

W0.10-

0.20

S31803 ... 0 .03 2. 00 0 .03

0

0.02

0

1.0

0

21.0-

23.0

4.5-6.5 2.5-3.5 0.08-

0.20

...

S32001 ... 0.03 4.0-

6.0

0.04

0

0.03

0

1.0

0

22.0-

23.0

1.00-

3.00

0 .60 0 .05-

0.17

1.00

S32205 2205 0 .03 2. 00 0 .03

0

0.02

0

1.0

0

19.5-

21.5

4.5-6.5 3.0-3.5 0.14-

0.20

...

S32304 2304 0 .03 2. 50 0 .04

0

0.03

0

1.0

0

21.5-

24.5

3.0-5.5 0.05-

0.60

0.05-

0.20

0.05-

0.60

S32520 ... 0 .03 1. 50 0 .03

5

0.02

0

0.8

0

24.0-

26.0

5.5-8.0 3.0-4.0 0.20-

0.35

0.50-

2.00

S32550 255 0 .04 1. 50 0 .04

0

0.03

0

1.0

0

24.0-

27.0

4.5-6.5 2.9-3.9 0.10-

0.25

1.5-2.5

S32750 2507 0 .03 1. 20 0 .03

5

0.02

0

0.8

0

24.0-

26.0

6.0-8.0 3.0-5.0 0.24-

0.32

0.50

S32760 ... 0 .03 1. 00 0 .03

0

0.01

0

1.0

0

24.0-

26.0

6.0-8.0 3.0-4.0 0.20-

0.30

0.50-

1.00

c

S32900 329 d 0 .06 1. 00 0 .04

0

0.03

0

0.7

5

23.0-

28.0

2.5-5.0 1.0-2.0 ... ...

S32950 ... 0 .03 2. 00 0 .03

5



THE DUPLEX STAINLESS STEELS FAMILYTHE DUPLEX STAINLESS STEELS FAMILY

Composition

Grade (%)

YS Rp 0,2

(Mpa)

UTS/Rm

(Mpa) A% USA EU

UR 35N (2304) > 400 > 600 > 25 32304 4362

UR 45N (2205) > 480 > 680 > 2531803, 32205

(+A923)4462

UR 47N+/52N+

(2507)> 560 > 760 > 25

32750

32550/32520

4410

4507

CompositionGrade (%) C Cr Ni Mo Cu N PREN

Corrosion

resistance similar

to

UR 35N (2304) 0,02 23 4 > 0,1 0,2 0,12 > 24 316L

UR 45N (2205) 0,02 22 5,7 3,1 - 0,18 33/36317L, 317LMN,

904…

UR 47N+/52N+

(2507)0,02 25,5 7 3,7 -/1,5 0,25 >40 904L, 6MO



HIGH STRENGTHHIGH STRENGTH

Duplex stainless steels yield strength rangeDuplex stainless steels yield strength range

from 400Mpa (UR 35N) to 560Mpa (UR 52N+)from 400Mpa (UR 35N) to 560Mpa (UR 52N+)

ASTMASTM

(47N+) 32550(47N+) 32550

(52N+) 32750(52N+) 32750

(B25) 31254

317LMN904L

304L

317L316L

40

50

60

70

80

Yield strength Yield strength

((KsiKsi))

30

20

(35N) 32304(35N) 32304

(45N) 31803(45N) 31803

3220532205

D U P L E X D U P L E X

Austenitic

EU StandardEU Standard

D U P L

E X

D U P L

E X

(47 N+) 4410(47 N+) 4410

(52 N+) 4507(52 N+) 4507

(45 N) 4462(45 N) 4462

(35 N) 4362(35 N) 4362

(B26) 4529

(B25) 45474439

(B6) 453944044307

200

300

400

500

600

RpRp 0.2%0.2%

(MPA)(MPA)

Austenitic





4

8

12

16

20

24

28

0 4 8 12 16 20 24 28 32 4036

32

AUSTENITE

A+M

MARTENSITE

A+M+F

FERRITE

Cr equivalent

317 LN

316L

304LUR 52N + /S32205

UR45N/S31803UR 35N/S32304

F+M

50 α / 50 γ

100%

UR B26 / 6 MoN08926

UR B66 /S31266

5%

20%

S32520/550/750/760

DUPLEX

Cr% + Mo% + 1,5 Si% + 0,5 Nb% + 5 V% + 3 Al%

UR B25 S31254

SR50A32050

Ni equivalent 0%

11-2.5

12-4.5.1.5 12-6.5.2.5

SCHAEFFLERSCHAEFFLER--DELONGDELONG DiagramDiagram



STRESS CORROSION CRACKINGSTRESS CORROSION CRACKING

Constant-strain tests in autoclave Above the curve, SCC occurs.Open circles mean no SCC for UNS 32520/UR 52N +

Duplex stainless steels behave much better than austenitic stainless steels

0 32

100 210

200 390

300°C

0.001 0.1 10 Cl- (%)

AISI 304

UR 35N

UR 45N

UR 52N +

No cracking

570

AISI 316



Duplex vs Austenitic SSUniversity of

Indonesia


Sifat korosi Duplex SS

Sifat ketahanan korosi

Memiliki ketahanan SCC lebih baik dari Austenitic SS

Memiliki ketahanan korosi pitting dan korosi batas butir





Pembentukan Fasa Kedua

(Intermetallic phase)

The high alloy content of duplex stainless

steels also makes them susceptible to

the formation of intermetallic phases from

extended exposure to high temperatures.

Significant intermetallic precipitation may lead

to

a loss of corrosion resistance

and sometimes to a loss of toughness.



PRECIPITATIONS OBSERVED IN FUNCTION OF TIME ANDPRECIPITATIONS OBSERVED IN FUNCTION OF TIME AND

TEMPERATURETEMPERATURE

(UR 45 N - 4462)

1 10 100 1000 104s

1 10 100 1000 104h 105hHours

0

300

500

800

1000

1200

1400

SAFE SERVICE RANGE (- 50, 300°C)

LONG TERM SERVICE ⇒ EMBRITTLEMENTα −−> α 'Cu --> ε EMBRITTLEMENTMaximum service limit

α −−−> σα −−−> χ

Heat treatment (α= γ = 50 %)

Increase of ferrite content

FERRITE 100 %

(Seconds)

Temperature (°C)

Time

LOW TEMPERATURE BRITTLENESS (θ <- 50°C)

Heat treatmentrange

DO NOTUSE



Possible Precipitates in Duplex SSUniversity of

Indonesia


10 100 1000 10000

1890

1470

1110

750

390

1000

800

600

200

400

Seconds10 hours

S32304 S31803 S32520

°C °F

TTTTTT diagramsdiagrams

SIGMA

Alpha’





Sigma Phase

Sigma (σ) = Fe-30Cr-4Ni and 4-7% Mo

Has harmful effects

on the mechanical properties, ductility and toughness

it is detrimental to corrosion resistance

Sigma phase precipitates in Duplex SS

over a wider temp range and in a shorter time

The presence of ferrite

enhances the precipitation of sigma phase

The diffusion rate of sigma phase forming elements(Cr, Mo, W) in ferrite is 100 times faster than inaustenite

Sigma phase can form from 600 – 1000OC



Morphology of Sigma Phase



Chi Phase

Chi (λ) phase

is commonly found in duplex SS but

it is usually in much smaller quantities than sigmaphase

It is also as harmful as sigma phase to the properties

It is more detrimental to pitting resistance than sigma

phase Chi and Sigma phase are not distinguishable using

optical microscope (can be distinguished by TEM)

Chi phase precipitates in the range 700 – 900OC

Upon long term aging, chi phase will convert to sigmaphase



Chi Phase





Welding

Duplex stainless steels have good weldability

They are not quite as easily welded as the

austenitic grades

but low thermal expansion in duplex grades

reduces distortion and residual stresses after

welding.

The recommended filler material for 2205

stainless steel is 2209



Welding Duplex Stainless Steels

Weldability of duplex stainless steels are better than

ferritic stainless steels, but generally not as good as

austenitic materials.

Control of heat input is important. Solidification

cracking and hydrogen cracking are concerns when

welding duplex stainless steels

Modifications to the alloy compositions have been

made to improve corrosion resistance, workability,

and weldability.

In particular, nitrogen additions have been effective in

improving pitting corrosion resistance and weldability

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