outokumpu corrosion management news acom 3 4 edition 2013

8/17/2019 Outokumpu Corrosion Management News Acom 3 4 Edition 2013

1/10

A CORROSION MANAGEMENT AND APPLICATIONS ENGINEERING MAGAZINE FROM OUTOKUMPU 3-4/2013

The two phasedoptimization of

duplex stainlesssteel


2/10

2

AbstractDuplex stainless steels are well known for high strength in

comparison to their austenitic counterparts. They also have good

cost efficiency providing required properties without the level of

exposure to nickel price volatility seen for many austenitic grades.

The development of these two phased grades is a continuous

process and a natural focus is to optimize the composition to

obtain the maximum possible benefit from the alloying elements.

One proven way of doing this is to decrease the content of nickel

while increasing the amount of other austenitizing elements such

as nitrogen and manganese. Nitrogen has a strong beneficial

influence on both strength and corrosion resistance. This alloying

concept has been used successfully in the lean duplex grades,

which have corrosion resistance on a par with standard austeniticgrades. The most recent contribution based on this concept is the

lean duplex grade LDX 2404® with enhanced strength but a

corrosion resistance which is still close to that of the standard

duplex grade 2205.

When testing the corrosion resistance in the duplex grades it is

important to consider whether there is an imbalance in the

corrosion resistance of the individual phases. The development of

the super duplex grade 2507 was reported to be based on the

concept that the grade exhibits optimal pitting corrosion resist-

ance when annealed at a temperature where the localized

corrosion resistance equivalent is equal in both phases. Such an

optimization naturally also involves other concepts such as phase

balance elemental partitioning and structural stability.This paper aims to take such considerations a step further,

using an analysis which acknowledges the different performance of

the austenite and ferrite phases in duplex grades and addresses

the possibility that they do not need to have the equal PRE to give

the alloy a maximum critical pitting temperature, CPT. This concept

has proven useful in duplex alloy development especially for

molybdenum alloyed grades where the resistance against pitting

corrosion is high. The paper exemplifies these considerations both

by examining a number of different variants of the standard duplex

grade 2205 and by evaluation of a series of model alloys. The

partitioning of alloying elements is determined by EDS/WDS

analysis and correlated to predictions using the thermodynamic

software Thermo-Calc®

. The location of pitting attack is evaluatedin the different cases and the discussion focuses on the possible

mechanisms behind the observed results.

The two phased optimizationof duplex stainless steel

Presenter: Jan. Y. Jonsson, Alexander Thulin,Sukanya HäggOutokumpu Stainless AB, Avesta Research Centre, Avesta, Sweden.

Rachel Pettersson, Jernkontoret,Swedish Steel Producers’ Association, Stockholm, Sweden.

IntroductionDuplex stainless steels are well known for high strength in

comparison to their austenitic counterparts. They also have good

cost efficiency, providing required properties without the level of

exposure to nickel price volatility seen for many austenitic grades.

The development of these two phased grades is a continuous

process and a natural focus is to optimize the composition to

obtain the maximum possible benefit from the alloying elements.

The term “maximum possible benefit” can be interpreted in many

ways; one may consider that a low price is beneficial while another

may think that e.g. higher impact toughness is more beneficial.

These different interpretations can be met by varying the chemical

composition of the steel melts. The duplex grade 2205 is a good

example of this; Outokumpu Avesta Works has 4 different meltcodes of the 2205. One is the Outokumpu standard while the

others fulfill three specific requirements; lower price, higher impact

toughness or lower ferrite content.

This paper focuses on the partitioning of the alloying elements

and the effect that this has on the pitting resistance of each

phase. The partitioning of alloying elements is determined by EDS

analysis and correlated to predictions using the thermodynamic

software Thermo-Calc®. The duplex grades LDX 2404® and

EDX 2304TM together with a set of experimental alloys are used

to exemplify the alloying concept.

3-4/2013 |


3/10

3

Materials and experimentalprocedureMaterials

The investigated materials are 2205 as well as the leaner duplexsteel grades; LDX 2404® and EDX 2304TM, the chemical composition

of the grades are presented in Table 1. All materials has been

examined after a solution annealing in 1050°C followed by 15s

in air and a final water quench.

The pitting resistance equivalent (PRE) is commonly used to

estimate the corrosion resistance of stainless steel grades. There

are different opinions on the exact formulation of the PRE formula,

as discussed in a review paper (Pettersson & Flyg, 2004) and this

work uses PRE(NMn) where the positive effect of nitrogen and a

slight negative effect of manganese is taken into account. It can

be noted that the overall effect associated with manganese

alloying can nevertheless be positive, since this element increases

the nitrogen solubility. The PRE(NMn) formula is given below Table 1and it contains a coefficient of 22 for nitrogen and -1 for the

manganese content. The most common PRE formula, PRE(N), uses

a factor 16 or 30 for nitrogen and it does not take into account

any effect of manganese. This common formula is used in various

situations where the general pitting resistance of a specific

material needs to be evaluated, a customer specification can e.g.

contain a requirement that sets a lower limit of PRE(N) that the

material needs to fulfill.

In addition to the commercially available alloys above, a set of

Table 1 Chemical composition in % by weight of the duplex

stainless steel materials investigated.

Materials Plate thickness (mm) Chemical composition (wt.%) PRE(NMn) *

2205 alloys Cr Ni Mo N Mn

Alloy A 10 22.6 4.6 2.6 0.20 1.5 34.0

Alloy B 10 22.9 4.9 2.6 0.19 1.6 34.0

Alloy C 10 22.3 5.2 2.8 0.18 1.4 34.2

Alloy D 10 22.4 5.7 3.2 0.17 1.5 35.1

Alloy E 10 22.3 6.3 3.1 0.20 1.4 35.7

LDX 2404® 10 24.1 3.6 1.6 0.27 2.9 32.3

EDX 2304TM 10 23.9 4.4 0.5 0.19 1.4 28.4

*Pitting resistance equivalent. PRE(NMn) = %Cr+3.3%Mo+22%N-%Mn.

Materials Chemical composition (wt.%) PRE(NMn) *

Experimental alloys Cr Ni Mo N Mn

Alloy I 27.6 9.6 1.9 0.02 0.6 33.5

Alloy J 27.7 8.5 2.2 0.06 0.6 35.6

Alloy K 27.7 6.4 2.6 0.10 0.6 37.8

Alloy L 27.7 7.5 2.3 0.16 0.6 38.3

* Pitting resistance equivalent. PRE(NMn) = % Cr+3.3%Mo+22%N-%Mn.Table 2 Chemical composition in % by weight of the duplex laboratory materials.

small (300 g) experimental alloys were used to show how the

optimization concept taken from the commercially available

materials can be used on a real case. These materials are shown

in Table 2. As for the commercial alloys, these materials were

investigated after solution annealing at 1050°C followed by 15

seconds in air prior to water quenching.

Metallographic examination

The microstructure was examined after a chemical etching in a

modified Beraha II solution (50ml HCl, 100ml H20 1.5g K

2SO

5).

The ferrite contents were evaluated with light optical microscopy

(LOM) using image analysis according to ASTM E 1245. The same

etchant was also used for detecting pit initiation sites after

corrosion testing.

For comparison of the pitting resistance in each phase using

the individual phase PRE the chemical composition of the

austenite and the ferrite was analyzed with scanning electron

microscope with energy dispersive spectroscopy (SEM-EDS).

Calibration was performed using actual plate material as referencesample. The N-content was analyzed using wavelength dispersive

spectroscopy, WDS. In this case a set of stainless steel materials

with known N-contents was used a reference material.

Corrosion testing

The CPT-testing has been performed according to ASTM G150 in

a 1M NaCl solution for the commercial alloys 2205, LDX 2404 ®

and EDX 2304®. A 1 cm2 test area was used for the commercial

steels and 10 cm2 for the set of laboratory materials.

3-4/2013 |


4/10

4

Table 3 Chemical composition % by weight in austenite and ferrite

analysed by SEM-EDS with WDS-analysis for N.

Materials Chemical composition (wt.%) PRE(NMn) CPT (°C)

2205 alloys Cr Ni Mo N Mn

Alloy A 22.6 4.6 2.6 0.20 1.5 34.0 52.9

BCC 24.7 3.3 3.3 0.02 1.3 32.9

FCC 20.9 5.6 2.1 0.40 1.7 35.3

Alloy B 22.9 4.9 2.6 0.19 1.6 34.0 53.3

BCC 25.2 3.5 3.2 0.02 1.4 33.3

FCC 20.9 6.1 2.1 0.42 1.8 35.8

Alloy C 22.3 5.2 2.8 0.18 1.4 34.2 56.4

BCC 24.8 3.6 3.6 0.02 1.2 34.4

FCC 20.6 6.3 2.3 0.35 1.6 34.2

Alloy D 22.4 5.7 3.2 0.17 1.5 35.1 58.8

BCC 24.9 4.0 4.0 0.03 1.2 35.8

FCC 20.5 7.0 2.5 0.32 1.7 34.0

Alloy E 22.3 6.3 3.1 0.20 1.4 35.7 57.1

BCC 25.3 4.2 4.1 0.02 1.1 36.7

FCC 20.8 7.3 2.6 0.33 1.5 35.0

LDX 2404® 24.1 3.6 1.6 0.27 2.9 32.3 42.3

BCC 26.5 2.5 2.0 0.03 2.5 29.7

FCC 22.4 4.4 1.3 0.56 3.2 36.1

EDX 2304TM

23.9 4.4 0.5 0.19 1.4 28.4 34.2

BCC 26.8 3.1 0.7 0.03 1.2 27.0

FCC 21.8 5.3 0.4 0.41 1.5 31.2

Results and DiscussionsThe commercial duplex alloys

The chemical analyses of the alloy and specifically of the

austenite and the ferrite are shown in Table 3 together with the

corresponding PRE(NMn) and the CPT results. It is noted that CPTincreases with the PRE(NMn) in all cases except for alloy E which

has a lower CPT in spite of the higher PRE(NMn).

The majority of the nitrogen found in a duplex alloys is located

in the austenite phase, which contains approximately 10 times the

nitrogen in the ferrite phase. It is noted that the addition of

nitrogen in duplex grades has many benefits, such as higher

strength as for LDX 2404®, but the resistance towards pitting is

not in general favored by this addition if the only other adjustment

is a decrease in nickel content, in part exemplified by alloys A-C.

An increase in nitrogen can improve the austenite PRE but

primarily increases the austenite fraction and will have to be

balanced with higher levels of ferrite stabilizers such as chromium

and molybdenum. Lowering the nickel content will lower the PRE

of the already weakest phase, the ferrite, even though nickel is

not a factor in the PRE formula, because of its influence on phase

balance and elemental partitioning. This is exemplified in Figure 1

below for an alloy space around 2205.

Looking at initiation sites of the corrosion samples it canbe seen that in samples from alloy E the austenite phase is

predominantly attacked and this the weaker phase, see Figure 2.

The ferrite phase is seen to be the weaker phase in all other

tested alloys.

The results have further been illustrated in Figure 3 to Figure 6

which show the results from Table 3 graphically. The figures show

the CPT as a function of the PRE(N) for the general composition,

which is the commonly used procedure, as well as the PRE(NMn)

in the individual phases; the austenite phase, the ferrite phase

and the weakest phase, which in the case of Alloy E is the

austenite but in all other steels is the ferrite.

3-4/2013 |


5/10

53-4/2013 |

Figure 2 Micro photos of surface sections of polished and etched CPT samples indicating pit initiation in the ferrite to the left for sample A and in the austenite

to the right for sample E (see arrow).

N

Ni

4.6 4.8 5 5.45.2 5.6 5.8

0.23

0.21

0.19

0.17

0.15

PREFCC

36.5 37

36

35.5 N

Ni

4.6 4.8 5 5.45.2 5.6 5.8

0.23

0.21

0.19

0.17

0.15

PREFCC

3.736.5 36

35

34.5

33

34

33.5

35.5

N

Ni

4.6 4.8 5 5.45.2 5.6 5.8

0.23

0.21

0.19

0.17

0.15

Ferrit

55

50 45

40

60

N

Ni

4.6 4.8 5 5.45.2 5.6 5.8

0.23

0.21

0.19

0.17

0.15

PRE Tot

34.5

35.5

35

36

Figure 1 Example of influence of Ni and N on ferrite content and PRE(NMn).

Investergation: 2205CPT22N100 (MLR) Contour Plot T = 1100, Cr = 22.5, Mo = 3


6/10

63-4/2013 |

C r i t i c a l p i t t i n g t e m p e r a t u r e G 1 5 0 ( ° C )

PRE(N)

65

60

55

50

45

40

35

30

26 2927 28 3130 3332 35 3634 37

R2 = 0.848

Figure 3 CPT vs. PRE(N) of the general chemical composition.

PRE(N)=%Cr+3,3%Mo+16%N.

C r i t i c a l p i t t i n g t e m p e r a t

u r e G 1 5 0 ( ° C )

PRE(N)

65

60

55

50

45

40

35

30

26 2927 28 3130 3332 35 3634 37

R2 = 0.228

Figure 4 CPT vs. PRE(NMn) of the austenite phase.


PRE(N)

65

60

55

50

45

40

35

30

26 2927 28 3130 3332 35 3634 37

R2 = 0.949

Figure 5 CPT vs. PRE(NMn) of the ferrite phase.


PRE(N)

65

60

55

50

45

40

35

30

26 2927 28 3130 3332 35 3634 37

R2 = 0.992

Figure 6 CPT vs. PRE(NMn) of the weaker phase. red dot represent Alloy E.

In Figure 3 it is indicated quite strongly that the overall PRE(NMn)

formula only moderately describes the resistance towards pitting

corrosion for the examined materials. Figure 4 indicate no direct

correlation between the CPT and the PRE(NMn) in the austenite

phase while Figure 5, on the contrary, shows quite a good

correlation. However, Figure 6 clearly shows that the PRE(NMn)

of the weaker phase correlates very well with the CPT results

from this investigation.

The PRE(NMn) in each individual phase and the resulting CPT

are further graphically shown in Figure 7. This view shows quite wellthat the CPT follows the PRE(NMn) of the ferrite phase except for

Alloy E where the CPT drops somewhat in spite of an increase in

the PRE(NMn) in both phases. As indicated by the metallographic

investigation, the austenite phase is the weaker phase in Alloy E

and the PRE(NMn) of this phase correlate very well with the slightly

lower CPT.

It is important to note that nickel and nitrogen contents are the

predominant variables in the alloys. The nickel content governs

the alloying distribution between phases and thus the PRE of

each phase, the higher the nickel content, the higher fraction of

chromium and molybdenum in the ferrite. This is quite clear when

looking at the higher alloyed grades in Figure 7: all have quite

similar PRE in the austenite phase while the ferrite phase PRE

is gradually increasing, notably because of the increasing nickel

content in the alloys, Table 1. The CPT also increases and

correlates quite well with the PRE except for Alloy E as shown

in Figure 5 and 6.

Figure 7 PRE(NMn) in ferrite phase and austenite phase from EDS analysis

and CPT.

P R E ( N M n )

C P T

( N M n )

24

40

38

36

34

32

30

28

26

30

70

65

60

55

50

45

40

35

E D X 2 3 0 4

( 3 4 . 2

° C )

L D X 2 4 0 4 @

( 4 2 . 3

° C )

A l l o y A

( 5 2 . 9

° C )

A l l o y B

( 5 3 . 3

° C )

A l l o y C

( 5 6 . 4

° C )

A l l o y D

( 5 8 . 8

° C )

A l l o y E

( 5 7 . 1

° C )

Ferrite phase

Austenite phase

CPT

The result indicate that a switch from ferrite to the austenite

as the weak phase does not occur until the local corrosion

resistance in the austenite is a certain degree lower than thatof the ferrite. It is not seen as soon as the PRE of the austenite

falls below that of the ferrite. The conclusion is therefore that the

pitting resistance of the austenite is better than the PRE suggests,

or conversely that the pitting resistance of the ferrite is lower than


7/10

73-4/2013 |

the PRE would indicate. The reasons for this are open to speculation

but it is hardly surprising that such a simplified, generalized

expression as PRE neglects to take into account the way in which

alloying elements can contribute to the formation, maintenance and

repair of the passive film on fcc and bcc substrates. For example,

it is conceivable that the nitrogen in ferrite has no positive effect,or that molybdenum is more efficiently utilized in fcc. In this

context it should, however, be borne in mind that AES studies

(Olsson, 1996) have shown good lateral mobility of elements

forming the passive film. A further consideration is the phase ratio:

Alloy E has the lowest ferrite content of the 7 alloys tested. In

purely statistical terms a higher percentage of austenite increases

the risk for the weakest “link” to be found in the austenite if the

two phases are equally resistant to pitting. If there is a protective

function of the ferritic phase this should also be lower with less

ferrite.

Looking at the leaner duplex steels, they all have a weaker

ferritic phase which follows which follows the general trend.

See example in Figure 8.

Concept of optimized alloying of a duplex steel

With the previous results in mind one strategy to optimize the

alloying content of a duplex grade is to aim to achieve the highest

pitting resistance for a certain overall PRE. The results from the

2205 variants indicate that the alloying content should be

designed for the ferrite to only just be the weak phase, without any

unnecessary over alloying of the austenite. It seems that if the

PRE in the ferrite phase is 1.5 to 2.5 units higher than that of

the austenite then the ferrite is still the weak phase but if the

difference is larger the austenite becomes the weak phase, this

will also result in a drop in pitting resistance. It should however

be pointed out that this difference for 2205-type grades, and theresult may not be directly applicable to other alloy systems and

duplex grades.

Using Thermo-Calc®, software for thermodynamic calculations,

examination to optimize an alloying window can be performed. As

an example optimizing Cr, Ni, Mo and N for ferrite content as well

as a specific difference between the PRE value in the ferrite and

the austenite have been done around the alloying range of 2205.

The results indicate that the window for optimization is quite

narrow, see Figure 9. Comparing Thermo-Calc® and SEM-EDS-

values a small difference can be seen but the overall trend is very

similar.

Laboratory meltsThe results have been used as a basis for preparation of a set

of laboratory alloys with somewhat higher total PRE(NMn) than

2205. The different PRE(NMn) values and the resulting pitting

temperatures can be seen in Figure 10.

The results indicate that pitting corrosion initiates in the

austenite for alloy I and J and in the ferrite for alloy K and L,

see Figure 11. The earlier interpretations of the 2205 pit initiation

sites are thus reinforced with these observations. A clearly lower

PRE(NMn) in the austenite phase than in the ferrite phase make

initiation take place in the austenite. By comparing the corrosion

result with phase PRE(NMn) it can be concluded that an optimized

alloy in this design window seems to need a PRE(NMn) difference

between phases of between 2 and 6. Additional melts are neededto achieve a more precise definition of the optimization window,

and the difference in corrosion resistance between alloy K and L

merits further elucidation.

Figure 8 Micro photos for surface sections of polished and etched

CPT tested samples indicating initiation in the ferrite (1) for EDX 2304TM

and (2) for LDX 2404®.

Figure 10 PRE(NMn) in ferrite phase and austenite phase from EDS analysis

for four laboratory alloys.

P R E ( M n )

C P

T

( ° C )

21

41

39

37

35

31

29

27

23

35

85

80

75

70

33 65

60

55

50

25 45

40

A l l o y I

( 4 9 ° C )

A l l o y J

( 6 2 ° C )

A l l o y K

( 7 6 ° C )

A l l o y L

( 7 6 ° C )

Ferrite phase Austenite phase CPT

The results show quite clearly that the traditional PRE formula

should generally only be used as a first approximation of pittingresistance. It can however without much change be used as

a good tool for optimization in a local alloying window and for

best use also by considering the resistance of each phase.

1

2


8/10

83-4/2013 |

Figure 11 Micro photos for surface sections of polished and etched CPT tested samples indicating initiations in the ferrite to the left

for sample I and in the austenite to the right for sample K.

Figure 9 Example of optimization using Thermo-Calc® where green areas represent alloys with difference in phase

PRE(NMn) of max 2.5 (higher PRE in the ferrite phase) and austenite PRE(NMn) of min 34 and a ferr ite PRE(NMn) of min 35.5.

CR

T

CR

T

CR

T950 1050 1150

22.8

22.6

22.4

22.2

22

N =

0 . 2

3

950 1050 1150

22.8

22.6

22.4

22.2

22

950 1050 1150

22.8

22.6

22.4

22.2

22

Ni = 4,5 Ni = 5.25 Ni = 6

CR

T

CR

T

CR

T950 1050 1150

22.8

22.6

22.4

22.2

22

N =

0 . 2

950 1050 1150

22.8

22.6

22.4

22.2

22

950 1050 1150

22.8

22.6

22.4

22.2

22

CR

T

CR

T

CR

T950 1050 1150

22.8

22.6

22.4

22.2

22

N =

0 . 1

7

950 1050 1150

22.8

22.6

22.4

22.2

22

950 1050 1150

22.8

22.6

22.4

22.2

22

Sweet Spot Criteria met 3 Criteria met 2 Criterion met 1 Mo = 3

Investergation: 2205CPT22N100 (MLR)

Sweet Spot = Femite (40 – 55) PREFCC (34 – 40). PREBCC (35,5 – 40). DiffPRE (5 – 25)


9/10

93-4/2013 |

CONCLUSIONS

• To optimize an alloy towards pitting corrosion a better tool than

using PRE-formula for the overall composition is to use a PRE-formula

for each phase.

• A key in optimization of duplex steel towards pitting corrosion is

to find the change from ferritic to austenitic pit initiation.

REFERENCES

Olsson, C.-O. A. (1996). Analysis by AES and XPS of the

influence of nitrogen and molybdenum on the passivation of

2205 austenot-ferritic stainless steels. Acciaio Inossidabile.

Pettersson, R., & Flyg, J. (2004). Electrochemical evaluation

of pitting and crevice corrosion resistance of stainless steels

in NaCl and NaBr. Acom.

This article was first published in the Proceedings of the Stainless Steel

World Conference & Expo 2013, 12th - 14th November, 2013, Maastricht,

The Netherlands © KCI Publishing, 2013.


10/10

[email protected]

outokumpu.com

Outokumpu works with its partners to create long lasting

solutions for the tools of modern life and the world’s most

critical problems: clean energy, clean water and efficient

infrastructure. Our goal is a world that lasts forever.

Working towards a worldthat lasts forever.

1 5 3 2 .E N - G B ,A r t 5 8 ,1 2 ,1 3 .

Information given in this brochure may be subject to alterations without notice. Care has been taken to

ensure that the contents of this publication are accurate but Outokumpu and its affiliated companies do

not accept responsibility for errors or for information which is found to be misleading. Suggestions for

or descriptions of the end use or application of products or methods of working are for information only

and Outokumpu and its affiliated companies accept no liability in respect thereof. Before using products

supplied or manufactured by the company the customer should satisfy himself of their suitability.

outokumpu corrosion management news acom 3 4 edition 2013

Documents