Super Austenitic Stainless

18

INTRODUCTION

The use of corrosion resistant stain-less steels and alloys increases inmarine technologies (offshore , des-alinisation, marines civil and milita-ry applications). This is due to theincreasing severity of corrosive envi-ronments and to the increased relia-bility requirements for the equip-ments. Natural sea water appearsparticularly corrosive for stainlesssteels and only highly alloyed stain-less steels can give satisfaction. DCNhas an important feedback on 2507superduplex UNS S32550, S32750,and on NiCrMo alloys, especially al-loy 625 (UNS N06625) and alloyA59 (UNS N06059). These NiCrMoalloys and 2507 superduplex were

used for sea water piping after va-rious studies of corrosion resistancein sea water (ref 1 and 2). A newstainless steel B66 (UNS S31266),seems to present very interestingperformances for marine applica-tions. DCN performed a first evalu-ation of this alloy (on forgings) forapplication in sea water piping, onthe basis of its knowledges of studyand use of 2507 superduplex, alloy625 and alloy A59 (ref 1 and 2).

MATERIALS AND SPECIMENS

Mechanical and metallurgical testswere carried out on two solution an-nealed B66 forgings, heavy and me-dium section (Figure 1). Test speci-mens sampling was performed in 3

different orientations: radial, tan-gential, longitudinal; as well underskin and in the centre of the for-gings, in order to check the productshomogeneity. The mechanical testsconsisted of tensile tests and impacttests. The micrographic examina-tions were carried out on radial cut.The inclusion cleanliness accordingto ref 4, the structure, the grain sizeaccording to ref 5, and the possibleprecipitation of intermetallic phases(�), were characterized. Samples we-re taken in the broken impact testspecimens and were examined withthe STEM to investigate the phasesmorphology, and to analyse themsemi-quantitatively with an X-EDSanalyser. The fine structure was exa-mined using a TEM on thin bladesor after extraction.The crevice corrosion tests were car-ried out on: • alloy 625 seamless tube, diameter

141.3 mm, thickness 9.5 mm, (re-melted ESR )

• superaustenitic B66 seamless tube,diameter 144 mm, thickness 20mm, (remelted ESR)

• The corrosion testing devices aredescribed Figure 11. The crevicecorrosion tests carried out are thesame ones as those presented inthe article ref 1 and 2. These testsare regularly used by DCN becau-se they gave satisfaction to rankthe sensitivity to crevice corrosionof stainless alloys in the conside-red environment i.e. for sea waterpiping:

• a temperature over 4 to 35°C, withtemperature peaks up to 55°C,

• a flow rate of aerated seawaterover 1 to 3 ms-1, and quasi per-manent circulation,

• 100 months without disassem-bling.

S T A I N L E S S S T E E L W O R L D M A R C H 2 0 0 6 www.stainless-steel-world.net

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stainless steel B66By Hervé Le Guyader, Valérie Debout , Anne-Marie Grolleau and Nicolas Dolignon, CETEC DCN Cherbourg.

Franck Demulder, DCN Engineering Cherbourg

Figure 1. Sampling locations in forgings

00_SuperAustenitic 01-03-2006 12:16 Pagina 18

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PRESENTED FEEDBACK

The types of assemblies, and crevicecorrosions met in operating condi-tions on alloy 625 are presented Fi-gure 2, Figure 3, Figure 4, Figure 5.A detail macrographic view of cor-rosion is presented Figure 6. Thecrevice corrosions encountered onthe superduplex 2507 are presentedFigure 7 and Figure 8.

DESCRIPTION OF CORROSION

TESTS

These tests were potentiostatic testswith a potential fixed at + 300mV/ECS, representative value of thenatural potential measured onstainless after colonization by thebiofilm (ref 1 or 2). All specimensurfaces were wet polished to a 600-grit finish, degreased in methanol,and rinsed in deionised water. Thesamples were put in sea water bathsat a temperature ranging from 15 to90°C.

Initiation testsThree types of assembly are used.Two assemblies simulate metal/gas-ket interfaces. The gaskets are madeeither of Glass Reinforced Plastic(GRP) or, synthetic and mineral fi-bres bonded with an acrylonitrileelastomer mix (FAE mix). The GRPgaskets are not used in the sea waterpiping, but they are used as a refe-rence by DCN because they promo-te the crevice initiation. The FAEmix gaskets are those used on seawater piping. The third type of as-sembly simulates a metal/metal in-terface like existing interfaces in seawater piping, like the flanges assem-blies and the piled up metallic an-nulus that exist in many sea waterpiping equipments (cf Figure 5, Fi-gure 7, Figure 8). The interfaces cle-anliness is also a parameter whichcan influence the crevice initiation.It is probable that in the works-hops, metal dust settles on the partsbefore assembly. It seemed thus, ju-dicious to evaluate the influence ofthe iron particles in the crevice. Soinitiation test were carried out onidentical devices to those presentedFigure 11, including iron particles.The purpose of these tests is to de-termine the critical crevice tempera-

Figure 2. Crevice corrosion on alloy 625

flange 1 – under gasket






annulus internal part – metal/metal intrface

Figure 6. Crevice aspect

Figure 7. Crevice corrosion on superdu-

plex 2507 – internal annulus part 1

Figure 8. Superduplex 2507 - internal

annulus part 2



ture of the various crevice corrosiondevices with uncertainty of 10°C,and also especially to determine ifthe insertion of iron particles is apropitious factor to crevice initia-tion. These tests were carried out atconstant temperature. They beginat 40°C, if an initiation is observed(increase of the measured currentand corroded aspect of the sample)the following test is carried out at a10°C inferior temperature, in theopposite case, the test temperatureis increased by 10°C until obtainingthe minimal temperature of initia-tion which is the CCT (Critical Cre-vice Temperature).

All the assemblies were tightened to25 N/m.

Environment: natural sea water

Test duration: from 100 to 1200hours according to obtained results.

Metal/gasket devices:A ring test specimen, of externaldiameter 30 mm and internal 10mm and thickness 5 mm, is tighte-ned between GRP or FAE mix gas-kets with titanium plates, nuts andbolts.

MMeettaall//mmeettaall ddeevviicceess::Two rings test specimens of the sa-me dimensions than the ring testspecimens used for the metal/gasketdevices are squeezed together. Theyare isolated from the tightening sys-tem constituted of reinforcing plateand titanium bolt, by lubricatedgaskets. For the polluted devices, a small

quantity of iron (0,5 mg or 1 mg) isintroduced into the interface gas-ket/metal or into the interface me-tal/metal.

Propagation testsThe purpose of these tests is tocompare the various kinetics of pro-pagation according to the variationof temperature, once the crevicecorrosions are initiated; and to de-termine if the interfaces can be pas-

sivated by decreasing the tempera-ture. The test devices are the sameones as for the initiation tests. Thecrevice corrosion initiation is car-ried out either by drops of hydroch-loric acid, or by ferrous pollution.These tests begin at a sufficientlyhigh temperature to ensure the cre-vice corrosion initiation, then thetemperature is decreased by stage inorder to highlight the corrosion ki-netic for each temperature.

RESULTS AND DISCUSSION

Experience feedback on thesensitivity to sea water corrosion ofalloy 625 and the superduplex 2507.The experience feedback showed animportant sensitivity of alloy 625 tocrevice corrosion in sea water. Crevi-ce corrosions were observed undergaskets of the flanges (Figure 2, Figu-re 3, Figure 4), into the metal/metalinterfaces of piled up metallic annu-lus (Figure 5, Figure 7), often near theO rings, and under bolts (Figure 8).

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Figure 10. Crevice devices for crevice corrosion test

Figure 11. Impact test results

Figure 9. Circuit crevice corrosion assemblies


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Superaustenitic B66 mechanicaland metallurgical characteristicsThe mechanical characteristics (cfTable 1) are very high for superaus-tenitic, equivalent to those of alloy625 grade 1, and only inferior by100 MPa than that of the superdu-plex 2507. The results show a goodhomogeneity of the mechanicalcharacteristics in heavy sectionthickness in any locations andorientations of the samplings. Theelongation (50%) is particularly im-portant. The impact test energy isvery high and not very sensitive tothe low temperatures (Figure 11).The metallurgical structure, illustra-ted on the figure 12, is austeniticwith twinned grain of AFNOR index2, under skin and in the centre. Thealloy is not very sensitive to inter-metallic precipitations in heavy sec-

tions. Precipitations of phases chiand sigma are observed in the cent-re of the forgings (Figure 12 and Fi-gure 13), without effects on the me-chanical characteristics. These re-sults confirm the good metallurgi-cal stability of the B66 stainlesssteel, described in article ref 5,which ensure good mechanical pro-perties and corrosion resistanceeven in the centre of heavy sectionforgings. The metallurgical stabilityof this alloy is, according to DCN,equivalent with that of alloy 625and much superior to that of thesuperduplex 2507.

Sea water crevice corrosionresistance of the superaustenitic B66Two criteria are discussed • The initiation conditions - The

results are gathered in table 3

• The crevice corrosion propaga-tion kinetics – The interpretationis based on the intensity curveobtained at different temperatu-res and on the corrosion aspectafter cleaning.

For the initiation phase, the me-tal/metal interfaces are more criticalthan the metal/FEA mix gaskets on-es. The severity of these metal/me-tal crevices is close to the severityof the metal/GRP crevices. Ferrouspollution under the sealing gasketsis a very critical factor for initiationof crevice corrosion of stainless al-loys in sea water. Under these seve-re conditions, alloy 625 initiate cre-vice very quickly even at low tem-peratures (�< 15°C). The B66 supe-raustenitic alloy shows a resistanceto crevice corrosion initiation supe-rior to alloy 625. Without ferrouspollution, its resistance is over 40°Cand approximately 30°C in theseverest conditions (cf table3).The results obtained on the curvesfigures 14 and 15 confirm that nodifference of corrosion kineticcould be highlighted according tosampling locations and orienta-tions. After initiation of crevice corrosion,the corrosion kinetics depends onthe types of interfaces and highlyon the temperatures. (cf figures 16,17, 18, 19). The corrosions obser-ved on the metal/metal test speci-men are extended on all the inter-face area whereas the corrosionsunder gaskets present more locali-sed attacks. The curves show thatwhen the temperature decreases be-low 40°C, the kinetic of corrosionof B66 decreases notably. The tran-sition is more obvious on the re-sults of the metal/gasket device.The alloy 625 conserves an impor-tant corrosion rate for temperatureFigure 12. Micrographic examination

Al

-

0,4

-

-

C

-

0,1

-

0,03

Cb

3,15

4,15

-

-

Mn

-

0,5

2

4

P

-

0,015

-

0,035

S

-

0,015

-

0,02

Si

-

0,5

-

1

Ti

-

0,4

-

-

Cu

-

-

1

2,5

Ni

64

-

21

24

Fe

20

23

23

25

Cr

8

10

5,2

6,2

Mo

-

-

0,35

0,6

N

-

-

1,5

2,5

W

-

0,4

-

UNS N06625

UNS S31266

PRENW

min

max

46,4

56,0

48,2

59,2

Table 1. Chemical composition of B66 SST and alloy 625 PRENW = %Cr + 3,3(%Mo + 0,5 %W) + 16%N



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of 30 or 20°C. At the temperatureslower than 40°C, the B66 alloy hascorrosion rate significantly lowerthan alloy 625, of a factor 500 formetal/gasket devices and 10 for themetal/metal devices, and tends cle-arly to repassivate. (Figure 17 and19). It should be noticed that, atthe temperatures higher than 40°C,the imposed potential of +300mV/ECS is not justified any morein practice, because the natural bio-film of sea water declines. Underthese conditions, rates of crevicescorrosion strongly decrease (cf ref.1)

CONCLUSION

DCN uses stainless steels and alloysfor its applications in sea water

(cooling sea water piping for boileror auxiliary). DCN has a broad ex-perience feedback on the stainlesssteels superduplex and the NiCrMo

alloys, in particular on alloy 625.These alloys present a significantsensitivity to sea water corrosion.DCN developed specific crevice cor-rosion tests to simulate the sensiti-vity to corrosion of various alloysunder representative conditions ofthe operating conditions (cf ref 1and ref 2). The results of tests werecorrelated with the experiencefeedback and are very satisfactory.In a costs optimization search pur-pose, DCN performed a first evalu-ation of a new superausteniticstainless steel UNS S31266 (B66).The mechanical and metallurgicalcharacteristics on heavy sectionand medium section forgings arehomogeneous. The tensile charac-teristics are quite high for this typeof alloy (yield strength > 420 MPa),equivalent to alloy 625 grade 1. Itscorrosion resistance in sea waterwas tested in comparison to the al-loy 625 for which the behaviour isknown in operating conditions. Su-peraustenitic B66 heavy section for-gings have a very good crevice cor-rosion resistance in sea water, atambient temperature, higher thanthe alloy 625. The price of B66 be-ing potentially notably lower (ap-proximately 30%) than alloy 625because of the composition (lessnickel and molybdene), comple-mentary studies will be undertakento assess the stability and the corro-sion resistance of B66 welding insea water. TIG welding with highlyalloyed filling wire, type UNSN10276 or N06059 is recommen-ded by the manufacturer andshould give satisfaction.

Figure 13. X-EDS analysis of � phase

Table 2. B66 parts - Chemical compositions

Skin Core Requirements

C 0.027 0.025 0.025 < 0.03

S < 0.001 0.001 < 0.005 < 0.02

P 0.020 0.020 0.021 < 0.035

Si 0.18 0.20 0.14 < 1.0

Mn 3.17 3.25 3.00 [2 : 4]

Cu 1.52 1.53 1.62 [1 : 2.5]

Ni 21.0 20.9 21.53 [21 : 24]

Cr 24.0 23.8 24.25 [23 : 25]

Mo 5.50 5.50 5.50 [5.2 : 6.2]

Co 0.076 0.077 0.060 -

Fe 42.5 42.7 Bal. Bal.

W 1.93 1.91 1.90 [1.5 : 2.5]

N 0.0449 0.0449 0.45 [0.35 : 0.6]

O 0.0052 0.0054 -

Forged part tube ASTM A182



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Figure 15. B66 Propagation on B66 - test at 50°C – metal/gasket assembly – different locations

Figure 14. B66 Propagation on B66 - test at 25°C – metal/gasket assembly – different locations



Figure 17. Propagation test with ferrous pollution on B66 – metal/gasket assembly

Figure 16. Propagation test with ferrous pollution on A625 – metal/gasket assembly

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Figure 18. Propagation test with ferrous pollution on B66 – metal/metal assembly

Part Localisation Test specimen Tensile strength Yield strength Elongation

orientation (MPa) (MPa) (%)

Radial 817 484 66

Skin 822 488 63

Tangential 813 459 64

813 463 65

Forging Longitudinal 799 461 65

804 470 65

Radial 808 459 64

Centre 813 463 65

Tangential 808 466 66

804 459 63

Tube Longitudinal 853 465 63

Figure 19. Propagation test with ferrous pollution on B66 – metal/metal assembly

Table 3. Tensile test results


ACKNOWLEDGMENTS

This work was funded by the Fren-ch ministry of Defense. Mr Heuzé(DGA/SPN) is acknowledged for hissupport of this project. The authorswould acknowledge, Aubert & Du-val Company for the alloy B66technical data provided. DCN/CES-MAN laboratory (Mr Corrieu) for itscontribution to the metallurgicalexaminations, and also expresstheir appreciation to the DCN/CE-TEC laboratory (Nicolas Dolignon,Emilie François, Béatrice Avaulée)for their efforts in conducting thecorrosion tests.

REFERENCES

1. H. Leguyader, V. Debout, A.M. Grolleau,

Crevice corrosion of Ni base alloys and

highly alloyed stainless steels in sea water,

Eurrocorr 1999, Event 227 in European

Federation

2. H. Leguyader, V. Debout, A.M. Grolleau,

Crevice corrosion properties of weld over-

lays of alloy 59 for alloy 625 flanges repair,

Eurrocorr 2001, Event ??? in European

Federation …

3. B Mayonobe, F. Roch, R. Cozar , NICRIM-

PHY B66 A 0,5% N Superaustenitic stain-

less steel optimized or heavy section forg-

ings presented at stainless steel 1999

-Science and market - Chia Laguna Sar-

dinia - Italy

4. NF A 04-106 Iron and Steel - Methods of

determination of content of non metallic

inclusions in wrought steel – Part II :

Micrographic method using standards dia-

grams

5. NF EN ISO 643 Micrographic determina-

tion of the apparent grain size

About the author

Franck Demulder has a Mastersof Science in physico-chemistryanalysis of material interfacesfrom the University of Marne laVallée (France 77). Mr Demulderspent 3 years at the Schlumberg-er Riboud Product centre(France), in charge of procure-ment and quality of metallicalloys for wireline tools. (Tools forcharacterisation of oil productionwells). After that he spent 5years at DCN (industrial primecontractor for integrated war-ships) in the engineering depart-ment as materialspecialist for war-ship sub-systemsand seawater pip-ing.

Crevice device

Alloy Metal/GRP Métal/gasket Metal/joint Metal/metal Metal/metal

with ferrous with ferrous

pollution pollution

625 alloy � 20°C � 70°C � 15°C � 30°C � 25°C

Super-austenitic B66 > 35°C � 80°C > 30°C > 40°C > 40°C

Table 4. Critical crevice temperature


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Super Austenitic Stainless