duplex stainless steel

Duplex Stainless Steels 97 - 5th World Conference

AbstractMaterial selection for chemical tanks isof prime concern when consideringbuilding costs, and maintenance costs.This paper outlines why duplex stainlesssteels have been more and more select-ed instead of 316 LN or 317 LN grades forthe erection of new chemical tankers.The main advantages of duplex stainlesssteels, including their resistance to differ-ent corrosion mechanisms, their highmechanical properties which allows thedesigner to reduce weight when properlyused, and their good weldability are em-phasized.The paper gives also some practical ad-vices when selecting duplex stainlesssteels, i.e. how to specify and how toweld them. Finally, some practical experi-ence gained from the use of the materialin marine chemical tankers is summa-rised.

1 Introduction

The number of products carried by chemicaltankers increases continuously with time.The cargoes range from very aggressive so-lution like phosphoric acid, sulphuric acid tomore complex compounds like molasses,fish oil, chemical products, lubricants,methanol or even wine. Some of the prod-ucts are heated and may contain severalquantities of highly corrosive elements likechloride, fluorides, combined with acids.The steel used for the tanks must then beresistant to several mechanisms of corro-sion in order to avoid contaminations effectsof the cargoes as well as damages on thetanks. Furthermore, the steel must also beresistant to sea-water. To-day, material se-lection must also take into account the fu-ture needs which will include the transporta-tion of again more sophisticated andpossible corrosive products as well as moretight regulations concerning the protectionof the nature.In the past, for most of cargo tanks, auste-nitic stainless steels were selected. This in-cluded the 304 LN grade, but more oftenthe 316 L or 316 LN with 2.5 or 2.75 Mominimum.

Creusot-Loire was the first steel company tointroduce duplex stainless steels for theerection of chemical tankers. This was thecase in 1970, when Dunkerque Shipyardbuilt the 3 chemical tankers (Zambeze,Zeebrugge and Zeeland), with UR 50 grade.Those ships have successfully been used inservice. In that time, the production of du-plex stainless steels was almost marginaland CLI was already the world leader in themanufacture of duplex stainless steels.Since that time, steel industries have madecontinuous developments of their process,up to the present time. This had led to a sig-nificant improvement in the control of theresidual elements such as oxygen, sulphur...while at the same time guaranteeing narrowcomposition range including that for nitro-gen.As will be seen below, the accuracy and re-producibility of the chemical compositionenable the amounts of the two phases aand g to be closely adjusted. Furthermore,increased control of nitrogen levels made itpossible to improve the corrosion resistanceand the high temperature stability of the du-plex structure, particularly in the heat affect-ed zones of welds. Finally, the reduction inthe levels of residuals has resulted in amarked beneficial effect on the hot worka-bility, making possible the production ofwide plates.All those improvements have also led to asignificant reduction in production costs,making them more and more cost savingmaterials. This explains why they have in-creased in popularity.

Duplex stainless steels for chemical tankersby Jacques Charles and Bruno Vincent (CREUSOT-LOIRE INDUSTRIE, France)

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Stainless Steel World 1997 KCI Publishing

Authorsindex

Contents

Figure 1.

Home page

Duplex stainless steels for chemical tankers

2 Duplex stainless steels compared toaustenitic stainless steels

Figure 1 shows micrographs of both duplexand austenitic alloys. The duplex stainlesssteel has a balanced 50 a / 50 g microstruc-ture obtained by controlled chemical analy-sis and heat treatment.Figures 2 and 3 present phase diagrams ofduplex alloys. It is observed that the duplexalloys have a lower nickel content while thechromium, molybdenum and nitrogen addi-tions which contribute to the corrosion re-sistance properties, are high. Furthermorenitrogen additions, increase the structurestability of the duplex grades at high tem-perature.Table 1 presents typical analysis of stainlesssteel grades. The PREN value =[%Cr]+3.3[% Mo]+16[%N] is also given.This index (PREN value) is the most com-monly used in order to compare the corro-sion resistance behaviour between thestainless steels.Table 2 presents the mechanical propertiesof duplex stainless steels compared to themof the austenitic stainless steels. The yieldstrength of the duplex stainless steels is al-most twice that of the austenitic grades.Table 3 gives some allowable design stress-es values, following several codes, for theerection of pressure vessels. Those resultsshow clearly that thickness reductions maybe considered with duplex grades whenvessels are properly designed.

3 Duplex stainless steels and structurestability

Figure 4 presents typical time temperatureprecipitations diagrams for duplex stainlesssteels. An increase of molybdenum andchromium contents makes the alloy moreprone to intermetallic phase precipitations.This is particularly the case for the650900 C temperature range where inter-metallic phases like s or c are formed in theferritic grains. This result, as observed in fig-ure 5, in a sharp decrease of the mechanicalproperties.This concerns particularly the toughnessproperties. With a s content of 5%, the alloymay suffer from big losses in toughness aswell as in corrosion resistance properties.When forming those plates containing 5%sigma phase, we can be faced to cracksformations particularly in the segregated ar-eas. Fatigue life properties may also be af-fected.

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Figure 2. Ni Cr equivalent.

Figure 3. Schematic effect of nitrogen additions on thepseudo binary Cr-Ni 68Fe phase diagram.

Figure 4. Typical time temperature precipitation dia-grams for duplex stainless steels.

Figure 5. Mechanical properties of UR52N+ duplex al-loy, as function of sigma phase precipitation by thermalageing.


CREUSOT-LOIRE INDUSTRIE aim is theproduction of hot rolled plates totally free ofintermetallic phases. This is obtained by aclose control of the chemistry, solidificationprocess and solution annealing treatmentperformed on the most efficient equipmentsin the world. The plates are fully waterquenched by immersion few seconds after aplate by plate heat treatment where thetemperature and holding time are optimisedfor each treatment i.e. plate sizes andchemistry. Since more than 30 years,CREUSOT-LOIRE INDUSTRIE has devel-oped the duplex grades because they arecost effectiveness, but always provided ahigh quality product. Duplex grades are lessforgiven than austenitic stainless steels.

4 Duplex stainless steels and physicalproperties

Table 4 presents typical physical propertiesof stainless and structural steels. The mostinteresting characteristics of the alloys in-clude:

A low thermal expansion coefficient forduplex stainless steels when comparedto that of the austenitic stainless steels.The thermal expansion coefficient is simi-lar to that of C.Mn steels. This makes itpossible to reduce thermal stresses whenthe cargoes carried are heated. As a re-sult the number of expansion joints arelimited.

A thermal conductivity similar to that ofaustenitic stainless steels (slightly higher)but much lower than that of the structuralsteel (C.Mn steel).

A strongly magnetic behaviour whencompared to austenitic stainless steels,enabling the use of magnetic clamps dur-ing machining.

5 Duplex stainless steels and corrosionresistance properties

When considering the different corrosionmechanisms, i.e. general corrosion, pitting,crevice, stress corrosion cracking or corro-sion fatigue resistance duplex stainlesssteel UR 45N performs better than austenit-ic 304L, 316LN or 317 LNM grades.

5.1 General corrosion resistance

5.1.1 Phosphoric acidGeneral corrosion resistance properties ofduplex stainless steel 31803 (UR 45N, 2205)

are generally much better than austenitic316 and 317LN grade. Figure 6 presents thecorrosion resistance properties of the du-plex grade UR 45N of CLI when comparedto 316L and 317LN grades in 54% P205 in-dustrial solution. Temperature and chloridecontents may be increased when consider-ing duplex grades compared to austeniticstainless steels.This is explained by the high chromium andmolybdenum contents of the duplex grades.Duplex alloy UR45 is thus particularly welldesigned for phosphoric acid transportationeven if chloride species are encountered.

5.1.2 Sulphuric acidThe behaviour of duplex UR 45N 2205 isalso particularly good when comparing thealloys for sulphuric acid transportation. Inthe case of diluted sulphuric acid solutions(when the cleaning operations occur) to-gether with an increase of temperature which always occur when the remainingacid is diluted with water the duplex gradeUR 45N performs very well when comparedto the more classical austenitic grades. Forthe most concentrated solutions > 95%H2SO4 duplex alloy UR45N behaves as wellas 316LN or 317LN grades (figure 7)

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Figure 6. 54% P205 H2SO4 < 4%Industrial solution.

Figure 7. % H2SO4 = 2.000 ppm Cl-.


5.1.3 Organic acids and caustic mediaGeneral corrosion resistance in organic so-lutions as well as in caustic solutions arealso increased when using UR45N grade in-stead of austenitic 316L grade as presentedfigure 8.

5.2 Localised corrosion resistanceFigures 9 and 10 present compared pittingand crevice corrosion resistance of austenit-ic 304L, 316L, 317LN grades and duplexgrades. Without any doubt, the high chromi-um, molybdenum and nitrogen contents ofthe duplex grade UR 45N, or better UR45N+, make those alloys very resistant topitting or crevice corrosion. This makesthem much more resistant in solutions con-taining chlorides, fluorides, as well as corro-sion under deposits or in crevice like config-urations. Such a resistance increases a lotthe safety of the tanks under nominal in-service conditions. Extra-pollution of thecargoes, including seawater, are accepted(without any heating).The use of 316 LN complementary product(bolts, nuts, clamps, pipes), combined withduplex grades is acceptable, but the 316 LNgrade will be more prone to localised corro-sion when considering heated chloride con-taining solutions.

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Figure 8. General corrosion resistance in organic andcaustic solutions.

Figure 9. Pitting corrosion resistance.(1) Sensitivity to pitting corrosion Effects of temperature and chloride content.(2) ASTM G48A test results 10% Fe Cl3 6H20.

Figure 10. Crevice corrosion resistance.(1) Depassivation conditions of several stainless steels Solution : 30 g/l Na Cl 20 C.(2) ASTM G78 Crevice 10% Fe Cl3 6 H20 solution.


The duplex stainless steels will also resistbetter when considering cleaning heatedseawater operations between two cargoes.We thus recommend the use of duplexstainless steels for all complementary prod-ucts.

5.3 Stress corrosion cracking resistanceStress corrosion cracking may occur gener-ally in heated solutions containing chloridesor other species like H2S and when theplates are stressed. Residual stresses orthermal stresses may in those conditions,particularly on pitting initiation areas, assistthe crack propagation.When they occur, those damages are partic-ularly dangerous and difficult to repair. Insuch conditions, here also duplex stainlesssteels present a very good behaviour (figure11). They perform much better than the aus-tenitic grades 304L or 316L. This is particu-larly due to their microstructure and lownickel content.

5.4 Corrosion fatigue resistanceDue to their high mechanical properties andtwo phases microstructure, fatigue proper-ties of duplex UR45N grade are twice thatof the 316 2.5Mo grade. That result hasbeen obtained on sea-water rotating beambending tests on smooth samples. Such aresult is particularly interesting when con-sidering the cyclic stresses induced on thetanks by the ocean (waves) figure 12

6 Duplex stainless steels and processing

6.1 FormingUR 45N can be cold formed without anyproblem on all equipments suited to workstainless steels. Duplex stainless steels, due

to their high mechanical properties, requiremore strength than austenitic stainlesssteels. Intermediate full annealing heat treat-ment at 1040/1080 C followed by waterquenching is required for cold working de-formations higher than 20%. The ductility ofduplex grades are lower than that of auste-nitic stainless steels, but equivalent to thatof normalised C.Mn steel which are oftenused on the yards. Hot forming should becarried out in a temperature range of 1150-950 C. A final full annealing heat treatmentfollowed by rapid quench to restore thephase balance and corrosion resistanceproperties is requested (1040/1080 C 1900/1980 F interval).

6.2 PicklingDuplex UR45N grade can be pickled withthe same solutions than that of alloy 316LNgrade. Due to the higher corrosion resis-tance properties, the pickling time will beabout twice that used for the 316LN grade.Typical pickling baths are 10% HNO3-2%HF with a temperature which may be in-

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Figure 11. Stress corrosion cracking resistance.

Figure 12. Synthetics sea-water rotating beam bendingof smooth samples.


creased up to 60 C. Final pickling opera-tions on the yard are very important to re-store to the grade, after weldings, all theproperties, including the corrosion resis-tance properties of the alloy.

7 Welding

Duplex grades, like UR45N are now stan-dard grades for most of the manufacturers.They can be welded following all of theprocesses, i.e. TIG-Manual or automatic,Plasma, MIG or manual arc welding withcovered electrodes or submerged arc weld-ing. CLI is ready to provide technical assis-tance staff for specific projects. The generalguide lines are now well known. They in-clude a control of the heat input which is al-most close to 10 KJ/cm and must be opti-mised following the plate thicknesses inorder to obtain a correct microstructure inboth HAZ and weld metal. Welding energyshould always be lower than 15 KJ/cm, in-terpass temperature lower than 150 C andbetter 120 C. Heat input will be particularlycontrolled for cross multipass weldingswhen multi HAZ may affect the microstruc-ture, and thus the corrosion resistance be-haviour, if not properly done.Pre or post weld heat treatments are notrecommended. Usual precautions includingcleaning and degreasing of weld areas, pro-tection against weld sputters must be takento ensure corrosion resistance of finishedproducts. Careful final descaling and/orcleaning of the welds is highly recommend-ed. In order to optimise the properties, theweld consumables will be slightly over-al-loyed in nickel and chromium + molybde-num.Nitrogen contents of the base metal shouldbe better controlled on the level of 0.16N inorder to obtain a better stability in the HAZ.The nitrogen content of the weld should becontrolled and limited to 0.22 maxi. Someadditions of nitrogen (2%) in the shieldinggas are recommended while hydrogen con-taminations will be strictly avoided in orderto obtain enough ductility of the welds andno cold crackings. Lower oxygen contentsare also recommended in order to have thebest properties including the corrosion re-sistance and ductility.One more important advantage of duplexstainless steels, when compared to other316LN grades: the yard may use only onefiller material i.e. the same welding consum-ables may be used for welding duplex gradewith duplex grade or duplex grade with

structural steel! This makes the erectionconditions much more easier and safer. Theuse of wrong welding consumable is nomore possible (two products are used toweld 316 LN grade or 317 LN grade withstructural steel in order to avoid martensiticstructures).

8 Summary: why duplex grades insteadof austenitic grades for chemicaltankers

Table 5 gives a summary of the advantagesof duplex UR45N grade when compared to316LN 2.5 Mo mini.Without any doubt, duplex stainless steelsare the right choice for multipurposes appli-cations for the new generation of chemicaltankers. They offer more safety at a lowercost when tanks are properly designed.

9 How to specify duplex stainless steelsfor chemical tankers

Generally, when specifying stainless steelsfollowing general specifications (Europeanor American standards) the steel industryprovides heats with the minimum allowablelevel of alloying elements. i.e. for a 316Lgrade, the molybdenum content will be 2.00%. For austenitic stainless steels, thisdoesnt affect the microstructure, but onlythe corrosion resistance properties. In orderto obtain the required properties, heats forchemical tankers are then specified with aminimum level or molybdenum (2.5% or2.75 or even higher).When specifying duplex stainless steels fol-lowing international standards, some morerequirements are needed in order to obtainthe right properties including an almost 50 a/50 g microstructure, and enough corrosionresistance properties.Table 6 gives the chemical analysis rangesfor Euronorm and American standards fortwo duplex grades as well as typical CLIchemical analysis.The most important point is the fact thatwhen considering the lowest level of alloy-ing elements for international codes bothstructure balance and corrosion resistanceproperties will be poor.Nitrogen additions in the base metal further-more increase the structure stability of theHAZ. For this reason, nitrogen additions willbe at least 0.15% and 0.16% will be an aim.In order to obtain the best product, the fol-lowing guidelines may be proposed whenspecifying duplex stainless steels.

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9.1 Chemical analysis duplex 31803 / 1.4462C 0.030 Ni 5/6.5Si 0.75 Cr 21.5/23P 0.030 Mo 2.7/3.5S 0.005 N 0.15/0.20Mn 2 PREN value = [%Cr] + 3.3[% Mo] +16[%N] 34

9.2 StructureThe plates will be delivered in the solutionannealed condition followed by waterquenching. They will be free of intermetallicphase precipitation. The microstructure willbe balanced in order to obtain an austeniticratio included in the 4757% range. The re-maining being ferrite.

9.3 Mechanical propertiesMechanical properties will be specified fol-lowing the tables 7 and 8.

9.4 Corrosion testsThe CPT temperature following ASTM G48Atest will be at least 25 C.At 25 C, no pits will be recorded after 24h.Immersion test and the weight loss will nev-er exceed 1 g/m2/24 h.

10 Some specific CLI assistance forchemical tankers orders

Technical assistance has always been amust when considering chemical tankerbusiness. In CLI, a team of about 100 peo-ples is prepared to provide all technical ad-vices needed. This includes a technical as-sistance for the forming and weldingoperations as well as a strong team involvedin the evaluation of the corrosion resistanceproperties of stainless steels in several me-dia. This experience has been centralisedand a service known as SVP Corrosion hasbeen built up. An extensive cargo list hasbeen made available to help the owners inthe selection of products to be carried withtheir chemical tankers. Special services andparticular requests, including corrosion testsresults for specific cargeos, are available.

11 Experiences

CLI duplex plates have been ordered for thethree first chemical tankers projects andbuilt in 1970 at the Dunkerque Yard. Sincethat time, more than 30 000 Tons of duplexstainless steels have been ordered to CLIfor chemical tankers. This includes 7 deck

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Figure 14. Erection stage at Danyard field. (PhotoDanyard)




cargeos tanks for Korean ship yards, 7ships for Stolt Nielsen order now in progressat Danyard, as well as part or the latest or-der placed again by Stolt Nielsen at theAteliers et Chantiers du Havre (ACH FRANCE). Figures 13, 14 and 15 presentsome of the erection stages of the Danyardfield.

12 Conclusion

Duplex stainless steels are used since 1970as cargo tank material. The first use was thebuilding of three chemical tankers atDunkerque shipyard with CLI grade UR50.Since that time, duplex stainless steelchemistry has been modified by increasingthe molybdenum and nitrogen contents.This has made the duplex grades even moreattractive and much more stable in micro-structure. As a result, the nowaday main ad-vantages of duplex stainless steel of the2205 type UR 45N compared to auste-nitic 316LN 2.5Mo grade or 317 LN gradeare

Much better corrosion resistance, partic-ularly when considering localised corro-sion (pitting, crevice...).

Much higher mechanical properties al-most twice of the austenitic grade whichmakes it possible to reduce weight whenthe vessels are properly designed.

Much more compatible with C.Mn struc-tural steel i.e. the thermal expansion co-efficients are closed which result in lowerthermal stresses.

Only one weld product for duplex-duplexor duplex-CMn steel welds which makesthe erection more efficient and safe.

Chemical analysis less affected by nickelprice escalation.

All those advantages make the duplex stain-less steels cost effective and explain whythey are now extensively used as cargo tankmaterial. The only weak point is the need ofhigh quality products i.e. free of intermetallicphases. This is the only way to obtain theoptimum properties including corrosion re-sistance, formability as well as fatigue prop-erties.Such a quality of steel is a CLI standard andachieved tanks optimum equipments whenconsidering melting facilities as well as heattreatment facilities. This is probably one ofthe main reasons why CLI is the worldleader in the manufacture of duplex plates,and that more than 30 000 Tons of duplexgrades have been placed with CLI in thechemical tank business.

References1 J. Charles, Duplex Stainless Steels for Chemical

Tankers Marichem Cologne November 1995.

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Duplex Stainless Steels 97 - 5th World Conference D97-018PAGE 735


Table 1. Chemical analysis of duplex and austenitic stainless steels.

Table 2. Mechanical properties of duplex and austenitic stainless steels.

Table 3. Allowable design stress values for several pressure vessel codes.

Table 4. Typical physical properties of stainless steel alloys.

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Table 5.

Table 6.

Table 7.

Table 8. Mechanical characteristics according to the temperature.

duplex stainless steel

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