informacion outokumpo

7/28/2019 Informacion Outokumpo

http://slidepdf.com/reader/full/informacion-outokumpo 1/12

acom AVESTA SHEFF I E L D CORROS ION MANAGEMENT AND APPL ICAT I ON ENGINEER ING 4-2000

Choice of Specificationsand Design Codesfor Duplex Stainless Steels

stainless structural steel, representing apotential area for further growth. Ingeneral the service of duplexinstallations has been very successfulresulting in wider use.

A rapid growth has involved manynew operators manufacturing andfabricating duplex steels includingthose with limited experience. The mainproblems encountered with duplex

stainless steels have been related to heattreatment and welding. Unsuitable heattreatment has resulted in precipitationof intermetallic phase and deteriorationof toughness and corrosion resistance(2,3). Although most welding methodscan be used to weld duplex steels, theyrequire special procedures for theretention of properties after welding.Deviations from established weldingprocedures have caused problems (4).As a result users have applied several

restrictions and tests that in some waycould be considered as over-specifying.

This paper reviews some of thespecifications and design codes anddiscusses their applicability.

GENERAL SPECIFICATIONSFOR DSSThe two principal internationalstandards describing duplex stainlesssteels are ASTM/ASME and EN. Someof the standards are presented andbriefly discussed below. To limit thelength of this paper the discussion ismainly confined to three duplex gradesoften listed as 2304, 2205 and 2507respectively. Other commondesignations are 23Cr, 22Cr and 25Cr.These grades cover most applicationareas although similar grades areavailable. The international steelnumbers of these steels are listed in

ASTM and EN standards in Table 1where the composition ranges areshown.

It was not until the 1980’s,coinciding with the gradual introductionof the second generation of duplexstainless steels, that they found a wideruse and were manufactured in largertonnage by several producers. Importantapplications were pipeline systems foroil and gas industry, tanks for marinechemical carriers and pressure vesselsfor pulp industry. These areas are stilldominating but many new applicationareas have been added. Standards,specifications and special requirementshave developed alongside with thegrowth of this family. However, lack of data in some areas and, to some extent,

conservatism restricts their full use. Thehigh mechanical strength of duplexsteels makes them very attractive as a

INTRODUCTIONDuplex stainless steel is the currentcommercial designation of this steelfamily originally introduced as ferritic-austenitic stainless steels. In somestandards they are also denominatedaustenitic-ferritic steels. The firstduplex stainless steels were producedcommercially almost seventy years agobut did not appear in national standardsuntil around 1950. The main advantagesidentified with the duplexmicrostructure were the high strengthand improved resistance tointergranular corrosion compared toaustenitic grades. The development of

this family of stainless steels into thesecond generation of duplex steel hasbeen described elsewhere (1).

By Mats Liljas, Avesta Sheffield AB,

and Göran Gemmel, Avesta Sandvik Tube AB, Sweden

Duplex stainless steels have gained a large commercial interest in the latest

decades due to their good combination of strength and corrosion resistance.

Being a relatively new type of material to most users, this family of steels has

been subjected to a large variety of tests and specifications.

The paper reviews and discusses some international standards and special

requirements regarding manufacturing process, composition, microstructure,

corrosion tests and mechanical requirements. Available international standards

are generally sufficient to specify for most applications.

The use of quantitative metallography in acceptance criteria is a source of

confusion and alternative methods are possible. It is recommended that

acceptance of a certain product or procedure should be based on testing of

engineering properties of practical concern.One advantage of using duplex steels in loaded constructions is the possibility

to reduce the wall thickness compared to austenitic stainless steels. It is well

known that European design rules allow thinner sections and thus better cost

benefits than ASME codes. Different regulations and their consequences for

duplex steels are discussed.



acom 4 2000

2

Chemical compositionThere is no difficulty in meetingstandard specifications of chemicalcompositions. Individual steelproducers have narrow target

compositions within the specification tomeet different criteria. Duplex steels aresensitive to variations in composition,particularly of those elementscontrolling the phase balance.Therefore, the relatively broad chemicallimits permit large variation inproperties. It is thus more important tospecify what property profile isexpected from the product rather than acertain composition range. To specifythe minimum pitting resistanceequivalent (PRE) value based on

compositional condition to attaincertain corrosion resistance may resultin harmful deviation in other properties.The PRE concept can be used only inranking alloys processed to optimisepitting corrosion resistance and shouldnot be included in specifications.By applying narrowed compositionranges such as in the new standardUNS S32205 such problems inover-specification are avoided.

The specified minimum mechanicalproperties at room temperature for plateand pipe in both standards are shown inTable 2. The values are quite similar inboth standards the only substantial

difference being the tensile strength of the superduplex grade UNS S32750.

MANUFACTURINGPROCESSESGeneralThe process routes for the production of duplex steels are, to a great extent,similar to those for austenitic stainlesssteels. Individual process stepsinfluence the product properties invarious ways. However, experienced

producers have established proceduresfor optimum properties at justifiedproduction costs.

Heat treatmentOne of the most crucial process stepsinfluencing the product properties is thefinal heat treatment. Duplex stainlesssteels are more vulnerable to deviationsin heat treatment than austeniticstainless steels and several problems

have been reported from differentprojects (3). The low strength atsolution annealing temperaturesinvolves a potential for undesireddeformation of the product.

A great concern is the risk of precipitation of intermetallic phaseswith detrimental effects on toughnessand corrosion resistance. In most casesexcursions from the specifiedtemperatures are the cause of thefailure. Another reason could bee tooshort holding times insufficient todissolve intermetallic phases. This mayoccur in a highly loaded furnace wherethe heating time is long and thematerial is exposed long time to atemperature region where intermetallic

phase precipitates. Recommendedtemperature ranges are listed in moststandards (Table 3). It should be notedthat individual manufacturingprocedures require a smaller range foroptimum and consistent result. At leastfor thin walled material the annealingtimes are short and if the heat treatmentis carried out in a continuous furnace,the upper part of the temperature rangespecified is usually preferred or even

Table 1.Specification limits of selected elements in modern grades of duplex stainless steels. Weight compositions, maximumunless otherwise stated.

UNS/EN C Si Mn Cr Ni Mo N Cu

S32304 0.03 1.00 2.50 21.5 – 24.5 3.0 – 5.5 0.05 – 0.60 0.05 – 0.20 0.05 – 0.601.4362 0.03 1.00 2.00 22.0 – 24.0 3.50 – 5.50 0.10 – 0.60 0.05 – 0.20 0.10 – 0.60

S31803 0.03 1.00 2.00 21.0 – 23.0 4.5 – 6.5 2.5 – 3.5 0.08 – 0.201.4462 1.00 2.00 21.0 – 23.0 4.50 – 6.50 2.50 – 3.50 0.10 – 0.22

S32205 0.03 1.00 2.00 22.0 – 23.0 4.5 – 6.5 3.0 – 3.5 0.14 – 0.20

S32750 0.03 0.80 1.20 24.0 – 26.0 6.0 – 8.0 3.0 – 5.0 0.24 – 0.32 0.51.4410 0.03 1.0 2.0 24.0 – 26.0 6.00 – 8.00 3.00 – 4.50 0.20 – 0.35

Table 2.Mechanical properties at room temperature according to ASTM1 and EN2 standards of plate and pipe

Grade YS, MPa TS, MPa Elongation, % Charpy V, JStandard ASTM EN ASTM EN ASTM EN EN

S32304 400 400 600 630 25 25 60

S31803 450 460 620 640 25 25 60

S32750 550 530 795 (800) 730 15 20 601 ASTM: A240 plate, sheet. A790 welded and seamless pipe.2 EN: 10028-7 plate, sheet and strip. EN 10217-6/7 welded and seamless pipe.



acom 4 2000

catastrophic oxidation. ASTM A790requires heat treatment of the pipe. InA928 heat treatment can be excludedby agreement. However, there arecircumstances when a heat treatment is

not necessary because the properties areonly marginally improved, or evenimpaired. Heat treatment of weldedpipe should only be required whenthere is enough evidence that animprovement is achieved and necessaryfor the application. Depending on howand where the pipes will be used it isup to the end user to decide which typeof heat treatment is necessary. Forheat-treated pipes there are nostandards, which prescribe the holdingtime. Pipes delivered to the same

specification can have varyingcorrosion resistance, and themechanical properties can differconsiderably, depending on whether thepipes have been induction heated in line(5–10 sec) or solution annealed (about3–5 min). Bright annealing of duplexwelded tubes, in line or separately, isnot recommended as this often meanstoo slow cooling rates, with impairedcorrosion resistance as a consequence.

WELDINGGeneralFusion welding of duplex stainlesssteels has been the subject of substantial research during the lastdecades. One reason is that it is afascinating metallurgical topic withmany interesting reactions occurring.Another reason, naturally, is that thisarea is crucial to control for fabricationof sound constructions. The area is nowwell understood and weldingprocedures are established. Theessential aim is to control that parentsteel properties are retained in the weld.Available matching filler metals aredesigned to produce a weld metal withadequate properties and should be usedfor most “practical” joints. Nitrogenaddition to the shielding gas is currentlyused in automatic welding to controlthe nitrogen level in the weld metal andhence the weld properties.

Well known are the effects of low

heat input giving a risk of high ferritein the high temperature heat-affectedzone (HAZ). This is a much lesser

material will deform heavily and theproducts have to be straightened.

Heat treatment of welded pipesThe main reason for heat treating highalloy austenitic welded pipes with lowC- and high Cr- and Mo-contents is torestore the resistance to pittingcorrosion of the weld by reducing thesegregation of Cr and Mo which haveoccurred during solidification. For theduplex grades the partitioning of Cr,Mo and N between the ferrite andaustenite must be balanced in order tooptimise the corrosion resistance in theweld. If over-alloyed filler metal isadded, heat treatment of pipes with wallthickness above about 3 mm is not

always necessary. By optimising thewelding procedure it is possible toensure that the necessary corrosionresistance and strength are achievedwithout heat treatment. Other reasonsfor solution heat treatment oftenmentioned are:– Reduced stresses and hardness caused

by the forming of the strip/plate to apipe.

– Dissolution of carbides and otherprecipitates in the HAZ.

These reasons are not always relevantand the small improvements must be judged compared to the possibledisadvantages, which may occur, suchas collapse of thin walled pipe, slowcooling rates or surface defects due to

exceeded. This is accepted andexpressed in a footnote in the ENstandard. The lower temperatureregions could be on the borderline fordissolution of intermetallic phase and

should be avoided, particularly for thehigher alloyed variants.The cooling rate from the solution

annealing temperature is also of importance and more critical for thehigher alloyed grades. Extremely rapidquenching, particularly from the uppersolution temperature region, can resultin precipitation of nitrides and loss inproperties. If the cooling rate is too lowthere will be a risk of precipitation of intermetallic phases and a risk of impaired properties. A slow cooling

may result in a precipitate freemicrostructure but less exposure timeavailable in the critical temperaturerange (700–900°C) for furtherprocesses such as welding. This is of most concern for the highly alloyedsuperduplex grades. For abovedescribed reasons there arespecifications requiring “waterquenching” of duplex products. Fortype 2205 the propensity for thisbehaviour is hardly a great problem. It

has been shown that also after amoderate cooling this steel can bewelded with very high arc energieswithout any sign of loss in toughnessand pitting corrosion resistance (5).One effect of the quenching is that the

3

Table 3.Heat treatment of duplex plate and pipe

Steel grade ASTM A790 EN Cooling1

S32304 925 – 1050°C 950 – 1050°C Air or water

S31803 1020 – 1100°C 1020 – 1100°C Air or water

S32750 1025 – 1125°C 1040 – 1120°C Air or water

1 Cooling rate shall be rapid.

Table 4.Temperature limits for duplex steels in pressure vessels.

EN UNS VdTÜV WB EN ASME

1.4362 S32304 –40 – 280°C –20 – 250°C –20 – 600°F

1.4462 S31803 –40 – 280°C –20 – 250°C –20 – 600°F

1.4410 S32750 –30 – 250°C –20 – 250°C –20 – 600°F



4

acom 4 2000

problem in second generation duplexgrades and particularly in superduplexgrades. Too high heat inputs may causeformation of intermetallic phase in thelow temperature (~700–900°C) HAZ.

As mentioned above, this is seldom aproblem in lean or medium alloyedduplex grades but of some concern forthe superduplex steels.

In principle, all methods used forwelding austenitic stainless steels canbe used for duplex steels, provided thespecific limits for duplex steels of eachprocess are known. For example, gasmetal arc welding (GMAW) has notbeen allowed for certain projects withthe argument that this method involveda risk of lack of fusion. Naturally, it is

required to perform a weld procedurequalification (WPQ) for any process toverify that desired properties areattained. Use of hydrogen addition tothe shielding gas is frequentlyprohibited due to the risk of hydrogenembrittlement. With control of steel andweld chemistry and of the proceduresthis risk is minimal and miles of weldedand annealed pipes have beenmanufactured using hydrogen in theshielding gas with no problems.

Examples of specificationsfor welded duplex pipe

Almost each oil company has its ownpipe specification for 22Cr duplexstainless steels (Table 5).

The requirements are usually amixture of own experience andconsultant’s ideas. All requirements areset in order to secure the materialproperties, which are:

– Strength– Toughness– Corrosion resistance

Chemical composition, welding andheat treatment are process parametersthat determine the properties. To controlall these parameters, somespecifications are “over-specified”,which sometimes lead to conflictingrequirements. One example is when anover-alloyed filler wire with 9% Ni is

required for the longitudinal weld, andnitrogen is added to the gas for pittingresistance, in which case the ferritecontent after heat treatment may fallbelow 30% in the weld. Mostspecifications allow minimum 30%ferrite in the weld, due to an expectedrisk for SCC, in spite of that the risk forSCC being negligible in the intendedprocess environment.

The NORSOK specification allowsmin. 25% ferrite, which is a reasonable

and safe level to achieve the expectedresistance to SCC. Specification of microstructure will be discussed furtherbelow. Our opinion is that mechanicaland pitting corrosion tests are enough tosecure a product with the necessary andexpected properties.

SPECIAL REQUIREMENTS AND METHODS FOR ASSESSMENTIn many projects using duplex stainlesssteels several special requirements are

included in the specifications. This maybe needed, as there is an ambition tocontrol those suppliers and fabricatorsthat are handling a less mature steelfamily properly but the result is alsothat the costs increase and theavailability will be restricted. Anexcellent review of testing methodsapplicable to duplex stainless steels wasmade in 1997 (6). The followingcriteria on special requirements wereput forward:

– Conservative capability to address theproperty of interest.

– Definition of a test sample, location,test frequency and acceptancecriterion.

– Applicability to all product forms andconstructions of interest.

– Cost.

Some further aspects of specialrequirements will be discussed below.

Microstructure

Naturally, the microstructure is of greatimportance for duplex steels as mostproperties are closely related to thephase balance between austenite andferrite. However, microstructure is not aproperty but a state of the metal andmuch confusion emanates from unclear

Table 5.Specifications for 22Cr welded pipe.

Pitting YS, Hard-Chemical Ferrite % test min. UTS ness Toughness CTOD Hydropr.Specifier comp. PREN base weld G48A MPa MPa max. base weld FL base test

A N>0.145 34 40 – 60 40 – 60 25°C 450 620 290 54 J at

–40°C

B N>0.14 35 35 – 55 30 – 65 30/22°C 450 620 28 Rc 60 J at 95% x YS

–50°C

C N>0.16 35 35 – 60 30 – 60 25°C Rt 0.5 yes

>450

D – 33 40 – 60 35 – 65 25°C 448 621 28 Rc 45 J at yes yes yes 95% x YS

–35°C

E N>0.14 – 35 – 55 25 – 60 No 450 620 28 Rc 45 J at yes yes

–46°C



5

acom 4 2000

or inadequate requirements onmicrostructure. There are relevanttechnological tests to verify that aproduct in a duplex stainless steel issuitable for its purpose without the

microstructure documentation. Becausethe phase balance in annealed millproducts has very low heat-to-heatvariation, specification or determinationof a certain range is of limited value.

PHASE BALANCE

For duplex steels the ferrite content islargely depending on the chemicalcomposition and the thermal history.For the parent steel it is controlledwithin a fairly narrow range. In manycases the measuring methods

commonly used give a greater variationthan the process variation. Theinfluence of variation (+/–15%) of thephase balance on properties such astensile strength and corrosion resistanceis small. The second generation of duplex stainless steels is in generalaimed to contain 40 – 50 % ferrite inthe parent steel resulting in optimumproperties. No ferrite content isprescribed in the internationalstandards. There are, however, otherspecifications requesting a ferrite range.

The most common specified rangesare 40–60% for the parent metal and30–60% for the weld area. However,some of the most experienced end usersof duplex and superduplex steels areusing the Norwegian standard NORSOKthat specifies 35–55% for the basemetal and 25–60% for the weld area.

The general opinion is that a too highferrite content, i.e. >70%, decreases thetoughness and pitting resistance, and atoo low ferrite content, i.e. < 25%,

decreases the SCC-resistance.What really matters is that the corrosionresistance and mechanical propertiesfulfil the engineering requirements. Theferrite content is not a property, but away to check that the welding and heattreatment have been properly done.Therefore the limits should be within areasonable range and be used forcontrol only, and in case of deviationlead to an extra check of the materialproperties.

MEASURING FERRITEAs stated above, measuring the ferritecontent and intermetallic phases can be

done for monitoring consistency inprocessing, but the microstructureshould not be a cause for rejection, aslong as the engineering properties arewithin the specified limits. Our

recommendation is that assessmentof microstructure should be of informative character and not a causeof rejection.

Magnetic methods are attractive, asthey are non-destructive andinexpensive. However, they only give aqualitative measure even if propercalibration standards are used. Forwelds where ferrite numbers (FN) arespecified magnetic methods areregularly recommended using forexample a measurement system defined

in ISO 8249 and AWS A4.2 (7). Theferrite number is then rather a physicalproperty and cannot directly betranslated into a ferrite content. A moredirect method for measuring the volumefraction of ferrite is metallographicexamination. The manual procedureaccording to ASTM E562 is currentlyreferred to in specifications but it istime consuming and to some extentsubjective. As the method involves arelative error of ±10%, at the best, the

requirements should involve an averagevalue and tolerated extremes. As anexample, if a minimum ferrite contentof 40% is required and themetallographic assessment gives35 ± 5% the product should beaccepted. Automatic methods adoptingimage analysis systems are usedregularly. However, there is no standarddirectly written for duplexmicrostructures. ASTM E1245 isprimarily designed for measuringinclusion content and is not fullyrelevant for ferrite measurement. Theetching technique used is very criticalto the result and details should beincluded in such a standard.

PRECIPITATES

As shown in several papers manydifferent phases can occur in duplexsteels and extensive research has beendevoted to defining their conditions andeffects (8, 9). Duplex steels are moresusceptible to precipitation of

intermetallic phases than austeniticstainless steels due to the high Cr- andMo-contents and high diffusion rates in

the ferrite phase. Problems haveoccurred with reduced toughness andcorrosion resistance due to excessiveamounts of intermetallic phase (10).The fact that such properties are

directly effected also makes it possibleto control the level of impairment bytechnological test methods. Theformation conditions of intermetallicsmay vary considerably depending onprocess thermal cycles resulting indifferent particle size, amount andcompositional gradients.

A certain percentage of secondphases has an ambiguous correlation toa property (11, 12). Thus, it is hardlypossible to judge from themicrostructure what amount of

intermetallic phase is acceptable for acertain purpose. It is also unrealistic torequire a microstructure completely“free from precipitates” as this is notquantifiable and therefore a requirementdifficult to interpret. A paper,recommended for publication byInternational Institute for Welding(IIW), Commission IX, regardingmetallography of weldments, supportsthis view (13).

A concept analogous to ASTM A262

for austenitic stainless steels is used inASTM A 923; metallographicexamination is used as a screening testand an affected microstructure is furtherevaluated by other methods. In thisstandard, mill products cannot berejected by the metallographicexamination per se but presence of detrimental intermetallic phases istested with impact testing or corrosiontesting. The standard thus offersdifferent methods to verify that an‘affected’ microstructure has nodetrimental effect on the material and,depending on the conditions, eithermethod can be used and be reported.This standard should be used only toassure the absence of harmfulintermetallic precipitates and not toassess the suitability for any service.For welds the situation is morecomplex and A923 is less applicable asa test for presence of intermetallicphase, as other microstructureconditions such as non-metallic

inclusions, nitrides or segregationeffects also could impair theproperties.



6

acom 4 2000

Charpy impact testingToughness criteria for steel aretraditionally set to assure that the riskof brittle fracture is avoided. Austeniticstainless steels do not show a brittle

transition and in general there are norequirements on toughness testing onparent material. In absence of completedata for duplex stainless steelsspecifications have been based on thosefor ferritic structural steels. This is aconservative approach considering thegradual transition behaviour for duplexsteel with substantial amounts of theductile austenite. EN specifiesminimum Charpy V impact toughnessNumber of CVN ≥ 60 J at roomtemperature as shown in Table 2

(transverse direction, CVN ≥ 100 J inlongitudinal direction). Theserequirements are reasonable. In thecoming EN pressure vessel standard60 J will be required in the joints.However, this could restrict the possiblewelding methods. Recent investigationsat TWI conclude that a relevantrequirement on duplex steels is CVN≥ 40 J at minimum operatingtemperature (14). Substantialdocumentation of thick plate confirms

that duplex steel and welds havingCVN ≥ 35 J still show ductilebehaviour (15).

It is suitable to use a differentcriterion when the purpose is to verifythat the product has undergone correctheat treatment. In ASTM A923, forexample, the 2205 parent metal shallpass an impact test (CVN ≥ 54 J at– 40°C) for acceptance.

In national standards there are no oronly room temperature toughnessrequirements on duplex stainless steels(Table 2). For certain application areas,particularly oil and gas industry, lowservice temperatures have necessitatedmore stringent specifications. Frequentrequirements for parent metal andwelds are minimum 45 J average at–46°C for both duplex and superduplexsteels.

CTODCrack tip opening displacement

(CTOD)-testing is a fracture toughnesstest developed for carbon steel and itsrelevance and usefulness for

specification of duplex steels is underdispute. Fracture toughness data arenecessary for design purposes andshould be collected for different steeltypes and products. Correlation between

CTOD and impact energy have beendeveloped for duplex steels (14) andtherefore it would be sufficient tospecify impact toughness testing as aroutine check of the material.

Hardness measurementSome pressure vessel standards includea maximum hardness value to verifythat the material has been properlyheat-treated and has not been subjectedto excessive cold deformation. EN hasno such requirement and it could be

argued if such a requirement will assureany improvement of the steel. There arealso hardness requirements for servicein environments containing hydrogensulphide involving a risk of sulphidestress corrosion cracking. Thisrequirement is based on the conditionthe actual steel was documentedaccording to NACE MR0175 (16). Asdescribed below under corrosion testingit should be noted that each steel gradehas its individual environmental limits

listed in NACE MR0175.NACE maximum requirements are36 HRC for UNS S31803 and32 HRC for UNS 32750.NORSOK requires for both parent andweld metal 28 HRC for UNS S31803and 32 HRC for UNS 32750 (17).

Corrosion testsCorrosion testing is frequently used torank alloys concerning their resistancein different aqueous solutions. Data areavailable in corrosion tables or curvesand diagrams (18, 19). If an end userwants to have information concerningthe performance of a steel in a certainservice environment a tailor made testcan be performed. Such testing isseldom included in a specification.Corrosion testing is also used to verifythat the steel has been properlymanufactured to achieve optimumproperties. Classical corrosion tests foraustenitic stainless steels are themethods to assess the susceptibility to

intergranular corrosion. The duplexmicrostructure possesses a much higherresistance to this form of corrosion and

the standard tests such as ASTM A262Ewill not give any response on duplexsteels.

PITTING CORROSION TESTS –ASTM G48A

Duplex steels are more frequentlyspecified to meet requirementsregarding pitting corrosion. The mostcommon specified corrosion test isferric chloride testing, e.g. ASTMG48A, and modified versions. This testis considered to be a check of themetallurgical condition of the steelrather than of its performance in anyservice environment. This is forexample the case of ASTM A923. Theferric chloride solution is very

aggressive and resembles very fewactual industrial service environments.

Specified test temperatures varyconsiderably and are 22.5–30°C for22Cr duplex and 35–60°C for 25Crduplex. Although a 72hour test period isstipulated in ASTM G48A, a 24hourexposure is usually specified. For 22Crduplex, the experience from service aswell as from testing according tovarious specifications is extensive andshows that a test temperature of

22.5–25°C is suitable to ensure thatmaterials properties after welding andheat treatment operations are within thelimits expected for 22Cr duplex. For25Cr duplex, a combination of laboratory test results and serviceexperience, although not as extensive asfor 22Cr duplex, strongly indicate that50°C is a suitable test temperature forsolution annealed products. Evaluationcriteria vary between test procedures.The most objective and reproduciblecriterion for approval is a maximumtolerable weight loss, as in e.g. ASTMA923, although it is not a fully relevantquantification of pitting corrosion.

STRESS CORROSION CRACKING (SCC)

It is well established and experienced inpractice that duplex stainless steelshave a superior resistance to chloride-induced SCC compared to conventionalaustenitic grades. The SCC resistancedepends on many environmental andmetallurgical factors. There is, however,

no simple accelerated laboratory testavailable today which generates aranking valid for all chloride



7

acom 4 2000

environments (20). Tests that have beenspecified in certain projects for duplexsteels are thus questionable and are notrecommended.

Another cracking mechanism of

concern for duplex steels is sulphideinduced stress corrosion cracking(SSCC). This is of relevance in the oiland gas industry handling hydrocarbonscontaining hydrogen sulphide (sourenvironment). The susceptibility toSSCC depends on environmentalfactors such as hydrogen sulphidepartial pressure and steel condition.Several duplex steels are approved foruse in sour service and are listed inNACE MR0175. The approvals can beregarded as procedure approvals and

are based on SSCC testing in certainenvironments and in differentcold-worked conditions. This is anextensive and costly documentation.Each steel grade is listed with allowedenvironmental conditions andmaximum product hardness accepted.Project specifications may includeSSCC testing in cases when theenvironment is beyond that in NACEMR0175 but this is a very time-consuming and expensive procedure.

DESIGN RULESTemperature limitations

Duplex steels are being used more andmore in critical components such aspressurised vessels and pipe systems.Naturally, restrictions in environmenthave to be set for various applications.The nature of the micro-duplexstructure limits the use of duplex steelsboth to low and elevated temperatures.

Use of the duplex steels outside thepermitted range could involve hazards.The German pressure vessel codes

(VdTÜV WB) specify differenttemperature ranges for parent steels of various duplex grades as shown inTable 4. For welds TÜV specifiesallowed maximum temperature 30°C

lower than for the parent material dueto a higher susceptibility toembrittlement in the weld metal. InASME and the coming EN standardsfor pressure vessels the temperaturelimits are not explicitly listed. However,the designer can conclude from thetemperature range that strengthrequirements are listed whattemperature is allowed. The maximumtemperature appears to be 250°C for allthree grades. This is at large inaccordance with the existing TÜV rules

for welded constructions.

Wall thicknessIt is well known that the pressure vesseldesigns using EN or ASME rules differsubstantially as the European system isbased on yield strength values whileASME rules are based on tensilestrength (21). The different concepts areboth results of old traditions difficult tochange. According to EN the exchangeof 316 with 2205 gives a reduction inwall thickness of up to about 35%

while ASME rules only permit about20% thinner wall. A certain vessel in22Cr duplex designed according to ENwill have 40% less wall thickness thandesigned according to ASME (21). Thisdifference remains with the change inJuly 1999 of ASME rules using a safetyfactor against the tensile strength of 3.5instead of 4. The different rule systemsmay be one reason why duplex steelshave been used more extensively inEurope. For offshore pipelines some oil

companies are using own design criteriamore based on European codes to takefull advantage of the material.

Dynamic loads

There are limited data in the literaturefor stainless steels in general to be usedin structural codes, particularly forwelded joints. The very limited data for

duplex stainless steels indicatecomparable performance to those of structural steels (22). Recent Europeanwork on several joint configurationsshow that fatigue data of duplexstainless steel fall within thescatterbands enclosing the extensivedatabases for structural steels. Thedesign S-N curves for welded structuralsteels are thus applicable also forduplex steels (23).

Welded pipe issuesASTM STANDARDS FOR WELDEDDUPLEX STAINLESS STEEL PIPE

All of the mentioned grades areincluded in the most common ASTM,ASME and ANSI standards. The twomost common ASTM specifications forwelded duplex stainless steel pipe areA790 and the new A928 for duplexpipe welded with filler.

The important differences betweenASTM A790 and A928 are described in

Table 6.These parameters are covered todifferent degrees in the various nationalstandards, however there are somesignificant differences, which should bepointed out. A specifier who wants onespecification for a package of pipe sizeslike NPS 1/2–36 (21.34–914.9 mm) inSch 10S and 40S will have problem if ASTM standards are used. A790, anoften-used ASTM specification includesboth seamless and welded pipe with noaddition of filler metal. When A790 isspecified for thick walled welded pipe,problems with undercut, incompletepenetration and lack of fusion may existon thickness above about 6 mm (1/4inch) if normal welding techniques, likeGTAW and PAW are used. When pipesare welded from plate it may bedifficult to get enough power to pressthe plate edges together in order toproperly fill the weld gap. The qualityof such welds is questionable becauseof the higher risk of weld defects.

It is much better to add filler metalwhen welding heavy walled pipebecause it will minimise the risk with

Table 6.The important differences between ASTM A790 and A928.

ASTM A 790 ASTM A 928

Filler metal No Yes

Weld classes One Five

Non-destructive test, NDT Hydrotest or EC HT or EC/Radiogr. exam.

Heat treatment Yes Yes/no



8

acom 4 2000

lack of fusion and an incomplete fill upof the weld. Filler metal also providesthe opportunity to improve corrosionresistance through the use of higheralloy filler metals.

Principally, there are only advantagesin using filler metal on thick walledpipe. An alternative to A790 for duplexheavy wall pipe is ASTM A928.However, as these two specificationshave different requirements on use of filler metal, heat treatment and NDT, achange will create new problems for themanufacturer. The pipe producer mustpropose deviations for some sizesbecause of restrictions regarding theaddition of filler metal and the amountand type of testing coupled to this.

Contractors and package buildersseldom have the authority to approvesuch deviations.

A possibility is that the pipespecification should have options to useeither seamless or welded, and alsooptions to use either A790 or A928.This will increase the availabilitywithout reducing the quality.

JOINT EFFICIENCY FACTOROR WELD FACTOR

ASME/ANSI rules allow a jointefficiency factor of 0.8 for hydro-testedwelded pipe and 1.0 for 100%-radiographed pipes. This means thatwhen calculating the wall thickness fora specified internal pressure, 100% of the strength can be utilised forwelded pipe, the same as for seamlesspipe, when the pipe weld is X-rayed100%.

HYDRO TEST (HT)

HT is used both as a tightness test and a

mechanical test of the pipe body. Thereare mainly two common requirements.Either a test pressure according to theASME or API rules is required, or apressure able to strain the pipe up to95% of the Y.S.

A pressure test up to 95% of the Y.S.can result in distortion of the pipe thatleads to out of tolerance. Therefore thetest can produce more problems than itsolves.

CONCLUSIONS

Duplex stainless steels have beencommercially available several decadesbut gained large interest first in the1980’s and 1990’s. This makes this

family of steel less mature than theaustenitic family and many differentspecifications have emerged. Theconclusions of this paper can besummarised as follows.• The international standards should be

used as far as possible. They includerequirements on chemistry andmechanical properties.

• Recommended procedures for heattreatment in standards andspecifications are not sufficient toguarantee a good result. Eachmanufacturer has to develop specificprocedures for optimum properties of each product.

• Specification on manufacturing andfabrication outside the standardsshould be based on relevantproperties of interest for theapplication.

• Requirements on microstructure areseldom necessary and if qualificationof mill products is requestedtechnological testing of properties is

preferred.• A pipe specification should have the

option to use either seamless orwelded pipes for best availability andquality.

• Duplex stainless steels have a veryattractive combination of mechanicalproperties that makes them suitableas a stainless structural steel. Designdata indicate that they so far areused too conservatively.



9

acom 4 2000

REFERENCES

1. Olsson J, Liljas M, “60 years of duplexstainless steel applications”,Corrosion’94, NACE, paper No. 395

2. Tystad M, “Application of duplex

stainless steels in the offshore industry”,5th World Conference on DuplexStainless Steels, Maastricht, 1997,Book 1, p.95

3. Egan F, “Service experience of superduplex stainless steel in seawater”,Stainless Steel World, December, 1997,p.61

4. Johansson KA, “Duplex stainless steelsin offshore applications – experiencesfrom projects and operations”, 4thInternational Conference, DuplexStainless Steels’94, Vol.3,

ISBN 1 85573 210 6, Glasgow,November, 1994, paper No. KVII5. Vishnu R, to be published6. Redmond JD, Davison RM, “Critical

review of testing methods applied toduplex stainless steels”, 5th WorldConference on Duplex Stainless Steels,Maastricht, 1997, Book 1, p.235

7. Kotecki DJ, “Ferrite determination instainless steel welds – advances since1974”, Welding Journal, Vol. 76, No.1,ISSN:0043-2296., 1997, p.24-s

8. Nilsson J-O, “Super duplex stainlesssteels”, Materials Science and

Technology, Vol.8, August 1992, p.6859. Nilsson J-O, “The physical metallurgy

of duplex stainless steels”, 5th WorldConference on Duplex Stainless Steels,Maastricht, 1997, Book 1, p.73

10. Bowden PL, Ward JL, “Experiences inwelding 25Cr superduplex stainless steelfor topsides hydrocarbon piping”, Conf.Proc. 25th Annual OTC, May, 1993,Houston, Texas, OTC 7316, p.545

11. Karlsson L, “Duplex stainless steel weldmetals – effects of secondary phases”,5th World Conference on Duplex

Stainless Steels, Maastricht, 1997,Book 1, p.4312. Ginn BJ, Gooch TG, “Effect of

intermetallic content on pittingresistance of ferritic austenitic stainlesssteels”, Proc. Stainless Steel ’99,Science and Market, Chia Laguna,Sardinia, ISBN 88-95298-34-6,June 1999, Vol. 3, p.81

13. van Nassau L, Melker H, “Positionstatement on the specification of metallographic properties of weldmentsin duplex and superduplex stainlesssteel”, Welding in the World, Vol. 43,

No. 2, 1999, p.11

14. Wiesner, CS, “Toughness requirementsfor duplex and super duplex stainlesssteels”, 5th World Conference onDuplex Stainless Steels, Maastricht,1997, Book 2, p.979

15. Deleu E, Dhooge A, “Fracture

toughness of welded thick walledduplex stainless steels”, 5th WorldConference on Duplex Stainless Steels,Maastricht, 1997, Book 1, p.387

16. NACE Standard MR0175-99, StandardMaterial Requirements, Sulfide StressCracking Resistant Metallic Materialsfor Oilfield Equipment, NACEInternational, ISBN 1-57590-021-1,1999

17. NORSOK Standard, NorwegianTechnology Standards Institution

18. Avesta Sheffield Corrosion Handbook,8th Edition, Avesta Sheffield AB,ISBN 91-630-8118-0, 1999

19. MTI Publication No.47, “Corrosiontesting of iron- and nickel-based alloys,Part II, Test Data”, MaterialsTechnology Institute of the ChemicalProcess Industries, Inc., 1996

20. Jargelius-Pettersson RFA, Linder J,Hertzman S, “Ranking the resistance of duplex stainless steels to chloride-induced stress corrosion cracking”,5th World Conference on DuplexStainless Steels, Maastricht, 1997,Book 2, p.585

21. Jonson J, “Stainless steel design stressesin EN and ASME pressure vesselcodes”, Proc. Stainless Steel ’99,Science and Market, Chia Laguna,Sardinia, ISBN 88-95298-34-6,June 1999, Vol. 3, p.313

22. Razmjoo G R, “Design guidance onfatigue of welded stainless steel joints”,Proc. OMAE, Vol. III, MaterialsEngineering, ASME, 1995, p.163

23. Maddox S J, “Fatigue design of weldedstainless steels”, EUR 18922 - ECSCInformation Day; Stainless Steels-NewProduct and Market Developments,6 October 1998, Seville, Edited byD. Naylor, E Nägile,ISBN 92-828-7106-1, 1999, p.63

This paper was earlier presented at the DuplexAmerica 2000 Conference in Houston in March.Published with the kind permission of Stainless

Steel World.



10

acom 4 2000

EDITORIAL

Dear Reader,

There have been some remarks from alert readers concerning data

presented in acom No. 2, “Corrosion testing in flash tanks”, earlier this year and I agree that some comments are worthwhile.

First of all, Table 2, which gives the chemical compositions of

grades included in the test. The composition given for 316L is not a

316L composition but rather “2304”, i.e. the grade presented on the

next line. The composition given for 2304 reflects the content of

2205. The consequence is that there is no composition for 316L in

the table so we have to assume a typical one, low carbon and around

17Cr-11Ni-2.2Mo.

A second remark is about the isocorrosion diagram for 304 and

316 in sodium hydroxide, Figure 8. The 1 mpy curve is rather

generous and should preferably not be used for the selection of

stainless steels in this type of environment. The isocorrosion

diagrams shown in Avesta Sheffield Corrosion Handbook are not

only more conservative; they are more realistic as well.

I also want to take this opportunity to thank the readers for being

observant. That strengthens my opinion that most of you find acom

worthwhile reading.

Yours faithfully,

Jan Olsson

Technical Editor



Although Avesta Sheffield has made every effort to ensure the accuracy of this publication, neither it nor any contributor can accept any legalresponsibility whatsoever for errors or omissions or information found to be misleading or any opinions or advice given.

11

acom 4 2000



acom is distributed free of charge to persons actively involved in process industrydevelopment and other areas where stainless steels are important.

acom appears four times a year, and we welcome applications from all interestedparties for additions to our mailing list.

Avesta Sheffield AB

Research and DevelopmentSE-774 80 Avesta, Sweden

Tel. +46 (0)226 810 00Fax +46 (0)226 813 05

www.avestasheffield.com

I S S N 1 1 0 1 – 0 6

8 1 . T e k n i s k i n f o r m a t i o n / C e n t r u m t r y c k A v e s t a 2 0 0 0

Name:Please type or write legibly.

CompanyPosition: activity:

Company:

Mailing address:

Postcode/City:

Country:

Please, add my name to your mailing list

I have changed my address as shown above.My previous label is enclosed. acom 4.2000

All rights reserved.

Comments and correspondence can be directed to Jan Olsson,Technical Editor, Avesta Sheffield AB, R&D, SE-774 80 Avesta, Sweden.

Tel. +46 (0)226 812 48. Fax +46 (0)226 813 05.

informacion outokumpo

Documents