Microstructure/Property Relationships inDissimilar Welds Between Duplex StainlessSteels and Carbon SteelsThe effect of weld metal microstructure on toughness and pittingcorrosion resistance is evaluated for both a duplex stainless steel and Ni-based filler metalBY E. J. BARNHOUSE AND J. C. LIPPOLD

E. J. BARNHOUSE is with Weirton Steel Corp., Weirton, W. Va. J. C. LIPPOLD is with The OhioState University, Columbus, Ohio.

ABSTRACT. The metallurgicalcharacteristics, toughness and corrosionresistance of dissimilar welds betweenduplex stainless steel Alloy 2205 andcarbon steel A36 have been evaluated.Both duplex stainless steel ER2209 andNi-based Alloy 625 filler metals wereused to join this combination using amultipass, gas tungsten arc welding(GTAW) process. Defect-free weldswere made with each filler metal. Thetoughness of both the 625 and 2209deposits were acceptable, regardless ofheat input. A narrow martensitic regionwith high hardness was observed alongthe A36/2209 fusion boundary. A similarregion was not observed in welds madewith the 625 filler metal. The corrosionresistance of the welds made with 2209filler metal improved with increasingheat input, probably due to higher levelsof austenite and reduced chromiumnitride precipitation. Welds made with625 exhibited severe attack in the rootpass, while the bulk of the weld wasresistant. This investigation has shownthat both filler metals can be used to joincarbon steel to duplex stainless steels,but that special precautions may benecessary in corrosive environments. Introduction Duplex stainless steels have becomeincreasingly attractive to a number ofindustry sectors due to their superiormechanical properties and corrosioncharacteristics relative to other stainlesssteels and structural steels. Although thejoining of duplex stainless steels tothemselves has been studiedextensively, the increased application ofthese steels will require a better

This study was designedto provide some insight intothe microstructure/ propertyrelationships in dissimilarfusion welds with duplexstainless steels. Thedissimilar materials selectedfor the overall study includeda plain carbon structuralsteel (A36), an austeniticstainless steel (Type 304L)and a martensitic stainlesssteel (Type 410). This reportfocuses on the dissimilarcombination of Alloy 2205and A36. A future paper willreport the results of studiesconducted on the othercombinations. KEY WORDS

Duplex Stainless Alloy 625 Dissimilar Metals CorrosionResistance ConstitutionsDiagrams Ferrite Number Filler Metal Carbon Steel

Stainless Steel/Carbon SteelDissimilar Joints Early investigations onthe joining of dissimilar

By plotting the Cr- andNi-equivalents for thematerials on the diagramand connecting the basemetals by a tie line, thedeposit microstructurecan be estimated byconnecting a point alongthat tie line (selected asthe midpoint in Fig. 1) toa tie line to the fillermetal composition. Theweld metal constitutionthen lies along the linebetween the filler metaland base metal midpointas dictated by the levelof dilution. The ability to predictmicrostructure using theSchaeffler Diagramprovided a valuable toolfor the selection of fillermetals and thedetermination of theeffects of base metaldilution. The diagramwas particularly usefulfor predicting the ferritecontent in austeniticstainless steel depositsand determining theconstitution of dissimilarcombinations of carbonsteels and austeniticstainless steels. The Schaeffler Diagramdoes not have a specificweighting factor fornitrogen, however, andas a result, a diagramwas proposed byDeLong in 1974 that

understanding of the issues associatedwith welds to dissimilar metals. Thejoining of dissimilar materials isgenerally more challenging than that ofsimilar materials because of differencesin the physical, mechanical andmetallurgical properties of the basemetals to be joined. These differencesmay also complicate the selection offiller metals compatible to both basemetals. Therefore, filler metal selectionis often a compromise between the twodissimilar metals. There are fewguidelines for dissimilar metal joiningand, in most cases, predicting themicrostructure and resultant propertiesof the weld deposit can be difficult.

the joining of dissimilarmetals were primarilydevoted to ferrous alloys;however, much of theemphasis was placed on theprevention of weld metalliquation cracking (oftenreferred to as microfissuring),heat-affected zone cracking,carbon migration and oxidepenetration, as discussed byPattee, et al. (Ref. 1). In the1940s, Schaeffler proposeda diagram for the selection ofelectrodes for the dissimilarjoining of plain carbon andstainless steels that relatedthe microstructuralconstitution of the welddeposit to its composition, asdictated by the relativeproportion of filler metals andbase metals (Ref. 2). Thisdiagram (Fig. 1), commonlyreferred to as the SchaefflerConstitution Diagram, can beused as a means ofpredicting the weld metalmicrostructure of dissimilarmetal welds in a select groupof alloys.

DeLong in 1974 thatincorporated N in the Ni-equivalent formula (Ref.3). This diagram allowedfor more accurateestimation of ferritecontent over a narrowercomposition range thanthe Schaeffler Diagram,improving and correctingthe limitationsassociated with theSchaeffler Diagram. TheDeLong Diagram waslater found tomisrepresent Mn, andFN (Ferrite Number)predictions of highlyalloyed compositionssuch as 309 stainlesssteel were found to beinaccurate (Ref. 4).Furthermore, its limitedcomposition range madeit difficult to utilize fordissimilar metalwelding. Siewert, et al. (Ref. 4),developed a modifiedprediction diagramcalled the WRC-1988diagram. This diagrammodified and greatlysimplified the Cr- and Ni-equivalent formulae andcorrected theoverestimation of FN forhigher alloyed weldmetals. Recently,Kotecki, et al. (Ref. 5),had shown Cu toinfluence the austeniteformation and thereforeadded a Cu factor in theNi-equivalent formula.This change resulted inthe WRC-1992 diagram,which is essentiallyidentical to the WRC-1988 with the addition ofa Cu factor in Ni-equivalent formula. Anextended version of thisdiagram (Fig. 2) allowsFN estimation indissimilar welds butdoes not contain otherconstitution regimes, asin the SchaefflerDiagram.

Fig. 1 - Schaeffler Diagram (Ref. 2) showing the dissimilarcombinations used in this investigation.

Fig. 2 - WRC-1992 diagram,extended version (Ref. 5)

Duplex Stainless Steel to Carbon Steel Recently, Odegard, et al. (Ref. 6),studied the joining of duplex Alloy SAF2507 to carbon steels with respect tofusion zone mechanical properties. Theyreported that the phase stability and theoverall properties of the fusion zone wereinfluenced by the welding parameters andthat low heat inputs were necessary toensure structural integrity and solidificationcracking resistance. Furthermore, theynoted that a highly ferritic fusion zoneresulted from high dilution by the carbonsteel, making the weld metalmicrostructure susceptible to secondaryaustenite formation in multipass weldingdue to reheating of the deposited weldmetal by subsequent passes. High heatinput welding schedules were reported toincrease the susceptibility to fusion zonesolidification cracking. Table 1 presents mechanical propertydata, collected by Odegard, et al. (Ref. 6),which includes tensile strength, yieldstrength and elongation values fordissimilar joints between the super-duplexAlloy 2507 and either carbon steel or Type316 austenitic stainless steel. No othermechanical property data for dissimilarwelds between duplex stainless steels andcarbon steels was found in the literature. Experimental Procedure Materials Welding trials were designed to study thedissimilar joining of duplex stainless steelAlloy 2205 to A36 carbon steel. Thisdissimilar combination was selected basedon a survey of industrial users. Both duplexstainless steel and Ni-based filler metalswere selected to join these metals. Thechemical compositions for the base andfiller metals are listed in Table 2. The base

Table 1 - Fusion Zone MechanicalProperties of Dissimilar Joints withAlloy 2507 (Ref. 6) Property 2507/Carbon

Steel2507/316

Tensilestrength, ksi 61.8 85.1Yield strength,ksi 43.4 54.5Elongation, % 23 45

Table 2 - Chemical Composition andCreq and Nieq of the Base and FillerMetals (wt-%) Element Alloy2205 A36 ER2209

ERNiCrMo-3

Cr 22.16 22.71 21.84Ni 5.64 8.23 64.46Mo 3.04 3.18 8.91Mn 1.46 0.627 1.64 0.02Si 0.48 0.236 0.52 0.03C 0.014 0.088 0.015 0.030N 0.18 0.160

materials were supplied in the form of 12.5-mm (0.5-in.) plate. These plates were cutinto 5 x 20 cm (2 x 7.8 in.) coupons with a37-deg bevel on each plate to provide a74-deg groove angle for a single-V-groovebutt joint configuration. The root face was1.6 to 2.4 mm (0.062 to 0.094 in.) with aroot opening of 2.4 mm.

(c)(Cr=1.5Mo=2Mn=0.255Si)/(2Ni+12C+12N). (d) PRE (Pitting ResistanceEquivalent)=Cr+3.3(Mo+0.5W)+16N.

properties(mainly thermalconductivity) theheat flowdifferedbetween thetwo filler metals,thus the rootpass heatinputs weredifferent. The2205/2209/A36combinationutilized a rootpass heat inputof 1.05 kJ/mm(26.7 kJ/in.),while that of the2205/625/A36combinationwas 1.90 kJ/mm(48.3 kJ/in.). MetallurgicalCharacterization OpticalMetallography Weld crosssections wereremoved fromeach of thecombinationsand polishedthrough 0.05-micron alumina.The sampleswereelectrolyticallyetched in 10%oxalic acid at 5-7 V for 20-25 s.Also, coloretchingtechniqueswere employedto provide betterresolutionbetween theferrite andaustenite.Thesetechniquesincluded a two-step modifiedMurakami's etch(Ref. 7), whichproduces acomposition-sensitive filmresulting in a

variety of colorson the ferritephase, whilethe austeniteremains white(uncolored). Aferro-fluidcolloidalsuspension ofFe3O4 was alsoused to revealthe duplexmicrostructure(Ref. 7).Residualmagnetismattracts Fe3O4particles to theferrite phase toprovide a colorof brown or darkblue in starkcontrast to theaustenite, whichis white.

(A)

(B)Fig. 3 - Cross section of2205/2209/A36 weld combination:A - Root pass; B - cover pass.

Fig. 4 - WRC-1992 diagram forA36/2209/2205 combination.

Fig. 5 - FN vs. location for the2205/2209/A36 weld combination.

Fig. 6 - Representativemicrograph of 2209/A36 fusionboundary region. Arrows indicatefusion boundary.

Fig. 7 - Charpy V-notch impacttoughness vs. temperature for the2205/2209/A36 weldcombination.

MicrohardnessSurveys

The second type was an all-weld-metal sample machined out of the

The major microstructural differencebetween these welds was that the

Hardness testing wascarried out on a LecoModel M-400microhardness unitusing a 1-kg load.Surveys wereconducted both acrossthe weld and from rootto cover pass within theweld deposit. Thedistance betweenindents wasdetermined by the welddimensions in order toobtain appropriate datafor each pass.Generally, the distancebetween indents was0.5 mm (0.02 in.) in thelower passes of theweld, 1.0 mm (0.04 in.)in the central portionsof the weld metal and2.0 mm (0.12 in.) in thecover pass of the weldmetal. Hardness wasalso measured alongthe fusion boundaryand across the heat-affected zone (HAZ)into the base metal. Hardness testingwas also conducted atreduced loads (25 and50 g) along the2209/A36 and 625/A36fusion boundaries. Thiswas done to moreclosely examine thehardness in themicrostructuraltransition regionbetween the weldfusion zone and HAZ. Ferrite NumberMeasurement The ferrite number(FN) in each weld layerwas determined usinga MagneGagecalibrated per AWSA4.2-91 according tothe method developedby Kotecki (Ref. 8). Allmeasurements wereobtained onlongitudinal sections inthe fusion zone ratherthan transverse

fusion zone through the thickness.Since the thickness of the materialwas 12.5 mm, standard 25 x 50mm (1 x 2 in.) specimens could notbe used, and samples of 12.5 x33.0 mm (0.5 x 1.3 in.) were used.The specimens were groundthrough 600-grit SiC and weighedprior to immersion in the solution.The pitting corrosion solution was6% FeCl36H2O. The specimenswere immersed in this solution for72 h and were removed andobserved every 12 h to note anychanges. Following the 72-h testperiod, the samples were carefullyscrubbed under running water andthen ultrasonically cleaned inmethanol for approximately 30min. Evaluation of pitting wasdetermined by optical microscopy.The pits were counted atmagnifications of 50X andmeasured at 400X. Pit density wasdetermined by dividing the numberof pits per unit area. A pit densitywas determined for each weldlayer so a comparison could bemade between the different basematerial combinations and the heatinput variations. Results 2205/2209/A36 Weldment Microstructure The dissimilar 2205/A36combination was welded withduplex filler metal ER2209. Figure3A shows the root passmicrostructure of this combinationresulting from a heat input of 1.05kJ/mm and consists of an austenitematrix (white) with both skeletaland acicular ferrite (dark etching).Ferrite measurements indicated20-25 FN in the root pass for bothwelding heat inputs of 1.57 KJ/mmand 2.60 KJ/mm. Based on this FNlevel, the WRC-1992 diagram (Fig.4) predicts that the dilution in theroot pass was approximately 33%,which corresponds well with the35-40% dilution that was estimatedmetallographically. Subsequentpasses in these weldmentsexhibited increased FN from theroot to the cover pass, ultimatelyreaching 92 FN. A micrograph of

lower heat input conditions resultedin the precipitation of morechromium nitride, presumably Cr2N,within the ferrite phase. Thisprecipitation was most pronouncedin the cover pass. The layersbeneath the cover pass exhibitedgreater volume fractions ofaustenite, which would thereforereduce the tendency for Cr2Nprecipitation because austenite hasa higher solubility for nitrogen thandoes the ferrite phase. The othermajor difference between the twoheat input levels was the level ofsecondary austenite ( 2). Bothwelds contained significant 2 inthe fill passes beneath the coverpass. This 2 formation occurs dueto reheating of weld metal duringsubsequent weld passes. Thisreheating and precipitation of 2alters the ferrite/austenite volumefraction relative to that of the initialweld deposit. Fusion Zone Hardness The resultant weld deposithardness of this combination variedwith heat input. The lower heatinput weld exhibited higherhardness in the weld metal andalong the A36 fusion boundary thanthe high heat input weld. Thehardness in the root pass variedfrom 261 to 287 DPH . Theremainder of the weld metalexhibited hardness levels between229 and 279 DPH. An averagehardness of 260 DPH was found inthe cover pass. Lower hardnesswas found in the center portions ofthe fusion zone (midway throughthe thickness), as averagehardness was 250 DPH. Thehardness along the fusion boundaryof the carbon steel was found to bemuch higher than the bulk fusionzone hardness and ranged from253-416 DPH. Generally, thehardness along the 2209/A36fusion boundary was above 300DPH, with average levels reaching410 DPH near the root pass. Softerregions along this boundary werefound to exist in the top third of theweld. Average hardness levelswithin and at the fusion boundary ofthe A36/2209/2205 weldments are

than transversesections in order tobetter quantify variationwithin a given pass.Four measurementswere taken at fourdifferent locationswithin each pass.These measurementswere then averaged toobtain the FN for eachpass. Charpy Impact ToughnessTesting Charpy V-notch(CVN) specimens weremachined from thewelded coupons.Samples wereprepared in the L-Torientation as perASTM E-23. The L-Torientation represents asample transverse tothe welding directionwith the notch locatedsuch that testing occursthrough the thickness ofthe weld from the rootto the cover passes. Allnotches were located inthe center of the welddeposit. Charpy V-notch testing wasperformed attemperatures from -196C (-320.8 F) to more

than 100 C (212 F) inorder to develop acomplete ductile-to-brittle transitiontemperature (DBTT)curve for eachcombination. Scanningelectron microscopy(SEM) was utilized toexamine the fracturesurfaces of the CVNimpact samples. Pitting Corrosion Testing Pitting corrosiontests were performed at50 C (122 F) asdescribed in ASTM G-48. Two types ofsamples were tested.The first type consistedof an entire weldment,

reaching 92 FN. A micrograph ofthe cover pass is shown in Fig. 3B.The FN transition from the root tothe cover pass is shown in Fig. 5.Note that the FN differentialbetween the root pass and coverpass is roughly 70 FN and that asignificant increase in FN occursbetween the last fill pass and thecover pass for both heat inputs. The filler metal composition,dilution and cooling rate are theprincipal factors that influence theweld metal ferrite/austenitebalance in duplex stainless steels.The lower heat input (1.57 kJ/mm)weld passes undoubtedly resultedin higher cooling rates than thehigher heat input (2.60 kJ/mm)weld. However, as shown in Fig. 5,the difference in heat input andcooling rate did not significantlyaffect the ferrite/austenite balance.

summarized in Table 4. The 2.60-kJ/mm weld exhibited alower average hardness, with rootpass values from 249 to 256 DPHand an average hardness of 250DPH (Table 4). The remainder ofthe weld metal exhibited hardnesslevels between 223 and 265 DPH.An average hardness of 245 DPHwas measured in the cover pass.Lower hardness was found in thecenter portions of the fusion zone(midway through the thickness),with an average hardness of 230DPH. The hardness along the2209/A36 fusion boundary in thisweld was also significantly lowerthan the lower heat input weld. Thefusion boundary region associatedwith the root pass exhibited thehighest hardness, 300 DPH.Hardness elsewhere along theboundary was below 300 DPH withthe lowest hardness exhibited in thefusion boundary adjacent to thecover pass.

of an entire weldment,including both basematerials and thefusion zone.

(A)

(B) Fig. 8 - CVN fracture behavior for 2205/2209/A36weldment: A - Lower shelf; B - fracture transitionrange.

(A)

(B)Fig. 9 - Average pit depth vs. weld layer for2205/2209/A36 combination.

Fig. 10 - Pit density vs. weld layer for the

(A)

(B)Fig. 11 - Microstructure of 2205/625/A36combination: A - Root pass; B - cover pass.

Fig. 12 - CVN impact toughness vs.temperature for the 2205/625/A36 weldcombination.

Fig. 13 - SEM fractograph of 2205/625/A36CVN specimen produced at 1.57 kJ/mmand tested at -196C.

2205/2209/A36 pitting samples: A - Weldmentsamples; B - all-weld-metal samples.

Fig. 14 - Average pit depth vs. weld layerfor 2205/625/A36 combination.

A representativemicrostructure along the2209/A36 fusion boundary regionis shown in Fig. 6. Thismicrograph clearly shows anacicular martensitic structure,which results in significantlygreater hardness along the2209/A36 fusion boundary thanalong the 2205 fusion boundary.The carbon steel average basemetal hardness (155 DPH) andthe fusion boundary hardness(250-410 DPH) transitions rapidlywithin a short distance from thefusion boundary. Type II grainboundaries (Ref. 13) are alsopresent along this fusionboundary. Type II boundarieshave been previously reported byWu and Patchett (Ref. 13) innickel-based alloy cladding onCr-Mo steels. Carbideprecipitation decorates theseboundaries resulting from carbonmigration out of the carbon steelinto the more highly alloyed, butlower carbon, fusion zone. Weld Metal Impact Toughness The CVN impact toughnessresults for the 2205/2209/A36weld combination at both heatinput levels are shown in Fig. 7.The lower shelf impact energywas roughly 7.5 ft-lb attemperatures below -140 C (-220F). The upper shelf of the DBTT

curve was roughly 240 ft-lb (325J) for the 1.57 kJ/mm heat inputand 250 ft-lb (339 J) for the 2.60KJ/mm heat input. These DBTTcurves also show the transitionimpact energy, midpoint betweenthe upper and lower shelf, to be125-130 ft-lb (169-176 J) and thecorresponding transitiontemperature to be between -45 to-50 C (-49 to -58 F) for both heatinput levels. Fracture surfaces from upperand lower shelf and transitionregion specimens were evaluatedin the SEM. The upper shelffracture surface exhibited ductile

The second pittingcorrosion test wasconducted on all-weld-metal samples machinedfrom the weldment. Thepitting corrosion resultswere tabulated as pitdensity and average pitdepth for the fusionzone. In the weldmentsample, the carbon steelwas preferentiallyattacked by the solution,resulting in completedissolution of the A36base metal. The fusionzone pitting corrosionresponse varied withheat input and corrosionsample. The average pitdepth of the weldmentsample increased as theheat input increased, asshown in Fig. 9.Conversely, the averagepit depth of the all-weld-metal samplesdecreased as the heatinput increased.However, this relation istrue only in weld layers 4and 5 since the generalattack experienced bythe samples causedsurface collapse in theinitial passes of bothheat input welds. The weldmentcorrosion samplesshowed a decrease in pitdensity in the 2.60kJ/mm weld ascompared to the 1.57kJ/mm weld. This isillustrated in Fig. 10Aand shows that thisrelation holds true for allweld layers with theexception of the secondpass due to acceleratedattack in this layer. The2.60 kJ/mm weld showsdensities below 2.0pits/mm2 for all weldlayers, while the lower

Alloy 625 filler metal is a Ni-Cr-Momaterial and consists nominally of65% Ni, 20-23% Cr and 8-10% Mo.This alloy solidifies entirely toaustenite with some possible lowermelting point eutectic and/orcarbide formation at the dendriteinterstices. The root passmicrostructure of this dissimilarweld joint is shown in Fig. 11A. It isfully austenitic with second phaseconstituents forming in the dendriteinterstices. The cover pass of the2205/625/A36 combination isshown in Fig. 11B. This fullyaustenitic microstructure isrepresentative of that of the fillpasses and exhibits a more distinctweld solidification substructure thanthe root pass. This differenceresults from the higher dilution ofthe filler metal by the basematerials in the root pass. Fusion Zone Hardness The fusion zone hardness for the2205/625/A36 welds wasunaffected by weld heat input,ranging from 190-220 DPH.Similarly, the hardness along thecarbon steel fusion boundaryregion was comparable to that ofthe fusion zone for both heat inputs,ranging from 180-220 DPH. Theaverage hardness values for thiscombination are also summarizedin Table 4. Weld Metal Impact Toughness Charpy V-notch test results forthe 2205/625/A36 dissimilar weldcombination are shown in Fig. 12.As expected, a DBTT is notobserved due to the fully austeniticstructure of the fusion zone. Theimpact toughness decreasesslightly with decreasingtemperature. At -196 C impacttoughness was 62 ft-lb (84 J) and57 ft-lb (77 J) for 1.57 kJ/mm and2.60 kJ/mm heat inputs,respectively, relative to 88 ft-lb (119J) and 76 ft-lb (103 J) at 25 C(77F). Note that the lower heat

fracture surface exhibited ductilefracture, as exemplified by adimple-type fracture morphology.The lower shelf exhibited acleavage-type fracture showing arelatively flat fracture surface asshown in Fig. 8A. Fracturesurfaces in the transition regionexhibited mixed mode fracture asshown in Fig. 8B. Portions of thefracture surface resemblecleavage fracture, while otherareas show ductile fracture. Thetransition region fracture behaviorwas similar for both heat inputs inthe temperature range from -20 to-90 C (-4 to -130 F). Pitting Corrosion Corrosion testing of the weldmetal was conducted in twoways. Initial testing wasconducted on the entire weldmentby immersing the entire jointcross section, including the basematerials, into 6% FeCl3 solutionat 50 C for 72 h.

layers, while the lowerheat input weld revealsdensities generallygreater. The all-weld-metal corrosion sampleresults are shown in Fig.10B. General attackcaused the surfacecollapse of the initiallayers preventing datacollection. Figure 10B alsoshows that no simplecorrelation can be drawnfrom the graph as pitdensity appearsscattered between thetwo heat inputs. Theweight lossmeasurements showedthat the lower heat inputweld resulted in greaterweight loss than that ofthe higher heat inputweld. The weight losseswere 2.529 g (36.5% ofinitial weight) and 2.374g (30.9% of initial weight)for the 1.57 and 2.60kJ/mm weldments,respectively, thusindicating higher pittingcorrosion resistance ofthe higher heat inputweld. 2205/625/A36 Weldment Microstructure The dissimilarcombination of Alloy2205 and A36 was alsowelded using the Ni-based filler metal 625.The heat inputs utilizedfor fill and cover passeswere the same as for the2205/2209/A36combination. The rootpass heat input, 1.90kJ/mm, was differentfrom the 2205/2209/A36combination due todifferences in thermalproperties of the filleralloys.

input, 1.57 kJ/mm, resulted inhigher impact toughness values atany given temperature. Arepresentative fracture surface forthis combination is shown in Fig.13. The fracture mode was ductilerupture over the entire range from -196 C to 25 C. Note the evidenceof the underlying solidificationsubstructure on this fracturesurface, probably due to fractureinitiation at the interdendriticeutectic constituent.

Pitting Corrosion The weldment corrosionresponse of the 2205/625/A36

Discussion The results of thisinvestigation have shown

Previous researchers havesuggested that the fusion zoneaustenite content is not strongly

response of the 2205/625/A36weldments also resulted, asexpected, in general attack ofthe carbon steel base metal.Attack was also verypronounced in the root of theweld and resulted in thecomplete dissolution of theroot pass. The root pass wasthe only pass attacked in thismanner and probably resultedfrom higher dilution of thispass by the A36 base metal.Figure 14 shows increased pitdepths with increased heatinput for both corrosionsample types. However, thepit depth difference is muchsmaller in the all-weld-metalsamples than the weldmentsamples when comparing thetwo heat inputs. The pittingdensity results vs. weld layerfor the weldment and all-weld-metal samples are shown inFig. 15. Pit density increasedwith increased heat input asshown for both corrosionsamples. The only exceptionis weld layer 2 in theweldment sample, as thehigher heat input resulted inlower pit density relative tothe 1.57 kJ/mm weld. The pitdensities of both corrosionsample types appear higherin the initial weld layers andgenerally tend to level outabove the third weld layer. The weight loss of the all-weld-metal corrosion sampleswas 1.402 g (21.5% of initialweight) and 2.212 g (20.6% ofinitial weight) for the 1.57kJ/mm and 2.60 kJ/mm heatinputs, respectively. Thecorrosion rate, expressed asweight percent, indicates thatno difference in pittingcorrosion is evident betweenthe two heat inputs as theweight loss percentagesresult in similar values.

Table 3 - GTAW ProcessParameters

Low HeatInput, 1.57

High HeatInput, 2.60

investigation have shownthat duplex stainless steelalloy 2205 can be joined toA36 carbon steel usingeither duplex filler materialER2209 or Ni-based Alloy625. Both combinationsexhibited good weldabilityand were free fromfabrication-related defectssuch as solidificationcracking, liquationcracking, porosity,incomplete fusion, etc. Ingeneral, from a proceduralpoint of view, there wereno problems welding thedissimilar combinationsusing either filler material. Microstructure Evolution Weld Metal Table 4 compares theFN and hardness for eachof the combinationsstudied relative to locationin the weld, i.e. root pass,fill passes and cover pass.Dilution of the Alloy 625filler metal is not sufficientto form ferrite within theweld deposit, and theentire fusion zone isaustenitic. The2205/2209/A36 weldexhibited 20-25 FN in theroot pass, 25-45 FN in thefill passes and increasedsignificantly to 95 FN inthe cover pass. The fillpasses showed a gradualincrease in FN with eachpass. This results fromboth a change in dilutionfrom bottom to top in themultipass weld and theformation of secondaryaustenite in underlyingweld passes that reducesthe FN in the underlyingpasses. The WRC-1992diagram can be used toestimate the dilutioneffects on FN, but thediagram is not effective inpredicting secondaryaustenite formation inmultipass welds. Thus,some deviation from the

dependent on heat input, and iscontrolled primarily by thecomposition of the weld metal (Refs.9-11). Composition analysis of theindividual passes was not performedand, thus, it is not apparent whatcomposition difference may existbetween the fill passes and coverpasses. The formation of secondaryaustenite in the underlying fill passesmay explain some of the difference,but does not help rationalize why thecover pass FN exceeds thatcalculated from the filler metalcomposition. The loss of an austeniteforming element, such as nitrogen,may explain the difference, but wasnot verified. The root pass of both weldcombinations exhibited slightlyhigher hardness than the fill passes.In the 2205/2209/A36 combination,this may be due to higher levels ofsecondary austenite ( 2) formationduring reheating by the subsequentpasses. The intragranularprecipitation of 2 results in somesecond phase strengthening and thusa small increase in hardness. Theincrease in hardness in the coverpass results from the high Cr2Nprecipitate concentration associatedwith highly ferritic weld metals. Thehardness increase is particularlypronounced in the low heat inputweld cover pass - Table 4.

Table 4 - FN and Hardness for the FusionZone and Boundary Region

RootPassFill

PassCoverPass FBR

Combination FNFPHFN DPHFNDPHDPH2205/2209/A36,1.57 kJ/mm 25 275

25-45 250 95 260 350

2.60 kJ/mm 23 250 25-45 230 95 245 2802205/625/A36,1.57 kJ/mm 0 220 0 205 0 200 2102.60 kJ/mm 0 240 0 200 0 195 210

Input, 1.57kJ/mm

Input, 2.60kJ/mm

Voltage 10V 11VCurrent 200A 300ATravelspeed

3.0 in./min(1.27 mm/s)

3.0 in./min(1.27 mm/s)

Wirefeedspeed

33 in./min(13.97mm/s)

50 in./min(21.17mm/s)

Interpasstemp.,max

150 C 150 C

predicted WRC-1992 FNbased strictly on dilutioncalculations would beexpected. In addition, thisdiagram predicted lowerFN for the cover pass thanwas actually measured. The cover pass FNincreased significantlyrelative to the last fill passfor the 2205/2209/A36combination. It isinteresting to note that thecover pass ferrite number(95 FN) is actually greaterthan the predicted FN ofundiluted ER2209 fillermetal. The reason for thishigh FN in the cover passis not clear. The fact thatthe cover pass FN iscomparable between heatinputs suggests thatcooling rate is not a factor.

The 2205/625/A36 weld combination exhibited a fullyaustenitic fusion zone and, hence, there was littlechange in hardness through the weld thickness. Slightlyhigher hardness in the root pass of this combination maybe attributed to higher dilution from the base materialsand reflects the difference in microstructure between theroot and fill passes. Fusion Boundary Region Microstructure The high hardness along the fusion boundary regionof both weld combinations is attributable to the formationof a narrow band of martensite at the dissimilar interface.This martensitic region is predicted in both cases by theSchaeffler Diagram - Fig. 1. By drawing tie lines betweenthe filler metal composition and midpoint of the basemetal compositions, it can be seen that for the2209/625/A36 combination, 15% of this tie line lieswithin a region where martensite is present. The2205/2209/A36 tie line, on the other hand, has more than65% of its length in a martensitic region. Thus, for the2209 filler metal the composition transition region overwhich martensite can form will be much wider. This mayexplain the pronounced martensite formation along the2205/2209/A36 fusion boundary region and the apparentmartensite-free fusion boundary region in the2205/625/A36 weld. Another factor that influencesmartensite formation is the difference in carbon diffusionrates in the different combinations. If carbon migration isreduced or restricted, the likelihood of martensiteformation will be similarly reduced. Gittos and Gooch (Ref. 12) studied the fusionboundary below stainless steel and nickel-alloycladdings on Cr-Mo steels. They reported narrow bandsof martensite formation along the interfaces of bothcladding alloys, with hardness levels in the as-welded

Toughness Behavior The CVN results forboth combinations aresummarized in Table 5.In the 2205/2209/A36combination, there waslittle apparent differencein toughness as afunction of heat input,which reflects thesimilarity inmicrostructure of theseweld deposits. Becauseof the L-T orientation ofthe test samples, thetoughness valuesrepresent a compositeof the entire welddeposit and are notindicative of localvariations due tomicrostructure. Forexample, lowertoughness might beexpected in the higherFN cover pass. It should be pointedout that the toughnessof the 2205/2209/A36may be significantlyaffected by theformation of martensitealong the A36 fusionboundary region. Thisnarrow band of

Yasuda (Ref. 17) and Ume (Ref.18) claim that pitting corrosiondecreases with increased heatinput due to slower cooling ratesand the formation of austeniterather than Cr2N precipitationwithin the ferrite phase of thefusion zone. Another beneficialeffect of lower cooling rates onpitting resistance is the healing ofchromium-depleted regionsaround any precipitates. Ume(Ref. 18) reported that the numberof initiation sites decrease withhigher heat inputs, therebysupporting the data collected inthe current study of the weldmentsamples. The all-weld-metal samplesexhibited increased pit densityand depth for the higher heatinput relative to the 1.57 kJ/mmweld. However, the all-weld-metalcorrosion data may be erroneousdue to the collapse of the surfacein many of these samples,thereby changing the kinetics ofthe corrosion testing relative tothe weldment samples. Table 8 compares the weightloss due to corrosion of the all-weld-metal samples. Thepercentage weight loss illustratesthat increased corrosionresistance is obtained withincreased heat input for the

condition ranging from 300-441 HV. This martensiticband was associated with the partial mixing regionbetween the fusion zone and base metal HAZ. It is interesting to note in Fig. 6 that Type II grainboundaries are present within the fusion zone. Wu andPatchett (Ref. 13) found similar Type II grain boundariespreviously in austenitic stainless steel cladding. Theyreported that Type II grain boundaries were formed dueto the crystallographic change from -ferrite, the existentphase of solid hypoperitectic base steel at the initialtransient of solidification, to -austenite as a result ofaccumulation of nickel at the solidification front. Theyreported that disbonding (i.e., cracking) was more severein nickel-based alloy deposits that solidified directly toaustenite where high dilution from the base metal wasincurred. However, they also claimed that the ferriticsolidification of duplex stainless steels eliminates Type IIgrain boundaries from occurring.

Table 7 - Pitting Corrosion Data for the Weldment and All-Weld-Metal Samples

Root Pass Fill Pass Cover PassCombination Density Depth Density Depth Density Depth

(pits/mm2) (m) (pits/mm2) (m) (pits/mm2) (m)2205/2209/A36 Weldment,1.57 kJ/mm 1.15 5.55 2.30 3.91 2.53 5.942.60 kJ/mm 0.89 6.21 1.49 5.78 1.13 5.62Weld Metal,1.57 kJ/mm - - 1.61 6.67 1.90 5.392.60 kJ/mm - - 2.16 5.41 2.08 5.402205/625/A36Weldment,1.57 kJ/mm - - 1.75 3.84 1.04 6.042.60 kJ/mm - - 1.77 7.18 1.33 5.24Weld Metal,1.57 kJ/mm - - 3.74 5.91 2.86 5.822.60 kJ/mm - - 6.36 8.13 4.30 7.58

This may suggest that the solidification behavior inthis partially mixed region adjacent to the fusionboundary may be different from that of the bulk fusionzone, or that solid-state transformations followingsolidification may result in the formation of theseboundaries. The nature of the Type II boundaries is thesubject of ongoing research (Ref. 14).

narrow band ofmartensite wouldundoubtedly result inreduced toughnessalong this particularregion relative to thefusion zone. However,no fusion boundaryCVN testing wasperformed to investigateany toughness anomalyin this region. As expected, thefully austenitic fusionzone of the2205/625/A36combination exhibitedgood impact toughnessover a wide range oftemperatures. Thismicrostructure does notexhibit upper/lowershelf transition behaviorsuch as ferritic (or highferrite) weld metals,and, instead, the impacttoughness exhibits agradual decrease withtemperature. It isinteresting that theupper shelf toughnessof the duplex 2209deposit far exceeds thatof the Alloy 625 depositand that the duplex fillermaterial providescomparable toughnessat temperatures down to-60 C. Table 6 comparesfusion zone impactenergies obtained inthis study to previouswork from duplexstainless base/fillermetal combinationsperformed by Kotecki(Ref. 15) andBonnefois, et al. (Ref.16). The impacttoughness datareported here arehigher than from thisprevious work and maybe attributed to higheraustenite contents(lower FN) in the rootand fill passes of thedissimilar welds. Mostimportantly, thesecomparisons suggest

increased heat input for the2205/2209/A36 combination.Sridhar, et al. (Ref. 19), showedthat pitting corrosion resistance,expressed as a percentage ofweight loss, increases with higherheat inputs. They claimed thatslower cooling rates resulting inincreased austenite and thedistribution of the variouselements between the ferrite andaustenite are the reasons for theincreased pitting resistance. The 2205/625/A36combination resulted in thelocalized attack of the root pass ineach of the corrosion samplestested. The root pass of the allweld metal corrosion sampleswas completely dissolved by theferric-chloride solution. This isapparently the result of higherdilution of the filler metal by thecarbon steel, as indicated by avariation in microstructure relativeto the subsequent corrosion-resistant fill passes. Thissuggests that a criticalcomposition change occursbetween the root pass and theremaining fill and cover passes.

(A)

(B) Fig. 15 - Pit density vs. weld layerfor 2205/625/A36 combination: A- Weldment sample; B - all weldmetal.

that dilution from adissimilar carbon steelbase metal does notcompromise thetoughness of the fusionzone relative to asimilar metalcombination.

Table 8 - Weight Loss fromthe All-Weld-MetalCorrosion Samples All-Weld-MetalCorrosion SamplesCombination

WeightLoss

grams wt-%2205/2209/A36, 1.57 kJ/mm 2.529 36.5 2.60 kJ/mm 2.374 30.92205/625/A36, 1.57 kJ/mm 1.402 21.5 2.60 kJ/mm 2.212 20.6

Corrosion Behavior The two types ofcorrosion samples(weldment and all weldmetal) and their pittingcorrosion data aresummarized in Table 7.As shown for the2205/2209/ A36material combination,the weldment sampleshows that forincreasing heat input,the pitting corrosionresistance increasedwhen considering thepit density. The averagepit depth, on the otherhand, increased withincreasing heat inputand may be due tohigher concentration onfewer initiated pits.

Summary and Recommendations The 2205/A36 base metal combination has been successfullyjoined with duplex stainless steel ER2209 and Ni-based Alloy 625filler metals using multipass GTAW. Heat input had only a minor effecton the microstructure and toughness for the ER2209 combination.However, the corrosion behavior showed a marked improvement for

References 1. Pattee, H. E.,Evans, R. M., andMonroe, R. E. 1968. TheJoining of DissimilarMetals. Defense MetalsInformation Center,

higher heat input welding parameters relative to the lower heat input. The choice of filler metals in joining 2205 to A36 will be primarilydependent on the service requirements needed. The strength andductility of nickel-based Alloy 625 and duplex filler metal ER2209 aresimilar, being roughly 110 ksi and 30-40% elongation. Therefore, abalance between toughness and corrosion resistance will determinethe use of either the duplex filler metal ER2209 or the nickel-basedAlloy 625. The recommendation for cryogenic temperatures would beto utilize Alloy 625 over the duplex filler metal due the increasedtoughness at these temperatures. However, dilution should beminimized in the root pass to limit the carbon steel dilution in thefusion zone to prevent the reduction in corrosion resistance. On theother hand, in service environment temperatures greater than -50C,the duplex filler metal ER2209 should be utilized due to highertoughness over the nickel-base Alloy 625. The heat input should beas high as possible in order to gain the maximum corrosion resistanceof the fusion zone. Conclusions 1) The fusion zone microstructures of dissimilar weld combination2205/2209/A36 resulted in a general increase in ferrite number witheach subsequent pass and a large increase in FN occurred betweenthe last fill pass and the cover pass. The measured FN in the coverpass was greater than that predicted by the WRC-1992 diagram. 2) The ferrite number and hardness of the 2205/2209/A36combination were similar for both the 1.57 and 2.60 kJ/mm heatinputs, suggesting that heat input is a secondary factor relative to weldmetal composition in controlling the ferrite/austenite phase balance. 3) The 2205/2209/A36 combination formed a narrow martensiticband adjacent to the A36 fusion boundary along the entire thicknessof the weld. No martensite was observed in welds made with the Alloy625 filler metal, possibly due to the smaller composition range overwhich martensite forms and lower carbon migration rates in Ni-basedvs. Fe-based alloys. 4) The 2205/2209/A36 fusion zone exhibited similar upper shelfenergies for the two heat inputs utilized, suggesting that heat inputand dilution from the dissimilar base metal had little or no effect on themechanical properties. 5) The 2205/625/A36 fusion zone toughness behavior was typicalof face-centered cubic (FCC) materials as impact energies decreased10-15 ft-lb (14-20 J) over a temperature range from 25 to -196 C. 6) Pitting corrosion resistance of the weldment samples utilizing2209 filler metal showed increased resistance as the heat inputincreased from 1.57 kJ/mm to 2.60 kJ/mm. The all weld metal samplesresulted in a lower percentage weight loss for the higher heat inputweld, signifying increased corrosion resistance with the 2.60 kJ/mmweld relative to the 1.57 kJ/mm weld. 7) The Alloy 625 fusion zone exhibited general attack in the rootpass for each type of corrosion sample tested. It appeared thatcorrosion resistance of the 2205/625/A36 combination decreases withincreasing heat input for the remaining fill and cover passes.

Acknowledgment This work was supported by the members of Edison WeldingInstitute through the Cooperative Research Program. The technicalsupport of Dr. Wangen Lin, formerly with EWI and currently with Pratt& Whitney, and other members of the EWI staff during the course ofthis investigation is greatly appreciated.

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213-224. 8. Kotecki, D. J. 1982.Extension of the WRCFerrite Number system.Welding Journal 61(11):352-s to 361-s. 9. Honeycomb, J., andGooch, T. G., 1985. Arcwelding ferritic-austeniticstainless steels:prediction of weld areamicrostructures. TheWelding InstituteResearch Report,286/1985. 10. Gooch, T. G. 1982.Proc. Conf. DuplexStainless Steels. StLouis, published by ASMInternational, MaterialsPark, Ohio, 1983. 11. Mundt, R., andHoffmeister, H. 1983.The continuous ferrite-austenite transformationduring cooling of ferritic-austenitic iron-chromium-nickel alloys, Arch.Eisenhuttenwes 54, No.7, pp. 291-294. 12. Gittos, M. F., andGooch, T. G. 1992. Theinterface below stainlesssteel and nickel-alloycladdings. WeldingJournal 71(12): 461-s to472-s. 13. Wu, Y., andPatchett, B. M. 1994.Solidificationmicrostructure andsources of Type II grainboundary disbonding inCRA cladding. Paperpresented at 1995 AWSAnnual Meeting,Cleveland, Ohio. 14. Nelson, T. W.,Mills, M., and Lippold, J.C. 1998. Weld interfacephenomena in dissimilarmetal welds. Acceptedfor publication in Scienceand Technology ofWelding and Joining. 15. Kotecki, D. J.1990. Weldability ofMaterials ConferenceProceedings, 127, Editedby R. A. Patterson and K.W. Mahin, ASM

International, MaterialsPark, Ohio. 16. Bonnefois, B.,Charles, J., Dupoiron, F.,and Soulignac, P. 1991.How to predict weldingproperties of duplexstainless steels. Proc.Duplex Stainless Steels'91, Beaune, Bourgogne,France, Vol. 1, pp. 347-361. 17. Yasuda, K.,Tamaki, K., Nakano, S.,Kobayashi, K., andNishiyama, N. 1986.Metallurgicalcharacteristics of weldmetals and corrosionperformance of girth weldjoints of duplex stainlesssteel pipes. DuplexStainless Steels, ASMInternational, MaterialsPark, Ohio, p. 201. 18. Ume, K., Seki, N.,Naganawa, Y., Hyodo,T., Satoh, K., and Kuriki,Y. 1987. Influence ofthermal history on thecorrosion resistance ofduplex stainless steellinepipe. MaterialsPerformance, No. 8, pp25-31. 19. Sridhar, N.,Flasche, L. H., and Kolts,J. 1984. Effect of weldingparameters on localizedcorrosion of a duplexstainless steel. MaterialsPerformance, pp 52-55.

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Microstructure/Property Relationships in Dissimilar Welds Between Duplex Stainless Steels and Carbon SteelsThe effect of weld metal microstructure on toughness and pitting corrosion resistance is evaluated for both a duplex stainless steel and Ni-based filler metal