How to Perform Weldingin Duplex Stainless Steels to ObtainOptimum Weld Metal Properties

byBjrn Holmberg,

Avesta Welding AB, P.O. Box 501, SE-774 27Avesta, Sweden

SummaryWelding of duplex grades, e.g. 2205,has been carried out using SMAW,GTAW and SAW. The most sensitivearea, with frequent problems, is theroot side in single-side GTA-weldedjoints. In this case the duplex filler inthe exposed area should either beover-alloyed or, in the case ofGTAW, the filler material should bewelded with a nitrogen-bearingshielding/purging gas. The impactstrength of the weld metal obtainedwith SMAW is normally lower thanthat of GTA welds. By using basiccovered electrodesor electrodeswhich give low oxygen or inclusioncontents in the weldsacceptablyhigh impact strength at low tem-peratures can be obtained. Contraryto expectations, the use of nickel basefiller for root runs can strongly reducethe notch toughness.

IntroductionThe requirements for modern duplexstainless steel weldments have inmany cases been too stringent in thepast. In some cases these stringentrequirements have in fact causedserious problems for welding en-gineers. The aim of this paper is topropose more realistic requirementsto designers and to help fabricatorsto select filler material, weldingmethods and welding parametersto fulfil those requirements. This isrealized by presenting a number ofwelding trials including evaluation ofproperties and examination ofmicrostructures.

MaterialsThe fillers and parent materials usedin the following trials are listed inTables 1 a and 1 b, page 2. Theshielding gas during GTAW waspure argon mixed with 3% nitrogen.Three different purging gases wereused: argon, argon + 3% nitrogen,and 90% nitrogen + 10% hydrogen(Formier gas).

WeldingFor the experiments, 3, 5, 10, 12 and30 mm plates were used. Pipewelding in fixed position (6G) wascarried out in 8", t = 3.7 mm andin 17", t = 10 mm pipes. Welds No.15-17 were carried out from bothsides, all others were single-sidewelded. The welding procedures areshown in Table 2, page 3. For thewelding of plate material SMAW,GTAW or SAW was used. For pipewelding GTAW and SMAW wereused.

TestingThe microstructure was studied byoptical microscopy and by amodified electron probe micro ana-lyser. The ferrite content in the weldmetal was measured with Feritscope(MP3). The ductility was measured bybend and impact testing.

ASTM G-48A testing was under-taken with test pieces in two condi-tions; with welded surfaces bothtotally cleaned and in weldedcondition but with the cap brushedwith a rotating disc. New specimenswere used at each exposure.

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Table 1a.Chemical composition of filler materials used and all weld metal impact strength.PRE(N) = %Cr + 3.3 x %Mo + 16 x %N.

Type Diam. Chemical composition, % PRE(N) ImpactAvesta Welding (mm) C Si Mn Cr Ni Mo N Nb strength (J)

+20C -40C2205-PW 2.50 .023 .89 .90 22.3 9.5 3.1 .168 - 35 - -

3.25 .025 .82 .82 22.6 9.8 3.1 .158 - 35 - -4.00 .029 .82 .81 23.4 9.8 3.0 .158 - 36 53 -

2205 BAS 2.50 .026 .43 .64 23.5 9.5 3.2 .16 - 36 - -3.25 .028 .44 .98 23.4 9.3 3.0 .146 - 36 84 604.00 .027 .34 .94 23.3 9.4 3.0 .156 - 36 67 50

2205 super 4.00 .022 .88 .60 23.5 9.4 3.6 .188 - 38 37 242507/P100 Rut. 4.00 .031 .46 1.37 25.4 10.3 3.6 .211 - 41 - -

2205 wire 1.6 .014 .49 1.58 22.5 8.8 3.1 .130 - 35 - 1502205N wire 1.6 .014 .49 1.58 22.5 8.8 3.1 .230 - 36 - -2205/Flux 805Cr 2.4 .016 .57 1.28 23.5 8.5 3.0 .128 - 35 183 149

2507/P 100 wire 1.6 .014 .39 .42 25.3 9.6 4.0 .280 43 - -PI 2 wire 1.6 .008 .04 .03 21.9 64.4 9.1 .01 3.7 52 - -P16 wire 1.6 .002 .02 .15 22.5 60.8 15.9 .03 - 76 120 110

Table 1b.Chemical composition of parent materialsPRE(N) =%Cr + 3.3 x %Mo + 16 x %N.

Type Thickn. Chemical composition, % PRE(N)Avesta Sheffield (mm) C Si Mn Cr Ni Mo N

2205 plate 3-30 .016-.022 .37-.53 1.40-1.52 21.5-22.2 5.5-5.7 2.9-3.1 .140-.184 34-35

ResultsX-rayIn the first trials, welds No. 19 and 20showed an unacceptable amount ofporosity. They were therefore re-welded. With the first welds, Nos 19and 20, 10 and 13 beads, respect-ively, were used. With the re-welding27 and 20 beads, respectively, wereused. The X-ray investigation resultafter re-welding showed no porosity.

MicrostructureAll welds for which duplex filler hadbeen used showed a ferrite levelinside the welds between 23 and53%. Traces of nitrides were found inthe HAZ in some welds. The highestamounts in the weld metal were

noticed in No. 2 and No. 4. Second-ary austenite was found in mostmultiple welds. Small areas withsigma phase and secondaryaustenite were also present in somewelds carried out with super duplexfillers.

In welds No. 1, 5 and 5b nickel-based fillers were used for the rootrun and the duplex filler for the otherruns. The first bead duplex filler ontop of the high molybdenum root runresulted in a ferrite with a highmolybdenum content, which trans-formed during cooling to sigmaphase. This structure already crackedduring welding. Micro-fissures insigma phase are shown in Figure 1.

The nitrogen content in the HAZwas measured in welds No. 1 and14. The nitrogen content in the ferrite

in weld No. 1 was 0.054% and inweld No. 14 it was 0.055%. Thenitrogen content in matrix ferrite inparent metal from weld No. 1 was0.047% and in weld No. 14 it was0.050%. The thin austenite phasealong the fusion line in weld No. 1had a nitrogen content of 0.21-0.25% and in weld No. 14 the samephase had 0.14-0.18% (see Figures2a and 2b, page 4).

The hardness measurements acrossthe weld were carried out 1 mm fromthe root side. The hardness variedfrom 245 to 308 Hv5. The highesthardness was measured in a pipe,weld No. 19. This result is entirelynormal because the severe restraintcondition in a circumferential multiplebead weld will cause great colddeformation of the root bead.

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Table 2.Welding procedures.

Weld Process* Filler** Gas types Heat input (kJ/mm) Total No Parent***No. Root Cap Root Cap Shielding Purging Root Cap/fill of beads material1 141 111 A D Ar Ar 0.7 0.5 2 PI 3.02 141 111 B E Ar Ar 0.7 0.7 2 PI 3.03 141 111 C E Ar + 3%N2 Ar + 3%N2 0.6 0.6 2 PI 3.04 141 111 C D Ar Ar 0.6 0.5 2 PI 3.0

3b 141 111 C D Ar + 3%N2 Ar + 3%N2 1.3 0.9 2 PI 5.03c 141 111 C D Ar + 3%N2 N2+ 10%H2 0.9 0.9 2 PI 5.0

5 141 111 A D Ar Ar 1.4 0.8-1.2 9 PI 125b 141 111 G D Ar + 3%N2 N2+ 10%H2 1.1 0.8-1.2 6 PI 106 141 111 B E Ar Ar 1.1 0.9-1.3 8 PI 127 141 111 C D Ar N2+ 10%H2 1.4 1.0-1.3 8 PI 128 141 111 B F Ar Ar 1.0 0.7-1.3 7 PI 129 141 111 H E Ar Ar 1.3 0.8-1.6 8 PI 1210 141 12 H 1 Ar Ar 1.5 0.9-2.0 5 PI 12

18 141 111 C E Ar + 3%N2 N2+ 10%H2 1.1 0.8-1.5 28 PI 30

11 141 141 B B Ar Ar 0.6-0.8 0.3-0.5 3 Pi 3.712 141 111 H D Ar Ar 0.5-1.2 0.6 2 Pi 3.713 141 141 C C Ar + 3%N2 Ar + 3%N2 0.5-0.9 0.4-0.5 3 Pi 3.713b 141 141 C C Ar + 3%N2 N2+ 10%H2 1.2 0.8 2 Pi 3.714 141 141 G G Ar + 3%N2 Ar + 3%N2 0.5-0.9 0.3-0.4 3 Pi 3.7

19 141 141 C C Ar + 3%N2 N,+ 10%H2 2.0 0.6-1.4 27 Pi 1020 141 111 C E Ar + 3%N2 N2+ 10%H2 2.0 0.6-0.9 20 Pi 10

15 111 111 J J - - 0.9 0.8-1.1 9 PI 1216 111 111 E E - - 1.0 1.0-1.3 8 PI 1217 111 111 F F - - 1.3 0.9-1.3 7 PI 12

* 111 = SMAW141 = GTAW 12 = SAW

** A=P12B = 2205 NC = 2205D = 2205-PW AC/DCE =2205 basicF = 2205 sup. rutileG=P16H=2507/P100I = 2205 + flux 805J = 2507/P 100rutile

***Pi = pipe; wall (mm)PI = plate; thickn. (mm)

N.B. Weld Nos 15, 16 and 17 are welded from both sides. All other welds are single-side welded.

Figure 1.Weld No. 5. First bead duplex with micro-fissure in sigma phase, 400x. Root:GTAW; filler PI 2 wire. Hot pass: SMAW;filler 2205-PW.

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Figure 2a.Micro-probe mapping of the HAZ in weldNo. 1 (GTAW, filler P 12 with Nb). Fusionline in the middle, weld metal below.P12 gave a slightly wider low-nitrogenferrite zone in the HAZ compared to weldNo.14.

Figure 2b.Micro-probe mapping in the HAZ in weldNo. 14 (filler P16, no Nb).

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Mechanical propertiesImpact strength. The pure GTAW(No. 19) and the SAW (No. 10) gavean impact strength at room tempera-ture which was higher than all otherwelds. Basic coated electrodes orrutile electrodes with low Si content(2507/P100) gave high impactstrength. At -20C all welds with onlyduplex fillers had a higher impactstrength than 40 J independent ofcoating type. The combination ofduplex and nickel base fillers gavethe lowest impact strength (Nos 5and 5b).

Ductility. The results are given inTable 3. The negative effect onductility when mixing duplex andnickel base fillers could also be seenduring this test. All other weldsshowed good ductility.

Table 3.Bend testing to 180 angle.

Weld Root Cap

No.

1 3xT cracked 3xT crackedat 80C at 103C

5 2xT cracked 3xT OKat 85C3xT small cracks

5b 2xT cracked 2xT crackedat 65C at 85C

Pitting corrosion resistanceBrushed and pickled test speci-mens. The root side in the single-sided GTA weldments was theweakest area from corrosion point ofview when standard 2205 filler andpure argon were used. In manycases, the cap-side weld metalwelded with covered electrodes hada higher pitting resistance than theadjacent HAZ. Nickel base fillers andsuper duplex fillers also gave en-hanced pitting resistance. The use ofa nitrogen-alloyed 2205 wire com-bined with a purging/shielding gasof pure argon improved the pittingresistance but only to a limited extent.The addition of nitrogen in theshielding gas seemed to have moreeffect.

Figure 3a. ppWeld metal impact strength from GTAW,SAW and SMAW basic coatings.

Figure 3b.Weld metal impact strength from superduplex fillers with higher silicon andoxygen contents.

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Table 4.Pitting corrosion test results in ASTM G48-A.

Weld Brushed and pickled Weight loss Pitting location As-welded root Weight loss Pitting locationNo. Test temperature (C)at pitting Test temperature at pitting

no pitting pitting (mg) cap root no pitting pitting (mg) cap root1 35.0 - - - 22.5 1.0 H F2 32.5 35.0 30.1 W F - 20.0 1.7 H, F3 25.0 27.5 1.8 H W3B 25.0 27.5 0.8 P 22.5 25.0 3.9 P, H W, F3C 22.5 25.0 0.2 P 22.5 25.0 3.8 P, H

4 17.5 20.0 0.4 F - 12.5 0.9 H5 32.5 35.0 73.4 H (W)5B 40.0 45.0 283.9 F P, H 27.5 32.5 52.2 H H6 - 22.5 1.0 W - 20.0 5.3 W7 27.5 - - - 22.5 1.8 H

8 - 22.5 40.7 W9 22.5 25.0 2.2 W 22.5 27.5 36.6 H, W H, W

10 25.0 27.5 0.7 H - 22.5 6.2 H, W11 20.0 22.5 4.2 W12 32.5 35.0 3.9 H F - 22.5 1.5 F

13 20.0 22.5 2.2 W 20.0 22.5 5.2 W13B 30.0 32.5 7.6 H 25.0 27.5 4.0 P14 30.0 32.5 1.5 H - 22.5 2.5 F15 27.5 30.0 2.5 H16 27.5 - -

17 30.0 32.5 5.7 H - 22.5 16.3 H H18 22.5 25.0 7.2 P, H 17.5 22.5 6.5 H, W H19 25.0 27.5 8.0 H 22.5 25.0 3.8 H H20 17.5 20.0* 1.1 P grinding 20.0 22.5 1.1 H

marks

* Not valid due to grinding marks H = HAZ; W = weld metal; F = fusion line; P = parent metal

Cap brushedroot "as-welded".Brushed and pickled samples gavesignificantly better values than theones only cleaned on the cap sidewith rotating fibre disc. Most pittingoccurred on the root side in the HAZor in the fusion line. These resultsshowed that pitting in the HAZ wasfound down to 12.5C. By using theaddition of nitrogen in the purging/shielding gas the pitting resistance inthe root could be increased up to atleast 22.5C. Super duplex and nickelbase wires combined with pure argongave the same pitting resistance.

DiscussionIn these types of weld metals thecontents of ferrite, secondaryaustenite, nitrides, oxides and sigmaphase will in varying degree affectthe mechanical properties andcorrosion resistance. It is also veryimportant to realize that, for example,ferrite content, secondary austenitelevel and so on, are not materialproperties in themselves. The ductilityis acceptable when duplex filler isused, but not when duplex fillers arewelded on top of nickel base fillers.The notably higher impact strength inweld Nos 10 and 19 is a result ofwelding procedures giving cleanweld metals with very few oxides.When standard rutile electrodes areused, the amount of oxides will

increase and consequently the impactstrength will be lower. To compensatethis in certain joints the weldingengineer can increase the number ofGTAW runs and lower the SMAWruns.

The result shows that fabricationspecifications stipulating "no pitting at22C on as-welded samples" can bevery difficult to pass independent ofthe filler metal used, because thepitting might start in the HAZ or in thefusion line. The use of a newspecimen for each temperature,instead of using the same piece andincreasing test temperature untilpitting occurs, most likely explainswhy this investigation shows a 5-10C lower pitting resistance resultthan some other laboratories.

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Joints welded from both sides givebetter pitting resistance in the weldwhen slag forming processes such asSMAW are used. Duplex fillers forSMAW and SAW with PRE(N) >35can give weld metals which are moreresistant to pitting than the HAZ.

Good pitting resistance wasobtained when the shielding/purginggas was mixed with nitrogen.Nitrogen addition in the gas reducesnitrogen loss in the weld pool. Therewas no significant difference in pittingtemperature when the purging gaswas Ar + 3 N2 or 90 N2+ 10 H2 butthere was a tendency that the rootweld metal had a better pittingresistance: pitting occurred in theHAZ instead. Addition of nitrogenseems to improve the pitting resist-ance in 6% ferric chloride (ASTMG48A) by about 5C.

When studying the total test resultsit is obvious that a weld metal fromstandard 2205 electrode in brushedcondition will be attacked at 22.5-27.5C. In pickled condition the resultwas always above 27.5C. For bothsurface conditions SMAW welds areequal or better than the HAZ. Theheat input level in the root run or thesecond run did not affect the pittingresistance (0.5-2.0 Id/mm) in thesetests. The same was valid for the caprun (0.3-2.0 Id/mm).

Pitting resistance of welds in the as-welded condition will give lowervalues than can be obtained withparent materials in pickled condition.In the case of single-sided weldedpipes it is realistic to expect that theGTAW welded sample can pass+20C without pitting when standardfiller and nitrogen addition in theshielding/purging gas are used. Theproblems with passing +20C duringCPT testing without pitting have beenreported earlier (1).

A reduction of nitrogen content inthe HAZ, when using Nb-stabilized

nickel base fillers, causing reductionin austenite content, could to someextent be illustrated (Figure 2). This isalso emphasised in literature (3).However, this investigation could notmeasure any difference in pittingresistance in the HAZ when a Nb-alloyedor Nb-freenickel base fillerwas used.

Conclusions Pitting resistance of single-sided

GTA-welds with conventional2205-fillers and pure argon may beenhanced by using either nitrogenadditions to the shielding- andbacking gas or by using superduplex fillers.

Pitting resistance increases by 2.5-5C when nitrogen is added to theshielding-purging gas.

GTAW weldments with standardduplex fillers in as-weldedcondition can give a CPT value of22.5C with N2 addition. The sametemperature is also valid for theHAZ/fusion line.

Standard duplex covered elec-trodes with PRE(N) >35 givesufficient pitting resistance.

The highest impact strength induplex weldments can be obtainedwith GTAW. Rutile coated elec-trodes give the lowest impactstrength.

Due to lack of ductility, duplexfillers should not be welded on topof nickel base fillers.

The corrosion resistance andmechanical properties do not varysignificantly for normal heat input(0.3-2.0 kJ/mm) or for a ferritelevel between 23 and 53%.

Welding with a few thick beadsincreases the tendency to porosity.

References1. R. Gunn: "Comparison of Corrosion and

Mechanical Properties of Weldments inWrought 25%Cr and Super DuplexStainless Steels", Duplex Conf. '94, Glasgow,UK, paper 32.

2. "Recommended Practice for Pitting CorrosionTesting of Duplex Stainless Steel Weldmentsby the Use of Ferric Chloride Solution", TWI5632/16/93.

3. L. degrd et al.: Proc. Duplex StainlessSteel '91,Beaune, France, Les ditions dePhysique P441.

This paper was first printed inStainless Steel World, March 1997,pp. 28-33. Reprinted with the kindpermission of the copyright holder.

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 gen.

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