Influence of Shielding Gas Composition Welding Parameters ...

ISIJ International. Vol. 36 (1996). No. 7, pp. 968-976

Influence of Shielding Gas Composition and Welding

on the N-content and Corrosion Properties of WeldsStainless Steel Grades

Parametersin N-alloyed

Staffan HERTZMAN.ERIKSSON1)

Rachel JARGELIUS

Swedish Institute for Metals Research,

1) Department of Welding Technology,

(Received on September29.

PETTERSSON.Roland BLOM.Esa KIVINEVA.1) and Jan

Drottning Kristinas Vag 48. S-1 1428 Stockholm. Sweden.Royai Institute of Technology, S-1 0044 Stockholm. Sweden.

1995, accepted in final form on March4. 1996)

TIG welding using different heat inputs, arc lengths and shielding gas nitrogen contents wasperformed,The aim was to evaluate the possibilities to avoid nitrogen losses on welding or even increase the weldmetal nitrogen content and thereby improve the corrosion properties and, in the case of duplex grades, also

to improve the phase balance, Three different nitrogen-alloyed duplex grades and one superaustenitic grade

were investigated, Thecorrosion resistance in terms of CPT, Critical Pitting Temperature, of the super duplexmaterial wasfound to be strongly correlated to the nitrogen content of the weld metal. In the case of thesuperaustenitic weld metal, the increased nitrogen content was found to be associated with an increased

pore formation, Ieading to a lower corrosion resistance and thereby masking the positive effect of theincreased nitrogen content.

In order to illuminate the nitrogen exchange reactions between the arc, weld pool, and shielding gas,stationary weld experiments were also performed. The results from these stationary trials indicated that the

net weld pool nitrogen content could be qualitatively understood if the various fluxes involved in the nitrogen

transport between the plasma arc, weld pool and weld pool-shielding gas were considered. At short timesthe weld pool was limited in area and the nitrogen content of the weld pool increased due to high nitrogenactivity in the arc. At longer times the nitrogen escaped from the weld pool to the shielding gas. This flux

becamethen the dominating factor due to the increased weld pool area exposed to the shielding gas. Thesituation then approachedthe equilibrium conditions that were expected from the gas nitrogen activity andweld pool alloy composition according to thermodynamiccalculations using the Thermo-Calc database.

KEYWORDS:TIG welding; welding; duplex stainless steels; superaustenitic stainless steel; pitting corrosionresistance, nitrogen uptake; shielding gas; root gas composition.

l. Introduction

Thechoice of welding parameters is important in orderto obtain weldment properties in parity with the basemetal. In the case of stainless steels the main ambitionis to obtain a microstructure with a corrosion resistance

equal to that of the base metal. Howevermechanicalstrength has also becomeincreasingly important for the

newly-developed nitrogen-containing duplex and super-austenitic grades. Oneparameter of special interest in

the welding of duplex stainless steels is the compositionof the shielding and root gases which can be designed to

maintain or control the nitrogen level of the weld metal.

This is emphasisedby the fact that nitrogen is firstly the

most effective strengthener of austenitic steels and sec-ondly strongly enhances austenite reformation in plex

grades i.e, nitrogen is essential in order to obtain goodweldability in these steels. Slnce nitrogen is a volatile,

rapidly diffusing species it can easily be drained from the

weld and a balanced nitrogen content of the shielding

and root gases should be utilised to maintain the nominalcomposition of the filler metal, if nitrogen alloyed, orthe base material if autogenous welding is performed.Alternatively there is the possibility to use the shielding

gas to actively increase the nitrogen content of the weld.

Plasma interaction and nitrogen uptake has beenreviewed in a paper by Alluml) and also been treated

by others e.g. Kuwanaet al.2)

Vasinen et al.3) has studied the nitrogen uptake fromshielding gases on welding and found a fairly linear

dependenceon nitrogen content in shielding gas up to

about 20 "/* N in the gas. At higher nitrogen contents in

the shielding gas the nitrogen content in the weld beadlevelled off. The objective of the present work was tostudy the effect of welding parameters such as shielding

and root gas composition (primarily nitrogen andhydrogen contents), arc length and arc energy on the

weld metal nitrogen uptake, and also the influence ofsteel composition on nitrogen uptake kinetics andsaturation levels. The resulting pitting corrosion re-

C 1996 ISIJ 968

ISIJ lnternational. Vol. 36 (1996), No. 7

sistance of the welds and the weld metal microstructure 2.2. procedure of Welding Trials and Resultswasexamined.

To investigate the effect of arc length, autogenous

2. Materials, Welding Procedure and ResultsTIG-welding was performed using 2.5 and 5mmarclengths and with weld parameters according to Table 2.

2.1. Materials Butt welding was performed without edge preparation

Thematerials used in the present investigation were a after cutting. In the table the results in terms of nitrogen

set of commercial steels, solution treated according to contents and phase fractions are also included. In Fig.

recommendedpractice. The chemical compositions are l, the nitrogen content of the weld metal is plotted vs.

presented in Table 1. 2304 is a Mo-free duplex stainless arc length.

steel, (UNSS32304), 2205 is a 22Cr3Moduplex steel Comparedto the basemetal,anitrogen loss isobserved

(UNSS31803)and 2507 is a super duplex stainless steel for the weld metal of 2507 whenusing a 2.5mmarc

(UNSS32750). SMOis a saltwater-resistant super aus- Iength for both 2and 5o/, N2' A balanced compositiontenitic grade (UNSS31254). Commonto all materials is is obtained by using 5mmarc length and 5o/o N2 in the

that they are nitrogen alloyed to different levels, shielding gas and a minor nitrogen deficit is observed for

Table l. Chemical composition of the materials investigated.

>~::.o

Cl)

(D(U

EC(D~COOC(DO)OZ

0.35

0.3

o25

0'2

0.15

0.1

0,05

o

Chemical com osition n]ass"/o

Material Heat thickn

mmc Si Mll P S Cr Ni Mo Cu N

Weldingtrials

2205 800656 3 O016 0.4 J5 o020 0,002 22.0 57 30 03 O1632507 462854 3 0,022 O, IO03 O015OOOl 24.2 66 37 o06 0.284

SMO 383541 3 0.016 03 o.4 0,023 O. OOl 19 8 i7 8 61 o.7 O189Stationary

arc weldin

2304 800233 4 o015 23 o 47 02 OIO!

2205 376572 6 0.019 21 9 5.5 30 O1302507 802694 45 o024 24 6 6.9 3.8 o257SMO 383495 5 0.0 15 19 8 18 o 60 o2]2

Table 2. Weldparameters, shieldinggascompositions (note thathydrogenis not present) and resulting weld metal nitrogen contents andaustenite fraction. Root gas 90N2+10H2'Autogenous TIGwelding. Travel speed 2mm/s.

Gascomosition

Ar 20/0N Ar 5010N,

Material Arc length arc arc arc N- are arc arC N-[mm] current voltage energy content current voltage energy content

[A] [V] [k J/mnl masso/o [A] [v] [k J/mm] masso/oolov ("/.y)

SMO 2.5 llO l 1.3 0.62 0.2 13 120 12 l o73 0.219O, 1890/0N 110 l I .2 0.62 0.213 120 12.5 0.75 0.242

5.0 ll5 13.7 o79 0.299 120 14 O o84 0.304120 14, l 0.85 0.296 125 14 l o88 0.264

2507 2.5 l lO 12.3 o68 0.239 llO 12 6 o69 0.2650.2840/0N 35.3) (41.9

2.5 130 13.0 0.85 0.23738 7

50 120 14.5 o87 0.248 120 15.5 o93 o300(40, I) (44 3)

120 15 2 0.91 0.302

Ar 80/0N2

5,0 ~29 0.3315 8 l Ol

o 1 2 3 4Arc length [mm]

5 6

' Ar2N2(SMO)c:1 Ar5N2(SMO)' baseM(SMO)c Ar2N2(2507)^ Ar5N2(2507)AAr8N2(2507)' baseM(2507) Fig. l.

Nitrogen content of alloys SMOand 2507after welding using 2.5 and 5mmarclengths and2, 5and8'/, N2in the shielding

gas. TIG welding, travel spe~d 2mm/s.

969 C 1996 ISIJ

ISIJ International. Vol. 36 (1996), No. 7

Table 3. Gascombinations, arc energies (kJ/mm), nitrogen content in the

autogenous TIG-welds and, for the 2507 material, the austenitefraction determined with magnetic balance.

Material.

BasemetalN*content

Shielding

gas

Ar+5H2+25N2 Ar+5H2+5N2Ar+5H2+8N2 Ar Ar

Root gas N2+IOH2 N2+IOH2 N2+IOH2 N2 N2+IOH2

SMO3mm

O189•/.N

kJ/mm

massQ/oN

0.3

O161

0.3

O. 173

0.3

O. 165

0.6

O, 157

0.6

O. 155

25073mnl

0.2840/0N

kJ/mmo/oY

masso/oN

0526

O, 196

0.5

29O. 194

0.5

28O. 182

o.5

o, 192

0.6

O. 176

Table 4. Infiuence of alloy composition andarc length on nitrogen uptakefrom shielding gas. Stationary arc with 60s welding time.Material 2205.

Weld parameters: U=13 and 15V, I=110A, gas flow

lO//min.

Arclen h2 5mu~U*i3V Arclen h5mmU=15VMaterial

Basemetal N*content

Gascon]position

Weldmeta]

oxygencontentmasso/o

Weld ITletal

nitrogeu

eontellt

mass"/o

Weldmetal

oxygeneontentmasso/o

Weldmetainitrogeu

eontentmasso/o

2205

O13'/•N2N 98Al' 0.09 O. 16 O. 14 O. 185N, 95Ar 0.07 O19 o08 O.2l8N. 92Ar 0.08 o20 OlO o27

2507

O257'/.N2N, 98Al 0.09 O, 17 O, IO 0.225N 95Ar o.09 021 0,09 0.268N 92Ar O, I l 0.25 O, 13 0.28

2304

OI{)1"/.N

2N, 98Al O16 O15

5N 95Ar O18 0.208N 92Ar O, 19 o25

SMO0.2 120/0N

2N 98Ar Oll O]l5N. 95Ar O14 O158N, 92Al 0.20 O, 18

the 2o/o N2 shielding gas. SMOhas a lower initial basemetal nitrogen composition and the weld metal nitro-

gen content is higher than the base metal values for all

combinations of shielding gas compositions and arclengths used.

The materials were autogenously TIG-welded with2.5mmarc length and with different combinations ofshielding and root-gases containing hydrogen and ni-

trogen, according to the schedule presented in Table3. The resulting nitrogen content of the welds, deter-

mined using LECOmelt evaporation, are also includedand should be comparedto the base metal compositionof the individual materials according to Table I .

Evidently the weld metal nitrogen content has been re-

duced on welding in all cases comparedto the base met-al. In order to investigate the austenite content of thewelds, magnetic balance measurementswere performedand the results from three welds are also presented in

Table 3. No large differences were observed and theresults did not showany correlation to the N-content,which is perhaps not surprising in view of the minordifferences in nitrogen content.

A Iack of correlation between shielding gas 'cornposi-

tion and weld metal nitrogen content and also a con-sistent loss of weld metal nitrogen content comparedtothe base metal can be observed in spite of the fairly high

C 1996 ISIJ 970

nitrogen partial pressure of the shielding gases. Seefurther discussion below, Note however that from in-

dustrial experience, hydrogen is knownto decrease thenitrogen content of welds.

2.3. Procedure of Stationary Arc Trials and Results

In order to elucidate the nitrogen exchangereactionsbetweenthe arc, weld pool, and shielding gas, stationaryTIG bead-on-plate weld experiments were also per-formed.

The influence of arc length and shielding gas com-position were studied by using two different shielding

gases and two arc lengths. The experiments were thusperformed with the arc in the sameposition for 60swhichconsequently eliminates the effect of travel speed. Themain investigation wasconcentrated on 2205, with 2507,2304and SMOincluded to investigate the effect of alloycomposition, see Table 4. Shielding gases with andwithout hydrogen additions and with two different ni-

trogen contents were chosen. Table 5. Furthermore thekinetics of nitrogen pick-up were studied by interruptingthe weld process after different times of 2.5, 5, lO, 30,60, and 120 s; these results are presented in Table 6.

The results are encouraging in the sense that they areconsistent, see discussion below, but a certain scatter is

observed, for example two series of 2205 in 2and 5olo

ISIJ International. Vol. 36 (1996), No. 7

nitrogen contained in one case O.16 and O.19 and in asecond series O.12 and O.18 respectively (Table 4). Thisis also demonstrated in Fig. 9 in the discussion section

below.

3. Corrosion Testing

Corrosion testing was performed in a 3o/o NaC1so-lution according to the Avesta method,4) whereby the

breakdownpotential is determined at various tempera-tures and the critica] pitting temperature defined as the

temperature at which a transition from transpassive

Table 5. Influence of hydrogen on nitrogen uptake in 2205from shielding gas with two nitrogen compositions,welding parameters; U=12.9V, I=109A, arclength 2.5mm, gas fiow lO//min, stationary arcwith an exposure time of 60 s.

Material

2205

O13'/.N

Shieidiug gascomposition

2N lOH,Ar5N. 10H, Ar

2N, Ar5N Ar

Weldmetal

ox, ygen contentmaSSo/o

o lO

O08

O12

o J3

We]dmetalnitrogen content

mass"/o

o073

o092

O12

O,18

Table

dissolution to pitting attack wasobserved. Test couponsfor corrosion testing werecut to dimensions 25 by 25mmfrom the 3mmplates and were then wet surface groundto 600meshsilicon carbide prior to testing. For the weldstested here (autogenous TIGbutt welds) there wasoften

no large discontinuity in the breakdownpotential at thetransition as has been observed in other materials.4) Themain differentiating criterion used was therefore the

shape of the polarization curves (transpassive dissolu-

tion being characterised by a gradual current increase

comparedwith the usually rapid current increase ac-companyingpit initiation). Testing was performed onspecimenswet ground to 600 meshwhich were mountedin a multicell allowing for simultaneous testing of up to

l2specimens. Crevice corrosion was avoided by allow-ing distilled water to slowly percolate through a filter

paper placed between the specimen and the cell.4) Thepotential scan rate employed was 20mV/minand the

currents to Individual specimens were registered using

a Sotelem CMP860 multichannel pitting corrosion sys-tem. The results from corrosion testing of the welds

are summarisedin Table 7and examples from different

specimens are shownin Figs. 2~t.

6. Influence of time on nitrogen pick-up during TIGbead-on-plate welding of 2205 using two different

shielding gases and a stationary arc. Weldingparameters; U=13V, I= I09 A, arc length 2.5 mm,gas flow 10 !Imin. '

2205

O13'/~N

Shie]ding gas composition

2N 98Ar 8N, 92ArTime [s] Weldmetal

oxygen contentmass'/o

Weldmetalnitrogen coutent

masso/o

Weldmetal

oxygen contentmasso/o

Weldmetalnitrogen eontent

masso/o

2.5 0.25 o28 O, 16 o23 *

5 O19 0.22 o28 0.27lO o.25 O, 18 ,O 18 o2420 o08 O14 O, 14 o2060 O, 12 O, 13 o 12 O19

l20 o 15 o. J5 O16 o20* erroneously sampled, parts ofmatrix inc]uded* erroneously sampled, parts ofmatrix inc]uded

Table 7, CPTmeasurementsin3"/,NaCl accordingto theAvestamethod.

Material Weidparameters

Arc length

[mm]Position Sltielding gas Nitrogen

coutent[masso/o

CPT['C]

SMO see 'l'ab[e 2 1'OOt 90N,-~ IOH? 65-70base metal o J89 -90

to 25N+5H,+Al 0,161 65.75

5N,+5H+Ar O173 >808N,+5H+Ar O165 75-80

see Table 3 base metal O189 -902.5 to 2N,+Al 0213 >6525 5N+Al o242 >805 2N,+Ar 0.296 >805 5N+A]' 0.304 >70

2507 see Tabie 2 root 90N,+IOH, 60-65base Dletal o284 80*85

to ) 25N+5H,+Ar O196 50*55tl 5N.+5H+Ar O, 196 50-55

8N+5H,+Ar O182 50-55

0.68kJ/mm 2.5 to 2N+Ar 0.239 60

O85kJ/mm 2.5 to 2N,+Ar 0.237 60-65

see Table 3 2.5 5N+Ar o265 -605 2N+Al 0.248 60*65

5 5N,+Ar o300 70

5 8N+Ar ~65

971 C 1996 ISIJ

ISIJ International, Vol. 36 (1996), No. 7

1200

DI2507base material $ 1~

o looo(1)

>E800 ~IL-J ~c5 ~.~c:o eoot-o

CL ~400

40

1200-

DTo loooa)

>E 800

co eoo

o_400

60 80

Temperature ['C]i Oo

1200

DTo Ioooa)>E:

800

c5

= eoo

oo_400

SMObase material

lp (~ -

~

~

40

1200

Luoloooa)

>E800

c5

co 600

o[l400

60 80

Temperature ['C]1oo

, $

~

250750kN2(2.5mm)0.27'kN 42'k v

~~~~

Fig. 2.

40 1OO60 80

Temperature ['C]

Breakdownpotential vs, temperature for material 2507

after different welding conditions given in the diagrams.

The critical pitting temperature (CPT) is indicated by

a vertical line in the diagrams, the criterion beingprimarily the shape of the polarisation curves.

SMO2'kN2 (5mm)0,30'kN

. $ 8,~~

~~~

OH,~O

90

80

70

60

50

40

Basemetal -\ I

~ll:

.

D

D

2507

AutogenousTIG-weld

Fig.

0,10 0,20 0,30 0,40Nitrogen content [mass o/o]

3. CPTaccording to the Avesta method vs, weld metalnitrogen content for autogenous TIG welds in alloy

2507,

In the case of 2507 it was apparent that a higherweld metal nitrogen content resulted in a higher austenitefraction, and that a relatively good correlation was ob-served between the measuredCPTvalues and the ni-

trogen content, Fig. 3.

The situation was howevermorecomplex for SMO,Fig. 4. The three welds (see Table 3) produced usinghydrogen additions to the shielding gas gave a slight loss

of nitrogen compared to the base metal but still arelatively good pitting corrosion resistance, with CPTvalues of 70-80'C comparedwith approximately 90'Cfor the base metal. The three welds produced withouthydrogen additions to the shielding gases exhibited ahigher nitrogen content. This was, however, associatedwith pronounced porosity which was evident in themicrostructures, andalso refiected in the corrosion results

40 60 8o Ioo

Temperature ['C]

Fig. 4. Breakdownpotential vs. temperature for SMOafter

different welding conditions given in the diagram. Thecritical pitting temperature (CPT) is indicated by avertical line in the diagrams, the criterion being pri-

marily the shape of the polarisation curves. Trans-passive potentials are given as filled symbols and the

breakdown potential due to pitting with open sym-bols,

in that these welds all showedan extremely large scatterin breakdownpotentials. Detailed examination of Fig.

4 shows that trans.passive breakdown potentials wereobserved up to relatively high temperatures (for example80'C for the specimens welded with 50/0N2 in theshielding gases and an arc length of 2.5 mm)indicating

an inherent relatively goodpitting resistance but this wasaccompaniedby a large numberof cases in which pitting

wasobserved at lower temperatures and presumedto beassociated with pores in the tested surface.

4. Discussion

As indicated in the above sections, the nitrogen pick-

up on welding is complex and is controlled by several

factors. A fairly complete review was presented byAllum.1) DenOuden5)gave a comprehensivepicture ofthe most important factors in the process, emphasisingstrongly the non-equilibrium character and indicatingthat nitrogen pick-up is a result of thermal history andthe net result of nitrogen pick-up from the arc andnitrogen entering through the pool surface, illustrated

by the arrows c-arc/pool and c-pool/gas in Fig. 5,

In order to understand the results of the presentinvestigation, the base metal nitrogen content must also

be considered, this is illustrated in Fig. 5by a third ar-

row c-pool/matrix. In a discussion concerning nitrogenflow between several different media it is convenientto introduce the term activity instead of the nitrogencomposition of the respective media. The reason is quite

C 1996 ISIJ 972

ISIJ International. Vol. 36 (1996). No. 7

SHIELDINGGAS~)-arc/gas -~ ARCCOLUMN(~)-arclpool LIQUIDPOOL

q)-pool/gas ^ BASEMETAL

(~)-pool/matrix

Fig. 5. Schematic of arc cotumnweld pool and nitrogen fiowindicated by arrows. Thearc diameter hes in the range2~mm.

simply that the direction of the nitrogen flux will betowards regions with lower nitrogen activity. The nitro-

gen activity is a measureof the nitriding potential of themedia, and is here related to the activity of nitrogen gas,the activity of pure nitrogen gas is unity.

4.1. The Nitrogen Activity of the Arc ColumnThe nitrogen activity of the arc column is dependent

on several factors, firstly of course the nitrogen contentof the shielding gas and also on the temperature. Thenitrogen mo]ecule is dissociated at high temperatures,for pure nitrogen, dissociation starts above roughly4000Kand is fairly complete at temperatures abovelOOOOK.This implies that in a flame the nitrogen is

hardly dissociated at all, whereas at temperatures en-countered in the plasma of a gas-shielded arc column,the nitrogen is fully dissociated. At about 5000K, N+starts to form but the amount is smal] and even at

lOOOOKthe N+ fraction amounts to only I o/o of theatomic nitrogen. At temperatures above 15 OOOK how-ever, N+dominates.5)

Thenitriding potential of the arc is howeverdependenton the temperature profile of the arc in addition to theequilibrium composition. At the surface of the melt the

temperature of the arc is probably muchlower than the

lOOOOKthat is malntained at somedistance above thesurface. The dissociated nitrogen is thus believed to befrozen in and as a result the nitriding potential is

controlled by the ratio (pN)2/pN..

DenOudensuggested an expression for the nltridlngpotential of the arc in the form of a modified Sieverts

Law which states that the equilibrium content of agaseousspecies in a metal, CNin thls case, is proportionalto the partial pressure of the species in the gas, or if the

gas is a diatomic molecule, the activity is instead the

square root of the partial pressure, as follows:

cN=K1~//~pN, +K2pN+K31'N-

where pN. represents the partial pressure of molecularnitrogen, pN atomic nitrogen and pN+ nitrogen ionsin the arc. The values of the indlvidual constants arehoweverunknown.

Since the dissociation and thus also the nitridingpotential of the arc are controlled by both the corn-position and temperature, weld parameters will be ofprime importance. In this context it is also relevant todiscuss the arc length slnce very clear increases in nitrogencontent are seen with increasing distance, as illustratedin Figs. 6to 8where the calculated equilibrium nitrogencontent is shown for comparison (Thermo-Calc data-

.)~~

u),,,(10

E:

C:~2co

o::o

O)OZ

Fig.

~~('

(hU)C:l

ECo

Co

OC(1)

O)OZ

0.40

0.35

O30

0.25

0.20

0.15

0.10

0,05

o

Nitrogen content in shielding gas [•/.]

O 16 36 644

25071673K

5mmth(rc/length

I~

A

773K

1873K

1oo

O 100.2 0.4 O6 0.8

Nitrogen activity

6. Nitrogen content in stationary arc welds in 2507 after60sexposure in three different shielding gases with 2.5and 5mmarc lengths, compared to the calculatedvariation of the melt nitrogen content with nitrogenactivity at three different temperatures.

Nitrogen content in shielding gas [o/o]

O 4 16 36 64 1OO

0.40

0.35

0.30

0.25

0.20

0.15

o lo

Q=05

o

2205

5mmarc length

A

~

1673K 1773K

1873K

O 0.2 0.4 0.6 1.O0.8

Nitrogen activity

Fig. 7, Nitrogen content in stationary arc welds in 2205 after

60s exposure to the arc in three different shielding

gases with 2.5 and 5mmarc lengths, comparedto thecalculated variation of the melt nitrogen content withnitrogen activity at three different temperatures,

base6)). This is probably attributable to an increasednitriding potential of the arc associated with a morepronounced shielding gas dissociation with increasingwelding voltage due to a higher plasma temperature. Inthe figures, 5mmarc length data are connected with aiine and the 2.5 mmdata are shownas single points. Theeffect of increasing the arc length from 2.5 to 5mmcanbe estimated as O.05-0.07010N. For SMO,Fig. 9, theeffect wassmaller, this might be related to the observed

pore formation in these welds.

973 O1996 ISIJ

ISIJ lnternational, Vol, 36 (1996), No. 7

~~O

U,,,,C:f

EC:(D

~COOOO)OZ

0.40

Nitrogen content in shielding gas [•/.]

O 16 36 1OO644

0.35

0.30

0.25

0.20

0.15

0.10

0.05

o

2304

5mmarc iength

A~

1673K1773K

\

1873K

O I .O0.2 0.4 0.6 0.8

Nitrogen activity

8. Calculated variation of nitrogen content vs. nitrogenactivity in 2304 at three different temperatures.Nitrogen activity is defined as the square root of the

nitrogen partial pressure.

Nitrogen content in shielding gas [o/o]

O 4 16 36 64 1OO

1600

Nitrogen content in shielding gas [•/.]

4 i6 36 64 1OO

O,D

=c:,

oQEoH

1500

1400

1300

1200

Fig.

)~oo

U'U)~5

ECO~:

OOC(DO)OZ

0.40

0.35

o.30

0.25

0.20

0.15

0.10

0.05

o

SMO

5mmarc length

1673K 1773K

1873K

O I .O0.2 0.4 0.6 0.8

Nitrogen activity

Fig. 9. Calculated variation of nitrogen content vs. nitrogenactivity in SMOat three different temperatures.Nitrogen activity is defined as the square root of the

nitrogen partial pressure.

4.2. The Nitrogen Activity of the Steel

Figure 10 presents the nitrogen activity of the different

steels in the present study. It is obvious that the activi-

ty differs substantially between the steels and this is

primarily due to the contents of nitrogen and the majoralloying elements chromium and nickel, the formerdecreases the nitrogen activity and the latter increases it

strongly. Themost relevant part of the diagram concernsthe liquid state, the upper part of the curves. Below the

uppermost break point, the primary solid phase will

influence the situation and this part describes thesituation on cooling and not in the region close to the

plasma. It also gives information on whether nitrogen is

SMO

2304

2507/

_../~

2205

1iOO

O O2 O4 O.6 0.8 1.O I .2 14Nitrogen activity

Fig. ro. Catculated variation of nitrogen activity in the ma-terials studied *s. temperature.

lost or gained from the high-temperature HAZ,which is

most relevant to the resulting HAZproperties. In the

case of duplex steels, the primary phase is usually ferrite,

leading to a rapid increase of the nitrogen activity oncooling. This indicates that nitrogen will be transportedinto the weld pool by diffusion through the solid phaseimmediately below the fusion line. This maylead to a10cal draining of nitrogen in the HT-HAZwith conse-quences for the austenite reformation7) jn this region.

4.3. The Nitrogen Activity and Hydrogen Contentof the Shielding Gas

As indicated above, the nitrogen activity of a gas is

simply related to its partial pressure and for nitrogen it

is given by the square root of the nitrogen content. Thenitrogen flux between the weld pool and shielding gas is

determined by the activity difference between the weldpool and shielding gas. In addition to the activity

differences, transport phenomena, including kineticrestrictions in the phase boundaries, also play a sig-

nificant role.

If the welds are produced in such a way that theshielding gas/weld pool interphase nitrogen flux is

substantial and thus controls the nitrogen content of theweld, the resulting nitrogen content can be estimatedfrom the equilibrium diagram presented in Fig. 11.

Thenitrogen content clearly approaches the shielding

gas/weld pool equilibrium values after long times. Twoobvious reasons are

(1) a larger interphase area between the v,'eld pooland the shielding gas with increasing time so the gas/weldpool equilibrium is approachedandcontrols the nitrogencontent of the weld pool. Dueto kinetic reasons, the arccannot maintain the high nitrogen level and

(2) a larger fused volumedilutes the nitrogen uptake,again the nitriding capacity of the arc is too low tomaintain the potential of the arc in the weld pool.

Fromthe abovediscussion it follows that the nitrogen

content after the shortest tirnes are those that mostcloselyreflect the nitriding potential of the arc, since the dilution

C 1996 ISIJ 974

IS]J International, Vo]. 36 (1 996), No. 7

~~eo

U)U),U

EC(D,:OOC:G)a)O

Z

0.40

Nitrogen content in shielding gas [•/.]

O 4 16 36 1OO64

0.35

0.30

0.25

0.20

0,15

0.10

0.05

o

2205

215s A

5s

10s

120s20s60s

1673K 1773K

1873K

O 0.2 0.4 0.6 1.O0.8

Nitrogen activity

Fig, Il. Nitrogen content of weld metal of liquid 2205 vs.

nitrogen activity with experimental data for 2 and8'/* Nin shielding gas included, Thenitrogen contentis achieved after different times of exposure to an arccolumn from 2.5 s up to 120s,

)~oo

U''OC!f

EC~2C:o

O~a)O

Z

0.40

Nitrogen content in shielding gas ['/.]

O 4 16 36 1OO64

0.35

0.30

0.25

0.20

0.15

0.10

0.05

o

2205

Nitrogen/

argon

1673K 1773K

1873K

Nitrogen/1 OHydrogen/Argon

is minimal and the shielding gas/weld pool interphase is

also minimal. Aminor reservation is howevermadedueto the somewhatlarger risk of atmospheric interactionsat the shortest welding times.

If hydrogen is added to the shielding gas, one canexpect several effects of which a reduced tendency tooxide formation is the one of major importance. Sinceoxide layers on the weld pool will reduce the escape ofnitrogen to the ambient shielding gas, conversely a lackof oxide will lead to a nitrogen content of the weld metalcontrolled by the weld pool/shielding gas reaction. Anocular inspection showedthat oxidation wasappreciablylower on the welds with hydrogen addedto the shielding

gas comparedto those with only nitrogen and argon. If

the data for the hydrogen-free and hydrogen-containingshielding gas welds are plotted in the same type ofdiagram as above, see Fig. 12, a clear tendency to ap-proach the equilibrium values is seen. The oxygen level

of the weld metal, after removal of the surface layer,

gives further support for the interpretation, see Table 5.

Hydrogencould also, in addition to the effect on oxideformation, have an infiuence on the nitriding potentialof the plasma.

If these results are transferred to the real weld situation

we mayexpect that weld parameters that promote alimited weld pool e.g. small shielding gaslpool surface

area, a limited dilution by a limited fused volume, will

yield the highest nitrogen pick-up from the arc.

5. Summaryand Conclusions

A study of nitrogen uptake on autogenous TIGwelding with different shielding gases has been under-taken and the resulting microstructures and pitting cor-rosion resistance have been evaluated. A super-duplex2507 grade, a conventional 2205 type duplex and a su-

O 0.2 0.4 0,6 1.O0.8

Nitrogen activity

Fig. 12. Effect of 100/0 hydrogen addition to the shielding gason nitrogen content in the weld metal after 60sexposure to a stationary arc. 2,5 mmarc length. Basemetal nitrogen content O, 13 o/, N,

peraustenitic 6"/* Mostainless steel were investigated.Factors controlling the nitrogen absorption from the archave been investigated by utilising a stationary arc anddifferent shielding gases. The main conclusions drawnare presented below.

(1) Evensmall amountsof nitrogen in the shielding

gas will, using conventional welding conditions, Iead toappreciable nitrogen levels in the welds, Ievels well abovethose obtained in a gas environment with the samenitrogen content. Thenitriding potential of aTIGplasmais probably morethan 4-5 times higher than the nitridingpotential of agas with the samecomposition, in the rangeunder study.

(2) The resulting nitrogen content of the weld metalhas been discussed in terms of arc length, nitrogen con-tent of the shielding gas and hydrogen content ofshielding gas. Its consequences for primarily threeinterphase reactions driven by nitrogen activities in the

arc, weld pool, material and shielding gas are dealt with.Themain findings are

- The nitriding potential of the arc increases withnitrogen content in the shielding gas which can lead tohigh nitrogen content in the weld metal if only a limitedvolume is melted.

- If welding is performed such that large amountsof basemetal are melted, the nitrogen flux through the arc is

too low to maintain the high concentration of nitrogenin the weld pool. This is due firstly to dilution by thebase metal and secondly to an increasing shieldinggas/weld pool area so losses to the shielding gas drainthe weld pool.

- If this reaction is unrestricted by oxides it will lead to

a nitrogen content of the weld pool in the vicinity ofthe values given by the gas-metal equilibrium.indirect support for the effect of hydrogen is given bythe experiments since welds welded with hydrogen in

the shielding gas exhibited less visible surface oxides

975 C 1996 ISIJ

ISIJ International. Vol.

(after cooling). They also had lower oxygen levels in

the weld metal itself. Shielding gases without hydrogenresulted in a higher nitrogen content in the weld.

Howeverit is also possible that the nitriding capacityof the arc is affected by hydrogen.

The above information indicates that in the range ofweld parameters and shielding gas compositions inves-

tigated, weld parameters that promote a limited weldpool volume will yield the highest nitrogen pick-up. I,e.

a small fused volume, a restricted shielding gas/weld pool

area, and the presence of oxygen in the shielding gaswhich promotes a protective cap to the shielding gas.

(3) Welding of a superaustenitic steel with shielding

gases and arc lengths yielding nitrogen contents in excessof O.2 "/*, gave porosity that resulted in a large scatter in

corrosion testing results. The expected improvement ofproperties by higher nitrogen contents was thus notobserved.

(4) 2507 exhibited a good correlation between the

weld metal nitrogen content and the pitting corrosionresistance. In no case were detrimental pores or nitrides

observed in these welds, a higher nitrogen content in the

shielding gas resulted only in an increase in the austenite

fraction.

36 (1996), No. 7

Acknowledgments

This work was performed under the auspices of the

Nordic Industriai Fund and financed in part by theparticipating companies AGA,AST, Avesta Sheffield,

ELGA.ESABand Sunds Defibrator. Thanks are also

extended to Sandvik Steel for performing the magneticbalance measurementsand to the programmecoordina-tor Lars Johansson, Svetskommissionen.

REFERENCESl) G. J. Auum.Nitrogen absorption from welding arcs, Interna-

tional Institute of welding, Report, July i987, nw DOC212-659-86.

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6)

7)

T. Kuwana,H. Kokawaand K. Naitoh: Trans. Jpn. Wc'ld. Soc.,

21 (1990). No. 2, 85.

p. Vasinen, H. Hdnninenand A. Kyrdlziinen: Nordic SeminaronWelding of Duplex Stainless Steels, Svetskommisionen, Stock-

holm, Sweden,(1993).

R. Qvarfort and E. Alfonsson: Proc. I Ith Scandinavian CorrosionCongress, U1landhaug, Stavanger, Norway, Norsk Korrosion-steknisk F6rening, Oslo, (1989), paper F-79.

G. den Ouden: P/7i!!ips Weld. Rep., 13 (1977), No, l, l.

B. Sundman,B. Jansson, J.-O. Andersson: Ca!p/7ad, 9(1985), 153.

S. Hertzman, B. Brolund and P. Ferreira: Ist EuropeanStainless

Steel Conf., AIM, Milano, (1993), 3.243.

@1996 ISIJ 976