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Gas-shielded Arc Welding
of Duplex Steels
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Contents
03 Introduction
03 Fields of application and typical properties
07 Weldingproceduresandtechniques
07 General recommendations
07 Cooling rate, t12/8 -concept
07 Preheating and interpass temperatures
08 ShieldinggasesforGMAW
09 ShieldinggasesforTIGwelding
10 Examplesofapplication
11 Shieldinggasesforrootprotection
11 Conclusion11 References
Stainshield Duplex is aspecialised welding gasfor TIG applications onduplex steels.
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Introduction
Fieldsofapplicationandtypicalproperties
Since their development, ferritic-austenitic materials have been
used at an ever-growing rate. Their applications, primarily in the
oil and gas industry, petrochemical industry and pharmaceutics,
are based on the good properties of corrosion resistance and
strength. Because of their properties, in many cases they are an
interesting alternative to common Cr-Ni steels and Ni-based alloys.
Duplex steels are of two-phase microstructure (hence the name
duplex), containing both austenite and ferrite. Because of suchspecic microstructure, duplex steels combine to a certain extent
the advantages of two different sides. On one side, there are the
ferritic and martensitic chromium-steels. Due to their Cr-contents
of 18% and higher, they provide relatively high toughness as
well as very good resistance against stress-corrosion-cracking in
chloride-containing agents. Their weldability, however, is limited.
Because of high cooling rates appearing at welding, such steels
have a strong tendency to hardening and embrittling through
formation of martensitic microstructure. On the other hand there
are the austenitic Cr-Ni steels. Generally, they provide good
weldability, good impact toughness and very good resistance
against chloride-induced pitting corrosion. Their usable yield
strengths and their resistance against stress-corrosion-cracking
are signicantly lower than those of Cr-steels.
The rst austenitic-ferritic steels (Duplex) had carbon contents
mainly in the range of 0.1 to 0.2% and were thus susceptible to
intercrystalline corrosion (IC). Therefore, austenitic-ferritic steels
with reduced carbon content and additions of nitrogen were
developed. Such materials combine good resistance against
IC and pitting corrosion, good weldability, fair mechanical and
technological properties and better workability. The chromium
content in duplex steels is in the range from 20 to 26%, while
the nickel content is in the range from 3 to 8%. Almost all grades
of duplex steels additionally contain between 1.5 and 5.5%
molybdenum. Such alloying further improves resistance against
pitting corrosion. An overview of currently used duplex steel
grades is given in Table 1.
Table1:Commonduplexsteelgrades
ENnumber ASTM/UNS ENshortname
Typicalchemicalcompositionin%
PREC Cr Ni Mo N other
1.4162 S32101 N/A 0.03 21.5 1.5 0.3 0.22 Mn: 5.0 26
1.4362 S32304X2 CrNiN234
0.02 23.0 4.8 0.3 0.10 Cu: 0.35 26
1.4410 S32750X2 CrNiMoN2574
0.02 25.0 7.0 4.0 0.27 42
1.4462 S32205 X2 CrNiMoN2253
0.02 22.0 5.7 3.1 0.17 35
1.4501 S32760X2 CrNiMoCuWN25-7-4
0.02 25.0 7.0 3.8 0.27Cu: 0.75W: 0.75
42
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An approximate and rough classication of duplex steels may
be made according to the Cr-content. Two main groups may be
recognised, i.e. steels with 22% Cr and steels with 25% Cr. A
further classication may be made according to the so-called
Pitting Resistance Equivalent (PRE, see Table 1). PRE is a number
calculated using an empiric formula in which the favourable
effect of particular alloying elements upon the pitting corrosion
resistance is taken into consideration. There are different
formulas but the one commonly used to classify duplex steels is:
PRE = % Cr + 3.3 x %Mo + 16 x %N
Austenitic-ferritic steels characterised by PRE < 40 are classiedas duplex steels, while those steels with a PRE value greater
than 40 are designated as super-duplex grades. Fig. 1 shows the
simplied correlation between the pitting corrosion resistance
(critical pitting temperature, CPT) of different grades of austenitic
and ferritic-austentic steels and a PRE formula with the factor
30 x %N.
In the meantime, a further class of duplex steel grades made
it to the market, the so-called lean duplex-steels. They have
a slightly lower content of chromium, while the nickel content
has been considerably decreased and partially replaced by
manganese. This type of duplex steel provides better corrosion
resistance and mechanical properties than a standard 304 - type
stainless steel, while its price is signicantly lower compared to
a standard duplex steel due to its lower nickel content.
Ideally, the microstructure of a duplex steel consists of
approximately 50% of austenite and 50% of ferrite, Fig. 2.
Such conditions can be obtained after annealing at temperature
of 1020C and 1100C for about 5 minutes and subsequent
quenching in water. In the Schaefer diagram, duplex steels are
located in the mid of the austenite + delta ferrite area. In Fig. 3.,
the position of a duplex steel grade AISI 2205 in the Schaefer
diagram is marked. For the calculation of the corresponding
Niequ value the nitrogen content has been additionally taken
into account (30 x %N2) /3/.
CriticalpittingtemperatureCPT[C]
PRE (%Cr + 3.3 x %Mo + 30 x %N)
100
80
60
40
20
25 30 35 40 45 50 50
1.4404
1.4435
1.4436
1.4429
1.44391.4462
1.4539
1.4563
1.4529
X3 CrNiMnMoNbN 23-17-5-3
FeCI3
x 6H2O, 10 wt.-%
24hs. testing time
Fig.1:ComparisonofCPTsofdifferentmaterialsindependence
ofthePRE[acc.toGRFEN/KURON]
Fig.2:Microstructureofaduplexsteel(2205),asdelivered.
EtchingBerahaII.
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Austenite grains appear white, ferrite grains appear dark.
Magnication 750:1
Initially, any duplex steel solidies from the liquid state
completely into delta ferrite, which is then in solid state partially
transformed into austenite on further cooling. In the state of
equilibrium the transformation temperature is approximately
at 1250C. The amount of austenite that will be present in the
microstructure at ambient temperature depends on content of
alloying elements and cooling conditions, i.e. the cooling rate.
As already mentioned, the maintaining of a well-balanced
ferrite/austenite ratio in the weld metal is very important forthe properties of a duplex steel weldment. There are, however,
different methods of determining the ferrite content in a material.
Originally, parent metals and weldments were often specied
to have a certain percentage of ferrite. The ferrite percentage is
usually determined using metallographic methods, which means
that a cross-section of the weld or the parent material is prepared
and then examined. The examination of the prepared sample is
usually done with manual or computerised planimetry, the result
being a direct ferrite percentage.
Using metallographic methods inevitably means destroying the
workpiece, which is undesirable in many cases. Non-destructive
methods of determining the ferrite content are usually based
on the ferromagnetic properties of ferrite. Early techniques
measured the amount of it takes to remove a probe, consisting
of a permanent magnet, from the workpiece. Since the correlation
between the amount of ferrite and the aforementioned
magnetic force is not exactly linear, the amount of ferrite was
indicated in FN (ferrite number). A rough conversion from FN to
volume percent for duplex steels is 70%. For example, 100FN is
approximately 70% ferrite /4/. Also, in most of the diagrams used
for ferrite prediction, such as the DeLong- or WRC1992-diagram,
the ferrite content is indicated in FN. Further information on
ferrite measurement and FN can be found in ISO 8249 /5/.
Today, most of the ferrite measuring devices make use of electric
elds rather than magnetic forces. All the user has to do is to
touch the workpiece with a small probe shaped like a ball-pen
and the device shows the ferrite content directly in FN or percent.
It important to mention that these devices need to be adjusted
and calibrated on a regular basis.
Fig.3:Constitutiondiagramacc.toSCHAEFFLER.Markedisthe
rangeofchemicalcompositionofa2205duplexsteel.The
nitrogencontentwastakenintoaccountbyaddingthefactor
30x%N2whencalculatingtheNieq.
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 400
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Niq=%Ni+30%C+0,5%Mn
: Boundaries of analyses for duplex steel 2205
CrEq = %Cr + %Mo + 1,5%Si + 0,5Nb + 2%Ti
Austenite
Austenite + Ferrite
Austenite+ Martensite
Martensite
Martensite
+ Ferrite Ferrite
Ferrite +
Martensite
0%Ferrit
e
5%
10%
20%
40%
80%
100%Austenite
+ Martensite+ Ferrite
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Fillermaterials
As a rule, the ller materials used for welding of duplex steels are
of same type as the base material. The ller material is usually
2-4% higher in Ni-content than the base material. This is to provide
a well-balanced austenite ferrite ratio in the weld metal through
the austenite-supporting effect of nickel. This ratio would be
excessively shifted into the higher ferrite content side due to the
high cooling rates encountered in welding. Table 2 gives a brief
overview over common duplex ller metals and their composition.
Table3:Recommendedcombinationsofbasemetalandfillermetal
Fillermaterial
2205 2507Zeron100
Basematerials
2304 + +
2507 + +
2205 + +
Zeron 100 + +
Occasionally, particularly for welding the root pass on the
22%Cr steels, ller materials with a higher Cr-content are used
with the intention to improve the pitting corrosion resistance /1/.
It must however be kept in mind, that these ller materials, just
like the respective base materials, are more prone to produce
intermetallic phases. Thus, impact toughness may be impaired,
and therefore welding parameters must be carefully selected
and closely controlled.
Filler materials for TIG and GMA welding are essentially the
same. In TIG welding (under certain conditions), it is feasible
to make a joint without applying a ller material, provided that
special shielding gases are used (Stainshield Duplex). Thus an
acceptable austenite-ferrite ratio in the weld deposit can be
maintained, see section 4.
The lean duplex steels such as 1.4162/S32101 are either welded
using a ller metal containing 23%Cr and 7% Ni or standard
22%Cr ller metals. Please consult the parent metal or welding
consumable manufacturer for further details on welding these
steel grades.
Table2:Commonlyusedfillermetalsforduplexsteels
MaterialClassification
EN-Designation
Chemicalcompositionin%
PREC Cr Ni Mo N Mn Other
2205 G/W 22 9 3 N L
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Generalrecommendations
Due to their chemical composition, duplex steels are susceptible
to formation of precipitations if they are exposed to too high
temperatures for a prolonged time. Here it is important to
mention the 475C-embrittlement and the formation of sigma-
and chi-phases. The risk of such phenomena increases with
higher Cr-contents. Therefore, service temperature for duplex
steels is limited to 250C and for super-duplex steel to 220C /3/.
The heat input that is encountered during welding may impair
corrosion resistance and mechanical properties, particularly
when interpass temperatures are specied too high, or if due
to the particular shape of the workpiece, the heat can not be
decreased efciently. So a general requirement is to apply a not
too high heat input during welding. Opposite requirements (i.e.
higher temperatures and lower cooling rates) would be necessary
for best transformation properties of these materials. Since
the solidication is primarily ferritic, and transformation into
austenite happens in the solid state, too high a cooling rate may
partially suppress formation of austenite, leading to unwanted
and increased content of ferrite in the weld metal.
The upper limit for the heat input is thus dened by start of
intermetallic phases formation, while the lower limit is set by therequirement to provide an acceptable austenite-ferrite ratio. In
the references, different values for the linear energy, a measure
for the total heat input per length of weld, may be found. EN
1011-3 recommends a linear energy range of 0.5 to 2.5 kJ/mm
for 22%Cr grades and a range of 0.2 to 1.5 kJ/mm for 25%Cr
superduplex grades. Due to the fact that there is a wide range
of possible parameters, there is no general recommendation
in terms of most appropriate values. For each particular job,
appropriate parameters should be chosen and tested.
Coolingrate,t12/8-concept
Besides considering the linear energy input, there is also the
concept of t12/8 -time for description of the cooling conditions.
The t12/8 -time denotes a time required for cooling down the
welding point from 1200C to 800C. This method of determining
the cooling conditions is generally rather complicated, since it
is done applying thermo-couples which are introduced into the
welding pool. As an acceptable value for the t12/8 -time range
of approximately 10s is given /2/. If the value is in this range,
acceptable properties of material would be achieved.
Preheatingandinterpasstemperatures
Preheating of base material is not generally required. If it
is taken into consideration that signicant ferrite-austenite
transformation occurs in the temperature range between 1200C
and 800C, preheating temperature of maximum of 200C can
not essentially decrease cooling rate. On the contrary, cooling
time in the temperature range between 800C and 500C will
be increased, and in this range major precipitation processes
are occurring. For this reason, a preheating is likely to have a
negative effect, in particular for superduplex.
For thicker sections of 10mm and above, preheating to 100Ccan be applied to reduce residual stresses and to slow down
the cooling rate, especially in the root pass.
Typical recommendations for maximum interpass temperature are
maximum 200-250C for duplex and lean duplex grades whereas
maximum 150C (or lower) should be used when welding
superduplex.
Welding procedures and techniques
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Generally the same shielding gases are used for welding of
duplex steel as for the austenitic steels, see Table 4.
GMA welding under pure argon shielding is hardly used anymore,
since the arc is unstable and the penetration is poor. Active gas
mixtures consisting of argon and additions of oxygen or carbon
dioxide are generally applied. In comparison to the gases used
for welding of unalloyed steel, the content of active gases is
lower. Argon-oxygen gas mixtures (most common percentage
of oxygen is between 1 and 3%) produce very stable arcs and
a spatter-free process. In comparison to the Ar/CO2
mixtures,
the penetration prole is less favourable and the weld surface
is more oxidised. The penetration depth might be increased
applying higher oxygen content, but then oxidation of the joint
surface is of course even stronger. Also, losses in toughness and
ductility have been reported. Because of this reason, Ar/CO2
gas mixtures with a CO2
content of 2-3% are widely used. This
Stainshield Light mixture provides better penetration with
lower oxidisation, Fig. 4.
Further improvement can be achieved through addition of helium
to the gas mixture. Compared to argon, helium has a higher
thermal conductivity and a higher ionisation potential. This leads
to better wetting capabilities and higher travel speeds. Another
effect that can be observed with a helium-bearing shielding gas
is that they seem to cause a more even distribution of oxides
on the weld surface, g. 4. Whereas in the rst two samples the
oxides appear to form islands on the weld surface, the surface
of the third sample appears less heavily oxidised, which with
helps cleaning and post welding treatment.
Shielding gases for GMAW
Table4:ShieldinggasesforGMAweldingofduplexsteels
Name ISO14175 AS4882-2003
Compositioninvol.%
Ar He O2 CO2
Stainshield Light M12 SG-AC-2.5 Bal. - - 2.5
Stainshield Heavy M12 (1) SG-AHeC-35/3 Bal. 35 - 2.8
Stainshield M12 (2) SGAO-1.5 Bal. - 1.5 -
Fig.4:Influencesofshieldinggasesonweldsurfaceandpenetrationprofileonstainlesssteel.Resultsonduplexarecomparable.
MechanisedGMAwelding,wirefeedspeed9m/min,platethickness10mm.
Stainshield Light(Ar + 2.5% CO
2)
Stainshield
(Ar + 1.5% O2)
Stainshield Heavy(Ar + 2.8% CO
2+ 35% He)
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Compared to the GMA process, heat input in TIG welding is
controllable in a wider range. However, the already described
rules regarding the linear energy input and cooling rate are valid
here too.
The standard shielding gas for TIG welding of duplex steels is
straight argon. With this gas the majority of welding jobs may
done safely and cost-effectively. Argon/Hydrogen (Argoplas 5)
mixtures that are frequently used for welding of austentic steels
with intention to increase welding speed, are not recommended,
because under certain circumstances hydrogen-induced cracking
may appear due to the high ferrite content in the material.
Argon/helium (Alushield) mixtures offer increased heat input,
particularly advantageous for the duplex steels, which has
favourable effect upon the viscosity of base material and provide
wider range of acceptable welding parameters. Arc voltage and
linear energy input is also increased with increased content of
helium. An overview of shielding gases used for TIG welding
of duplex steels is given in Table 5.
Duplex steels are TIG welded applying ller material in most
cases. Duplex ller material usually contains a slightly higher
percentage of Ni than the base material. In contrast to the GMA
process, in certain cases use of ller material may be avoided,
for example in orbital welding of tubes. The advantage in such
cases is that welding speed may be increased if ller material
is not applied. This leads to shorter welding time, resulting in
cost reduction. This welding technique is possible through use
of nitrogen-containing shielding gases (Stainshield Duplex).
While in non-autogenous welding the increased content of
nickel in the ller material provides balanced ferrite/austenite
ratio, in autogenous welding this task is taken by the nitrogen
in the shielding gas. Nitrogen is a strong austenite-promoting
element, and, as a shielding gas component, can help achieving
well-balanced austenite-ferrite-ratios. It should be noted that
tungsten tip wear is more intensive because of nitrogen content,
i.e. the electrode requires re-grounding more frequently than if
welding in pure argon shielding is used.
Table5:ShieldinggasesforTIGweldingofduplexsteels
Name AS4882-2003
CompositioninVol.%
Ar He N2
Argon SG-Ar 100 - -
Alushield Light SG-He-27 Bal. 27 -
Alushield Universal SG-He-50 Bal. 50
Stainshield Duplex SG-AN-2 Bal. 2
Shielding gases for TIG welding
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As an example, N-containing shielding gases have already been
used in orbital TIG welding of tubes made of a 22% Cr grade
2205 duplex steel, Fig. 5. The wall thickness of the tube was
2 mm; outer diameter was 54 mm. Welding without ller material
has been applied. Measuring of ferrite content in the weld metal
was not made through metallographic examination, but with a
magneto-inductive method instead. The presented values of mean
ferrite content may be taken only as indications of tendencies.
It is noticeable how strong nitrogen reduces ferrite content in
the weld metal in comparison to pure argon, and with how the
addition of helium (Specshield N2He20*) stabilises the welding
process and improves fusion.
*Please note that Specshield N2He20 is not a stock item but can
be made on request.
The second example includes welding of an overlap joint without
ller material, again on the 2205 duplex steel grade. In this
particular case, welding was performed applying a shielding gas
containing up to 10% of N2
(balance was Ar). The effect of the
nitrogen may be easily recognised, as presented in Fig. 6. The
customers requirements for this application were that ferrite
content in weld metal must not exceed 70%, and this target could
be achieved through use of Stainshield Duplex shielding gas. A
cost analysis pointed out that the achieved increase of welding
speed from 7 cm/min to 13 cm/min provided a signicant cost
cut. Additionally, a content of nitrogen in the shielding gas can
improve the corrosion resistance according to the CPT-test.
Examples of application
Fig.6:ApplicationexampleoverlapweldwithN2-containinggases,withoutfillermetal
Base metal:1.4462 / AISI 2205 (duplex)
Welding process:TIG-orbital, pulsed, butt joint,w/o filler metal
Travel Speed:4.5 cm/min
Pulse frequency: 2.2 Hz
Base- / Pulse current:30A/60A
Wordpiece dimensions:Pipe 54 x 2 mm
Sheilding gas
Backing gas
Mean ferrite content
1) measured with a magneto-inductive measuring device
Argon
Argon
58.4%
Nitrogen Nitrogen
42.8% 48.3%
StainshieldDuplex Specshield N2He20
Ferrite
contentinweldmetal[%]
permissionFerritecontent
Basematerial
50
51
Argon
74
75
StainshieldDuplex
68
71
N2-3%
62
70
N2-5%
61
65
N2-10%
60
60
30%
70%
20
30
40
50
60
70
80
90Platethickness:
Material:
2,0mm1,5mm
2205
Fig.5:ApplicationexampleTIG-orbitalweldingwithN2-containinggases,withoutfillermetal
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To retain the corrosion resistance of duplex steel, proper root
shielding always must be applied. By application of appropriate
shielding gas for the root side, air is removed from the root and
formation of corrosion inducing layer of tarnish is effectively
reduced or suppressed. Generally, the same gases may be used
for root shielding as for the austenitic grade steels. However, in
this case too, hydrogen content in the root shielding gas must be
limited because of higher content of ferrite in the base material,
to exclude the danger of hydrogen induced cracking. Therefore,
argon and nitrogen, or their mixtures might be used. For
further elimination of tarnish and improvement of corrosion
resistance, residual oxygen must not exceed concentration of
30 ppm at the root side. There is a general rule that pitting
corrosion resistance increases with lower residual oxygen on the
root side. Appropriate devices for the measurement of residual
oxygen are commonly available on the market.
Shielding gasesfor root protection
Conclusion
Due to their two-phase micro-structure consisting
of ferrite and austenite, duplex steel grades possess
excellent mechanical and technological properties and
corrosion resistance. Particular attention should be paid
to heat input, selection of ller material and shielding
gas in order to retain these interesting properties at
gas shielded arc welding processes. The most important
requirement at welding is to provide the most balanced
ferrite-austenite ratio in both weld metal and heat
affected zone. The linear energy input and interpass
temperature must be limited. Generally, ller materials
of identical or similar composition but over-alloyed in
Ni should be selected. Ar-CO2
(Stainshield Light) or
Ar-He-CO2
(Stainshield Heavy) gas mixtures are applied
for GMA welding, while for TIG welding Ar, Ar-He
(Alushield) or Ar-N2
(Stainshield Duplex) mixtures may
be used.
In certain cases, use of ller material may be omitted
in TIG welding, resulting in certain cost reducing effects.
In such cases, shielding gases should contain nitrogen.
AuthorThomas Ammann
References1. Noble, D.N., Gunn, R.N.: Welding of duplex stainless steels. A readers
digest. Stainless Steel Europe, August 1992.
2. Geipl, H.: MAGM-Schweien von korrosionsbestndigen Duplex-Sthlen22Cr5(9)Ni3Mo. Einu von Schutzgas- und Verfahrensvarianten. Linde -Sonderdruck Nr. 146. Hllriegels-kreuth, 1989.
3. Folkhard, Erich: Metallurgie der Schweiung nichtrostender Sthle. Wien,New York: Springer Verlag, 1984.
4. Jippold, J. C., Kotecki, D. J.: Welding metallurgy and weldability of stainlesssteels. John Wiley & Sons, New Jersey, 2005.
5. ISO 8249: Welding Determination of Ferrite Number (FN) in austenitic andduplex ferritic-austenitic Cr-Ni stainless steel weld metals.
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