BOC 216521 Gas Shielded ArcWelding of Duplex Steels BrochureAUS

8/2/2019 BOC 216521 Gas Shielded ArcWelding of Duplex Steels BrochureAUS

1/12

Gas-shielded Arc Welding

of Duplex Steels


2/12

02

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.


3/12

03

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


4/12

04

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.


5/12

05

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


6/12

06

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


7/12

07

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


8/12

08

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)


9/12

09

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


10/12

10

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


11/12

11

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.


12/12

012

For more information contact theBOCCustomerServiceCentreon:

Australia

[email protected]

www.boc.com.au

BOC is a trading name of BOC Limited, a member of The Linde Group. BOC Limited 2010. Reproduction without permission is strictly prohibited.

Details given in this document are believed to be correct at the time of printing. Whilst proper care has been taken in the preparation, no liabilityfor injury or damage resulting from its improper use can be accepted

BOCLimitedABN 95 000 029 729

Riverside Corporate Park10 Julius AvenueNorth Ryde, NSW 2113Australia

MP09-0433

EQAUS04105K

Documents

BOC 216521 Gas Shielded ArcWelding of Duplex Steels BrochureAUS