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Welding Superduplex Stainless Steels

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Graham HollowayMetrode Products Ltd

UK

Jakarta, Indonesia28th November 2006

Superduplex

Introduction

Base material.Code & specification requirements.Welding consumables.Procedural guidelines.Objectives of the welded joint.Joint properties.Achieving code requirements.Realistic specifications.Conclusions.

Base materials

1701523---2.51217316L

47030360.18--3.2523S32205

400.2620.33725S32974

400.22-1.83.55.525S32550

410.260.70.73.5725S3276055060

420.26--3.8725S32750

Min Proof MPa

CPT°C

PRENNWCuMoNiCrUNS

Duplex microstructure Welding processes

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Welding consumables

1.6 & 2.4mm wire with fully basic agglomerated flux.

SAW

1.2mm rutile wires for downhand welding and all-positional pipework.

FCAW

2.5-5.0mm coated electrodes - basic for ASME 5G/6G welding & optimum toughness, rutile for optimum operability.

SMAW

0.8, 1.0 & 1.2mm spooled wire for pulsed GMAW and mechanised GTAW.

GMAW

1.6, 2.4 & 3.2mm diameter cut lengths for manual welding.

GTAW

Properties specified

Strength.Toughness (Charpy test).Hardness.Ferrite.Corrosion (G48A test).Other tests – NDT, bends etc.

Butt weld objectives Procedural control

Preheat:Normally none.

Interpass:150°C maximum.Forced air cooling has been used.

Heat input:0.5-2.5 kJ/mm.Normally < 1.75 kJ/mm.Normally > 0.75 kJ/mm.

Weld toughness40J average (30J min) at -50°C.Duplex alloys do not have a ductile-brittle transition like CMnsteel:

Gentle sloping transition.Low scatter in individual sets of Charpy’s.

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MMA / SMAW toughness

0

25

50

75

100

125

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30

Temperature, °C

Ab

sorb

ed E

ner

gy, J

2507XKS

Zeron 100XKS

Weld toughness - superduplex

30JSMAW (rutile)30JFCAW50JSMAW (basic)50JSAW60JGMAW150JGTAW

SuperduplexProcess

Approximate Charpy value at –50°C.

Effect of oxygen on toughness

0

50

100

150

200

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

Deposit Oxygen, %

Impa

ct e

ner

ggy

at -

50o C

, J MIGFCWTIGSAWMMA

Weld metal hardness

Most operators have there own limits.Many originate from NACE MR0175.Ironically NACE MR0175 no longer imposes a restriction (except for cold drawn tubulars – 36HRC).

Weld metal hardness -superduplex

Typical restrictions:NORSOK M601 – 330HV / 32HRC.Shell ES106 & 247 – 32HRC.NACE MR0175 – None.TOTAL (GS PVV614) – 350HV.BP Rhum – 330HV.

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Weld hardness measurement

Specifications often in Rockwell C.Conversions from HRC into HV.ASTM E140 HV-HRC conversion applies to CMn.Use TWI hardness conversion.

TWI hardness conversion

10

20

30

40

50

200 300 400 500Hardness, HV

Har

dnes

s, H

RC

TWIE140

TWI hardness conversion

HRC = 0.091 HV – 2.4

28 HRC = 334 HV32 HRC = 378 HV

Weld metal hardness

Hardness higher in the root.This is the result of strain hardening.Fewer runs - lower hardness.

Weld metal hardness

250

275

300

325

350

375

400

0.25 0.5 0.75 1 1.25 1.5 1.75 2

Thickness / Number of Runs

Har

dnes

s, H

V(1

0)

Weld MetalHAZ

Weld metal hardness

Superduplex stainless steels are high strength.Avoidance of stress corrosion cracking (SCC) – limit hardness to 36HRC.

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Weld and HAZ ferrite

Typical oil & gas requirements:30-60%.

Ferrite not specified in standards.Normally use point counts in procedures.Some codes have production checks.

Ferrite measurement

Point counting:ASTM E562.

Magnetic measurement:Ferritescope.Magnegage.

Prediction based on analysis:WRC diagram.DeLong.Espy.

Point counting

ASTM E562.Round robin tests show significant lab-to-lab variations.Use sensible magnifications to resolve microstructure.Carry out sufficient tests to get true average ferrite value.

Weld ferrite content

±1136Weld root±861Weld cap±1854HAZ±456Parent

Ferrite, Range

% Ferrite,average

Area

Magnetic measurement

Ferritescope or magnegage.Calibrated against secondary standards.

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Percent ferrite versus FN Lab-to-lab variation

Material finish Predictive methods

Used for estimation only.The WRC diagram has shown good correlation to Ferritescope measurement.

WRC predicted ferrite Weld & HAZ ferrite content

Primary effect - Analysis.Ferrite increases with:

> Cr equivalent< Ni equivalent.

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Weld & HAZ ferrite content

Secondary effect – Procedure:Heat input.Interpass temperature.Preheat.Joint thickness.

Faster cooling rate produces higher ferrite.

HAZ heating & cooling cycles

Location of measurement

As-deposited weld metal:Cap.

Reheated weld metal:Root and mid-section.

Diluted / undiluted weld metal.

Ferrite measurement location

Norsok M601 Rev 3 (Jan 2004).Root:

Higher dilution.Reheated.

Final bead:Not reheated.

Reheated weld metal Final bead ferrite

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Final bead % ferrite

Reh

eate

d %

fer

rite

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IIW position statement

30-70 FN (~22-55%).Ferrite should be secondary.Toughness, G48A etc should be primary factors.

Corrosion testing

ASTM G48A ferric chloride pitting test.No pitting (x20).Maximum weight loss:

4 g/m2 (NORSOK).20 mg (Shell & others).10 mdd = 1 g/m2/day (ASTM A923).

Corrosion testing

20 mg weight loss on standard size specimen = 4 g/m2.

Passing the G48A test

Correct root welding procedure.Use Ar 1-2%N2 shielding gas.Proper sample preparation:

Pickle sample if permitted.1200 grit finish on cut surfaces.

Realistic test temperature:Maximum of 40˚C superduplex.Control temperature ± 0.5˚C.

Purging

Argon.Argon-Nitrogen.Nitrogen.

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PurgingPurge

- argon versus nitrogen

Critical region of a pipe joint Root welding

Passing the G48A test

0.1NoYes3.1NoNoAr+N2

5.5NoYes31.9NoNoAr

Wt loss, g/m2

PittingPicklingGas

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Microstructure

No third phases.Nitrides.Carbides.Sigma.Intermetallics.

Not normally a problem.Secondary austenite.

Secondary austenite

Ferrite – austenite (S32750)

39.40.243.424.32nd austenite

41.70.074.027.4Ferrite

45.80.523.326.6Austenite

PRENMoCr

Sigma

Conclusions

Superduplex stainless steel is readily weldable.Good quality stainless steel fabrication practice.Select correct process and consumable.Welder guidance and training is essential.Engineers & supervisors should be trained.Consistent weld procedure control is essential.

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Graham Holloway

In Conjunction With:

PT Alfa Metalindo IndonesiaIWS

Komunitas MIGAS Indonesia

Welding Superduplex Stainless Steel

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