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Welding Superduplex Stainless Steels
Successfully
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.
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).
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.
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.