Welding of Corrosion Resistant Alloys, CRA (duplex stainless steels) Lars M. Haldorsen Leading Advisor – Metallic materials and welding 2016-01-21 Classification: Internal

Agenda

1. CRAs – types and chemistry

2. Statistics related to faults and challenges

3. Short overview of microstructure challenges in duplex steels and metallographic examination of these

4. Precipitations and secondary phases in duplex steels

5. Welding of duplex stainless steels – Challenges

6. Recommended welding parameters for duplex steels

2016-01-21 2 Classification: Internal

3 Classification: Internal

Fe-Cr-Ni Phase Diagram

• Ferritic stainless steels

− ~30%Cr

• Duplex steels

− 22-27%Cr, 3-9Ni, 3 Mo

• Austenitic stainless steels

− 18-20%Cr, 10-20%Ni, 3-6% Mo

• Martensitic Stainless Steels

− 13%Cr, 4-6,5%Ni, 0-2,5%Mo + C

Why Corrosion Resistant 1

Protection of the substrate by formation of chromium oxide:

• Very thin

• Passive

• Impervious (ugjennomtrengelig)

• Self repairing instant after damage

4 Classification: Internal

Some statistics – CRAs (in-house failure analysis)

2015-04-15 5 Classification: Internal

13Cr 3 %

17-4 PH / 13-8 PH and eqvivalent

4 % 6Mo 7 %

AISI 300-series 49 %

AISI 400-series 1 %

Duplex 18 %

Superduplex 6 %

Nickel alloys 12 %

Why CRA components fail (In-house failure analysis)

2015-04-15 6 Classification: Internal

Corrosion (SCC, pitting, other) 33 %

Overload (ductile and brittle) 11 %

Fatigue 38 %

Wear (SPE, abrasive, other) 4 %

Hydrogen related 8 %

Other (creep, LME, etc..) 6 %

Root causes

2015-04-15 7 Classification: Internal

Subquality material 11 %

Poor design 21 %

Operational conditions

32 %

Welding and fabrication

17 %

Material selection 9 %

Other (maintenance etc.) 10 %

Microstructural challenges in CRAs

8 Classification: Internal

Ferritic and austenitic SS Duplex steels

Ti and Nb are added to form TiC and NbC in lieu of CrC

Source: Charles J., “The Duplex Stainless Steels: Materials to Meet Your Needs,” Proceedings of the Conference Duplex Stainless Steels '91, Beaune, France, 1991

Metallographic examination of DSS/SDSS 22

Cr

Fine

gra

ined

(In

term

etal

lics)

25C

r C

oars

e gr

aine

d (n

itrid

es)

20% NaOH electrolytic 10% oxalic acid electrolytic Murakami’s, 80 °C

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Etching techniques to investigate precipitations and micro structures in duplex steels

• Based on in-house testing

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• For good delineation of austenite/ferrite-, intermetallic- and chromium nitride- phases combined with good contrast between these, the use of a two-step procedure involving the following etchants (in sequence) may furthermore be found advantageous:

− (i) 10 % Oxalic acid electrolytic (typically at 5-6 V for 5-60 s)

− (ii) 20 % NaOH electrolytic (typically at 1.5-3.0 V for 5-10 s)

Metallographic examination of DSS/SDSS

11 -

10 % Oxalic + 20 % NaOH

Precipitations and secondary phases in duplex steels

12

Classification: Internal 2011-10-22

Precipitations and secondary phases in duplex steels

Precipitation of intermetallic phases in duplex steels

Light optical microscope images showing; (a) typical microstructure of a wrought 22Cr duplex stainless steel following correct solution annealing heat treatment; and (b) microstructure of the same material following incorrect heat treatment. Coarse intermetallic precipitates, predominantly σ-phase, are present in the latter microstructure. It can also be noted that a shift in the austenite-ferrite balance has accompanied the precipitation of these secondary phases. From Statoil’s internal investigations.

• Main challenge, depletion of Cr in the neighborhood of the grain boundaries

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15 Classification: Internal

Intermetallic phases – depletion of Chromium

TEM-EDX line profile acquired across boundary between σ-phase and ferrite. A chromium depleted zone surrounding the σ-phase is evident. [C.J.Park et al].

16 Classification: Internal

Intermetallic phases - effect on Critical Pitting Temperature, CPT

Measured reductions in critical pitting temperature (CPT) in ferric chloride for superduplex stainless steels as function of volume fraction intermetallic phases relative to solution annealed condition. From [TWI].

Critical crevice corrosion temperature (CCT) for superduplex stainless steel in sea water as function of ageing time at 800 °C (potentiostatic exposure testing, 500 mV ref. SCE). Corresponding room-temperature impact toughness values are included for comparison. From [M.E. Wilms et al].

Intermetallic phases - effect on impact toughness (22Cr)

Precipitation of chromium nitrides in duplex steels

• Nitrogen supersaturation in ferrite phase during rapid quenching from high temperatures forms Chromium nitride, Cr2N

18

From: John C. Lippold and Damian J. Kotecki. Welding Metallurgy and Weldability of

Stainless Steels.

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Chromium nitrides - effect on corrosion, G48-test

G. Byrne et al, Paper 2052, DUPLEX 2000, Houston

Sensitive above a certain amount of Cr nitrides

(UNS S32760-Forgings)

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Chromium nitrides - effect on impact toughness,CVN -46°C

Statoil choke module incident

G. Byrne et al, Paper 2052, DUPLEX 2000, Houston (25Cr, UNS S32760-Forgings)

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Chromium nitrides - effect on HISC, SSRT*

Looks sensitive at low amounts of Cr nitrides…; act as reversible hydrogen traps in the ferritte? Both Foinhaven (BP) og Garn West (Shell) HISC failures had 25Cr forgings with considerable amounts of Cr nitrides….

G. Byrne et al, Paper 2052, DUPLEX 2000, Houston

(UNS S32760-Forgings)

* Slow Strain Rate Testing

Heat treatment of superduplex to avoid CrN

22 -

1160 °C

1120 °C

1080 °C Effect of solution heat treatment temperature

Cooling of superduplex to avoid CrN

23 -

Step quenching Solubility in ferrite vs austenite as a func. of temperature

Cooling of duplex to avoid intermetallic phases

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Tem

pera

ture

Sigma

Time

Rapid cooling low σ

Slow cooling σ

Welding of duplex steels - – Challenges

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Welding of duplex stainless steels -challenges • Duplex steels are among the most challenges

materials to weld due to the following:

− Dual microstructure

− Precipitate intermetallic phases and secondary microstructural constituents (e.g. secondary austenite, chromium nitrides)

• Latest update of NORSOK M-601 ASTM G48 test temperature was reduce 40°C to 35°C.

− Argumentation:

• one the borderline related to success or failure

• caused extensive project cost and time consuming due to fine tuning of welding parameters during welding qualification

• weld parameters are later loosen up during establishment of WPS and the actual welding

2016-01-21 26 Classification: Internal

Welding of duplex stainless steels - challenges • Examples of challenges (From: Aker Solution)

− WPQ Corrosion test experience and results for 25Cr Super Duplex piping

− According to NORSOK M-601 Rev.5 and ASTM G48 Method A- 40C

− Experience and results for 5 ea. main Norwegian test laboratories in the period 2012-2014

− Average result from WPQ laboratory testing to 40C = 29.5 % failed corrosion testing.

− Main corrosion test failure: Pitting in root and/or weigthloss > 4g/m2

2016-01-21 27 Classification: Internal

Welding of duplex stainless steels - Challenges

The following challenges are identified:

• Effect on HAZ from welding

• Ferritic/ austenitic weld microstructures heat input, interpass temperature and their tendency of cause precipitation of intermetallic phases and secondary microstructures

• Welding technique (GTAW) Weld pool flow, root and side wall penetration, inter run penetration

• Welder skills and training

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LOF 1

LOF 2

LOF 2

Corrosion testing of SDSS weld with sec. austenite

Defects (inter run lack of Fusion) in SDSS weld

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Effect on HAZ from welding • Beneficial situation from

fracture mechanics point of view since the HT HAZ close to the weld is free from σ-phase (reflecting isotherms from 1500 to about 950°C).

− I.e. σ-phase is NOT exposed to large and sharp weld defects (LOF etc), only tiny surface irregularities and roughness of the base metal fitting itself.

3 mm

1 mm

σ-free HAZ

Example of 1-3 mm wide precipitation-free HTHAZ zone of 3” thin-walled DSS elbow

AaHa SDSS

• 6’’ spool:

• Secondary Austenite in root

• Testet at 35 °C → failed (pitting 15.3 g/m2)

2016-05-11 30 Classification: Internal

AaHa SDSS

• 6’’ spool:

• Secondary austenite in root

• Tested at 40 °C → failed (12.1 g/m2; pitting)

2016-05-11 31 Classification: Internal

AaHa SDSS

• 20’’ spool:

• Sigma phase

• Cold flow

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Mariner SDSS • 6’’ spool:

• High ferrite content in the cap

• 90% surface austenite in root.(N2 purge gas)

• Sigma phase precipitations at the root

• Cold flow.

• G48-tested at 40 °C → failed (2.6 g/m2, but pitting in the root area due to presence of sigma phase

2016-05-11 33 Classification: Internal

AaHa SDSS • 20’’ spool:

• Secondary austenite in root.

• Fitness for purpose tested at 25 °C

• → pass

2016-05-11 34 Classification: Internal

Recommended welding parameters for duplex steels

2016-01-21 35 Classification: Internal

Critical welding parameters – Joint configuration

Joint configuration must be design to optimise low heat input in combination with good penetration in the root and side walls:

• Bevel angle should be 30°±5° (i.e Groove angle 50-70°)

• Root opening should be 2-5 mm dependent on the wall thickness and pipe diameter. Smaller root opening is used on thinner wall thicknesses

• Root face should be 0-2 mm to avoid excessive base metal mix-up in the weld deposit

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Critical welding parameters – Root welding parameters

Root welding parameters must be selected to get the right thickness of the root and to optimise the metallurgical microstructure of the root. This is optimised by:

• Heat input 0.5 to 1.0 kJ/mm dependent of the wall thickness

• Filler wire should be Ø1.6 – 2.0 mm to allow for wire control and bridging of the root opening.

• The root layer thickness should be minimum 2 mm for thin walled and 3 mm for WT > 7mm

2016-01-21 37 Classification: Internal

Root weld

Critical welding parameters – Cold pass welding

Cold pass welding parameters must be selected to optimise good binding towards the base material and to avoid excessive re-heating of root bead. This is optimised by using:

• Heat input of 75-80% of root bead

• Heat input should be 0.4 to 0.8 kJ/mm dependent of the wall thickness

• Welding consumable wire shall be Ø1.6 – 2.0 mm to better control and restrict the heat input.

• The cold pass layer thickness should be 1,5 mm for thin walled pipes and 2 mm for WT > 7mm

• Bead width should be max 8mm. For wider grooves splitting can be applied

2016-01-21 38 Classification: Internal

Cold pass

Critical welding parameters – 1st fill welding

1st fill welding parameters must be selected to optimise good binding towards the base material and to avoid excessive re-heating of cold pass and root bead. This is optimised by :

• Heat input of 0.5 to 1.0 kJ/mm, dependent of the wall thickness

• Filler wire should be Ø1.6 – 2.0 mm to better control and restrict the heat input (cold pass metallurgical effect).

• Layer thickness should be minimum 2 mm for thin walled and 3 mm for WT > 7mm

• Splitting of layer is allowed

2016-01-21 39 Classification: Internal

1st fill

Critical welding parameters – fill and cap welding

The remaining fill and cap shall be welded as follows:

• Heat input of 1.0 to 1.5 kJ/mm, dependent of the wall thickness

• Filler wire should be Ø 2.4mm

• Layer thickness 2 mm for thin walled and 3 mm for WT > 7mm

• Splitting of layer is allowed

2016-01-21 40 Classification: Internal

Remaining fill and cap

Critical welding parameters – shielding gases

Shielding gasses should be selected as follows:

• Shielding gas:

• Ar + 2-5% N2

• Backing gas:

• 100% N2 or Ar + N2 Mixtures • 100% N2 mat give too much

Austenite in the root

2016-01-21 41 Classification: Internal

Backing gas

Presentation title

Presenters name Presenters title E-mail address ……@statoil.com Tel: +4700000000 www.statoil.com

2016-01-21 42 Classification: Internal