Key aspects in welding of 5 & 9% Ni steel for LP-, and LNG equipment

V. Gross Metallurgist, Application Technology Hamm, Germany

TEHRAN Seminar, Nov..2007

content •

General criteria for material selection for liquid gas service

•

Features of Ni-alloyed ferritic steels

•

Criteria for choice of filler metals

•

Weld metal properties

•

recommendations

TEHRAN Seminar, Nov..2007

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steel grades •

High strength values

•

Sufficient toughness at cryogenic temperatures – Embrittlement temp.below design temperature

•

Weldability (HAZ, hardening)

•

Price

TEHRAN Seminar, Nov..2007

3

development has led to ferritic steels with Ni •

featuring – High purity (oxides) – Low level of tramp elements – Small grain size

•

achieved by sophisticated metallurgy – Vacuum treatment (degassing), forming processes 160

CVN [J]

treatment 120

Rpo.2

Rm

A

(MPa)

(MPa)

(%)

>515

690 – 825

>20

>585

690 – 825

>20

Ni-content 80

N+N+T* 9,0

40

5,0

0 - 200 - 150

3,5

Q+T** - 100

- 50

±0

Test temperature [°C] TEHRAN Seminar, Nov..2007

+ 50 *process equip.

** storage tanks 4

criteria for filler metal selection •

Mechanicals – – – – –

•

Strength Elongation Toughness Crack sensitivity (wall thickness up tp ~60mm!) Thermal expansion

Welding – Deposition rate – Atmospheric site condition – weldability

TEHRAN Seminar, Nov..2007

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filler metal process •

SMAW/ SAW/FCAW – Weld pool is protected by slag, not affected by wind – High rate of deposit for SAW

•

GTAW/GMAW – Very sensitive to turbulances for weld pool protection – Low rate of deposit for GTAW (GTAW hot wire equipment to sensitive for site conditions. – GMAW good compromise for shop fabrication in terms of quality and deposit rate.

TEHRAN Seminar, Nov..2007

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filler metal grades to meet 5/9% Ni steel requ. •

Matching ferritic grades do not meet toughness/ elongations requirements •

alternatively

Choice: Austenitic grades, Ni-base grades (depending on calculation factors)

Homogenous microstructure respond positively to: -strength and toughness requirements -and impact due to dilution

TEHRAN Seminar, Nov..2007

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Min project requirements for mechanical properties

TEHRAN Seminar, Nov..2007

5%Ni

9%Ni

Rpo.2 (MPa)

390

430

Rm (MPa)

570

690

Elong. (%)

25

35

Av (J)

-140°C 34

-196°C 70

Experienced filler metal grades structure Class.AWS

austenitic

austenitic

Ni-base

Ni-base

Ni-base

Ni-base

na

NiCr3

NiCrMo-3

ENiCrMo-6

ERNiCrMo-4

ER316LN

TPUT

Thermanit process

5% Ni

19/15 SMAW, SAW GMAW

17/15TT Nicro 82 SAW GMAW

X

9% Ni

* in case pwht is required TEHRAN Seminar, Nov..2007

X

SMAW, SAW GMAW

625 SAW SMAW

X*

13/65TTW150 Nimo C 276 SMAW

SAW

X X

X

X

Example 5% Ni steel •LPG tankers

•~95% of all welds by GMAW Thermanit 17/15 TT •~5% of all welds by SAW Thermanit 17/15 TT/ M 104 TEHRAN Seminar, Nov..2007

Weld Thermanit 17/15 TT C

Mn

Cr

Ni

W

0,2

10

17

15

4

GMAW

SAW

•Dia 1/ 1,2mm

•With basic flux

•Process: impuls;200A;22V, 8m/min

•Dia 3mm, 380A, 28V, 40cm/min

•Gas: Arg/ He/Co2 Preheat.: -/-, interpass: max 150°C

•Positional weld.

requ.

GMAW

SAW

Rpo.2 (MPa)

>390

Rm (MPa)

>570

644

690

Elong. (%)

>25

39

35

TEHRAN Seminar, Nov..2007

449

430

CVN [J]

200 150

SAW

100

GMAW 50 0 -196

-140-120 RT Test temperature [°C]

~plate

thickness range for ~150.000m³ LNG tank ~35mm bottom, 10mm top

TEHRAN Seminar, Nov..2007

Filler metals for 9% Ni in LNG tank/equip. •

Overview of most common filler metals, experienced in ~30 years SMAW, 3G up

SAW, 2G

ENiCrMo-6

ENiCrMo-3

ERNiCrMo-3

ERNiCrMo-4

2,5-5

2,5-5

1,6

1,6-2,4

TPUT, Thermanit

13/65TTW150

625

625

Nimo C 276

Cr

15,5

21,8

21,5

15,5

Mo

5,5

9

9

16,1

Ni

Bal.

Bal.

Bal.

Bal.

Nb

1,3

3,2

3

-/-

W

1,3

-/-

-/-

3,5

Rpo.2 (MPa)

480

520

466

455

Rm (MPa)

710

782

712

720

Elong. (%)

42

45

38

42

Av (J)

85

90

95

110

dia (mm)

TEHRAN Seminar, Nov..2007

Attention •

Properties can be safely met, but: – – – – – – –

Restraint condition due high strength of 9%Ni steel Ni-base filler metal show hot crack tendency Restriction in heat input required Max interpass temperature to be respected Control of dilution Magnetic arc blow Close to technological strength limit of the weld

¾ Filler metal as well as parameters have to be selected to satisfy these features !!!! ¾ QA activities have to be established as well !!!!

TEHRAN Seminar, Nov..2007

Electrode Thermanit 13/65TTW150 for 3 G up

•Pure Ni core wire •Low electrical resistance •Stable arc over the entire length •Recovery is at 150% •Suitable for AC/ DC •Due to use of multiple generators at site AC welding avoids influence of magnetisation and prevents arc blow TEHRAN Seminar, Nov..2007

Typical weld build up for 3G up,10mm

•Electrode dia: 3,2mm only •dia 4mm only in flat position! •Bead height to be controlled •~2,5mm •!! Restriction in heat input !! TEHRAN Seminar, Nov..2007

Typical weld build up for 3G up,10mm

439

TEHRAN Seminar, Nov..2007

709

35,7

Usual Ni-alloys for SA welding in 2G pos •Thermanit 625 (AWS A 5.14 ER NiCrMo-3) •constant and high meachnical properties •good weldability in AC and DC+ •wire dia restricted to 1,6mm due to control of dilution and hot crack.

•Thermanit Nimo C 276 (AWS A 5.14 ER NiCrMo-4) •comparable properties to 625 alloy, but: •wire dia can be increased to 2,4mm with AC polarity due to flexibilty of the microstructure TEHRAN Seminar, Nov..2007

Influence of dilution to the hot cracking tendency

Hot crack tendency

[%] Dilution

BM: 6Mo (S31254) FM: ENiCr20Mo9Nb or ENiCrMo3

TEHRAN Seminar, Nov..2007

20

80

30

70

40

60

50

50

60

40

70 80

30 20

10

20

30

40 50 60 [%] Ni in the weld seam

70

[%] Filler metal in the weld seam

Thermanit 625

Hot cracking tendency/ MVT-Test Base metal: alloy 625 Filler metal: Thermanit 625 SMAW SAW GMAW GTAW Base metal

MVT-Test (7,5 kJ/cm) 30

Total cracking length [mm]

25

20

15

10

5

0 0

0,5

1

2 Total strain [%]

TEHRAN Seminar, Nov..2007

3

4

SA welding of 9%Ni in 2G pos

Thermanit 625 dia: 1,6mm, DC+ 350A,30V,40cm/min

Tht. Nimo C 276 dia: 2,4mm, AC 400A, 31V, 50cm/min

TEHRAN Seminar, Nov..2007

ERNiCrMo-3

ERNiCrMo-4

dia (mm)

1,6

1,6-2,4

Thermanit

625

Nimo C 276

Cr

21,5

15,5

Mo

9

16,1

Ni

Bal.

Bal. 3,5

W Nb

3,5

-/-

YS (MPa)

466

455

TS (MPa)

712

720

Elon. (%)

39

42

Av (J)

100

110

Schematic range of welding parameters (e.g. amperage, preheating, interpass temp., pwht) 13%Cr •Thickness •Restraint condition

Ni-base, 6Mo,SDSS, 9Ni

•Yield strength

DSS, 5Ni Low alloy steel C-steel

-

optimum parameters

+

QA activities have to be developped accordingly TEHRAN Seminar, Nov..2007

Summary •to meet required strength properties only Ni-base filler metals can be apllied •Controlled heat input guarantees: •sufficient strength level as the filler are operating at their technological limit •ensures crack free welds in highly restrained welds

•Future: matching filler metals? •Disadvantage: only GTAW process applicable in terms of toughness, low elongation TEHRAN Seminar, Nov..2007