Technical Highlight Vol.1

1 Preface

Because growing populations in developing countries create ever more demand for energy, investments in mining, transportation and storage of energy resources and relevant equipment are expected to increase. Used for stable and continuous transport of crude oil and natural gas, pipelines, in small and large sizes, play a key role in energy infrastructure worldwide.

There are two major types of pipeline: on-shore, pictured in Figure 1, and subsea pipelines. Two methods of installing subsea pipelines are used. One is called the S-Lay or J-Lay method, in which pipes are girth-welded on board a ship and then lowered to the seabed. In contrast, the spooled pipe method has the pipes girth-welded into one long pipe (a few km in length). It is wound or spooled onto a bobbin and carried to a ship for installation under the seabed.

SMAW, GTAW, GMAW and SAW are the processes most often applied for welding the pipes used in pipelines. However, SAW tends to be limited to welding the longitudinal pipe seams whereas the other processes are used for circumferential girth welding. This article focuses on the up-to-date welding consumables for onsite girth welding.

2 Girth welding and requirements for welding consumables

The specifications required of pipelines vary, depending on materials (strength requirements), sizes (pipe diameter), construction site conditions (temperature, on-shore or seabed), and service condition (pressure). Most countries also have their own particular regulations and requirements that will influence a pipeline’s specifications – and raise issues when pipelines cross borders.

Cellulose type covered electrodes, used in vertical downward position (from 12 to 6 o’clock), have been preferred for girth welding from the start of pipeline history because of their fast welding speed. Their use, however, is limited to warm areas due to their low crack resistance and that a certain degree of welding skill is necessary. More recently they are being replaced by self-shielded flux cored wires (SS-FCWs). Nevertheless, both of these welding consumables still account for 70 to 80 % of total girth welding.

Because construction, transportation and installation of pipelines require significant investments of time and money, demand is high for the most efficient girth welding processes. As seen in Figure 2, the application of exclusive girth welding equipment with automatic GMAW has gradually been expanding and replacing covered electrodes and SS-FCWs. The use of solid wires as well as metal type FCWs that enable vertical downward welding is also rising.

Because pipe constructed with the spooled pipe method is wound on a bobbin after welding, mechanical properties like ductility and strength as well as the inspection methods of the weld metal have to be carefully considered when designing the weld metal and setting-up the welding procedures. For example, vertical downward welding by conventional GMAW with solid wires often suffers from lack-of-penetration that can increase the need for future repairs. Therefore, all-positional FCWs that enable vertical upward welding with deep and stable penetration are preferable. Furthermore, because higher strength pipe like API 5L X80 grade is being considered for actual pipeline projects, FCWs with the quality and efficiency suitable for higher strength pipes are now a must for development.

The grades of carbon steel that are suited to pipeline pipes range from API 5L X52 to X100, as shown in Tables 1 and 2. Clad pipes (Ni base) are also available from the view point of corrosion resistance.

3 Carbon steel welding consumables suitable for girth welding

Kobe Steel has been marketing welding consumables for girth welding for decades. Table 1 shows the typical covered electrodes for girth welding while Table 2 shows the TIG and MAG wires for particular grades of steel.

4 Up-to-date welding consumables for girth welding

4.1. LB-52NSU

When pipe root pass welding must be conducted from outside rather than inside in order to form the back bead inside the pipe, GTAW or SMAW are generally favored. Kobe Steel has long marketed FAMILIARC™ LB-52U as well as FAMILIARC™ LB-62U for this purpose, and they still serve as Kobe’s most reliable, “one and only” products worldwide.

In addition to meeting the always-changing and diverse specifications of pipelines, TRUSTARC™ LB-52NSU has been developed specifically for root pass welding of pipes for low temperature service. It is a covered, low hydrogen type electrode equivalent to AWS A5.5 E7016-G. It offers superb notch toughness at -60°C and very low diffusible hydrogen content of about 3.0ml/100g. The chemistries, mechanical properties of all weld metal and diffusible hydrogen content of TRUSTARC™ LB-52NSU are shown in Tables 3, 4 and 5 respectively.

Table 1: Covered electrodes for girth welding

API 5L pipe grade	Welding pass	Low hydrogen type		High cellulose type
		Vertical upward position	Vertical downward position
X42-X52	Root	LB-52U LB-52NSU	LB-78VS	KOBE-6010
	Hot	LB-52-18 LB-52NS
	Filler & Cap
X56-X60	Root	LB-52U LB-52NSU		KOBE-6010 KOBE-7010S
	Hot	LB-52-18 LB-52NS		KOBE-7010S
	Filler & Cap
X65	Root	LB-52U	LB-88VS	KOBE-7010S KOBE-8010S
	Hot	LB-62 LB-62D
	Filler & Cap			KOBE-8010S
X70	Root	LB-62U		KOBE-7010S KOBE-8010S
	Hot	LB-62 LB-62D
	Filler & Cap			KOBE-8010S
X80	Root	LB-62U	LB-98VS LB-108VS	——
	Hot	LB-65D LB-106
	Filler & Cap
X100	Root	——	LB-118VS	——
	Hot	LB-80L LB-116
	Filler & Cap

Table 2: TIG and MAG wires for girth welding

API 5L pipe grade	Welding pass	Temperature (°C)
		-20	-40	-60
X42-X56	Root & Hot	TG-S50 MX-100T	TG-S1N MX-A55T
	Filler & Cap	DW-A50 DW-A50SR	DW-A55E DW-A55ESR	DW-A55L DW-A55LSR DW-A81Ni1
X60	Root & Hot	TG-S62	TG-S60A
	Filler & Cap	DW-A55E DW-A55ESR		DW-A55L DW-A55LSR DW-A81Ni1
X65	Root & Hot	TG-S62	TG-S60A
	Filler & Cap	DW-A55E DW-A55ESR		DW-A55L DW-A55LSR DW-A81Ni1
X70	Root & Hot	TG-S62	TG-S60A
	Filler & Cap	DW-A70L		DW-A55L DW-A81Ni1
X80	Root & Hot	TG-S80AM
	Filler & Cap	DW-A70L		——
X100	Root & Hot	TG-S80AM
	Filler & Cap	DW-A80L	——	——

Table 3: Chemistries of TRUSTARCTM LB-52NSU all weld metal (mass %)

C	Si	Mn	P	S	Ni	Ti	B
0.06	0.62	1.25	0.016	0.004	0.50	0.014	0.0027

Table 4: Mechanical properties of TRUSTARCTM LB-52NSU all weld metal

Tensile properties				Notch toughness
0.2% PS (MPa)	TS (MPa)	EI (%)	RA (%)	Absorbed energy:J (Brittle fracture: %)			FATT (°C)
				-80°C	-60°C	-40°C
511	598	32	78	43(60) 55(60) 41(60) Av.46(60)	44(55) 72(55) 58(52) Av.58(54)	70(50) 137(35) 144(35) Av.117(40)	-53

Table 5: Diffusible hydrogen content of TRUSTARCTM LB-52NSU (ml/100g)

Electrode dia (mm)	1	2	3	4	Ave.
3.2	2.8	3.3	3.5	3.0	3.2

Note: Tested method: According to AWS A4.3.(Gas chromatography)
            Welding current: 120 A (DCEP)
            Welding atmosphere: 21°C x RH10%

In welding a butt joint on a 25 mm thick plate, 3.2 mm dia. TRUSTARC™ LB-52NSU was used for the root pass with DC 95 amp, and 3.2 mm dia. TRUSTARC™ LB-52NS was used for the second pass onwards with DC 110 amp in the vertical upward position. The preheating and interpass temperatures were kept between 115 and 135°C. Figure 3 shows the groove shape and the pass sequence and Figure 4, the macrostructure of the weld metal. The chemistries and the tensile properties are shown in Tables 6 and 7, respectively and the notch toughness properties and the transition curve of the butt joint weld metal are shown in Table 8 and Figure 5, respectively. (Note: both TRUSTARC™ LB-52NSU and TRUSTARC™ LB-52NS are specified as AWS A5.5 E7016-G).

Table 6: Chemistries of butt joint weld metal (mass %)

Location	C	Si	Mn	P	S	Ni	Ti	B
Face	0.07	0.31	1.40	0.008	0.003	0.50	0.013	0.0022
Reverse	0.08	0.30	1.36	0.009	0.003	0.43	0.014	0.0023

Table 7: Tensile properties of butt joint weld metal

Location	Tensile properties
Center	0.2%PS (MPa)	TS (MPa)	El (%)	RA (%)
	506	577	25	81

Table 8: Notch toughness properties of butt joint weld metal

Location	Notch toughness
	Absorbed energy:J (Brittle fracture: %)			FATT (°C)
	-80°C	-60°C	-40°C
Face side	47(56) 73(64) 71(55) Av. 64(58)	169(26) 145(30) 167(26) Av. 160(27)	162(22) 172(16) 160(26) Av. 165(21)	-75
Reverse side	17(79) 108(56) 49(73) Av. 58(69)	93(53) 92(56) 82(50) Av. 89(53)	167(26) 114(40) 169(26) Av. 150(31)	-58

4.2. DW-A70L

TRUSTARC™ DW-A70L was developed by Kobe Steel in order to meet the need of pipeline-constructors for high quality and efficiency in welding high strength pipes. A rutile type FCW for all position welding that was designed exclusively for pipeline girth welding. TRUSTARC™ DW-A70L is well suited for welding high strength pipes and for complying with the NACE MR0175 requirement that specifies total Ni content in the weld metal of not more than 1%. The diffusible hydrogen content of TRUSTARC™ DW-A70L all weld metal is as low as 4ml/100g.

Table 9 shows the classification of TRUSTARC™ DW-A70L and Tables 10, 11 and 12, the chemistries, the mechanical properties and the diffusible hydrogen content of TRUSTARC™ DW-A70L all weld metal, respectively.

Table 9: Classification of TRUSTARCTM DW-A70L

Wire diameter	1.2 mm dia.
Shielding gas	80%Ar-20%CO2
Welding position	All position
Classification	AWS A5.29 E101T1-GM ISO 18276 -A- T 62 5 Mn1NiMo P M 2 H5

Table 10: Chemistries of TRUSTARCTM DW-A70L all weld metal (mass %)

C	Si	Mn	P	S	Ni	Mo
0.05	0.36	1.90	0.008	0.011	0.97	0.46

Table 11: Mechanical properties of TRUSTARCTM DW-A70L all weld metal

Tensile properties				Notch toughness
0.2% PS (MPa)	TS (MPa)	EI (%)	RA (%)	Absorbed energy:J (Brittle fracture: %)			FATT (°C)
				-50°C	-40°C	-30°C
663	739	21	63	75(23) 76(23) 66(30) Av.72(25)	88(23) 89(18) 84(18) Av.87(20)	95( 8 ) 92(13) 92(13) Av.93(11)	<-50

Table 12: Diffusible hydrogen content of TRUSTARCTM DW-A70L (ml/100g)

Wire dia (mm)	1	2	3	4	Ave.
1.2	3.5	3.7	3.9	3.6	3.7

Note: Tested method: According to AWS A4.3.(Gas chromatography)
            Welding current: 200A-24V-300mm/min
            Wire stick-out length: 25 mm

Using TRUSTARC™ DW-A70L FCW, API 5L X65 pipe was girth-welded with a CRC Evans M300-C welding machine (as shown in Figure 6), and successful results were obtained. Table 13 shows the tested welding conditions. The macrostructure and the bead appearance are shown in Figures 7 and 8, respectively and the chemistries, in Table 14. The mechanical properties and the transition curve of the weld metal are shown in Table 15 and Figure 9 respectively.

Table 13: Tested welding conditions

Base metal	API 5L X65-PSL1 273.1 mm dia.× 21.4 mm wall thickness
Welding position	5G (Pipe is fixed in horizontal position.)
Welding equipment	M-300-C External Pipe Welding System (CRC-EVANS)
Groove shape
Root & hot passes	TRUSTARCTM TG-S60A (2 layers) Welding parameters: 150 A-10 V-70 mm/min
Polarity	DCEP
Welding parameters	200A-23.5V
Heat input	1.7 kJ/mm
Pass sequence (FCW)	8 passes / 5 layers
Preheating & Interpass temp.	100 -130 °C
Shielding gas	80%Ar-20%CO2 , 25 L/min.
PWHT	None (As-welded)

Table 14: Chemistries of weld metal (mass %)

C	Si	Mn	P	S	Ni	Mo
0.05	0.30	1.77	0.008	0.006	0.89	0.42

Table 15: Mechanical properties of weld metal

Tensile properties				Notch toughness
0.2% PS (MPa)	TS (MPa)	EI (%)	RA (%)	Absorbed energy:J (Brittle fracture: %)			FATT (°C)
				-60°C	-50°C	-40°C
627	691	29	66	57(37) 63(44) 54(48) Av.58(43)	63(38) 70(37) 49(45) Av.61(40)	82(22) 86(23) 82(34) Av.83(26)	<-60

As seen in Table 14, Ni content of 0.89 % in the weld metal complies with the NACE requirement. The mechanical properties such as strength (0.2%PS as well as TS) and notch toughness as low as -60°C, are also satisfactory, thanks to the optimization of the alloying elements including minor components in the TRUSTARC™ DW-A70L flux. Finally, the amount and composition of slag in TRUSTARC™ DW-A70L flux is optimum and provides good weldability in all position welding as can be seen in Figures 7 and 8, the macrostructure of the weld metal at 3 o’clock position as well as the bead appearance.

4.3. DW-N625P

Depending on where it is drilled, crude oil or natural gas may sometimes contain substances that can corrode pipes. In such cases, the inner pipe has to be corrosion-resistant, so clad pipes in which the inner surface is overlay-welded are generally used. For girth welding of corrosion-resistant pipes as well as clad steel pipes, Ni-Cr-Mo 625 alloy is normally applied due to its excellent corrosion resistance. Its strength is usually designed to be equal to or better than the pipes being welded.

Until recently, an FCW with good weldability, corrosion resistance as well as appropriate mechanical properties for girth welding was not available in the market. However, Kobe Steel’s newly-developed PREMIARC™ DW-N625P flux cored wire fulfills all the requirements mentioned above. Table 16 shows the classification of PREMIARC™ DW-N625P and Tables 17 and 18, the chemistries and the mechanical properties of PREMIARC™ DW-N625P all weld metal, respectively.

Table 16: Classification of PREMIARCTM DW-N625P

Wire diameter	1.2 mm
Shielding gas	75-80%Ar+Bal.%CO2
Welding position	All position
Classification	AWS A5.34/A5.34M: ENiCrMo3T1-4 ISO 12153 T Ni 6625 P M21 2

Table 17: Chemistries of PREMIARCTM DW-N625P all weld metal (mass%)

Elements	C	Si	Mn	P	S	Cu	Ni
DW-N625P	0.031	0.21	0.02	0.007	0.004	0.01	65.2
ENiCrMo3Tx-y	≤0.10	≤0.50	≤0.50	≤0.02	≤0.015	≤0.05	≥58.0
Elements	Cr	Mo	Ti	Fe	Nb+Ta		Others
DW-N625P	21.3	8.8	0.17	2.0	3.23		——
ENiCrMo3Tx-y	20.0 -23.0	8.0 -10.0	≤0.40	≤5.0	3.15-4.15		≤5.0

Table 19: Welding conditions of girth welding

Welding position	5G (6 →12 o’clock)	Pass sequence
Type of steel	Carbon steel*
Pipe size	Wall thickness 30 mm Outer diameter 267 mm
Welding process	1 -3 passes: GTAW 4-10 passes: FCAW
Wire	1-3 passes: TG-SN625 2.4 mm dia. (AWS A5.14 ERNiCrMo3) 4-10 passes: DW-N625P 1.2 mm dia.
Shielding gas	1-3 passes: 100%Ar (Back purge: 100%Ar) 4 -10 passes: 80%Ar-20%CO2 (25 l/min)
Wire stick-out	4 -10 passes: 15 mm (160A)
Torch angle	10°back-hand
Interpass temp.	150°C max.
* For checking the usability of DW-N625P only

Table 18: Mechanical properties of PREMIARCTM DW-N625P all weld metal

	Tensile properties			Notch toughness
	0.2% PS (MPa)	TS (MPa)	EI (%)	Absorbed energy:J
				-196°C	-100°C	0°C
DW-N625P	479	765	45	Av.70	Av.78	Av.84
ENiCrMo3Tx-y	Not required	≥690	≥25	Not required

Figure 10 shows pipe being girth-welded on a Magnatech machine with PREMIARC™ DW-N625P in 5G position. GTAW and FCAW were used to conduct welding according to the welding conditions listed in Table 19. GTAW was used for the root, hot and 3rd passes (3 passes) with PREMIARC™ TG-SN625 rod, and FCAW was used from the 4th pass to the cap pass (10th pass) with PREMIARC™ DW-N625P.

The bead appearances from 6 to 3 o’clock of the 4th pass and the cap pass are shown in Figures 11 and 12, respectively. The macrostructures of 6, 4 and 3 o’clock positions are shown in Figures 13, 14 and 15, respectively. Table 20 shows the impact test results of the 3 o’clock position in the different temperatures down to -196°C.

Table 20: Impact test results of girth welding

Position	Tested temp. (°C)	Absorbed energy (J)
3 o’clock	0	Av. 96
	-30	Av. 93
	-100	Av. 87
	-196	Av. 82

These tests show that girth welding was able to obtain excellent bead appearance at the 6, 4 and 3 o’clock positions which are the most difficult positions in which to achieve defect-free welds.

5 Postscript

Whereas most offshore structures are constructed according to the same specifications and, therefore, will utilize the same welding processes and consumables, pipeline projects are more likely to apply specifications set forth by the particular project owner. For this reason, one project may differ significantly in terms of welding from another. It can be assumed, however, that future pipeline projects will specify ever higher quality requirements.

Demand for more stable and more efficient welding consumables and processes shall not stop and Kobe Steel is always ready to challenge the limits of current technology.

With special thanks, photographs courtesy of:
Pipeline Service S.r.I., Manufacturer of the Proteus FAP Magnatech International B.V.

Page Top