Technical Highlight Vol.15

Supply and demand for LNG and implications for 7% Ni TMCP steel and welding consumables

1. Preface

Three and a half years after describing Kobelco’s welding consumables for liquefied natural gas (LNG) storage tanks made of 9% Ni steel in the Kobelco Welding Today, Vol.14, No.2, (KWT14-2) (2011) issue, the global market for LNG has changed significantly.

Not only has the supply and demand situation changed, but so have the properties of the steel used for storage tanks. 7% Ni Thermo Mechanical Control Process (TMCP) steel has successfully been introduced in Japan in order to reduce the Ni content, which is expensive and susceptible to fluctuations in the market. The specification of 7% Ni TMCP steel is already covered by Japanese Industrial Standard (JIS) regulations as well as non-Japanese specifications such as ASTM.

This article briefly introduces the welding consumables that are suitable for 7% Ni TMCP steels and provides some up-to-date technical data.

2. Recent LNG supply and demand

Figure 1 shows that LNG exports increased sharply in 2010. Total worldwide LNG exports in 2013 reached 237 million tons per annum (MTPA) as shown in Figure 2, reflecting an increase in global demand, mainly in Asia and particularly in China.

The export of LNG from floating storage units (FSUs) (Figure 1) is also a recent supply trend. In most cases, FSUs or floating storage and gasification units (FSRUs) are converted out of LNG ships, which reduces both cost and time associated with construction and, therefore, keeps up with the current supply and demand of LNG.

Situation in Asia

Due to the large increase in natural gas consumption, 7% Ni Thermo Mechanical Control Process (TMCP) has also increased and is expected to grow, particularly in Asia and the Pacific as shown in Figure 3.

Accordingly, the need for LNG storage yards and transportation systems such as LNG carriers (oceangoing and domestic) will increase.

Figure 4 shows China’s primary energy consumption plan, based on the twelfth five year plan (2011-2015). China’s LNG imports are forecast to increase 50% each year, from 14.7 million tons in 2012 to a maximum 100 million tons per year. Naturally, a large number of LNG terminals and LNG carriers (for oceangoing and domestic use) will be required in due course.

LNG tanks are classified roughly into three types: Membrane, Moss and IMO (International Maritime Organization)-type A, B or C tanks. While membrane and Moss tanks are applied on oceangoing LNG carriers, the third type is for the smaller-sized, domestic carriers, as shown in Table 1. Figure 5 shows a typical domestic LNG carrier and Figure 6, some IMO–type C tanks.

Table 1: Types of LNG tanks

	Type of LNG tank
Oceangoing LNG carrier	Membrane and Moss
Domestic LNG carrier	IMO ‒ type A, B, C

Figure 7 shows a newly-developed tri-lobe tank, which will be equipped on a liquefied ethylene gas (LEG) ship for the transport of LEG in the near future.

Development and specifications of 7% Ni TMCP steel

For safe operations under cryogenic conditions, LNG storage tanks are generally made of 9% Ni steel plates. Recently, however, 7% Ni TMCP steel plate has been developed, which reduces the content of expensive Ni by nearly 20%.

7% Ni TMCP steel was standardized as SL7N590 in JIS G3127, "Nickel steel plates for pressure vessels for low temperature services," in March 2013, when application of this product began in Japan. Around the same time in the USA, ASTM standardized 7% Ni TMCP steel as Gr. G Class 9 and Class 10 in A841, "Standard Specification for Steel Plates for Pressure Vessels, Produced by Thermo-Mechanical Control Process (TMCP)."

The JIS and ASTM specifications of 7% Ni TMCP and 9% Ni steels are shown in Table 2 for reference.

Table 2: Specifications for 7% Ni TMCP and 9% Ni steels

Specification	ASTM			JIS G 3127
	A553 Type I	A841 Grade G		SL9N 590	SL7N 590
		Cl.9	Cl.10
Plate thickness (mm)	50 max.	50 max.		100 max.	50 max.
Process	QT	TMCP		QT	TMCP
C (%)	0.13 max.	0.13 max.		0.12 max.
Si (%)	0.15-0.40	0.04-0.15		0.30 max.
Mn (%)/td>	0.90 max.	0.60-1.20		0.90 max.	1.20 max.
P (%)	0.035 max.	0.015 max.		0.015 max.
S (%)	0.035 max.	0.015 max.		0.015 max.
Ni (%)	8.50-9.50	6.00-7.50		8.50-9.50	6.00-7.50
0.2%PS (MPa)	585 min.	585 min.	620 min.	590 min.
TS (MPa)	690-825	690-825	750-885	690-830
El (%); Thick(mm)	20 min.	20 min.		21 min. (t ≤ 16) 25 min. (t > 16)
IV (J) at -196°C	34 min.	34 min.		41 min.
LE*1 (mm) at -196°C	0.38 min.	0.38 min. (t ≤ 32) 0.48 min. (t=50)*2		-	-
Note: *1: LE: Lateral Expansion *2: LE value between the plate thickness 32 and 50 shall be determined　by linear interpolation.

Results of tests comparing 9% Ni and 7% Ni TMCP steels are described below.

4-1. Basic features of 7% Ni TMCP steel

Figure 8: Microstructure comparison

Steel	7% Ni TMCP	9% Ni
Microstucture
Residual γ (%)	8.5	3.2

In order to maintain the same high toughness as 9% Ni steel, TMCP technology allows for much residual austenite (γ) to be distributed in the base structure of 7% Ni TMCP steel.

As seen in Figure 8, the lath structure is refined in 7% Ni TMCP steel, resulting in the increase of residual γ.

4-2. Basic performance evaluation

Tests were carried out on several properties related to the basic performance of 7% Ni TMCP steel, as shown in Table 3. The test results shown in Tables 4 and 5 prove that 7% Ni TMCP steel performs as well as 9% Ni steel.

Table 3: Tests for evaluation of performance

	Basic	Resistance to brittle fracture
Plate	・Tensile test	・CTOD test
	・CTOD test	・Duplex ESSO test
Welded joint	・Tensile test	・CTOD test
	・CTOD test	・Cross weld notched wide plate test

Table 4: Results of tensile tests

Steel	Thickness (mm)	0.2%PS (MPa)	TS (MPa)	EL (%)
7% Ni TMCP	40	655	738	31
9% Ni	36	726	743	23
SL7N590		590 min.	690-830	21 min.
Note: Position: 1/4 t Direction: Parallel to rolling direction

Table 5: Results of notch toughness tests

Steel	Thickness (mm)	IV (J)at -196°C	BA(%) at -196°C
7% Ni TMCP	40	Avg 256	0
9% Ni	36	Avg 243	0
SL7N590		41 min.	-
Note: BA: Brittle fracture appearance value Position:1/4 t Direction: Parallel to rolling direction

4-3. Brittle fracture resistance

7% Ni TMCP and 9% Ni steels were compared for brittle fracture resistance, as shown in Table 3.

Resistance to brittle crack initiation and cracking were evaluated by CTOD test and a Duplex ESSO test, respectively. For reference, a schematic drawing of the Duplex ESSO test is shown in Figure 9. The results of both the CTOD and Duplex ESSO tests show basic equivalence between 7% Ni TMCP and 9% Ni steels as shown in Tables 6 and 7, respectively.

Figure 9: Schematic drawing of Duplex ESSO test

Excellent property	Poor property

Table 6: Results of CTOD tests

Steel	Thickness (mm)	Critical CTOD value (mm) at -165°C
7% Ni TMCP	40	1.18; 1.05; 1.18
9% Ni	36	0.65; 0.70; 0.68
SL7N590	41 min.	-
Note: Direction: Parallel to rolling direction

Table 7: Results of Duplex ESSO tests

Steel	Thickness (mm)	Temperature (°C)	Applied stress (MPa)	Judgment
7% Ni TMCP	40	-196	392	No-Go
9% Ni Steel	36	-196	392	No-Go

4-4. Properties of butt joint welding with 7% Ni TMCP steel

Double V butt joint welding was performed on 7% Ni TMCP steel plate using PREMIARC™ NI-C70S, 4 mm dia. covered electrodes in the vertical upward position (3G). The welding conditions are shown in Table 8.

Table 8: Welding conditions

Direction of welding	Welding process	Product name	Φ mm	Welding position	Heat input (kJ/mm)
Transverse to rolling direction	SMAW	NI-C70S	4.0	3G uphill	4.4 max.

Figure 10 shows the schematic cross-sectional weld metal and location of notch toughness test specimens.

The notch toughness test results are shown in Figure 11. All values fulfill the requirement of SL7N590 (34J min. and 41J average at -196°C).

4-4-2 Brittle fracture resistance

Resistance to brittle fracture was tested by CTOD, and all values were found to exceed the requirements of a 140,000m³ LNG tank (0.085 mm min. at -196°C) as shown in Figure 12.

Welding consumables for 7% Ni TMCP steel

All welding consumables recommended by Kobe Steel for 9% Ni steels are also suitable for welding 7% Ni TMCP steels without exception. Typical welding consumables recommended for 7% Ni TMCP steels are listed in Table 9.

5-1. PREMIARC™ DW-N709SP

The AWS specification (A5.34) of ENiMo13-T, under which PREMIARC™ DW-N709SP is included, has formally been issued. It is now classified as ENiMo13T1-4/0-1 as shown in Table 9. Recent test results of welding by DW-N709SP and of comparing the efficiency of DW-N709SP with that of a covered electrode are described below.

Table 9: Typical welding consumables for 7% Ni TMCP steel

	FCAW	SMAW	GTAW	SAW
Product name	DW-N709SP	NI-C705	TG-S709S	PF-N4 (flux) / US-709S (wire)
Features	・Hastelloy type ・Ar-CO2 gas for all position welding and CO2 gas, for 1G, 1F and 2F welding	Inconel type	・Hastelloy type ・Suitable for automatic-TIG welding	・Hastelloy type ・Suitable for 2G position welding
Polarity	DCEP	AC	DCEN	DCEP
Ni (%)	62.5	63.4	70.4	64.0
Cr (%)	6.5	16.6	2.0	1.7
Mo (%)	17.6	5.3	19.0	17.2
W (%)	2.4	0.7	3.0	2.7
Nb+Ta (%)	-	1.1	-	-
Fe (%)	7.9	9.9	5.5	14.9
0.2%PS (MPa)	447	430	460	410
Ts (MPa)	723	705	730	680
El (%)	51	41	47	43
IV(J) at -196°C	89	62	160	70

As seen in Figure 13, Ar-CO₂ shielding gas does not provide sufficient penetration at the corner in horizontal fillet position (2F) welding. When full penetration is required, 100%CO₂ shielding gas is recommended.

5-2. Butt joint welding on 10 mm thick plate

Table 10: Welding conditions

Product name			DW-N709SP
Shielding gas & flow rate			80%Ar-20%CO2 & 25l/min
Welding position			3G uphill
Interpass temperature			150°C max.
Polarity			DCEP
Welding parameters	Face side	1st layer	140A-24V-17 cm/min
		2nd layer	160A-26V-16 cm/min
	Back side	Final layer	160A-26V-15 cm/min

Butt joint welding in the 3G position was conducted on a 10mm thick plate. The welding conditions are shown in Table 10, the groove shape and macro structure, in Figure 14 and the welded joint properties, in Table 11, respectively.

Table 11: Welded joint properties

Properties	Measurements
TS (MPa)	759; 764 (Fractured at base metal) *1
Notch toughness (J) at -196°C	62, 65, 60 (Avg. 62) *2
Longitudinal bending, 180°	No defect
Note: *1: Due to plastic constraint, the weld metal strength is increased. *2: Specimen size is 7.5mm x 10mm

5-3. Welding efficiency comparison of SMAW and FCAW (DW-N709SP)

Table 12: Comparison of efficiency

	DW-N709SP (1.2mmΦ)	SMAW (4mmΦ)
Product quantity (kgs)	125	200
Arc time (hour)	29.4	71.4
Deposition rate (g/min)	75 (at 200 A)	34 (at 150 A)
Deposition efficiency (%)	85	50

SMAW and DW-N709SP were compared in terms of the product quantity and arc time needed to obtain 100 kgs of weld metal. Table 12 shows the results. DW-N709SP was found to be excellent in deposition rate, arc time as well as deposition efficiency.

Notes on usage

When using 7% Ni TMCP steels, users must take the same precautions they would for 9% Ni steels, which were described in KWT14-2.

(1) Easy magnetization

Residual magnetism in 7% Ni TMCP steel will cause magnetic arc blow. For welding, it is advisable to use AC polarity as much as possible for SMAW and SAW.

(2) Crater crack

It is strongly recommended that users grind the crater down each time the arc stops, in order to avoid crater cracks.

(3) Dilution

Dilution of the base metal into the weld metal by the arc causes changes in the weld metal chemistry, resulting in the decrease of weld metal tensile strength. Users must ensure that the tensile strength and 0.2% proof strength fulfill the requirements in the procedure test in advance.

Postscript

This article discussed the recent global supply and demand of LNG as well as the application of 7% Ni TMCP steel for cryogenic uses. As a clean source of energy, the natural gas demand is expected to increase further, requiring the development of many new technologies. Kobe Steel will continue cultivating new welding technologies, in accordance with the needs of our users.

References

[1]-[4] JOGMEC (Japan Oil, Gas and Metals National Corporation), Trend of LNG, 2014
[5] Kobe Steel engineering reports, Vol. 64, No. 1 (2014)
[6] Sinopacific Offshore & Engineering Co., Ltd.

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