Technical Highlight Vol.15

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

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

1. Preface

Worldwide trends in LNG exports [1] Note: MTPA: Million tons per annum

Figure 1: Worldwide trends in LNG exports [1]
      Note: MTPA: Million tons per annum

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 2: Worldwide LNG exports in 2013 [2]

Figure 2: Worldwide LNG exports in 2013 [2]

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

Figure 3: Worldwide gas-liquefaction capacity [3]

Figure 3: Worldwide gas-liquefaction capacity [3]

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.

Figure 4: Forecast of primary energy consumption in China [4] Note: *1: Million tons of oil equivalent.

Figure 4: Forecast of primary energy consumption in China [4]
      Note: *1: Million tons of oil equivalent.

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.

Figure 5: Typical domestic LNG carrier [6]

Figure 5: Typical domestic LNG carrier [6]

Figure 7: Tri-lobe tank [6]

Figure 7: Tri-lobe tank [6]

Figure 6: IMO-type C tanks [6]

Figure 6: IMO-type C tanks [6]


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 Figure 8: Microstructure comparison Figure 8: Microstructure comparison
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
Figure 9: Schematic drawing of Duplex ESSO test Figure 9: Schematic drawing of Duplex ESSO test
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.

Figure 10: Schematic location of test specimens

Figure 10: Schematic location of 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).

Figure 11: Results of notch toughness tests

Figure 11: Results of notch toughness tests


4-4-2 Brittle fracture resistance

Figure 12: Results of CTOD tests

Figure 12: Results of CTOD tests

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
Figure 13: Comparison of horizontal fillet welding by shielding gas

Figure 13: Comparison of horizontal fillet welding by shielding gas

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.

Figure 14: Groove configuration and macro structure

Figure 14: Groove configuration and macro structure

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.


Page Top

Products

Welding Robot/Software
japanese_scenery KOBELCO ARC over the last decade (2008~)