What is 9Ni steel?
9% nickel steel (9Ni steel) is the standard material for liquefied natural gas (LNG) ship storage tanks because it offers exceptional fracture toughness and ductility at low temperatures. Even when exposed to LNG at -163°C, it prevents brittle fracture and provides a good balance between structural integrity and cost-effectiveness compared to alternative materials such as aluminum.
The 9% nickel steel is a low-carbon nickel alloy steel with nominal composition of 9Ni, which is particularly used for low-temperature or cryogenic pressure vessels and plants for processing, transportation, and storage of LNG down to -196°C [-320°F].
It also has high melting point and retention of strength at elevated temperatures, which provide fire-safe integrity for the LNG pressure vessels. Besides, the relatively low thermal expansion, low thermal conductivity and good corrosion resistance make the steel applicable for ethylene, methane, LNG, oxygen, and nitrogen applications.
ASTM A553 / ASME SA553: The Standard for 9Ni Steel
When sourcing 9Ni steel for LNG projects, it is typically produced under the ASTM A553 Type I or ASME SA553 specification. These standards guarantee:
Quenched and Tempered (Q&T) heat treatment for maximum toughness.
Strict chemical control (Nickel 8.5% – 9.5%).
Mandatory Charpy V-notch impact testing at cryogenic temperatures.
Technical Comparison: 9Ni Steel vs Alternatives
| Material | Temp Limit | Strength | Cost | Best Use Case |
| 9% Ni Steel | -196°C | High | Moderate | Large LNG Cargo Tanks |
| Stainless 304L | -269°C | Medium | High | Small Piping / Small Tanks |
| Aluminum 5083 | -269°C | Low | Moderate | Moss-type Spherical Tanks |
| Invar (36% Ni) | -250°C | High | Very High | Membrane-type Liners |
Typical Chemical Composition of 9% Nickel Steel
| Element | C | Mn | P | S | Si | Ni |
|---|---|---|---|---|---|---|
| Content, wt. % | ≤0.13 | ≤0.90 | ≤0.015 | ≤0.015 | 0.15~0.30 | 8.5~9.5 |
Typical Mechanical Properties of 9% Nickel Steel
| Tensile Strength, ksi [MPa] | Yield Strength, ksi [MPa] | Elongation in 2", % |
|---|---|---|
| ≥ 100 [690] | ≥ 75 [515] | ≥ 20.0 |
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Key Material Advantages of 9% Nickel Steel：
❄️ Extreme Low-Temperature Resistance: Can operate long-term in -163°C LNG environments and withstand temperatures as low as -196°C;
🧱 High Structural Stability: Coefficient of linear expansion is lower than that of aluminum alloys and slightly better than that of austenitic stainless steel, with minimal deformation at low temperatures;
💰High Overall Cost-Effectiveness: Compared to aluminum alloys and specialty stainless steels, it offers lower costs and better machinability and formability under equivalent operating conditions;
🔒Excellent Crack Resistance: Produced through a high-purity smelting process with extremely low impurity levels, it is resistant to cracking and leakage even in extreme environments.
Applications of 9Ni Steel in LNG Ships
In LNG carriers, bunkering vessels, and powered vessels, 9Ni steel is the exclusive primary material used for tanks. It comes into direct contact with liquid natural gas at -163°C and serves as the final line of defense for vessel safety. Its mainstream applications include:
- LNG carriers: inner tanks of medium- and large-sized standalone liquid cargo tanks;
- LNG bunkering vessels: mobile bunkering tanks designed for offshore refueling operations;
- LNG-powered merchant vessels: Type B/C independent fuel tanks, currently the mainstream high-end tank configuration.
Manufacturing and Welding Requirements for 9Ni Steel
Welding Methods for 9Ni Steel:
Shielded Metal Arc Welding (SMAW),
Submerged Arc Welding (SAW),
Tungsten Inert Gas Welding (GTAW),
Gas Metal Arc Welding (GMAW)
In the actual production process of welding 9Ni steel, both tungsten inert gas welding (GTAW) and gas metal arc welding (GMAW) use inert gas to protect the molten pool during welding. Harsh outdoor conditions can compromise the quality of gas-shielded weld joints, and welding thick plates can also easily lead to poor fusion.
Tungsten inert gas (TIG) welding can produce high-quality joints in narrow-gap welding, but it lacks cost-effectiveness and has low welding efficiency, making it suitable only for small-scale piping and thin-gauge materials.
Currently, the most widely used and prevalent welding methods for 9Ni steel are shielded metal arc welding (SMAW) and submerged arc welding (SAW).
Welding Consumables for 9Ni Steel
Currently, there are four types of welding materials commonly used for 9Ni steel in arc welding, namely:
- ferritic type with w(Ni) = 11%;
- austenitic stainless steel type with w(Ni) = 13% and w(Cr) = 16%;
- nickel-based type (Ni-Cr-Mo alloy) with w(Ni) > 60%; and
- Fe-Ni-based (Fe-Ni-Cr alloys) with w(Ni) ≈ 40%.
Common welding materials for 9Ni steel:
- Welding electrodes: ENiCrMo-3 and ENiCrMo-6,
- Welding wires: ERNiCrMo-3 and ERNiCrMo-4.
ASME, ASTM Standards & International Equivalent
| ASTM/ASME Grade | Product Form |
|---|---|
| ASTM A353, ASME SA-353 Grade 9%-Ni Steel | Double-normalized and tempered 9% nickel steel pressure vessel plates |
| ASTM A553/ ASME SA-553 Type I | Quenched and tempered 9% nickel steel pressure vessel plates |
| ASTM A333/ ASME SA-333 Grade 8 | Seamless and welded 9%-Ni alloy steel pipes for low-temp. service |
| ASTM A334/ ASME SA-334 Grade 8 | Seamless and welded 9% nickel steel tubes for low-temp. service |
| ASTM A420/ ASME SA-420 Gr. WPL8 | Wrought pipe fittings for low-temp. service |
| ASTM A522/ ASME SA-522 Type I | 9% nickel alloy steel forged flanges, fittings, valves, and parts. |
| ASTM A844/ ASME A844 9Ni | 9%-Ni steel pressure vessel plates, by direct-quenching process |
| ASTM A352/ ASME SA-352 Gr. LC9 | 9% nickel steel castings for low-temp. service |
| ASTM A671/ ASME SA-671 Gr. CG 100 & Gr. CH 100 | EFW 9%-Ni steel pipe for low-temp. |
If you are designing or building LNG storage systems, ASME SA553 (9Ni Steel) is the gold standard because it combines extreme cold resistance, high structural strength, and cost-effectiveness.
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PACKAGING & SHIPPING
FAQ:
Are you looking for certified ASME SA553 Type I plates for your next LNG project?
We provide high-quality 9% Nickel steel with full MTC (Mill Test Certification), impact testing at -196°C, and third-party inspection (SGS/TUV/BV) compliance.
What is 9 ni material?
9%Ni steel was first applied to liquid oxygen tanks in 1952, and it has been used as a main material of the inner walls of LNG tanks ever since as a ferritic steel for ultra-low-temperature use.
What temperature can 9 nickel steel be used down to?
At very low temperatures, nickel steel is strong and ductile. Studies indicate that steels with 5–9% nickel maintain satisfactory properties down to −260°F. For lower temperatures, down to −320°F, properties such fatigue and fracture resistance may be degraded for the 5% steel.