What are the differences between LNG stainless steel pipes and ordinary stainless steel pipes

LNG stainless steel pipes (used for -162°C low-temperature LNG storage and transportation systems) have significant differences in design goals, performance requirements, and manufacturing standards compared to ordinary stainless steel pipes (such as industrial pipes, construction pipes, etc.). The core differences lie in four aspects: low-temperature adaptability, material selection, process control, and quality standards. Specifically: 

I. Differences in Core Functions and Application Scenarios 

LNG stainless steel pipe: Specifically designed for ultra-low temperature (-162) LNG medium, it needs to withstand thermal contraction and low-temperature brittleness risks under extreme low temperatures, while meeting the corrosion resistance and sealing requirements of LNG (containing trace amounts of moisture, CO₂, H₂S, etc.), and is applied in core systems such as LNG storage tanks, transportation pipelines, pump valve connections, etc. 

Common stainless steel pipes: These are mainly used in normal temperature or medium-low temperature (such as 0-100) conditions, such as chemical pipelines, water supply, and architectural decoration, etc. Their functions are mainly to resist common corrosion and provide structural support. There is no need to consider ultra-low temperature toughness and thermal contraction stability. 

II. Differences in Material Selection 

LNG stainless steel pipes

Common grades: 316L, 304Lmod (modified 304L), 904L

Carbon content: ≤ 0.03% (ultra-low carbon, avoiding low-temperature embrittlement and sensitization)

Key alloy elements: contain Mo (2-3%, enhancing low-temperature corrosion resistance), strictly control Ni (8-12%, ensuring austenitic stability)

Low-temperature toughness requirements: -196 Shore impact energy ≥ 47J (ASTM A370)

Non-metallic inclusions: grade ≤ 2 (preventing low-temperature crack sources) 

Common stainless steel pipes

Common grades: 304, 316, 201, 321, etc.

Carbon content: Can be higher (e.g., for 304, the carbon content is ≤ 0.08%)

Key alloy elements: Some grades do not contain Mo (e.g., 304), and the Ni content can be lower

Low-temperature toughness requirements: No mandatory low-temperature impact requirements (impact at room temperature is sufficient)

Non-metallic inclusions: Lower requirements (usually ≤ 3-4 grades) 

III. Differences in Manufacturing Process and Performance Control 

Low-temperature toughness guarantee

LNG pipes: Must undergo 1050-1100 degree solid solution treatment followed by water quenching to ensure complete dissolution of carbides and obtain a single austenitic structure (grain size refined to ≥7 levels), avoiding the precipitation of brittle phases (such as σ phases) at low temperatures;

Ordinary pipes: The solid solution treatment temperature can be slightly lower (e.g. 1000-1050), with more relaxed cooling requirements (even air cooling), and no strict control over grain size and low-temperature structure. 

Welding quality control

LNG pipes: The welding process uses TIG as the base welding + MIG for filling, with 99.99% pure argon as the shielding gas (to prevent alloy burn-off). The welds must undergo 100% radiographic testing (RT) and low-temperature impact tests (-196°C) to ensure there are no defects such as incomplete fusion or pores;

Ordinary pipes: Stick electrode arc welding (SMAW) can be used. The protection requirements are lower, and the weld inspection rate is usually 10-20% (random inspection), without the requirement for low-temperature impact tests. 

Dimension accuracy and stability

LNG pipe: Outer diameter tolerance ±0.1mm, wall thickness tolerance ±5%, roundness (ellipticity) ≤0.5% (to reduce low-temperature assembly stress);

Ordinary pipe: Tolerances are relatively loose (such as outer diameter ±0.5mm, wall thickness ±10%), lower requirements for roundness. 

Surface treatment

LNG pipes: Must undergo acid pickling and passivation + pure water rinsing to form a uniform Cr₂O₃ passivation film (thickness 5-10nm), and the inner wall needs to be polished (Ra ≤ 0.8μm to avoid LNG liquid accumulation and impurity deposition);

Ordinary pipes: The surface treatment is simpler (such as sandblasting, simple acid pickling), and the roughness requirement for the inner wall is low (Ra ≤ 3.2μm is sufficient). 

IV. Differences in Quality Inspection Standards 

Requirements for LNG stainless steel pipes

Low-temperature impact test: Mandatory -196 test, impact energy ≥ 47J

Pressure resistance test: Maintain 1.5 times the design pressure (usually 1.6 - 4.0 MPa) for 30 minutes

Leak detection: Helium leak detection (leak rate ≤ 1×10⁻⁹ Pa·m³/s)

Intergranular corrosion test: Required (e.g., ASTM A262 E method, no intergranular corrosion)

Low-temperature tensile test: Test tensile strength and elongation at -162 degrees 

General requirements for ordinary stainless steel pipes:

Low-temperature impact test: Only normal temperature impact (20°C), no unified low-temperature requirements

Pressure resistance test: Maintain 1.5 times the nominal pressure for 10-15 minutes

Leak detection: Mostly water pressure or air pressure testing, no helium leak detection requirement

Intergranular corrosion test: Only required under specific conditions (such as chemical pipeline), not mandatory

Low-temperature tensile test: Only tensile test at normal temperature (20°C) 

V. Summary: The core difference lies in "low-temperature reliability".

LNG stainless steel pipes are customized extreme environment pipes. Through material purification, process optimization, and strict testing, they ensure no brittleness, no leakage, and corrosion resistance at -162 degrees. While ordinary stainless steel pipes only meet the requirements of normal working conditions and cannot withstand the material brittleness and thermal stress risks brought by ultra-low temperatures. Therefore, the two cannot be interchanged - if ordinary stainless steel pipes are used in the LNG system, they may fracture due to insufficient low-temperature toughness, leading to serious safety accidents.