What are the application limitations of 304/304L stainless steel pipes in the LNG system?

The 304/304L stainless steel pipe, due to its certain low-temperature toughness and corrosion resistance, has applications in some low-temperature fields. However, in the LNG (liquefied natural gas, with a working temperature of approximately -162℃) system, its performance has significant shortcomings and its application is strictly limited. The following elaborates in detail from the aspects of low-temperature toughness, corrosion risk, and mechanical properties: 

1. Insufficient low-temperature toughness, with a risk of brittle fracture

Core issue: In the LNG operating condition at -162℃, the impact toughness of 304/304L fails to meet safety requirements. 

The brittle transition temperature (DBTT) of 304 stainless steel (unstabilized steel) is relatively high, typically ranging from -100℃ to -70℃. When the temperature drops to -162℃, its impact absorption energy (Ak) will sharply decrease to below 20J (even lower than 10J), which is far below the Ak ≥ 40J requirement for LNG systems (as per ASME B31.3 and other standards). This makes it prone to brittle fracture without plastic deformation, potentially leading to serious accidents such as leaks. 

The low-temperature toughness of 304L (low-carbon type) is slightly better than that of 304, but it still cannot completely overcome the problem of brittle transition. The impact performance at -162℃ fluctuates significantly, and the toughness of the heat-affected zone (HAZ) in welding is even more pronounced, making it difficult to pass the low-temperature impact tests (such as the V-notch impact test). 

Comparison: The commonly used materials in LNG systems, such as 304Lmod (modified 304L), 316L, and 9% Ni steel, have a DBTT (Deformation-Induced Breakdown Temperature) as low as below -196℃. At -162℃, the impact energy remains above 80J, effectively avoiding the risk of brittle fracture. 

2. Limited resistance to low-temperature corrosion, susceptible to impurity erosion

Although methane is the main component in the LNG system, it may contain trace amounts of H₂S, CO₂, moisture, etc. At low temperatures, it is prone to form corrosive media (such as the mixture of liquid water and H₂S forming an acidic solution). The corrosion resistance of 304/304L is insufficient to cope with this: 

Sulfide Stress Corrosion Cracking (SSCC): Although the Cr content (18-20%) in 304/304L can form a passive film, in the low-temperature and high-pressure environment with H₂S present, the passive film is prone to be destroyed, and its austenitic structure has a higher sensitivity to SSCC. Long-term use may result in intergranular or transgranular cracks.

Pitting and Crevice Corrosion: If there is a trace of chloride ions in the system (such as those brought in by seawater cooling leakage), the pitting resistance equivalent (PREN ≈ 18-20) of 304/304L is relatively low, far lower than that of 316L (PREN ≈ 24-26). It is more likely to undergo pitting corrosion at low temperatures (with higher ionic activity), especially at the crevices of flanges and valves. 

III. Limitations of Welding and Processing Properties

Welding joint embrittlement: When welding 304/304L, if proper protection is not provided, it is prone to form Widmanstätten structure or ferrite segregation, resulting in a further decrease in the toughness of the welding joint at low temperatures. Moreover, the corrosion resistance of the weld zone may be reduced due to the loss of Cr and Ni, becoming a corrosion weak point.

The influence of cold work hardening: LNG pipelines often require bending and shaping during cold processing. After cold processing, 304/304L is prone to generating residual stress, and the austenitic structure may partially transform into martensite (especially 304), leading to a deterioration in low-temperature toughness and increasing the risk of stress corrosion cracking. 

IV. Clear Limitations of Standards and Specifications

International design standards for LNG systems (such as ASME B31.3, EN 13480, API 620) have strict requirements for materials: 

The specific requirement is that the low-temperature impact toughness of the steel used for the low-temperature equipment must meet -196℃: Ak ≥ 27J (the minimum value), and 304/304L usually cannot meet this standard.

For pipelines in contact with sulfur-containing LNG, low-alloy high-strength steel resistant to SSCC or nickel-based alloys should be used. 304/304L is excluded from the mainstream selection due to its sensitivity to SSCC. 

Summary: The application scenarios of 304/304L in LNG systems are limited.

It can only be used in non-core, low-pressure, short-cycle auxiliary sections (such as the constant-temperature accessory connection pipes of LNG storage tanks, and the instrument pipelines away from the low-temperature zone), and it needs to strictly control the purity of the medium (without H₂S, Cl⁻) and the working temperature (not lower than -100℃).

The core systems (such as cryogenic storage tanks, main transportation pipelines, pump and valve connection pipes) must use dedicated cryogenic steel (such as 9% Ni steel) or high-alloy stainless steel (such as 316L, 304Lmod) to ensure safety and reliability.