Liquid nitrogen is -196℃, and liquid oxygen and liquid argon also fall in this range. Carbon steel will brittle-fracture like glass at this temperature, while austenitic stainless steel (300 series) has no obvious ductile-brittle transition — this is why it has become the main material for cryogenic equipment.
One-sentence judgment
For -196℃, use 304L or 316L (low-carbon grades), not ordinary 304/316. When the medium is corrosive (e.g., containing chlorine or sulfur), use 316L; for pure cryogenic storage and transport (liquid nitrogen, liquid oxygen, liquid argon), 304L is sufficient. The ferrite content in the weld seam must be controlled, and low-temperature impact testing must be performed for verification.
Why low carbon is key
Austenite has a face-centered cubic structure with many slip systems and is not prone to brittle fracture at low temperatures. But this advantage has two enemies:
- Carbon: High carbon will form carbides at grain boundaries, reducing toughness and corrosion resistance, and can also cause sensitization during welding; therefore, cryogenic equipment uses 304L / 316L (C ≤ 0.03%);
- Ferrite: If a significant amount of δ ferrite remains in austenitic stainless steel (commonly found in weld seams), the ferrite phase will become brittle at low temperatures and become a crack source. Therefore, the ferrite content in the weld seam must be controlled (usually desired to be low; specific limits according to project technical specifications).
What to inspect for acceptance
- Low-temperature impact test: Charpy V-notch, measuring impact energy at the service temperature (e.g., -196℃). This is the most core acceptance item for cryogenic equipment and cannot rely solely on room-temperature mechanical properties;
- Chemical composition: Confirm it is L grade (C ≤ 0.03%);
- Weld seam ferrite: Measure with a ferrite meter or metallographic method to see if it is within the allowable range of the technical specifications;
- Non-destructive testing: Cryogenic equipment usually requires RT/UT; crack-type defects will propagate rapidly at low temperatures.
Welding key points
The weld seams of cryogenic equipment are often the weakest link:
- Use matching low-carbon filler metals (ER308L / ER316L);
- Control heat input and interpass temperature to avoid coarse grains;
- Clean thoroughly after welding and perform pickling and passivation (oxide scale is the starting point of corrosion);
- Perform post-weld solution treatment when necessary (for high-requirement applications), but pay attention to deformation control.
Comparison with other cryogenic materials
- 9% nickel steel: Dedicated to LNG (-162℃), with high cost and process requirements, mostly used for large storage tanks;
- Aluminum alloy: Good low-temperature toughness and light weight, but low strength and significantly reduced welded joint performance;
- Austenitic stainless steel: Usable at -196℃ or even lower, corrosion-resistant, easy to process, weldable, and a common choice for small and medium-sized cryogenic equipment and piping.
Common pitfalls
- Using ordinary 304 instead of 304L: High carbon, toughness of weld seam and heat-affected zone fails to meet standards;
- Only performing room-temperature tensile testing, not low-temperature impact testing — the core of acceptance for cryogenic equipment is precisely impact testing;
- Weld seam ferrite is too high (wrong filler metal or poor protection), becoming a crack source at low temperatures;
- Ignoring medium corrosion: Liquid oxygen systems are extremely sensitive to oil contamination (safety risk); 316L is more reliable;
- Support structure and piping materials do not match, causing additional stress due to differences in thermal expansion and contraction.
Tell us the medium, minimum temperature, volume, and applicable standards, and we will provide grade and filler metal recommendations based on cryogenic conditions, and can provide impact test data at the corresponding temperature — the cost of cryogenic equipment is almost entirely in "verification"; the material itself is not the major part.
