Liquid nitrogen is -196℃, and liquid oxygen and liquid argon also fall in this range. Carbon steel will 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 forms carbides at grain boundaries, reducing toughness and corrosion resistance, and also causes 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 becomes brittle at low temperatures and becomes a crack initiation site. Therefore the ferrite content in the weld seam must be controlled (generally desired to be low; specific limits according to project technical specifications).
What to look for in acceptance
- Low-temperature impact test: Charpy V-notch, measuring impact energy at the service temperature (e.g. -196℃). This is the most critical acceptance item for cryogenic equipment; room-temperature mechanical properties alone are not sufficient;
- Chemical composition: Confirm it is an L grade (C ≤ 0.03%);
- Weld seam ferrite: Measured with a ferrite meter or metallographic method to check whether it is within the range permitted by 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 demanding applications), but pay attention to distortion 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℃ and 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 requirements;
- Only performing room-temperature tensile testing and not low-temperature impact testing — the core of cryogenic equipment acceptance is precisely impact testing;
- Weld seam ferrite too high (wrong filler metal or poor shielding), becoming a crack initiation site at low temperatures;
- Ignoring medium corrosion: liquid oxygen systems are extremely sensitive to oil contamination (safety risk), and 316L is more reliable;
- Support structure and piping materials not matching, causing additional stress due to differences in thermal expansion and contraction.
Tell us the medium, minimum temperature, volume and applicable standard, and we will provide grade and filler metal recommendations for cryogenic service, and can provide impact test data at the corresponding temperature — the cost of cryogenic equipment lies almost entirely in "verification"; the material itself is not the major part.
