Choosing Alloys for Cryogenic Applications

## The Cryogenic Challenge BCC metals (carbon steel, ferritic stainless) undergo a ductile-to-brittle transition (DBTT). FCC metals (aluminum, copper, austenitic stainless) do NOT have a DBTT and retain ductility to absolute zero. ## Alloy Selection by Temperature ### Down to -46 degrees C: Normalized Carbon Steel **SA-516 Grade 70 (Normalized)**: Adequate when impact tested per ASME UCS-66. ### Down to -100 degrees C: 3.5% Nickel Steel **SA-203 Grade D**: Charpy 27 J at -100 degrees C. Used for ethylene storage (-104 degrees C). ### Down to -165 degrees C: 9% Nickel Steel **SA-553 Type I**: The standard for LNG storage tanks worldwide. 690 MPa tensile, 100+ J impact at -196 degrees C. Requires nickel alloy weld filler (not matching ferritic filler). ### Down to -269 degrees C: FCC Metals **304L and 316L stainless**: No DBTT. 150+ J impact at -196 degrees C. **Aluminum 5083-O**: FCC structure, excellent cryogenic toughness. Actually strengthens at low temperature (290 to 434 MPa tensile). Used for LNG ship cargo tanks. **Invar 36 (36% Ni)**: Ultra-low CTE (1.2 ppm/degree C). Used for LNG membrane tanks where dimensional stability during cooling to -163 degrees C is critical. ## Welding Considerations For 9% Ni steel, nickel alloy filler (ERNiCrMo-6, Inconel 625 type) is mandatory because matching ferritic filler cannot achieve sufficient cryogenic toughness. ## Common Mistakes - Using carbon steel below -29 degrees C without impact testing - Specifying matching 9Ni weld filler (must use Ni-alloy filler) - Selecting duplex stainless for cryogenic service (ferrite has DBTT; limit to -50 degrees C) - Ignoring thermal contraction in piping design (30 m of stainless at -196 degrees C contracts by approximately 33 mm)