A333Gr6 cryogenic tubes are seamless steel tubes designed for cryogenic applications, conforming to ASTM A333/ASME SA333 standards. They are primarily used in petrochemical equipment (such as reactors, heat exchangers, and separators), natural gas processing, and cryogenic equipment manufacturing in environments ranging from -45℃ to -195℃. Common specifications include an outer diameter of 21.3-762mm and a wall thickness of 2.0-140mm. They are manufactured using hot rolling, cold drawing, or hot expansion processes and require normalizing heat treatment at 900℃~930℃ upon delivery.
Material Properties and Chemical Composition
1. Composition Design
Gr.6 uses a low-carbon steel base (C≤0.30%), achieving low-temperature stability through Ni-Cr-Mo composite microalloying. Typical compositions are:
- C: 0.19%-0.25%
- Mn: 0.70%-1.05%
- Si: 0.15%-0.35%
- Ni: 0.40%-0.70%
The addition of nickel refines the grain size and lowers the ductile-brittle transition temperature, while limiting phosphorus and sulfur content (both ≤0.025%) reduces the risk of impurity segregation.
2. Mechanical Properties
- Tensile strength ≥ 415 MPa, Yield strength ≥ 240 MPa
- Elongation ≥ 22% (reflects plastic deformation capacity)
- Hardness ≤ 197 HB (avoids cold cracking during welding)
Performance Characteristics
Low-Temperature Toughness: SA333Gr.6 seamless steel pipe possesses excellent low-temperature toughness, suitable for low-temperature pressure vessel pipelines and low-temperature heat exchanger pipelines in temperatures ranging from -45℃ to -195℃.
Corrosion Resistance: This steel pipe maintains good corrosion resistance even in extremely cold environments, without brittle fracture or other problems.
High Strength and High Toughness: Yield strength can generally reach over 200 MPa, and tensile strength can reach over 400 MPa, meeting the application requirements in various complex environments.
Key Control Points in Production Process
1. Smelting Process: An electric arc furnace + LF refining double-stage process is adopted, controlling the oxygen content to ≤ 30 ppm, combined with calcium treatment to improve sulfide morphology. A leading domestic manufacturer has reduced hydrogen content to below 1.5 ppm through vacuum degassing (VD), significantly reducing white spot defects.
2. Hot Working Technology
- Hot rolling temperature range: 900℃-1200℃; final rolling temperature: ≥850℃ to avoid mixed grains.
- Normalizing treatment (890℃±10℃) optimizes grain size to grade 7 or higher.
- ACC (Accelerated Accelerated Cooling) process improves low-temperature toughness; one case showed a 30% increase in impact energy at -60℃.
3. Welding Key Points
Recommended welding material: AWS A5.28 ER80S-Ni1; preheating temperature: 100℃-150℃; interpass temperature: ≤200℃. Special attention should be paid to post-weld hydrogen removal treatment (250℃×2h) to prevent delayed cracking.