ASTM A333 Gr.3 and Gr.6 are two commonly used low-temperature carbon steel pipes, with main differences in chemical composition, mechanical properties, applicable temperature range, and corrosion resistance. The following is a detailed comparison:
Chemical Composition
Carbon Content: Gr.3 carbon content ≤0.19%, Gr.6 ≤0.30%. Gr.6 allows for higher carbon content to improve strength, but requires heat treatment to optimize toughness.
Alloying Elements: Gr.6 contains trace amounts of nickel (≤0.40%), molybdenum (≤0.12%), etc., while Gr.3 contains none or lower amounts. Since 2016, Gr.6 has added elements such as Cr, V, and Nb to further enhance its low-temperature performance.
Mechanical Properties
Tensile Strength: Gr.6 ≥415 MPa, Gr.3 ≥450 MPa.
Gr.3, due to its low-carbon design, has slightly higher strength but superior toughness.
Impact Toughness: Gr.6 has an impact energy ≥18J at -45°C, while Gr.3 maintains ≥18J at -100°C.
Gr.3 is suitable for even lower temperature environments (such as liquid nitrogen storage tanks).
Applicable Temperature Range
Gr.6: -45°C to -195°C (e.g., petrochemical pipelines, natural gas transportation)
Gr.3: -100°C to -195°C (ultra-low temperature equipment such as LNG storage tanks)
Corrosion Resistance and Application Scenarios
Gr.6: Due to its nickel and molybdenum content, its corrosion resistance is superior to Gr.3, making it suitable for corrosive environments such as chemical and oil/gas industries.
Gr.3: Primarily used in purely cryogenic environments, such as nuclear power plant cooling systems.
Heat Treatment and Process
Gr.6: Requires normalizing (≥815°C) or normalizing + tempering to refine grains and improve toughness.
Gr.3: Usually only requires normalizing; the process is simpler.
Summary
The differences between the two are significant: Gr.6 emphasizes strength and corrosion resistance, suitable for medium- and low-temperature corrosive environments; Gr.3 is designed for ultra-low temperatures, offering superior toughness but slightly lower strength.