1. What are the main material grades covered by the ASTM A691 standard?
Answer:
ASTM A691 covers the following key material grades (listed in ascending order of alloy content):
Carbon Steel: Grade 1 (CM65)
Carbon-Molybdenum Steel: Grade 2 (C½Mo)
Chromium-Molybdenum Steel:
Grade 11 (1¼Cr-½Mo)
Grade 12 (1Cr-½Mo)
Grade 22 (2¼Cr-1Mo)
High-Alloy Steel:
Grade 5 (5Cr-½Mo)
Grade 9 (9Cr-1Mo)
Grade 91 (Improved 9Cr-1Mo-V-Nb)
2. What are the key differences between the different material grades?
Answer:
Compositional differences:
Carbon steel (CM65) contains ≤0.30% C and is unalloyed;
Chromium-Molybdenum steel (such as Grade 22) contains 2.25% Cr and 1% Mo, significantly improving its high-temperature oxidation resistance. Applicable Temperature:
Grade 1 is suitable for temperatures ≤ 450°C;
Grade 91 can be used in supercritical power plants ≤ 600°C.
3. How should the material grade be selected?
A: The selection should be based on a comprehensive consideration of the following factors:
Design Temperature/Pressure: For high temperature and high pressure (e.g., > 500°C), Grade 91 is preferred.
Corrosive Environment: For sulfur-containing media, Grades 5/9 are recommended.
Cost Consideration: Carbon steel (Grade 1) has the lowest cost but poor temperature resistance.
4. Will the latest version (2025) include new material grades?
A: As of August 2025, ASTM A691-23 has not added any new grades, but has revised the following:
Grade 91 has stricter impact energy requirements (≥ 40J at -29°C);
Added optional digital material certificates with blockchain traceability support.
5. How do the material grades compare with those in other standards (e.g., ASTM A335)? Answer:
Similarities: Grades 22/91 and P22/P91 share the same composition as A335.
Differences:
A691 is designed specifically for electric fusion welded pipes and requires more stringent weld inspection.
A335 is suitable for seamless pipes and has no welding process restrictions.
