1. Standard Specification and Process Definition
ASTM A691 is a high-temperature and high-pressure pipeline standard developed by the American Society for Testing and Materials, designed for carbon steel and chromium-molybdenum alloy steel pipes manufactured by the electric fusion welding (EFW) process.
EFW (Electrofusion Welding) uses an electric arc to melt the material to form a weld. There is no additional pressure during the whole process. The inner wall of the weld is smooth and has excellent sealing. It is suitable for high-pressure scenarios (working pressure 0.6-1.6MPa).
2. Material composition and grade system
Alloy type: Covering carbon steel to 9% chromium-molybdenum alloy steel (such as 1Cr, 5Cr, 9Cr), containing strengthening elements such as molybdenum (0.45%-1.5%), vanadium, and niobium.
Grade identification: named "A691+chromium content+grade" (such as A691 5Cr CL22), and the CL suffix represents the heat treatment and inspection grade requirements.
3. Key technologies of manufacturing process
Forming technology: JCOE (progressive bending) or UOE (compression molding) is used, and the roundness accuracy is controlled at ±0.5mm.
Welding and post-processing: Double-wire submerged arc welding (SAW) or high-frequency welding (HFW) to ensure double-sided penetration welds.
After welding, normalizing (920℃±10℃) + tempering (680-720℃) heat treatment must be performed to optimize the metallographic structure and eliminate stress.
Quality inspection: 100% radiographic testing (RT) and ultrasonic testing (UT), impact toughness must meet ≥54J at -20℃.
Water pressure test reaches 1.5 times the design pressure, and the pressure is maintained for ≥30 minutes.
Main application areas
1. Energy and power fields
Supercritical boiler pipelines: high-pressure pipeline systems with steam temperature ≥550℃ (such as main steam pipes with wall thickness ≤150mm).
Nuclear power plant auxiliary system: cooling water pipelines outside the nuclear island, which must meet the requirements of -40℃ low-temperature impact toughness.
2. Petrochemical field
Hydrogenation reactor pipelines: withstand sulfur-containing oil and gas environments with H₂S partial pressure ≥0.01MPa, and corrosion rate as low as 0.03mm/year.
Catalytic cracking unit: high-temperature (≤650℃) high-pressure reactor connecting pipelines.
3. Industrial transportation system
High-pressure gathering and transportation trunk lines: long-distance oil and gas transportation pipelines with pressure ≥15MPa.
Key points for project selection and acceptance
Grade matching:
For low temperature conditions (below -29°C), CL22 grade (such as A691 CM-65 CL22) is preferred to ensure impact toughness.
For high temperature scenarios (＞550°C), high chromium alloys (such as 5Cr/9Cr) must be selected.
Document verification:
It is necessary to request a heat treatment report (normalizing + tempering temperature curve) and a non-destructive testing certificate (in accordance with ASME SE94 standard).
Welding compliance:
The construction party must hold a special equipment manufacturing license, and environmental parameters (temperature, humidity, voltage) must be recorded and archived.
