What is ASTM A691 Pipe?

ASTM A691 is a specification covering carbon and alloy steel pipes, electric-fusion-welded (EFW) with added filler metal. These pipes are fabricated from pressure vessel quality plate for high-pressure service at high temperatures.

The EFW process makes pressure vessel plates from either ASTM A387 (alloy) or A516 (carbon). Using submerged arc welding with filler metal, plates are rolled into cylinders and then welded longitudinally. This results in a strong weld that can handle tough service conditions.

Pipes are classified by steel grade and designated in classes based on heat treatment, radiographic examination and pressure testing. The class system reflects the level of quality and the tests that need to be done.

Key applications include high-pressure steam lines, power plants, refineries and petrochemical plants. A691 EFW pipes serve as equivalent to seamless A335 grades for welded applications in large diameters.

ASTM A691 Pipe Grades

ASTM A691 covers two main grade categories: Carbon-Molybdenum (CM) and Chrome-Molybdenum (Cr) grades. Each grade corresponds to a specific plate material such as ASTM A204 for CM grades and A387 for Cr grades.

ASTM A691 CM65, CM70, CM75 (Carbon-Molybdenum)

This is a carbon-molybdenum steel grade for moderate temperature service. It is made from ASTM A204 Grade A (CM65), B (CM70), C (CM75) plates. Tensile strength ranges from 450-515 MPa. Molybdenum addition provides creep resistance. Applications include pressure vessels and moderate temperature piping systems.

ASTM A691 1-1/4Cr (Grade 11) Chrome Moly Pipe

ASTM A691 1-1/4Cr pipe is a 1.25% chromium, 0.5% molybdenum alloy steel. It is made from ASTM A387 Grade 11 plate that is rolled into a pipe and then welded. Equivalent to seamless A335 P11 pipe. Tensile strength is 415 MPa minimum. Applications include power plant steam lines and boiler piping operating up to 600°C.

ASTM A691 2-1/4Cr (Grade 22) Chrome Moly Pipe

ASTM A691 2-1/4Cr pipe is a 2.25% chromium, 1% molybdenum alloy steel. It is made from ASTM A387 Grade 22 plate. Equivalent to seamless A335 P22 pipe. Tensile strength is 415 MPa minimum. Applications include high-temperature power plant piping and refinery heaters.

ASTM A691 5Cr (Grade 5) Chrome Moly Pipe

ASTM A691 5Cr pipe is a 5% chromium, 0.5% molybdenum alloy steel. It is made from ASTM A387 Grade 5 plate. Equivalent to seamless A335 P5 pipe. It has enhanced oxidation and corrosion resistance. Applications include refinery service and high-temperature hydrocarbon processing where sulfur attack is a concern.

ASTM A691 9Cr (Grade 9) Chrome Moly Pipe

ASTM A691 9Cr pipe is a 9% chromium, 1% molybdenum alloy steel. It is made from ASTM A387 Grade 9 plate. Equivalent to seamless A335 P9 pipe. Air-hardenable steel requires careful heat treatment and preheat for welding. Applications include high-temperature refinery service and hydrogen environments.

ASTM A691 Grade 91 (9Cr-1Mo-V) Pipe

ASTM A691 Grade 91 pipe is a modified 9Cr-1Mo with vanadium and niobium additions for enhanced creep strength. It is made from ASTM A387 Grade 91 plate. Equivalent to seamless A335 P91 pipe. Tensile strength is 585 MPa minimum. Applications include ultra-supercritical power plants and advanced steam cycles operating above 600°C.

IBR Approved ASTM A691 EFW Pipes

IBR (Indian Boiler Regulations) certification is mandatory for ASTM A691 pipes used in power plants and boiler applications in India. IBR approval ensures safety, quality and legal compliance for pressure piping systems.

Chhajed Steel Corporation supplies IBR certified A691 pipes with Form 3A/3D documentation. Third-party inspection by government-approved agencies is available. We work with NABL approved laboratories for testing and certification.

Difference Between ASTM A691 and ASTM A335

The main difference between A691 and A335 lies in the manufacturing process:

Aspect ASTM A691 ASTM A335
Manufacturing EFW (Welded) Seamless
Size Range 16" and larger 1/2" to 26"
Wall Thickness Up to 3" (75mm) Per schedule
Raw Material Pressure vessel plate Billet/Round
Weld Seam Longitudinal None
Equivalence A691 2-1/4Cr = A335 P22 -

A691 EFW is cost-effective alternative to seamless for large diameter pipes. Both meet the same design stress requirements when properly specified.

Quality Testing & Certification

As per ASTM A691, the following tests are required:

  • Tensile Test: On plate and weld metal to verify strength properties
  • Guided Bend Test: On weld specimens – face and root bends
  • Radiographic Examination: Required for Class 22, 32, 42, 52 – full length of welds
  • Hydrostatic Test: Each pipe tested to specified pressure
  • Hardness Test: For heat-treated grades, per specification limits
  • Impact Test: When specified, CVN testing on weld and HAZ

Available certifications:

  • IBR Certification (Form 3A/3D)
  • Govt. Lab Test Certificate (NABL Approved)
  • EN 10204/3.1B Material Test Certificate (MTC)
  • Third Party Inspection Report (TPI)
  • 100% Radiography Test Report
  • PWHT Chart & Certificate

ASTM A691 Pipe Applications

ASTM A691 EFW pipes are used extensively in high-temperature, high-pressure applications. These pipes find applications in the following 5 industries:

Power Generation Plants

ASTM A691 pipe is used in main steam lines, hot reheat piping, boiler headers and superheater connections. Grade 91 is specified for ultra-supercritical plants operating above 600°C. Large diameter headers use A691 for cost-effective fabrication.

Refineries & Petrochemical Plants

This pipe is used in process heaters, reformer piping, cracking unit headers and high-temperature hydrocarbon service. 5Cr and 9Cr grades are preferred for hydrogen environments where sulfidation and hydrogen attack resistance are critical.

High-Pressure Steam Systems

Steam headers, main steam piping, HP/IP turbine connections use ASTM A691 pipe. 1-1/4Cr and 2-1/4Cr grades handle conventional steam temperatures up to 565°C with excellent creep resistance.

Chemical Processing

This pipe is used in reactor piping, heat exchanger headers, and high-temperature process lines. Various grades are selected based on temperature and corrosion requirements of specific chemical processes.

Pressure Vessel Connections

ASTM A691 alloy steel EFW pipes are used in nozzle connections, shell penetrations and header connections. Material matches vessel plate for compatible weldability and similar thermal expansion characteristics.

Frequently Asked Questions

  • What is ASTM A691 pipe used for?
    ASTM A691 pipes are used for high-pressure, high-temperature service in power plants, refineries and petrochemical plants. Common applications include main steam lines, boiler headers, and process piping in thermal power stations.
  • What is the difference between ASTM A691 and A335?
    ASTM A691 covers EFW (Electric Fusion Welded) pipes while A335 covers seamless pipes. A691 is used for large diameter pipes (16" and above) as a cost-effective alternative to seamless. Both have equivalent grades (e.g., A691 2-1/4Cr = A335 P22).
  • What does Class 22 or Class 42 mean in ASTM A691?
    The class designation indicates quality level. Classes 22, 32, 42, 52 require radiographic examination of welds. Higher class numbers indicate killed steel with higher tensile strength. Class 56 requires quenching and tempering heat treatment.
  • Is ASTM A691 Grade 91 suitable for supercritical boilers?
    Yes, ASTM A691 Grade 91 (9Cr-1Mo-V) is specifically designed for ultra-supercritical and supercritical power plants. It has high creep strength up to 600°C and is commonly used in main steam and hot reheat piping systems.
  • What plate material is used for ASTM A691 2-1/4Cr pipe?
    ASTM A691 2-1/4Cr (Grade 22) pipes are fabricated from ASTM A387 Grade 22 pressure vessel plate. This plate contains 2.00-2.50% chromium and 0.90-1.10% molybdenum for elevated temperature service.