What Is ASTM A691? High-Pressure EFW Pipe
ASTM A691 is the standard specification for carbon and alloy steel pipe manufactured by the electric fusion welding (EFW) process for high-pressure service at high temperatures. The specification covers chrome-moly grades (1Cr through 9Cr) that operate in creep-range conditions, making it the primary EFW pipe standard for power plant steam lines, refinery heater transfer lines, and high-temperature process piping.
The pipe is produced from alloy steel plate (ASTM A387 grades) formed into cylinders and welded longitudinally using submerged arc welding (SAW) with filler metal. ASTM A691 differs from ASTM A672 in that it covers alloy steels designed for elevated-temperature service rather than carbon steels for moderate temperatures.
| Property | ASTM A691 |
|---|---|
| Pipe type | EFW (Electric Fusion Welded) |
| Welding process | SAW with filler metal |
| Base plate spec | ASTM A387 |
| Service | High-pressure, high-temperature |
| Temperature range | Up to 593°C (1100°F) |
| Size range | NPS 16 and larger (typically) |
| Classes | 10, 12, 20, 22, 30, 32, 40, 42 |
| NDT | Per class: RT and/or UT on weld seam |
| PWHT | Required for Classes 30+ |
Common Grades
ASTM A691 grades derive directly from ASTM A387 plate grades. The grade designation indicates the chromium content.
| Grade | Material | Plate Spec | MDMT | Max Temp |
|---|---|---|---|---|
| 1CR | 1Cr-0.5Mo | A387 Gr. 12 | -29°C | 593°C |
| 1.25CR | 1.25Cr-0.5Mo | A387 Gr. 11 | -29°C | 593°C |
| 2.25CR | 2.25Cr-1Mo | A387 Gr. 22 | -29°C | 593°C |
| 3CR | 3Cr-1Mo | A387 Gr. 21 | -29°C | 593°C |
| 5CR | 5Cr-0.5Mo | A387 Gr. 5 | -29°C | 593°C |
| 9CR | 9Cr-1Mo-V (P91) | A387 Gr. 91 | -29°C | 593°C |
| 91 | 9Cr-1Mo-V-Nb | A387 Gr. 91 | -29°C | 649°C |
Class System
The class suffix determines the level of testing, heat treatment, and examination required.
| Class | Radiography | PWHT | Hydro Test |
|---|---|---|---|
| 10 | None | None | Yes |
| 12 | None | None | Yes |
| 20 | Spot RT | None | Yes |
| 22 | Spot RT | Yes | Yes |
| 30 | 100% RT | None | Yes |
| 32 | 100% RT | Yes | Yes |
| 40 | 100% RT | None | Yes |
| 42 | 100% RT | Yes | Yes |
For critical high-temperature piping, Class 42 is the standard requirement in most EPC specifications. It combines 100% radiographic examination with mandatory PWHT and hydrostatic testing.
Applications
- Superheater and reheater piping in thermal power plants
- High-temperature transfer lines in refineries
- Reactor effluent piping in hydrocracking and hydrotreating units
- Main steam and hot reheat piping (Grade 91 for supercritical plants)
- Large-diameter alloy headers where seamless pipe is unavailable
ASTM A691 fills a critical niche: large-diameter alloy pipe for high-temperature, high-pressure service where seamless manufacturing is impractical or unavailable above NPS 24.
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