ASTM A671 vs A672 vs A691

Quick Answer

ASTM A671, A672, and A691 all cover electric fusion welded (EFW) pipe made from steel plate. The difference is the service condition: A671 covers atmospheric and low-temperature service, A672 covers high-pressure moderate-temperature service, and A691 covers high-pressure high-temperature service using chrome-moly alloy steels.

All three specifications use the same manufacturing process: steel plate is formed into a cylinder and welded longitudinally with filler metal, typically by submerged arc welding (SAW). The specifications share an identical class system (Classes 10 through 42+) that defines the level of radiographic examination, heat treatment, and testing.

Side-by-Side Comparison

Feature	ASTM A671	ASTM A672	ASTM A691
Service	Atmospheric and lower temps	High pressure, moderate temps	High pressure, high temps
Temperature range	Down to -46°C	-29°C to 343°C	Up to 593°C
Base plate	A285, A515, A516, A537	A285, A515, A516, A537	A387 (Cr-Mo alloy)
Material type	Carbon steel	Carbon steel	Chrome-moly alloy steel
Typical grades	CB60, CB65, CB70	B60, B65, B70, C55, C60	1CR, 1.25CR, 2.25CR, 5CR, 9CR
Common sizes	NPS 16 to NPS 60+	NPS 16 to NPS 60+	NPS 16 to NPS 48+
PWHT	Per class (32, 42)	Per class (32, 42)	Per class (32, 42)
Hydrostatic test	Required all classes	Required all classes	Required all classes
Weld factor (E)	0.85-1.0	0.85-1.0	0.85-1.0

Grade Naming Convention

Each specification uses a different grade prefix linked to its base plate.

Specification	Grade Example	Base Plate	Explanation
A671	CB65	A516 Gr. 65	”C” = A516, “B” = killed steel, 65 = tensile class
A671	CA55	A515 Gr. 55	”C” = A515 prefix, “A” = semi-killed
A672	B60	A516 Gr. 60	”B” = killed steel, 60 = tensile class
A672	C65	A516 Gr. 65	”C” = normalized, 65 = tensile class
A691	1.25CR	A387 Gr. 11	Direct alloy designation (Cr content)

How to Select the Right Spec

The selection depends on three factors: operating temperature, pressure, and whether alloy steel is required.

Choose ASTM A671 when:

• The pipe operates at atmospheric pressure or low-pressure utility service
• The service temperature is at or below ambient, including low-temperature applications down to -46°C
• The piping is non-critical (no high-pressure design requirement)

Choose ASTM A672 when:

• The system operates at high pressure with moderate temperatures (up to 343°C)
• Carbon steel is adequate for the corrosion environment
• Applications include high-pressure water, steam, and process piping in refineries and power plants

Choose ASTM A691 when:

• The system operates at high pressure and high temperature (above 343°C)
• Chrome-moly alloy steel is required for creep resistance
• Applications include superheated steam lines, reactor effluent piping, and transfer lines

Pro Tip

All three specs are EFW pipe. For large-diameter carbon steel pipeline applications (oil and gas transmission), API 5L LSAW pipe is the standard choice, not ASTM A671/A672. The ASTM EFW specs are primarily used in process plant piping designed per ASME B31.3 or B31.1.

Class Selection

The class suffix is identical across all three specifications. For process piping designed per ASME B31.3, the joint efficiency factor (E) depends on the class:

Class	RT Requirement	PWHT	Joint Factor (E)
10, 12	None	No / No	0.85
20, 22	Spot	No / Yes	0.90
30, 32	100%	No / Yes	1.00
40, 42	100%	No / Yes	1.00

Specifying Class 32 or 42 allows a joint factor of 1.0, reducing the required wall thickness and pipe weight. For most EPC projects, Class 22 (spot RT + PWHT) is the minimum for process piping; Class 42 (100% RT + PWHT) is standard for critical service.

Read the full guide to pipe types