ASTM A691 Electric-Fusion-Welded Pipe

Basic Overview

ASTM A691 is the standard specification for carbon and alloy steel pipe, electric-fusion-welded (EFW) with filler metal added, fabricated from pressure-vessel-quality plate . This specification is specifically designed for high-pressure service at high temperatures, making it essential for demanding applications in power generation, refineries, and petrochemical plants .

The designation "electric-fusion-welded" distinguishes this process from ERW (which uses no filler metal) and indicates that the pipe is manufactured by forming pressure vessel quality plates into a cylinder and welding the longitudinal seam using an electric arc process with added filler metal .

Key Features

Grade Designation System

ASTM A691 uses a two-part designation system: Grade (indicating the plate material) and Class (indicating heat treatment and inspection level) .

Grades (Plate Material)

Class Designations (Heat Treatment & Inspection)

The class numbering system follows a logical pattern where the first digit indicates heat treatment type and the second digit indicates inspection level :

Common selections for high-temperature service :

Class 22: Stress relieved, fully inspected – standard for many applications

Class 32: Normalized, fully inspected – when grain refinement is needed

Class 42: Normalized and tempered, fully inspected – for enhanced elevated-temperature properties

Chemical Composition (Typical for Common Cr-Mo Grades)

*Note: Actual values vary by plate specification. Tighter limits apply for PSL2-equivalent quality.*

Mechanical Properties (Typical for Cr-Mo Grades)

Note: Higher strength grades (e.g., 91) have higher strength requirements. Elevated temperature properties (creep strength) are the primary design consideration.

Dimensional Tolerances

Testing & Inspection Requirements

Comparison: ASTM A691 vs. A671 vs. A672

All three specifications use the same manufacturing process but serve different temperature ranges :

Common Applications

Key Advantages

Important Considerations

1. Temperature Limitations

ASTM A691 is specifically for high-temperature service (up to 593°C / 1100°F)

Different grades have different maximum temperature limits – consult applicable codes

For service above 540°C (1000°F), higher alloy grades (9CR, 91) are typically required

2. Grade Selection

½ CR, 1CR, 1¼ CR: Mild high-temperature service, good weldability

2¼ CR: Excellent creep resistance, standard for power plant piping

5CR, 9CR: Enhanced oxidation and corrosion resistance, suitable for high-sulfur environments

91: Advanced 9Cr-1Mo-V steel for ultra-high temperature applications requiring superior creep strength

3. Class Selection

The class numbering system makes selection straightforward:

First digit = heat treatment: 10 (as-welded), 20 (stress relieved), 30 (normalized), 40 (normalized & tempered), 50 (quenched & tempered)

Second digit = inspection: 0 (none), 1 (RT only), 2 (RT + hydrotest), 3 (hydrotest only)

Most common selections for high-temperature service :

Class 22: Stress relieved, fully inspected – standard for many applications

Class 32: Normalized, fully inspected – when grain refinement is needed

Class 42: Normalized and tempered, fully inspected – for enhanced elevated-temperature properties

4. Joint Efficiency Factor

For process piping designed per ASME B31.3, the joint efficiency factor (E) depends on the class :

Class 10/12: E = 0.85

Class 20/22: E = 0.90

Class 30/32/40/42/50/52: E = 1.00

Specifying Class 32 or 42 allows a joint factor of 1.0, reducing required wall thickness and pipe weight.

5. Code Compliance

ASME Code: Often adopted for Boiler and Pressure Vessel Code applications

Consult applicable construction codes (ASME B31.1 Power Piping, B31.3 Process Piping) for acceptance and design factors

6. Quality Assurance

Mill Test Certificate: EN 10204 Type 3.1 or 3.2 typically required for critical applications

Ensure all required testing (RT, hydrotest, impact, PMI) per specified Class is documented

PWHT (Post-Weld Heat Treatment) curves and records required for verification

Summary

ASTM A691 EFW pipe is the standard specification for large-diameter, electric-fusion-welded carbon and alloy steel pipe fabricated from pressure vessel quality plates for high-pressure service at high temperatures. It is distinguished by its Grade (indicating the plate material, e.g., 2¼ CR from A387 Grade 22 for elevated temperature service) and Class (indicating heat treatment and inspection level, e.g., Class 22 for stress-relieved with radiography and hydrotest) .

Available in diameters from 16 inches (400 mm) to over 48 inches with wall thicknesses up to 75 mm, A691 pipes are manufactured using double-sided submerged arc welding (DSAW) with filler metal added. The Class system allows engineers to select the appropriate combination of heat treatment and inspection based on service criticality, with higher classes achieving a joint efficiency factor of 1.0 .

When to specify ASTM A691:

The system operates at high pressure and high temperature (above 343°C / 650°F)

Chrome-moly alloy steel is required for creep resistance and oxidation resistance

Applications include superheated steam lines, refinery heater transfer lines, and high-temperature process piping

Large diameters and thick walls are required where seamless pipe is not feasible

When ordering, specify: ASTM A691, Grade [e.g., 2¼ CR], Class [e.g., 22], Size (OD x WT), Length, End Finish, Heat Treatment, and any supplementary requirements .