ASTM A691 Electric Fusion Welded Alloy Steel Pipe For High-Pressure Service

ASTM A691 electric fusion welded alloy steel pipe is manufactured from pressure-vessel-quality alloy steel plate with filler metal added during welding. It is designed primarily for high-pressure piping systems operating at elevated temperatures where large diameter, controlled Cr-Mo chemistry, weld integrity, and specified post-weld heat treatment are required.

ASTM A691/A691M nominally covers pipe with an outside diameter of 400 mm and above and wall thickness up to 75 mm. Other dimensions may be supplied when all applicable requirements of the specification are satisfied. Common Cr-Mo pipe grades include 1CR, 1 1/4CR, 2 1/4CR, 5CR, 9CR and Grade 91, with the pipe grade determined by the ASTM pressure-vessel plate used for manufacture.

Specifications

Main Characteristics

1. Large-Diameter High-Pressure Construction

ASTM A691 is specifically structured for electric-fusion-welded pressure pipe manufactured from pressure-vessel-quality plate. Its nominal dimensional scope begins at approximately 400 mm outside diameter and extends to wall thicknesses of 75 mm, making it particularly suitable for large-bore high-temperature pressure piping where seamless production becomes less practical.

2. Cr-Mo Alloy Selection Based on Service Temperature

The A691 grade system includes multiple chromium-molybdenum levels. Representative plate relationships include:

1CR: ASTM A387 Grade 12, approximately 1% Cr-0.5% Mo

1 1/4CR: ASTM A387 Grade 11, approximately 1.25% Cr-0.5% Mo

2 1/4CR: ASTM A387 Grade 22, approximately 2.25% Cr-1% Mo

5CR: ASTM A387 Grade 5, approximately 5% Cr-0.5% Mo

9CR: ASTM A387 Grade 9, approximately 9% Cr-1% Mo

Increasing chromium generally improves oxidation resistance, while molybdenum contributes to elevated-temperature strength and resistance to creep deformation.

3. Defined Heat-Treatment and Inspection Class System

ASTM A691 does not use the pipe class number as a material grade. The class identifies the manufacturing controls applied after welding.

Classes 10-13: No pipe heat treatment.

Classes 20-23: Stress relieved.

Classes 30-33: Normalized.

Classes 40-43: Normalized and tempered.

Classes 50-53: Quenched and tempered.

The second digit defines additional examination requirements. Classes ending in 1 or 2 include radiographic examination of the weld, while classes ending in 2 or 3 include the specified pressure test. For example, Class 22 represents stress-relieved pipe with radiographic examination and pressure testing.

Our Advantages

1. Class-Controlled Weld Quality for Critical Pressure Service

For high-pressure piping, buyers commonly need clear confirmation of whether the longitudinal weld has received full radiographic examination and whether each required pipe length has been pressure tested. ASTM A691 resolves this through its class designation system.

For critical service, classes ending in X2 can be specified where both radiographic examination and pressure testing are required. This provides a clearly defined procurement basis instead of relying on a generic statement such as "NDT tested."

2. Traceable Plate-to-Pipe Material Control

A691 alloy grade selection starts with pressure-vessel-quality plate rather than a generic alloy description. Each Cr-Mo pipe grade is connected to a defined ASTM plate specification and plate grade.

This permits material traceability from incoming plate heat number through welding, heat treatment, dimensional inspection and final testing. Procurement documents can therefore identify the A691 pipe grade, corresponding plate specification, pipe class and supplementary examination requirements separately.

3. Dimensional Control Suitable for Large-Bore Fabrication

Large-diameter EFW pipe can create fit-up problems when ovality, alignment and diameter are insufficiently controlled. ASTM A691 establishes an outside-diameter variation of ±0.5%, maximum out-of-roundness of 1%, and alignment deviation limited to approximately 3 mm over a 3 m straightedge.

These dimensional controls are important when pipes are prepared for circumferential welding into high-pressure headers, process piping assemblies and large-bore fabricated systems.

Execution Standards

ASTM, EN, JIS, GOST and GB specifications are not automatically interchangeable. Material substitution should be verified against chemistry, mechanical properties, heat treatment, welding route, allowable stress, NDE requirements and the governing pressure design code.

Application

Main Steam Header in a Thermal Power Plant

ASTM A691 Cr-Mo electric fusion welded pipe can be used for a large-diameter main steam header carrying high-pressure, high-temperature steam from boiler or superheater sections into the power-generation steam piping system. Grades such as 1 1/4CR or 2 1/4CR are selected according to the design temperature, pressure, allowable stress and project piping code.

Chemical Composition

Mechanical Properties

Because ASTM A691 Cr-Mo pipe grades are defined by the pressure-vessel plate used for manufacture, the base-material mechanical properties are tied to the applicable ASTM A387/A387M plate strength class. The table below gives representative Class 2 plate requirements for five common A691 Cr-Mo grades.