BS 6323-7 Submerged Arc Welded Steel Tubes for Automobile and Mechanical Engineering

BS 6323-7 submerged arc welded steel tubes are longitudinally welded carbon steel tubes intended for automobile, mechanical, and general engineering applications. The standard specifically applies to submerged arc welded tubes with an outside diameter of 150 mm and above and defines two material grades: SAW 4 and SAW 5.

The tubes are manufactured by longitudinal butt welding preformed steel plate or skelp using an automatic submerged arc welding process. They may be supplied as-welded, hot sized, or cold sized. For cold-sized tubes, heat treatment may be applied when required.

BS 6323-7:1982 is a historical British Standard and has been withdrawn. For new projects, purchasers should confirm whether the original BS 6323-7 specification is contractually required or whether the replacement BS EN 10296-1 should be used.

Specifications

Main Features

Controlled Large-Diameter Geometry

BS 6323-7 is specifically intended for tubes of 150 mm outside diameter and above. Standard-listed commonly used dimensions extend to 2020 mm OD, making the specification suitable for relatively large welded mechanical tube sections.

The outside diameter tolerance is controlled to ±0.75% or ±10 mm, whichever produces the smaller permitted deviation. Wall thickness is controlled to +10% and -5%, apart from external reinforcement at the weld.

Defined Tensile Strength Levels

SAW 4 has a minimum yield strength of 235 MPa and a minimum tensile strength of 410 MPa.

SAW 5 provides a higher minimum yield strength of 340 MPa and a minimum tensile strength of 490 MPa.

This distinction allows designers and purchasers to select the strength level according to the mechanical loading requirements of the finished component.

Submerged Arc Weld Integrity

The tube is produced by automatically submerged arc welding a longitudinal butt joint in preformed plate or skelp. The process is particularly suitable for comparatively large-diameter and heavier-wall tubing because welding parameters can be controlled continuously along the longitudinal seam.

In addition to tensile testing, BS 6323-7 requires a strip bend test. The bend-test bar diameter is six times the thickness of the test piece, and the specimen must not show significant cracking or flaws.

Our Advantages

1. Control of Weld-Seam Quality for Subsequent Fabrication

One of the main concerns when purchasing large-diameter SAW mechanical tubing is whether the longitudinal weld will tolerate cutting, welding, bending, and fabrication without developing defects.

Production can therefore be controlled around weld alignment, heat input, weld reinforcement, penetration, and dimensional stability. BS 6323-7 additionally requires a strip bend test, providing a direct method of evaluating weld-zone ductility and soundness.

For critical orders, supplementary NDT such as ultrasonic examination of the longitudinal weld can be specified separately in the purchase order.

2. Dimensional Control for Assembly and Machining

Large-diameter welded tubes can create assembly problems when ovality, wall variation, straightness, or cut-length deviation is excessive.

BS 6323-7 provides defined OD, thickness, and length tolerances, while BS 6323-1 establishes a general straightness limit of 1.5 mm per metre. Tighter tolerances can be agreed when tubes are intended for machining, fitted mechanical assemblies, or controlled welding fixtures.

Dimensional inspection can include OD, circumference, wall thickness, straightness, length, and weld-seam geometry according to the purchaser's inspection plan.

3. Clear Grade and Inspection Traceability

Professional purchasers frequently need confirmation that the tube supplied corresponds to the ordered grade rather than only to a commercial carbon-steel description.

Orders can therefore be identified by BS 6323-7 grade, heat number, dimensions, delivery condition, and inspection requirements. Chemical composition and tensile properties can be verified against the corresponding SAW 4 or SAW 5 requirements.

Where project documentation requires modern specifications, the purchaser should state this before production because BS 6323-7 has been withdrawn and replaced for comparable mechanical-engineering applications by BS EN 10296-1.

Execution Standards

These standards should not be treated as automatically interchangeable. Grade equivalence, welding process, dimensions, testing requirements, and delivery condition should be checked separately before substitution.

Application

Heavy-Duty Automotive Drive-Shaft Housing

One specific application is a large mechanical tubular housing used in heavy-duty automotive driveline assemblies.

The tube provides the cylindrical structural body required to transmit or support mechanical loads while maintaining controlled diameter, straightness, and wall thickness for subsequent machining, welding, or assembly. SAW 5 can be selected where a higher minimum yield strength is required than SAW 4.

Final suitability should be confirmed from the actual component load calculation, fatigue requirement, weld design, and applicable vehicle engineering specification.

Chemical Composition

BS 6323-7 specifies only two steel grades, SAW 4 and SAW 5. Therefore, adding five grades would require mixing material specifications from unrelated standards and could mislead purchasers. The table below intentionally contains only the two grades formally specified by BS 6323-7.

Mechanical Properties