BS 6323-6 Standard DOM Steel Tube with 6-350mm OD and CEW1a-CEW5a Grades for Automotive Applications

Cold-finished electric resistance welded drawn-over-mandrel steel tubing is manufactured from ERW mother tube and subsequently cold drawn through a die and over a mandrel. This process refines the outside diameter, inside diameter, wall thickness, roundness and surface finish while reducing the dimensional effects of the original weld.

Produced to BS 6323-6, the tubing is intended for automotive, mechanical and general engineering components requiring tighter dimensional control than standard as-welded ERW tubing. Available grades include CEW1a, CEW2a, CEW3a, CEW4a and CEW5a, with delivery conditions selected according to the required strength, machinability and forming performance. BS 6323-6 specifically covers cold-finished electric resistance welded, including induction-welded, steel tubes.

The commercial dimensional range of OD 6-350 mm, wall thickness 1-35 mm and lengths up to 12,000 mm is consistent with the referenced BS 6323-6 product range.

The standard identifies cold-finished hard, cold-finished soft, annealed and normalized conditions for CEW grades.

Drawing the welded mother tube through a die and over a mandrel controls both the external diameter and the bore. This makes the product suitable for components in which piston clearance, bearing fit or machining allowance depends on the actual internal diameter rather than nominal wall thickness alone.

Cold drawing corrects part of the ovality inherited from strip forming and ERW welding. Roundness can therefore be controlled more closely than on conventional as-welded tubing, reducing setup adjustments during honing, turning or centerless grinding.

The initial ERW weld is consolidated during the drawing operation. The internal weld profile becomes less pronounced, producing a more uniform bore for mechanical applications. The weld remains metallurgically present, so its quality must still be verified through process control and nondestructive testing.

The five CEW grades cover low-carbon forming grades through higher-strength carbon-manganese grades. Carbon limits range from 0.13% for CEW1a to 0.25% for CEW4a, while CEW5a permits manganese up to 1.50%.

BK and BKW conditions retain different levels of cold-working strength, while GBK and NBK conditions improve ductility and dimensional stability. The delivery condition should therefore be stated on the purchase order rather than specifying only the steel grade.

A common procurement problem is ordering only by outside diameter and nominal wall thickness, although the component is assembled or honed according to its inside diameter.

We can define the tube by:

• OD × wall thickness
• OD × ID
• OD tolerance plus minimum wall thickness
• finished bore size with honing allowance

This prevents accumulated OD and wall-thickness tolerances from producing an unacceptable bore.

A tube ordered in BK condition may provide high strength but insufficient ductility for severe bending, flaring or end forming. Conversely, unnecessary annealing can reduce strength and add processing cost.

The delivery condition can be selected according to the downstream operation:

• BK for machining and high cold-worked strength
• BKW for moderate forming
• GBK for improved ductility
• NBK for stable mechanical properties and more uniform microstructure
• GZF or NZF when a scale-free surface is required

A tube can meet tensile requirements but still create production problems because of excessive eccentricity, local weld-area hardness or inconsistent straightness.

Inspection can therefore combine:

• eddy-current testing of the weld and tube body
• OD, ID and wall-thickness mapping
• ovality and eccentricity measurement
• weld-area hardness checks
• straightness inspection
• lot-based tensile, flattening or flaring tests

This provides evidence that both the material and the finished tube geometry are suitable for the intended component.

The following maximum ladle-analysis limits are based on the CEW grade table in BS 6323-6. A dash indicates that the standard does not specify a separate silicon limit for that grade.

• CEW1a: lowest carbon content; preferred for forming and lower-strength components.
• CEW2a: low-carbon grade with a moderate increase in strength.
• CEW3a: general-purpose medium-strength mechanical grade.
• CEW4a: higher carbon and manganese limits for increased strength.
• CEW5a: higher manganese grade for components requiring greater normalized strength.

Mechanical properties depend on the specified delivery condition. The table below gives the minimum normalized-condition values shown for the five CEW grades, allowing a consistent comparison between grades. The purchase order and inspection certificate should state the actual delivery condition because BK, BKW, GBK and NBK values are not interchangeable.