BS 7668: Weldable Structural Steel in Hot Finished Hollow Sections — Grade WR 355

Quick Answer

BS 7668:1994 is the BSI standard for weldable structural steel in the form of hot-finished hollow sections with enhanced atmospheric corrosion resistance — colloquially known as weathering steel. The standard specifies a single grade, WR 355, which corresponds to a yield-strength class of 355 MPa with a defined chemistry of copper, chromium, and nickel additions that drive the formation of a tightly adhering, self-sealing rust patina on the exposed surface.

BS 7668 is the hollow-section counterpart to EN 10025-5 (which covers weathering steel flat and long products). It references EN 10210-1 for the general delivery conditions of hot-finished hollow sections. The standard is maintained by BSI and remains the UK reference for architects and structural engineers specifying exposed weathering steel tubes and sections.

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Scope and Applicability

BS 7668 applies to:

• Hot-finished structural hollow sections: circular (CHS), rectangular (RHS), and square (SHS)
• Grade WR 355 only
• Sections intended for use in exposed environments where the patina can form and stabilise without regular immersion or burial

The standard does not apply to:

• Cold-formed hollow sections (no weathering steel equivalent in BS/EN cold-formed standards)
• Hollow sections used underground, permanently submerged, or in enclosed non-aerated environments (patina cannot form; no corrosion benefit)
• Weathering steel flat products (covered by EN 10025-5, grades S235W, S355W, S355WP, S355WD)

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Grade Coverage

BS 7668 defines a single grade:

The "WR" prefix indicates Weathering Resistant. The "355" is the nominal yield strength in MPa at t ≤ 16 mm, consistent with S355 in the EN 10025 system.

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Chemical Composition Requirements

Ladle analysis limits for Grade WR 355 per BS 7668:1994. All values wt% maximum unless a range is stated.

Carbon Equivalent (CEV):

CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ≤ 0.47 (t ≤ 16 mm)

The CEV limit ensures weldability without preheat for the majority of practical section sizes and thicknesses.

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Mechanical Properties

Properties are specified by nominal wall thickness. Hot-finished hollow sections to BS 7668 must meet the following minimum values:

Charpy test sub-grade: WR 355 specifies a test temperature of −20 °C, equivalent to the J2 sub-grade in EN 10025 terminology. The test is performed per EN ISO 148-1 on a longitudinal specimen. Individual specimen minimum is 70% of the average value.

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Impact Test Requirements

Charpy V-notch impact testing is mandatory for all WR 355 material:

The impact requirement is fixed at −20 °C for all thicknesses; there is no JR (room temperature) sub-grade option within BS 7668.

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Additional Tests

The following supplementary tests may be invoked by purchase order reference to the applicable EN standard:

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Corrosion Resistance and Patina Formation

The corrosion protection mechanism of WR 355 differs fundamentally from coated steels:

Patina formation process: On initial exposure, WR 355 rusts in the same way as carbon steel. Over 3–5 years of cyclic wet–dry exposure, the copper, chromium, and nickel additions promote the formation of a dense, tightly adhering inner layer of goethite (α-FeOOH) between the steel surface and the outer rust layer. This inner layer acts as a diffusion barrier that reduces the corrosion rate by a factor of 3–8 compared to plain carbon steel.

Corrosion index (I): ASTM G101 defines a corrosion index for weathering steels:

I = 26.01(Cu) + 3.88(Ni) + 1.20(Cr) + 1.49(Si) + 17.28(P) − 7.29(Cu)(Ni) − 9.10(Ni)(P) − 33.39(Cu)²

WR 355 typically achieves I ≥ 6.0, which is the threshold for beneficial weathering performance in most temperate atmospheric environments.

Environments where WR 355 performs well:

• Rural and urban atmospheres (ISO 9223 Categories C1–C3)
• Industrial atmospheres with adequate wet–dry cycling
• Coastal locations set back from the shoreline (> 1–2 km from sea spray)

Environments where WR 355 should not be used without coatings:

• Direct marine spray zones (chloride-accelerated corrosion prevents stable patina)
• Permanently wet or buried conditions (no wet–dry cycling; no patina stabilisation)
• Enclosed crevices and water-ponding details (trapped moisture causes ongoing corrosion)
• High-chloride industrial environments (ammonia, SO₂ above C4)

For hollow sections specifically, sealing both ends is critical. Open-ended hollow sections trap moisture internally and corrode from the inside; internal corrosion is not arrested by the weathering mechanism.

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UKCA vs CE Marking

BS 7668 is a BSI product standard. Hollow sections manufactured to BS 7668 that are construction products under the UK CPR require UKCA marking when placed on the Great Britain market (post 1 July 2025). The regime is identical to that described for BS EN 10025 structural steel. The UK DoP must reference BS 7668 and name the UKAS-approved body responsible for FPC certification (System 2+).

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Cross-Standard Equivalents

Corten A vs Corten B: These are trade names, not standards. "Corten A" corresponds roughly to ASTM A242 / EN S235W — lower strength, higher P content for patina. "Corten B" corresponds roughly to ASTM A588 / EN S355W — higher strength, lower P, adds Cr and Ni. WR 355 in BS 7668 is closest to the Corten B / A588 / S355W family.

Key equivalence caveats:

• EN 10025-5 covers flat and long products; BS 7668 is specifically hollow sections. A hollow section labelled "S355W" will typically be manufactured to EN 10025-5 combined with EN 10210-1, not BS 7668.
• ASTM A588 has a yield strength of 345 MPa (50 ksi) vs 355 MPa for WR 355 — a minor difference rarely significant in design.
• Chemical limits for P and Cu differ between A588 and WR 355; verify actual compositions on the MTC before substitution.

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MTC Verification Checklist

A compliant mill test certificate for BS 7668 WR 355 material should contain:

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Frequently Asked Questions