Introduction to Weathering Steel and REDCOR Grades
Weathering steel is a family of low-alloy steels designed to form a protective, rust-like patina when exposed to atmospheric conditions. This patina, formed through controlled corrosion, eliminates the need for protective coatings, making weathering steel ideal for outdoor applications such as bridges, architectural structures, and public art installations. The patina not only enhances aesthetic appeal but also significantly reduces maintenance costs by preventing further corrosion.
REDCOR, developed by BlueScope Steel, is a premium range of weathering steel tailored for structural applications in Australia and New Zealand. The grades WR350B, WR350L0B, and WR350L20B are high-strength variants within this range, conforming to AS/NZS 3678 standards. These grades are distinguished by their yield strength and impact properties, with WR350L0B and WR350L20B offering enhanced toughness at low temperatures (0°C and -20°C, respectively). Their corrosion resistance and ability to develop a consistent patina make them suitable for harsh environmental conditions.
Welding these steels requires careful consideration to preserve their mechanical properties and corrosion resistance. This guide provides an in-depth look at the welding procedures, consumable selections, and best practices for WR350B, WR350L0B, and WR350L20B, drawing on guidelines from AS/NZS 1554.1:2014.
Welding Characteristics of REDCOR Weathering Steel
According to BlueScope’s technical note, REDCOR weathering steel grades WR350B, WR350L0B, and WR350L20B exhibit welding characteristics similar to conventional hot-rolled steels, such as AS/NZS 3678 350. However, their elevated phosphorus and copper content, which enhances corrosion resistance, introduces specific welding challenges. These challenges include the potential for hot cracking in thinner sections and the need for careful consumable selection to match the steel’s patina and corrosion properties.
Key Precautions
- Consumable Selection: Hydrogen-controlled welding consumables are recommended for these high-tensile steels to minimize the risk of hydrogen-induced cracking. The suitability of consumables must be verified through weld procedure qualification tests as outlined in AS/NZS 1554.1:2014, Clause 4.7. This ensures that the weld metal meets the required mechanical properties and corrosion resistance.
- Preheat Requirements: For sections up to 50mm thick, preheating may be necessary to prevent hardening in the weld heat-affected zone (HAZ) and delayed cracking. For thicknesses exceeding 50mm, WR350B is classified under Weldability Group 6 for preheat calculations, as per Clause 5.3.4(a)(ii) of AS/NZS 1554.1. If the carbon equivalent (CE) exceeds 0.48, adjustments based on heat certificate data or WTIA Technical Note 1 may be required.
- Hot Cracking Risk: For steels up to 12mm thick, heat inputs exceeding 2.5kJ/mm increase the risk of hot cracking due to higher phosphorus and copper levels. Fabricators should conduct hot cracking tests, such as the Transvarestraint test or AS 2205.9.1, to assess and mitigate this risk when higher heat inputs are unavoidable.
Welding Procedures per AS/NZS 1554.1:2014
AS/NZS 1554.1:2014 is the authoritative standard for welding steel structures in Australia and New Zealand, covering materials, procedures, quality assurance, and inspection. It applies to statically and dynamically loaded structures, including those made from weathering steels like REDCOR grades.
Qualification of Welding Procedures
Clause 4.1 of AS/NZS 1554.1 mandates that welding procedures be qualified before fabrication begins. This involves creating a Welding Procedure Specification (WPS) and a Procedure Qualification Record (PQR) to document the welding parameters and test results. The qualification process ensures that welds meet the required mechanical properties and quality standards, particularly for critical applications.
For REDCOR grades, qualification tests should include:
- Macro Testing (Clause 4.7.4): To assess weld imperfections and ensure compliance with permissible levels (Tables 6.2.1 and 6.2.2).
- Tensile and Bend Tests (Clauses 4.7.5 and 4.7.6): To verify the strength and ductility of the weld.
- Hardness Testing (Clauses 4.7.8 and 4.7.9): To ensure the weld metal and HAZ hardness do not exceed specified limits (e.g., 350 HV 10 for HAZ).
Prequalified Procedures and Consumables
AS/NZS 1554.1 allows for prequalified welding procedures and consumables under specific conditions (Clause 4.3), reducing the need for extensive testing. For REDCOR steels, prequalified consumables are listed in BlueScope’s technical note, ensuring compatibility with the steel’s properties. These consumables are detailed in the following section.
Specific Welding Guidelines for WR350B, WR350L0B, and WR350L20B
BlueScope’s technical note provides detailed recommendations for welding these grades, focusing on consumable selection to achieve welds that match the steel彼此
Consumable Selection for Matching Patina and Corrosion Resistance
To ensure welds develop a patina and corrosion resistance similar to the parent material, nickel-bearing low-alloy steel electrodes are recommended, particularly for multi-run welds. For single-run welds, dilution from the base metal may suffice, especially for high-phosphorus weathering steels like WR350 grades. The following tables summarize prequalified consumables for various welding processes, as per BlueScope’s technical note and AS/NZS 1554.1.
Table 1: Prequalified Consumables for REDCOR Weathering Steels
| Steel Grade | Process | Consumables (Examples) |
|---|---|---|
| AS/NZS 3678 WR350 (A and B) | MMAW | A E42 2 3Ni, A E46 3 3Ni |
| FCAW | A T42 2 2Ni, A T46 2 2Ni | |
| GMAW | A G42 2 2Ni2, A G46 2 2Ni2 | |
| SAW | A – S2Ni2, A – S2Ni3 | |
| AS/NZS 3678 WR350L0 (A and B) | MMAW | B E49XX N5, B E55XX N5 |
| FCAW | A T42 2 3Ni, A T46 2 3Ni | |
| GMAW | B G492U SN2, B G552U SN2 | |
| SAW | A – S3Nil.5, B – SUN2 | |
| AS/NZS 3678 WR350L20 (A and B) | MMAW | B E49XX N7, B E55XX N7 |
| FCAW | B T492U N2, B T552U N2, B T492U N3, etc. | |
| GMAW | B G492U SN3, B G552U SN3, B G492U SN5, etc. | |
| SAW | B – SUN3, B – SUN31, B – SUN5, B – SUN7 |
Table 2: AWS D1.1 Consumables for Matching Patina on Multipass Welds
| Process | AWS Filler Metal Specification | Approved Electrodes |
|---|---|---|
| SMAW (MMAW) | A5.5 | B2L, C1, C1L, C2, C2L, C3, WX analysis |
| SAW | A5.23 | Ni1, Ni2, Ni3, Ni4, WX analysis |
| FCAW | A5.29, A5.36 | B2L, K2, Ni1, Ni2, Ni3, Ni4, WX analysis |
| GMAW | A5.28, A5.36 | B2L, G1, Ni1, Ni2, Ni3, analysis |
These consumables are selected based on their ability to provide weld metal with corrosion resistance and patina characteristics similar to the parent steel. For single-run fillet or butt welds without weaving, consumables from Table 4.6.1(A) of AS/NZS 1554.1 are suitable, while Table 4.6.1(C) applies to single-run welds with weaving or capping runs on multi-run welds.
Weld Imperfections and Quality Control
AS/NZS 1554.1 specifies permissible levels of weld imperfections for two categories: General Purpose (GP) and Structural Purpose (SP). For critical applications involving WR350 grades, SP category welds are typically required, necessitating stricter inspection and acceptance criteria (Tables 6.2.1 and 6.2.2). Common imperfections include:
- Cracks: Not permitted in SP welds.
- Undercut: Limited to t/10 (max 1.5mm) for SP welds.
- Porosity: Limited to two surface pores per 12t length for SP welds, with a maximum loss of area of 5%.
Non-destructive examination (NDE) methods, such as radiography (Clause 6.3), ultrasonic testing (Clause 6.4), magnetic particle examination (Clause 6.5), and liquid penetrant examination (Clause 6.6), are recommended to detect internal and surface imperfections. For SP welds, 10-50% of welds may require NDE beyond visual inspection, as per Table 7.4.
Best Practices for Welding REDCOR Weathering Steel
To ensure high-quality welds and minimize defects, fabricators should adhere to the following best practices:
- Surface Preparation: Clean welding surfaces to remove contaminants like oil, grease, rust, or scale, which can compromise weld quality. Millscale that withstands wire brushing may remain, as per Clause 5.1.1.
- Joint Fit-Up: Ensure accurate alignment and minimal gaps (max 5mm, or 8mm for thicker sections with sealing welds or backing material) to prevent burn-through and ensure proper fusion (Clause 5.2.3).
- Welding Parameter Control: Maintain recommended current, voltage, and travel speed to control heat input, avoiding hot cracking in thinner sections. For multi-arc processes, calculate total arc energy as the sum of individual arcs (Clause 5.3.4).
- Preheat and Interpass Temperature: Apply preheat as determined by Tables 5.3.4(A) and 5.3.4(B), ensuring the full thickness reaches the specified temperature (Clause 5.3.3). For seismic-resisting steels, maintain a maximum interpass temperature of 300°C unless qualified otherwise (Clause 5.3.5).
- Post-Weld Cleaning: Remove slag and tightly adhering spatter to facilitate inspection and maintain aesthetics (Clause 5.11). Welds should not be painted until inspected and accepted.
- Inspection and Testing: Conduct 100% visual inspection for all welds (Clause 7.3) and additional NDE for SP welds as specified. Ensure inspectors are qualified per Clause 7.2, with certifications like those from the Welding Technology Institute of Australia.
- Avoiding Lamellar Tearing: For T-joints and cruciform connections, select materials with adequate through-thickness ductility (Z-values) as per Appendix H. Ultrasonic examination of parent metal can help identify inclusions that may lead to lamellar tearing (Appendix H3).
Conclusion
Welding REDCOR weathering steel grades WR350B, WR350L0B, and WR350L20B requires adherence to specific guidelines to preserve their corrosion resistance and aesthetic patina. By selecting appropriate hydrogen-controlled consumables, applying necessary preheat, controlling heat input, and following AS/NZS 1554.1:2014 standards, fabricators can produce high-quality welds suitable for structural applications. The use of nickel-bearing electrodes for multi-run welds ensures matching corrosion properties, while rigorous inspection and testing guarantee weld integrity. For detailed consumable recommendations and technical guidance, refer to AS/NZS 1554.1:2014.