Corten steel, or weathering steel, is prized for its ability to form a protective, rust-like patina that eliminates the need for painting. However, for fabricators and welders, this unique chemical composition presents a specific challenge: How do you join two pieces of weathering steel so that the joint is as strong as the base metal and, more importantly, weathers to the same color?
At Conquest Steel & Alloys, we understand that a project’s success depends on the details of its execution. Whether you are working with Corten A (ASTM A242) or Corten B (ASTM A588), following these best practices will ensure your welds are structurally sound and visually seamless.
1. The Core Challenge: Color Matching
The most common mistake in Corten fabrication is using standard carbon steel filler metals (like E7018). While these will physically join the plates, the weld bead will not develop the same protective patina. Over time, the base metal will turn a deep chocolate brown, while the weld remains a bright, contrasting silver or orange-black.
To prevent this, you must use filler metals specifically designed with weathering properties, typically containing around 0.5% Copper ($Cu$) and 0.5% Nickel ($Ni$).
2. Selecting the Right Filler Metal
The choice of electrode or wire depends on the structural requirements and the thickness of the material.
For Single-Pass Welds:
If you are performing a single-pass weld on thin-gauge material (under 6mm), standard mild steel electrodes (like E7018 or ER70S-6) are often acceptable. This is because the weld pool will pick up enough alloying elements from the surrounding Corten base metal to weather similarly.
For Multi-Pass Welds:
For thicker plates or structural joints, you must use “weathering grade” consumables.
- SMAW (Stick): Use E7018-W or E8018-W electrodes. The “-W” suffix indicates weathering properties.
- GMAW (MIG): Use wires that meet AWS A5.28 standards, such as ER80S-G or ER80S-Ni1.
- FCAW (Flux-Cored): Look for wires like E81T1-W2.
3. Pre-Welding Preparation
Corten steel is sensitive to surface contaminants. To ensure a high-quality weld, follow these preparation steps:
- Remove the Mill Scale: Use a grinder to remove the dark mill scale and any pre-formed rust at least 25mm back from the weld joint. Welding through rust causes porosity and brittle joints.
- Edge Preparation: For plates thicker than 6mm, a V-groove or U-groove preparation is essential to ensure full penetration.
- Moisture Control: Ensure the joint is dry. Hydrogen-induced cracking is a risk in high-strength low-alloy (HSLA) steels like Corten.
4. Preheating Requirements
Preheating helps slow the cooling rate of the weld, reducing the risk of cracking in the Heat Affected Zone (HAZ).
- Thin Sections (<19mm): Generally do not require preheat if the ambient temperature is above 10°C.
- Thick Sections (>19mm): For Corten B (ASTM A588), preheating to 100°C – 150°C is recommended.
- High Constraint Joints: If the joint is under heavy tension or part of a rigid frame, preheat regardless of thickness to prevent “underbead cracking.”
5. Managing the Welding Process
When welding Corten, the goal is to maintain the mechanical properties of the HSLA steel while ensuring the patina-forming elements are evenly distributed.
Heat Input
Keep heat input within a moderate range. Excessive heat can enlarge the grain structure in the HAZ, weakening the joint. Conversely, too little heat can lead to “cold laps” or lack of fusion.
Shielding Gas
For MIG welding, a standard 75% Argon / 25% $CO_2$ mix works well. However, for a cleaner finish and better penetration on thicker plates, a 90% Argon / 10% $CO_2$ mix is often preferred.
6. Post-Weld Cleaning and Finishing
What you do after the weld is just as important for the aesthetic.
- Slag Removal: For Stick or Flux-Core welding, thoroughly chip away all slag. Remaining slag will prevent the patina from forming under it, leading to “spotty” weathering.
- Blending: If the project is architectural (like a facade), grind the weld flush. If the weld is left proud, it may weather at a slightly different rate than the flat surface due to the texture difference.
- Sandblasting: For the most uniform patina, sandblast the entire assembly after fabrication. This removes oils, fingerprints, and mill scale, allowing the steel to start weathering at the exact same time across the entire surface.
7. Mechanical Fastening: An Alternative
In some cases, welding is not ideal. If you are joining Corten to another material (like galvanized steel or stainless steel), welding can cause galvanic corrosion.
- Bolting: Use weathering steel bolts (meeting ASTM A325, Type 3). These bolts have the same chemistry as the plates and will blend in perfectly.
- Thermal Expansion: Remember that steel expands and contracts. In long architectural facades, use slotted holes to allow for movement without buckling the panels.
8. Summary Checklist for Success
| Step | Action |
| Material | Identify if it is Grade A or Grade B. |
| Consumable | Use “-W” series electrodes for multi-pass welds. |
| Prep | Grind away all rust and scale before striking an arc. |
| Heat | Preheat thick plates (>19mm) to prevent cracking. |
| Finish | Sandblast for a perfectly uniform, 100% matching patina. |
Why Source from Conquest Steel & Alloys?
Welding success starts with material consistency. At Conquest Steel & Alloys, our Corten plates are manufactured to strict chemical tolerances, ensuring that the Phosphorus and Copper levels are consistent from the center to the edge of the plate. This consistency means your welding parameters stay the same from the first joint to the last.
Whether you are building a bridge that must last a century or a sculpture that must look perfect in a gallery, our technical team is here to support your fabrication journey.
Contact Conquest Steel & Alloys today for high-grade corten steel and the technical expertise to join it right.
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