Welding and Fabrication Tips for Corten Steel Sheets

When corrosion caused by rain, snow, humidity, etc., commonly referred to as atmospheric corrosion, is of concern in a welding application, weathering steels are often a viable option for the job. These materials feature additions of copper, nickel, and chromium that help improve the atmospheric corrosion resistance of the steel. As a result, weathering steel structures are often left largely unpainted. This reduces or even eliminates a source of added time and cost. Because many of these materials also provide good mechanical properties, weathering steels are commonly used for bridges and other load-bearing structures.

When corrosion resistance is an issue of concern in specific welding applications,

Weathering steels are an option to consider.

Weathering steels have small additions of copper, nickel, and chromium.

This helps improve the corrosion resistance of the steel to atmospheric sources.

Common weathering steel specifications

Common weathering steel specifications used in structural fabrication include ASTM A242, A588, A606, and A847. Specifications common to bridge fabrication include ASTM A709 and AASHTO M270. Several manufacturers also use trade names to indicate their unique patented formulations of weathering steel, such as U.S. Steel's COR-TEN®.

Choosing the filler metal

When choosing the appropriate filler metal for welding weathering steels, there are numerous factors to consider:
• The code or specification requirements that exist.
• The strength and toughness level required.
• Where the weld will be located and if it will be painted.
• The joint geometry and required weld size.
• Color match.

Alongside design requirements, welding codes and specifications provide unbreakable rules contractors must abide by during welding and fabrication that override all other technical advice. The two most common steel welding codes, American Welding Society (AWS) D1.1 and D1.5, govern structural and bridge welding. These codes directly address the use of weathering steels and provide welding methods for different service environments for these materials.

One major challenge in Corten fabrication is the visual contrast: initial welds often appear glossy grey against the matte brown steel. To achieve 'Color Matching,' use copper-nickel-chromium-bearing filler metals (marked with a 'W' designator). This ensures the weld develops the same protective patina as the base metal over time.

Matching strength and corrosion resistance

Low alloy filler metals are available with higher tensile and yield strengths than carbon steel filler metals, allowing the properties of the filler metal to match those of the base metal when welding the higher-strength grades of weathering steel, such as HPS 70W of the AASHTO M270 specification.
Commonly used low-alloy filler metals for weathering steel applications include those with a minimum nominal nickel content of one percent. That alloy content is sufficient to provide atmospheric corrosion resistance similar to the weathering steel, and the cost is typically less than other low-alloy filler metals with acceptable properties. The most common alloy designators with this one percent nominal content are Ni1 (for wires) and C3 (for stick electrodes), although many other designators indicate filler metals with sufficient nickel in addition to other elements.

Balancing toughness, cost and alloy composition

Filler metals with a higher nickel content can be used successfully. They may be demanded in certain applications, since higher nickel content enhances toughness. However, the price of a filler metal typically increases significantly as the nickel and general alloy content also increases.
Copper-nickel-chromium-bearing low-alloy steel filler metals have nominal alloy content. This is similar to many weathering steels and is indicated by the "W" alloy designator. These filler metals are almost exclusively used in applications where color match (discussed below) is a primary concern. However, they tend to cost more, are often less readily available, and are somewhat more crack-sensitive than alternative filler metals.

For most structural applications, low-alloy filler metals with 1% nominal nickel content (Ni1 or C3) provide sufficient corrosion resistance at a lower cost. However, for projects in high-stress environments, higher nickel content is recommended to enhance impact toughness, even if it increases the filler metal price.

Filler metals for color-matching applications

Eventually all welds adopt a patina, regardless of whether carbon steel or low alloy filler metals are used. However, welds on weathering steels are initially highly visible. This is due to the contrast between the matte brown material and the glossy grey color of the weld metal. In some applications, such as sculptures or certain architectural details having welds in the public eye, it is desirable to have the appearance of the weld match the base material both in color and texture as soon as possible. The ability for this to occur quickly with a seamless appearance is known as color matching.

Strength over-match

Many commercially available low-alloy filler metals for welding weathering steel offer 80-plus ksi tensile strength and 60-plus ksi yield strength. But these filler metals are overmatched (higher strength than required) for the base metal when used to weld on 50 ksi yield weathering steel.

Maximize the benefits

Choosing the appropriate filler metal plays a key role in realizing the benefits offered by weathering steels. When selecting a filler metal, consider all aspects of the welding application. For the best results consider code, strength, toughness requirements, and aesthetics. When in doubt, contact the technical support services of filler metal manufacturers. Their staff can help identify which products in their portfolio will help maximize the performance of the welding application.

Pro Tips for Outdoor Corten Welding:

• Surface Prep: Always clean the weld area of any existing rust or scale to ensure deep penetration.
• Code Compliance: Ensure your process follows AWS D1.1 or D1.5 for structural or bridge projects.
• Overmatching Risk: Be careful when using high-strength fillers on 50 ksi yield steel; "overmatching" can lead to unnecessary stress.

Facing Challenges with Corten Fabrication?

Selecting the right filler metal and welding code is critical for the integrity of your structure. Whether you are building a coastal bridge or a public sculpture, Promisteel provides the technical expertise and high-quality Corten sheets you need.

[Consult Our Technical Team for Fabrication Support]