Weld for the Outdoors: Welding Corton Steel
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, reducing or eliminating 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.
welding applications, weathering steels are an option to
consider. Weathering steels have small additions copper,
nickel and chromium that help improve the corrosion resistance
of the steel to atmospheric sources.
Weathering steels do not prevent rusting, but rather change the way that it occurs so that it can be used beneficially. In essence, the alloys in weathering steels make the rust/oxide layer (known as a patina) form quicker and thicker, and also make it more difficult to remove by rain, wind or light abrasion than the rust of non-weathering grade steels. The patina acts like a natural, durable layer of paint that prevents the exposure of unreacted iron — found in the underlying base material — to the atmosphere. The result is a fabricated structure that is more resistant to losing material thickness, and load carrying capacity, caused by the elements.
Sculptors and architects may also choose to use weathering steels simply for aesthetics. The texture and colors of the patina provides the unique appearance and character of a structure with significant age in significantly less time.
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
• If “color match” is desired
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, which respectively govern structural and bridge welding, directly address the use of weathering steels and provide welding methods appropriate to different service environments for these materials.
Weathering steels are available in a wide variety of yield strength levels — typically ranging from 50 to 100 ksi — and toughness intensities. However, filler metal requirements vary by application. Matching the mechanical properties of the filler metal — the tensile strength, yield strength and toughness — to the design requirements is critical to ensuring the intended performance of the structure. It is common practice for the filler metal to at least meet the minimum mechanical properties of the base material, but design requirements may require or allow under-matching or over-matching in certain instances. Always confirm the exact specification and grade of material being welded, and all design requirements prior to selecting a filler metal.
Two other important considerations when selecting a filler metal are if the weld will be located away from sources of atmospheric corrosion and whether it will be painted. Welds in a less exposed area, and particularly those that will be painted over, may not require a corrosion resistant filler metal like welds left unpainted and exposed to atmospheric elements.
When choosing a filler metal there are additional questions to consider as they relate to joint geometry and required weld size, relative to material mechanical properties.
• Does the base material have a minimum yield strength requirement of 50 ksi?
• Will the weld be completed in one pass, or one pass on each side of the weldment?
• If the weld is a fillet weld, will the leg sizes be less than or equal to 5/16 inch when using
solid wire, metal-cored wire or flux cored wires, or 1/4 inch when using stick electrodes?
Carbon steel filler metals
Answering yes to the above questions indicates welds that are single pass, small and on a lower strength grade of weathering steel, which means a low-cost carbon steel filler metal without significant alloying elements may be used, provided the product still meets minimum mechanical property requirements.
Welding small-sized, single-pass welds typically causes a high amount of base metal dilution; a large amount of base metal is melted and mixed with the weld metal. As a result, the weld picks up a sizable amount of alloying elements from the weathering steel, which can provide the weld metal with the same atmospheric corrosion resistant properties as the base metal itself.
Some of the most common carbon steel filler metals suitable for small welds on low-strength weathering steels include AWS E7018 stick electrodes, ER70S-3/6 solid wires, E70C-6M metal-cored wire and E71T-1/9 C/M flux-cored wires.
Low alloy filler metals
Answering no to the above questions about joint geometry, weld size and material strength indicates that a low alloy filler metal may be needed to provide the appropriate corrosion resistance and/or mechanical properties when welding weathering steel.
for aesthetics. The texture and colors of the patina provides
the unique appearance and character of a structure with
significant age in significantly less time.
Large, multi-pass welds typically do not experience a significant amount of base metal dilution. For that reason, carbon steel filler metals cannot pick up enough alloying elements from the base material to provide the necessary atmospheric corrosion resistance. Instead, a low alloy filler metal is recommended to ensure that the weld will have the same corrosion resistance as the weathering steel.
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.
Filler metals with a higher nickel content also can be used successfully, and 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 similar to many weathering steels and are indicated by “W” alloy designators. 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.
Many of the most 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 over-matched (higher strength than required) for the base metal when used to weld on 50 ksi yield weathering steel. While this over-match doesn’t provide a structural advantage, many codes and engineers allow it to ensure corrosion-resistant alloying of welds with more readily available filler metals. For example, on 50 ksi yield materials, when permissible, combining carbon steel and low alloy filler metals can provide good results and lower filler metal costs, particularly on very large multi-pass welds. Use a carbon steel filler metal for all welding up to the final two layers of the weld joint, then weld the last two layers using the more expensive low alloy filler metal to ensure that the weld face has not been diluted and still has the elements it needs to resist corrosion.
Filler metals for color-matching applications
Given enough time, all welds will eventually adopt a patina of their own regardless of whether carbon steel or low alloy filler metals are used. However, welds on weathering steels are initially highly visible due to the contrast between the matte brown color of the 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 seamless appearance is known as color matching.
Low alloy steel filler metals with a “W” designator are typically used for these applications, as they are capable of forming a patina more quickly that matches the unique colors of the weathering steel than other alternatives.
Small welds with high dilution also are likely to color match relatively quickly, even when not made with “W” designator filler metals. Still, they may not blend as seamlessly and as quickly as when using “W” filler metals.
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 — from code, strength and toughness requirements to aesthetics — to gain the best results. 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.
Related Articles:
Weathering Steel: Taking the Guesswork Out of Filler Metal Selection
Welding Stainless Steel Tube and Pipe: Maintaining Corrosion Resistance and Increasing Productivity