Guide: What is Weathering Steel and How Do You Weld It?

May 03, 2024 · Leave a comment · Peter Germanese
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Weathering Steel

Welding is a career full of challenges. Some of those challenges involve building up a stable client base and consistent work; others involve learning the technical skills of the trade. When you encounter a new metal or a metal you aren’t necessarily familiar with, you need to learn how to weld it properly – if it can even be welded in the first place – if you want to handle it effectively. One such material you may or may not have ever encountered is weathering steel.

What is weathering steel, how does it differ from regular steel, when might you encounter it, and how can you weld it? Let’s dig into the details.

What is Weathering Steel?

Steel is a fantastic material for construction. It’s very strong and has a good amount of resilience to all kinds of stress, and while it’s very heavy, it’s also very cheap, so it’s frequently used in everything from building frames to bridges to automotive construction.

One of the biggest problems with steel, however, is that it’s largely based on iron (usually with some carbon thrown in) and, as we all know, iron is susceptible to oxidation. When exposed to the elements, like the oxygen in the air and in water and moisture, steel rusts. That rusting and corrosion will eventually degrade the material until it’s no longer structurally sound, and that is what we in the industry call “a problem.”

There are many different ways to protect steel from corrosion. A common method is to paint or grease it, coating it with a material that forms a barrier between the steel and atmospheric moisture and oxygen. Steel can also be galvanized or anodized, forming a similar coating but with more of a chemical reaction instead of just a surface layer. 

The biggest drawback of these methods is that if there’s a gap or damage to the coating, it will rust through. They need to be well-maintained and continually inspected, lest they suffer enough damage to catastrophically fail.

Weathering Steel in Construction

To solve this issue, materials engineers turned to another metal commonly used around the world: copper.

Copper is also susceptible to oxygen. Whenever you see a copper construction with a green tint, that’s copper oxide forming a patina over the surface of the copper. Unlike steel, though, this patina doesn’t continue to corrode. Instead, it forms a naturally protective shell around the copper. This shell inhibits most of the further corrosion, and if the patina is damaged, it “self-heals” by forming more patina in the gap.

Engineers asked: what if we could make steel do the same thing? Through development nearly a century ago, a material was created out of steel and various alloyed metals, including copper, chromium, silicon, and phosphorus. The result is a form of steel that has essentially all of the material properties of low-carbon steel, except instead of rusting like normal steel, it forms a brown patina that serves as a protective coating.

This material was originally developed and patented in 1933 under the name Cor-Ten, which stands for Corrosion Resistance and Tensile Strength. Cor-Ten, also commonly referred to as Corten steel, was eventually standardized by the American Society for Testing and Materials, making it broadly available for widespread use, which is why, today, you may encounter it and the need to weld it.

When is Weathering Steel Used?

Because of its natural resistance to corrosion and its mildly self-healing properties, weathering steel is most often used in places where it will be exposed to the elements but where regular inspection and maintenance are often sidelined. You frequently see it in engineering and architecture, particularly as exposed and decorative elements, in sculpture, for roofs and walls, and various utility designs like planters, bicycle corrals or hoops, and tree grilles. It’s also frequently used in marine transportation, and you see it frequently as the structural material for short, often pedestrian bridges over rivers and streams.

Overall, it’s valuable for any case where you need a steel structure that is robust and strong, while also benefitting from both a longer maintenance and upkeep schedule and potentially the handsome uniform orange/brown appearance. It’s frequently seen in nature parks where the brown blends nicely with surrounding trees.

Using Weathering Steel

Three potential drawbacks limit the places where weathering steel can be used.

  • It needs to get wet and then dry out for the patina to form, so in areas where it rarely gets wet, like deserts, it remains less protected from the elements.
  • It needs to get wet and then dry out for the patina to form, so in areas where it never dries out (like coastlines or underwater), it never dries enough to form the patina.
  • It’s very susceptible to salt, so it should not be used on seashores where saltwater air can accelerate corrosion, or near roads where winter snowmelt can corrode it further.

The corrosion can also drip off of the material, which can stain surrounding concrete and other surfaces, which isn’t a structural concern but can be an aesthetic concern.

ASTM has several specifications for variations of weathering steel, including A588, A242, A606-4, A847, A871-65, and A709-50W. These include forms like steel panels and sheets, thick plates, tubes, pipes, coils, and more. It’s divided into grades A and B, where B is generally more suitable for load-bearing uses.

How Can You Identify Weathering Steel?

The only true way to identify Corten or weathering steel is by metallurgic testing. However, you can generally tell at a glance if a piece of steel is likely to be weathering steel based on its distinct color and uniform appearance.

How to Identify Weathering Steel

If you have a welding specifications document for your project, it should identify the material as well. 

Can You Weld Weathering Steel?

Yes. In fact, welding weathering steel is the primary way it is attached to itself for various constructions. Done properly, weathering steel can be welded in an almost seamless and smooth fashion that provides a strong and enduring joint.

Welding Steel

One primary consideration is preparation. Since the primary benefit of weathering steel is the formation of a patina, that patina is likely to have formed on the materials you’re working with before you begin working. However, as any experienced welder knows, one of the worst things you can do when welding is try to weld a rusty piece of steel. That rust, if not cleaned off, can form inclusions that damage and compromise the finished product. Therefore, you need to clean the area of the weld thoroughly before you weld it, usually with grinding.

What Process is Best for Welding Weathering Steel?

Since weathering steel is essentially just a low-carbon steel alloy, it can be welded using virtually any process. However, as you likely have guessed, some processes work better and provide longer-lasting welds than others.

Welding Steel

That said, 90% of the time, you’re going to want to use MIG for welding weathering steel. It needs to be handled properly, but that’s true of any welding. TIG welds on weathering steel tend to fail prematurely, and stick doesn’t provide benefits over MIG that make it worth using unless you’re somehow very uncomfortable with MIG welding.

What Filler Should You Use on Weathering Steel?

The choice of filler depends heavily on the purpose of the joint. First, start by considering the following factors.

  • Are there any specifications from a WPS or building code that need to be followed?
  • What are the strength and toughness requirements of the finished product?
  • Where will the weld be located (indoor versus outdoor, exposed or not, etc.)
  • Will the joint be painted or otherwise coated?
  • What is the geometry of the joint, and how big will it be?
  • Do you have aesthetic concerns regarding the color matching of the joint?

The interplay of these factors will determine what kind of filler material you should use for your joint.

The first possibility is carbon steel fillers such as AWS E7018, ER70S, or E70C-6M. These are most useful when you need a weld that is performed in a single pass, with a small or lower-strength grade of weathering steel. The resulting weld will be a little weaker than it could be, but cheaper. Moreover, the low-carbon steel filler mixes very well with the weathering steel and picks up the alloying elements, allowing the joint to maintain a similar level of corrosion resistance as the base materials themselves.

Filler for Weathering Steel

The second possibility is a low alloy filler metal. These come in a variety of forms based on their alloy materials but are, in general, best used when you have higher requirements for strength, corrosion resistance, or other mechanical properties. When you’re performing a weld that uses large or multi-pass welding techniques, the base metal doesn’t dilute into the filler metal as much. This means that if you use carbon steel filler for a large weld, it won’t have the same corrosion resistance and will be more susceptible to damage. It would need to be painted or coated in another way to stay solid alongside the weathering steel base.

  • Nickel-based low alloys are useful for providing atmospheric corrosion resistance on par with that of weathering steel. These can be useful when corrosion is the primary concern.
  • High nickel alloys can be used for certain applications where toughness is more required, but the cost of a higher nickel alloy can make it prohibitive for certain projects.
  • Copper-Nickel-Chromium alloy fillers are very similar to weathering steel and even tend to be indicated with a W designator. They are, however, not prized for structural use or for corrosion resistance but rather for color matching with weathering steel. If you don’t want the joints of your project to be visible over time, this kind of filler is a better option.

Another option, recently developed, is to use proprietary filler metals. For example, Central Steel Service developed and released a product called Cor-Match, which is a series of welding filler materials designed to be as close a match to Corten steel as possible. They are designed to be low-alloy steel fillers meant specifically for weathering steel purposes.

Are There Other Concerns for Welding Weathering Steel?

Potentially. Depending on the filler you’re using and the process, the weld shape, and design, you may have to restrict yourself to a flat and horizontal weld procedure. This means you may need welding positioners to hold your workpieces in position, which may or may not be possible, depending on the construction.

Another consideration is preheating. You generally don’t need to preheat weathering steel when you’re welding it unless it’s particularly thick weathering steel pieces that need to be joined. The thicker the material, the more likely you need to preheat it to prevent the temperature differential from cracking the piece as it cools. This can also vary between different kinds of weathering steel.

You may also need to consider a post-treatment for the weld joint. Weathering steel protects itself from the environment by forming a patina, but a joint needs to be prepared for welding by removing that patina to avoid inclusions and faulty welds. So, when the weld is done, you have an exposed joint. This will naturally form a patina over time, but before it does, it may look displeasing or not meet aesthetic demands. Certain post-treatments can accelerate the formation of the patina.

A Person Welding Weathering Steel

Whatever your considerations, one thing is always true: in order to weld any material, from mild steel to weathering steel to aluminum and more, you need the right kind of welding gear. If you don’t have a good welder and the right tools and PPE, you aren’t going to be able to weld appropriately. 

That’s where we come in. At Red-D-Arc, we provide rental welding equipment for a fraction of the cost of buying new. You can rent a machine for one-off projects or rent a machine for full duty until you determine if it’s the kind of machine you want. If it suits your needs, keep renting it, or buy a used one from our store; if it’s not suitable, return it and try something else. And, of course, if you have any questions, don’t hesitate to reach out; our welding experts can guide you and answer any questions you may have.

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