Welders may know weld slag as a bothersome byproduct of gas-shielded and self-shielded flux-cored processes. It frequently requires chipping or grinding to remove it after welding or in between passes. But slag is much more than that. It plays an important role in ensuring good weld quality. Additionally, the various slag systems also impact a wire’s welding characteristics.

So, what exactly is slag and what does it do?

Slag by definition, purpose and chemistry

The American Welding Society (AWS) defines slag as “a nonmetallic byproduct of the mutualdissolution of flux with nonmetallic impurities in welding and brazing processes.” In short, it is the hardened layer left on the top of weld made during flux-cored welding (FCAW). It is also present in shielded metal arc welding (SMAW), submerged arc welding (SAW) and other welding and brazing processes and can have a slightly different makeup depending on process or product. 

Removing welding slag is a mechanical process, completed with chipping hammers, wire brushes or wheels, or a needle scaler. Some filler metals, however, have self-peeling slag that releases from the weld on its own.

Slag’s purpose is two-fold. It protects the weld from oxidation and contamination from the atmosphere. It also helps keep the molten weld pool in the joint as it cools. This is especially important for out-of-position welding.

Slag forms when heat from the arc breaks down the filler metal and base material, forming a molten weld pool. Through thermodynamic processes, elements that are not included in the formation of the weld pool are essentially pushed out and forced to the surface. There, the elements solidify into slag that includes nonmetallic elements. Including oxides of aluminum, silicon and calcium oxides, and may also pull nitrogen, hydrogen and some carbon out with it.

The flux inside of both gas-shielded and self-shielded flux-cored wire impacts the way slag behaves. For example, wires with higher levels of titanium dioxide (TiO₂) have a faster freezing slag. That makes the wire better for welding out of position. However, too much titanium can make the slag sticky. Further, wires with higher levels of silica tend to generate a slag that is more difficult to remove. However, those with higher levels of lime have slag that is easier to remove.

Slag systems and their characteristics

Flux-cored wires are available with rutile (acidic) or basic slag systems. Each of which affects the wire’s weldability, including arc performance. Each system also has its advantages and limitations.

Rutile slag systems are composed mostly of titanium dioxide from the mineral rutile, along with other minerals, such as silica, alumina or zircon. Wires with a rutile slag system have good weldability with low spatter and a soft arc. They have excellent welder appeal. This is because they are easier to use than flux-cored wires with a basic slag system, particularly out of position. But wires with a rutile slag system don’t offer as good mechanical properties for achieving low-temperature toughness as their basic counterparts. Wires with a basic slag system are composed of minerals of alkali elements, like fluorspar, limestone, magnesite or dolomite. Basic slag systems are highly effective at lowering weld metal oxygen. This is because they provide superior weld metal toughness but low freezing temperature slags.

Rutile Wires: T-1, T-9 and T-12

Some of the most common flux-cored wires with rutile slag systems include those classified under AWS A5.20: Specification for Carbon Steel Electrodes for Flux-Cored Arc Welding and feature designators of T-1, T-9 and T-12.

T-1 flux-cored wires provide good wetting action and are generally reserved for less critical applications since they don’t offer the best toughness. T-9 wires have to meet the same chemistry requirements as a T-1, along with additional requirement of 20 ft. lb. at -20°F for their Charpy V-Notch testing. T-12 wires are very similar to T-1 and T-9 wires, but they have been modified to provide similar or better impact properties than the T-9. They also have lower manganese content, which allows the wires to meet the requirements of ASME Section IX for boiler and pressure vessel welding. They tend to require the base material to be relatively clean to achieve X-ray quality.

Basic slag systems are made up primarily of limestone, particularly calcium carbonate (CaCO₃). This helps provide good impact properties at low temperatures. This slag system also offers a low-hydrogen weld deposit to provide excellent resistance to hot and cold cracking. Many T-5 wires are limited to flat and horizontal welding positions. Overall, they are more prone to generating spatter and are considered less welder-friendly than flux-cored wires with a rutile slag system.

Preventing welding slag inclusions

Welding slag inclusions can affect the quality of welds produced with flux-cored processes and occur when the slag becomes trapped in the weld when it solidifies. There are several best practices and techniques to help minimize this risk.

First, always fully remove the slag after welding during multi-pass welds. This prevents it from being trapped inside the weld during subsequent passes.

During flat and horizontal welding, use a drag technique with a gun angle of zero (straight up and down) to 10 degrees. The technique for vertical up may vary according the gas- or self-shielded flux-cored wire being used. A drag technique still works well for vertical-up welding if the wire produces a heavy slag. For T-1 wires in a vertical weld, typically a zero-degree travel angle will produce quality bead shapes.

For other wires, such as T-5 wires, a zero-degree travel angle, to a slight push (around 5 degrees) works. This technique puts more heat into the base material, allowing the slag to come to the surface of the weld pool.

Using the right welding parameters and travel speeds is also key to reducing the risk of slag inclusions. Travel speeds that are too fast or current that is too high (which directly affects wire feed speed) can create a convex bead profile with dips at the toes of the weld that trap slag. Current that is too low can cause the same problem.

If your voltage is too high, it tends to make the weld bead more concave. This can sometimes lock slag into the weld as well.

Following best practices

As with any welding process, the key to good welding performance is selecting the right flux-cored wire for the application and using it according to the filler metal manufacturer’s recommendations. Knowing the slag type of each wire also helps you understand its weldability and how much post-weld and interpass cleanup it requires. Always follow cleaning best practices to avoid inclusions that can lead to downtime and costly rework.

FAQs

What is weld slag?

Weld slag is the hardened, nonmetallic layer that forms on top of a weld made with flux-based processes like flux-cored arc welding (FCAW), shielded metal arc welding (SMAW) and submerged arc welding (SAW). It protects the weld from oxidation and contamination. Additionally, it helps hold the molten weld pool in the joint as it cools, which matters most for out-of-position welding.

What is the difference between rutile and basic slag systems?

Rutile slag systems use mostly titanium dioxide and give a soft arc, low spatter and easier out-of-position welding. Basic slag systems use minerals like limestone and fluorspar. They lower weld metal oxygen for better low-temperature toughness and a low-hydrogen deposit. However, they produce more spatter, and many are limited to flat and horizontal welding.

Why is some slag harder to remove than others?

It comes down to the flux chemistry. Wires with higher silica levels tend to produce slag that is harder to remove, while higher lime levels make it easier. Too much titanium can make slag sticky. Some filler metals produce a self-peeling slag that releases from the weld on its own.

What causes slag inclusions in flux-cored welds?

Slag inclusions happen when slag gets trapped in the weld as it solidifies. Common causes are not fully removing slag between passes on multi-pass welds, travel speeds that are too fast, and current or voltage settings that create convex beads with dips at the toes or overly concave beads where slag can lock in place.

What gun angle should I use to prevent slag inclusions with flux-cored wire?

For flat and horizontal welding, use a drag technique with a gun angle between zero (straight up and down) and 10 degrees. For vertical-up welding, T-1 wires typically do well at a zero-degree travel angle. T-5 wires work best between zero degrees and a slight push of around 5 degrees. This puts more heat into the base material so slag can rise to the surface.

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