How To Avoid Slag Inclusions In Stick Welding

Stick welding is one of the oldest but still most popular arc welding methods. It is widely used worldwide, and its low costs and ability to weld outdoors on dirty and rusty metal make it valuable in various industries.

Slag is an essential part of any flux welding process, including Stick welding. As long as it peels easily after the weld, you won't have to worry about it.

However, mistakes in weld preparation and welding techniques can create a defect known as slag inclusions. Tiny bits of slag stay trapped, compromising the structural integrity of the weld. Today, we'll discuss slag and slag inclusions and teach you how to avoid slag inclusions in Stick welding.

Stick Welding Pipe Process
Photo by kenny.steww (IG)

What is Slag in Welding?

Slag is a non-metallic substance that forms on top of the weld. It is a byproduct of welding flux and appears in flux welding processes such as Stick welding (Shielded Metal Arc welding), Electroslag welding, Submerged Arc welding, or Flux-cored arc welding.

Slag in Stick Welding
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The slag comes from flux, which is crucial in Stick welding. SMAW uses self-shielded electrodes coated with flux. As the flux melts, it goes through several phases that include reaction, wetting, rising to the top, and freezing

Flux reacts with oxygen and nitrogen inside and on top of the weld to form oxides and nitrates. These oxides further react with other elements, such as calcium and aluminum, to form slag.

Due to its low density, flux rises to the top of the weld. As it reaches the surface, it creates a protective layer known as slag. Slag protects the surface of the weld from oxidation, atmospheric contamination, and reactive gases.

As a result, you don't need external shielding gas protection like with MIG or TIG Welding. That means you can Stick weld outdoors, in the wind or draft, without the risk of contamination or external shielding gas protection.

Stick Welding Outdoors
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Is Slag Bad?

Slag is vital to Stick welding. When welded correctly, you can easily remove slag with a chipping hammer or a wire brush. A strong, high-quality stick weld is uniform and free of defects or internal or external discontinuities that affect the micro or macrostructure.

As long as slag peels easily and the weld underneath it is uniform, without pinholes, cracks, or discontinues, you don't need to worry about it. However, if you notice needle-shaped or spherical pieces in your weld, you are likely dealing with a weld defect known as slag inclusions.

Slag Peeling while Stick Welding
Source: https://www.youtube.com/shorts/9YUlAOTk55I

How To Remove Slag?

You can remove most slag with simple hand tools, such as a chipping hammer or wire brush. These are helpful tools for any welding, and easy-to-peel slag will come right off after a few strikes of your hammer. Just wait for the welds to cool down before striking them with a pointy part of the hammer.

Once you are dealing with hard slag, you will need the bigger guns. Flux with higher lime content creates a hard slag, so you will need power tools such as an angle grinder or rotating wire brush head.

Power tools are ideal for removing large amounts of hard spatter quickly, but they require higher skills to run. For hard-to-reach places, you can use a needle scaler. This is an ideal tool for hard spatter in corners and tight spaces, but it requires a steady hand and expertise.

Use a Needle Scaler to Remove the Welding Slag
Source: https://www.youtube.com/watch?v=el0dZu9PuFE

What Are The Slag Inclusions?

Slag inclusions represent non-metallic particles of slag trapped in the weld metal or on the surface of the weld. In most cases, they occur as flux doesn't correctly migrate to the surface and create a slag layer. But why are slag inclusions so bad?

How Slag Inclusion Happens
Source: https://www.youtube.com/watch?v=us66qtKRODY

The first issue with slag inclusions arises due to the composition of the flux. Welding flux is comprised of a combination of carbonate and silicate materials. Once the oxides (like lime), silicon, and other non-metal elements stay trapped, they influence the structural integrity of the weld.

Slag inclusions cause concentrated stress, reducing the weld's durability. They also prevent natural interactions between individual grains, further affecting the weld's mechanical integrity. Thus, inclusions create weak spots in the weld, regardless of whether they appear inside or on the weld's surface.

Source: https://weldingofwelders.com/slag-inclusions-in-welding/

In addition, slag inclusions allow liquids or gases to pass through them, which is a highly unfavorable condition for pressure vessels or pipes that transfer them. Over time, inclusions can cause corrosion in the weld or create more surface area for corrosion.

Therefore, slag inclusions clearly affect the structural integrity of the weld, weakening its overall strength and quality. You should prevent this defect whenever possible, and we'll explain how later in the text.

Source: https://tft-pneumatic.com/blog/identify-welding-defects/

How To Detect Slag Inclusions?

In most cases and your home applications, you can visually detect slag inclusions. These defects appear as clear, glass-like thin lines or pinholes on the weld's surface. Inclusions are typically scattered randomly across the weld, so always pay close attention.

However, the more significant issue creates slag inclusions trapped inside the weld. While as dangerous as surface inclusions, the internal inclusions are more complex to identify. These often occur in industrial applications that involve multi-pass welding. Failing to properly clear slag after each pass leaves it trapped inside the weld, creating a weak spot in the weld.

To identify internal slag inclusions, you will need non-destructive testing methods, such as dye penetrants or radiographic testing. These will help you pinpoint the location of inclusions, which can make rework easier. However, repairing internal inclusions can be costly and time-consuming, so it is always better to prevent them in the first place.

Non-Destructive Testing of Welds
Source: https://www.zetec.com/blog/destructive-and-nondestructive-testing-of-welds-how-ndt-ensures-quality/

What Causes Slag Inclusions?

The first step in dealing with slag in Stick welding is understanding what causes the inclusions. But the things are not so simple. The slag inclusions are caused by a combination of several factors, with the most important ones being:

• Flux composition

• Welding parameters

• Welding technique

• Bad preparation and cleaning

Each can play a crucial role in the formation of slag inclusions, and here is how and why.

Source: https://www.youtube.com/watch?v=4YtpBLTnoDQ

Composition of Flux

A welding flux consists of minerals, alloying materials, and chemical additives. Each flux component shows different freezing rates, wettability, and surface tension, but it also indicates a higher or lower risk of slag inclusion.

Close-up View of Flux-coated Stick Electrodes
Source: https://www.tianqiaowelding.com/news/the-composition-of-the-welding-electrode/

For example, rutile flux has high surface tension, yielding a flat and wide weld bead. Due to the lower bead profile, the flux rapidly rises to the surface, creating slag and lowering the risk of inclusions.

Conversely, fluxes with high calcium carbonate content have increased wettability. Because of this, the flux dips inside the weld puddle rather than rising to the top. As a result, calcium carbonate fluxes are infamous for slag inclusions.

Flux with a higher silicate content creates glass-like slag that tends to self-detach more quickly. Easy peel helps you remove the slag more rapidly and lowers the risk of inclusions. Meanwhile, flux coatings with more lime content often generate hard slag that is difficult to remove. In multi-pass applications, hard slag can remain trapped and cause inclusions.

E7018 Electrode Slag Peel
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Welding Parameters

Although stick welders are easy to set, as there are only a few options, the welding parameters can play an essential role in slag formation. Out of them, the amperage can have different effects on slag inclusions.

The amperage in Stick welding directly impacts the energy and how fast the flux melts and rises from the puddle. First, you will need a proper electrode diameter suitable for the given job and material thickness.

Source: https://www.kingsofwelding.com/guides/welding-rod-sizes-amperage-metal-thickness-chart/

When setting the amperage, avoid using the lower end of the supported amperage whenever possible. Low amperage can cause slag inclusions, undercutting, improper fusion, and penetration.

Low amperage means low energy and heat. Flux trails the weld puddle instead of moving with it, causing it to mix with the bead. Additionally, low heat causes faster puddle cooling. The flux doesn't have enough time to reach the surface, so it solidifies and stays trapped inside.

Stick Welding Puddle at Too Low Amperage
Source: https://www.youtube.com/watch?app=desktop&v=l4SKUEoF2AA&t=0s

Welding Technique

A Stick welding technique comprises several different factors, such as travel angle, travel speed, arc length, or travel direction. Each aspect can lead to slag inclusions, so you should be careful.

Photo by @weldd1 (TikTok)

The first rule of using slag welding processes is: "If there is a slag, you drag!" If you push the stick weld, the slag can melt and get trapped inside the weld, causing inclusions.

Stick welding works best with a 5-15-degree travel angle. If you use a too-wide angle, slag can fall near or ahead of the weld pool, causing inclusion. Using a too-steep angle causes spatter and messy welds that are hard to control, including the slag positions.

Source:https://weldingpros.net/how-to-stick-weld/

Welding is all about consistency, and uncontrollable speed can also cause slag inclusions. Moving too fast across the weld joint has similar effects to low amperage. The weld doesn't have enough time to penetrate the sides, and the slag remains trapped inside the weld or rests on the cold sides instead of a molten and hot puddle. Traveling too slowly causes high depositions and excessive reinforcement. The arc can burn through the weld or cause excessive spatter and slag formation.

The gap between the electrode tip and base metal affects the voltage. Keeping the distance consistent yields a consistent voltage. On the other hand, moving closer or further to the weld will cause voltage variations, which can impact the formation of slag and cause inclusions.

The Relationship Between Arc Length and Voltage
Source: https://www.megmeet-welding.com/en/news/How-Does-Arc-Length-Affect-a-Weld

Bad Preparation And Cleaning

The truth is many welders turn to Stick welding when there are light layers of contamination on the metal surface. The stick can burn through oil, paint, dirt, or rust, but it can yield unpredictable results.

Stick Welding Process on Rusty Metal
Photo by @buddymar18 (TikTok)

Dirty and irregular surfaces can cause slag inclusions. Contamination can react with the flux or prevent it from reaching the surface and solidifying. Uneven surfaces with gaps and craters can trap slag or cause undercutting.

Slag inclusions commonly occur in multi-pass welding due to poor cleaning. Welders use multi-pass welding to fill wide gaps and large joints. By beveling the pieces and leaving a root opening, you can achieve strong results on thick metal.

However, failing to clean the slag correctly after each pass can trap it inside the weld. When using weaving beads, failing to overlap the patterns creates cavities in the weld toes. Slag can get trapped inside these cavities, so running multi-pass welding requires a lot of expertise and experience.

Source: https://weldguru.com/slag-inclusion/

How To Prevent Slag Inclusions In Stick Welding?

Understanding the causes of slag inclusions is the first step in avoiding them. As noted, preventing them is a much more effective solution than repairing and reworking them, so here are some tips on how to avert slag inclusions in stick welding.

Tips and Basic Techniques for Correct Stick Welding
Source: https://www.youtube.com/shorts/hytUoe2df-w

1. Carefully Choose Your Stick Electrodes

Since flux composition and electrode diameter can cause inclusions, you should choose your electrodes carefully. First, make sure your electrode size is compatible with metal thickness. A too-big electrode can cause high buildup and thick slag, while an electrode that is too tiny yields various other defects.

Source: https://www.weldingtipsandtricks.com/arc-welding.html

Most beginners use either E6013 or E7018 rods due to their ease of use. Both create smooth welds with easily peelable slag, so as long as your technique is correct, you don't need to worry too much about slag inclusions. However, some report that inclusions are more common with 6013s.

If the slag builds on the tip of your electrode, use a strong file to remove it before continuing to weld. Welding with contaminated electrodes can cause slag inclusions.

Use a file to remove the slag on the tip of your electrode.
Source: https://www.youtube.com/watch?v=tC7PmksNDhY

2. Increase Amperage

Stick weld at the upper end of amperage to avoid inclusion. High amps will provide enough force to melt the pieces and force the flux out of the weld puddle. Additionally, you can keep a tight arc and create a clean-finished weld. Of course, make sure that you don't blast through the base metal.

Newer Stick welders, such as the YesWelder ARC205DS-Pro , feature additional settings, such as hot start or arc force, to help you achieve even better penetration and heat. This ensures that you are not running cold welds that lack penetration or trap the slag inside the weld puddle.

ARC-205DS-PRO Digital Display 110V/220V Stick& Lift TIG In One Stick Welder

3. Practice the technique

Understanding all aspects of the welding technique is crucial to avoiding slag inclusions. First, make sure you drag or pull the torch towards you at a travel angle of 5-15 degrees.

Move moderately across the joint without traveling too fast or too slow. The speed should be high enough to focus the flux inside the weld puddle and provide enough penetration and good tie-in. The flux should pass through all stages: reaction, wetting, rising to the top, and freezing. If you feel you are missing some of them, adjust the speed.

Keep the arc length consistent across the joint. The proper stick arc length should be the same diameter as the core diameter of the electrode. For example, for a 1/8" electrode, you should keep a 1/8" distance between the tip of the rod and the base metal. That way, you provide consistent voltage, enough to drive the flux to the surface of the weld.

Source: https://www.wcwelding.com/arc-welding-tips.html

4. Plan your welds and multi-pass welding

Although stick rods burn through light contamination, such as dirt, oil, or paint, it is always better to clean the surfaces before welding. Clean surfaces yield clean welds with little spatter and easily peeling slag.

Make sure your surfaces and pieces have an even gap between them. Even joint gap produces even results, with no cavities that can pick up slag inclusions. When multi-pass stick welding, plan your passes carefully. The wider the joint, the more passes you'll need, and the higher the risk of slag inclusions.

Your planning should include thorough slag cleaning after each pass and weld bead lining. Lining the welds limits the number of cavities between two parts, lowering the risk of inclusions. Meanwhile, thoroughly cleaning each pass ensures that no slag is left in the middle of the weld.

Overhead Multi-pass E7018 Stick Welding
Source: https://www.youtube.com/watch?v=7HTxpLJ7qD4

5. Visually inspect pieces

Most slag inclusions you'll encounter in your home or hobby welding are surface defects. That means you can notice the inclusions on the surface of the weld right after peeling the slag.

Uneven beads, holes, lines, or pinholes in your welds are never a good sign. You can quickly notice these by visually inspecting the welds, and it is crucial to repair all mistakes before proceeding.

Welds in critical applications, such as pressure vessels or pipes, are subjected to a series of destructive and non-destructive tests. Professional inspectors use various tools to check for internal impurities and inclusions and offer solutions for further repair or rework.

Source: https://www.trccompanies.com/insights/source-inspection-non-destructive-testing/

Final Thoughts

Slag is a paramount part of Stick welding, as it protects the molten weld puddle from atmospheric gases and contamination. When welded properly, slag peels off quickly, and you move on.

However, many beginners make mistakes by improperly setting up the machine, ignoring preparation planning and cleaning, or simply using the wrong technique. In these cases, a specific defect called slag inclusions appears.

Slag inclusions affect the structural integrity of the weld and create weak spots in the welds. You can prevent them by using the proper technique, tuning up the amperage, and cleaning the slag and pieces before each weld.