What Is Slag in Welding?

This article provides detailed information on welding slag, how it is developed, its benefits and uses, and the problems that can arise with slag inclusions. Furthermore, it also sheds light on how flux composition affects slag and other aspects of welding slag. 

So, what is slag in welding? Consumable electrodes often have flux that melts while welding. This forms shielding gas and slag that safeguard the weld pool and the arc. The slag is chipped off after welding is complete. The flux also supplies deoxidizers, scavengers, and alloying elements for the weld region. 

There are many more details on the welding slag that have been discussed below. Read on to find out more. 

How is welding slag formed and what is its purpose?

After striking an arc, the resultant heat melts both the filler metal and slag. The filler metal is collected into the molten weld pool. Flux coating melts at the same time to generate slag and shielding gas to cover the hot weld pool from atmospheric elements and contamination. The slag solidifies as it cools. Once the weld pass is complete, it must be chipped off. The solidified weld must be removed after completing welding or if another welding pass is required.

Slag is required for specific welding processes because it protects the hot weld joint against environmental oxidation. 

From the chemical point of view, slag is a nonmetallic byproduct. Since it is nonmetallic, it must be removed, especially if there is a need to make another welding pass. Otherwise, it can significantly weaken the applied weld layer. 

The solidified slag contains not just the flux, but also the atmospheric gases and impurities that were absorbed by the flux during melting. The combination of the slag and shielding gas layer provides an effective barrier against corrosion. 

What are some conventional welding processes that can produce slag?

Many welding methods produce weld slag. Examples of such welding processes include flux-cored arc welding, electroslag welding, shielded metal arc welding, and submerged arc welding. 

Both self-shielded flux-cored arc welding and gas-shielded flux-cored arc welding produce slag. The weld slag thickness depends on the kind of flux as well as its relative amount in the electrode. 

Does weld slag cause any problems?

Although weld slag serves many useful purposes, it can also cause specific problems under different circumstances. Weld slag inclusion is one such defect that can adversely affect the quality and reliability of the weld joint. 

If the molten slag material fails to rise to the weld pool surface when cooling down, slag inclusions may develop. The most common causes for weld slag inclusions are improperly handled flux, poor flux quality, and poor welding technique. Although slag inclusions are most commonly associated with submerged arc welding, flux-cored arc welding and stick welding, they can also transpire in MIG welding. 

Weld slag inclusions can lead to corrosion in the weld joint. This can weaken the weld over a while. Crevices can develop as a result of slag inclusions, which can become sites providing extra surface area for corrosion.

Slag is normally visible as a layer that runs along the surface of the weld joint. It may be continuous or discontinuous.      

How are slag inclusions developed?

Here are more details on how slag inclusions are formed. Slag is essentially the residue that develops from the flux layer on the consumable electrode. It comprises mostly of deoxidation products that arise due to a chemical reaction between surface oxide, air, and flux. 

Slag may get trapped within the weld joint if two adjacent weld layers are applied with insufficient overlap so that a void is developed. After the successive layer is applied, the trapped slag may not be able to escape towards the surface. 

Slag may also get trapped within cavities, as a result of multiple welds passes with weld toe undercut. It can also form due to the irregular surface profile of the previous weld layer. 

This kind of weld defect is influenced by several factors, including access restrictions, welding position, and joint configuration. 

What is the effect of flux coating on weld slag?

The primary purpose of the flux layer is to generate a molten slag that can flow freely over the molten weld pool and cover it evenly. This is necessary to keep the molten weld pool safe from oxidation by atmospheric gases and contaminants. 

The slag layer also influences other characteristics of the consumable electrode. As a result, freezing rate and surface tension are essential slag properties that must be taken into account to ensure consistently good welding quality. 

The surface tension property, in particular, is necessary for more difficult welding positions. Slags with higher viscosity are preferred to perform welding in a horizontal or vertical position. The resultant slag layer is less likely to get trapped and will create a smoother weld joint profile. On solidifying, it will also be easier to remove it. 

In vertical welding applications, the slag should have lower viscosity for better flow. It should also have higher surface tension to support the weld pool and prevent it from flowing away. It must have a fast freezing rate so that it can quickly contain the weld pool and prevent it from leaking away from the heat-affected zone. 

The flux coating composition can also affect the slag inclusion risk. Flux composition has a direct impact on the ease with which the slag can be removed and on the weld joint profile as well. A weld pool with a low oxygen content is likely to exhibit higher surface tension. As a result, it will not be able to wet the parent material sufficiently. 

Therefore, an oxidizing flux should be employed, one that contains iron oxide, for instance. This will help create a weld pool with low surface tension. The weld joint profile will also be more concave and will enhance the ability of the weld pool to wet the base metal properly.

Flux with high silicate content generates a glass-like slag that can self detach more quickly. Flux coatings with more lime material create a slag that is difficult to remove.  

How easy or difficult is it to remove common flux types?

Acid fluxes or rutile

Specific flux compositions contain large quantities of titanium oxide, which is also known as rutile. It can also be mixed with a certain percentage of silicates. The resultant weld pool has a sufficiently high oxygen level to produce a slightly convex or flat weld joint profile. The ease with which the slag flows depends on the calcium fluoride levels. Flux coatings may be designed for supine welding positions to produce the right weld bead shape. 

Fluoride-free coatings are utilized for smooth weld joint profiles and to remove the slag with ease. Flux coatings with higher fluoride content are more fluid in nature. These flux coatings are less easy to chip off.  

Basic fluxes

This kind of flux composition contains high levels of calcium fluoride (fluorspar) and calcium carbonate (limestone). This reduces oxygen content within the molten weld pool, which directly affects surface tension. Compared to rutile coating, the slag produced with these compositions is more fluid. Quick freezing also helps with welding in the overhead and vertical positions. However, the slag layers thus formed are more challenging to remove. 

As a consequence of the properties described above, the risk of slag inclusions is higher with elemental fluxes. This can also be attributed to the convex weld joint profile, as well as the difficulty involved in chipping off the slag from the weld surface. This problem is more pronounced where multiple welding passes are made.  

What is the effect of welding technique on slag inclusions?

Welding techniques can have a profound effect on the risk of slag inclusion. Correct electrode handling can lead to adequate weld bead overlap and the right shape as well. These characteristics ensure that pockets do not develop. Slags can get trapped in these pockets. 

Specific steps should be taken to minimize the occurrence of slag entrainment. The correct electrode size should be selected according to the nature of the joint. Also, the proper welding angle will ensure sufficient weld penetration as well as a smooth weld profile. 

For multiple vertical welding passes, especially when basic electrodes are used, you should weld so that slag pockets and undercut are minimized. While using the weaving technique, you should take care to prolong the dwell time slightly at the ends of the weave pattern. This will result in improved sidewall fusion as well as a flatter weld joint profile. 

It is worth noting that high welding speed and excessive current may cause sidewall undercutting. Due to this, slag removal will become difficult. 

Before applying the next welding pass, ensure that all slag is removed from the surface. You can accomplish this with chipping, grinding, or just plain wire brushing. The cleaning tools used for this purpose must be used only for materials on which they were initially used. 

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