Slag In Welding

What Is Slag Inclusion in Welding? and How to Prevent it

Xiris Automation Inc.
Written by Xiris Automation Inc. on June 11, 2024

Welding may appear straightforward until common defects arise, sometimes unavoidably or accidentally. Slag inclusion is one of the most common defects in self-shielded and gas-shielded flux-cored welding. The American Welding Society (AWS) defines slag as “a non-metallic by-product of the mutual dissolution of flux with non-metallic impurities in welding processes.” 

Slag typically appears in spherical or needle-shaped forms, varying in shape and size according to the type of product used in the welding process and the type of process followed. It is important to understand the difference between slag and slag inclusion. Slag is a residual of welding and serves two purposes:

(a) It protects the weld from contaminations and oxidation

(b) Helps keep the molten weld pool in the welded joint as it cools down

Slag is a result of a thermodynamic process when elements not part of the weld pool are pushed out to the surface. These elements subsequently solidify and form non-metallic elements of calcium oxides, silicon, or aluminum. It becomes imperative to remove slag by brushing or grinding the weld bead before proceeding with further welding processes. When another layer of weldment is placed without removing the slag from the previous layer, slag inclusion can occur.  Depending on the formation, slag inclusion can be point-like, chain-like, linear, and dense; other reasons for slag inclusion can be: 

  • Relatively high welding speed which results in slag running ahead of the weld; 
  • Poor quality flux; 
  • Improper de-slagging of weldment between layers; 
  • Convex passes which create slag pockets; and 
  • Unfavorable bead sequences. 

How To Prevent Slag Inclusion In Welding? 

Often, a reliable method of preventing slag inclusion is to implement better monitoring of the weld process. Using a Xiris weld camera to remotely view the welding process can assist operators in looking for slag remnants which are seen as the continuous/discontinuous lines appearing along the weld length. This can happen when slag may be seen running ahead of the welding torch, or when inadequate de-slagging of the previous layer of weldment can be seen as the new layer is about to be applied.   

Welding experts recommend using the correct electrode size, ensuring accurate angles, and following welding techniques that produce smooth weld beads. These practices can help reduce the risk of slag inclusion or slag pockets in welding.

For instance, when making a concave weld joint, using a flux with iron oxide content helps create a lower surface tension weld pool enabling it to wet the base metal efficiently. The shape of the weld joint can be easily monitored on a regular basis by the operator using a Xiris weld camera.  If the process goes awry, it can be paused and corrected before defects are created. 

Modifying the speed of travel can change the mixture between slag and weld pool and can limit slag from running forward of the torch. Monitoring the process with a Xiris weld camera allows the operator to track the slag is relative to the torch. These cameras can be mounted at the trailing view (behind the torch) to look forward to the welding process. Additionally, confirmation of slag removal can be done by monitoring the weld process in real-time using Xiris weld cameras. 

The simplest way to prevent welding over slag is to clean and remove the slag thoroughly during multiple-pass welding. In addition, slowing down the rate of cooling of the weld pool can be beneficial. Operating at higher voltages makes the weld bead concave, which can lead to slag being locked into the weld. Mounting a Xiris weld camera can help in detecting the presence of this slag, providing an early warning to the operator before depositing the next layer of weldment. 

Slag can be avoided or reduced in the case of horizontal and flat welding, by using the drag technique whereby the arc is kept on the leading edge of the weld puddle with a weld torch angle of 0-10°. With the push technique in the flat position, the risk of slag in welding increases. The drag technique is also suitable for vertical-up welding if the wire is producing heavy slag. 


Fig. 1: Push Technique and Drag (Pull) Technique of Welding. Source: 


How To Remove Slag From Welding? 

Before removing the slag make sure the weld is cooled down. There are two ways one can get rid of the slag:  

  • Manual Tools: The most common way of removing slag is by using: a wire brush to brush off the welded surface, a chipping hammer to strike the large pieces of slag, or a needle scaler to reach the tight corners of the welded surface. 
  • Power Tools: To quickly remove large amounts of slag an angle grinder is used which grinds the surface slag from the workpiece.  

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Effects Of Slag Inclusion Defects 

Slag inclusion in a welded piece can directly impact the quality of the joint, as it decreases the strength of the weld making it easier to crack or fail with stress. Furthermore, slag in welding reduces the durability of the metal and with time can also give rise to corrosion. Porosity is another common welding defect that increases with slag inclusion. Porosity is when small gas pockets are formed inside the cooled weld pool to create voids that impact the strength of the resulting welded joint. 

Fig. 2: Common Welding Defects: Slag Inclusions and Porosity. Source:  


In certain cases, slag inclusion can also affect the appearance of the joint, as it can result in uneven and rough surfaces making the joint look rough and unpleasant. In the automotive, aviation, aerospace, and art industry the appearance of the joint is a critical consideration. The only resource to fix weld defects is to cut off the defective portion and re-weld it to enhance the strength and quality of the joint. 

Surface welding defects like slag and porosity can be promptly detected using Xiris weld monitoring cameras which allow for real-time monitoring of the weld formation process. 


Slag inclusion is one of the prevalent weld defects, occurring when non-metallic by-products are trapped in the weld pool. This happens due to factors such as improper de-slagging, poor quality flux, or high welding speed. Welders can prevent slag inclusion by using proper cleaning techniques on slag and ensuring the weld area is free of slag prior to the next pass of welding.  This can be done by monitoring the welding process with Xiris weld cameras to look for the presence of slag before a new layer is put down. Removing slag is usually a manual process involving tools like wire brushes or power tools like angle grinders. If left unremoved, the slag on a weld can become slag inclusion, which can weaken the welds, increase porosity, and can over time lead to corrosion, impacting both strength and appearance of the joint. Real-time monitoring with tools like Xiris weld cameras helps detect and prevent such defects, ensuring high-quality welds. 






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