What Is Slag In Welding

By Joe Stephens •  Updated: 09/15/19 •  2 min read

what is slag in welding

Slag is a byproduct of various welding processes. Essentially when the area you just welded cooled down, the remaining flux leftover is called the slag. The flux is very important when it comes to welding.

What is Flux

The welding flux is a combination of materials used when welding which actually shields the weld from different atmospheric gases. While shielding the weld, the flux will also pull the impurities out of the weld when the heat of the weld reaches the flux. This will also prevent oxidation. The flux will solidify on top of the weld further protecting the weld while it cools.

What Welding Processes Cause Slag

There are essentially four different welding processes that can produce use flux and produce slag. Those processes are:

  • Shielded Metal Arc Welding (SMAW)
  • Flux-Core Arc Welding
  • Submerged Arc Welding
  • Electroslag Welding

Slag Removal

Now most welders can agree that it is necessary to remove the slag for a number of reasons, which include:

  • The obvious one is so that you can inspect your weld. The typical welder likes to look at their welds to see if they need to be fixed or adjusted. With the slag in the way, you wouldn’t be able to do this.
  • Secondly, if you need to make a second pass over the weld you just did. Welding over the slag can cause serious defects.
  • Leaving the slag can also affect the overall look or aesthetic of the assembly
  • If after the welding is done and you want to apply oil or paint, the slag can cause some issues.

You might be wondering how to remove the slag. This process is honestly pretty easy and takes only seconds. The slag is not greatly bonded to the weld itself. Because of this all that is needed is something like a chipping hammer or wire brush. Also, I should point out that the better the weld, the less it’ll be bonded to the slag. So if your welds are good, you should only need to tap it.

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Joe Stephens

Joey has over 20 years of experience working in the welding industry and now works with providing readers with intensive reviews. Joey has also self-published an e-book and has written countless articles regarding welding information and safety.