Almost anyone associated with welding has heard the term “slag.” Those involved in shielded metal arc welding (stick), flux-cored arc welding, and submerged arc welding will have first-hand knowledge.
For those new to the welding trade and anyone considering it as a career path, here is a primer that defines slag, how it is developed, its pros and cons, and the problems that can come about with slag inclusions.
What is slag in welding?
Weld slag forms when flux, a solid shielding material (as opposed to shielding gas) used to protect the weld pool and the arc during the welding process, melts into or on top of the weld area. As the weld zone cools, the remaining flux solidifies over the weld bead.
Slag is that remaining flux that forms over the weld bead as the weld area cools. Since it does not result in a surface that is receptive to coatings, the slag must be removed before other coating processes are attempted.
Weld Slag (Picture Credit: Welding Productivity)
What is the welding slag composition?
After the weld area cools, slag is the remaining flux that has solidified. And welding flux is composed of a combination of carbonate and silicate materials that shield the weld from atmospheric gases. As soon as the heat from the weld zone reaches the flux, the flux melts and outgasses, and those gases push back the atmospheric gas, preventing oxidation.
The melted flux covers the molten metal in the weld zone. Flux materials are chosen to keep the density of the melted flux lower than the metal being welded. This allows the flux to float to the top of the weld puddle, leaving relatively pure metal to solidify below it. The covering flux also helps to insulate the weld thermally, reducing the cooling rate.
What is slag inclusion in welding?
Slag inclusions are the nonmetallic particles (slag) that are trapped in the weld or between layers. They are easy to detect (see below) since they are needle-shaped or spherical. Sometimes they penetrate the entire thickness of the weld. Slag inclusions in welding cause concentrated stress, reducing the durability of the weld metal.
Slag inclusions can lead to corrosion in the weld, weakening the weld over time. Crevices can sometimes develop from slag inclusions, and these can provide more surface area for corrosion. As with other welding defects, slag inclusions typically affect the reliability and quality of the weld joint adversely.
Slag Inclusion (Photo Credit: Welding Answers)
What causes slag inclusions?
The flux composition can increase the risk of slag inclusion. It directly impacts how easily the slag can be removed and also on the weld joint profile. If the weld pool has a lower oxygen content, it will probably exhibit higher surface tension. Because of this, it cannot wet the parent material sufficiently.
To prevent this, using a flux that contains iron oxide will help to create a weld pool with lower surface tension and a weld joint profile that is more concave, allowing the weld pool to wet the base metal properly. Flux with a higher silicate content creates a slag that is glass-like and tends to self-detach more quickly. Flux coatings with more lime content often generate slag that is difficult to remove.
Proper welding techniques are essential to preventing slag inclusions
With appropriate technique, slag inclusions will come to the surface of the weld pool. Handling the electrode correctly is a good start since it can result in the right weld bead shape and adequate overlap. Using the best methods can also ensure that pockets that can trap slag do not form.
The correct electrode size, selected according to the nature of the joint, is critical. Also, keep in mind that the proper welding angle ensures adequate weld penetration and a smooth weld profile.
One way that experienced welders avoid problems is by planning their welds for multiple passes. Lining up their beads correctly prevents flux from being trapped inside the weld bead of the next pass.
Welding at the upper range of your amperage settings ensures you can keep a tight arc and create a clean weld. A welder working with lower amperage will not have sufficient power to force the slag, fuse the metal, or keep a focused arc.
While you need enough power to push the slag to the top of the weld puddle, you also must allow the right amount of time for the slag to rise. By welding at a steady travel speed, you will ensure that you won’t burn through the metal, and you’ll give the slag enough time to come to the top. Moving too fast will leave the slag in your finished weld.
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