What is slag in welding? A full guide to welding slag.
Slag in welding discussion is a type of slag, or vitreous material, produced as a byproduct of some arc welding processes, specifically shielded metal arc welding (also known as stick welding), submerged arc welding, and flux-cored arc welding. Slag is formed when the molten, solid shielding material used in the welding process melts inside or on top of the weld. Slag is the solid state of the alloy remaining after the weld has cooled.
What is Flux in Welding?
Welding flux is a combination of carbonate and silicate materials used in welding processes to protect the weld from atmospheric gases. When the heat flux reaches the weld spot, the flux melts and flows out. The gases produced displace the outside gas, preventing oxidation (and reaction with nitrogen).
Molten flux covers the molten metal in the weld space. The flux material is chosen so that the density of the molten flux/slag is lower than the density of the metal to be welded, so that the flux floats to the very top of the weld bead and leaves pure or nearly pure metal to solidify at the bottom.
Flux components can contribute to the metallurgical properties of molten metal, including physical or chemical changes in the molten metal.
The flux coating helps to thermally isolate the weld and reduce the cooling rate.
Inclusion of welding slag
Weld slag inclusions are welding defects that occur when the slag does not float to the top of the molten weld pool.Â
These inclusions may be visible on the surface of the weld, or they may be hidden in the metal and only detectable by x-raying the weld.Â
Inclusions can have a negative effect on weld quality and may need to be removed by drilling or grinding before the section can be reworked. Common causes of weld lag inclusions include using poor quality flux, handling the flux incorrectly, or using poor welding techniques.
Procedures
Flux is used in four welding techniques that result in slag:
- welding using electro-slag
- Submerged arc welding
- Another name for flux-core arc welding is FCAW or FC
- The term “shielded metal arc welding,” or “SMAW,”
Slag removal
The slag after the welding process does not contribute to the strength or protection of the metal; it is waste material. Slag removal is necessary for four reasons:
- Ability to inspect weld zone quality;
- aesthetics or visual appearance;
- If the second layer, or welding pass, is made above the first,
- To make the surface clean and transparent for coatings like paint or oil.
Removal is usually done manually or using mechanical tools. Hand tools may include a welding or chipping hammer, which has a pointed head on one end for efficiently breaking large slag chunks, or wire brushes. Mechanical equipment includes grinding discs or wire brush wheels with angle grinders.
FAQ
What type of welding produces slag?
Slag is produced by a variety of welding techniques, such as electro-slag welding, flux-cored arc welding (gas- and self-shielding), and submerged arc welding. The type and amount of flux used in the electrode determines the thickness of the slag produced.
What Use Does Slag Have in the Welding Process?
Welding slag, which is formed by the molten flux coating during the welding process, is to protect the molten metal from contamination and oxidizing atmospheric gases. Slag is a metal by-product that must be removed to leave a clean metal joint, especially if additional welding passes are to be made, as any slag left behind can weaken the subsequent weld layer applied. In addition to containing flux, the slag contains atmospheric gases and impurities that are absorbed by the flux during melting.
What Takes Place When Slag Is Welded Over?
It is recommended to remove the welding slag between welding passes to avoid the risk of non-metallic inclusions from the flux entering directly into your weld, which will cause weld defects that will weaken the joint.
Is slag weldable over it?
Although it is technically possible to weld over slag, it is generally not recommended because any slag left at the bottom of the weld puddle, especially if it is heavy, will likely contaminate the new weld. An experienced welder may be able to complete multiple passes without cleaning the slag from the previous weld bead, but the arc energy must be sufficient to push the weld bead back from the joint; The more power is used, the more slag is formed and pressure jumps can cause spalling.
Does MIG welding contain slag?
Although flux-cored arc welding, stick welding, and submerged arc welding are the common processes linked to slag (and slag inclusions), MIG welding can also produce inclusions.
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