Slag
(welding)
Welding slag is a form of slag, or vitreous material
produced as a byproduct of some arc welding processes, most specifically
shielded metal arc welding (also known as stick welding), submerged arc
welding, and flux-cored arc welding. Slag is formed when flux, the solid
shielding material used in the welding process, melts in or on top of the weld
zone. Slag is the solidified remaining flux after the weld area cools.
Flux
Shielded metal arc welding process, showing slag
Welding flux is a combination of carbonate and silicate
materials used in welding processes to shield the weld from atmospheric gases.
When the heat of the weld zone reaches the flux, the flux melts and outgasses.
The gases produced push the atmospheric gas back, preventing oxidation (and
reactions with nitrogen).
The melted flux covers the molten metal in the weld zone.
Flux materials are chosen so that the density of the melted flux / slag is
lower than that of the metal being welded, so that the flux floats to the top
of the weld puddle and leaves pure or nearly pure metal to solidify below.
Flux materials may also contribute to metal behavior in the
molten metal, with physical or chemical alterations to the molten metal.
The flux cover also helps thermally insulate the weld and
reduce the cooling rate.
Flux-core arc welding process, showing slag
Inclusions
It is possible for areas of slag to become embedded within
the solidified metal, if it did not float to the top of the molten metal for
some reason. These are called inclusions and are a form of welding defect.
Inclusions may be visible on the surface after cleaning, or may be completely
contained within the metal, in that case they can only be detected on X-rays of
the weld, requiring grinding or drilling to remove (followed by re-welding that
section).
Processes
Electroslag welding process, showing slag
Four welding processes use flux in slag-producing manners:
Shielded metal arc welding, also known as SMAW
Flux-core arc welding, also known as FCAW or FC
Submerged arc welding
Electroslag welding
Slag does not contribute to strength or protection of metals
after the welding process; it is waste material. Removal of the slag is
necessary for four reasons:
ability to inspect the quality of the weld area;
aesthetics, or visual appearance;
to clean and clear the surface for coatings such as paint or
oil.
Removal is usually done using manual or power tools. Manual
tools may include a welding or chipping hammer, which has a pointed tip on one
end to break up large chunks of slag efficiently, or wire brushes. Power tools
include angle grinders with grinder disks or wire brush wheels.
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