Because there are several types of welding, the prerequisites for reversing a weld vary. Generally, welding may be removed using plasma cutting, torch cutting, grinding, circular saws, drills, or oxy-acetylene torches. The type of metal being welded affects which methods can be used to remove it.
Welding is the process of joining two pieces of metal together by melting filler material such as wire or powder and then allowing the metal to cool and solidify. The two main types of welding are fusion welding and adhesive bonding. In fusion welding, the metals melt and fuse together with no remaining seam, while in adhesive bonding only the surface tension of the molten metal causes the metals to stick to one another without any additional substances needed.
Filler materials can be added to welding projects for various reasons. For example, if filler material is added to a weld pool, it will prevent the metal from cooling too quickly which could cause cracks to form in the weld. Fillers can also be used when welding certain metals that would otherwise oxidize during combustion processes such as oxygen-deprived atmospheres or electric arc welding.
The most common types of welding used in construction are gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), flux-cored wire welding (FCAW), and laser welding.
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Weld slag must thus be removed using grinding, abrasive blast cleaning, wire brushing, flame cleaning, or chipping. As a result, only welds done using flux-coated wire or a flux blanket are prone to weld slag development. Flux acts as a slag shield during welding.
If you plan to paint or otherwise finish the weld, then slag must be removed because it contains elements that prevent proper adhesion of subsequent layers. These may include metal, which can cause galls on finished parts; silica, which can break down into silicon dioxide when heated; and phosphorus, which can evaporate at high temperatures. The removal of slag is called "welding prep" and involves either sanding or grinding away any excess material.
Weld slag has many uses after removal from the weld. It can be sold for its gold content or used as an aggregate in concrete or asphalt. The process of removing slag will also remove some amount of metal from the weld zone which should be considered when estimating the amount of filler required for a repair job.
Weld slag is different from burn-off or smoke produced during welding operations. Burn-off occurs when enough heat is applied to a small volume of material to cause it to vaporize or burn.
The removal of weld metal and base metal from the weld root side of a welded joint to allow for full fusion and joint penetration when subsequent welding is done from that side. The backing gouge weld process is used to provide deep welds in hard materials such as steel and stainless steel, while keeping the heat impact on surface quality at a minimum. The backing gouge weld process was developed by Miller Electric Company in the early 1950's.
Backing gouges are specially designed cutting tools with abrasive surfaces used to remove material from the root side of a weld. The tool is rotated either manually or by electric motor drive to produce a swirling action which removes metal without damaging the surrounding area. Backing gouges can be flat or angled. Angled tools include chisels, riffles, and scratch plates. Flat tools include hacksaws, scraper blades, and spade bits.
Welding-type metals such as carbon steel and stainless steel cannot be backed-gouged due to their ductility. Therefore, they must be straightened after welding before being backed-gouged. This process is called hot working. Metals that can be backed-gouged include alloy steels, tool steels, and high speed steel.