Welding slag is a residue formed during some welding processes. It is a result of flux melting. Some electrodes often have flux as a safeguard from oxidation.
When the flux melts, it forms a shielding gas and slag to shield the weld pool (molten metal) and the arc from direct contact with oxygen. The flux is also responsible for supplying deoxidezers, alloying, and scavengers to the weld area.
How is welding slag formed? What purpose does it serve? What welding processes produce slag? Does slag cause any problems? Is welding slag magnetic? We’ll take a look at some questions related to slag formation and more.
The formation of slag and its purpose
The filler metal and flux coating both melt at the same time after striking an arc, when the heat intensifies. The metal forms a molten weld pool and the flux coating melts generating shielding gas and slag.
They shield the hot weld pool of molten metal from contamination and atmospheric elements that can cause a reaction. Slag protects the weld joint and area from oxidation due to exposure to the environment.
As the metal joint cools, the slag also solidifies and then it’s chipped off. Slag is usually a non metallic by product and needs to be removed, especially if there is another weld pass to be made. It can weaken the weld layer.
Is welding slag magnetic? We’ll see. Its purpose is served during the process itself. The slag and shielding gas are an effective barrier against oxidation. It can also be an effective shield against corrosion.
What welding processes produce slag?
Many methods of welding can produce slag. Shielded metal arc welding, submerged arc welding, flux cored arc welding, electroslag welding are some examples of welding processes that produce slag.
The welding slag thickness varies depending on the kind of flux and its amount in the electrode.
Does welding slag cause any problems?
Weld slag serves many purposes that are useful. However, sometimes it can cause specific problems as well. One such problem is weld slag inclusion. It adversely affects the quality of the weld joint.
Slag inclusions may happen when the slag that melts during welding process fails to rise to the surface of the weld pool.
This results in slag remaining inside the joint and thus forming a weak joint. The reasons for this happening can be poor flux quality, improper handling, or poor welding technique. Slag inclusions can also cause corrosion in the joints over time.
Is welding slag magnetic?
As mentioned above, the slag is usually a non metallic by product. So, is welding slag magnetic? No, it isn’t magnetic. However, if there are remnants of iron in the slag, it can attract towards magnets. Sometimes silicate is a component of welding slag.
Silicates which are iron-bearing are para-magnetic at all temperature. In such cases this can be different. But, welding slag is mostly not magnetic.
How does flux coating affected weld slag?
The main purpose of flux layer is to produce a molten slag. This slag covers the molten weld pool evenly and keeps it safe from atmospheric gases and contamination. It acts as a shield to prevent oxidation.
The composition of the flux coating can also affect the risk of slag inclusion. It also directly impacts the ease of removing the slag after weld joint solidifies.
If the weld pool is low in oxygen, it can exhibit higher surface tension, resulting in insufficient covering of the parent material. Due to this reasons, an oxidizing flux should be used, for example one that has iron oxide.
It helps create low surface tension in the weld pool, making the weld joint more concave. It also helps the weld pool to wet the surface of the base metal properly.
Flux with hugh silicate can produce glass-like slag which self-detaches more quickly. If the flux coating has lime material, the slag created can be more difficult to remove.
Common types of flux and how easy or difficult it is to remove them
Acid fluxes or rutile
Some specific compositions contain titanium oxide in large quantities, also known as rutile. This results in a weld pool containing sufficiently high level of oxygen, producing a slightly convex or sometimes flat weld joint.
The ease to remove the slag depends on the levels of calcium fluoride. Flux coatings that contain high fluoride content are less easy to remove.
Basic fluxes
These compositions contain high calcium fluoride and calcium carbonate content which reduces oxygen in the molten weld pool. It directly affects the surface tension of the weld pool.
These compositions are more fluid and allow quick freezing, which is helpful when welding in vertical or overhead positions. However, the slag formed is more difficult to remove.
Do welding techniques affect slag inclusions?
The risk of slag inclusion can be affected by the welding techniques used. Proper handling of electrodes and correct techniques to ensure that pockets don’t develop during the process should be taken to prevent slags getting trapped inside.
The correct electrode size and welding angle should be taken in consideration to prevent this from happening. The minimization of undercut and slag pockets should be kept in mind when basic electrodes are used and during multiple welding passes.
When welding using the weaving technique, the dwell time should be slightly prolonged towards the end of the weave pattern ro ensure improved side wall fusion and a flatter joint.
High welding speed and excessive current can also cause sidewall undercutting, making it difficult to remove the slag, which should also be noted.
In multiple welding passes, the slag should be properly removed before applying the next pass. Chipping, grinding, or wire brushing can be used for slag removal.
What happens when welding over slag?
If you are welding over slag, the arc power should push the slag on the weld puddle from the alloy joint and straight back. More energy will result in more slag, given that the weld joint doesn’t burst or burn off during the process.