What is Submerged Arc Flux?
Granular flux used in welding is a type of granular insulative materiall that is made up of numerous small particles. In Submerged Arc Welding (SAW), the granular flux provides a blanket over the weld, which protects against sparks and spatter. In SAW, the granular flux is frequently the means for achieving high deposition rates. The flux is also instrumental in producing the type of quality weld that is common in this particular welding process.
Functions of Flux in Sub Arc Welding
The effect of gravity on the flux feeding into the weld area and the molten weld pool limits the versatility of Submerged Arc Welding. This process must be performed in the flat and horizontal fillet positions only, except in special cases. These special cases include vertical and horizontal welds using special equipment, such as belts or shoes, to hold the flux in position.
The granular flux used in SAW serves several functions. In addition to providing a protective cover over the weld, the flux shields and cleans the molten puddle. The flux also affects the chemical composition of the weld metal, the weld bead shape, and the mechanical properties of the weld.
Another function of granular flux is to act as a barrier that holds the heat in and concentrates the heat into the weld area to promote deep penetration.
Types of Granular Fluxes
The methods used to manufacture fluxes determine the flux types. There are fused fluxes, bonded fluxes, agglomerated fluxes, and mechanically mixed fluxes.
When manufacturing fused fluxes, raw materials are melted into a liquid state with a high temperature electric furnace. The material is then cooled and crushed or ground into the desired particle size.
When making bonded fluxes, the ingredients are dry mixed, then glued together with a liquid binder. This binder may be a liquid such as sodium silicate. After the particles are bonded, they are baked and then sifted through a sieve to attain flux particles of the desired size.
Agglomerated fluxes are manufactured much the same way that bonded fluxes are made. However, instead of a liquid binder, a ceramic binder is used. A higher drying temperature is used, too. (The higher drying temperature limits the use of deoxidizers and alloy elements.)
Fluxes that are mechanically mixed are combinations of two or more bonded or agglomerated fluxes. Although mechanically mixed fluxes make it possible to create special mixtures for more sensitive welds, these fluxes may separate during storage, use, and recovery of flux.
Fused Fluxes versus Bonded Fluxes
Among the various types of fluxes use in Submerged Arc Welding are the fused flux and the bonded flux. Each of these fluxes offers some advantages and some disadvantages.
When making fused fluxes, the raw materials are dry mixed together, and then they are fused or melted into a liquid state by using a high temperature furnace. After fusion is complete, the fluxes are cooled. This may be accomplished by using a stream of water or with big chill blocks.
Once the fluxes are cooled, they are crushed or ground into particles. A variety of particle sizes are made to ensure optimal performance for different applications.
Advantages of fused fluxes include:
- The non-hygroscopic flux particles do not absorb moisture and, therefore, any surface moisture can be eliminated merely by drying the particles at a low temperature oven setting of 300 degrees F.
- Low temperature drying of condensation on the fused flux particles provides better protection against hydrogen cracking.
- Flux particles create welds that are chemically consistent.
- Recycling of fused flux particles through the flux recovery systems can be achieved without losing sizing or composition.
A disadvantage of fused fluxes is that the high temperature used during the manufacture process makes it difficult to add alloys and deoxidizers.
The manufacture of bonded fluxes involves combining the dry ingredients, then using a liquid binder such as sodium silicate or potassium silicate to glue the ingredients together. After the bonded mix is made into pellets, the pellets are baked at a low oven temperature. Once the drying of the pellets is complete, the pellets are broken up by using a sieve to attain the desired particle size. The particles are then packaged for shipping.
Advantages of bonded fluxes include:
- Deoxidizers are present in bonded fluxes, protecting against rust and mill scale. These deoxidizers also help to prevent welds from becoming porous.
- Alloys can be added to bonded fluxes. Alloy elements may improve chemical and mechanical properties of the flux.
- Bonded fluxes allow for a thicker flux layer when welding.
- Bonded fluxes can be identified by color.
- Bonded fluxes typically provide better peeling properties than fused fluxes.
There are at least two disadvantages of using bonded fluxes. These are:
- They absorb moisture.
- They can change in composition due to segregation or loss of fine particle size.
About the Submerged Arc Welding (SAW) Process
Submerged Arc Welding (SAW) is a common welding process that is commonly used in the structural and vessel construction industries. Originally developed by the Linde - Union Carbide Company, it is commonly used in beam, boom, tractor, and multi-head type rigs. Also known as Sub Arc or SAW, this process uses a blanket of granular fusible flux beneath which both the weld and the arc zone are protected or “submerged.” This flux blanket offers the following advantages:
- Guards against atmospheric contamination
- Stabilizes the arc during welding
- Prevents splatter and sparks from flying about
- Suppresses radiation and fumes that are typical of the shielded metal arc welding process (SMAW)
How it works
Sub Arc welding requires a continuously-fed tubular or consumable solid electrode and may be fully automatic or semi-automatic. The arc is flat and is maintained between the end of a bare wire electrode and the weld. The electrode is constantly fed into the arc and as it is melted, a layer of granular flux provides a protective cover beneath which the welding occurs. The blanket is created as some of the flux becomes molten. This fusible flux may consist of lime, silica, manganese oxide, calcium fluoride, and other compounds. In a molten or melted state, the flux becomes conductive. This allows it to supply a constant current between the electrode and the welding work. The remainder of the flux is recovered and reused, unless it has become contaminated.
In the automatic version of SAW, the process is performed with a set of rollers driven by a controlled motor to ensure that the wire is fed into the arc at a speed rate that is equivalent to the rate at which the electrode is melted. The arc length remains constant as a result. The SAW process is usually automated; however, there are semi-automated systems available, too.
Properly performed Sub Arc welding should consistently result in mechanical properties that are at least equal to that of the base metal. Ductility and impact resistance should be good, and bead appearance should be uniform.
Variables of the SAW Process
There are some key variables of the submerged arc welding process. These variables include:
- The arc voltage
- The wire feed speed
- Travel speed
- Contact tip to work (CTTW) or electrode stick-out (ESO)
- Polarity and current type (may be either AC or DC), as well as variable balance AC current
In automatic submerged arc welding, there are three types of guns that are generally used. These include the side flux delivery gun, the deep groove gun, and the concentrated flux delivery gun.
The concentrated flux delivery gun deposits the flux around the wire. With both the side flux delivery gun and the deep groove gun, the flux is fed from an overhead gravity hopper to the gun’s flux shut-off assembly.
The type of gun chosen for a certain job may be dependent upon the joint design and/or the welding operator’s preference.
SAW material applications include carbon steels, low alloy steels, stainless steels, nickel-based alloys, and surfacing applications (wear-facing, buildup, and corrosion-resistant overlay of steels). SAW is frequently used in heavy structural construction. It is also used in the pressure vessel industry, chemical plants, and shipbuilding.
Properly performed Sub-Arc welding should consistently result in mechanical properties that are at least equal to that of the base metal. Ductility and impact resistance should be good, and bead appearance should be uniform.
Variations of the Submerged Arc Welding Process
The submerged arc welding process may be varied in a number of ways to give it more capabilities. These include, but are not limited to, varying the number of wires and power sources, adding iron powder to the flux, and using a strip electrode for surfacing.
Multi-wire systems offer advantages, because the use of more electrodes can improve deposition rates and travel speeds. The utilization of more than one electrode in submerged arc welding may be accomplished with either a single power source or separate power sources for each wire.
The use of multiple power sources with two or more electrodes allows for the utilization of different polarities on the electrodes. Also, with separate power sources for two electrodes, alternating current may be used on one, while direct current is used on the other electrode. Typically, when three wires are used in the tandem position (one electrode is placed in front of the other), alternating current is used. The electrodes are connected to three-phase power systems, which are used for making high-speed longitudinal seams on large pipes and fabricated beams.
Adding iron powder to the flux increases deposition rates of submerged arc welding, but it does not decrease the properties of the weld metal.
The utilization of a strip electrode for surfacing may be done to save money. This particular welding system uses the strip electrode and flux to make a corrosion-resistant overlay on a less expensive base material such as stainless steel. During this procedure, a wide, uniform bead is produced that has minimum penetration. The uniform bead is necessary to provide a smooth overall surface. The strip welding system is often used for overlaying the inside of vessels. The flux that is used in strip surfacing is made specifically for that purpose.
Advantages of Submerged Arc Welding
Some of the advantages of submerged arc welding include:
- Strong, sound welds are readily made
- Minimal welding fume is emitted
- Minimal arc light is emitted
- SAW is suitable for both indoor and outdoor works
- Less distortion
- Deep weld penetration
- Minimal edge preparation
- High deposition rates are possible
- Thick materials may be welded
- At least half or more of the flux is recoverable
Disadvantages of Submerged Arc Welding
There are a few limitations with submerged arc welding. One issue is that welding can normally be performed only in the flat position. The use of a granular flux and the fluidity of the molten weld pool mean that welding is limited to positions 1F, 1G, and 2F.
Another disadvantage of SAW is that welding is normally limited to long, straight seams or rotated vessels or pipes. Flux handling systems can be quite bothersome, as well.
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