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Submerged arc welding or SAW is a form of welding that takes place underneath a layer of flux material. This material could be any of a variety of different compounds. The flux material acts as a guard against contamination in the weld, also creates sparks to shield the arc.
Why do we need Submerged Arc Welding?
Today, many factories have high demand for products that require welding. Submerged arc welding allows for those to be carried out quickly and consistently. Submerged arc welding is also a much safer way of performing welds using arc welding.
There is a variety of saw equipment available, including welding tractors that are used to automatically weld a seam, and flux recovery units, that use a vacuum to "hoover"up excess flux from the weld.
Often used alongside positioning equipment, like column and booms and pipe rotators, sub arc equipment is used in pipe and vessel fabrication within a variety of fabrication industries.
Need more information? Read this Blog on Submerged Arc or contact us to speak to our expert team on the Sub-Arc process.
Learn more about the Sub-Arc process here.
Submerged Arc Welding (SAW)
Sub-Arc welding is a common arc welding process. It requires a continuously fed consumable solid or tubular (flux cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being “submerged” under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the SMAW (shielded metal arc welding) process.
SAW is normally operated in the automatic or mechanized mode, however, semi-automatic (hand-held) SAW guns with pressurized or gravity flux feed delivery are available. The process is normally limited to the Flat or Horizontal-Fillet welding positions (although Horizontal Groove position welds have been done with a special arrangement to support the flux). Deposition rates approaching 100 lb/h (45 kg/h) have been reported — this compares to ~10 lb/h (5 kg/h) (max) for shielded metal arc welding. Although Currents ranging from 300 to 2000 A are commonly utilized, currents of up to 5000 A have also been used (multiple arcs).
Single or multiple (2 to 5) electrode wire variations of the process exist. SAW strip-cladding utilizes a flat strip electrode (e.g. 60 mm wide x 0.5 mm thick). DC or AC power can be utilized, and combinations of DC and AC are common on multiple electrode systems. Constant Voltage welding power supplies are most commonly used, however Constant Current systems in combination with a voltage sensing wire-feeder are available.
Electrode
SAW filler material usually is a standard wire as well as other special forms. This wire normally has a thickness of 1/16 in. to 1/4 in. (1.6mm to 6mm). In certain circumstances, twisted wire can be used to give the arc an oscillating movement. This helps fuse the toe of the weld to the base metal
Key SAW process variables
- Wire Feed Speed (main factor in welding current control);
- Arc Voltage;
- Travel Speed;
- Electrode Stick-Out (ESO) or Contact Tip to Work (CTTW);
- Polarity and Current Type (AC or DC) & Variable Balance AC current.
Other factors
- Flux depth/width;
- Flux and electrode classification and type;
- Electrode wire diameter;
- Multiple electrode configurations.
Material applications
- Carbon steels (structural and vessel construction);
- Low alloy steels;
- Stainless steels;
- Nickel-based alloys;
- Surfacing applications (wearfacing, build-up, and corrosion resistant overlay of steels).
Advantages
- High deposition rates (over 100 lb/h (45 kg/h) have been reported);
- High operating factors in mechanized applications;
- Deep weld penetration;
- Sound welds are readily made (with good process design and control);
- High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible;
- Minimal welding fume or arc light is emitted.
Practically no edge preparation is necessary · The process is suitable for both indoor and outdoor works. · Distortion is much less. · Welds produced are sound, uniform, ductile, corrosion resistant and have good impact value. · Single pass welds can be made in thick plates with normal equipment. · The arc is always covered under a blanket of flux, thus there is no chance of spatter of weld.
Limitations
- Limited to ferrous (steel or stainless steels) and some nickel based alloys;
- Normally limited to the 1F, 1G, and 2F positions;
- Normally limited to long straight seams or rotated pipes or vessels;
- Requires relatively troublesome flux handling systems;
- Flux and slag residue can present a health & safety issue;
- Requires inter-pass and post weld slag removal.
Manufacturers: Lincoln ESAB Miller SAF Linde-Union Carbide Fronius Gullco
A very useful Submerged Arc welding handbook from ESAB covers all the subarc welder needs here
Equipment for Submerged-arc Welding
The submerged-arc welding (SAW) process is similar to MIG where the arc is formed between a continuously-fed wire electrode and the work piece, and the weld is formed by the arc melting the work piece and the wire. However, in SAW a shielding gas is not required as the layer of flux generates the gases and slag to protect the weld pool and hot weld metal from contamination. Flux plays an additional role in adding alloying elements to the weld pool.
Essential equipment
Essential equipment components for SAW are:
- power source
- SAW head
- flux handling
- protective equipment
As SAW is a high current welding process, the equipment is designed to produce high deposition rates.
Power source
SAW can be operated using either a DC or an AC power source. DC is supplied by a transformer-rectifier and AC is supplied by a transformer. Current for a single wire ranges from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter wire). In practice, most welding is carried out on thick plate where a single wire (4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a twin wire system operating between 800 and 1200A.
In DC operation, the electrode is normally connected to the positive terminal. Electrode negative (DCEN) polarity can be used to increase deposition rate but depth of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for surfacing applications where parent metal dilution is important. The DC power source has a 'constant voltage' output characteristic which produces a self-regulating arc. For a given diameter of wire, welding current is controlled by wire feed speed and arc length is determined by voltage setting.
AC power sources usually have a constant-current output characteristic and are therefore not self-regulating. The arc with this type of power source is controlled by sensing the arc voltage and using the signal to control wire feed speed. In practice, for a given welding current level, arc length is determined by wire burnoff rate, i.e. the balance between the welding current setting and wire feed speed which is under feedback control.
Square wave AC square wave power sources have a constant voltage output current characteristic. Advantages are easier arc ignition and constant wire feed speed control.
Welding gun
SAW can be carried out using both manual and mechanised techniques. Mechanised welding, which can exploit the potential for extremely high deposition rates, accounts for the majority of applications.
Manual welding
For manual welding, the welding gun is similar to a MIG gun, with the flux which is fed concentrically around the electrode, replacing the shielding gas. Flux is fed by air pressure through the handle of the gun or from a small hopper mounted on the gun. The equipment is relatively portable and, as the operator guides the gun along the joint, little manipulative skill is required. However, because the operator has limited control over the welding operation (apart from adjusting travel speed to maintain the bead profile) it is best used for short runs and simple filling operations.
Mechanised welding - single wire
As SAW is often used for welding large components, the gun, wire feeder and flux delivery feed can be mounted on a rail, tractor or boom manipulator. Single wire welding is mostly practised using DCEP even though AC will produce a higher deposition rate for the same welding current. AC is used to overcome problems with arc blow, caused by residual magnetism in the work piece, jigging or welding machine.
Wire stickout, or electrode extension - the distance the wire protrudes from the end of the contact tip - is an important control parameter in SAW. As the current flowing between the contact tip and the arc will preheat the wire, wire burn-off rate will increase with increase in wire stickout. For example, the deposition rate for a 4mm diameter wire at a welding current of 700A can be increased from approximately 9 kg/hr at the normal 32mm stickout, to 14 kg/hr at a stickout length of 178mm. In practice, because of the reduction in penetration and greater risk of arc wander, a long stickout is normally only used in cladding and surfacing applications where there is greater emphasis on deposition rate and control of penetration, rather than accurate positioning of the wire.
For most applications, electrode stickout is set so that the contact tube is slightly proud of the flux layer. The depth of flux is normally just sufficient to cover the arc whose light can be seen through the flux.
Recommended and maximum stickout lengths:
|
Wire diameter mm
|
Current range A
|
Wire stickout
|
|
Normal mm
|
Maximum mm
|
| 0.8 |
100 to 200 |
12 |
- |
| 1.2 |
150 to 300 |
20 |
- |
| 1.6 |
200 to 500 |
20 |
- |
| 2.0 |
250 to 600 |
25 |
63 |
| 3.2 |
350 to 800 |
30 |
76 |
| 4.0 |
400 to 900 |
32 |
128 |
| 4.75 |
450 to 1000 |
35 |
165 |
Mechanised welding - twin wire
Tandem arc connections
SAW can be operated with more than one wire. Although up to five wires are used for high deposition rates, e.g. in pipe mills, the most common multi-wire systems have two wires in a tandem arrangement. The leading wire is run on DCEP to produce deep penetration. The trailing wire is operated on AC which spreads the weld pool, which is ideal for filling the joint. AC also minimises: interaction between the arcs, and the risk of lack of fusion defects and porosity through the deflection of the arcs (arc blow). The wires are normally spaced 20mm apart so that the second wire feeds into the rear of the weld pool.
Gun angle
In manual welding, the gun is operated with a trailing angle, i.e. with the gun at an angle of 45 degrees (backwards) from the vertical. In single wire mechanised welding operations, the gun is perpendicular to the work piece. However, in twin wire operations the leading gun is normal to the work piece, with the trailing gun angled slightly forwards between an angle of 60 and 80 degrees. This reduces disturbance of the weld pool and produces a smooth weld bead profile.
Flux handling
Flux should be stored in unopened packages under dry conditions. Open packages should be stored in a humidity-controlled store. While flux from a newly-opened package is ready for immediate use, flux which has been opened and held in a store should first be dried according to manufacturer's instructions. In small welding systems, flux is usually held in a small hopper above the welding gun. It is fed automatically (by gravity or mechanised feed) ahead of the arc. In larger installations the flux is stored in large hoppers and is fed with compressed air. Unused flux is collected using a vacuum hose and returned to the hopper.
Note: Care must be taken in recycling unused flux, particularly regarding the removal of slag and metal dust particles. The presence of slag will change the composition of the flux which, together with the wire, determines the composition of the weld metal. The presence of fine particles can cause blockages in the feeding system.
Protective equipment
Unlike other arc welding processes, SAW is a clean process which produces minimum fume and spatter when welding steels. (Some noxious emissions can be produced when welding special materials.) For normal applications, general workshop extraction should be adequate.
Protective equipment such as a head shield and a leather apron are not necessary. Normal protective equipment (goggles, heavy gloves and protective shoes) are required for ancillary operations such as slag removal by chipping or grinding. Special precautions should be taken when handling flux - a dust respirator and gloves are needed when loading the storage hoppers.
