Introduction
Submerged arc is the process of electric arc welding with consumable electrodes, in which the electric arc and the weld pool are protected from the environment by the products resulting from the burning of a flux that is added independently of the electrode. It can be used as an electrode: solid wire, tubular wire or tape.
Details of the process
Submerged Arc Welding (SAW) joins metals by heating and melting them with an electric arc (or arcs) established between a bare electrode (or multiple electrodes) and the base metal.The arc is submerged (covered) by a layer of fusible granular material which is known as flux; therefore, the melting regime is mixed: by joule effect (ie thermal) and by electric arc.
Automatic devices ensure the feeding of the electrode (or electrodes) at a convenient speed in such a way that their ends constantly dip into the bath of molten flux.
The movement of the welding head in relation to the workpiece advances the weld pool step by step, which is always covered and protected by a slag that is formed by the molten flux and impurities. The figure below shows this process.
Advantages of the process:
- High quality welds.
- Extremely high deposition rate and travel speed.
- No visible welding arc, minimizing protection requirements.
- Little or no smoke.
- May use multiple wires (Optional).
A utility of the submerged arc welding process is the welding of thick steel sheets, for example, pressure vessels, tanks, large diameter pipes and beams.
Care must be taken with drying and maintenance of fluxes drying to avoid introducing hydrogen into the solder.
The flow that was not used can be reused but there are criteria. The first is obviously that it hasn't fused (or been used). The second is that there are limits on how many times the flux can be used because the more reuse, the more moisture contamination (for example).
Welding Equipment
Submerged arc welding is a normally automatic process and can be found as semi-automatic, in which the consumable feed and arc length are controlled by the wire or ribbon feeder and the power source.In the automatic process, an advance mechanism moves the welding head along the part and, normally, a system to recover the unused granular flux. In cylinder union welding, the welding head remains fixed and the assembly moves through swivel positioners.
- A variable voltage from a DC generator or rectifier.
- A direct voltage from a DC generator or rectifier.
- Um AC transformer.
Power sources provide high working currents. Most welding is done in a range of 350 to 2000A. Direct current welding allows better control of weld pass shape, penetration depth and welding speed.
Direct current welding normally takes place with reverse polarity (electrode positive, DC+). Alternating current has the advantage of reducing magnetic blow (deflection of the arc from its normal path due to magnetic forces).
Fluxes for submerged arc welding also change the chemical composition of the weld and influence its mechanical properties. Flux characteristics are similar to coatings used in the coated electrode arc welding process.
The different types of flow are listed below:
- Melted;
- Agglutinated;
- Mechanically mixed.
The possibility that the process presents of using several wire-flux combinations, as both are individual, gives the process great flexibility to achieve the desired properties for the weld.
Process Control
The following observations are important for mastering the submerged arc welding technique:- The greater the current intensity (I) the greater the penetration;
- The greater the voltage (V) the greater the arc length and consequently the greater the width of the pass;
- The greater the stick out (distance between the electrode and the part) the greater the deposition rate;
- The higher the welding speed, the lower the penetration and the smaller the pass width;
- The smaller the electrode diameter, the greater the penetration;
- Direct current, reverse polarity (DC+) produces lower deposition rate and higher penetration.
Features and uses
Submerged arc welding can be used for many industrial applications, which include ship fabrication, structural element fabrication, pressure vessels, etc. The process can be used to weld thin sections as well as thick sections (5 mm up to over 200 mm).The process is mainly used in carbon, low-alloy and stainless steels.
Not suitable for all metals and alloys.
Listed below are some of the various classes of base metals that can be welded by this process:
- Carbon steel up to 0.29% C.
- Heat treated carbon steels (normalized or tempered – tempered).
- Low alloy steels, quenched and tempered, yield strength up to 700 Mpa (100,000 psi).
- Chrome-molybdenum steels (1/2% at 9%Cr and 1/2% at 1% Mo).
- Austenitic stainless steels.
- Níquel and lines of Níquel.
By this process, butt welds, angle welds, plug welds can be carried out, as well as surface deposits on the base metal (coating).
When welding open root butt joints, a copper joint is used to support the molten metal. Instead of a copper joint it is possible to use a TIG root weld to withstand the heat of the submerged arc.
In coating welding to provide desired properties to a surface, for example corrosion or erosion resistance, the filler metal used is normally a tape. The deposition rate can vary from 5.0 kg/h, using semi-automatic processes, to a maximum of approximately 85 kg/h, when using automatic processes with several conjugated arcs.
Preparing and cleaning the gasket
Joint cleaning and machine alignment with the joint are particularly important in submerged arc welding.As far as cleaning is concerned, any residual contamination not removed can result in porosity and inclusions. Therefore, for submerged arc welding, all the recommendations made for welding with coated electrode prevail, namely:
- Parts to be welded must be free of oil, grease, rust, residue from examination by penetrating liquid, sand and soot from gas preheating, in a strip of at least 20 mm on each side of the edges, and demagnetized.< /li>
- Irregularities and slag from the oxy-cut should be removed, at a minimum, by grinding.
- Carbon, slag and copper deposits resulting from carbon electrode cutting must be removed.
- Incorrect machine/joint alignment results in lack of penetration and lack of fusion at the root. If the weld is highly restricted, cracks can also arise due to misalignment.
Process-induced discontinuities
In submerged arc welding, like welding with coated electrode, almost any type of discontinuity can occur, at least the most common ones. The welding inspector must pay attention to some main aspects:Lack of Fusion
It can occur in the case of a thick bead executed in a single pass or in very fast welds, that is, in cases of low welding energy.Lack of penetration
As I mentioned before, the lack of penetration, when it happens, is due to an incorrect alignment of the welding machine with the joint to be welded.Slag Inclusion
It can occur when slag removal in multi-pass welding is not perfect. We must take care that all the slag is removed, bearing in mind that there are regions where this operation is more difficult: the region between passes and that between the pass and the face of the chamfer executed in the base metal.Undercut
They happen with some frequency in submerged arc welding, when the welding proceeds quickly and when the current is very high.Porosidade
It occurs frequently, having as main causes the high speed of advance of the machine and the fast cooling of the weld. These are gas bubbles trapped under the slag. We can eliminate porosity by changing the granulation (less fines) or the flow composition.Other ways to avoid porosity are: adequate cleaning of the joint, reduction of the machine's forward speed, use of wires with higher deoxidizing content and adequate flow height.
Trinca
In submerged arc welding, cracks can occur at high temperatures or at low temperatures. Crater cracks normally occur in submerged arc welding, unless the operator has a perfect crater filling technique.In practice, we use appendix plates (run-on and run-off tabs) to move the beginning and end of the welding operation away from the parts that are actually being welded.
Edge cracks and root cracks often occur some time after the welding operation and in this case are due to hydrogen. Often the cause is moisture in the stream.
Double rollings, non-base metal slabs and lids
May lead to cracks in submerged arc welding. Such discontinuities appear in the form of notches that tend to initiate cracks in the weld metal. Double laminations associated with high welding stresses can result in interlamellar cracking.Conditions for personal protection
With the submerged arc (invisible because it is below the flux) welding is normally performed without fumes, projections and other inconveniences commonly seen in other electric arc welding processes.Hence, we do not need masks and other protective devices other than safety glasses. They should be darkened to protect against glare in the event that an arc starts inadvertently without coverage flow.
Submerged arc welding can produce toxic fumes and gases. It is always advisable to ensure that there is adequate ventilation of the welding place, especially in the case of confined areas.
The operator and others involved in the operation of welding equipment must be familiar with the manufacturer's operating instructions. Particular attention must be paid to the precautionary information contained in the operating manual.
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