What is Submerged Arc Welding (SAW)?

Submerged arc welding(SAW) is one of the most important arc welding methods. It has the characteristics of high efficiency, good welding quality, and good working conditions. It is widely used in boilers, pressure vessels, shipbuilding, locomotive vehicles, offshore platform manufacturing and other fields. It can also be used for surfacing welding of wear-resistant or corrosion-resistant coatings and repairing the surface of workpieces, which plays a very important role in industry.

Basic knowledge of SAW

Submerged arc welding (SAW) is “an arc welding process that uses an arc or arcs between a bare metal electrode or electrodes and the weld pool. The arc and molten metal are shielded by a blanket of granular flux on the workpieces. Pressure is not used in the process and filler metal is from the welding wire or sometimes from a supplemental source (flux or metal granules).”

In submerged arc welding, the end of the electrode and the welding zone are always surrounded and shielded by an envelope of molten flux on which is superimposed a layer of unfused flux in a granular state. The electrode is not in contact with the workpiece; the current is carried across the gap through the fluid flux. Fluxes are specially manufactured mineral compositions formulated so that, even when they are brought to the high temperature of the welding zone, appreciable amounts of gas are not evolved. Fluxes are laid either manually or automatically along the seam to be welded in advance of the electrode.

Since the end of the electrode and the welding zone are completely covered at all times during the actual welding operation, the weld is made without the sparks, spatter, smoke or flash commonly observed in other arc welding processes. No protective shields or helmets are necessary; safety glasses should be worn as routine protection for the eyes and may be tinted for protection against flash from adjacent arc welding operations. Since welding in general may produce fumes and gases hazardous to health, it is common practice to provide adequate ventilation, especially where submerged arc welding may be done in confined areas.

Material for SAW

Submerged arc welding materials include flux and welding wire. The combined use of these two not only affects the stability of the welding process, but also determines the chemical composition and mechanical properties of the weld.

The flux protects the molten pool against the atmosphere by sealing it in an envelope of molten flux. Flux acts as a cleanser for the weld metal, modifies its chemical composition and influences the shape of the weld metal area.

Submerged arc welding generally uses solid welding wire, which is produced by drawing through hot-rolled wire. The main function of submerged arc welding wire is to fill the weld with metal. For example, when submerged arc welding is used for welding stainless steel, stainless steel submerged arc welding wire with the same composition as the base metal should be used.

Welds made in this manner, under the protective layer of flux, have unusually good ductility, impact strength, uniformity, density and corrosion resistance, as well as low nitrogen content.

Multi-wire system of SAW

The multi-wire systems offer advantages since deposition rates and travel speeds can be improved by using more electrodes. Figure 1 shows the two methods of utilizing two electrodes, one with a single-power source and one with two-power sources. When a single-power source is used, the same drive rolls are used for feeding both electrodes in the weld. When two power sources are used, individual wire feeders must be used to provide electrical insulation between the two electrodes. With two electrodes and separate power, it is possible to utilize different polarities on the two electrodes or to utilize alternating current on one and direct current on the other. The electrodes can be placed side by side, which is called transverse electrode position, or they can be placed one in front of the other in the tandem electrode position.

Figure 1  Two electrode wire system

The two-wire tandem electrode position with individual power sources is used where extreme penetration is required. The leading electrode is positive, with the trailing electrode negative. The first electrode creates a digging action and the second electrode will fill the weld. When two DC arcs are in close proximity, there is a tendency for arc interference between them. In some cases, the second electrode is connected to alternating current to avoid the interaction of the arcs.

The Skills for SAW

One of the major applications for submerged arc welding is on circular welds where the parts are rotated under a fixed head. These welds can be made on the inside or outside diameter. Submerged arc welding produces a large molten weld pool and molten slag which tends to run. This dictates that on outside diameters the electrode should be positioned ahead of the extreme top, or 12-o’clock position, so that the weld metal will begin to solidify before it starts the downside slope. This becomes more of a problem as the diameter of the part being welded is smaller. Improper electrode position will increase the possibility of slag entrapment or a poor weld surface. The angle of the electrode should also be changed and pointed in the direction of travel of the rotating part. When the welding is done on the inside circumference, the electrode should be angled so that it is ahead of bottom center, or the 6-0 ’ clock position. Figure 2 illustrates these two conditions.

Figure 2  Welding on rotating circular parts

Sometimes the work being welded is sloped downhill or uphill to provide different types of weld bead contours. If the work is sloped downhill, the bead will have less penetration and will be wider. If the work is sloped uphill, the bead will have deeper penetration and will be narrower (Figure 3).

Figure 3  Angle of slope of work versus weld

The weld will be different depending on the angle of the electrode with respect to  the work when the work is level. This is the travel angle, which can be a drag or push angle, and has a definite effect on the bead contour and weld metal penetration. Figure 4 shows the relationship.

Figure 4  Angle of electrode versus weld

One-side welding with complete root penetration can be obtained with submerged arc welding. When the welded joint is designed with a tight root opening and a fairly large root face , high current and electrode positive should be used. If the joint is designed with a root opening and a minimum root face, it will be necessary to use a backing bar since there will be nothing to support the molten weld metal. The molten flux is very fluid and will run through narrow openings. If this happens, the weld metal will follow and the weld will burn through the joint. Backing bars are needed whenever there is a root opening and a minimum root face.

Copper backing bars are useful when welding thin steel. Without backing bars, the weld would tend to melt through and the weld metal would fall away from the joint. The backing bar holds the weld metal in place until it solidifies. The copper backing bars may be water cooled to avoid the possibility of melting and copper pickup in the weld metal. For thicker materials, the backing may be submerged arc flux or other specialized type flux.

Advantages and disadvantages of SAW

(1) Advantages:

Submerged arc automatic welding is one of the earliest automatic welding methods applied. Since the flux is used to protect the arc and the molten pool, it effectively prevents air pollution. Therefore, the submerged arc welding seam has a good quality. The main advantages are as follows.

① The weld quality is good. The slag wall has a good protection effect and the weld content is low; and the welding parameters are automatically adjusted, the shape and size of the weld are uniform, the surface is smooth, and there is no flying particles on the workpiece.

② The welding speed is fast. The welding current is large and the thermal efficiency is high, so the welding speed is fast.

③ There is only a small amount of welding fumes.

④ No arc light, no need for arc protection, good working conditions.

⑤ High utilization rate of welding wire. Without splashing, the molten wire metal all enters the molten pool.

⑥ It is easy to realize automation and the load continuity rate is high.

⑦ No special operation technique is required.

(2) Disadvantages:

Submerged arc welding is protected by flux covering the welding part. The use of flux limits the use of this method in certain fields. Therefore, submerged arc welding has the following disadvantages.

①It can only be used in flat welding position or near flat welding position. In other welding positions, the flux cannot cover the weld bead.

② It can not be used for welding of active metals. There are a lot of oxides in the flux, and the reactive metal and the flux react violently.

③ When welding quenched and tempered steel, the large heat input and slow cooling rate of this welding method will cause some problems. Therefore, when submerged arc welding is used to weld the quenched and tempered steel, the heat input should be strictly limited. In this way, the low-carbon steel adopts the thickness of single pass welding, and the quenched and tempered steel may require multiple passes.

④ The electric field strength of the arc is large, and it is unstable at small currents, so it is not possible to weld thin plates.

⑤ Not suitable for welding short and complicated welds.

⑥ For semi-automatic submerged arc welding, since the arc and molten pool cannot be seen, it is difficult to weld the root weld bead, and it is difficult to fill the groove to the best condition with the last weld bead.

Application of submerged arc welding

Submerged arc welding is widely used for the welding of thick workpieces, including the construction of steel structures, longitudinal joints of large-diameter thick-walled pipes, various heavy equipment components, boiler steam drums, pressure vessels, and storage containers. In addition, submerged arc welding can also be used for surface overlay welding, surface repair welding, etc.

Submerged arc welding is mainly used to weld low carbon steel, medium carbon steel, low alloy steel, quenched and tempered steel and most stainless steels. It can also be used to weld certain copper alloys and nickel alloys.