What is SAW Welding used for?
Submerged Arc Welding (SAW) is a welding process where the tubular electrode is fed continuously creating an arc to join two metals by generating heat between a wire electrode and bare metal. There are two welding consumables involved in the process, the electrode, and the flux. It’s commonly used in industries where thick steel sheets are involved or where long welds are required. It’s normally operated in the alternating current automatic mode or direct current semi-automatic mode. The self-adjusting arc principle is used to keep the arc length constant. If the arc length drops, the arc voltage and arc current rises, and the burn-off rate rises, leading the arc to extend. If the arc length exceeds the normal, the opposite happens.
Granular flux is deposited on the work surface ahead of the electrode. The arc forms within the flux, thereby melting a portion of the flux blanket that shields the molten weld pool and adjacent base metal. The flux hopper is where the flux is stored, and it controls the rate of flux deposition on the welding joint. The critical role of molten granular flux is deoxidizing, alloying, shaping and it creates a gas shield and slag for protection from atmospheric contamination. The method is generally mechanized and is employed to achieve very high deposition rates, e.g., for heavy section material. The process is generally characterized by high heat input, high penetration, and, thus, dilution of the molten pool by base metal.
As with the shielded metal arc welding (SMAW) process, submerged arc welding is commonly applied to stainless steels and nickel alloys. Nevertheless, it can be used to join low and medium carbon steel, tempered steels, quenched steel, stainless steel, mild steel, medium and high tensile alloys, and low alloy steels (LAS), including high-strength LAS. Some copper alloys, nickel alloys, and even uranium have been used, but not widely adopted. One flux metal reaction is usually complex, and welding conditions have been closely controlled to obtain consistent weld metal composition. The effects of the high heat input are normally considered before selecting this process.
History of SAW Welding
One of the first and notable uses was on T34 tanks in WWII, the first licensed right for use involved granular flux overlaying an electric arc, registered in 1935.
Benefits of SAW Welding
The main benefits are speed, efficiency, and quality. When large amounts of filler material are required such as polarity, long stick out, flux additives, additional electrode, the sub arc process provides high deposition rates with deep weld penetration. The process produces little fumes and no spatter, sparks, fumes, or UV radiation meaning that minimal welding safety equipment is required, for example, welding helmets and jackets, and It’s not affected by external elements such as wind. Due to its deep and broad penetration and deposition rate, this process is often the greatest fit for longitudinal and circumferential welds.
Disadvantages of Sub-Arc Welding
- Flat or horizontal-fillet and butt in flat welding positions only
- Can only weld thicker metals
- Limited to welding steel, stainless steels, and some nickel-based alloys
- Requires removal of slag post-weld and inter-pass
- Requires separate flux handling systems
- Arc is not visible to the operator in semi-automatic mode
SAW Welding Equipment
- Power source
- Welding torch/gun and cable assembly
- Flux recovery unit, flux hopper and feeding
- Travel mechanism for automatic welding
- Welding head
Submerged Arc Fabrication Applications
- Pipes
- Penstocks
- Boilers
- Structural shapes
- Pressure vessels
- Railroads
- Rotary kilns
- Earthmovers
- Cranes
- Girders
- Bridges
- Locomotives
Sub-Arc Welding Processes
Machine method and automatic method are the two most common techniques. The most popular approach is the machine method, in which the operator maintains an eye on the welding process. The automated approach uses push-button technology to apply the procedure semi-automatically, however it's not a widely used form.
Because it's not possible to handle an invisible arc, the process cannot be done manually. The limitation is imposed because large molten pools and slag are formed, making it difficult to keep them in place, the flat and horizontal fillet position are preferred. In some instances, the horizontal position is possible. Because it cannot keep flux and molten metal in place in vertical or above the head situations.
SAW Semi-Automatic Mode
The flux powder and wire electrode are fed into the process by a handheld semiautomatic weld head. A wire feeder and a copper contact tube are used to supply the wire electrode. Because the welder cannot see the welding arc or weld pool, it's critical to set and ensure the location of the wire electrode, current, arc voltage, and travel speed to make a quality weld. The welding head may include a start switch for commencing the weld, or the system may begin feeding the flux automatically when the electrode encounters the work piece.
Sub-Arc Automatic Mode
The welding is performed without the need of a welder to manage or regulate the process. To achieve exceptionally high production, expensive, self, or auto-regulated equipment is employed. This system will have a flux feed system, a wire electrode, and a flux recovery system. The flux hopper is mounted in front of the welding head and features magnetically controlled valves that the control system may open or close. A constant speed is applied to the wire electrode. The welding head is moved over the stationary work piece by a separate driving system, or the work piece is moved or rotated below a fixed welding head.
Sub-Arc Machine Mode
Equipment used in this process includes a flux hopper, a wire feeding unit, an automated arc creation unit, and a flux recovery unit. However, by situating the work, beginning, and halting the welding, and altering the controls and speeds for each welding, the welder must keep track of the process. A constant predetermined speed is applied to the wire electrode. The welding head is moved over the stationary work piece by a separate driving system, or the work piece is moved or rotated below a fixed welding head. Normally, it’s carried out in an automated or machine manner. Because it's virtually impossible for a welder to manage an invisible welding arc, the semiautomatic approach is not widely applied.
SAW Welding Duty Cycle
Because the process is continuous and weld times can exceed 10 minutes, the power supply must be qualified for a 100 percent duty cycle. If a 60 percent duty cycle power source is employed, it must be downgraded for 100 percent operation according to the duty cycle.
Alternating Current Power Source
These are typically transformers with rated currents ranging from 800 to 1500 amperes at 100% duty cycle. When greater amperes are necessary, these devices can be joined in parallel. A voltage detecting device can be employed in the wire electrode feeder system when a continuous current source (DC or AC) is used to keep a constant arc length. A basic fixed-speed wire feeding system may be used with a constant voltage source, and the constant voltage system is only employed with a direct current (DC) source. When utilising an AC source, a specialised kind of circuit is necessary for functioning with numerous electrodes. Additional fittings are required for several wire electrodes.
Direct Current Power Source
A transformer-rectifier or motor-generator that produces a constant voltage (CV) or constant current (CC) or a device with selectable CC or CV can be used as a DC source. The transformer-rectifier configuration is more common. DC constant voltage power sources are obtainable in transformer-rectifier and motor-generator configurations, with capacities ranging from 400 to 1500 amperes. These are suitable for semi-automatic welding with currents ranging from 300 to 600 amperes and wire electrode sizes ranging from 1.6 to 2.4mm. Automatic welding is indicated for wire electrode diameters ranging from 2.4mm to 6.4mm and currents ranging from 300 amperes to 1000 amperes. Due to the risk of serious arc blow at high currents, applications using currents more than 1000 amperes are limited.
Because voltage or current sensing is usually not needed, the wire electrode unit can be made using simple materials with feed controls. With rated outputs up to 1500 amperes, DC constant current power sources are accessible in both transformer-rectifier and motor-generator types. The process may be done in both polarities, DCEN (direct current electrode negative) and DCEP (direct current electrode positive). DCEN produces a fast rate of weld metal deposition as well as greater yield and weld metal toughness. DCEP has a poorer yield and a lower rate of weld metal deposition.
Sub-Arc Welding Travel Carriage
The travel carriage can be a simple tractor or a complex, specialised system. The welding head mounted on the tractor moves over the weld length, and the work piece is stationary. The welding head is still above the work piece, simultaneously the work piece moves or revolves below the welding head.
SAW Welding Wire Feed Electrode Unit
This unit guarantees that the wire electrode is continuously fed to the welding site, and the feed rate may be set to constant or variable. The feed unit is made up of a pool of wire, a driving motor, and straightening and pushing rollers that feed the wire through the welding head. Wire electrodes of several types are employed depending on the weldment. The wire electrode's size is determined by the type of metal to be welded and its thickness. The wire electrode unit may be connected to the main system through a loop for adjusting the speed to maintain a constant voltage or current.
Submerged Arc Welding Flux Recovery & Flux Hopper
The flux recovery may be integrated into the welding head and used to re-circulate unwanted flux grains. The flux is kept in the flux hopper, and it’s transported to the welding head either by gravity or force. Magnetic valves can manage the rate at which flux is deposited on the welding line.
Sub-Arc Welding Head
The welding head is set up to provide for a constant flow of flux and a constant feed of the wire electrode. The welding head may additionally have a flux recovery device.
SAW Deposition
When compared to other arc welding methods, this process has the highest rate of deposition due to its polarity, flux additives, long stick out and additional electrode.
Negative direct current electrodes have the highest deposition rate (DCEN). Between DCEP and DCEN lies the alternating current deposition, and the negative pole is influenced by the polarity with the most heat. With any welding, the rate of deposition increased when stick out was extended. Stick out is the point when electricity is introduced into the electrode, an arc is formed. By employing extra electrodes and adding additives to the flux, the deposition rate can be improved.
This form of welding produces high-quality weld metal. The weld metal's strength and ductility are superior to mild steel and low alloy materials which occurs when the right mix of an electrode, flux, and power source is combined. When a machine operates the welding, human error is eliminated, and the welding is more consistent and defect-free.
More arcing and arc flash will occur because of the thin layer of flux, increasing porosity. A thin and rounded welding bead will result from the high flux and pec markings appear on the bead due to small impurities in the flux.
It produces substantially larger weld bead than other types of arc welding. Because the heat input is a lot hotter, cooling takes longer, there is ample time for the gases to escape and the slag in this area is less dense and rises above the bead.
Submerged Arc Circular Welds
Circular welds, where the pieces are spun under a fixed head, are one of the most common applications. On the inner or outside diameter, these welds can be produced. It generates a large molten weld puddle as well as slag that drips. This means that on outer diameters, the electrode should be positioned ahead of the top, or 12 o'clock position, so that the weld metal begins to solidify before it begins to descend. The electrode should be oriented such that it’s ahead of bottom centre, or the 6 o'clock position, when welding on the inside circle.
Sub-Arc Welding Backing Bar
If a suitably big root face is utilised while welding thicker metal, the backing bar may be omitted. Backing bars are recommended when welding from one side to ensure thorough penetration. A backup weld that will fuse into the initial weld to give full penetration can be made where both sides are accessible.
Because there is nothing to hold the molten weld metal if the connection is built with a root aperture and a minimal root face, a backing bar is required. The molten flux is incredibly fluid, and it will flow through small gaps. If this occurs, the weld metal will follow, and the joint will be burned through. When there is a root aperture and a minimum root face, backing bars are required. When welding thin steel, copper backing bars come in handy. The weld would melt through if there were no supporting bars, and the weld metal would drop away from the joint. The weld metal is held in place by the supporting bar until it hardens. To minimise melting and copper pickup in the weld metal, the copper backing bars can be cooled with water. Submerged arc flux or other specialty type flux can be used as a backing for thicker metals.
Difficulties of Submerged Arc Welding
The wire could become curved when exiting the end of the gun and could become deposited in the wrong place. Depositing the correct amount of filler wire without over or under filling due to being unable to see the weld joint visually. Single passes can become cracked if the weld is contaminated
Sub-Arc Welding Variables
With a few exceptions, the welding variables are comparable to those of other arc welding methods. The electrode type and flux are selected based on the metal to be welded and the electrode is related to the size of the weld joint and the appropriate current. The number of passes and sizes of the bead are considered while deciding on the joint and the same dimension weld can be completed in several or few passes, depending on the metallurgy. Multiple passes usually result in a higher-quality weld and decision on polarity must be made first, whether maximum penetration or maximum deposit rate is required.
Sub-Arc Welding Current
The welding current is critical since it must be adequate for sufficient penetration without scorching the base metal in a single-pass weld. The deeper the penetration, the higher the current. The current for a multiple pass weld should be adjusted to the size of the weld in each pass. The electrode size might be a factor in determining the amount of current used in a weld.
Submerged Arc Welding Voltage
The variance in arc voltage is kept to a minimum, the breadth and form of the bead is determined by the arc voltage, which causes the bead to be flat and broad. Increased arc voltage can induce cracking due to excessive flux melting and deoxidizers being transmitted to the welding zone, reducing ductility. When the arc voltage is high, more flux is utilised and the low voltage creates a stronger arc, which improves penetration, low voltage produces a thinner bead with a high crown and makes slag removal harder.
SAW Travel Speed
The weld bead and penetration are affected by travel speed. The smaller the bead is and the less penetration it has, the faster the speed. When a tiny bead with minimal penetration is required in sheet metal, this is a good circumstance. Because of the rapid freezing, excessively fast speeds might result in undercuts and porosity. Bad beads, heavy spatter, and flash result from slow speeds.
Types of Flux
Fused fluxes are made by melting the appropriate materials in an electric furnace, then cooling and grinding it to the specified particle size. This flux is noted for producing a smooth and steady arc, the capacity to work at greater currents, and consistent weld metal characteristics. Bonded flux is made up of dry components bonded together using a low melting point substance such as sodium silicate. To avoid porosity in the weld metal, most bonded fluxes contain metallic deoxidizers. Flux typically contains manganese, calcium, silica, aluminium, rutile, calcium carbonate, silicon, magnesium, and fluoride.
Multi-Wire System
By employing additional electrodes, the multi-wire method boosts deposition rates, and the same drive roll is utilised for both electrodes with a single power supply. Individual wire feeders are utilised to provide insulation between two electrodes in the weld when two power sources are utilised. Different polarities can be used and both electrodes can be placed side by side. This is referred to as a transverse electrode position. In the tandem electrode configuration, one electrode is placed in front of the others.
Two-Wire Tandem System
Each wire with its own power source can be employed for thorough penetration in the two-wire tandem approach. The front electrode is positive, whereas the one behind is negative. The first electrode penetrates, while the second electrode fills the gaps to complete the weld. Interference between the two arcs can occur due to their proximity, and in certain circumstances, the back electrode is connected to AC to reduce this interference.
Three-Wire Tandem System
All three electrodes are commonly connected to three-phase AC power circuits when using the three-wire tandem technique (two wires behind the other). For large pipes and beams, this approach can be employed for high-speed longitudinal seam welding. High current, as well as high travel speed and weld metal deposition rates, are conceivable. Weld deposition rate and travel rates are increased when multi-wire electrodes are used. Multi-wire electrodes can be used with either a single common power source for all wire electrodes or multiple power sources.
The same roll feeds more than one wire electrode into the weld while using a single source of power. Individual wire feed systems should be employed to isolate the electrodes from each other when using multiple power sources. When more than one power source is employed, various polarities can be used, or one power source can be AC and the other DC, resulting in higher weld metal deposition and weld travel speed. The wire electrodes can be adjacent to each other or one in front of the other (tandem).
Strip Welding
This procedure is used on alloy and mild steel items with a coating of stainless steel. This process produces a broad, consistent, and low-penetrated weld bead. This procedure may be used to cover a low or medium alloy vessel with stainless steel. It may be beneficial from the corrosion resistance of stainless steel as well as the strength and economy of low and medium alloy steels. This procedure makes use of a strip electrode feeder and a specific flux. A magnetic arc oscillator is utilised to provide accurate penetration and proportional melting of strip electrodes with a width more than 50mm.
Iron Base Under Flux
We can increase the deposition by adding an iron base material to join under the blanket of flux. The iron will melt here and become part of a metal weld. The metal deposition increases without decreasing base material properties.
Cold Filler Wire
To speed up the metal deposition, an alloy of electrically cold filler rod is introduced. It enhances the deposited material's characteristics, and a flux-cored electrode can be used.
Double Submerged Arc Welding
Double sub-arc welding involves welding the piece being welded on both sides to join substantially thick metals, such as in circumferential welding.