PRINCIPLE OF SUBMERGED ARC WELDING
- Submerged Arc Welding is a process where a bare coiled electrode wire is automatically fed into the weld joint where the arc melts it under a layer of granular flux that is metered into the weld zone by gravity through a nozzle. SAW is usually carried out fully mechanised (or automatically). It is commonly used to join thick sections in the flat position.
- Fusion takes place beneath the flux without sparks, spatter, or flash. This means that arc welding helmets and shields are not required while welding instead light goggles can be used to prevent from flashes that occurs ocassionally.
- As the ‘Automation’ in the SAW Process increases, the direct influence of the operator decreases and the precise setting of the parameters becomes much more important as compared to the manual welding process.
- Submerged arc fluxes have a melting temperature lower than that of metal. This insures that no solid particles flux will be trapped within the weld pool.
- Widely used ingredients in granular fluxes include lime, silica, manganese oxides and calcium flourides.
- The flux has important mettalurgical, as well as shielding functions to perform, such as supplying deoxidizers or alloying elements.
- Submerged arc welding is generally performed indoors in fabrication shops. Working outdoors always carries the risk of undesirable levels of moisture finding their way into the joint or flux and resulting in porosity in the weld. If submerged arc welding cannot be avoided outdoors, special measures should be taken, such as the construction of a roof over the work area.
- In Submerged Arc Welding, the qualities of weld deposits are determined by the following parameters.
- Welding Current
- Welding Arc Voltage
- Electrode stick out
- Grade of wire
- Travel speed
- Type of flux
- Size of electrode
- Flux layer depth
Submerged Arc Welding (SAW) Process |
SUBMERGED ARC WELDING EQUIPMENT
A Submerged Arc Welding Equipment can be of manually operated, Semi-automatic and fully automated machines.
- Manual welding: In Manual welding process the operator holds the gun and places it on the weld path and he guides it along the path throughout the process. This requires high skilled operators to handle the complete process.
- Semi-automatic: Semi-automatic equipment is commonly called manual “Squirt” which maintains a preset current and voltage. The operator must strike the arc, guide the welding gun along the welding seam and manually apply the travel speed. A remote electrode feeding mechanism feeds the electrode and automatically controls the current and voltage. The gun is mounted on a small tractor that rides on the work and follows the weld path. This relieves the operator from holding and guiding the gun, and reduces his function to setting the guide rolls for the joint, starting the welding and monitoring the progress as the tractor rides the seam.
- Automated welding: In this method generally the head is equipped to feed single electrode. The same head can be used to feed two electrodes by changing the contact nozzle and changing the wire drive rolls. This is known as twin electrode submerged arc welding. The electrical control system maintains the preset current and voltage, strikes the arc, starts the travel and controls the travel speed. Either the head is stationary and the work moves or the work is stationary and the head moves.
- Long - Stickout Welding: The fastest deposition rate of submerged arc welding is enhanced by the application of “long-stickout” principle to the process. This principle, which is also widely used with the self- shielded flux cored electrode process, which enables increasing the welding speed by as much as 50%. In semi-automatic and fully automatic welding, electric current is fed into the electrode at the point of electrical contact within the gun or nozzle. The current must then travel to the tip of the electrode to reach the arc. The distance between the point of the electric contact and electrode tip is referred to as “stickout” or “electrical stickout”. Deposition rates with long stickout welding are increased around 25% to 50% without increase in welding current. This method provides the greatest and economic in fast filling application that require a large volume of weld metal.
Submerged Arc Welding (SAW) Equipment |
SAW TORCHES
The submerged Arc welding torch consist of following setup
- Flux Hopper
- Nozzle
- Contact tip
- Wire feeder
- Electrode guide
The flux hopper supplies flux during the process of welding and the wire which is fed goes out through the electrode guide and the contact tip and finally reaches the work piece. A schematic representation of submerged arc welding torch is shown below,
Submerged Arc Welding (SAW) Torch |
CURRENT IN SAW
Currents ranging from 300A to 3000A are commonly used. Currents up to 5000A can also be used in multiple arc welding. DC or AC powers as well as a combination of both are common sources of power on multiple electrode systems. Constant voltage power supply is a most commonly used source. However, constant current system in combination with a voltage sensing wire-feeder is also utilized in semiautomatic systems.
ELECTRODE & FLUX
SAW is normally carried out with a single wire electrode on either AC or DC current but the other options are also in use:
- Twin wire
- Triple wire
- Single wire with hot wire addition
- Metal powder addition
All contribute to improve the productivity through a marked increase in weld metal deposition rate or travel speed. Other factors includes
- Flux depth/width
- Flux and electrode classification and type
- Electrode wire diameter
- Multiple electrode configurations
Various formulations that are used as flux are,
- Calcium silicate
- Manganese silicate
- Aluminate rutile or basic
- Basic fluorides
Fluxes termed "neutral" or "active" according to their potency in modifying weld composition. They are also categorized as "basic" or "acid" based on their chemical reaction. "Acid" silicate fluxes are active types. Active fluxes and/or electrodes deoxidized with silicon and manganese are useful when making single pass welds on scaled or rusty steel plate. However, Si and Mn build up may give poor toughness and soundness in multi-pass welds Basic fluxes give optimum strength and toughness in steel welds
ADVANTAGES
- Molten flux provides very suitable conditions for high current to flow.
- Great intensities of heat can be generated and kept concentrated to weld thicker sections with deep penetrations.
- Because of high heat concentration, considerably higher welding speeds can be caused and because of high welding speeds weld distortion is much less.
- High metal deposition can be achieved.
- Single pass welds can be made in thick plates with normal equipment.
- Welding is carried out without sparks, smoke, flash or spatter.
- Weld metal deposit possesses uniformity, good ductility, corrosion resistance and good impact strength.
- Very neat appearance and smooth weld shapes can be obtaimed.
- The submerged process can be used for welding in exposed areas with relatively high winds.
- Practically, no edge preparation is necessary for materials under 12 mm in thickness.
DISADVANTAGES
- Since the operator cannot see the welding being carried out, he cannot judge the progress of welding accurately. Therefore accessories like jigs and fixtures, pointers, light beam focusing devices or roller guides may be used for proper welding at the joint.
- The flux needs replacing of the same on the joint which is not always possible.
- The progress is limited to welding in flat position and on the metal more than 4.8 mm thick. In small thicknesses burn through is likely to occur.
- The process requires edge preparation and accurate fit up on the joint. Otherwise the flux may spill through the gap and arc may burn the work piece edges.
- Flux is subjected to contamination that may cause weld porosity.
- Weld metal chemistry is difficult to control. A change in welding variables especially when using alloyed fluxes may affect weld metal composition adversely.
- Cast iron, Al alloys, Mg alloys, Pb and Zn cannot be welded by this process.
APPLICATION
Submerged Arc Welding is generally used in,
- Joining heavy sections in steel, stainless steels
- Pressure vessel & piping circumferential & longitudinal seams
- Plate girder fabrication
- Ship panel subassembly
- Multi-wire & strip cladding variants
- The method is suitable for butt welding and fillet welding
- Manufacture of bridge beams.
- Thin sheet shells
- Welding carbon and low alloy steels
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Email us: [email protected]
Arcraft Plasma Equipments ( I ) Pvt Ltd
124 Diamond Industrial Estate,
Ketkipada , Dahisar ( East )
Mumbai 400068
Tel 0091 22 28963247
[email protected]
www.arcraftplasma.com
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