submerged arc welding

Submerged arc welding (SAW) is a process utilising an electric arc beneath a granulated flux. Patented by Jones, Kennedy and Rothermund, the process feeds a consumable electrode, whilst the arc zone is protected from the atmosphere by being submerged under a flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive. The flux covers the molten metal and prevents spatter.

What is submerged arc welding (SAW)?

This process can be either automatic or mechanised, however, hand-held guns with gravity flux feed delivery can be used. The process is normally limited to the horizontal-fillet welding positions. Deposition rates approaching 45 kg/h have been achieved and currents ranging from 300 to 2000 A are normal. 

 

Principles of submerged arc welding

The cold flux is a non-conductor of electricity and the arc is struck by touching the electrode with the work piece. The flux becomes highly conductive and hence the current flow is maintained between the electrode and the target. The upper portion of the flux, in contact with atmosphere, is recyclable. The lower, melted flux becomes waste material and is called slag.

The electrode is continuously fed to the joint. In semi-automatic welding, the head is moved manually along the joint. In automatic welding a separate drive moves either the welding head over the stationary target or the target moves under the head.

Arc length is maintained by using the principle of a self-adjusting arc. If the arc length decreases, the arc voltage will increase, arc current and therefore burn-off rate will rise, resulting in a longer arc. A backing plate of steel or copper controls penetration and supports large amounts of molten metal.

 

 

Industrial applications using submerged arc welding

Maximising output whilst retaining quality in advanced manufacturing requires careful selection of process, procedures, kit, and consumables. Submerged arc welding is a highly efficient process, that is used in heavy industrial applications, including pressure vessel fabrication and offshore structural fabrication.

The following materials that can be joined include; carbon steels, low alloy steels, stainless steels, nickel-based alloys and formulation of surfacing protection e.g. corrosion on steels.

Generally, this process is used for joining low carbon and low alloy steels, or in combination with specific fluxes it is an appropriate process for welding stainless steels, copper and titanium. Medium carbon steels, heat resistant and very high strength steels can also be processed. Potentially, it is also feasible to join nickel and monel (a copper and nickel alloy).

Straight and long welds with a plate thicknesses between 5 to 50 mm are favourable. Additionally, butt and fillet welds within heavy industries like shipbuilding, pressure vessel fabrication, structural engineering, pipe welding, and storage tanks.

Material thickness determines that either single-pass, or a multi-pass weld procedures is used. Welded materials also include non-ferrous materials which is dependent upon the combination of electrode filler wire and flux.

 

Components within a SAW system

The welding head

This feeds flux and filler metal to the joint. The filler metal, or electrode is stimulated at this point.

 

Flux and flux hopper

The flux prevents molten weld contamination from the surrounding air. The flux also cleans weld metal. It consists of fused, bonded or mechanically mixed type e.g. fluorides of calcium and oxides of calcium, magnesium, silicon, aluminium and manganese. A flux with fine and coarse particle sizes is recommended for welding heavier and smaller thickness respectively. The hopper stores the flux and controls the rate of deposition at the joint.

 

Electrode

Filler material usually is a standard wire with a thickness of 1.6 mm to 6 mm. Twisted wire can be used to give the arc an oscillating movement. This helps fuse the toe of the weld to the base metal. Alloying elements may be added in the electrodes and they are available to weld mild steels, high carbon steels, low and special alloy steels, stainless steel and some of the nonferrous of copper and nickel. Electrodes are generally copper coated to increase their electrical conductivity. The electrode diameters are a typically; 1.6, 2.0, 2.4, 3, 4.0, 4.8, and 6.4 mm.

Process variables of submerged arc welding

Wire feed speed and arc voltage are critical variables, including the travel speed. Additionally, the following must be considered:

  • Electrode stick-out (ESO) or contact tip to work (CTTW)
  • Polarity and current type (AC or DC) and variable balance AC current

 

Advantages and benefits of SAW process

High deposition rates (over 45 kg/h) are a huge advantage for SAW. Additional benefits include deep weld penetration, automation, leading to consistent quality and standardisation. High speed welding of thin sheet steels up to 5 m/min is possible, releasing minimal welding fume and arc light. The process is suitable for both indoor and outdoor processes – offering excellent, uniform, ductile and considerable corrosion resistance. Aesthetically, the arc is always covered by the flux, and hence there is minimal spatter produced. More than half of the flux volume is recyclable.

 

Submerged arc welding efficiency and optimisation

Stick out

A simple optimisation can be via increasing stick-out (length between the tip and target material).  The 3/4 in alteration to 1.5in would be an initial optimisation experiment. In this instance the welding wire is pre-heated, countering the extra resistance. Simultaneously, the wire feed must be upped, for a given current level, outputting a greater deposition rate.

 

Twin wire

To achieve higher deposition rates, you can utilise twin-wire. The feeding of two electrodes, at equal velocity, into the same welding pool and retaining the same electrical potential. The deposition and penetration ratio, allows either AC or DC. Twin-wire utilises the same flux as the single-wire process. Wire straighteners can aid complications caused by twin wires. The contact block can be opened or closed to force a spacing between the wires.

 

Disadvantages and limitations of SAW

There are limitations to the process e.g. potential materials include only ferrous and some nickel-based alloys. Positionally, this process is limited to 1F, 1G, and 2F configurations. It also needs relatively complex flux handling systems and constant, inter-pass and post weld slag removal. A disadvantage also stems from the requirement of backing strips, to achieve deep root penetration.

 

 

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References

  1.  US 2043960, Jones, Lloyd Theodore; Harry Edward Kennedy & Maynard Arthur Rothermund, “Electric welding”, published 1935-10-09, issued 1936-06-09
  2. ^ Kalpakjian, Serope, and Steven Schmid. Manufacturing Engineering and Technology. ‘5th ed’. Upper Saddle River, NJ: Pearson Prentice Hall, 2006.
  3. ^ Jeffus, Larry. Welding: Principles and Applications. Florence, KY: Thomson Delmar Learning, 2002.
  4. ^ “Resources Recovered Calculator”Weld Engineering Co. Retrieved 5 March 2015.
  5. https://en.wikipedia.org/wiki/Submerged_arc_welding
  6. https://www.thefabricator.com/thewelder/article/consumables/using-cored-wire-in-submerged-arc-welding-applications
  7. https://www.yourarticlelibrary.com/welding/submerged-arc-welding/submerged-arc-welding-saw-equipment-and-applications/96695
  8. https://www.thefabricator.com/thefabricator/article/arcwelding/3-ways-to-boost-submerged-arc-welding-performance