The first patent on the submerged-arc welding (SAW) process was taken out in 1935 and covered an electric arc beneath a bed of granulated flux. Developed by the E O Pa- ton Electric Welding Institute, Russia, during the Second World War, SAW’s most fa- mous application was on the T34 tank.
Similar to MIG welding, SAW involves formation of an arc between a continuously-fed bare wire electrode and the workpiece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not re- quired. Prior to welding, a thin layer of flux powder is placed on the workpiece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hop- per. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction.
SAW is usually operated as a fully-mechanised or automatic process, but it can be semi- automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration and chemical composition of the deposited weld metal. Be- cause the operator cannot see the weld pool, greater reliance must be placed on pa- rameter settings.
According to material thickness, joint type and size of component, varying the following can increase deposition rate and improve bead shape.
SAW is normally operated with a single wire on either AC or DC current. Common vari- ants are:
- Twin wire
- Triple wire
- Single wire with hot wire addition
- Metal powdered flux addition
All contribute to improved productivity through a marked increase in weld metal deposition rates and/or travel speeds.
Fluxes used in SAW are granular fusible minerals containing oxides of manganese, sili- con, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as ‘active’ if they add manganese and silicon to the weld, the amount of manganese and silicon added is influ- enced by the arc voltage and the welding current level. The the main types of flux for SAW are:
- Bonded fluxes – produced by drying the ingredients, then bonding them with a low melting point compound such as a sodium silicate. Most bonded fluxes con- tain metallic deoxidisers which help to prevent weld porosity. These fluxes are ef- fective over rust and mill scale.
- Fused fluxes – produced by mixing the ingredients, then melting them in an elec- tric furnace to form a chemical homogeneous product, cooled and ground to the required particle size. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes.
SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint prepa- ration is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non- ferrous materials with judicious choice of electrode filler wire and flux combinations.