Submerged-arc Welding
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 Paton Electric Welding Institute, Russia, during
the Second World War, SAW's most famous application was on the T34 tank.
Process features
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 required. 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 hopper. 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.
Operating characteristics
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. Because the operator
cannot see the weld pool, greater reliance must be placed on parameter
settings.
Process variants
According to material thickness, joint type and size of component,
varying the following can increase deposition rate and improve bead
shape.
Wire
SAW is normally operated with a single wire on either AC or DC current.
Common variants 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.
Flux
Fluxes used in SAW are granular fusible minerals containing oxides of
manganese, silicon, 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 influenced 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 contain metallic
deoxidisers which help to prevent weld porosity. These fluxes are
effective over rust and mill scale.
- Fused fluxes - produced by mixing the ingredients,
then melting them in an electric 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.
Applications
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 preparation 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.
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