SAW or Submerged Arc Welding

SAW, Submerged Arc Welding, Submerged Arc Welded Tubulars Submerged arc

welding is a process in which the joining of metals is produced by heating with an arc or arcs

between a bare metal electrode or electrodes and the work.

Originally devolved by the Linden - Union Carbide Company.

It requires a continuously fed consumable solid or tubular (flux cored) electrode.

SAW is normally operated in the automatic or mechanized mode, however, semi-automatic

(hand-held) SAW guns with pressurized or gravity flux feed delivery are available. The

process is normally limited to the Flat or Horizontal-Fillet welding positions (although

Horizontal Groove position welds have been done with a special arrangement to support the

flux).

Contents:-

Equipment

Principles of Operation.

Deposition rates and weld quality.

Material applications

Advantages

Limitations

Equipment

Power supply

Start plate

Copper mold

Electrode

Guide tube

Wire feed

Power source

SAW head

Flux handling

Protective equipment

Poqwer supply:-

Deposition rates approaching 100 lb/h (45 kg/h) have been reported — this

compares to ~10 lb/h (5 kg/h) (max) for shielded metal arc welding. Although Currents

ranging from 300 to 2000 A are commonly utilize. Currents of up to 5000 A have also been

used (multiple arcs).Single or multiple (2 to 5) electrode wire variations of the process exist.

SAW strip-cladding utilizes a flat strip electrode (e.g. 60 mm wide x 0.5 mm thick). DC or

AC power can be used, and combinations of DC and AC are common on multiple electrode

systems. Constant Voltage welding power supplies are most commonly used; however,

Constant Current systems in combination with a voltage sensing wire-feeder are available.

Variations of the process:-

There are a large number of variations to the process that give submerged arc

welding additional capabilities. Some of the more popular variations are:

1. Two-wire systems--same power source.

2. Two-wire systems--separate power source.

3. Three-wire systems--separate power source.

4. Strip electrode for surfacing.

5. Iron powder additions to the flux.

6. Long stickout welding.

7. Electrically "cold" filler wire.

Electrode:-

SAW filler material usually is a standard wire as well as other special forms. This

wire normally has a thickness of 1/16 in. to 1/4 in. (1.6 mm to 6 mm). In certain

circumstances, twisted wire can be used to give the arc an oscillating movement. This helps

fuse the toe of the weld to the base metal.

Other factors

Flux depth/width

Flux and electrode classification and type

Electrode wire diameter

Multiple electrode configurations

Flux:

Fluxes used in SAW are granular fusible minerals containing oxides of

manganese, silicon, titanium, aluminum, 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. 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 deoxidizers

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 chemically 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.

In closing, the first patent on Submerged Arc Welding was taken out in 1935. Although

little has changed from the original concept when it was first developed, it has and will

continue to be heavily used in our industrial work environment.

Principles of Operation:-

The submerged arc welding process is shown by figure 10-60. It utilizes the heat of an

arc between a continuously fed electrode and the work.The arc melts the surface of the base

metal and the end of the electrode. The metal melted off the electrode is transferred through

the arc to the workpiece, where it becomes the deposited weld metal.Shielding is obtained

from a blanket of granular flux, which is laid directly over the weld area. The flux close to the

arc melts and intermixes with the molten weld metal, helping to purify and fortify it. The flux

forms a glass-like slag that is lighter in weight than the deposited weld metal and floats on the

surface as a protective cover. The weld is submerged under this layer of flux and slag, hence

the name submerged arc welding. The flux and slag normally cover the arc so that it is not

visible. The unmelted portion of the flux can be reused. The electrode is fed into the arc

automatically from a coil. The arc is maintained automatically. Travel can be manual or by

machine. The arc is initiated by a fuse type start or by a reversing or retrack system.

Operating characteristics:-

SAW is usually operated as a fully-

mechanized 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.

Deposition Rates And Weld Quality:-

The deposition rates of the submerged arc welding process are higher than any other

arc welding process.

The quality of the weld metal deposited by the submerged arc welding process is

high.

The weld metal strength and ductility exceeds that of the mild steel or low-alloy base

material when the correct combination of electrode wire and submerged arc flux is

used.

When submerged arc welds are made by machine or automatically, the human factor

inherent to the manual welding processes is eliminated. The heat input is higher and

cooling rates are slower.

For this reason, gases are allowed more time to escape.

Additionally, since the submerged arc slag is lower in density than the weld metal, it

will float out to the top of the weld. Uniformity and consistency are advantages of this

process when applied automatically.

Several problems may occur when using the semiautomatic application method. The

electrode wire may be curved when it leaves the nozzle of the welding gun. This

curvature can cause the arc to be struck in a location not expected by the welder.

There is another quality problem associated with extremely large single-pass weld

deposits

Material applications:-

Carbon steels (structural and vessel construction)

Low alloy steels

Stainless steels

Nickel-based alloys

Surfacing applications (wear-facing, build-up, and corrosion resistant overlay of

steels)

Advantages:-

High deposition rates (over 100 lb/h (45 kg/h) have been reported).

High operating factors in mechanized applications.

Deep weld penetration.

Sound welds are readily made (with good process design and control).

High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible.

Minimal welding fume or arc light is emitted.

Practically no edge preparation is necessary.

The process is suitable for both indoor and outdoor works.

Distortion is much less.

Welds produced are sound, uniform, ductile, corrosion resistant and have good impact

value.

Single pass welds can be made in thick plates with normal equipment.

The arc is always covered under a blanket of flux, thus there is no chance of spatter of

weld. 50% to 90% of the flux is recoverable.

Limitations:-

Limited to ferrous (steel or stainless steels) and some nickel based alloys.

Normally limited to the 1F, 1G, and 2F positions.

Normally limited to long straight seams or rotated pipes or vessels.

Requires relatively troublesome flux handling systems.

Flux and slag residue can present a health & safety issue.

Requires inter-pass and post weld slag removal.