Submerged Arc Welding (SAW)

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Submerged Arc Welding (SAW)

Summary:

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.

The arc is shielded by a blanket of granular fusible material on the work.

Pressure is not used.

Filler metal is obtained from the electrode or from a supplementary welding rod.

Arc Welding Video Demonstration

Equipment

The SAW equipment components required for submerged arc welding are shown by figure 10-59.

Equipment consists of a welding machine or power source, the wire
feeder and control system, the welding torch for automatic welding or
the welding gun and cable assembly for semiautomatic welding, the flux
hopper and feeding mechanism, usually a flux recovery system, and a
travel mechanism for automatic welding.

The power source for submerged arc welding must be rated for a 100
percent duty cycle, since the submerged arc welding operations are
continuous and the length of time for making a weld may exceed 10
minutes.

If a 60 percent duty cycle power source is used, it must be
derated according to the duty cycle curve for 100 percent operation.

When constant current is used, either ac or dc, the voltage sensing electrode wire feeder system must be used.

When constant voltage is used, the simpler fixed speed wire
feeder system is used. The CV system is only used with direct current.

Both generator and transformer-rectifier power sources are used, but the rectifier machines are more popular.

Welding machines for submerged arc welding range in size from 300 amperes to 1500 amperes.

They may be connected in parallel to provide extra power for high-current applications.

Direct current power is used for semiautomatic applications, but
alternating current power is used primarily with the machine or the
automatic method.

Multiple electrode systems require specialized types of circuits, especially when ac is employed.

SAW Equipment

Shown: SAW or submerged arc welding equipment completing a weld. The weld starts on the right and moves left. The gray colored powder is flux.NearEmptiness

For semiautomatic application, a welding gun and
cable assembly are used to carry the electrode and current and to
provide the flux at the arc.

A small flux hopper is attached to the end of the cable assembly.

The electrode wire is fed through the bottom of this flux hopper through a current pickup tip to the arc.

The flux is fed from the hopper to the welding area by means of gravity.

The amount of flux fed depends on how high the gun is held above the work.

The hopper gun may include a start switch to initiate the weld or
it may utilize a "hot" electrode so that when the electrode is touched
to the work, feeding will begin automatically.

For automatic welding, the torch is attached to the
wire feed motor and includes current pickup tips for transmitting the
welding current to the electrode wire.

The flux hopper is normally attached to the torch, and may have
magnetically operated valves which can be opened or closed by the
control system.

Other pieces of equipment sometimes used may
include a travel carriage, which can be a simple tractor or a complex
moving specialized fixture. A flux recovery unit is normally provided to
collect the unused submerged arc flux and return it to the supply
hopper.

Submerged arc welding system can become quite
complex by incorporating additional devices such as seam followers,
weavers, and work rovers.

SAW Welding Diagram

Figure 10-59. Block diagram of SAW (submerged arc welding) Equipment.

Advantages

The major advantages of the SAW or submerged arc welding process are:

  1. high quality metal weld.
  2. extremely high speed and deposition rate
  3. smooth, uniform finished weld with no spatter.
  4. little or no smoke.
  5. no arc flash, thus minimal need for protective clothing.
  6. high utilization of electrode wire.
  7. easy automation for high-operator factor.
  8. normally, no involvement of manipulative skills.

 

SAW Pipe Welding

SAW welding process to build long steel piles to support ocean platform.

Major Uses

The submerged arc process is widely used in heavy
steel plate fabrication work. This includes the welding of structural
shapes, the longitudinal seam of larger diameter pipe, the manufacture
of machine components for all types of heavy industry, and the
manufacture of vessels and tanks for pressure and storage use. It is
widely used in the shipbuilding industry for splicing and fabricating
sub-assemblies, and by many other industries where steels are used in
medium to heavy thicknesses. It is also used for surfacing and buildup
work, maintenance, and repair.

SAW Welding


In SAW welding the flux and wire are separate. Both impact the properties of the weld, requiring the selection of the optimal combination by the engineer for each project.

Process Limitations

A major limitation of SAW (submerged arc welding) is its limitation of
welding positions. The other limitation is that it is primarily used
only to weld mild and low-alloy high-strength steels.

The high-heat input and slow-cooling cycle can be a
problem when welding quenched and tempered steels. The heat input
limitation of the steel in question must be strictly adhered to when
using submerged arc welding. This may require the making of multipass
welds where a single pass weld would be acceptable in mild steel. In
some cases, the economic advantages may be reduced to the point where
flux-cored arc welding or some other process should be considered.

In semiautomatic submerged arc welding, the
inability to see the arc and puddle can be a disadvantage in reaching
the root of a groove weld and properly filling or sizing.

Submerged Arc Welding


Demonstration of Saw Welding Process.

Photo Credit:Arcadia Industrial

Principles of Operation

Process

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.

Submerged Arc Welding Process Diagram

Figure 10-60: Process Diagram for SAW (submerged arc welding)

The heat of 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.

Normal Method of Application and Position Capabilities

The most
popular method of SAW application is the machine method, where the operator
monitors the welding operation.

Second in popularity is the automatic method, where welding is a
pushbutton operation. The process can be applied semiautomatically;
however, this method of application is not too popular.

The process cannot be applied manually because it is impossible
for a welder to control an arc that is not visible. The submerged arc
welding process is a limited-position welding process.

The welding positions are limited because the large pool of
molten metal and the slag are very fluid and will tend to run out of the
joint. Welding can be done in the flat position and in the horizontal
fillet position with ease.

Under special controlled procedures, it is possible to weld in the horizontal position, sometimes called 3 o'clock welding.

This requires special devices to hold the flux up so that the
molten slag and weld metal cannot run away. The process cannot be used
in the vertical or overhead position.

Metals Weldable and Thickness Range

Submerged arc welding is used to weld low- and medium-carbon steels,
low-alloy high-strength steels, quenched and tempered steels, and many
stainless steels.

Experimentally, it has been used to weld certain copper alloys,
nickel alloys, and even uranium.

Metal thicknesses from 1/16 to 1/2 in. (1.6 to 12.7 mm)
can be welded with no edge preparation. With edge preparation, welds
can be made with a single pass on material from 1/4 to 1 in. (6.4 to
25.4 mm).

When multipass technique is used, the maximum thickness is
practically unlimited. This information is summarized in table 10-22.
Horizontal fillet welds can be made up to 3/8 in. (9.5 mm) in a single
pass and in the flat position, fillet welds can be made up to 1 in. (25
mm) size.

Joint Design

Although the submerged arc welding process can utilize
the same joint design details as the shielded metal arc welding process,
different joint details are suggested for maximum utilization and
efficiency of submerged arc welding. For groove welds, the square groove
design can be used up to 5/8 in. (16 mm) thickness.

Beyond this thickness, bevels are required. Open roots are used
but backing bars are necessary since the molten metal will run through
the joint.

When welding thicker metal, if a sufficiently large root face is
used, the backing bar may be eliminate. However, to assure full
penetration when welding from one side, backing bars are recommended.
Where both sides are accessible, a backing weld can be made which will
fuse into the original weld to provide full penetration.

Welding Circuit and Current

The SAW or submerged arc welding process uses either
direct or alternating current for welding power. Direct current is used
for most applications which use a single arc. Both direct current
electrode positive (DCEP) and electrode negative (DCEN) are used.

The constant voltage type of direct current power
is more popular for submerged arc welding with 1/8 in. (3.2 mm) and
smaller diameter electrode wires.

The constant current power system is normally used
for welding with 5/3 2 in. (4 mm) and larger-diameter electrode wires.
The control circuit for CC power is more complex since it attempts to
duplicate the actions of the welder to retain a specific arc length. The
wire feed system must sense the voltage across the arc and feed the
electrode wire into the arc to maintain this voltage. As conditions
change, the wire feed must slow down or speed up to maintain the
prefixed voltage across the arc. This adds complexity to the control
system. The system cannot react instantaneously. Arc starting is more
complicated with the constant current system since it requires the use
of a reversing system to strike the arc, retract, and then maintain the
preset arc voltage.

For SAW ac welding, the constant current power is
always used. When multiple electrode wire systems are used with both ac
and dc arcs, the constant current power system is utilized. The constant
voltage system, however, can be applied when two wires are fed into the
arc supplied by a single power source. Welding current for submerged
arc welding can vary from as low as 50 amperes to as high as 2000
amperes. Most submerged arc welding is done in the range of 200 to 1200
amperes.

Deposition Rates and Weld Quality

The deposition rates of the submerged arc welding process are higher
than any other arc welding process. Deposition rates for single
electrodes are shown by figure 10-62. There are at least four related
factors that control the deposition rate of submerged arc welding:
polarity, long stickout, additives in the flux, and additional
electrodes. The deposition rate is the highest for direct current
electrode negative (DCEN). The deposition rate for alternating current
is between DCEP and DCEN. The polarity of maximum heat is the negative
pole.

The deposition rate with any welding current can be
increased by extending the "stickout." This is the distance from the
point where current is introduced into the electrode to the arc. When
using "long stickout" the amount of penetration is reduced. The
deposition rates can be increased by metal additives in the submerged
arc flux. Additional electrodes can be used to increase the overall
deposition rate.

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 weld will be more uniform and free from inconsistencies. In general,
the weld bead size per pass is much greater with submerged arc welding
than with any of the other arc welding processes. 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. When welding
in fairly deep grooves, the curvature may cause the arc to be against
one side of the weld joint rather than at the root. This will cause
incomplete root fusion. Flux will be trapped at the root of the weld.
Another problem with semiautomatic welding is that of completely filling
the weld groove or maintaining exact size, since the weld is hidden and
cannot be observed while it is being made. This requires making an
extra pass. In some cases, too much weld is deposited. Variations in
root opening affect the travel speed. If travel speed is uniform, the
weld may be under- or overfilled in different areas. High operator skill
will overcome this problem.

There is another quality problem associated with
extremely large single-pass weld deposits. When these large welds
solidify, the impurities in the melted base metal and in the weld metal
all collect at the last point to freeze, which is the centerline of the
weld. If there is sufficient restraint and enough impurities are
collected at this point, centerline cracking may occur. This can happen
when making large single-pass flat fillet welds if the base metal plates
are 45º from flat. A simple solution is to avoid placing the parts at a
true 45º angle. It should be varied approximately 10º so that the root
of the joint is not in line with the centerline of the fillet weld.
Another solution is to make multiple passes rather than attempting to
make a large weld in a single pass.

Another quality problem has to do with the hardness
of the deposited weld metal. Excessively hard weld deposits contribute
to cracking of the weld during fabrication or during service. A maximum
hardness level of 225 Brinell is recommended. The reason for the hard
weld in carbon and low-alloy steels is too rapid cooling, inadequate
postweld treatment, or excessive alloy pickup in the weld metal.
Excessive alloy pickup is due to selecting an electrode that has too
much alloy, selecting a flux that introduces too much alloy into the
weld, or the use of excessively high welding voltages.

In automatic and machine welding, defects may occur
at the start or at the end of the weld. The best solution is to use
runout tabs so that starts and stops will be on the tabs rather than on
the product.

Weld Schedules

The submerged arc welding process applied by machine or
fully automatically should be done in accordance with welding procedure
schedules. All of the welds
made by this procedure should pass qualification, tests, assuming that
the correct electrode and flux have been selected. If the schedules are
varied more than 10 percent, qualification tests should be performed to
determine the weld quality.

Welding Variables

The welding variables for submerged arc welding are similar to the other arc welding processes, with several exceptions.

In submerged arc welding, the electrode type and
the flux type are usually based on the mechanical properties required by
the weld. The electrode size
is related to the weld joint size and the current recommended for the
particular joint. This must also be considered in determining the number
of passes or beads for a particular joint. Welds for the same joint
dimension can be made in many or few passes, depending on the weld metal
metallurgy desired. Multiple passes usually deposit higher-quality weld
metal. Polarity is established initially and is based on whether
maximum penetration or maximum deposition rate is required.

The major variables that affect the weld involve
heat input and include the welding current, arc voltage, and travel
speed. Welding current is the most important. For single-pass welds, the
current should be sufficient for the desired penetration without
burn-through. The higher the current, the deeper the penetration. In
multi-pass work, the current should be suitable to produce the size of
the weld expected in each pass. The welding current should be selected
based on the electrode size. The higher the welding current, the greater
the melt-off rate (deposition rate).

The arc voltage is varied within narrower limits
than welding current. It has an influence on the bead width and shape.
Higher voltages will cause the bead to be wider and flatter. Extremely
high arc voltage should be avoided, since it can cause cracking. This is
because an abnormal amount of flux is melted and excess deoxidizers may
be transferred to the weld deposit, lowering its ductility. Higher arc
voltage also increases the amount of flux consumed. The low arc voltage
produces a stiffer arc that improves penetration, particularly in the
bottom of deep grooves. If the voltage is too low, a very narrow bead
will result. It will have a high crown and the slag will be difficult to
remove.

Travel speed influences both bead width and
penetration. Faster travel speeds produce narrower beads that have less
penetration. This can be an advantage for sheet metal welding where
small beads and minimum penetration are required. If speeds are too
fast, however, there is a tendency for undercut and porosity, since the
weld freezes quicker. If the travel speed is too slow, the electrode
stays in the weld puddle too long. This creates poor bead shape and may
cause excessive spatter and flash through the layer of flux.

The secondary variables include the angle of the
electrode to the work, the angle of the work itself, the thickness of
the flux layer, and the distance between the current pickup tip and the
arc. This latter factor, called electrode "stickout," has a considerable
effect on the weld. Normally, the distance between the contact tip and
the work is 1 to 1-1/2 in. (25 to 38 mm). If the stickout is increased
beyond this amount, it will cause preheating of the electrode wire,
which will greatly increase the deposition rate. As stickout increases,
the penetration into the base metal decreases. This factor must be given
serious consideration because in some situations the penetration is
required.

The depth of the flux layer must also be
considered. If it is too thin, there will be too much arcing through the
flux or arc flash. This also may cause porosity. If the flux depth is
too heavy, the weld may be narrow and humped. Too many small particles
in the flux can cause surface pitting since the gases generated in the
weld may not be allowed to escape. These are sometimes called peck marks
on the bead surface.

Tips for Using the Process

One of the major applications for submerged arc
welding is on circular welds where the parts are rotated under a fixed
head. These welds can be made on the inside or outside diameter.
Submerged arc welding produces a large molten weld puddle and molten
slag which tends to run. This dictates that on outside diameters, the
electrode should be positioned ahead of the extreme top, or 12 o'clock
position, so that the weld metal will begin to solidify before it starts
the downside slope. This becomes more of a problem as the diameter of
the part being welded gets smaller. Improper electrode position will
increase the possibility of slag entrapment or a poor weld surface. The
angle of the electrode should also be changed and pointed in the
direction of travel of the rotating part. When the welding is done on
the inside circumference, the electrode should be angled so that it is
ahead of bottom center, or the 6 o'clock position.

Sometimes the work being welded is sloped downhill
or uphill to provide different types of weld bead contours. If the work
is sloped downhill, the bead will have less penetration and will be
wider. If the weld is sloped uphill, the bead will have deeper
penetration and will be narrower. This is based on all other factors
remaining the same.

The weld will be different depending on the angle
of the electrode with respect to the work when the work is level. This
is the travel angle, which can be a drag or push angle. It has a
definite effect on the bead contour and weld metal penetration.

One side welding with complete root penetration can
be obtained with submerged arc welding. When the weld joint is designed
with a tight root opening and a fairly large root face, high current
and electrode positive should be used. If the joint is designed with a
root opening and a minimum root face, it is necessary to use a backing
bar, since there is nothing to support the molten weld metal. The molten
flux is very fluid and will run through narrow openings. If this
happens, the weld metal will follow and the weld will burn through the
joint. Backing bars are needed whenever there is a root opening and a
minimum root face.

Copper backing bars are useful when welding thin
steel. Without backing bars, the weld would tend to melt through and the
weld metal would fall away from the joint. The backing bar holds the
weld metal in place until it solidifies. The copper backing bars may be
water cooled to avoid the possibility of melting and copper pickup in
the weld metal. For thicker materials, the backing may be submerged arc
flux or other specialized type flux.

Variations of the SAW 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.

Multi-wire Systems

The multi-wire systems offer advantages since
deposition rates and travel speeds can be improved by using more
electrodes. Figure 10-68 shows the two methods of utilizing two
electrodes, one with a single-power source and one with two power
sources. When a single-power source is used, the same drive rolls are
used for feeding both electrodes into the weld. When two power sources
are used, individual wire feeders must be used to provide electrical
insulation between the two electrodes. With two electrodes and separate
power, it is possible to utilize different polarities on the two
electrodes or to utilize alternating current on one and direct current
on the other. The electrodes can be placed side by side. This is called
transverse electrode position. They can also be placed one in front of
the other in the tandem electrode position.

Two-wire Tandem

The two-wire tandem electrode position with
individual power sources is used where extreme penetration is required.
The leading electrode is positive with the trailing electrode negative.
The first electrode creates a digging action and the second electrode
fills the weld joint. When two dc arcs are in close proximity, there is a
tendency for arc interference between them. In some cases, the second
electrode is connected to alternating current to avoid the interaction
of the arc.

 

Three-wire Tandem System

The three-wire tandem system normally uses ac power
on all three electrodes connected to three-phase power systems. These
systems are used for making high-speed longitudinal seams for
large-diameter pipe and for fabricated beams. Extremely high currents
can be used with correspondingly high travel speeds and deposition
rates.

The Strip Welding System

The strip welding system is used to overlay mild
and alloy steels usually with stainless steel. A wide bead is produced
that has a uniform and minimum penetration. This process variation is
shown by figure 10-69. It is used for overlaying the inside of vessels
to provide the corrosion resistance of stainless steel while utilizing
the strength and economy of the low-alloy steels for the wall thickness.
A strip electrode feeder is required and special flux is normally used.
When the width of the strip is over 2 in. (51 mm), a magnetic arc
oscillating device is used to provide for even burn-off of the strip and
uniform penetration.

Other Options

Another way of increasing the deposition rate of
submerged arc welding is to add iron base ingredients to the joint under
the flux. The iron in this material will melt in the heat of the arc
and will become part of the deposited weld metal. This increases
deposition rates without decreasing weld metal properties. Metal
additives can also be used for special surfacing applications. This
variation can be used with single-wire or multi-wire installations.

Another variation is the use of an electrically
"cold" filler wire fed into the arc area. The "cold" filler rod can be
solid or flux-cored to add special alloys to the weld metal. By
regulating the addition of the proper material, the properties of the
deposited weld metal can be improved. It is possible to utilize a
flux-cored wire for the electrode, or for one of the multiple electrodes
to introduce special alloys into the weld metal deposit. Each of these
variations requires special engineering to ensure that the proper
material is added to provide the desired deposit properties.

Typical Applications

The submerged arc welding process is widely used
in the manufacture of most heavy steel products. These include pressure
vessels, boilers, tanks, nuclear reactors, chemical vessels, etc.
Another use is in the fabrication of trusses and beams. It is used for
welding flanges to the web. The heavy equipment industry is a major user
of submerged arc welding.

Materials Used

Two materials are used in submerged arc welding: the welding flux and the consumable electrode wire.

Submerged arc welding flux shields the arc and the
molten weld metal from the harmful effects of atmospheric oxygen and
nitrogen. The flux contains deoxidizers and scavengers which help to
remove impurities from the molten weld metal. Flux also provides a means
of introducing alloys into the weld metal. As this molten flux cools to
a glassy slag, it forms a covering which protects the surface of the
weld. The unmelted portion of the flux does not change its form and its
properties are not affected, so it can be recovered and reused. The flux
that does melt and forms the slag covering must be removed from the
weld bead. This is easily done after the weld has cooled. In many cases,
the slag will actually peel without requiring special effort for
removal. In groove welds, the solidified slag may have to be removed by a
chipping hammer.

Fluxes are designed for specific applications and
for specific types of weld deposits. Submerged arc fluxes come in
different particle sizes. Many fluxes are not marked for size of
particles because the size is designed and produced for the intended
application.

There is no specification for submerged arc fluxes
in use in North America. A method of classifying fluxes, however, is by
means of the deposited weld metal produced by various combinations of
electrodes and proprietary submerged arc fluxes. This is covered by the
American Welding Society Standard. Bare carbon steel electrodes and
fluxes for submerged arc welding. In this way, fluxes can be designated
to be used with different electrodes to provide the deposited weld metal
analysis that is desired.

References for SAW

Submerged Arc Welding Process

Superior Consumables

 



Page Author: Jeff Grill