Submerged Arc Welding Introduction
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| Submerged Arc Welding Introduction |
Submerged Arc Welding Introduction (SAW) is a welding process that involves the formation of an arc between a continuously fed wire electrode and the workpiece. The arc is submerged under a blanket of granular flux, which helps protect the molten weld pool from atmospheric contamination and provides a slag layer to protect the weld as it cools.
Submerged Arc Welding
- Submerged arc welding was developed in the Soviet Union
during the 2nd world War for the welding of thick section steel.
• The process is normally mechanized.
• The process uses amps in the range of 100 to over 2000, which
gives a very high current density in the wire producing deep
penetration and high dilution welds.
• A flux is supplied separately via a flux hopper in either
fused or agglomerated.
• The arc is not visible as it is submerged beneath the flux layer
and no eye protection is required
SAW Principle of operation
Factors that determine whether to use SAW chemical
composition and mechanical properties required for the weld
deposit
• thickness of base metal to be welded
• joint accessibility
• position in which the weld is to be made
• frequency or volume of welding to be performed
Submerged Arc Welding
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| Submerged Arc Welding |
SAW process variables
- welding current
- current type and polarity
- welding voltage
- travel speed
- electrode size
- electrode extension
- width and depth of the layer of flux
SAW process variables
Welding current
- controls the depth of penetration and the amount of
base metal melted & diluted
SAW operating variables
Current type and polarity
- Usually DCEP, deep
penetration, better
resistance to
porosity
•DCEN increase
deposition rate but
reduce penetration
(surfacing)
•AC used to avoid
arc blow; can give
unstable arc
SAW Consumables
(Covered in detail in Section 14)
Fused fluxes advantages:
- good chemical homogeneity
- easy removal of fines without affecting flux composition
- normally not hygroscopic & easy storage and handling
- readily recycled without significant change in particle size or composition
Fused fluxes disadvantages:
- difficult to add deoxidizers and Ferro-alloys (due to segregation or extremely high loss)
- high temperatures needed to melt ingredients limit the range of flux compositions
SAW Consumables
Agglomerated fluxes advantages:
• easy addition of deoxidizers and alloying elements
• usable with a thicker layer of flux when welding
• color identification
Agglomerated fluxes disadvantages:
• tendency to absorb moisture
• possible gas evolution from the molten slag leading to porosity
• possible change in flux composition due to segregation or removal of fine mesh particles
SAW equipment
Power sources can be:
• transformers for AC
• transformer-rectifiers for DC
Static characteristics can be:
• Constant Voltage (flat) – most of the power sources
• Constant Current (drooping)
Constant Voltage (Flat Characteristic) power sources:
• most commonly used supplies for SAW
• can be used for both semiautomatic and automatic welding
• self-regulating arc
• simple wire feed speed control
• wire feed speed controls the current and power supply
controls the voltage
• applications for DC are limited to 1000A due to severe arc
blow (also thin wires!)
ARC CHARACTERISTICS
Constant Current (Drooping Characteristic) power sources:
• Over 1000A – breakneck speed required – control of burn-off rate and stick-out length
• can be used for both semiautomatic and automatic welding
• not self-regulating arc
• must be used with a voltage-sensing variable wire feed speed control
• more expensive due to more complex wire feed speed control
• arc voltage depends upon wire feed speed while the power source controls the current
• cannot be used for high-speed welding of thin steel
SAW equipment
Welding heads can be mounted on a: Tractor-carriage
• provides travel along straight or gently curved joints
• can ride on tracks set up along the joint (with grooved wheels) or on the workpiece itself
• can use guide wheels as a tracking device
• due to their portability, they are used in field welding or where the piece cannot be moved
SAW operating variables
Welding current
•too high current: excessive excess weld metal
(waste of electrode), increase weld shrinkage and
causes greater distortions
•excessively high current: digging arc, undercut,
burn through; also a high and narrow bead &
solidification cracking
•too low current: incomplete
fusion or inadequate penetration
• shallow current: unstable arc
SAW operating variables
Welding voltage
- welding voltage controls arc length
•an increased voltage can increase pick-up of alloying elements from an alloy flux
• An increase in voltage produces a flatter and wider bead
•increase in voltage increase flux consumption
• An increase in voltage tends to reduce porosity
•an increased voltage may help bridge an excessive root gap
SAW operating variable
Welding voltage
•low voltage produces a
“stiffer” arc & improves
penetration in a deep
weld groove and resists
arc blow
•excessive low-voltage
produce a high narrow
bead & difficult slag
removal
- excessively high voltage
produce a “hat-shaped” bead
& tendency to crack
•excessively high voltage
increase undercut & make slag
removal difficult in the groove
welds
•excessively high voltage
produce a concave fillet weld
that is subject to cracking
Travel speed
•increase in travel speed: decrease heat input & less
filler metal applied per unit of length, less excess
weld metal & weld bead becomes smaller
- excessively high speed
lead to undercut, arc
blow and porosity
•excessively low speed
produce “hat-shaped” beads
danger of cracking
•excessively low speed produces rough beads and
lead to slag inclusions
SAW operating variables
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Electrode size
•at the same current, small electrodes have higher
current density & higher deposition rates
SAW operating variablesElectrode extension
•increased electrode extension adds resistance in the
welding circuit I increase in deposition rate, decrease in
penetration and bead width
•to keep a proper weld shape, when electrode extension is
increased, voltage must also be increased
•when burn-through is a problem (e.g. thin gauge), increase
electrode extension
•excessive electrode extension: it is more difficult to
maintain the electrode tip in the correct position
SAW operating variablesDepth of flux
•depth of flux layer influences the appearance of weld
•usually, the depth of flux is 25-30 mm
•if the flux layer is to keep the arc is too confined, the result is
a rough ropelike appearing weld
•if the flux layer is to keep the gases from escaping & the
surface of molten weld metal becomes irregularly
distorted
•if flux layer is too shallow, flashing and spattering will
occur, give a poor appearance and porous weld
SAW variants
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