5_The Submerged Arc Welding
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Transcript of 5_The Submerged Arc Welding
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The submerged arc welding (SAW) Process
Submerged Arc Welding:
Submerged arc welding (SAW) is an arc welding process that fuses together the parts tobe welded by heating them with electric arcs between bare electrodes and the work piece.
The submerged arc welding process utilizes the heat of an arc between a continuously fed
electrode and the work. The heat of the arc melts the surface of the base metal and theend 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 weldarea. The flux close to the arc melts and intermixes with the molten weld metal and helps
purify and strengthen it. The flux forms a glasslike slag that is lighter in weight than thedeposited 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 shields the molten pool from atmospheric contamination, cleans impurities from theweld metal, and shapes the weld bead. Depending on the design of the flux, it can also
add alloying elements to the weld metal to alter the chemical and mechanical properties
of the weld.
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Figure: Submerge arc welding process
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)
Process and Equipment Fundamentals:
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The principles of the submerged arc process are shown schematically below. A power
source P, is connected across the contact nozzle on the welding head and the work piece. The
power source can be a transformer for AC welding, or a rectifier (or motor generator) for DCwelding. The filler materials are an uncoated continuous electrode and a granular welding flux
fed down to the joint by way of a hose from the flux hopper. To prevent the electrode
overheating at high currents the welding current is transferred at a point very close to the electricarc. The arc is burning in a cavity filled with gas (CO2, CO, etc.) and metal fumes. In front, thecavity is walled in by unfused parent material and behind the arc by solidifying weld metal. The
covering over the cavity consists of molten slag. The diagram below also shows the solidified
weld and the thin covering of solid slag, which has to be detached after the completion of eachrun.
Figure: Schematic diagram of SMAW process
Since the arc is completely submerged by the flux there is no irritating arc radiation thatis characteristic of the open arc process - welding screens are therefore unnecessary.
The welding flux is never completely consumed so the surplus quantity left can be
collected, either by hand or automatically, and returned to the flux hopper to be used
again. Although semi-automatic submerged arc welding equipment exists and is convenient for
certain applications, most submerged arc welding uses fully mechanized weldingequipment. One of the main virtues of the submerged arc process is the ease with which it
can be incorporated into fully mechanized welding systems to give high deposition rates
and consistent weld quality. Weld metal recovery approaches 100% since losses throughspatter are extremely small.
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Heat losses from the arc are also quite low due to the insulating effect of the flux bed andtherefore the thermal efficiency of the process can be as high as 60%, compared with
about 25% for MMA welding.
Flux consumption is approximately equal to the wire consumption, the actual ratio -weight of wire consumed: weight of flux consumed - being dependent on the flux type
and the welding parameters used. Welding parameters are maintained at their set values by the arc control unit. A feed back
system is usually used to maintain a stable arc length so that a change in arc length
(corresponding to a change in arc voltage) will produce an increase or decrease in thewire feed speed until the original arc length is regained.
Joint Preparation:
Joint preparation depends on plate thickness, type of joint e.g. circumferential orlongitudinal and to some extent on the standards to which the structure is being made.Plates of up to 14mm thick can be butt welded without preparation with a gap not
exceeding 1mm or 10% of the plate thickness, whichever is the greater. Thickerplates need preparation if full penetration is to be obtained. Variable fit up cannot betolerated.
A welder using stick electrodes can adjust his technique to cope with varying jointgaps and root faces or varying dimensions. Not so an automatic welding head. If
conditions are set up for a root gap of 0.5mm and this increase to 2 or 3mm, burnthrough will occur unless an efficient backing strip is used.
All plate edges must be completely clean and free from rust, oil, millscale, paint, etc.If impurities are present and are melted into the weld, porosity and cracking can
easily occur. Time spent in minimizing such defects by careful joint preparation and
thorough inspection prior to welding is time well spent since cutting out weld defects
and rewelding is expensive and time consuming.
Welding procedure:
In general the more severe the low temperature notch toughness requirements, the lower themaximum welding current that can be used. This is to minimise heat input and means that a
multipass technique may be required. When welding stainless steels the heat input should be kept
low because it has poor thermal conductivity and a high coefficient of expansion compared withmild steel. These two effects lead to overheating and excessive distortion if large diameter wires
and high currents are used. Multi-run welds using small diameter wires are therefore
recommended for stainless steels and high nickel alloys such as Inconel.
1. http://www.youtube.com/watch?v=5yQdI94THNk&NR=1&feature=endscreen
(SAW (manual)Good video) (Upto here on Jan 21,2014)
2. http://www.youtube.com/watch?feature=endscreen&v=j-hfExEmGsE&NR=1
(VERY GOOD SAW for CLASS DEMO)
Welding Parameters:
http://www.youtube.com/watch?v=5yQdI94THNk&NR=1&feature=endscreenhttp://www.youtube.com/watch?feature=endscreen&v=j-hfExEmGsE&NR=1http://www.youtube.com/watch?feature=endscreen&v=j-hfExEmGsE&NR=1http://www.youtube.com/watch?feature=endscreen&v=j-hfExEmGsE&NR=1http://www.youtube.com/watch?v=5yQdI94THNk&NR=1&feature=endscreen -
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Selection of the correct welding conditions for the plate thickness and joint preparation to be
welded is very important if satisfactory joints free from defects such as cracking, porosity and
undercut are to be obtained. The process variables, which have to be considered, are:
a. Electrode polarity.
b.
Welding current.c. Electrode diameter.
d. Arc voltage.
e. Welding speed.f. Electrode extension.
g. Electrode angle.
h. Flux depth.
These are the variables that determine bead size, bead shape, depth of penetration and in some
circumstances metallurgical effects such as incidence of cracking, porosity and weld metal
composition.
Electrode polari ty
The deepest penetration is obtained with DC reverse polarity (DC electrode positive,DCEP) which also gives the best surface appearance, bead shape and resistance to
porosity.
Direct current straight polarity (DC electrode negative, DCEN) gives faster burn off(about 35%) and shallower penetration since the maximum heat is developed at the tip of
the electrode instead of at the surface of the plate. For this reason DC electrode negative
polarity is often used when welding steels of limited weldability and when surfacing /cladding since, in both cases, penetration into the parent material must be kept as low as
possible. The flux/wire consumption ratio is less with electrode negative polarity (DCEN) than
with electrode positive polarity (DCEP), so that alloying from the flux is reduced.
With DC polarity the maximum current used is 1000 amperes due to arc blow problems.In changing from positive to negative polarity some increase in arc voltage may be
necessary to obtain a comparable bead shape.
Alternating current (AC) gives a result about half way between DC electrode positive andDC electrode negative and usually gives a flatter, wider bead. It can be used on multiheadsystems and is particularly useful when arc blow is a problem. It is often used in tandem
arc systems where a DC positive electrode is used as the leading electrode and an AC
electrode as the trail.
Welding current
Increasing the wire feed speed increases the welding current so that the deposition rate increases
as the welding current increases. The wire feed speed is the most influential control of fusion and
penetration. The current density controls the depth of penetration - the higher the current densitythe greater the penetration. For a given flux, arc stability will be lost below a minimum threshold
current density so that if the current for a given electrode diameter is too low arc stability is lost
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and a uneven, irregular bead is obtained. Too high a current density also leads to instability
because the electrode overheats and undercutting may also occur.
Electrode Diameter
For given current, changing the electrode diameter will change the current density. Therefore alarger diameter electrode will reduce penetration and the likelihood of burn-through, but at the
same time arc striking is more difficult and arc stability is reduced.
Ar c voltage
Bead on plate welds and square edged closed butt welds have increased width and dilution as the
arc voltage increases, but depth of penetration remains the same. If the joint preparation is open,
for example in a butt joint with a small angled 'V' preparation, increasing the arc voltage can
decrease the penetration.
The arc voltage controls the arc length, flux consumption and weld metal properties. Increasingthe arc voltage increases the arc length so that the weld bead width is increased, reinforcement is
decreased, flux consumption is increased and the probability of arc blow is also increased. Whenalloying fluxes are used arc lengths, and hence arc voltage, is very important since at high arc
voltages more flux is melted so that more alloying elements enter the weld metal. Thus arc
voltage can affect weld metal composition.
Weldi ng speed
Welding speed or travel speed controls depth of penetration. Bead size is inversely proportional
to travel speed. Faster speeds reduce penetration and bead width, increase the likelihood of
porosity and, if taken to the extreme, produce undercutting and irregular beads. At high weldingspeeds the arc voltage should be kept fairly low otherwise arc blow is likely to occur. If the
welding speed is too slow burn-through can occur. A combination of high arc voltage and slow
welding speed can produce a mushroom shaped weld bead with solidification cracks at the beadsides.
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El ectrode extension
Also known as electrode sticks out and alters the tip to work distance. Electrode extension
governs the amount of resistance heating which occurs in the electrode. If the extension is short
the heating effect is small and penetration is deep. Increasing the extension increases the
temperature of the electrode, which decreases the penetration, but deposition rates are increased.Increased extension is therefore useful in cladding and surface applications, but steps have to be
taken to guide the electrode, otherwise it wanders. For normal welding the electrode extensionshould be 25 - 30mm for mild steel and less, about 20 - 25mm, for stainless steel. This is because
the electrical sensitivity of stainless wire is appreciably greater than that of mild steel wire.
Electrodeangle
Since the angle between the electrode and the plate determines the point of application anddirection of the arc force it has a profound effect on both penetration and undercut. The first
figure shows the effect on horizontal/vertical fillet welds and the second figure compares the
effect obtained with a vertical arc with those obtained with leading and trailing arcs. The effecton undercutting can be particularly marked.
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F lux depth
The depth of the flux, or flux burden, is often ignored and the powder piled around the wire until
the arc is completely covered. If optimum results are to be obtained the flux depth should be justsufficient to cover the arc, although the point where the electrode enters the flux bed light
reflected from the arc should be just visible. Too shallow a flux bed gives flash-through and cancause porosity because of inadequate metallurgical protection of the molten metal. Too deep aflux bed gives a poor bead appearance and can lead to overflow on circumferential welds. On
deep preparations in thick plate it is particularly important to avoid excessive flux depth
otherwise the weld bead shape and slag removal can be unsatisfactory.
Fluxes
Fluxes are graded by basicity index and in two types - agglomerated and fused. Particle size is
important with larger currents requiring finer fluxes. Fused fluxes are dark brown or black in
colour with a glasslike surface and flakey in shape. Fused fluxes give a good surface profile and
reasonable properties. Fused fluxes are general purpose fluxes that require no preheating.Agglomerated fluxes are light in colour and roughly spherical in shape. Agglomerated fluxes
give the best mechanical properties and low hydrogen potential possible requiring the flux to be
preheated (baked). Agglomerated fluxes absorb moisture so at the end of work they must beremoved and dried.
Submerged Arc Fluxes:
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Basicity
The term basicity is commonly used to describe the chemical and metallurgical nature of a
flux. The following formula is generally used to measure the basicity of a submerged arc flux:
B = [CaO + MgO + Na2O + K2O + CaF2+ (MnO + FeO)] / [SiO2+ (Al2O3+ TiO2+ ZrO2)]
This calculation defines the ratio between acid and basic oxides present in the flux and can be
used to determine the usability of the flux. Basicity can be used to determine the relative
impact toughness a flux can provide.
Influence of grain size
Grain size is usually designated by a number that signifies the range of particle sizes thatsignify the high and low end of the range that is within the package, for example 14X65. Each
number indicates the number of openings per inch of screen. The first number indicates the
largest particle permitted, while the second number indicates the smallest particle permitted.
Grain size can affect how well the flux delivers through a delivery system, how well a
weldment de-gases, and the wetting performance of the flux. A coarse grain size is better suited
to single wire, low current applications. A fine grain size provides better edge wetting for
multi-wire, high current applications.
Different types of flux
Bonded
Bonded fluxes are made by dry mixing the ingredients, then bonding them together with a low
melting point compound. Most bonded fluxes contain metallic deoxidizers that prevent weld
porosity, especially important in fillet welds. Fine ingredients are mechanically bonded intolarger particles for good performance with one mesh size.
http://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id4367f2a9784824.24488975/path.filler_metals_submerged_arc_fluxes_bondedhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id4367f2a9784824.24488975/path.filler_metals_submerged_arc_fluxes_bonded -
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Bonded Flux Features:
Contain metallic deoxidizers
May contain alloying agents
Flat, low gloss, or dry particle appearance
Each flux particle has a unique chemistry.
Bonded Flux Benefits:
Presence of deoxidizers provides good performance over rust and mill scale and helps
prevent weld porosity.
Usually provides better peeling properties than fused fluxes.
Alloying elements can be added to provide improved chemical and mechanical
properties.
Usually exhibit lower flux consumption than a fused flux welded at the same current and
voltage.
Fused
Fused fluxes are made by mixing the ingredients, then melting them together to form achemically homogenous product. Because the ingredients are completely reacted in the
manufacture, you get smooth stable performance and consistent weld metal properties.
Fused Flux Features:
Non-hygroscopic
Fully reacted
Chemically homogenous
Contain no metallic deoxidizers
Glass-like appearance, high grain strength
Fused Flux Benefits:
Particles are non-hygroscopic and do not absorb moisture, therefore only a low
temperature (300F/150C) drying cycle is required to remove surface
moisture/condensation, providing increased protection against hydrogen cracking.
Provide smooth, stable performance even at extremely high welding currents (up to 2,000
amps).
Flux particles are chemically identical, providing more consistent welds.
Fused fluxes are less susceptible to particle breakdown due to flux recycling, reducing the
creation of fine dust particles.
Active fluxesActive fluxes are those fluxes that add manganese and silicon to the weld deposit in proportion to
the arc voltage. As voltage increases, the amount of flux consumed during the welding process
also increases, which leads to more Mn and Si added to the weldment. The addition of Mn and Si
make active fluxes well suited to welding over rust, mill scale and light oil. They also provide
excellent welder appeal. However, due to the alloying tendency of active fluxes, they can add
excessive amounts of Mn and Si, which can lead to weld embrittlement and/or cracking. As a
http://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id4367f2a9796837.40569041/path.filler_metals_submerged_arc_fluxes_fusedhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id4367f2a9796837.40569041/path.filler_metals_submerged_arc_fluxes_fused -
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result, active fluxes are recommended only for use below 36 volts and for single or multiple pass
welds up to 1-inch thickness.
Advantages of Active Fluxes:
Good for use over rust, mill scale, even light rust
Excellent slag peeling characteristics High speed capability
Improved weld metal wetting
Neutral Flux
A neutral flux does not cause a significant change in weld chemistry as a result of changes to arc
voltage or of the amount of flux consumed during welding. As with any flux, a neutral flux does
affect the weld deposit chemistry. The levels of alloying elements added to the weld are
generally consistent across even significant changes in voltage. Therefore, the deposit chemistry
will not match the wire chemistry. Neutral fluxes can be used in multiple pass applications of
unlimited plate thickness without the concern for alloy buildup, as with active fluxes. Neutralfluxes are generally not designed to handle rust and mill scale tolerance, and therefore should be
used on clean plate.
Advantages of Neutral Fluxes:
Unlimited number of weld passes
Unlimited plate thickness allowed
Weld deposit chemistry not sensitive to changes in voltage/flux consumed
Storage and Handling of F luxes
Storage
Unopened flux bags must be stored in maintained storage conditions as follows:
Temperature: 68F, +/- 18F (20C, +/- 10C)Relative humidity: As low as possible - not exceeding 60% max.
Fluxes should not be stored longer than 3 years.
The content of unheated flux hoppers must, after an 8 hours shift, be placed in a drying
cabinet or heated flux hopper at a temperature of 300F, +/- 45F (150C +/- 25C).
Remaining flux from unopened bags must be placed at a temperature of 300F, +/- 45F
(150C +/- 25C).
Re-Cycling
Moisture and oil must be removed from the compressed air used in the re-cycling system.
Addition of new flux must be done with the proportion of at least one part new flux toone parts re-cycled flux.
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Foreign material, such as millscale and slag, must be removed by a suitable system, such
as sieving or magnetic separator.
Re-Drying
When handled and stored as above, the new fluxes can normally be used straight away.
In severe applications, stipulated by the applicable material specification, re-drying of theflux is recommended.
Furthermore, if the flux has somehow picked up moisture, redrying can return the flux to
its original moisture content.
Re-drying shall be performed as follows:
Agglomerated fluxes: 570F, +/- 45F (300C +/- 25C) for about 2-4 hours.
Fused fluxes: 390F, +/- 90F (200C +/- 50C) for about 2-4 hours.
Re-drying must be done either in equipment that turns the flux so that the moisture canevaporate easily or in an oven on shallow plates with a flux height not exceeding 2 in (5
cm).
Re-dried flux, not immediately used, must be stored at 300F, +/- 45F (150C +/- 25C)before use.
Disposal
Discard any product, residue, disposable container or liner in an environmentally
acceptable manner, in full compliance with federal and local regulations.
Please address your local disposal company for prescribed disposal.
Information on product and residues are given in the Safety Data Sheets
3. http://www.youtube.com/watch?v=WMZGdI_93TM&NR=1&feature=endscreen SAW
Reclaiming Slugs---Good Class Video)
http://www.youtube.com/watch?v=WMZGdI_93TM&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=WMZGdI_93TM&NR=1&feature=endscreenhttp://www.youtube.com/watch?v=WMZGdI_93TM&NR=1&feature=endscreen -
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Submerged Arc Wire:
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.
Soli d Wir e for Carbon Steel: variety of formulations for single or multipass welding on
carbon steel applications.
http://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id44059f3282a204.99939221/path.filler_metals_submerged_arc_wire_solid_wire_carbon_steelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id44059f3282a204.99939221/path.filler_metals_submerged_arc_wire_solid_wire_carbon_steel -
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Solid Wire for Low Alloy Steel:Low alloy wires are designed for single or multipass
welding on low alloy steels.
Wire forStainless Steel:Stainless Steel Submerged Arc Wires are manufactured under a
carefully administered, high standard quality control program.
Wire forNickel:Nickel-based alloys are specially formulated and designed to meet a
variety of corrosion resistant and low temperature cryogenic applications.
Advantages of SAW:
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 thefluxis recoverable.
Limitations of SMAW:
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 concern.
Requires inter-pass and post weld slag removal.
http://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id4405a09e617486.90942069/path.filler_metals_submerged_arc_wire_solid_wire_low_alloy_steelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id461f82b62b10b3.98238718/path.filler_metals_submerged_arc_wire_stainless_steelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id461f82b62b10b3.98238718/path.filler_metals_submerged_arc_wire_stainless_steelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id461f877761c0b3.03508688/path.filler_metals_submerged_arc_wire_nickelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id461f877761c0b3.03508688/path.filler_metals_submerged_arc_wire_nickelhttp://en.wikipedia.org/wiki/Flux_(metallurgy)#Flux_Recoveryhttp://en.wikipedia.org/wiki/Flux_(metallurgy)#Flux_Recoveryhttp://en.wikipedia.org/wiki/Flux_(metallurgy)#Flux_Recoveryhttp://en.wikipedia.org/wiki/Flux_(metallurgy)#Flux_Recoveryhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id461f877761c0b3.03508688/path.filler_metals_submerged_arc_wire_nickelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id461f82b62b10b3.98238718/path.filler_metals_submerged_arc_wire_stainless_steelhttp://products.esabna.com/EN/home/filler_metals_catalog/filler_metals_secondary/q/display_id.id4405a09e617486.90942069/path.filler_metals_submerged_arc_wire_solid_wire_low_alloy_steel -
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