5_The Submerged Arc Welding

8/9/2019 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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5_The Submerged Arc Welding