equipment for submerged

8/13/2019 Equipment for Submerged

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Equipment for Submerged-arcWelding

The submerged-arc welding(SAW) process is similar to MIG where the arc isformed between a continuously-fed wire electrode and the wor piece! and theweld is formed by the arc melting the wor piece and the wire" #owe$er! in SAWa shielding gas is not re%uired as the layer of flu& generates the gases and slagto protect the weld pool and hot weld metal from contamination" 'lu& plays anadditional role in adding alloying elements to the weld pool"

Essential equipment

ssential e%uipment components for SAW are• power source•

wire gun• flu& handling• protecti$e e%uipment

As SAW is a high current welding process! thee%uipment is designed to produce highdeposition rates"

Power source

SAW can be operated using either a *+ or an A+ power source" *+ is supplied

by a transformer-rectifier and A+ is supplied by a transformer" +urrent for asingle wire ranges from as low as , A (."/mm diameter wire) to as high as. A (/" mm diameter wire)" In practice! most welding is carried out on thicplate where a single wire (0" mm diameter) is normally used o$er a more limitedrange of / to 1 A! with a twin wire system operating between 2 and ., A"

In *+ operation! the electrode is normally connected to the positi$e terminal"lectrode negati$e (*+ 3) polarity can be used to increase deposition rate but

depth of penetration is reduced by between , and ,45" 'or this reason! *+ 3is used for surfacing applications where parent metal dilution is important" The*+ power source has a 6constant $oltage6 output characteristic which produces a

self-regulating arc" 'or a gi$en diameter of wire! welding current is controlled bywire feed speed and arc length is determined by $oltage setting"

A+ power sources usually ha$e a constant-current output characteristic and aretherefore not self-regulating" The arc with this type of power source is controlledby sensing the arc $oltage and using the signal to control wire feed speed" Inpractice! for a gi$en welding current le$el! arc length is determined by wire



burnoff rate! i"e" the balance between the welding current setting and wire feedspeed which is under feedbac control"

S%uare wa$e A+ s%uare wa$e power sources ha$e a constant $oltage outputcurrent characteristic" Ad$antages are easier arc ignition and constant wire feed

speed control"

Welding gun

SAW can be carried out using both manual and mechanised techni%ues"Mechanised welding! which can e&ploit the potential for e&tremely highdeposition rates! accounts for the ma7ority of applications"

Manual welding

'or manual welding! the welding gun is similar to a MIG gun! with the flu& which

is fed concentrically around the electrode! replacing the shielding gas" 'lu& is fedby air pressure through the handle of the gun or from a small hopper mounted onthe gun" The e%uipment is relati$ely portable and! as the operator guides the gunalong the 7oint! little manipulati$e s ill is re%uired" #owe$er! because the operatorhas limited control o$er the welding operation (apart from ad7usting tra$el speedto maintain the bead profile) it is best used for short runs and simple fillingoperations"

Mechanised welding - single wire

As SAW is often used for welding large components! the gun! wire feeder andflu& deli$ery feed can be mounted on a rail! tractor or boom manipulator" Singlewire welding is mostly practised using *+ 8 e$en though A+ will produce a



higher deposition rate for the same welding current" A+ is used to o$ercomeproblems with arc blow! caused by residual magnetism in the wor piece! 7iggingor welding machine"

Wire stic out! or electrode e&tension - the distance the wire protrudes from the

end of the contact tip - is an important control parameter in SAW" As the currentflowing between the contact tip and the arc will preheat the wire! wire burnoff ratewill increase with increase in wire stic out" 'or e&ample! the deposition rate for a0mm diameter wire at a welding current of 9 A can be increased fromappro&imately 1 g:hr at the normal ;,mm stic out! to .0 g:hr at a stic outlength of .92mm" In practice! because of the reduction in penetration and greaterris of arc wander! a long stic out is normally only used in cladding and surfacingapplications where there is greater emphasis on deposition rate and control ofpenetration! rather than accurate positioning of the wire"

'or most applications! electrode stic out is set so that the contact tube is slightly

proud of the flu& layer" The depth of flu& is normally 7ust sufficient to co$er the arcwhose light can be seen through the flu&"

<ecommended and ma&imum stic out lengths

Wire diameter mm Current range AWire stickout

Normal mm Maximum mm

0.8 100 to 200 12 -

1.2 150 to 300 20 -

1.6 200 to 500 20 -

2.0 250 to 600 25 63

3.2 350 to 800 30 76

4.0 400 to 900 32 128

4.75 450 to 1000 35 165

Mechanized welding - twin wireTandem arc connections

SAW can be operated with more than one wire" Although up to fi$e wires areused for high deposition rates! e"g" in pipe mills! the most common multi-wiresystems ha$e two wires in a tandem arrangement" The leading wire is run on*+ 8 to produce deep penetration" The trailing wire is operated on A+ which



spreads the weld pool! which is ideal for filling the 7oint" A+ also minimisesinteraction between the arcs! and the ris of lac of fusion defects and porositythrough the deflection of the arcs (arc blow)" The wires are normally spaced, mm apart so that the second wire feeds into the rear of the weld pool"

Gun angleIn manual welding! the gun is operated with a trailing angle! i"e" with the gun atan angle of 04 degrees (bac wards) from the $ertical" In single wire mechanisedwelding operations! the gun is perpendicular to the wor piece" #owe$er! in twinwire operations the leading gun is normal to the wor piece! with the trailing gunangled slightly forwards between an angle of / and 2 degrees" This reducesdisturbance of the weld pool and produces a smooth weld bead profile"

Flu handling

'lu& should be stored in unopened pac ages under dry conditions" =penpac ages should be stored in a humidity-controlled store" While flu& from anewly-opened pac age is ready for immediate use! flu& which has been openedand held in a store should first be dried according to manufacturer6s instructions"In small welding systems! flu& is usually held in a small hopper abo$e the weldinggun" It is fed automatically (by gra$ity or mechanised feed) ahead of the arc" Inlarger installations the flu& is stored in large hoppers and is fed with compressedair" >nused flu& is collected using a $acuum hose and returned to the hopper"

Note: +are must be ta en in recycling unused flu&! particularly regarding theremo$al of slag and metal dust particles" The presence of slag will change the

composition of the flu& which! together with the wire! determines the compositionof the weld metal" The presence of fine particles can cause bloc ages in thefeeding system"

Protecti!e equipment

>nli e other arc welding processes! SAW is a clean process which producesminimum fume and spatter when welding steels" (Some no&ious emissions canbe produced when welding special materials") 'or normal applications! generalwor shop e&traction should be ade%uate"

8rotecti$e e%uipment such as a head shield and a leather apron are notnecessary" 3ormal protecti$e e%uipment (goggles! hea$y glo$es and protecti$eshoes) are re%uired for ancillary operations such as slag remo$al by chipping orgrinding" Special precautions should be ta en when handling flu& - a dustrespirator and glo$es are needed when loading the storage hoppers"

equipment for submerged

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