Welding Processes - Word 6

8/13/2019 Welding Processes - Word 6

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USE OF MATERIALS COURSEWELDING PROCESSES NOTES

INDEX

Page

1. Metal-Arc Weldig !it" C#$ered Electr#de%................................. 3

1.1 The Process .........................................................................3 1.2 Welding Positions ................................................................4 1.3 Functions of the Electrode Coating .....................................5 1.4 Types of Electrode Coating .................................................6 1.5 Electrode Classification yste!..........................................." 1.6 The #nfluence of Welding Current .....................................11 1.$ %rc &ength .........................................................................12

1.' &o( )ydrogen Electrodes .................................................14 1." *eep Penetration Welding ................................................15 1.1+ )ard Facing .......................................................................15 1.11 ,ra-ity Welding .................................................................16

&. S'()erged Arc Weldig 1'

2.1 The Process .......................................................................1'2.2 aterials /oined ................................................................1"2.3 Flu0es ................................................................................1"

2.4 Welding )ead %rrange!ents ............................................1"2.5 perating ariales ..........................................................2+

2.5.1 Welding Current ...............................................2+ 2.5.2 %rc oltage .......................................................2+ 2.5.3 Tra-el peed ....................................................21 2.5.4 Electrode ie ..................................................21 2.5.5 Electrode E0tension .........................................22 2.5.6 Type of Electrode .............................................22 2.5.$ Width and *epth of Flu0 ..................................22

*. Ga%-S"ielded Metal-Arc Weldig.................................................. 25

3.1 The Process .......................................................................25 3.2 Electrodes ..........................................................................26 3.3 Transfer odes .................................................................2$

3.3.1 pray Transfer ..................................................2$ 3.3.2 hort Circuit or *ip Transfer ............................2' 3.3.3 e!ishort Circuiting %rc .................................2" 3.3.4 Pulsed %rc pray ..............................................2"

3.4 hielding ,ases.................................................................2" 3.5 perating ariales............................................................32

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3.5.1 %rc oltage .......................................................32 3.5.2 %rc &ength ........................................................32 3.5.3 Current ..............................................................32 3.5.4 Tra-el peed ....................................................32 3.5.5 Electrode E0tension .........................................32

3.5.6 Electrode ie ..................................................33

3.6 %d-antages and &i!itations of the ,%W Process .........33 3.$ Cored and elfhielded Wires ........................................34

+. T'g%te Iert Ga% Arc Weldig .....................................................3$

,. A't#)atic Weldig ...........................................................................3"

. Electr#%lag Weldig..........................................................................4+

6.1 The Process .......................................................................4+ 6.2 Welding (ith Consu!ale ,uides or oles ..................41

. Electr#ga% Weldig ...........................................................................43

/. Oe Side Weldig !it" 0acig.......................................................44

2. C#%')a(le% ad P#!er S#'rce%...................................................4$

".1 Care and storage of consu!ales ....................................4$ ".2 Po(er ources ..................................................................4'

".3 %rc 7lo( ............................................................................4"

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1. METAL-ARC WELDING WIT3 CO4ERED ELECTRODES

1.1 T"e Pr#ce%%

8no(n in the 9% as hielded etal %rc Welding :%W; and else(here

as !anual !etal arc (elding :%; this (elding process is y far the !ost(idely used< especially for short (elds in production< !aintenance< repairand construction in the field :see Figure 1;. Welds can e !ade in areas ofli!ited access and the e=uip!ent is relati-ely si!ple< ine0pensi-e andportale. Welding in any position is possile pro-ided appropriateelectrodes are chosen. The process !ay e applied to the !ost co!!onlyused !etals and alloys such as caron and alloy steels< stainless steelsel and their alloys. #t is not suitale for lo( !elting !etals suchas Tin< &ead or ?inc or the !ore o0ygen reacti-e !etals such as alu!iniu!< the -oltage drops to the arc-oltage. The po(er supply can e alternating or direct current and in thelatter case the electrode !ay e connected positi-ely or negati-ely.,enerally %C see!s to e fa-oured in the 98 (hile *C is !ore co!!onlye!ployed in the 9%.

Fig're & Ma'al Metal Arc !eldig

For flat (elding< !etal transfer across the arc is attriuted to gra-ity< gase0pansion< electric and !agnetic forces and surface tension ut in otherpositions gra-ity (ill (or> against the other pheno!ena. The centre of thearc has a te!perature of at least 5+++6+++A C< (ell ao-e the !elting pointof any !etal.

1.2 Weldig P#%iti#%

The specification of (elding positions is i!portant for t(o reasons. First< the!anufacturer needs to define the positions for (hich his electrodes aresuitale. econd< the (elderBs s>ills and =ualifications are to a large e0tentdeter!ined y the position at (hich he can produce an acceptale (eld.

Thus for e0a!ple !uch greater s>ill is re=uired to (eld in an o-erheadposition as co!pared (ith a flat or do(nhand position.

#t should e pointed out that the (elding position is not li!ited y theprocess itself ut y the sie and type of electrode.

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There are no asolute definitions of (elding positions< ut< in principle< suchdefinitions are all si!ilar< -ariations arising only fro! !inor differences inangles. The (eld slope !ay e defined as the angle et(een the line of theroot of the (eld and the horiontal. The (eld rotation is defined y dra(inga line fro! the root of the (eld so that it isects the (eld profile and is at

right angles to the (eld line. The angle that this line !a>es (ith the -erticalis the angle of (eld rotation. #nter!ediate positions not specified !ay ereferred to as inclined. :ee Figure 3 and Tale 1;.

Fig're * Weldig P#%iti#%

Ta(le 1 Weldig P#%iti#%

Positions for Plates lope otation &y!ol

#y!ol

%Wy!ol

Flat +5A +1+A * P% 1,)oriontal-ertical +5A 3+"+A D PC 2,

ertical up '+"+A +1'+A u PF 3,

ertical do(n '+"+A +1'+A d P, 3,

-erhead +15A 1151'+A PE 4,

ote for fillet (elds< horiontal-ertical has the sy!ol P7 and o-erheadP*.

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1.3 F'cti#% #7 t"e Electr#de C#atig

The functions of the electrode coating are !any and -aried ut the t(o of!ost significance are

:a; to pro-ide a gas to shield the arc and pre-ent e0cessi-e at!osphericconta!ination of the !olten filler !etal tra-elling across the arcG:; to i!pro-e the s!oothness and staility of the arc.

ther i!portant aspects are

:c; to produce a slag lan>et to protect the hot (eld !etal fro! the air< toallo( slo(er cooling and to enhance ead shape and surfacecleanliness of the (eld !etalG

:d; to pro-ide flu0es< sca-engers and deo0idisers to cleanse the (eld andpre-ent e0cessi-e grain gro(thG

:e; to allo( alloying ele!ents to e added to change the co!position ofthe (eld !etal.

The use of %C (ill also affect the de!ands on the coating since the arc ise0tinguished and !ust e reignited e-ery half cycle. Therefore the arcat!osphere !ust contain a suitale ionised gas to !a>e this possile.Coatings containing iron po(der !ay e used to increase the rate ofdeposition and to i!pro-e efficiency in the use of arc energy.

1.4 T86e% #7 Electr#de C#atig

There are three !ain classes of electrode coatingG

The first< BCell'l#%icB< contains a large proportion< up to aout 35H< of theorganic co!pound Cellulose :C6)1+5;n< together (ith slag for!ing ite!sprincipally utile< a !ineral for! of Titaniu! *io0ide Ti 2. Cellulose is anaturally occurring constituent of (ood (hich at high te!peratures (illdissociate into o0ides of caron and (ater. #n ferrous (elding the latter (illreact (ith iron to produce !etallic o0ide and the gas hydrogen< (hich toso!e e0tent (ill e asored into the (eld. uch coatings (ill produce ali!ited a!ount of slag and they tend to e restricted to s!aller dia!etersand !ay e used for (or> in all positions.

#n the second type of coating< B0a%icB< a protecti-e gas is produced y thedissociation of asic caronates< !ainly Calciu! Caronate CaC3< (hichat high te!perature is con-erted to Calciu! 0ide Ca and Caron *io0ideC2. These coatings usually include so!e Calciu! Fluoride or Fluorsparel steels< and the classification is indicated y the !a@or alloyingele!ents of the (eld deposit. The first three character sets indicate the

no!inal Chro!iu!< ic>el and olydenu! contents respecti-ely. Theletter & or ) is then added for lo( or high caron content respecti-ely. therche!ical sy!ols such as n< Cu< follo( if re=uired. Finally appropriateelectrode coating sy!ols such as 7 for asic< for rutile etc. appear. :e.g.1".12.3..;

The %W pecification for these electrodes is %5.4. %s efore they aredenoted y the prefi0 E follo(ed y a specification nu!er for the (eld!etal deposit. These specification nu!ers are the sa!e as those of the%## series for stainless and alloy steels. :e.g. E316&;

Tale 4 gi-es details of -arious alloy and stainless steel co!positionsrelating to the # sy!ol on (hich the 7ritish and European tandards areased. The %W e=ui-alents are included.

#t (ill e appreciated that the foregoing descriptions relate only to theirarest essentials. #t cannot e e!phasised too strongly that (hen dealing(ith tandard pecifications it is i!portant that they are studied in theirofficial for! and their re=uire!ents fully understood.

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Ta(le + Staile%% Steel C#)6#%iti#%

#y!ol

Co!position of deposited !etali!ilar

%W :2;

standardC a0H Cr H i H o H ther

ele!ents13 +.12 1114 E 41+

1$ +.1+ 151' E 43+

3+ +.1+ 1$3+1"." +.+' 1'21 '11 E 3+'

1"." & +.+4 1'21 '11 E 3+'&1"." +.+' 1'21 '11 :1; E 34$

1"." & +.+4 1'21 '11 :1;

1".12.2 +.+' 1$2+ 1114 22.5 E 3161".12.2 & +.+4 1$2+ 1114 22.5 E 316&

1".12.2

+.+' 1$2+ 1114 22.5 :1; E 31'

1".13.4 +.+' 1$21 1115 3.55.5

1".13.4 & +.+4 2226 1115 3.55.5

E 31$

23.12 +.15 2226 1115

23.12 & +.+4 2226 1115 E 3+"

23.12 +.12 2225 1115 :1;

23.12.2 +.12 2225 1115 23

1'.' n +.2+ 1$2+ $1+ n 5'

25.2+ +.2+ 242' 1'22 E 31+25.2+ & +.+4 242' 1'22

25.2+ +.12 242' 1'22 :1;

25.2+.2 +.12 252' 1'22 23

25.2+ C +.25I+.45 242' 1'22

2+." +.13 1'21 '1+ +.35+.65

:1; E 34"

2"." +.15 2'32 '12 E 3121'.36 +.25 141" 333'

:1; 7 content M !in '0C content and !a0. 1.2H. part of can ereplaced y Ta.

:2; %!erican Welding ociety.

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1.6 T"e I7l'ece #7 Weldig C'rret

Tales are a-ailale as guides to the appro0i!ate currents to e used (ith-arious types and sies of electrodes< although the actual -alues e!ployed(ill depend to a great e0tent on the (or> to e done. ,enerally the higher

the current in the range gi-en for a particular electrode sie< the deeper the6eetrati#and the faster the rate of deposition. Too high a current canlead to %6atter and 'derc'ttig ut too lo( a current (ill result ininsufficient penetration and too s!all a deposit of (eld !etal. %s a rule thearc -oltage (ill e oser-ed to increase slightly (ith increase in electrodedia!eter.

perating re=uire!ents are in-arialy clearly stated y the electrode!anufacturer and these are est adhered toG ho(e-er< as a guide< Tale 5sho(s so!e suggested -alues.

Ta(le , T86ical Electr#de C'rret%Electrode*ia!eter:!!;

1.6 2.5 3.25 4 5 6

or!aleco-eryTypeElectrode

3+5+% 5+11+% 11+15+%

14+2++%

2++26+%

22+34+%

)igheco-eryElectrode

13+16+%

1"+22+%

26+32+%

33+3'+%

The dia!eter of the electrode to e used (ill depend on the (eldingposition< thic>ness and the type of @oint. #n the o-erhead< -ertical andhoriontal-ertical positions< o(ing to the effects of gra-ity< !olten !etaltends to run out of the @oint and control y (ay of s!aller !olten !etal poolas pro-ided y lo(er currents and s!aller dia!eter electrodes is necessary.

The (elding current !ay e DC positi-e or negati-e< or AC. o!eelectrodes !ay e used (ith DC or AC< ut others (ill e li!ited in thisrespect. The energy cost is lo(er (hen (elding (ith AC ut as thisrepresents only a -ery !inor part of the total (elding costs it is unli>ely toe a significant factor (hen choosing the current type. ,enerally all

electrodes can e used (ith DC(hich pro-ides a steadier arc and s!oother!etal transfer than AC. #t also produces a good (etting action and a unifor!(eld ead shape. #t is considered etter for -ertical and o-erhead (or>< and(here a short arc is ad-antageous. Thin sheet is easier to (eld (ith DC.)o(e-er< there is the disad-antage of Karc lo(L (here !agnetic effectsinfluence the direction of the arc !a>ing it difficult to control< especially(hen (elding near the edges of ferro !agnetic !etals using high currents.:ee ection 12;. This prole! does not arise (ith AC.

When the electrode is connected to DC negati-e :%!erican ter!inology

DCstraight polarity< DCSP; aout t(o thirds of the heat is at the (or>piece(hich (ill gi-e deeper penetration. n the other hand if the electrode isconnected to DCpositi-e :US DCre-erse polarity< DCRP; t(o thirds of the

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heat (ill e at the electrode< thus increasing the electrode !elting rate utreducing penetration.

1.$ Arc Legt"

The arc length is the distance fro! the !olten tip of the electrode core (ireto the surface of the !olten (eld pool. Proper arc length is i!portant inotaining a sound (elded @oint. etal transfer fro! the tip of the electrodeto the (eld pool is not a s!ooth< unifor! action< and instantaneous arc-oltage -aries as droplets of !olten !etal are transferred across the arc direction and intensity< (hich (ill tend to spatterthe !olten !etal as it !o-es fro! the electrode to the (eld. The spatter!ay e hea-y and deposition efficiency lo(. %lso< the gas and flu0

generated y the co-ering are not as effecti-e in shielding the arc and the(eld !etal fro! air. The poor shielding can cause porosity andconta!ination of the (eld !etal y o0ygen or nitrogen< or oth and the=uality of the (eld (ill e poor.

Control of arc length is largely a !atter of (elder s>ill< in-ol-ing the (elderBs>no(ledge< e0perience< -isual perception and !anual de0terity. %lthoughthe arc length does change to so!e e0tent (ith changing conditions certainfunda!ental principles can e gi-en as a guide to the proper arc length fora gi-en set of conditions.

For do(nhand (elding< particularly (ith hea-y electrode co-erings< the tipof the electrode can e dragged lightly along the @oint. The arc length< in thiscase< is auto!atically deter!ined y the depth of the cup at the tip of theelectrode and the !elting rate of the electrode. For -ertical or o-erhead(elding< the arc length is al(ays gauged y the (elder. The proper arclength< in such cases< is the one that per!its the (elder to control the sieand !otion of the !olten (eld pool. The sa!e is true for the root passes ingroo-e and fillet (elds.

The -arious classifications of electrodes ha-e (idely different operatingcharacteristics< including arc length. #t is i!portant< therefore< for the (elderto e fa!iliar (ith the operating characteristics of the types of electrodes heuses in order to recognise the proper arc length and to >no( the effect ofdifferent arc lengths. The effect of a long and a short arc on eadappearance (ith a !ild steel electrode is illustrated :see Figure 4;.

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Fig're + E77ect% #7 $ar8ig c'rret: arc legt" ;arc $#ltage< ad tra$el%6eed ill'%trated (8 %'r7ace% ad cr#%%-%ecti#al $ie!% #7 %"ielded)etal-arc !eld%= le7t t# rig"t - c'rret: arc legt" ad tra$el %6eed#r)al> c'rret t## l#!> c'rret t## "ig"> arc legt" t## %"#rt> arclegt" t## l#g> tra$el %6eed t## %l#!> tra$el %6eed t## "ig"

Ta(le I7l'ece #7 Arc Legt" # Weld Metal Aal8%i%

%rc Type C 2 n i P

hort +.+'5 +.++" +.$2 +.53 +.+1' +.+16

or!al +.+'+ +.+15 +.$1 +.54 +.+1$ +.+2$

&ong in still air +.+$5 +.+4' +.64 +.3" +.+1' +.+25

&ong in (indy conditions +.+55 +.+6" +.63 +.15 +.+1' +.+1':*eposition (ith a nor!al asiccoated electrode;

1.' L#! 38dr#ge Electr#de%

%t high te!peratures hydrogen< unli>e o0ygen and nitrogen< does not for!any co!pounds (ith iron and has a high soluility in the austenite phase.)ydrogen has a lo(er soluility in steel after transfor!ation at lo(erte!peratures< on cooling. This hydrogen (ill cause e!rittle!ent in steel. #na (eld and the surrounding regions the presence of hydrogen (ill also

increase the tendency to crac>ing. #t is i!portant in all critical structures to>eep hydrogen to a !ini!u!. The prole! can e !ini!ised y e!ployingasiccoated electrodes (hich ha-e een a>ed in !anufacture andsuse=uently >ept dry. )ere the coating consists of calciu!< and othercaronates< and fluorspar onded (ith sodiu! or potassiu! silicate. #n theheat of the arc< the caronates dissociate releasing caron dio0ide (hichacts as the shielding gas. To further reduce !oisture content they arefre=uently a>ed i!!ediately efore use. &o( hydrogen electrodes arenor!ally tested to de!onstrate a (eld !etal hydrogen content of less than15 c!3I1++g of deposited !etal.

Tale $ indicates the effect of different coatings and a>ing te!peratures onhydrogen content of the (eld !etal.

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Ta(le T86ical 38dr#ge C#tet%

Coating type )ydrogen c!3per 1++ g deposited!etal

Cellulosic N $+utile N 2+

7asic *ried 1++15+AC 1+ 157asic *ried 35+45+AC 3 1+

% 7asic slag is relati-ely thic> and -iscous (hich !a>es the electrodesco!parati-ely difficult to use. They can ho(e-er e e!ployed for (elding inall positions and the (eld !etal has e0cellent !echanical properties. uchelectrodes are often used for (elding structures e0posed to high stressesand are usually specified (hen there are re=uire!ents for i!pact -alues atlo( te!peratures. 7asic< lo( hydrogen coatings are used for electrodes todeposit high strength steel (eld !etal.

1." *eep Penetration Welding

With co!!on (elding practices it !ay e e0pected that penetration (ill eof the order of 1 !! per 1++ a!ps of current. % deep penetration electrodeis defined in 7 4"" as K% co-ered electrode in (hich the co-ering aids theproduction of a penetrating arc to gi-e a deeper than nor!al fusion in theroot of the @oint.L uch electrodes can e used to produce faultless utt(elds in s=uare utt @oints (hich ha-e een set up correctly.

The deep penetration electrodes are so!eti!es gi-en the classification Ppiece. For such electrodes an arc -oltage of 6+$+is usual co!pared (ith 2+3+ for the nor!al type of electrode. #n generalthe (eld !etal (ill contain a large proportion of !elted parent !etal and (illtherefore ha-e a co!position closely related to that of the parent !etal.

1.1+ 3ard Facig

K% (elding is a often used for applying surface layersL to !etals to

i!pro-e the resistance to arasion< i!pact< corrosion and heat. Thead-antage of the !ethod is that the surface can e deposited on a cheaperase !etal to gi-e (ear resistance or other =ualities< e0actly (herere=uired< (ith great financial sa-ings. %lso (orn parts can e uilt up (ithsustantial reductions in ti!e and replace!ent costs. ery hard surfacesare nor!ally re=uired for good arasion resistance ut high hardness-alues are usually acco!panied y poor resistance to i!pact. Con-erselygood i!pact resistance is not allied to e0tre!e hardness and it is thereforenecessary to deter!ine (hich =uality is of greater i!portance.

i!ilarly consideration !ust e gi-en to the re=uire!ents of corrosion and

heat resistance and to the co!position of the ase !etal< the need for preheating and the possiility of post(elding heat treat!ent. The telliteseries of alloys (hich !ay e nic>el or coalt ased are (ell >no(n for

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hardfacing applications (hich !ay include caterpillar trac>s< e0ca-atoruc>ets< rail(ay points< roc> crushers etc. Cutting tools for lathes and!illing !achines etc. can e !ade y depositing a layer of high speed toolsteel onto a shan> of lo(er caron steel.

#t (ill e realised that the parent !etal (ill dilute the deposited !etal and to!ini!ise this effect three hard facing layers should e applied (herepossile. The total thic>ness of the hard facing layers should nor!ally note0ceed aout 6 !! and (here a thic>er deposit is re=uired it should first euilt up (ith lo( hydrogen (eld !etal.

1.11 Gra$it8 Weldig

This is a si!ple !ethod for econo!ically (elding long fillets in the flatposition using gra-ity to feed the electrode and to tra-erse the (eld poolalong the @oint. %n operator can loo> after t(o or !ore !achines at any one

ti!e< for e0a!ple< one on each side of a plate< gi-ing sy!!etrical (elds andreducing stress and distortion. The electrode holder is !ounted on a allearing carriage and slides s!oothly do(n a guide ar< the angle of (hichcan e ad@usted to gi-e faster or slo(er tra-erse and thus -ary the length ofdeposit of the electrode and the leg length of the (eld :see Figure 5;.

Fig're , Gra$it8 Weldig

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Electrodes of $++ !! and !ore in length are a-ailale in dia!eters of 3.5 et(een aconsu!ale (ire electrode and the (or>.

The process is suitale for (elding alu!iniu!< !agnesiu! alloys< plain andlo(alloy steels< stainless and heatresistant steels< copper and rone< the-ariations eing filler (ire and type of shielding gas.

The continuous consu!ale electrode (ire is !echanically fed fro! a spoolto a !anually or !echanically controlled gun through a fle0ile guide tuey !otordri-en rollers of ad@ustale speed. The rate of urnoff of theelectrode (ire !ust e alanced y the rate of (ire feed (hich deter!inesthe current used.

#n addition< a shielding gas or gas !i0ture is fed to the gun together (ith(elding current supply< cooling (ater flo( and return :if the gun is (atercooled; and a control cale fro! the gun s(itch to control contactors :seeFigures ' and ";.

Fig're / C#)6#et% #7 ga% %"ielded )etal arc !eldig 6r#ce%%

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3.3 Tra%7er M#de%

3.3.1 S6ra8 Tra%7er

#n !anual !etal arc (elding< !etal is transferred in gloules or droplets

fro! the electrode to (or>. #f the current is increased to the continuouslyfed< gasshielded (ire< the rate at (hich such droplets are pro@ected acrossthe arc increases and they eco!e s!aller in -olu!e and the transfer isthen in the for! of a fine spray :see Figure l+a;.

The type of gas eing used as a shield greatly affects the -alues of currentat (hich spray transfer occurs ut they are usually !ore than 2++ a!ps.uch greater current densities are re=uired (ith C2 than (ith argon!i0tures to otain the sa!e droplet rate. The arc is continuous duringoperation< arc energy output is high< the rate of deposition of !etal is highing the process suitale for thic>er sections< !ostly in the flat position.

The high currents used produce strong !agnetic fields and a -erydirectional arc. With argon shielding the forces on the droplets are (ellalanced during transfer so that they !o-e s!oothly fro! (ire to (or> (ithlittle spatter. With C2shielding the forces on the droplet are less alancedso that the arc is less s!ooth and spatter tendency is greater.

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;a< S6ra8 tra%7er= arc $#lt% &-+, 4. S"ieldig ga%e%= arg#: arg#- 1 #r& #98ge: arg#- &B CO&: arg#- & #98ge- , CO&. 3ig" c'rret ad

de6#%iti# rate: '%ed 7#r 7lat !eldig #7 t"icer %ecti#%

;(< S"#rt-circ'it #r di6 tra%7er= arc $#lt% 1,-&& 4. S"ieldig ga%e% a% 7#r%6ra8 tra%7er. L#!er "eat #'t6't ad l#!er de6#%iti# rate t"a %6ra8 tra%7er.Mii)i%e% di%t#rti#: l#! dil'ti#. U%ed 7#r t"ier %ecti#% ad 6#%iti#al!eldig #7 t"icer %ecti#%

Fig're 1B T86e% #7 arc tra%7er

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3.3.2 S"#rt Circ'it #r Di6 Tra%7er

With lo(er arc -olts :1522; and currents usually less than 2++ a!pses place in gloular for! ut (ith inter!ittent shortcircuiting ofthe arc :see Figure 1+;. The (ire feed rate !ust @ust e0ceed the urnoff

rate so that the inter!ittent shortcircuiting (ill occur. When the (iretouches the pool and shortcircuits the arc there is a !o!entary rise ofcurrent< (hich !ust e sufficient to !a>e the (ire tip !olten< a nec> is thenfor!ed in it due to !agnetic pinch effect and it !elts off in the for! of adroplet eing suc>ed into the !olten pool aided y surface tension. The arcis then reestalished< gradually reducing in length as the (ire feed rategains on the urnoff until shortcircuiting again occurs. The po(er source!ust supply sufficient current on shortcircuit to ensure !eltoff or other(isethe (ire (ill stic> in a solidified (eld. #t !ust also e ale to pro-idesufficient -oltage i!!ediately after shortcircuit to reestalish the arc.

The shortcircuit fre=uency depends upon

the arc -oltage and current

type of shielding gas

dia!eter of (ire

po(er source characteristics

ut (ill e aout 5+ to 2++ ti!es per second. The heat output of this type ofarc is !uch less than that of the spray transfer type and !a>es the processsuitale for the (elding of thinner sections and for all positional (elding< inaddition to !ultirun thic>er sections< and it gi-es !uch greater (elding

speed than !etalarc(elding (ith co-ered electrodes on light gauge steel of fusion defectseep stale (elding conditions (ith a lo( -oltage arc :1$2+ ;(hich is eing rapidly shortcircuited< the po(er source !ust ha-e the rightcharacteristics. #f the shortcircuit current is lo( the electrode (ill freee tothe plate (hen (elding (ith lo( currents and -oltages. #f the shortcircuitcurrent is too high a hole !ay e for!ed in the plate or e0cessi-e spatter!ay occur due to scattering of the arc pool (hen the arc is reestalished.

The po(er supply !ust fulfil the follo(ing conditions

1. *uring shortcircuit the current !ust increase enough to !elt the (iretip ut not so !uch that it causes spatter (hen the arc is reestalished.

2. The inductance of the circuit !ust store enough energy during theshortcircuit to help to start the arc again and assist in !aintaining itduring the decay of -oltage and current

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3.3.3 Semi-short Circuiting Arc

#n et(een the spray transfer and dip transfer ranges is an inter!ediaterange in (hich the fre=uency of droplet transfer is approaching that of sprayyet at the sa!e ti!e shortcircuiting is ta>ing place< ut is of -ery short

duration. This se!ishort circuiting arc has certain applications< as fore0a!ple the auto!atic (elding of !ediu!thic>ness steel plate (ith C 2asthe shielding gas.

3.3.4 P'l%ed Arc S6ra8

This syste! allo(s allposition (elding at higher energy le-els than shortcircuit transfer. The po(er source pro-ides t(o current le-els< a steadyKac>groundL le-el too lo( to produce spray transfer and a Kpulsed pea>Lcurrent (hich is superi!posed on the ac>ground at regular inter-als. Thepulsed pea> is (ell ao-e the transition current and usually one drop of

!etal is transferred during each pulse. The co!ination of t(o currentle-els produces a steady arc (ith a0ial spray transfer at currents elo(those re=uired for con-entional spray arc (elding.

3.4 S"ieldig Ga%e%

%s o0ygen and C2are not inert gases the ter! Metal-Arc Iert Ga% :MIG;is not applicale (hen either of these gases is !i0ed (ith %rgon< or C 2isused on its o(n. The ter! Metal-Arc Acti$e Ga% ;MAGpiece< :the electrode eing positi-e; to gi-e an irregular (eld profile.%rgon plus 1H or 2H o0ygen gi-es a higher arc te!perature and theo0ygen acts as a (etting agent to the !olten pool !a>ing it !ore fluidand stailising the arc.

3eli') is so!eti!es added to !i0ed gases. #ts presence increases thearc -oltage and conse=uent heat input. i0ing it (ith %rgon< 0ygen or

C2

controls the pool te!perature< increases (etting and stailises thearc.

Car(# Di#9ide CO&has the ad-antage of eing the cheapest shieldinggas and it can e used for (elding oth alloy and plain caron steels upto +.4HC. There is so!e dissociation of C2in the arc producing caron!ono0ide and o0ygen (hich re=uires the filler (ire to e ade=uatelydeo0idised to pre-ent porosity. o!e (ires rely solely on n and i forthis deo0idation. thers include the !ore efficient ele!ents %l< Ti and ?rin -arying proportions. ,enerally the arc is less s!ooth (ith C2 than(ith %rC2and %r2. The arc conditions are !ore critical and there is!ore spatter.

%rgon plus 5H C2or %rgon J 2+H C2for steel i!pro-es the (ettingaction< reduces surface tension and !a>es the pool !ore fluid. 7oth

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!i0tures are e0cellent (ith spray or dip transfer< they gi-e a s!ootherless critical arc than pure C2and reduce spatterG ut naturally they are!ore e0pensi-e than pure C2.

eco!!ended gases and gas !i0ture for -arious !etals and alloys are

sho(n in Ta(le / (el#!=etal Type ,as hield e!ar>sCaron and lo(alloy steels

C2

%r15I2+HC2

%r5HC2%r5H2

%r5HC2 2H2

For dip transfer< and spray transfer spatterprole!s. 9se deo0idied (ire.

For dip or spray transferini!u! spatter

For dip and spray transferpray transfer. )igh i!pact properties.

For pulsed arc and thin sections.

tainless teels %r1I2H2$5H)e 23.5H%r1.5HC2

$5H)e 24H%r 1H2

pray transfer)igh =uality dip transfer. For thin sectionsand positional (or>.

,ood profile.

%lu!iniu! andits alloys

%rgon)eliu!

$5H )e 25H%r

tale (ith little spatter)otter arc< less preheat< !ore spattertale arc< high heat input. ,oodpenetration. eco!!ended for

thic>nesses ao-e 16 !!.

agnesiu! andits alloys

%rgon$5H )e 25H%r

tale arc)otter arc. &ess porosity.

Copper and itsalloys

%rgon

)eliu!$5H )e 25H%r

For sections up to ".5 !! thic>ness

For !ediu! and hea-y sections. )igh heatinput.

ic>els and itsalloys

%rgon

$+H %r 3+H)e25H %r $5H)e

ections up to ".5 !! thic>ness Pulsedarc)igh heat input less crac>ing in thic>ersections of "H i

Cupronic>elTitaniu!

Welding Processes - Word 6

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