Welding Processes Module

8/12/2019 Welding Processes Module

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SHIELDED METAL ARC WELDING

General Description

Shielded metal arc welding oins metals !" heating them with an

electric arc !etween a co#ered metal electrode and the wor$% Themelted electrode metal is trans&erred across the arc into the moltenpool o& !ase metals' !ecoming the deposited weld metal% A slag&ormed &rom the electrode coating and the !ase metal imp(rities&loats to the s(r&ace and co#ers the deposit' protecting it&rom)atmospheric contamination and controlling the cooling rate%Shielding o& the weld comes &rom the !rea$down' ordecomposition' o& the electrode co#ering% *(alit" welds re+(ire aprotecti#e shield &rom the harm&(l gases in the s(rro(nding air%,iller metal comes &rom the electrode core wire and co#ering'

which is made (p o& iron powder and allo"ing elements%

SMAW is sometimes re&erred to as -stic$- welding% It is the mostwidel" (sed o& all the processes we will tal$ a!o(t !eca(se o& thesimplicit" o& the e+(ipment' high strength and +(alit"' and the lowcost% It has ma.im(m &le.i!ilit" and welds most metals in a widerange o& thic$ness% Welding with this process can !e done almostan"where and (nder e.treme conditions% It can !e powered !"gasoline or diesel i& necessar"%SMAW is (sed e.tensi#el" in ind(strial &a!rications' str(ct(ral steel

erections /!(ildings' !ridges' etc%0 !o. cars' tr(c$s' dams' andother commercial weldments%

Sheilded Metal Arc Welding

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E+(ipment

The SMAW process is (s(all" man(all" operated' !(t it can !ea(tomated with' machine or gra#it" ) stic$ &eeder% The !asice+(ipment consists o& a power so(rce' ca!les' an electrode holder'a gro(nd clop' and the electrode% The power s(ppl" can (s either

alternating c(rrent' direct c(rrent electrode negati#e /straightpolarit"0' or direct c(rrent electrode positi#e /re#erse polarit"0'depending on the o! re+(irements% Direct c(rrent power s(ppliesare o&ten pre&erred !eca(se the" are more #ersatile%

Shielded Metal Arc Welding E+(ipment

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The electrode in the SMAW process ser#es se#eral important&(nctions% It esta!lishes the arc and s(pplies &iller metal &or the welddeposit% The co#ering on the electrode' $nown as the &l(.' also hasspecial characteristics that "o( sho(ld !e aware o&% Here is a list o&some o& these characteristics1

(1) It provides a gas that shields the arc from'atmospheric contamination.

(2) It provides scavengers, deoxidizers, and fluxingagents to clean the weld and prevent excessivegrain growth.

(3) It estalishes the electrical characteristics of theelectrode, meaning that the electrode coveringdetermines if ac or dc power supplies are to eused.

(!) It provides a lan"et of slag that protects the metal

while it is cooling, and controls the cooling rate.(#) It provides a wa$ to add allo$ing elements tostrengthen the weld.

Co#ered electrodes are classi&ied according to speci&icationsiss(ed !" the American Welding Societ"% Commercialspeci&ications &or co#ered electrodes can !e &o(nd in the AWS A2speci&ications series%

Welding Characteristics3Applications

It is important &or a welding inspector to remem!er thatthe SMAW process ma" ha#e man" #aria!les to consider% ,orinstance' it can !e (sed on a wide #ariet" o& ointcon&ig(rations )) nearl" e#er" standard oint &o(nd in thewelding ind(str" 4 and there is a wide #ariet" o& possi!le !asemetal and &iller metal com!inations% 5ccasionall"' se#eral t"peso& electrodes are (sed &or a partic(lar weld% 6o( as a weldinginspector' m(st !e $nowledgea!le a!o(t the speci&ication (sed

&or the o! so "o( will $now how these #aria!les a&&ect weld+(alit"%

The SMAW process has all)position capa!ilities% I t can !e (sed &orwelding most steels and some o& the non&erro(s metals' as well as&or s(r&acing% 7ase metal thic$ness weld capa!ilit" ranges &rom

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appro.imatel" 839: in% minimall" to o#er ; in%' depending on heat ordistortion control re+(irements and techni+(es% Heat inp(t controld(ring welding is a maor &actor with some materials' s(ch as+(ench)and)tempered steels' precipitation hardened and otherstainless steels' and low allo" steels containing mol"!den(m%Improper control o& heat inp(t d(ring welding' when re+(ired' caneasil" ca(se crac$ing or' as in the case o& corrosion resistantstainless steels' a loss o& !ase metal prime properties /e%g%'corrosion resistance0% Electrode melting rate is one o& the &actorsthat determines the process selection !" the welding engineer% Itres(lts in a meas(rement called -deposition rate'- which ranges&rom :)83: to 8: l!3hr &or SMAW' depending on electrode selection%

The SMAW process is limited somewhat !eca(se all welding

manip(lations are controlled !" the welder% There&ore' m(ch o& thes(ccess o& this process depends on the s$ill and techni+(e o& thewelder% There are &o(r !asic areas that the welder m(st ha#e s$illin controlling1

(1) %ength of the arc(2) &ngle of the electrode(3) peed of travel(!) &mperage setting

Remem!er that e#en tho(gh "o(' as a welding inspector' are notdirectl" in#ol#ed with the welding process' it is still important &or "o(to ta$e sa&et" preca(tions when watching the process% 6o( m(st(se &ilter lenses o& the proper densit" to watch the electric arc /see

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The shielded metal arc weldments can contain almost an"discontin(it"% 7elow is a)list o& some common discontin(ities thatma" !e &o(nd))when this process is (sed and an e.planation o&how each is ca(sed or how each can !e a#oided ma" !e &o(nd atthe end o& this mod(le lesson%

(1) orosit$(2) luster porosit$(3) iping porosit$(!) Inclusions(#) Incomplete fusion(*) Inade+uate oint penetration(-) ndercut, underfill, overlap(/) %amellar tearing(0) rac"ing

Shielded Metal Arc Welding

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S?7MERGED ARC WELDING

General Description

S(!merged arc welding' or SAW' oins metals !" heating themWith an electric arc' or ar$s !etween a !are metal electrode

/or electrodes0 and the !ase metal% The arc is s(!merged in andshielded !" a !lan$et o& gran(lar' &(si!le material on the wor$$now as &l(.% In the SAW process' "o( cannot see the arc !etweenthe electrode and the wor$ piece' as it is hidden% The electrodeis not in contact with the wor$ piece% The melted electrode metalis trans&erred across the arc into the molten pool o& !asemetals and &l(.' !ecoming the deposited weld metal% The &l(.that melts close to the arc intermi.es with the molten metal''p(ri&"ing and &orti&"ing the metal )) m(ch the same as whathappens in the SMAW process% A slag is &ormed here &rom the &l(.and imp(rities that also &loats on the s(r&ace and co#ers thedeposit' protecting it%

S(!merged Arc Welding E+(ipment

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An ad#antage o& s(!merged arc welding is its deep penetration%Also' the high deposition rates o& SAW red(ce the total heat inp(tinto a oint% Welds that wo(ld re+(ire m(ltiple passes !" shieldedmetal arc welding can !e deposited in one pass !" s(!merged arcwelding% The welder or welding operator does not ha#e to wear ahelmet with this process' !(t' since he cannot see thro(gh the &l(.'he ma" ha#e a di&&ic(lt time directing the arc i& it gets o&& trac$%7eca(se the arc is hidden &rom #iew and re+(ires a trac$ings"stem' SAW has limited &le.i!ilit"% 7(t this is o&&set !" se#eralmaor ad#antages' s(ch as1

(1) igh weld metal +ualit$ and strength(2) xtremel$ high deposition rate and travel speed(3) o arc flash 44 minimizing protection re+uirements(!) %ittle, if an$, smo"e(#) asil$ automated, reducing manipulative s"ill needs.

SAW also welds a wide range o& thic$nesses' and most o& thesteels' &errite and a(stenitic' are welded !" SMAW%

A maor (se o& the SAW process is in the &a!rication o& hea#" steelplate weldments' e%g%' press(re #essels and tan$s' large diameterpipes' maintenance and repair' and' in ship!(ilding' s(!assem!l"&a!rication%

S(!merged Arc Welding

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The power s(ppl" &or SAW ma" !e one o& the &ollowing1

(1) & variale voltage dc generator or rectifier(2) & constant voltage dc generator or rectifier(3 )&n ac transformer

These power so(rces sho(ld pro#ide the high c(rrents at the d(t"c"cle re+(ired% Most welding is done in a range &rom @ to 82amperes%

The SAW process can !e (sed with ac or dc power so(rces' andeach o& these has certain ad#antages4 there&ore' it is important toselect the one proper &or the o!% Welding with dc power pro#ides!etter control o& the weld !ead shape' depth o& penetration' andwelding speed% Direct c(rrent re#erse polarit" /DCRB0 pro#ides

!etter arc sta!ilit"' and the res(lt is !etter control o& the !eadshape% Highest deposition rates are gained with dc straight polarit"/DCSB0' !(t penetration is low%

Alternating c(rrent prod(ces penetration somewhere !etweenDCRB and DCSB' and has the ad#antage o& lessening arc !low%S(!merged arc electrodes are s(pplied as !are solid wires or ascomposite electrodes' (s(all" #er" similar in chemistr" to that o& the!ase metals /Re&% AWS A2%80% SAW &l(.es also change thechemical composition o& the weld metal and in&l(ence its

mechanical properties% ,l(. characteristics are similar to those o&the &l(. in SMAW% The di&&erent t"pes o& &l(.es are listed as &ollows1

(1) 5used(2) 6onded(3) &gglomerated(!) 7echanicall$ mixed

The composition o& the weld is altered !" s(ch &actors as themelted !ase metal chemical reactions !etween the elements in the

electrode and &l(.' and allo"s added thro(gh the &l(.% ?se o& a&l(. and &iller wire gi#es the process great &le.i!ilit" inreaching the desired weld properties%

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As a welding inspector' it will !e "o(r responsi!ilit" to #eri&" that theproper wire and &l(. com!ination is (sed to o!tain the speci&iedweld composition%


S(!merged arc welding can !e (sed &or man" ind(strialapplications% That incl(des ship!(ilding' railroad car &a!rication' orsteel str(ct(ral mem!ers% The process can !e (sed to weld thin aswell as thic$ sections /838< in% to o#er ; in%0% SAW is (sed mostl"on car!on and low allo" steels and stainless steel% It is not s(ita!le&or all metals and allo"s% D(e to the nat(re o& the SAW process'most o& the welding is done in the &lat position%

Welds made with the SAW process (s(all" ha#e good d(ctilit"'high impact strength' (ni&orm densit"' low nitrogen content' andhigh corrosion resistance% Broperties that are at least e+(al tothose o& the !ase metal are o!tained%

It is partic(larl" important with s(!merged arc welding that all wor$s(r&aces !e clean% The aligning o& the machine tra#el with the ointis also critical% The welding operator m(st ha#e good training incontrolling all the #aria!les that are possi!le with this process%

S(!merged arc welding is (sed &or ma$ing groo#e' &illet' and pl(gwelds' and &or s(r&acing% When open)root !(tt oints are welded' a!ac$ing is (sed to s(pport the molten metal%

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SAW is (sed &or oining man" &erro(s and non&erro(s metals andallo"s% It is also (sed to appl" cladding to !ase metals to pro#idethe desired properties o& the s(r&ace materials% The general classeso& !ase metals that are welded are as &ollows1

(1) aron steels up to 8.20 percent caron(2) eat4treated caron steels (normalized or

+uenched and4tempered)(3) %ow allo$ steels, +uenched4and4tempered up to

188,888 psi $ield strength(!) hromium4mol$denum steels (192 percent to 0

percent chromium, and 192 percent to 1 percentmol$denum)

(#) &ustenite chromium4nic"el stainless steels(*) ic"el and nic"el allo$s (solid solution t$pes)

Electrode deposition rates &or SAW range &rom minim(m o& 83:l!3hr /semia(tomatic0 to a ma.im(m o& appro.imatel" 8 l!3hr(sing tandem arcs /a(tomated0% Remem!er that sa&et" preca(tionsare still important with the SAW process e#en tho(gh the arccannot !e seen% Goggles m(st !e worn in the shop' !(t it is notnecessar" to (se &ilter plates%

Welds ma" e.hi!it all o& the common discontin(ities% A list o&SAW)related discontin(ities &ollows1

(1) rac"s (crater, root, throat, toe)(2) Incomplete fusion(3) Incomplete penetration(!) %amellar tearing(#) %aps(*) orosit$(-) lag inclusions(/) ndercut

GAS METAL ARC WELDING

Gas metal arc welding' or GMAW' (ses the heat o& an electric arc!etween a contin(o(sl")&ed' !are &iller metal electrode and the

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!ase metal% The heat melts the electrode end and the !ase metals(r&ace to &orm the deposited weld metal% Shielding o& the arc andthe molten weld pool comes entirel" &rom an e.ternall" s(ppliedgas' which ma" !e inert' acti#e' ora mi.t(re% This process is sometimes re&erred to as -MIG- welding%Slag &ormed d(ring SAW and SMAW processes does not &orm withGMAW' as there is no &l(. (sed% Howe#er' a glass)li$e &ilm o& silica&orms &rom high silicon electrodes' which m(st !e treated li$e slag%

Gas Metal Arc Welding

GMAW is +(ite a #ersatile process% Maor ad#antages incl(de1higher deposition rates than SMAW4 red(ction o& smo$e and&(mes4 high #ersatilit"4 !road application a!ilit"4 welds a wide rangeo& thic$ness and metals% It can !e !" a semia(tomatic' machine' or

a(tomatic method% In the semia(tomatic method' the electrode is&ed a(tomaticall" thro(gh a hand)held g(n% The welder controls theinclination and distance o& the g(n &rom the wor$ as well as thetra#el speed and manip(lation o& the arc% In the machine method' awelding operator monitors a mechani>ed tra#el operation &ornecessar" ad(stments% In the a(tomatic method' the entireoperation is &(ll" machine controlled% GMAW can also !e (sed &ors(r&acing applications%

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E+(ipment

Gas metal arc welding e+(ipment consists o& a welding g(n' apower s(ppl"' a shielding gas s(ppl"' and a wire)dri#e s"stem%

Gas Metal Arc Welding E+(ipment

The g(n contains a contact t(!e to transmit the welding c(rrent tothe electrode and a gas no>>le to direct the shielding gas% The wire&eed is made (p o& small motors and dri#e wheels% The shieldinggas &low is reg(lated !" &low meters and press(re)red(cingreg(lators% This pro#ides a constant deli#er" o& gas to the g(nno>>le%

Most GMAW applications re+(ire direct c(rrent re#erse polarit"/DCRB0 power% DCRB has a more sta!le arc' smooth trans&er' lowspatter' and good !ead characteristics% Direct c(rrent straightpolarit" /DCSB0 is not o&ten (sed' and alternating c(rrent is notconsidered at all &or this process%

Welding G(n ControlThere are &o(r wa"s /-modes-0 in which the gas metal arc weldingprocess can deposit weld metal in a oint4

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The &irst is !" glo!(lar trans&er' which occ(rs with low c(rrent inrelation to the si>e o& the electrode% Metal trans&ers &rom electrodeto wor$ piece as glo!(les' each larger in diameter than theelectrode% The glo!(les trans&er to the pool witho(t m(ch directionand spatter is +(ite e#ident%

Spra" trans&er occ(rs with a high c(rrent% The molten &iller metaltrans&ers across the arc as &ine droplets%

Short circ(iting trans&er ma" re+(ire a special power so(rce% As theelectrode melts' each drop shorts o(t the arc d(ring its trans&er tothe pool% This is a relati#el" cold process that can !e (sed in allpositions to p(t weld metal where#er the welder wants it% /Low heatinp(t restricts the thic$ness o& the weld &or good penetration%0

B(lsed arc welding holds a low c(rrent arc as the !ac$gro(ndcondition and inects high c(rrent p(lses (pon that c(rrent% Trans&ero& &iller metal is !" spra" o& droplets d(ring each p(lse% This lowersthe a#erage heat inp(t &rom the c(rrent' ma$ing #ertical weldingpossi!le !" (sing larger electrode wires%

Most spra" t"pe GMAW is done in the &lat position% B(lsed andshort circ(iting GMAW are s(ita!le &or all)position welding% Whenwelding is done in the o#erhead position' small diameter electrodesare (sed with the short)circ(iting method% Spra" trans&er ma" !e

(sed with p(lsed direct c(rrent%

The primar" p(rpose o& the shielding gas in GMAW is to protect themolten weld &rom atmospheric contamination% The gas shield alsoa&&ects the t"pe o& trans&er' depth o& penetration' and !ead shape%

Argon and heli(m are shielding gases (sed &or welding mostnon&erro(s metals% Car!on dio.ide is widel" (sed &or the welding o&mild steels% When selecting a shielding gas' the most important&actor to $eep in mind is the -hea#ier- a gas' the more e&&ecti#e it isas an arc shield%

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The electrodes &or gas metal arc welding are similar or identical incomposition to those o& other !are &iller metal welding processes%

As a r(le' the composition o& the electrode and the !ase metal areas ali$e as possi!le% (:ef. &; .0, .18, and .1/.)


The gas metal arc welding process prod(ces high +(alit" weldswith proper welding proced(res% 7eca(se there is no &l(. co#eringon the electrode' the possi!ilit" o& slag incl(sions is lessened%H"drogen in the weld is practicall" eliminated%

GMAW is an all)position welding process' depending (pon theelectrode and gas/es0 (sed% It can !e (sed to weld most metals'incl(ding s(r&acing and metal !(ild(p% Metal thic$ness weldcapa!ilit" ranges generall" &rom :@ gage /%:2 in%0 with short

circ(iting trans&er' and (pward% Metal deposition rate rangesnormall" &rom 93@ l!3hr to 9 l!3hr' depending (pon selection o&electrode' arc mode o& trans&er' and gas (sed%

Gas metal arc weldments ma" contain an" o& the &ollowing commondiscontin(ities1

(1) rac"s (crater, root, toe, underead)(2) Incomplete fusion

(3) Incomplete penetration(!) %amellar tearing(#) %aps(*) orosit$(-) inclusions (silica)(/) ndercut(0) nderfill

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,L? C5RED ARC WELDING

,l(. cored arc welding /,CAW0 oins metals (tili>ing the heat o& anelectric arc !etween a contin(o(sl" &ed' internall" &l(.ed &iller metalelectrode and the !ase metal% Heating and melting are similar togas metal arc welding' !(t the di&&erence lies in how the weld is

shielded% Shielding comes &rom a &l(. contained with the electrode%,l(. cored electrodes are sel&)shielding or s(pplementall" shielded!" an e.ternall" s(pplied gas% S(pplemental shielding ma" !eo!tained &rom an e.ternal gas s(pplied to control depth o&penetration as well as to help protect the molten pool% Ingredientswithin the electrode pro#ide deo.idi>ers' ioni>ers' p(ri&"ing agents'and allo"ing elements% This res(lts in a thin slag co#ering to protectthe solidi&"ing weld metal' m(ch as what happens in the SMAWprocess%

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,le.i!ilit" o& the ,CAW process is e.tremel" good% Some o& itsman" maor ad#antages incl(de high +(alit" weld metal at relati#el"high tra#el speeds and lower cost than with the SMAW process%,CAW is more &orgi#ing than GMAW and is' o& co(rse' more&le.i!le than the SAW process% This process also prod(cese.cellent weld !eads in (ni&ormit"' smoothness' and generalappearance% It is adapta!le to a wide #ariet" o& car!on' allo"'stainless' and other steels and is an all)position weld process%

E+(ipment

,CAW is (s(all" applied as semia(tomatic welding% The g(n ishand)held and directed !" the welder% The process is also (sed inmachine welding where the operator monitors the operationcontin(o(sl" d(ring mechani>ed tra#el%The !asic e+(ipment &or sel&)shielded and gas shielded ,CAW issimilar% The onl" di&&erence !etween the two is a s"stem to pro#idethe e.ternal gas &or shielding% The recommended power so(rce isthe direct c(rrent' constant #oltage t"pe which is similar to those &orGMAW% The power s(ppl" sho(ld !e capa!le o& operating at thehighest c(rrent needed &or the speci&ic application% Mostsemia(tomatic set(ps operate at less than


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Most steels that are welded !" the SMAW' GMAW' or SAWprocesses can !e welded with ,CAW% ,CAW is #ersatile !eca(seso man" ingredients can !e incl(ded in the core o& the ro(ndelectrodes% Broper selection o& the ingredients in the core can ha#ea n(m!er o& important res(lts1

Brod(ce welding characteristics ranging &rom highdeposition rates in the &lat position to proper &(sion and!ead shape in the o#erhead posit

Brod(ce electrodes &or #ario(s gas shielding mi.t(res and

&or sel&)shielding%

Far" the allo" content o& the weld metal &rom mild steel &or

certain electrodes to high allo" stainless steel &or others%

The primar" &(nctions o& the &l(.)cored ingredients are to1

Bro#ide the mechanical' metall(rgical' and corrosionresistant properties o& the weld metal !" ad(sting thechemical composition%

Bromote weld metal so(ndness !" shielding the molten

metal &rom o."gen and nitrogen in the air%

Sca#enge imp(rities &rom the' molten metal !" (se o&

&l(.ing reactions%

Brod(ce slag co#er to protect the solidi&"ing weld metal

&rom the air' and to control the shape and appearance o&the !ead in the di&&erent welding positions &or which theelectrode is s(ited%

Sta!ili>e the arc !" pro#iding a smooth electrical path to

red(ce spatter and aid the deposition o& (ni&orml" smooth'properl" si>ed !eads%

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Car!on dio.ide :0 is the most widel" (sed shielding gas &or the&l(. cored arc welding process% It is ine.pensi#e and prod(ces deepweld penetration% It gi#es a glo!(lar t"pe o& metal trans&er%Gas mi.t(res are also (sed with ,CAW% The higher the percentageo& inert gas in mi.t(res with car!on dio.ide or o."gen' the higherwill !e the trans&er e&&iciencies o& the deo.idi>ers contained in thecore% Strong deo.idi>ers m(st !e present in the electrode toprod(ce a so(nd weld% Argon is capa!le o& protecting the moltenweld pool at all welding temperat(res%


,CAW can (se the same oint designs (sed !" SMAW andGMAW' with slight #ariations in &it)(p details /i& an"0' depending onthe t"pe and si>e o& electrode (sed%

Also' electrode t"pe' si>e' shielding method /sel& shielding alone' or

with e.ternal shielding o& C: or other gases0 determines thewelding positions' penetration depth' and metal thic$ness rangesthat can !e welded% Bosition o& welding depends mostl" onelectrode si>e%

Low)to)medi(m car!on steels' low allo" high)strength and+(ench)and)tempered steels' some stainless steels' and cast ironare among the most common metals welded !" ,CAW' incl(dings(r&acing and metal !(ild(p%

Generall"' metal thic$ness capa!ilit" !" the ,CAW process ranges&rom 838< in% (pward% This process does prod(ce more smo$e thanSr@AW' re+(iring proper attention !" the welder to techni+(es thatred(ce the smo$e +(antit" reaching the !reathing >one% E.ternalshielding with C: prod(ces the greater penetration and thegreater +(antit" o& smo$e%

An important element o& the ,CAW process' o&ten ignored oro#erloo$ed' is the need &or control o& -electrical stic$o(t- )the length

o& electrode wire !etween the point o& electrical contact in the g(n'or welding head' and the arc% Electrical stic$o(t pro#ides resistanceheating o& the electrode which preheats the electrode wire' ma$ingit operate more e&&icientl"%

7esides a&&ecting heat inp(t' changes in c(rrent res(lt in the&ollowing1

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Increasing the c(rrent increases the electrode

deposition rate%

Decreasing the c(rrent increases penetration%

E.cessi#e c(rrent prod(ces con#e. weld !eads with

poor appearance%

Ins(&&icient c(rrent prod(ces large droplets' trans&er'

and e.cessi#e spatter% Ins(&&icient c(rrent can res(lt in pic$(p o& large

amo(nts o& nitrogen and also porosit" in the weld metalwhen welding with sel&)shielded &l(. cored electrodes%

The t"pe o& c(rrent (sed with ,CAW is (s(all" direct c(rrentre#erse polarit" /DCRB0%

Metal deposition rate ranges normall" &rom : l!3hr to @ l!3hr'depending (pon selection o& electrode% /Remem!er' normal sa&et"preca(tions m(st !e &ollowed when welding or o!ser#ing the,CAW process%0 Discontin(ities that ma" !e &o(nd in weldmentswhen (sing the ,CAW process incl(de the &ollowing ones alsoassociated with GMAW' e.cept that incl(sions wo(ld !e slag ratherthan silica1

(1) rac"s (crater, root, toe, (underead)(2) Incomplete fusion(3) Incomplete penetration

(!) %amellar tearing(#) %aps(*) orosit$(-) lag inclusions(/) ndercut(0) nderfill

It sho(ld !e noted here that some discontin(ities &o(nd in ,CAWma" ha#e !een ca(sed !" a #oid in the &l(.ed core' or a missing'ins(&&icient ingredient in the &l(.% A good reason to periodicall"

chec$ new &iller metals when recei#ed%

Gas T(ngsten Arc Welding

Gas t(ngsten arc welding /GTAW0' sometimes re&erred to as -TIG-welding' oins metals !" heating them will) an arc !etween a nocons(ma!le t(ngsten electrode and an' wor$% Shielding is o!tained

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&rom an inert gas or inert gas mi.t(re% ,iller metal ma" !e added4 itis not trans&erred across the arc' !(t is melted !" the arc% Theelectrode that carries the c(rrent is a p(ts t(ngsten or t(ngstenallo" rod%


The arc area is protected &rom atmospheric contamination !" the

gas shielding' which &lows &rom the no>>le o& the torch% The gasdisplaces the air' eliminating nitrogen' o."gen' and h"drogen &romcontact with either the rotten metal or the not t(ngsten electrode%There is little or no spatter and little or no smo$e% The as welded!ead is smooth and (ni&orm' re+(iring little &inishing' i& an"The GTAW process can on (sed to ma$e high +(alit" welds inmost all metals and allo"s (sed in all positions% It also is theslowest o& the non)mechani>ed processes%

E+(ipment

Welding with a Non cons(ma!le t(ngsten electrode shielded withan inert gas to generall" a man(al process' !(t ma" !emechani>ed or e#en a(tomated% The e+(ipment needed incl(des1

(1) &n electrode older with gas passages and anozzle to direct too shielding gas around the

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arc and a gripping mechanism to energize andhold a tungsten electrode.

(2) & suppl$ of shielding gas.(3) & flowmeter and gas pressure4reducing regulator(!) & power ource.(#) & suppl$ of cooling water if the electrode holder

is water4cooled.

The power so(rce &or AC and DC gas t(ngsten arc welding is(s(all" a drooping #oltage t"pe% In this t"pe o& power s(ppl"' theshape o& the #olt)ampere c(r#e is steep so that a change in arcLength will not create a maor change in c(rrent%

E+(ipment &or GTAW

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!ead tends to !e red(ced and penetration increases% Howe#er' i& it!ecomes too pointed and remains that wa" too long' c(rrentdensit" !(ilds (p at the tip' e#ent(all" raising the temperat(rea!o#e the electrode melting point' and the tip drops o&& into theweld pool /t(ngsten incl(sion0% Then the tip re&orms into anhemispherical shape%

Non)Cons(ma!le Electrode

The GTAW process can weld almost all metals or allo"s that can!e &(sed !" an electric arc' that do not #apori>e rapidl" &rom thehigh heat o& an arc' and that can !e welded witho(t crac$ing%

,iller metals &or oining a wide #ariet" o& metals and allo"s area#aila!le &or (se with GTAW% I& &iller metals are (sed' the" are(s(all" similar to the metal !eing oined%

The shielding gases commonl" (sed &or GTAW are argon' heli(m'and mi.t(res o& the two% Argon is sometimes pre&erred o#er heli(m!eca(se it has se#eral ad#antages1

(1) moother, +uieter arc action

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This process is also (sed to weld &erro(s and non&erro(s pipe% Theroot passes o& s(!stantial +(antities o& car!on and stainless steelpiping' especiall" &or critical applications' =are o&ten welded with theGTAW process%

Altho(gh GTAW has a high initial cost and low prod(cti#it"/prod(ction speed0' these are o&ten o&&set !" the a!ilit" to weld soman" t"pes o& metals in thic$ness and positions not possi!le !"other processes' as well as the high +(alit" and high strengtho!tained with GTAW%

GTAW readil" lends itsel& to oining al(min(m' magnesi(m'titani(m' copper' and stainless steels' as well as the &erro(s car!onsteels and man" di&&ic(lt)to)weld metals%Some metals can !e welded in all positions' depending on the

welding c(rrent and welder s$ill%

C(rrent (sed with GTAW ma" !e ac or dc% Either straight orre#erse polarit" ma" !e (sed with dc c(rrent% Howe#er' sinceDCSB /straight polarit"0 prod(ces minim(m heat in the electrodeand ma.im(m heat in the !ase metal' smaller electrodes ma" !e(sed while o!taining deeper penetration in the !ase metal than withDCRB /re#erse polarit"0 or ac c(rrent% The opposite is tr(e whenshallow penetration is desired' s(ch as in thin)gage al(min(m%

Non&erro(s metals re+(ire e.tremel" clean s(r&aces to weld' andthe DCRB and ac c(rrents pro#ide an additional cleaning actionespeciall" necessar" &or al(min(m' magnesi(m' and copper%The thic$ness range &or which GTAW is Applica!le /depending onc(rrent t"pe' electrode si>e' &iller diameter' !ase metal' and gasselection0 r(ns &rom a!o(t %2 in% minim(m to a!o(t : in% Whenthic$ness e.ceeds 83@ in%' preca(tions m(st !e ta$en to controlheat !(ild(p in m(ltipass welds%Depending on the same &actors as listed &or thic$ness' thedeposition rate o& GTAW process ranges &rom 83: l!3hr to almost

9l!3hr%

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Weldments !" the GTAW process ma" contain one discontin(it"that is pec(liar to the GTAW process )) t(ngsten incl(sions' ares(lt o& pieces o& t(ngsten electrode dropping into the weld pool%5ther than slag incl(sion' most o& the discontin(ities listed &or theother arc welding process ma" !e &o(nd in GTAW weldments%


Welding Processes Module

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