welding final gate

8/9/2019 Welding Final Gate

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WELDING

2.1 INTRODUCTION

Welding is a process for joining two similar or dissimilar metals by fusion. It joins dierent metals/alloys wit! or wit!out t!e application of pressure andwit! or wit!out t!e use of "ller metal. #!e fusion of metal ta$es place bymeans of !eat. #!e !eat may be generated eit!er from combustion of gases electric arc electric resistance or by c!emical reaction. During sometype of welding processes pressure may also be employed but t!is is notan essential re%uirement for all welding processes. Welding pro&ides apermanent joint but it normally aects t!e metallurgy of t!e components.2.2 BASIC CONCEPT OF WELDINGWelding is a process of joining two similar or dissimilar metals wit! t!e !elpof !eat or pressure or by some ot!er means. #!e cost of welding is &eryless as compared to ot!er processes and forms a strong joint.'or t!is reason it is largely used in t!e following "elds of engineering(). *anufacturing of mac!ine tools auto parts cycle parts etc.+. 'abrication of farm mac!inery , e%uipment.-. 'abrication of buildings bridges , s!ips.

. onstruction of boilers furnaces railways cars aeroplanes roc$ets andmissiles.0. *anufacturing of tele&ision sets refrigerators $itc!en cabinets etc.1 weld will in!erit t!e common drawbac$ of brittleness w!ic! is adisad&antage. #!eemp!asis in welding tec!ni%ue s!ould t!erefore be pre&enting t!isbrittleness to t!e ma2imum possible e2tent.2.3 CLASSIFICATION OF WELDING PROCESSES*any types of welding processes !a&e been de&eloped depending upon t!e"eld of t!eir applications3#able 4.)5. 6ut t!e welding is broadly di&ided into following two groups.

1. Forge or Pressure Weldig !7nder pressure wit!out additional "llermetal53a5 'riction welding3b5 Electric resistance welding3c5 6lac$smit!s forge welding3d5 old pressure welding2. Fusio or o"#ressure $eldig 3Wit! additional "ller material53a5 Gas welding 38eat created by Gas53b5 Electric arc welding 38eat created by electrically53c5 #!ermite welding 38eat created by c!emical 9eaction5

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WELDING

2.% GAS WELDING PROCESSES

1 fusion welding process w!ic! joins metals using t!e !eat of combustionof an o2ygen /air and fuel gas 3i.e. acetylene !ydrogen propane or butane5mi2ture is usually referred as Cgas welding. #!e intense !eat 3ame5 t!usproduced melts and fuses toget!er t!e edges of t!e parts to be weldedgenerally wit! t!e addition of a "ller metal. :peration of gas welding iss!own in 'ig. . #!e fuel gas generally employed is acetyleneF !owe&er

gases ot!er t!an acetylene can also be used t!oug! wit! lower ametemperature. :2y


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WELDING

about 04??


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'. elding torch or blow pipe with a set o no**les and gas lighter

+. Trolleys or the transportation o oxygen and acetylene cylinders

,. -et o eys and spanners

1/. 0iller rods and luxes

11. rotectie clothing or the welder (e.g.3 asbestos apron3 gloes3 goggles3 etc.

1cetylene and o2ygen gas is stored in compressed gas cylinders.O'(ge )(lider

• 9ig!t !and side t!read

• 6lac$ colour

• )+? Hg/cm+ pressure

• ? litres capacityA)e*(lee!C22/ )(lider

• Left !and side t!read

• *aroon colour

• )0 Hg/cm+ pressure

• ? litres capacity

:2ygen gas cylinders are usually e%uipped wit! about ? litres of o2ygen ata pressure of about )0 Hgf/cm+ at +). #o pro&ide against dangerouslye2cessi&e pressure suc! as could occur if t!e cylinders were e2posed to"re e&ery &al&e !as a safety de&ice to release t!e o2ygen before t!ere isany danger of rupturing t!e cylinders. 'ragile discs and fusible plugs areusually pro&ided in t!e cylinders &al&es in case it is subjected to danger.

Chemistry of Oxy Acetylene Process

The most common uel used in welding is acetylene. 4t has a two stage reaction5 the irst stageprimary reaction inoles the acetylene disassociating in the presence o oxygen to produceheat3 carbon monoxide3 and hydrogen gas.

26272 8 2O2 9 "6O 8 272 8 7eat (1)

A secondary reaction ollows where the carbon monoxide and hydrogen combine with moreoxygen to produce carbon dioxide and water apor.

"6O 8 272 8 !O2 9 "6O2 8 272O 8 7eat (2)

hen you combine equations (1) and (2) you will notice that about # parts o oxygen isnecessary to consume 2 parts o acetylene

26272 8 #O2 9 "6O2 8 272O 8 7eat (!)

• 'rom abo&e for complete combustion of ) unit &olume of C22re%uires +.0 unit of o2

• :ut of t!is I unit &olume of o2 is obtained from o2 cylender andremaining from atmosp!ere.

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WELDING

Types of flames

Based on amount of o2 flame produced can be divided into 3 types

1. Natural flame

I. O2 : C2H2 = 1:1

II. Lent! of inner cone "#$ 1% to 1& cm

III. Inner cone 'ill be red or yello'

I(. )a*imum temperature 32+%%C

The neutral flame is commonly used for the welding of

Both ferrous and non ferrous metals

"i$ )ild ,teel

"ii$ ,tainless steel

"iii$ Cast iron

"iv$ Copper

"v$ -luminum

• ;uring


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WELDING

• WELDI! TO"C# $ %LOW PIPE

A welding torch mixes oxygen and acetylene in the desired proportions3 burns the mixture atthe end o the tip3 and proides a means or moing and directing the lame.

There are two types o welding torches3 namely:

a) 7igh pressure (or equal pressure) type

b) >ow pressure (or in


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WELDING

welding dierent materials) and si*es. These consumable iller rods may be bare3 or they maybe coated with lux. The purpose o the lux

a. 4s to retard oxidation o the suraces o the parts being welded3 by generating gaseousshield around the weld *one.

b. The lux also helps to dissole and remoe oxides and other substances from t!ewor$ piece and so contributes to t!e formation of a stronger joint.

Characteristics of good fl)x

a. #!e melting point of a u2 must be lower t!an t!at of eit!er t!e metal ort!e o2ides formed so t!at it will be li%uid.

b. #!e ideal u2 !as e2actly t!e rig!t uidity w!en t!e weldingtemperature !as been reac!ed.

c. #!e u2 will protect t!e molten metal from atmosp!eric o2idation.d. 'u2 will remain close to t!e weld area instead of owing all o&er t!e

base metal for some distance from t!e weld.

Composition of &l)xes

'lu2es dier in t!eir composition according to t!e metals wit! w!ic! t!ey are to beused. In cast iron welding a slag forms on t!e surface of t!e puddle. #!e u2

ser&es to brea$ t!is up. E%ual parts of a carbonate of soda and bicarbonate of soda ma$e a good compound for t!is purpose. Nonferrous metals usually re%uire au2. opper also re%uires a "ller rod containing enoug! p!osp!orous to produce ametal free from o2ides. 6ora2 w!ic! !as been melted and powdered is often usedas a u2 wit! copper alloys. 1 good u2 is re%uired wit! aluminum because t!ereis a tendency for t!e !ea&y slag formed to mi2 wit! t!e melted aluminum andwea$en t!e weld. 'or s!eet aluminum welding it is customary to dissol&e t!e u2in water and apply it to t!e rod. 1fter welding aluminum all traces of t!e u2s!ould be remo&ed.

Characteristics of the oxy*acetylene +elding process incl)de(

o The use dual oxygen and acetylene gases stored under pressure in steel cylinders5

o 4ts ability to switch quicly to a cutting process3 by changing the welding tip to a cutting

tip5

o The high temperature the gas mixture attains (?#+//@0)5

o The use o regulators to control gas low and reduce pressure on both the oxygen and

acetylene tans5

o The use o double line rubber hoses to conduct the gas rom the tans to the torch5

o elting the materials to be welded together5

o The ability to regulate temperature by ad


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WELDING

Neutral Flame < 1 neutral ame is produced w!en appro2imately e%ual&olumes of o2ygen and acetylene are mi2ed in t!e welding torc! and burntat t!e torc! tip. 3*ore accurately t!e o2ygen


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WELDING

Oxidising Flame < If after t!e neutral ame !as been establis!ed t!esupply of o2ygen is furt!er increased t!e result will be an o2idising ame.1n o2idising ame can be recognied by t!e small w!ite cone w!ic! iss!orter muc! bluer in colour and more pointed t!an t!at of t!e neutralame. #!e outerame en&elope is muc! s!orter and tends to fan out at t!e end on t!e

ot!er !and t!e neutral and carburiing en&elopes tend to come to a s!arppoint. 1n o2idising ame burns wit! a decided loud roar. 1n o2idising ametends to be !otter t!an t!e neutral ame. #!is is because of e2cess o2ygenand w!ic! causes t!e temperature to rise as !ig! as -0??. #!e !ig!temperature of an o2idiingame 3:+( +8+ ).0( )5 would be an ad&antage if it were not for t!e factt!at t!e e2cess o2ygen especially at !ig! temperatures tends to combinewit! many metals to form !ard brittle low strengt!o2ides. *oreo&er an e2cess of o2ygen causes t!e weld bead and t!esurrounding area to !a&e a scummy or dirty appearance. 'or t!ese reasonsan o2idising ame is of limited use in welding. It is not used in t!e welding

of steel. 1 slig!tly o2idising ame is !elpful w!en welding most 3i5 opperbase metals 3ii5 Minc base metals and 3iii5 1 few types of ferrous metalssuc! as manganese steel and cast iron #!e o2idiing atmosp!ere in t!esecases creates a base metal o2ide t!at protects t!e basemetal. 'or e2ample in welding brass t!e inc !as a tendency to separateand fume away.

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WELDING

Use of flux:/lu* is employed in t!e 'eldin of suc! metal as cast iron. ,ome alloy steel and non0ferrous metals

to dissolve suc! as:

1. emove impurities.

2. Control surface tension.

3. ive protection from atmosp!ere.

It is usually in t!e format paste in '!ic! t!e rod is dipped.

*et!od of welding using o2y


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WELDING

Forehand welding

-. Fillet welding: 4!e fillet 'eld is t!e most popular of all types of 'elds because t!ere isnormally no preparation re6uired.

. Horiontal !osition welding: In !ori7ontal 'eldin t!e 'eld a*is is appro*imately !ori7ontal but t!e 'eld type dictates t!e complete definition. /or a fillet 'eld 'eldin is performed on t!e

upper side of an appro*imately !ori7ontal surface and aainst an appro*imately vertical surface.

/or a roove 'eld t!e face of t!e 'eld lies in an appro*imately vertical plane

0. Flat !osition welding: 4!is type of 'eldin is performed from t!e upper side of t!e 8oint. 4!eface of t!e 'eld is appro*imately !ori7ontal.

@. er*i)+l #osi*io $eldig4 In &ertical position t!e plane of t!ewor$piece is &ertical and t!e weld is deposited upon a &ertical surface. It isdiJcult to produce satisfactory welds in t!is position due to t!e eect of t!e force of gra&ity on t!e molten metal. #!e welder must constantlycontrol t!e metal so t!at it does not run or drop from t!e weld. erticalwelding may be of two types &i. &ertical


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WELDING

4. O5er 0e+d #osi*io $eldig4 #!e o&er!ead position is probably e&enmore diJcult to weld t!an t!e &ertical position. 8ere t!e pull of gra&ityagainst t!e molten metal is muc! greater. #!e force of t!e ame againstt!e weld ser&es to counteract t!e pull of gra&ity. In o&er!ead position t!eplane of t!e wor$piece is !oriontal. 6ut t!e welding is carried out from t!eunderside. #!e electrode is !eld wit! its welding end upward. It is a goodpractice to use &ery s!ort arc and basic coated electrodes for o&er!eadwelding.

Ad,antages of Oxyacetylene Process

1) ;oes not require electricity5

2) The equipment is portable3 easy to transport5

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WELDING

!) elder has considerable control oer the rate o heat input3 the temperature o the weld*one3 and the oxidi*ing or reducing potential o the welding atmosphere5

") Oxyacetylene process is ideally suited to the welding o thin sheet3 tubes3 and smalldiameter pipe. 4t is also used or repair wor3 maintenance and in body shops5

#) ;issimilar metals can easily be


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WELDING

gi&ing iron o2ide wit! t!e following reactions(- 'e O + :+


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WELDING

#!oug! t!e gas cutting is more useful wit! t!ic$ plates t!in s!eets 3lesst!an - mm5 can also be cut by t!is process ta$ing special precautions. #ipsie c!osen s!ould be as small as possible. If small tips are not a&ailablet!en t!e tip is inclined at an angle of )0 to +? degrees. Gas cutting can bedone manually or by a mac!ine. #!e manual cutting is used for general

purpose wor$ and for straig!t line cutting. In mac!ine cutting t!e torc! ismounted on a rail and bot! rail and t!e torc! can mo&e simultaneouslyalong t!e two mutually perpendicular a2es in t!e !oriontal plane wit! t!e!elp of ser&o motors. #!ereis pro&ision in t!e mac!ine to !old more t!an one torc! so t!at largenumber of identical pieces can be cut at t!e same time.Ar)"Weldig

I*rodu)*io1rc welding is t!e fusion of two pieces of metal by an electric arc betweent!e pieces being joined R t!e wor$ pieces R and an electrode t!at is guidedalong t!e joint between t!e pieces. #!e electrode is eit!er a rod t!at simply

carries current between t!e tip and t!e wor$ or a rod or wire t!at meltsand supplies "ller metal to t!e joint.Pri)i#le o Ar) #!e basic arc welding circuit is an alternating current 315 or direct current

3D5 power source connected by a Swor$T cable to t!e wor$ piece and by a

S!otT cable to an electrode. W!en t!e electrode is positioned close to t!e

wor$ piece an arc is created across t!e gap between t!e metal and t!e !ot

cable electrode. 1n ionied column of gas de&elops to complete t!e circuit.

'igure 1rc welding setup

#!e arc produces a temperature of about @??? to 4??? at t!e tip andmelts part of t!e metal being welded and part of t!e electrode. #!isproduces a pool of molten metal t!at cools and solidi"es be!ind t!eelectrode as it is mo&ed along t!e joint. #!ere are two types of electrodes. onsumable electrode tips melt andmolten metal droplets detac! and mi2 into t!e weld pool. Non


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WELDING

#!e strengt! of t!e weld is reduced w!en metals at !ig! temperatures

react wit! o2ygen and nitrogen in t!e air to form o2ides and nitrides. *ost

arc welding processes minimie contact between t!e molten metal and t!e

air wit! a s!ield of gas &apour or slag. Granular u2 for e2ample adds

deo2idiers t!at create a s!ield to protect t!e molten pool t!us impro&ing

t!e weld.

Ar) Weldig E,ui#-e* #!e main re%uirement in an arc welding setup is t!e source of electricpower. #!ey are essentiallyof two types(a5 1lternating urrent *ac!ines). #ransformer+. *otor or engine dri&en alternatorb5 Direct urrent *ac!ines). #ransformer wit! D recti"er

+. *otor or engine dri&en generatorIn 1 welding normally transformer is used. It !as following operationalc!aracteristics.). No mo&ing parts and less noiseF+. Less maintenanceF-. 8ig!er eJciencyF. !eaper power source.In D arc welding a recti"er or a generator can be used to supply t!ere%uired D power. 1t "rst input &oltage is stepped down to re%uired&oltage and t!en t!roug! silicon controlled recti"er 3=95 is con&ertedfrom 1 to D. Its c!aracteristics are

). ompact setup+. 8ig!ly reliable and eJcient-. Less noise. ostly setup #!e welding mac!ine can be of two types.). onstant current welding mac!ines or droopers+. onstant &oltage welding mac!inesIn constant current welding mac!ine t!e c!ange in arc current magnitudedue to c!ange in &oltage across t!e electrodes is &ery small. #!is mac!ineis &ery essential for manual arc welding processes since t!e maintenanceof constant arc is nearly impossible by a !uman welder. Wit! t!e &ariationof electrode distance from t!e base plate in manual arc welding t!e &oltageacross t!e arc gap c!anges continuously but t!e magnitude of currentremains almost constant due to w!ic! good %uality of weld can be made.In constant &oltage welding mac!ines small c!ange in &oltage ma$es for ane2tremely large c!ange in t!e output currents. #!eses mac!ines aregenerally preferred in t!e automatic mac!ines since t!ey become self correcti&e. W!en t!e electrode comes a bit closer to t!e wor$ t!e arc&oltage drops raising t!e output current to &ery !ig! &alue. #!is currentinstantly melts t!e electrode and t!us maintains t!e arc gap.

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WELDING

'igure ( onstant current c!aracteristics

'igure ( onstant &oltage c!aracteristics

#!oug! D arc welding is more e2pensi&e t!an 1 welding it is generallypreferred because of t!e control of t!e !eat input oered by it. If more !eatis re%uired at t!e wor$piece side suc! as for t!ic$er s!eets or for t!e wor$materials w!ic! !a&e !ig!er t!ermal conducti&ity suc! as aluminium andcopper t!e wor$piece can be made as anode liberating large !eat near it.

#!is is termed as s*r+ig0* #ol+ri*( or dire)* )urre* ele)*rodeeg+*i5e !DCEN5. #!is gi&es rise to !ig!er penetration of weld metal. 'ort!inner materials w!ere less !eat input is re%uired in t!e weld one t!epolarity could be re&ersed by ma$ing t!e wor$piece as negati&e. #!is istermed as re5ersed #ol+ri*( or dire)* )urre* ele)*rode #osi*i5e!DCEP/.In t!is case weld metal penetration is small. In case of 1 weldingt!e bead obtained is somew!ere in between t!e abo&e two types. D arcwelding is preferred for diJcult tas$s suc! as o&er!ead welding since itcan maintain a stable arc.

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WELDING

'igure ( Weld penetration

1 &oltage of t!e order of ? to 0? s!ould be enoug! for starting an arcw!ereas for continuous welding +? to -? is suJcient. #!e minimum&oltage m can be calculated as

- 6 27 8 7.7% I 9w!ere I is t!e load current in amperes. #!e rated current speci"es t!e ma2imum current in amperes t!at a weldingmac!ine is capable of supplying at a gi&en &oltage. #!e preferred currentratings as per Indian standard are )0? +?? -?? ?? 0?? @?? and >?? 1.Du*( C()le4Duty cycle is t!e ratio of arcing time to t!e weld cycle time multiplied by)??. Welding cycle time is eit!er 0 minutes as per European standards or)? minutes as per 1merican standard and accordingly power sources aredesigned. It arcing time is continuously 0 minutes t!en as per European

standard it is )??Q duty cycle and 0?Q as per 1merican standard. 1t )??Qduty cycle minimum current is to be drawn i.e. wit! t!e reduction of dutycycle current drawn can be of !ig!er le&el. #!e welding current w!ic! canbe drawn at a duty cycle can be e&aluated from t!e following e%uationF

Duty cycle and associated currents are important as it ensures t!at powersource remains

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WELDING

safe and its windings are not getting damaged due to increase intemperature beyondspeci"ed limit. #!e ma2imum current w!ic! can be drawn from a powersource dependsupon its sie of winding wire type of insulation and cooling system of t!e

power source.

Bower sources produce D wit! t!e electrode eit!er positi&e or negati&e or

1. #!e c!oice of current and polarity depends on t!e process t!e type of

electrode t!e arc atmosp!ere and t!e metal being welded.

So-e I-#or*+* De:i*ios Arc-on time: W!en t!e welder !olds an arc between t!e electrode and t!ewor$ pieceIdling time: W!en welding e%uipment is ready for use but is not generatingan arcOperating factor: #!e ratio of arc


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WELDING

eJciency Q mass of metal deposited/mass of metal of t!e electrodeconsumed2 )??Wit! ordinary electrodes t!e eJciency &aries from 40 to >0 Q but wit!electrodes containing metallic components in t!e co&ering t!e eJciencycan approac! +?? Q3e.g. electrodes containing iron powder5.#!e electrodesare mar$ed wit! a @ digit numeral associated by a pre"2 and a suJ2. #!e

meaning of t!ese and t!e &arious &alues are s!own in "gure .

Ele)*rode s#e)i:)+*io"#+ge


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WELDING

inuenced by t!e lines of forces crossing t!e arc gap. 1s t!e weld joins t!epieces toget!er t!ere is less and less c!ance t!at t!e magnetic "eld willconcentrate in t!e arc gap. 1s t!e weld is "lling t!e gap of t!e joint itpus!es t!e magnetic u2 a!ead of t!e arc. 1s long as t!e u2 can tra&elno serious arc blow will interrupt t!e weld. W!en u2 ceases to mo&e itpiles up and a magnetic "eld of considerable strengt! de&elops. #!e

buildup of t!e u2 causes a deection of arc column as it pills away fromt!is !ea&y concentration of magnetic forces. Ionied gases t!at carry t!earc from t!e end of electrode to t!e wor$piece are acting as e2ibleconductors. #!is concentration of u2 t!at pulls t!e arc from its intendedpat! is called 1rc 6low.

S#+**er1t t!e conclusion of a weld small particles or globules of metal maysometimes be obser&edscattered around t!e &icinity of t!e weld along its lengt!. #!is is $nown asspatter and may occur t!roug!(). 1rc blow ma$ing t!e arc uncontrollable.+. #!e use of too long an arc or too !ig! an arc &oltage.-. #!e use of e2cessi&e current.

'igure ( =patterDuty cycle

Po$er E;)ie)(

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WELDING

Welding power sources draw power w!en idling. EJciency is greater w!enidling is reduced and t!e operating factor is close to )?? percent. #!e!ig!er t!e operating factor t!e more eJcient t!e process. #!e followingare ways to impro&e eJciency(U 7se t!e most eJcient welding process. 7se gas metal arc welding3G*1W5 instead of s!ielded metal arc welding 3=*1W5. #ypically operating

factors for =*1W fall between )? to -? percentF operating factors for G*1Wfall between -? to 0? percent.U 7se multi


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WELDING

Ad5+*+ges). =*1W is t!e simplest and most &ersatile of t!e arc welding

processes.+. #!esimplicity and portability of =*1W e%uipment allow use of t!is

process in a wide &ariety of applications from re"nery piping to crosscountry pipelines and e&en underwater to repair os!ore structures.

-. =*1W can be used in any position or location t!at can be reac!edwit! an electrode. Points in blind areas can be welded including t!ebac$ sides of pipes in restricted areas t!at are inaccessible for mostot!er welding processes by using bent electrodes.

. =*1W is used to join a wide &ariety of ferrous and nonferrousmaterials including carbon and low alloy steels stainless steelsnic$el based alloys cast iron and some copper alloys

0. =*1W is used to join a wide &ariety of ferrous and nonferrousmaterials including carbon and low alloy steels stainless steels

nic$el based alloys cast iron and some copper alloys.

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WELDING

Dis+d5+*+ges). E&en t!oug! =*1W is a !ig!ly &ersatile process it !as se&eral

c!aracteristics t!at ma$e t!e deposition rate lower t!an wit! semi<automatic or automatic processes

+. Electrodes are of "2ed lengt! and welding must be stopped after eac!electrode !as

been consumed.-. #!e stub of t!e electrode is lost and time is lost for c!angingelectrodes.

. #!e slag must be remo&ed from t!e weld after eac! pass beforesubse%uent passes can be deposited. #!ese steps lower weldingeJciency by about 0?Q.

0. =mo$e and fumes present a problem wit! =*1W and &entilation isre%uired in con"ned spaces.

@. #!e &iew of t!e weld puddle is somew!at obscured by t!e protecti&eslag t!at co&ers t!e freeing weld metal and by t!e smo$e.

4. E2tra welder s$ill is needed to ma$e radiograp!


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WELDING

G#1W !as been adapted to automatic welding. 1utomation of t!e processre%uires a programmed power source and controls a wire feeder andmac!ine guided tra&el. It !as been used to ma$e !ig! %uality tube


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WELDING

+. In addition G#1W re%uires closer control of joint "t


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WELDING

• urrent Density < #o understand w!y gas metal arc welding can deposit weld metal

at a rapid rate it is necessary t!at t!e term current density be understood.

'igure s!ows a )/ coated electrode and a )/)@ solid wire drawn to scale. 6ot!

are capable of carrying ?? amperes. Notice t!at t!e area of t!e )/)@ wire is only

)/)@ t!at of t!e core wire of t!e coated electrode. We can say t!at t!e current

density of t!e )/)@ wire is )@ times.

• EX7IB*EN# 1ND :BE91#I:N < #!e e%uipment used for gas metal arc welding is

more complicated t!an t!at re%uired for s!ielded metal arc welding. Initial cost is

relati&ely !ig! but t!e cost is rapidly amortied due to t!e sa&ings in labor and

o&er!ead ac!ie&ed by t!e rapid weld metal deposition.

• #!e e%uipment necessary for gas metal arc welding is listed below(

)5 Bower source

+5 Wire feeder

-5 Welding gun

5 =!ielding gas supply

05 =olid electrode wire

@5 Brotecti&e e%uipment

• #!e basic e%uipmentnecessary for semiautomatic

gas metal arc welding is s!own in 'igure .

• =8IELDING G1=E= < In gas metal arc welding t!ere are a &ariety of

s!ielding gases t!at can be used eit!er alone or in combinations of &arying

degrees. #!e c!oice is dependent on t!e type of metal transfer employed

t!e type and t!ic$ness of metal.

• carbon dio2ide 3:+5 is often used for s!ort circuiting arc welding because

of its low cost.

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WELDING

• *i2ing argon in proportions of 0?


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WELDING

*at!ematically 8 It I3I95t I+9t

W!ere 8 !eat generated in joulesI current in 1mp.9 resistance in o!mst time of current ow in seconds.1..1 T(#es o Resis*+)e $eldig #!e major types of resistance welding are gi&en as under(3)5 =pot Welding3+5 =eam Welding3-5 Brojection Welding35 9esistance 6utt Welding305 'las! 6utt Welding3@5 Bercussion Welding345 8ig! 're%uency 9esistance Welding

3A5 8ig! 're%uency Induction Welding=ome of t!e abo&e important welding processes are discussed as under

1..1.1 S#o* WeldigIn t!is process o&erlapping s!eets are joined by local fusion at one or morespots by t!e concentration of current owing between two electrodes. #!isis t!e most widely used resistance welding process. 1 typical resistancespot welding mac!ine is s!own in 'ig. )4.+-. It essentially consists of twoelectrodes out of w!ic! one is "2ed. #!e ot!er electrode is "2ed to a roc$erarm 3to pro&ide mec!anical ad&antage5 for transmitting t!e mec!anicalforce from a pneumatic cylinder. #!is is t!e simplest type of arrangement.

#!e ot!er possibility is t!at of a pneumatic or !ydraulic cylinder beingdirectly connected to t!e electrode wit!out any roc$er arm. 'or weldinglarge assemblies suc! as car bodies portable spot welding mac!ines areused. 8ere t!e electrode !olders and t!e pneumatic pressuriing systemare present in t!e form of a portable assembly w!ic! is ta$en to t!e placew!ere t!e spot is to be made. #!e electric current compressed air and t!ecooling water needed for t!e electrodes is supplied t!roug! cables and!oses from t!e main welding mac!ine to t!e portable unit. In spot weldinga satisfactory weld is obtained w!en a proper current density is maintained. #!e currentdensity depends on t!e contact area between t!e electrode and t!e wor$<piece. Wit! t!e continuous use if t!e tip becomes upset and< t!e contactarea increases t!e current density will be lowered and conse%uently t!eweld is obtained o&er a large area. #!is would not be able to melt t!e metaland !ence t!ere would be no proper fusion. 1 resistance welding sc!eduleis t!e se%uence of e&ents t!at normally ta$e place in eac! of t!e welds. #!e e&ents are(). #!e s%ueee time is t!e time re%uired for t!e electrodes to align andclamp t!e twowor$


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WELDING

!eated to t!e melting temperature.-. #!e !old time is t!e time w!en t!e pressure is to be maintained on t!emoltenmetal wit!out t!e electric current. During t!is time t!e pieces are e2pectedto beforged welded.

. #!e o time is time during w!ic! t!e pressure on t!e electrode is ta$eno so t!att!e plates can be positioned for t!e ne2t spot.

S#o* $eldig ele)*rodes=pot welding electrodes are made of materials w!ic! !a&e3)5 8ig!er electrical and t!ermal resisti&ities and

3+5 =uJcient strengt! to wit!stand !ig! pressure at ele&ated temperatures.opper base alloys suc! as copper beryllium and copper tungsten arecommonly usedmaterials for spot welding electrodes. 'or ac!ie&ing t!e desired currentdensity It is importantto !a&e proper electrode s!ape for w!ic! t!ree main types of spot weldingelectrodes are usedw!ic! are pointed domed and at electrodes.A##li)+*ios o S#o* Weldig3i5 It !as applications in automobile and aircraft industries3ii5 #!e attac!ment of braces brac$ets pads or clips to formed s!eet


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WELDING

1..1.2 Resis*+)e Se+- WeldigIt is a continuous type of spot welding w!erein spot welds o&erlap eac!ot!er to t!e desirede2tent. In t!is process coalescence at t!e faying surfaces is produced byt!e !eat obtained fromt!e resistance to electric current 3ow5 t!roug! t!e wor$ pieces !eld

toget!er under pressureby circular electrodes. #!e resulting weld is a series of o&erlappingresistance


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WELDING

t!ic$ness. #!e welding of a nut on t!e automoti&e c!asis is an e2ample of projection welding.Ad5+*+ges +d dis+d5+*+ges o resis*+)e $eldig

C 1d&antagesD =imple low power re%uirementsD 8ig! speed , low cost

D Not !aardous no e2tra material cost C Disad&antagesD :nly butt joint is possibleD *olten metal e2pulsion i.e. as! needs to be remo&edD ery rigid mac!ine is re%uired

C 1pplicationsD ombinations of metals can be weldedD Broduction of s!afts gears , &al&esD Broduction of cutting tools , t!eir bodiesD Welding toget!er t!e small forgings

1.


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WELDING

D =imple low power re%uirementsD 8ig! speed , low costD Not !aardous no e2tra material cost

C Dis+d5+*+gesD :nly butt joint is possibleD *olten metal e2pulsion i.e. as! needs to be remo&ed

D ery rigid mac!ine is re%uired C A##li)+*iosD ombinations of metals can be weldedD Broduction of s!afts gears , &al&esD Broduction of cutting tools , t!eir bodiesD Welding toget!er t!e small forgings

T0er-i*e Weldig #!ermite welding 3#W5 3sometimes called t!ermit welding5 is a processw!ic! joins metals by !eating t!em wit! super !eated li%uid metal from ac!emical reaction between a metal o2ide and aluminum or ot!er reducing

agent wit! or wit!out t!e application of pressure. 'iller metal is obtainedfrom t!e li%uid metal. #!e !eat for welding is obtained from an e2ot!ermicreaction or c!emical c!ange between iron o2ide and aluminum. #!isreaction is s!own by t!e following formula(A1) O -fe-? >'e O 1)+?- O 8eat #!e temperature resulting from t!is reaction is appro2imately +A+. #!esuper !eated steel is contained in a crucible located immediately abo&e t!eweld joint. #!e e2ot!ermic reaction is relati&ely slow and re%uires +? to -?seconds regardless of t!e amount of c!emicals in&ol&ed. #!e parts to bewelded are aligned wit! a gap between t!em. #!e super !eated steel runs

into a mold w!ic! is built around t!e parts to be welded. =ince it is almosttwice as !ot as t!e melting temperature of t!e base metal melting occursat t!e edges of t!e joint and alloys wit! t!e molten steel from t!ecrucible.Normal !eat losses cause t!e mass of molten metal to solidifycoalescence occurs and t!e weld is completed. If t!e parts to be weldedare large pre!eating wit!in t!e mold ca&ity may be necessary to bring t!epats to welding temperature and to dry out t!e mold. If t!e parts are smallpre!eating is often eliminated. #!e t!ermit welding process is applied onlyin t!e automatic mode. :nce t!e reaction is started it continues untilcompletion. #!emite welding utilies gra&ity w!ic! causes t!e molten metal to "ll t!e

ca&ity between t!e parts being welded. It is &ery similar to t!e foundrypractice of pouring a casting. #!e dierence is t!e e2tremely !ig!temperature of t!e molten metal. #!e ma$ing of a t!ermit weld is s!own in"gure . W!en t!e "ller metal !as cooled all unwanted e2cess metal maybe remo&ed by o2ygen cutting mac!ining or grinding. #!e surface of t!ecompleted weld is usually suJciently smoot! and contoured so t!at it doesnot re%uire additional metal "nis!ing.

*E*:9; In


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WELDING

Thermite Welding Equipment T!" #!ermite material is a mec!anical mi2ture of metallic aluminum andprocessed iron o2ide. *olten steel is produced by t!e t!ermite reaction in a

magnesite


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WELDING

C Ad5+*+ges

D No costly power supply on site repairs/welding is possible

C Dis+d5+*+ges

D Economical for !ea&ier sections , t!at too for ferrous

metals only.

C A##li)+*ios

D 9ail

welding final gate

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