Ch-19 Gas Welding, Gas Cutting & Arc Welding.pptx

8/9/2019 Ch-19 Gas Welding, Gas Cutting & Arc Welding.pptx

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ngBy S K

Mondal





Welding DefnitionWelding is a process by which two materials,

usually metals, are permanently joined togetherby coalescence, which is induced by a

combination of temperature, pressure, andmetallurgical conditions.

The particular combination of these variables canrange from high temperature with no pressure to

high pressure with no increase in temperature.Welding positive process!

Machining negative process!

"orming, casting #ero process!



Requirement or a high quality welding

$. % source of satisfactory heat and&or pressure,

'. % means of protecting or cleaning the metal,

and

(. )aution to avoid, or compensate for, harmful

metallurgical e*ects.



Classifcation o welding processes+y fuel gas welding +"W!

%rc welding %w!

-esistance welding

Solid state welding friction welding, ultrasonicwelding, forge welding etc.!

ni/ue process Thermit welding

0aser beam welding

1lectroslag welding

"lash welding

2nduction welding

1lectron beam welding



IES - 2!2

The advantage of the welding process isa! 2t relieves the joint from residual stresses

b! 2t helps in chec3ing of distortion of wor3 piece

c! 0arge number of metals and alloys, bothsimilar and&or dissimilar can be joined.

d! 4eat produced during the welding does notproduce metallurgical changes.



Welda"ility # $a"rication %rocesses

The weldability of a material will depend on thespeci5c welding or joining process beingconsidered.

"or resistance welding of consistent /uality, it isusually necessary to remove the oideimmediately before welding.

"abrication weldability test is used to determine

mechanical properties re/uired for satisfactoryperformance of welded joint.

The correct se/uence of the given materials inascending order of their weldability is

&luminum ' copper ' cast iron ' (S)ontd6



Case o &luminium The oide coating on aluminum alloys causes some

di7culty in relation to its weldability.

2t also has high thermal conductivity and a very shorttemperature range between li/uidus and solidus and

when li/uid its viscosity is very low.%luminium is poor absorber of laser light.

8uring fusion welding, the aluminum would oidi#eso readily that special 9ues or protective inert:gas

atmospheres must be employed.

"riction welding and T2; welding is good foraluminium.

"or aluminium %) current plus high fre/uency is

must.



Case o Cast Iron)ast iron is more di7cult to weld because of its high

carbon content and brittleness poor ductility!

Massive carbon deposits have a tendency to form inthe areas adjacent to the weld, and high:carbonmartensite tends to form in the heat:a*ected #ones.

These microstructures are very brittle and maycrac3 spontaneously while welding is in progress orlater when load is applied to the wor3piece.

)ast iron can be joined by the oyacetylene bra#ingprocess and shielded metal:arc welding stic3!process.

Some cases preheating and&or post heating is

re/uired.



Case o Stainless SteelStainless steel is a di7cult metal to weld

because it contains both nic3el and chromium.

The best method for welding stainless steel is

T2; welding. The electric arc is also preferred for welding

stainless steels. % heavily coated welding rod,which produce a shielded arc, is employed.

<ou must do a better job of pre:cleaning.sing a low arc current setting with faster

travel speeds is important when weldingstainless steel, because some stainless steelsare subject to carbide precipitation. )ontd6..



Case o Stainless Steel The ferritic stainless steels are generally less

weldable than the austenitic stainless steel andre/uire both preheating and postweld heattreatments.

Welds of ferritic stainless steel can be by

i! autogenously i.e. without the addition of 5llermetal!

ii! with an austenitic stainless steel

iii! using a high nic3el 5ller alloy.

iv! Type =>? 5ller low $$@ )r, low carbon and small>.'@ %l!

Welding processA T2;, M2;, Shielded:metal arcwelding and lasma arc welding



21S '>$>

&ssertion )&*+ It is generally di,cult toweld &luminum parts "y normal arcwelding process

Reason )R*+ .ard and "rittle &luminum-

o/ide flm is ormed at the welded 0oints

a! Both % and - are individually true and - isthe correct eplanation of %

b! Both % and - are individually true but - isC+T the correct eplanation of %

c! % is true but - is false

d! % is false but - is true



1&E-2!3 The major di7culty during welding of

aluminium is due to its

a! high tendency of oidation

b! high thermal conductivityc! low melting point

d! low density



IES 2!!)onsider the following statements.

)ast iron is di7cult to weld, because of

$. 0ow ductility

'. oor fusion(. Tendency to crac3 on cooling

Which of these statements are correct E

a! $, ' and (b! $ and ' only

c! ' and ( only

d! $ and ( only



IES - 2!2

Which of the following factors improve weldability of steelE

$. 0ow carbon content

'. 4igh carbon content(. ;ood a7nity content

=. oor a7nity to oygen

a! $ and ( b! ' and (

c! $ and = d! ' and =



21S:'>$(

Welda"ility depends on

$. Thermal conductivity

'. Surface condition

(. )hange in microstructure

a! $ and ' only b! $ and ( only

c! ' and ( only d! $, ' and (



1as $lame %rocesses+

Welding4 Cutting and Straightening5/y-uel gas Welding +"W!A 4eat source is

the 9ame produced by the combustion of a

fuel gas and oygen.

+"W has largely been replaced by otherprocesses but it is still popular because of its

portability and the low capital investment.

%cetylene is the principal fuel gas employed.



)ombustion of oygen and acetylene )'4'! in

a welding torch produces a temp. in a twostage reaction.

2n the 5rst stage

F 4eat This reaction occurs near the tip of the torch.

2n the second stage combustion of the )+ and4' and occurs just beyond the 5rst combustion

#one.

')+ F +' → ')+' F 4eat

4' F +' → 4'+ F 4eat

+ygen for secondary reactions is obtained

C H O CO H+ → +2 2 2 2

2

1

2







hree types o 6ames can "e o"tained "y

7arying the o/ygen#acetylene )oro/ygen#uel gas* ratio

2f the ratio is about $ A $ to $.$? A $, allreactions are carried to completion and aneutral 9ame is produced.

Most welding is done with a neutral 9ame. 2t ischemically neutral and neither oidi#es or

carburi#es the metal being welded.

5/y-acetylene gas welding neutral 6ame



% higher ratio, such as $.? A $, produces an

oidi#ing 9ame, hotter than the neutral 9ameabout ((>>o)! but similar in appearance.

sed when welding copper and copper alloysbut harmful when welding steel because theecess oygen reacts with the carbon,decarburi#ing the region around the weld.

5/y-acetylene gas welding 5/idising 6ame



1cess fuel, on the other hand, produces a

carburi#ing 9ame. )arburi#ing 9ame cancarburi#e metal also.

The ecess fuel decomposes to carbon andhydrogen, and the 9ame temperature is not as

great about (>>>o)!."lames of this type are used in welding Monel

a nic3el:copper alloy!, high:carbon steels, andsome alloy steels, and for applying sometypes of hard:facing material.

5/y-acetylene gas welding Car"uri8ing 6ame



Metal Flame

M S N

High carbon steel R

Grey cast iron N, slightly oxidizing

Alloy steel N

Aluminium Slightly carburizingBrass Slightly oxidizing

Copper, Bronze N, slightly oxidizing

Nickel alloys Slightly carburizing

Lead N



IES 29 Con7entional1plain the three types of oy:acetylene

9ames. 2ndicate with the help of s3etches

the various #ones, respective temperature

ranges and applications of each type of

9ame.

G '> H Mar3sI





8iagram



:ses4 &d7antages4 and ;imitations

+"W is usion welding. <o pressure is in7ol7ed.

"iller metal can be added in the form of a wireor rod.

"lues may be used to clean the surfaces andremove contaminating oide. The gaseousshield produced by vapori#ing 9u can prevent

oidation during welding, and the slagproduced by solidifying 9u can protect theweld pool. "lu can be added as a powder, thewelding rod can be dipped in a 9u paste, or

the rods can be pre:coated. )ontd6



1poser of the heated and molten metal to the

various gases in the 9ame and atmosphere ma3es

it di7cult to prevent contamination.

4eat source is not concentrated, a large area of the

metal is heated and distortion is li3ely to occur.

"lame welding is still /uite common in 5eld wor3, in

maintenance and repairs, and in fabricating small/uantities of speciali#ed products.



5/y acetylene welding equipment

+ygen is stored in a cylinder at a pressureranging from $(.J Ma to $J.' Ma .

8ue to high eplosiveness of free acetylene it

is stored in a cylinder with J>:J?@ porouscalcium silicate and then 5lled with acetonewhich absorb upto ='> times by its volume ata pressure $.? Ma .

%t the time of acetylene release if acetonecomes with acetylene the 9ame would give apurple colour.

%nother option is acetylene generator.2 2 2 2 2

2 ( )CaC H O C H Ca OH + → +



%ressure 1as Weldingressure gas welding ;W! or

+yacetylene ressure Welding is a processused to ma3e butt joints between the endsof objects such as pipe and:railroad rail.

The ends are heated with a gas 9ame to atemperature below the melting point, and

the soft metal is then forced together underconsiderable pressure.

This process, therefore, is actually a Lform ofsolid:state welding.



21S '>$>

he ratio "etween 5/ygen and&cetylene gases or neutral 6ame ingas welding is

a! ' A $ b! $ A 'c! $ A $ d! = A $



IES-29

=y which one o the ollowing methodsgray cast iron is usually welded>

a! T2; welding b! M2; welding

c! ;as welding d! %rc welding



IES - 2!2

Statement 2!A 2n gas welding the metal to be joined gets oidi#ed or carburi#ed

Statement 22!A The neutral 9ame a*ects nochemical change on the molten metal.

a! Both Statement 2! and Statement 22! areindividually true and Statement 22! is the correcteplanation of Statement 2!

b! Both Statement 2! and Statement 22! areindividually true but Statement 22! is not thecorrect eplanation of Statement 2!

c! Statement 2! is true but Statement 22! is false

d! Statement 2! is false but Statement 22! is true



5/ygen orch Cutting )1as Cutting*

2ron and steel oidi#e burn! when heated to atemperature between J>>>) to $>>>>).

4igh:pressure oygen jet (>> Ka! is directed

against a heated steel plate, the oygen jetburns the metal and blows it away causing thecut 3erf!.

"or cutting metallic plates shears are used.

These are useful for straight:line cuts and alsofor cuts up to => mm thic3ness.

)ontd6



"or thic3er plates with speci5ed contour,

shearing cannot be used and oy:fuel gascutting +")! is useful.

;as:cutting is similar to gas welding ecepttorch tip.

$ig- di?erences in torch tips or gas welding andgas cutting

)ontd6





0arger si#e ori5ce produces 3erf width wider and

larger oygen consumed.%t 3indling temperature about J>o)!, iron form

iron oide.

-eactionA

("e F '+' → "e(+= F. MN&3g of iron

The other reactionsA

'"e F +' → '"e+ F (.$J MN&3g of iron

="e F (+' → '"e'+( F =.O MN&3g of iron%ll eothermic reactions preheat the steel.

)ontd6



"or complete oidation >.'J m( oygen&3g ofiron is re/uired

8ue to unoidi#ed metal blown away theactual re/uirement is much less.

Torch tip held vertically or slightly inclined inthe direction of travel.

Torch position is about $.? to ( mm verticalfrom plate.

)ontd6



The drag lines shows the characteristics of the

movement of the oygen stream.

8rag is the amount by which the lower edge ofthe drag line trails from the top edge.

;ood cut means negligible drag.

$ig- positioning o cutting torch in o/y- uel gas

cutting

)ontd6



2f torch moved too rapidly, the bottom does

not get su7cient heat and produces largedrag so very rough and irregular:shaped:cutedges.

2f torch mo7ed slowly a large amount of slag

is generated and produces irregular cut.

)ontd6



;as cutting is more useful with thic3 plates.

"or thin sheets less than ( mm thic3! tip si#eshould be small. 2f small tips are not available

then the tip is inclined at an angle of $? to '>degrees.

$ig Recommended torch position or cutting thinsteel





&pplicationseful only for materials which readily get

oidi#ed and the oides have lower melting

points than the metals.Widely used for ferrous materials.

)annot be used for aluminum, bron#e,

stainless steel and li3e metals since they resist

oidation.





"or high carbon steel material around the cut

should be preheated about '?> to (>>o)! and

may post heat also necessary.

)utting )2 is di7cult, since its melting temp. islower than iron oide.

2f chromium and nic3el etc are present in

ferrous alloys oidation and cutting is di7cult.





IES 2@Consider the ollowing statements in respect o o/y-acetylene welding+

$. The joint is not heated to a state of fusion.

'. Co pressure is used.

(. +ygen is stored in steel cylinder at a pressure of $=Ma.

=. When there is an ecess of acetylene used, there is adecided change in the appearance

of 9ame. Which of the statements given above are correctE

a! $, ' and ( b! ', ( and =

c! $, ( and =d! $, ' and =



IES-2A

Consider the ollowing statements+

$. 2n gas welding, the torch should be held at an

angle of (>P to =?P from the hori#ontal plane.

'. 2n gas welding, the Si#e of the torch dependsupon the thic3ness of metal to be formed.

(. 8rag in gas cutting is the time di*erence

between heating of the plate and starting the

oygen gas for cutting. Which of the statements given above are correctE

a! $, ' and (b! $ and '

c! ' and (d! $ and (



%owder Cutting

)ast iron, stainless steel, and others high alloysteels are di7cult to cut by oy:fuel cutting andwe can use powder cutting.

By injecting a 5nely divided '>>:mesh iron powder

into the 9ame, a lower melting point eutecticoide is formed at the cutting interface, whereadditional iron:oygen reaction is generated andcutting proceeds in a similar way of oy:fuel

cutting. The heat and the 9uing action of the burning iron

powder enable the cutting oygen stream tooidi#e the base metal continuously, just as in

cutting carbon steel.



%lasma Cutting

ses ioni#ed gas jet plasma! to cut materialsresistant to oy:fuel cutting,

4igh velocity electrons generated by the arc impactgas molecules, and ioni#e them.

The ioni#ed gas is forced through no##le upto ?>>m&s!, and the jet heats the metal, and blasts themolten metal away.

More economical, more versatile and much faster

? to J times! than oyfuel cutting, produces narrow3erfs and smooth surfaces.

4%Q is $&( to R th than oyfuel cutting.

Maimum plate thic3ness '>> mm





Electric &rc Welding



Electric &rc Welding

$ig =asic circuit or arcwelding



%rinciple o &rc%n arc is generated between cathode and

anode when they are touched to establish the

9ow of current and then separated by a smalldistance.

?@ to ?@ heat is generated at the anode.

2f 8) is used and the wor3 is positive the

anode of the circuit!, the condition is 3nown as

straight polarity S8)!. )ontd6



Wor3 is negative and electrode is positive is

reverse polarity -8)!.

S8) conditions are preferred.

8) arc:welding maintain a stable arc and

preferred for di7cult tas3s such as overhead

welding.

"or a stable arc, the gap should be

maintained.

)ontd6



Manual arc welding is done with shielded

covered! electrodesBare:metal wire used in automatic or

semiautomatic machines.

Con consumable electrodes e.g tungsten! isnot consumed by the arc and a separate metalwire is used as 5ller.

There are three modes of metal transfer

globular, spray and short:circuit!.

h d t l t



hree modes o metal transerduring arc welding

(a0or $orces taBe part in (etal



(a0or $orces taBe part in (etalranser

i! gravity force

ii! Surface tension

iii! electromagnetic interaction

iv! hydrodynamic action of plasma





W( 2!

&ssertion )&* + =ead is the metal addedduring single pass o welding

Reason )R* + =ead material is same as "ase

metala! Both % and - are individually true and - isthe correct eplanation of %

b! Both % and - are individually true but - is

C+T the correct eplanation of %c! % is true but - is false




&rc welding equipments$. 8roopersA )onstant current welding

machines

;ood for manual welding

'. )onstant voltage machines ;ood for automatic welding

)ontd6



Fig. Machine with diferent settings

$ig Characteristic cur7e o a constant 7oltage arc-



"ormula

1V I

OCV SCC + =



-e/uires a large current $?> to ! %!,

voltage is between (> and => , actual

voltage across the arc varying from $' to (> .

To initiate a weld, the operator stri3e the

electrode and start arc.





21S '>$>

In arc welding4 the arc length should "eequal to

a! =.? times the rod diameter

b! ( times the rod diameter

c! $.? times the rod diameter

d! -od diameter



IES-2AConsider the ollowing statements+

$. 2n arc welding, ?@ to ?@ heat isgenerated at the anode.

'. 8uty cycle in case of arc welding is the

cycle of complete welding of wor3 piecefrom the beginning.

(. %rc blow is more common with 8) welding.

Which of the statements given aboveare correctE

a! $, ' and ( b! $ and '

c! ' and ( d! $ and (



1&E -2!2 Same in 1&E -2!2 )%I*

In a DC arc welding operation4 the 7oltage-arc length characteristic was o"tained asarc F 2 G A; where the arc length ; was

7aried "etween A mm and @ mm .ere arc

denotes the arc 7oltage in olts he arccurrent was 7aried rom 3 & to A &&ssuming linear power source

characteristic4 the open circuit 7oltage andthe short circuit current or the weldingoperation are

a! =? , =?> % b! ? , ?> %

c! O? , O?> % d! $?> , $?>> %



1&E-!992% low carbon steel plate is to be welded by themanual metal arc welding process using a linear : 2 characteristic 8) ower source. The

following data are available A+) of ower source '

Short circuit current $(> %

%rc length, 0 = mm

Traverse speed of welding $? cm/s

17ciency of heat input J?@

oltage is given as '> F $.? 0

)alculate the heat input into the wor3price



1&E-2!3 )%I*In an arc welding operation4 carried out with a

power source maintained at 3 7olts and 3

amperes4 the consuma"le electrode melts and

0ust flls the gap "etween the metal plates to

"e "utt-welded he heat transer e,ciency

or the process is H4 melting e,ciency is

and the heat required to melt the electrode is2 #mm I the tra7el speed o the electrode

is 3 mm#s4 the cross-sectional area4 in mm24 o

the weld 0oint is JJJJJJJ



Duty Cycle The percentage of time in a ? min period that

a welding machine can be used at its ratedoutput without overloading.

Time is spent in setting up, metal chipping,cleaning and inspection.

"or manual welding a >@ duty cycle issuggested and for automatic welding $>>@

duty cycle.

)ontd6



-e/uired duty cycle,

Where ,T rated duty cycle

2 rated current at the rated duty cycle

2o Maimum current at the rated dutycycle

= ÷

2

a

a

IT T

I



Electrode$. Con:consumable 1lectrodes

'. )onsumable 1lectrodes

<on-consuma"le ElectrodesMade of carbon, ;raphite or Tungsten.

)arbon and ;raphite are used for 8.).

1lectrode is not consumed, the arc lengthremains constant, arc is stable and easy tomaintain.

)ontd



Consuma"le Electrodes

rovides 5ller materials.Same composition.

This re/uires that the electrode be moved

toward or away from the wor3 to maintainthe arc and satisfactory welding conditions.

)ontd6



Consuma"le electrodes are three Binds+

a! Bare

b! "lued or lightly coated

c! )oated or etruded & shielded

"or automatic welding, bare electrode is in theform of continuous wire coil!.



Electrode coating characteristic

$. rovide a protective atmosphere.

'. Stabili#e the arc.

(. rovide a protective slag coating to

accumulate impurities, prevent oidation,and slow the cooling of the weld metal.

=. -educe spatter.

?. %dd alloying elements.

. %*ect arc penetration

. 2n9uence the shape of the weld bead.

J. %dd additional 5ller metal.



Electrode coatingsl Slag $orming Ingredients. asbestos, mica,

silica, 9uorspar, titanium dioide, 2ron oide,magnesium carbonate, )alcium carbonate andaluminium oide.

2 &rc Sta"ili8ing Ingredients or ioni#ingagentsA potassium silicate, Ti+' F Qr+'

-utile!, Mica, )alcium oide, sodium oide,magnesium oide, feldspar K%2 Si( +J!

)ontd6



Deo/idi8ing Ingredients )ellulose, )alcium

carbonate, dolo: mite, starch, detrin, wood 9our,graphite, aluminium, ferromanganese.

3 =inding (aterials Sodium silicate, potassiumsilicate, asbestos.

A &lloying Constituents to Impro7e Strength oWeld

L Ti+' and potassium compounds increase the melting

rate of the base metal for better penetration.

@2ron powder provides higher deposition rate.

)ontd6



)ontd6



The slag is then easily chipped.

)oatings are designed to melt more slowlythan the 5ller wire.



Binders

%) arc welding used potassium silicate binders.

8) arc welding used sodium silicate binders.

otassium has a lower ioni#ation potential as

compared with sodium.



IES-!99@&ssertion )&*+ he electrodes o ac arc welding arecoated with sodium silicate4 whereas electrodesused or dc arc welding are coated with potassiumsilicate "inders

Reason )R*+ %otassium has a lower ioni8ationpotential than sodium

a! Both % and - are individually true and - is thecorrect eplanation of %

b! Both % and - are individually true but - is not thecorrect eplanation of %

c! % is true but - is false




Welding $lu/

%vailable in three forms

;ranular

1lectrode wire coating

1lectrode core

;ow .ydrogen



;ow .ydrogen

Electrode The basic coatings contain large amount ofcalcium carbonate limestone! and calcium9uoride 9uorspar! and produce low

hydrogen.But it can absorb moisture therefore

coated low hydrogen electrodes are bac3ed

before use to a temperature of '>>o

) to(>>>) and stored in an oven at $$>o) to$?>o)

+ther types of electrode release large

amount of hydrogen, which can dissolve in

W ldi % iti



Welding %ositions

$ig he position o electrode or hori8ontalwelding

$ig %ositioning o electrode or welding in 7ertically



Welding Current

Welding current depends uponA the thic3nessof the welded metal, type of joint, weldingspeed, position of the weld, the thic3ness andtype of the coating on the electrode and itswor3ing length.

Welding current, 2 3. d, amperesU d isdia. mm!



Welding oltage The arc voltage depends only upon the arc length

3$ F 3'l olts

Where l is the arc length in mm and 3$ and 3' are

constants,

3$ $> to $'U and 3' ' to (

The minimum %rc voltage is given by

min '> F >.>= l! olt



&rc ;ength"or good welds, a short arc length is

necessary, becauseA

$. 4eat is concentrated.

'. More stable (. More protective atmosphere.

)ontd6



& long arc results in

0arge heat loss into atmosphere.nstable arc.

Weld pool is not protected.

Weld has low strength, less ductility, poorfusion and ecessive spatter.





%rc length should be e/ual to the diameter of theelectrode si#e

Bead width should be e/ual to three diameter of theelectrode si#e



;%T1:'>>', )onventional

The arc length:voltage characteristic of a 8) arc

is given by the e/uationA '= F =0, where

is voltage in volts and 0 is arc length in mm. The

static volt:ampere characteristic of the power

source is approimated by a straight line with a

no load voltage of J> and a short circuitcurrent of 600A. Determine the optimum arc

length for maximum power.



1&E-2! )%I*

8uring a steady gas metal arc welding with directcurrent electrode positive polarity, the weldingcurrent, voltage and weld speed are $?> %, (> and m&min, respectively. % metallic wire electrode ofdiameter $.' mm is being fed at a constant rate of $'m&min. The density, speci5c heat and meltingtemperature of the wire electrode are >>> 3g&m(,?>> N&3go) and $?(>o), respectively. %ssume theambient temperature to be (>o) and neglect the

latent heat of melting. "urther, consider that two:thirdof the total electrical power is available for melting ofthe wire electrode. The melting e7ciency inpercentage! of the wire electrode is

a! (O.?J b! =?.'? c! =O.(J d! ?=.OJ



1&E-2HIn arc welding o a "utt 0oint4 the weldingspeed is to "e selected such that highestcooling rate is achie7ed (eltinge,ciency and heat transer e,ciency areA and @4 respecti7ely he area o theweld cross section is A mm2 and the unitenergy required to melt the metal is !

#mmI

I the welding power is 2 BW4 thewelding speed in mm#s is closest to

a! = b! $=c! '=d! (=



&rc "low in DC arc welding

)ontd6



%rc blow occurs during the welding of magneticmaterials with 8).

The e*ect of arc blow is maimum when weldingcorners where magnetic 5eld concentration ismaimum.

The e*ect is particularly noticeable when weldingwith bare electrodes or when using currents belowor above

%gain the problem of arc blow gets magni5ed whenwelding highly magnetic materials such as Ci alloys,because of the strong magnetic 5elds set up bythese metals.

)auseA nbalanced magnetic forces. )ontd6



E?ect o arc "low

0ow heat penetration.1cessive weld spatter.

inch e*ect in welding is the result of

electromagnetic forcesWeld spatter occurs due to

4igh welding current

Too small an electrode arc

)ontd6



he e?ects o arc "low can "e minimi8ed

with DC welding "yShortening the arc.

-educe current

-educing weld speed.

Balance magnetic 5eld by placing one groundlead at each end of the wor3 piece.

Wrapping the electrode cable a few turns

around the wor3 piece.

21S '>$(



21S:'>$(Statement )I*+ The de9ection of %rc from its

intended path is called V%rc blow.Statement )II*+ The chances of %rc blow iscommon in %.). %rc welding.


b! Both Statement 2! and Statement 22! are

individually true but Statement 22! is not thecorrect eplanation of Statement 2!





1as shields

%n inert gas is blown into the weld #one to

drive away other atmospheric gases.

;ases are argon, helium, nitrogen, carbondioide and a miture of the above gases.

%rgon ioni#es easily re/uiring smaller arc

voltages.2t is good for welding thin sheets.

)ontd6



4elium, most epensive, has a better thermal

conductivity, is useful for thic3er sheets,

copper and aluminium welding, higher

deposition rate.

The arc in carbon dioide shielding gas is

unstable, least epensive, deoidi#ers needed.

2t is a heavy gas and therefore covers the

weld #one very well.



Car"on &rc welding

%rc is produced between a carbon electrode

and the wor3.

Shielding is not used.Co pressure

With or without 5ller metal

May be used in Xtwin arc methodX, that is,

between two carbon graphite! electrodes.



ungsten Inert 1as welding )I1*

%rc is established between a non:consumabletungsten electrode and the wor3piece.

Tungsten is alloyed with thorium or #irconium

for better current:carrying and electron:emission characteristics.

%rc length is constant, arc is stable and easyto maintain.

With or without 5ller.

)ontd6

ery clean welds



ery clean welds.

%ll metals and alloys can be welded. %l, Mg

also!

Straight polarity is used.

Weld voltage '> to => and weld current $'?

% for -8) to $>>> % for S8).

Shielded ;asA %rgon

Torch is water or air cooled.



$ig I1



21S:'>$(Statement )I*+ Con consumable electrodes,



used in arc welding are made of high meltingpoint temperature materials, even then thelength of electrode goes on decreasing withpassage of time.

Statement )II*+ The electrode material gets

oidi#ed and melts on the weld material to forma strong 9u.

a! Both Statement 2! and Statement 22! areindividually true and Statement 22! is the correct

eplanation of Statement 2!

b! Both Statement 2! and Statement 22! areindividually true but Statement 22! is not the

correct eplanation of Statement 2!



1as (etal &rc Welding )1(&W* or (I1

% consuma"le electrode in a gas shield.%rc is between wor3piece and an

automatically fed bare:wire electrode.

%rgon, helium, and mitures of the two can beused.

%ny metal can be welded but are usedprimarily with the non:ferrous metals.

When welding steel, some +' or )+' is usuallyadded to improve the arc stability and reduceweld spatter.

)ontd6



"ast and economical.

% reverse:polarity dc arc is generally usedbecause of its deep penetration, spray

transfer, and ability to produce smoothwelds with good pro5le.



$ig (I1



21S '>$(



21S:'>$( Consider the ollowing statements+

In metal are welding

$. tili#es a consumable electrode

'. % welding torch used is connected to

acetylene gas supply(. The electrode and wor3:piece are connectedto the welding power supply

Which of these statements are correctEa! $, ' and ( b! $ and ' only

c! $ and ( only d! ' and ( only



IES 2!2



IES - 2!2Statement 2!A 8) with reverse polarity is used inM2; welding

Statement 22!Ase of 8) with reverse polarityenables deeper penetration and a clean Surface


b! Both Statement 2! and Statement 22! are

individually true but Statement 22! is not thecorrect eplanation of Statement 2!





Su"merged &rc welding )S&W*

% thic3 layer of granular 9u is deposited just

ahead of a bare wire consumable electrode,

and an arc is maintained beneath the blan3et

of 9u with only a few small 9ames being

visible.

% portion of the 9u melts. Molten 9u and

9u provides thermal insulation, slows cooling

rate and produce soft, ductile welds.)ontd6



(ost suita"le or 6at "utt or fllet weldsin low car"on steel )' M car"on*

he process is not recommended or

high-car"on steels4 tool steels4

aluminum4 magnesium4 titanium4 lead4 or

8inc









Characteristic o su"merged arc welding

4igh speeds,

4igh deposition rates,

8eep penetration,4igh cleanliness due to the 9u action!.

&d7antages



&d7antages

Wire electrodes are inepensive.

Co weld spatter.

Cearly $>>@ deposition e7ciency.0esser electrode consumption.

;imitations



;imitations

1tensive 9u handling,

)ontamination of the 9u by moisture.

0arge:grain:si#e structures.Welding is restricted to the hori#ontal position.

)hemical control is important

IES 2!!



IES 2!! The welding process in which bare wire is usedas electrode, granular 9u is used and theprocess is characteri#ed by its high speedwelding, is 3nown asA

a! Shielded arc weldingb! lasma arc welding

c! Submerged arc welding

d! ;as metal arc welding

&t i . d ldi )&.W*



&tomic .ydrogen welding )&.W*

%n a.c. arc is formed between two tungstenelectrodes along which streams of hydrogen are fedto the welding #one. The molecules of hydrogen aredissociated by the high heat of the arc in the gap

between the electrodes. The formation of atomichydrogen proceeds with the absorption of heatA

4' '4 : ='$.' 3 N / mol

This atomic hydrogen recombines to form molecular

hydrogen outside the arc, particularly on therelatively cold surface of the wor3 being welded,releasing the heat gained previouslyA

'4 F ='$.' 3 N / mol.

H2

)ontd6



Temperature of about (>>o).

4ydrogen acts as shielding also.

sed for very thin sheets or small diameter

wires.

0ower thermal e7ciency than %rc welding.

)eramics may be arc welded.%) used.

IES 2A



IES-2A

In atomic hydrogen welding4 hydrogenacts as

a! % heating agent

b! +ne of the gases to generate the 9amec! %n e*ective shielding gas protecting theweld

d! % lubricant to increase the 9owcharacteristics of weld metal



Documents

Ch-19 Gas Welding, Gas Cutting & Arc Welding.pptx