1oxyacetylen cutting and welding

8/7/2019 1oxyacetylen cutting and welding

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Oxyacetylene welding andOxyacetylene welding and

CuttingCutting


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Oxyfuel gas welding (OFW)Oxyfuel gas welding (OFW)

Oxyfuel gas welding (OEW) is a group of welding processesOxyfuel gas welding (OEW) is a group of welding processeswhich join metals by heating with a fuel gas flame or flareswhich join metals by heating with a fuel gas flame or flareswith or without the application of pressure and with or withoutwith or without the application of pressure and with or withoutthe use of filler metal.the use of filler metal.

OFW includes any welding operation that makes use of a fuelOFW includes any welding operation that makes use of a fuelgas combined with oxygen as a heating medium. The processgas combined with oxygen as a heating medium. The processinvolves the melting of the base metal and a filler metal, ifinvolves the melting of the base metal and a filler metal, ifused, by means of the flame produced at the tip of a weldingused, by means of the flame produced at the tip of a weldingtorch.torch.

Fuel gas and oxygen are mixed in the proper proportions in aFuel gas and oxygen are mixed in the proper proportions in amixing chamber which may be part of the welding tipmixing chamber which may be part of the welding tipassembly. Molten metal from the plate edges and filler metal,assembly. Molten metal from the plate edges and filler metal,if used, intermix in a common molten pool. Upon cooling, theyif used, intermix in a common molten pool. Upon cooling, theycoalesce to form a continuous piece.coalesce to form a continuous piece.


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There are Four major processes within thisThere are Four major processes within this

group: Oxyacetylene welding,group: Oxyacetylene welding,

OxyhydrogenOxyhydrogen welding,welding,Pressure gas welding andPressure gas welding and

Welding withWelding with methylacetylenemethylacetylene--

propadienepropadiene gas (MAPP gas)gas (MAPP gas)


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AdvantagesAdvantages

(1)Advantage of this welding process is the control a welder can exercise(1)Advantage of this welding process is the control a welder can exerciseover the rate of heat input, the temperature of the weld zone, and theover the rate of heat input, the temperature of the weld zone, and theoxidizing or reducing potential of the welding atmosphere.oxidizing or reducing potential of the welding atmosphere.

(2) Weld bead size and shape and weld puddle viscosity are also controlled(2) Weld bead size and shape and weld puddle viscosity are also controlledin the welding process because the filler metal is added independently ofin the welding process because the filler metal is added independently of

the welding heat source.the welding heat source. (3) OFW is ideally suited to the welding of thin sheet, tubes, and small(3) OFW is ideally suited to the welding of thin sheet, tubes, and small

diameter pipe. It is also used for repair welding. Thick section welds,diameter pipe. It is also used for repair welding. Thick section welds,except for repair work, are not economical.except for repair work, are not economical.

(4)The welding is portable,i.e welding can be done at the site.(4)The welding is portable,i.e welding can be done at the site.

(5)The same equipment with the range of torch can be used for oxygen(5)The same equipment with the range of torch can be used for oxygen

cutting,brazing as well as braze welding.cutting,brazing as well as braze welding. (6)Since souce of heat is seperated from filler rod,filler metal can be(6)Since souce of heat is seperated from filler rod,filler metal can be

properly controlled and heat properly adjusted giving rise to satisfactoryproperly controlled and heat properly adjusted giving rise to satisfactorybead.bead.


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ApparatusApparatus

The apparatus used in gas welding consists basically ofThe apparatus used in gas welding consists basically of

an oxygen source and a fuel gas source (usuallyan oxygen source and a fuel gas source (usually cylinderscylinders),),

twotwopressurepressure regulatorsregulators and two flexible hoses (one of each forand two flexible hoses (one of each for

each cylinder), andeach cylinder), and a torch.a torch.

This sort of torch can also be used forThis sort of torch can also be used forsolderingsoldering andandbrazingbrazing..The cylinders are often carried in a special wheeledThe cylinders are often carried in a special wheeled trolleytrolley..

There are also examples of pressurized liquid fuel cuttingThere are also examples of pressurized liquid fuel cutting

torches, usually using gasoline. These are used for theirtorches, usually using gasoline. These are used for theirincreased portabilityincreased portability


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RegulatorRegulatorThe regulator is used to control pressure from theThe regulator is used to control pressure from the

tanks by reducing pressure and regulating flow rate.tanks by reducing pressure and regulating flow rate. Gas hosesGas hoses

The hoses used are specifically designed for weldingThe hoses used are specifically designed for weldingand cutting. The hose is usually a doubleand cutting. The hose is usually a double--hose design,hose design,

meaning that there are two hoses joined together.meaning that there are two hoses joined together. TheThe oxygen hose is greenoxygen hose is green and theand the fuel hose is redfuel hose is red..

The type of gas the hose will be carrying is importantThe type of gas the hose will be carrying is importantbecause the connections will have different threadsbecause the connections will have different threadsfor different types of gas.for different types of gas.

Fuel gases (red) will use leftFuel gases (red) will use left--hand threads and ahand threads and agroove cut into the nut, while the oxygen (green) willgroove cut into the nut, while the oxygen (green) willuse rightuse right--hand threads. This is a safety precaution tohand threads. This is a safety precaution toprevent hoses from being hooked up the wrong way.prevent hoses from being hooked up the wrong way.


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NonNon--return valvereturn valve

Between the regulator and hose, and ideallyBetween the regulator and hose, and ideallybetween hose and torch on both oxygen andbetween hose and torch on both oxygen and

fuel lines, afuel lines, a flashback arrestorflashback arrestorand/or nonand/or non--

return valve should be installed to preventreturn valve should be installed to preventflame or oxygenflame or oxygen--fuel mixture being pushedfuel mixture being pushed

back into either cylinder and damaging theback into either cylinder and damaging the

equipment or making a cylinder explode.equipment or making a cylinder explode.


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TorchesTorches TheThe torchtorch is the part that the welder holds andis the part that the welder holds and

manipulates to make the weld.manipulates to make the weld.

It has a connection and valve for the fuel gasIt has a connection and valve for the fuel gasand a connection and valve for the oxygen, aand a connection and valve for the oxygen, a

handle for the welder to grasp, a mixinghandle for the welder to grasp, a mixing

chamber (set at an angle) where the fuel gaschamber (set at an angle) where the fuel gas

and oxygen mix, with a tip where the flameand oxygen mix, with a tip where the flameforms.forms.


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Welding torchWelding torch

A welding torch head is used to weld metals. ItA welding torch head is used to weld metals. It

can be identified by having only one or twocan be identified by having only one or twopipes running to the nozzle and no oxygenpipes running to the nozzle and no oxygen--

blast triggerblast trigger


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Typical torch stylesTypical torch styles

A small welding torch, with throttle valvesA small welding torch, with throttle valves

located at the front end of the handle. Ideallylocated at the front end of the handle. Ideally

suited tosuited to sheet metal weldingsheet metal welding. Can be fitted. Can be fitted

with cuttingwith cutting

attachment in place of the welding headattachment in place of the welding head

shown. Welding torches of this general designshown. Welding torches of this general design

are by far the most widely used. They willare by far the most widely used. They will

handle any oxyacetylene welding job, can behandle any oxyacetylene welding job, can be

fitted with multiflame (Rosebud) heads forfitted with multiflame (Rosebud) heads for

heating applications, and accommodateheating applications, and accommodatecutting attachments that will cut steel 6 in.cutting attachments that will cut steel 6 in.

thick.thick.

A fullA full--size oxygen cutting torch which has allsize oxygen cutting torch which has all

valves located in its rear body. Another stylevalves located in its rear body. Another style

of cutting torch, with oxygen valves located atof cutting torch, with oxygen valves located at

the front end of its handle.the front end of its handle.


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hoice of fuel gasChoice of fuel gas

Fuel gas combustion occurs in two distinct zones. In the inner cone orFuel gas combustion occurs in two distinct zones. In the inner cone orprimary flame, the fuel gas combines with oxygen to form carbonprimary flame, the fuel gas combines with oxygen to form carbonmonoxide and hydrogen which for acetylene, the reaction is given bymonoxide and hydrogen which for acetylene, the reaction is given by

2C2C 22 HH 22 + 2O+ 2O 22 ----4CO + 2H4CO + 2H 22 Combustion also continues in the secondary or outer zone of the flame withCombustion also continues in the secondary or outer zone of the flame with

oxygen being supplied from the air.oxygen being supplied from the air.

4CO+2H4CO+2H22 +3O+3O22 --4CO4CO22 +2H+2H22OOThus, fuel gases are characterised by theirThus, fuel gases are characterised by their

flame temperatureflame temperature -- the hottest part of the flame is at the tip ofthe hottest part of the flame is at the tip ofthe primary flame (inner cone)the primary flame (inner cone)

fuel gas to oxygen ratiofuel gas to oxygen ratio -- the amount of fuel gas required forthe amount of fuel gas required forcombustion but this will vary according to whether the flame is neutral,combustion but this will vary according to whether the flame is neutral,oxidising or reducingoxidising or reducing

heat of combustionheat of combustion -- heat of combustion is greater in the outerheat of combustion is greater in the outerpart of the flamepart of the flame


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Commonly used fuel gasesCommonly used fuel gases

The five most commonly used fuel gases areThe five most commonly used fuel gases are

Acetylene,Acetylene,

HydrogenHydrogen

Propane,Propane,

MAPP (methylacetyleneMAPP (methylacetylene--propadiene),propadiene),

Propylene andPropylene and

Natural gas.Natural gas.

The relative performance of the fuel gases in terms of pierceThe relative performance of the fuel gases in terms of piercetime, cutting speed and cut edge quality, is determined by thetime, cutting speed and cut edge quality, is determined by theflame temperature and heat distribution within the inner andflame temperature and heat distribution within the inner andouter flame conesouter flame cones


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AcetyleneAcetylene

Acetylene produces theAcetylene produces the highest flame temperaturehighest flame temperature of all theof all thefuel gases. The maximum flame temperature for acetylene (infuel gases. The maximum flame temperature for acetylene (inoxygen) is approximatelyoxygen) is approximately 3,1603,160CC compared with a maximumcompared with a maximumtemperature of 2,810temperature of 2,810C with propane.C with propane.

The hotter flame produces more rapid piercing of the materialsThe hotter flame produces more rapid piercing of the materials

with the pierce time being typically one third that producedwith the pierce time being typically one third that producedwith propane.with propane.

The higher flame speed (7.4m/s compared with 3.3m/s forThe higher flame speed (7.4m/s compared with 3.3m/s forpropane) and the higher calorific value of the primary flamepropane) and the higher calorific value of the primary flame(inner cone) (18,890kJ/m 3 compared with 10,433 kJ/m 3 for(inner cone) (18,890kJ/m 3 compared with 10,433 kJ/m 3 for

propane) produce a more intense flame at the surface of thepropane) produce a more intense flame at the surface of themetal reducing the width of the Heat Affected Zone (HAZ)metal reducing the width of the Heat Affected Zone (HAZ)and the degree of distortion.and the degree of distortion.


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HydrogenHydrogen

HydrogenHydrogen has a clean flame and is good for use onhas a clean flame and is good for use on aluminiumaluminium. It can be used at a higher. It can be used at a higherpressure than acetylene and is therefore useful for underwater welding and cutting.pressure than acetylene and is therefore useful for underwater welding and cutting.

It is a good type of flame to use when heating much material. The flame temperature is high,It is a good type of flame to use when heating much material. The flame temperature is high,about 2,000about 2,000 C for hydrogen gas in air at atmospheric pressure, and up to 2800C for hydrogen gas in air at atmospheric pressure, and up to 2800 C when preC when pre--mixed in a 2:1 ratio with pure oxygen (mixed in a 2:1 ratio with pure oxygen (oxyhydrogenoxyhydrogen).).

For some oxyhydrogen torches the oxygen and hydrogen are produced byFor some oxyhydrogen torches the oxygen and hydrogen are produced by electrolysiselectrolysis ofofwater in an apparatus which is connected directly to the torch.water in an apparatus which is connected directly to the torch.

The oxygen and the hydrogen are led off the electrolysis cell separately and are fed into theThe oxygen and the hydrogen are led off the electrolysis cell separately and are fed into the

two gas connections of an ordinary oxytwo gas connections of an ordinary oxy--gas torch. This happens in thegas torch. This happens in the water torchwater torch, which is, which issometimes used in small torches used in makingsometimes used in small torches used in makingjewelryjewelry andand electronicselectronics..

The oxygen and hydrogen are drawn mixed from the electrolysis cell and are led mixed into aThe oxygen and hydrogen are drawn mixed from the electrolysis cell and are led mixed into aspecial torch designed to prevent flashbackspecial torch designed to prevent flashback

The weld pool is not protected from the atmosphere when the oxygen for combustion isThe weld pool is not protected from the atmosphere when the oxygen for combustion iscompletely provided by torch itself.So generally oxygen in an amount slightly less than thatcompletely provided by torch itself.So generally oxygen in an amount slightly less than thatrequired for complete combustion,whereas the atmospheric oxygen accounts for the burningrequired for complete combustion,whereas the atmospheric oxygen accounts for the burningof remaining hydrogen.This gives rise to protective preheating flame that surrounds mainof remaining hydrogen.This gives rise to protective preheating flame that surrounds main

flame.B

ut this reduces flame temperature.flame.B

ut this reduces flame temperature. Because of lower flame temperature,the oxy hyrogen welding is generally slow.Because of lower flame temperature,the oxy hyrogen welding is generally slow.

It is normally used to weld thin sheets of steels and alloys with low melting point.It is normally used to weld thin sheets of steels and alloys with low melting point.


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ropaneropane

Propane produces a lower flame temperature than acetylenePropane produces a lower flame temperature than acetylene

(the maximum flame temperature in oxygen is(the maximum flame temperature in oxygen is 2,8282,828CCcompared with 3,160compared with 3,160C for acetylene).C for acetylene).

It has a greater total heat of combustion than acetylene but theIt has a greater total heat of combustion than acetylene but theheat is generated mostly in the outer cone .heat is generated mostly in the outer cone .

The characteristic appearance of the flames for acetylene andThe characteristic appearance of the flames for acetylene and

propane are shown inpropane are shown inFigs.2 and 3Figs.2 and 3 where the propane flamewhere the propane flameappears to be less focused. Consequently, piercing is muchappears to be less focused. Consequently, piercing is muchslower but as the burning and slag formation are effected byslower but as the burning and slag formation are effected bythe oxygen jet, cutting speeds are about the same as forthe oxygen jet, cutting speeds are about the same as foracetylene.acetylene.

Propane has a greater stoichiometric oxygen requirement thanPropane has a greater stoichiometric oxygen requirement thanacetylene; for the maximum flame temperature in oxygen, theacetylene; for the maximum flame temperature in oxygen, theratio of the volume of oxygen to fuel gas are 1.2 to 1 forratio of the volume of oxygen to fuel gas are 1.2 to 1 foracetylene and 4.3 to 1 for propaneacetylene and 4.3 to 1 for propane


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Fig.2. Ocyacetylene gas jet and nozzle de

Fig.3. Propane gas jet and nozzle design


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MAPPMAPP

MAPP gas is a mixture of various hydrocarbons, principally,MAPP gas is a mixture of various hydrocarbons, principally,methylacetylene and propadiene.methylacetylene and propadiene.

It produces a relatively hot flame (2,976It produces a relatively hot flame (2,976C) with a high heat release in theC) with a high heat release in theprimary flame (inner cone) (15,445kJ/m 3 ), less than for acetyleneprimary flame (inner cone) (15,445kJ/m 3 ), less than for acetylene(18,890kJm 3 ) but much higher than for propane (10,433kJm 3 ).(18,890kJm 3 ) but much higher than for propane (10,433kJm 3 ).

The secondary flame (outer cone) also gives off a high heat release, similarThe secondary flame (outer cone) also gives off a high heat release, similarto propane and natural gas. The combination of a lower flame temperature,to propane and natural gas. The combination of a lower flame temperature,more distributed heat source and larger gas flows compared with acetylenemore distributed heat source and larger gas flows compared with acetyleneresults in a substantially slower pierce time.results in a substantially slower pierce time.

As MAPP gas can be used at a higher pressure than acetylene, it can beAs MAPP gas can be used at a higher pressure than acetylene, it can beused for underwater cutting in deep water as it is less likely to dissociateused for underwater cutting in deep water as it is less likely to dissociateinto its components of carbon and hydrogen which are explosive.into its components of carbon and hydrogen which are explosive.


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Natural GasNatural Gas

Natural gas has the lowest flame temperatureNatural gas has the lowest flame temperature

similar to propane and the lowest total heatsimilar to propane and the lowest total heatvalue of the commonly used fuel gases, eg forvalue of the commonly used fuel gases, eg for

the inner flame 1,490kJ/mthe inner flame 1,490kJ/m33 compared withcompared with

18,890kJ/m18,890kJ/m33 for acetylene. Consequently,for acetylene. Consequently,

natural gas is the slowest for piercing.natural gas is the slowest for piercing.


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Types of flameTypes of flame The welder can adjust the oxyThe welder can adjust the oxy--acetylene flame to beacetylene flame to be carbonizingcarbonizing

(reducing), neutral, or oxidizing.(reducing), neutral, or oxidizing. Adjustment is made by adding more orAdjustment is made by adding more or

less oxygen to the acetylene flame.less oxygen to the acetylene flame. TheThe neutral flameneutral flame is the flame most generally used when welding oris the flame most generally used when welding or

cutting. The welder uses the neutral flame as the starting point for all othercutting. The welder uses the neutral flame as the starting point for all otherflame adjustments because it is so easily defined.flame adjustments because it is so easily defined.

This flame is attained when welders, as they slowly open the oxygen valveThis flame is attained when welders, as they slowly open the oxygen valveon the torch body, first see only two flame zones. At that point, theon the torch body, first see only two flame zones. At that point, the

acetylene is being completely burned in the welding oxygen andacetylene is being completely burned in the welding oxygen andsurrounding air. The flame is chemically neutral.surrounding air. The flame is chemically neutral.

The two parts of this flame are the light blue inner cone and the darker blueThe two parts of this flame are the light blue inner cone and the darker blueto colorless outer cone. The inner cone is where the acetylene and theto colorless outer cone. The inner cone is where the acetylene and theoxygen combine. The tip of this inner cone is the hottest part of the flame.oxygen combine. The tip of this inner cone is the hottest part of the flame.It is approximately 6000 degrees F and provides enough heat to easily meltIt is approximately 6000 degrees F and provides enough heat to easily meltsteel . In the inner cone the acetylene breaks down and partly burns tosteel . In the inner cone the acetylene breaks down and partly burns to

hydrogenhydrogen andand carbon monoxidecarbon monoxide, which in the outer cone combine with, which in the outer cone combine withmore oxygen from the surrounding air and burnmore oxygen from the surrounding air and burn


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Parts of the OxyParts of the Oxy--Acetylene FlameAcetylene Flame


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In the inner cone or primary flame, the fuel gasIn the inner cone or primary flame, the fuel gas

combines with oxygen to form carboncombines with oxygen to form carbon

monoxide and hydrogen which for acetylene,monoxide and hydrogen which for acetylene,

the reaction is given bythe reaction is given by

2C2C22HH22 + 2O+ 2O 22 4CO + 2H4CO + 2H22

Combustion also continues in the secondary orCombustion also continues in the secondary or

outer zone of the flame with oxygen beingouter zone of the flame with oxygen being

supplied from the air.supplied from the air. 4CO+2H4CO+2H22 +3O+3O22 4CO4CO22 +2H+2H22OO


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Chemistry of the FlameChemistry of the Flame

Inner Cone (primaryInner Cone (primary

combustion)combustion)

CC22HH22 + O+ O22 HH22 + 2CO+ 2CO

Envelope (secondaryEnvelope (secondarystage of combustion)stage of combustion)

2 CO + O2 CO + O 2CO2CO22

2H2H22 + O+ O22 2H2H22OO


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Neutral FlameNeutral Flame

Has only an inner coneHas only an inner cone

and an envelopeand an envelope

60006000F at the end of theF at the end of the

inner coneinner cone

Inner cone has roundedInner cone has rounded

blunt shapeblunt shape

Neutral flame is used for most welding

operations and is highly

suited for welding mild steels and cast irons,

stainless steel,copper and aluminium. Even

during flame cutting of steels, the

pre-heating flame may be a neutral flame.


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Carburising flameCarburising flame AnAn excess of acetyleneexcess of acetylene creates a carbonizing flame. This flamecreates a carbonizing flame. This flame

is characterized by three flame zones; the hot inner cone, ais characterized by three flame zones; the hot inner cone, awhitewhite--hot "acetylene feather", and the bluehot "acetylene feather", and the blue--colored outer cone.colored outer cone.

The unburned carbon insulates the flame and drops theThe unburned carbon insulates the flame and drops thetemperature to approximately 5000 degrees F. The reducingtemperature to approximately 5000 degrees F. The reducingflame is typically used forflame is typically used forhardfacinghardfacing operations or backhandoperations or backhand

pipe welding techniques.pipe welding techniques. The feather is caused by incomplete combustion of theThe feather is caused by incomplete combustion of the

acetylene to cause an excess of carbon in the flame. Some ofacetylene to cause an excess of carbon in the flame. Some ofthis carbon is dissolved by the molten metal to carbonize it.this carbon is dissolved by the molten metal to carbonize it.The carbonizing flame will tend to remove the oxygen fromThe carbonizing flame will tend to remove the oxygen from

iron oxides which may be present, a fact which has caused theiron oxides which may be present, a fact which has caused theflame to be know as a "reducing flameflame to be know as a "reducing flame


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Carburizing FlameCarburizing Flame

Flame includes theFlame includes the

acetylene featheracetylene feather

55005500 F F

Length of feather variesLength of feather varies

by the amount ofby the amount of

acetyleneacetylene


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A carburising flame is mostly used for welding aluminium,

monel metal, stainless steel, white metal, die cast metals and

several other non-ferrous metals besides the high carbon steels.

The carburising flame prevents excessive formation of oxideson non-ferrous metals, which interfere with proper fusion of

metal since oxides of non-ferrous metals have very high

melting points because of which they are difficult to melt by

oxy-acetylene flames.

Carburizing flame gives a slight casehardening effect oncertain steels. It is also used for hard facing of steels withsatellite rods.


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Oxidizing flameOxidizing flame

The oxidizing flame is the third possible flame adjustment.The oxidizing flame is the third possible flame adjustment.

It occurs when the ratio of oxygen to acetylene required for a neutral flameIt occurs when the ratio of oxygen to acetylene required for a neutral flamehas been changed to give an excess of oxygen. This flame type is observedhas been changed to give an excess of oxygen. This flame type is observedwhen welders add more oxygen to the neutral flame.when welders add more oxygen to the neutral flame.

This flame is hotter than the other two flames because the combustibleThis flame is hotter than the other two flames because the combustible

gases will not have to search so far to find the necessary amount of oxygen,gases will not have to search so far to find the necessary amount of oxygen,nor heat up as much thermally inert carbon.nor heat up as much thermally inert carbon. [1][1]

It is called an oxidizing flame because of its effect on metal. This flameIt is called an oxidizing flame because of its effect on metal. This flameadjustment is generally not preferred.adjustment is generally not preferred.

The oxidizing flameThe oxidizing flame creates undesirable oxides to the structural andcreates undesirable oxides to the structural andmechanical detriment of most metalsmechanical detriment of most metals. In an oxidizing flame, the inner cone. In an oxidizing flame, the inner coneacquires a purplish tinge, gets pinched and smaller at the tip, and the soundacquires a purplish tinge, gets pinched and smaller at the tip, and the soundof the flame gets harsh.of the flame gets harsh.

A slightly oxidizing flame is used in brazeA slightly oxidizing flame is used in braze--welding and bronzewelding and bronze--surfacingsurfacingwhile a more strongly oxidizing flame is used in fusion welding certainwhile a more strongly oxidizing flame is used in fusion welding certainbrasses and bronzesbrasses and bronzes [1][1]


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Oxidizing FlameOxidizing Flame

Has only inner cone andHas only inner cone andenvelopeenvelope

Inner cone is short andInner cone is short and

pointed or sharppointed or sharp Inner cone is very brightInner cone is very bright

whitewhite

Has a roar or hissingHas a roar or hissing

soundsound 63006300 F F


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The oxidising flame is used where maximum temperature is

desired or in situations where oxidising effect is not harmful,

rather proves beneficial, for example, a slightly oxidisingflame is

used in welding of non-ferrous metals particularly copper base

metals as brasses & bronzes and zinc base metals, where it is

desirable to have oxidising flame giving oxide film to check

vaporisation of zinc and also to reduce further oxidation after

oxide film is formed. This flame is also used for pre-heating

purposes during flame cutting of steels.


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Chemistry of the FlameChemistry of the Flame


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Acetylene storageAcetylene storage Free acetylene is highly explosive,if stored ar presureFree acetylene is highly explosive,if stored ar presure

more than 200kpa,where it becomes unstable andmore than 200kpa,where it becomes unstable and

likely to explode.likely to explode.

Henece acetylene is to be carefully stored in strongHenece acetylene is to be carefully stored in strong

cylinder,filled with 80cylinder,filled with 80--85% porous material such as85% porous material such ascalcium silicate and then filled with actone which cancalcium silicate and then filled with actone which can

absorb upto 420 timesits volume at acetylene atabsorb upto 420 timesits volume at acetylene at

pressure of 1.75 Mpa.pressure of 1.75 Mpa.

Acetylene molecules fir inbetween the acetoneAcetylene molecules fir inbetween the acetonemolecules.This help in storing acetylene at muchmolecules.This help in storing acetylene at much

higher pressure then permitted when it is in free form.higher pressure then permitted when it is in free form.


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Acetylene would be released from acetone at a slowAcetylene would be released from acetone at a slow

rate and thus would not form any pockets of highrate and thus would not form any pockets of highpressure acetylene.pressure acetylene.

The rate of release depends on temp. of gas.hence rateThe rate of release depends on temp. of gas.hence rate

of consumption should be under rate of release whichof consumption should be under rate of release which

is normally about 1/7is normally about 1/7thth of capacity of cylinder perof capacity of cylinder perhour.hour.

However when acetylene is drawn at rapidHowever when acetylene is drawn at rapid

rate,acetone may also come along with acetylene.rate,acetone may also come along with acetylene.

The presence of acetone in flame would give it purpleThe presence of acetone in flame would give it purple

colour.It is not desirable as it reduces flamecolour.It is not desirable as it reduces flame

temperature.temperature.


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Acetylene GeneratorAcetylene Generator

It is possible to have acetylene generator in theIt is possible to have acetylene generator in the

place of acetylene cylinderplace of acetylene cylinder

Acetylene is normally produced by reactionAcetylene is normally produced by reaction

between calcium carbide and water which isbetween calcium carbide and water which is

instantaneous as shown belowinstantaneous as shown below

CaCCaC22 + H+ H22OO CC22 HH22 + Ca (OH)+ Ca (OH) 22

Acetylene generator consist of cylinder whichAcetylene generator consist of cylinder which

is partially filled by water. The Calciumis partially filled by water. The Calciumcarbide is stored in Hopper near the top ofcarbide is stored in Hopper near the top of

generatorgenerator


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A pressure regulated valve controls the flow ofA pressure regulated valve controls the flow of

calcium carbide into water,depending on pressure ofcalcium carbide into water,depending on pressure ofacetylene in the generator.acetylene in the generator.

The acetylene is taken out through the gas pipe.The acetylene is taken out through the gas pipe.

The generator would be permitted to produceThe generator would be permitted to produce

acetylene to safe pressure of 100kPa.acetylene to safe pressure of 100kPa. Oxygen and acetylene from two cylinders are broughtOxygen and acetylene from two cylinders are brought

through two separate hose pipes to welding torch.through two separate hose pipes to welding torch.

In torch two gases are mixed and flowed out throughIn torch two gases are mixed and flowed out through

nozzle at the torch tip.nozzle at the torch tip.


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Oxy Acetylene Welding TechniqueOxy Acetylene Welding Technique

To light the flame,the acetylene valve on the torch isTo light the flame,the acetylene valve on the torch isopened slightly and lightened with the help of frictionopened slightly and lightened with the help of frictionspark lighterspark lighter

The flame draws oxygen from atmospheric airandThe flame draws oxygen from atmospheric airandthus results in reducing flame.thus results in reducing flame.

Then acetylene valve is opened to get required flowThen acetylene valve is opened to get required flowof acetyleneof acetylene

The oxygen valve is then slowly opened till theThe oxygen valve is then slowly opened till the

intermediate flame feather of reducing flame recedesintermediate flame feather of reducing flame recedesinto the inner white cone.into the inner white cone.

The actual adjustment of flame depend on type ofThe actual adjustment of flame depend on type ofmaterial to be joined.material to be joined.


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The choice of torch size depends on thicknessThe choice of torch size depends on thickness

of the metal .of the metal . Larger torch tip sizes cause higher amount ofLarger torch tip sizes cause higher amount of

oxygen and fuel to flow out causing release ofoxygen and fuel to flow out causing release of

more heat. Thus for thicker section larger tipmore heat. Thus for thicker section larger tipradii are usedradii are used

Except for outside corner joint s all other jointsExcept for outside corner joint s all other jointsrequire filler metal to be added to fill the joint.require filler metal to be added to fill the joint.

This is done with welding rod ,whoseThis is done with welding rod ,whosecomposition depends on the parent metal ofcomposition depends on the parent metal ofthe joint.the joint.


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Oxyfuel CuttingOxyfuel Cutting

The oxyfuel process is the most widely appliedThe oxyfuel process is the most widely appliedindustrial thermal cutting process because itindustrial thermal cutting process because itcan cut thicknesses from 0.5mm to 250mm,can cut thicknesses from 0.5mm to 250mm,

the equipment is low cost and can be usedthe equipment is low cost and can be usedmanually or mechanised.manually or mechanised.

There are several fuel gas and nozzle designThere are several fuel gas and nozzle designoptions that can significantly enhanceoptions that can significantly enhanceperformance in terms of cut quality and cuttingperformance in terms of cut quality and cuttingspeed.speed.


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Fig.1. Diagram of oxyacetylene cutting proces


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The cutting process is illustrated inThe cutting process is illustrated inFig. 1Fig. 1. Basically,. Basically,a mixture of oxygen and the fuel gas is used toa mixture of oxygen and the fuel gas is used topreheat the metal to its 'ignition' temperature which,preheat the metal to its 'ignition' temperature which,for steel, isfor steel, is 700700CC -- 900900CC (bright red heat) but well(bright red heat) but well

below its melting point.below its melting point. A jet of pure oxygen is then directed into theA jet of pure oxygen is then directed into the

preheated areapreheated area instigating a vigorous exothermicinstigating a vigorous exothermicchemical reactionchemical reactionbetween the oxygen and the metalbetween the oxygen and the metalto form iron oxide or slag.to form iron oxide or slag.

The oxygen jet blows away the slag enabling the jetThe oxygen jet blows away the slag enabling the jetto pierce through the material and continue to cutto pierce through the material and continue to cutthrough the material.through the material.


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Reactions Occuring During CuttingReactions Occuring During Cutting

After steel is heated to its kindling temperatureAfter steel is heated to its kindling temperature

which is about 8700C ,it gets readily combinedwhich is about 8700C ,it gets readily combined

with oxygen giving iron oxide with thewith oxygen giving iron oxide with the

following reactionsfollowing reactions

3Fe + 2O3Fe + 2O22----FeFe33OO44+6.67MJ/Kg of Iron+6.67MJ/Kg of Iron

Other reactions taking place are:Other reactions taking place are:

2Fe+O2Fe+O222FeO+3.18MJ/Kg of Iron2FeO+3.18MJ/Kg of Iron

4Fe +3O4Fe +3O22----2Fe2Fe22OO33+4.9 MJ/Kg of Iron+4.9 MJ/Kg of Iron


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Cutting torchCutting torch

A cutting torch head is used to cutA cutting torch head is used to cut metalmetal. It is similar to a welding torch.. It is similar to a welding torch.OxygenOxygen is combined with the acetylene in the torch, which produces a highis combined with the acetylene in the torch, which produces a hightemperaturetemperature flameflame. It can be identified by having three pipes that go to a 90. It can be identified by having three pipes that go to a 90degree nozzle and by the oxygendegree nozzle and by the oxygen--blast trigger.blast trigger.

The metal, (only iron and steel can be cut with this method) is heated untilThe metal, (only iron and steel can be cut with this method) is heated untilit is cherry red, once this temperature is attained, gently at first oxygen isit is cherry red, once this temperature is attained, gently at first oxygen isapplied by pressing the "oxygenapplied by pressing the "oxygen--blast trigger" this oxygen reacts with theblast trigger" this oxygen reacts with themetal forming iron oxide and producing heat. It is this "heat" whichmetal forming iron oxide and producing heat. It is this "heat" whichcontinues the cutting process. The cutting torch only heats the metal to startcontinues the cutting process. The cutting torch only heats the metal to startthe process, further heat is provided by the "burning metal".the process, further heat is provided by the "burning metal".

The melting point of the iron oxide is around half of that of the metal, asThe melting point of the iron oxide is around half of that of the metal, asthe metal burns, it immediately turns to liquid Iron oxide and flows awaythe metal burns, it immediately turns to liquid Iron oxide and flows awayfrom the cutting zone. However some of the Iron Oxide remains on thefrom the cutting zone. However some of the Iron Oxide remains on thework piece forming a hard "slag" which can be removed by gentle tapping,work piece forming a hard "slag" which can be removed by gentle tapping,and/or a grinder.and/or a grinder.


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The tip is provided with a center hole throughThe tip is provided with a center hole through

which a jet of pure oxygen passes.which a jet of pure oxygen passes.

Mixed oxygen and acetylene pass throughMixed oxygen and acetylene pass throughholes surrounding the center holes for theholes surrounding the center holes for the

preheating flames.preheating flames.


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There is a wide variety of cutting tip styles and sizesThere is a wide variety of cutting tip styles and sizesavailable to suit various types of work.available to suit various types of work.

The thickness of the material to be cut generallyThe thickness of the material to be cut generally

governs the selection of the tip.governs the selection of the tip. The cutting oxygen pressure, cutting speed, andThe cutting oxygen pressure, cutting speed, and

preheating intensity should be controlled to producepreheating intensity should be controlled to producenarrow, parallel sided kerfs.narrow, parallel sided kerfs.

Cuts that are improperly made will produce ragged,Cuts that are improperly made will produce ragged,irregular edges with adhering slag at the bottom ofirregular edges with adhering slag at the bottom ofthe plates.the plates.


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NozzleNozzleThe primary functions of the nozzle are to provide:The primary functions of the nozzle are to provide:

a method of preheating the metal to its ignition temperaturea method of preheating the metal to its ignition temperature a jet of oxygen to react with the material to be cut and at a flow rate sufficient toa jet of oxygen to react with the material to be cut and at a flow rate sufficient to

blow away the slagblow away the slag

Each torch should be fitted with the appropriate nozzle for the type of fuel gas.Each torch should be fitted with the appropriate nozzle for the type of fuel gas.

The nozzle type will depend on:The nozzle type will depend on:

fuel gasfuel gas

manual or machine operationmanual or machine operation

manufacturer's preferencemanufacturer's preference

Acetylene nozzles are usually oneAcetylene nozzles are usually one--piece but twopiece but two--piece nozzles similar to those forpiece nozzles similar to those forother fuel gases are produced for machine cutting.other fuel gases are produced for machine cutting.

The diameter of the cutting oxygen hole is selected according to the materialThe diameter of the cutting oxygen hole is selected according to the materialthickness. There are two types of nozzle; standard and high speed. The standardthickness. There are two types of nozzle; standard and high speed. The standard

nozzle usually has a parallel sided, central bore for the oxygen jet, which isnozzle usually has a parallel sided, central bore for the oxygen jet, which issurrounded by an annulus or a ring of smaller diameter ports for the presurrounded by an annulus or a ring of smaller diameter ports for the pre--heating gasheating gasmixture, Fig. 2. There are many designs and arrangements of the preheating portsmixture, Fig. 2. There are many designs and arrangements of the preheating portsthat focus the flame for heating and to protect the oxygen jet from air entrainment.that focus the flame for heating and to protect the oxygen jet from air entrainment.


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Standard nozzle with central bore forStandard nozzle with central bore for

oxygen jet and a ring of ports for the preoxygen jet and a ring of ports for the pre--

heating gas mixtureheating gas mixture


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DRAGDRAG

Drag is amount by which lower edge of dragDrag is amount by which lower edge of drag

line trails from the top edge.line trails from the top edge.

A good cut is characterised by very small orA good cut is characterised by very small ornegligible drag.negligible drag.


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Cutting conditions are normally set to produce an acceptableCutting conditions are normally set to produce an acceptablecut surface finish for the application but at the highest cuttingcut surface finish for the application but at the highest cuttingspeed. It is, therefore, essential that consideration is given tospeed. It is, therefore, essential that consideration is given tothe following settings for the material thickness and the cuttingthe following settings for the material thickness and the cuttingspeed:speed:

nozzle distancenozzle distance -- too high ortoo lowwill disturboxygen flowtoo high ortoo lowwill disturboxygen flowpreheat flamepreheat flame -- too high a flow can cause top edge meltingtoo high a flow can cause top edge meltingcutting oxygencutting oxygen -- too low a flow can causepoorslagremovaltoo low a flow can causepoorslagremoval--too high a flow can resultin poorcutfinishtoo high a flow can resultin poorcutfinish

The typical appearances of a good and poor quality cut surfaceThe typical appearances of a good and poor quality cut surfacefor manual cutting are shown in Fig. The principal features arefor manual cutting are shown in Fig. The principal features are

described together with their cause and remedial measuresdescribed together with their cause and remedial measuresnecessary to produce the ideal square edge, smooth surfacenecessary to produce the ideal square edge, smooth surfacecut.cut.


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Surface Appearance Features CauseSurface Appearance Features Cause

Ideal Cut ProfileIdeal Cut Profile

Square edge, smooth cut surface, underside free of slag, small drag lines


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Cutting Too FastCutting Too Fast

When torch is moved too rapidly,the metal atWhen torch is moved too rapidly,the metal at

bottom does not get sufficient heat to getbottom does not get sufficient heat to get

oxidised and cut hence there is large dragoxidised and cut hence there is large drag

The result is coarse drag lines at angle toThe result is coarse drag lines at angle to

surface with excessive amount of slag stickingsurface with excessive amount of slag sticking

to bottom edge of plateto bottom edge of plate

Oxygen jet trailing with insufficient oxygen reaching bottom of the cut


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Too high nozzle to plate distanceToo high nozzle to plate distance

Uneven cut surface with heavy melting of topUneven cut surface with heavy melting of top

edge, coarse drag lines at bottom cut surfaceedge, coarse drag lines at bottom cut surface

Preheat is not focused on plate surface, oxygen jet easily disturbed


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Too High Oxygen FlowToo High Oxygen Flow

Excessive slag adhering to cut face, localExcessive slag adhering to cut face, local

gouging, excessive top edge meltinggouging, excessive top edge melting

Turbulence between the preheat flame and the cutting jet


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Slow Movement ofTorchSlow Movement ofTorch

When torch is moved slowly,all the preheatedWhen torch is moved slowly,all the preheated

metal is burnt awayby oxygen jet and largemetal is burnt awayby oxygen jet and large

amount of slag is generated.amount of slag is generated.

Since material is not preheated,oxygen supplySince material is not preheated,oxygen supply

has to be cut off to continue the cutting.has to be cut off to continue the cutting.

Jet is started only after material reachesJet is started only after material reaches

kindling temperature.kindling temperature.

This starting OFF and ON of oxygen jet givesThis starting OFF and ON of oxygen jet givesrise to irregular cut.rise to irregular cut.


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Requirements for oxyRequirements for oxy--fuel cuttingfuel cutting

The ignition temperature of the material must be lower than itsThe ignition temperature of the material must be lower than itsmelting point otherwise the material would melt and flowmelting point otherwise the material would melt and flowaway before cutting could take placeaway before cutting could take place

The oxide melting point must be lower than that of theThe oxide melting point must be lower than that of thesurrounding material so that it can be mechanically blownsurrounding material so that it can be mechanically blownaway by the oxygen jetaway by the oxygen jet

The oxidation reaction between the oxygen jet and the metalThe oxidation reaction between the oxygen jet and the metalmust be sufficient to maintain the ignition temperaturemust be sufficient to maintain the ignition temperature

minimum of gaseous reaction products should be produced sominimum of gaseous reaction products should be produced soas not to dilute the cutting oxygenas not to dilute the cutting oxygen


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As stainless steel, cast iron and nonAs stainless steel, cast iron and non--ferrousferrousmetals form refractory oxides ie the oxidemetals form refractory oxides ie the oxide

melting point is higher than the material,melting point is higher than the material,

powder must be injected into the flame to formpowder must be injected into the flame to form

a low melting point, fluid slag.a low melting point, fluid slag.


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Purity of oxygenPurity of oxygen

The cutting speed and cut edge quality areThe cutting speed and cut edge quality areprimarily determined by the purity of theprimarily determined by the purity of theoxygen stream. Thus, nozzle design plays aoxygen stream. Thus, nozzle design plays a

significant role in protecting the oxygen streamsignificant role in protecting the oxygen streamfrom air entrainment.from air entrainment.

The purity of oxygen should be at least 99.5%.The purity of oxygen should be at least 99.5%.A decrease in purity of 1% will typicallyA decrease in purity of 1% will typicallyreduce the cutting speed by 25% and increasereduce the cutting speed by 25% and increasethe gas consumption by 25%.the gas consumption by 25%.


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Precaution during OxyacetylenePrecaution during Oxyacetylene

cuttingcutting It is worth noting several things at this point:It is worth noting several things at this point:

The oxygen flowrate is criticalThe oxygen flowrate is critical too little will make a slow ragged cut; too muchtoo little will make a slow ragged cut; too muchwill waste oxygen and produce a wide concave cut. Oxygen Lances and otherwill waste oxygen and produce a wide concave cut. Oxygen Lances and othercustom made torches do not have a separate pressure control for the cutting oxygen,custom made torches do not have a separate pressure control for the cutting oxygen,so the cutting oxygen pressure must be controlled using the oxygen regulator. Theso the cutting oxygen pressure must be controlled using the oxygen regulator. Theoxygen cutting pressure should match the cutting tip oxygen orifice. Consult the tipoxygen cutting pressure should match the cutting tip oxygen orifice. Consult the tip

manufacturer's equipment data for the proper cutting oxygen pressures for themanufacturer's equipment data for the proper cutting oxygen pressures for thespecific cutting tipspecific cutting tip [1][1]..

The oxidation of iron by this method is highly exothermic. Once started, steel canThe oxidation of iron by this method is highly exothermic. Once started, steel canbe cut at a surprising rate, far faster than if it was merely melted through. At thisbe cut at a surprising rate, far faster than if it was merely melted through. At thispoint, the prepoint, the pre--heat jets are there purely for assistance. The rise in temperature willheat jets are there purely for assistance. The rise in temperature willbe obvious by the intense glare from the ejected material, even through properbe obvious by the intense glare from the ejected material, even through propergoggles. (goggles. (AAthermic lancethermic lance is a toolwhich alsouses rapidoxidation of iron to cutis a toolwhich alsouses rapidoxidation of iron to cutthrough almostany material.through almostany material.))

Since the melted metal flows out of the workpiece, there must be room on theSince the melted metal flows out of the workpiece, there must be room on theopposite side of the workpiece for the spray to exit. When possible, pieces of metalopposite side of the workpiece for the spray to exit. When possible, pieces of metalare cut on a grate that lets the melted metal fall freely to the ground. The sameare cut on a grate that lets the melted metal fall freely to the ground. The sameequipment can be used for oxyacetylene blowtorches and welding torches, byequipment can be used for oxyacetylene blowtorches and welding torches, byexchanging the part of the torch in front of the torch valvesexchanging the part of the torch in front of the torch valves


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SafetySafety

Gas welding/cutting goggles and safety helmetGas welding/cutting goggles and safety helmet

Proper protection should be worn at all times, including toProper protection should be worn at all times, including toprotect the eyes against glare and flying sparks.protect the eyes against glare and flying sparks.

Oxyacetylene welding/cutting is not difficult, but there are aOxyacetylene welding/cutting is not difficult, but there are agood number of subtle points that should be learned such asgood number of subtle points that should be learned such as

More than 1/7 the capacity of the cylinder should not be usedMore than 1/7 the capacity of the cylinder should not be usedper hour. This causes the acetylene to rapidly come out ofper hour. This causes the acetylene to rapidly come out ofsolution, likesolution, like carbon dioxidecarbon dioxidebubbles violently fizzing from abubbles violently fizzing from afizzy soft drink that has just been shaken.fizzy soft drink that has just been shaken.

Acetylene is dangerous above 15 psi pressure. It is unstableAcetylene is dangerous above 15 psi pressure. It is unstable

and explosively decomposes.and explosively decomposes. Proper ventilation when welding will help to avoid largeProper ventilation when welding will help to avoid large

chemical exposurechemical exposure


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Leftward (or Forehand) welding

It is the oldest method practiced by welders. In this method,

welding is started from the right hand side end of the weld joint. Thewelding torch is held at an angle of 30 to 45' with the work-piece.

The flame spreads on the work-piece joint and thus pre-heats the area

ahead of the torch flame cone under which welding takes place.

The flame is given a circulatory, rotational or side-to-side motion to obtain uniform fusion on each side of the workpiece

plates.

The filler rod (or welding rod) is held at about 300

to the work-piece. It is important that the cone of the flame

should never go outside the puddle, which is a small pool of

molten metal created due to the intense heat of the flame.


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The forehand welding is usually employed for thin jobs upto 5

mm thickness because when welding jobs are over 6.5 mm

thick, good penetration is not obtained and hence the quality of

weld decreases as job thickness increases.

For plates thicker than- 3 beveling of the plate. edges toproduce V-joint is needed. Good welding bead with niceappearance are the qualities of this method. The forehandwelding, however, needs careful manipulation of torch tosafeguard against excessive heating of base metal resultinginto too much mixing of base metal and filler rod metal. Also,in this method, the view of the joint edges is interrupted whichslows down the process.


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Rightward (orBackhand) welding

The torch and welding rod are held at an angle of 30' to 45' with theworkpiece.

In this case, the flame is directed back on the weld

portion, which has just been completed. The welding rod is

given a circulatory motion while the torch moves in a straight

line. The method is suitable for welding thicker sections over 5 mm. No edge preparation is needed upto 8 mm thick plates.

With proper edge preparation, plates upto 16 mm thickness can

be welded in one pass. A larger size-welding torch is used in

this method. Since the welding torch is moved in a straight line,

the molten pool of metal below the flame is least agitated and

hence the oxidation losses on the weld metal are reduced.


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Rightward (orBackhand) Welding.


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Benefits of backhand Welding areas

follows

(i) In backhand welding, the flame is directed on the just welded

bead, the flame thus gives an annealing effect on the weld

metal relieving the welding stresses to a great extent.

(ii) The direction of flame helps the welder in forming good bead and better penetration since the molten pool is clearly

visible and better control on the weld is thus obtained.

(iii) The technique is suitable for welding thicker sections, over 5

mm and upto 25 mm

(iv) Welding speeds are about 20% higher than leftward welding.

(v) Gas consumption is reduced by 15 to 25%.

(vi) It provides better shielding against oxidation.

1oxyacetylen cutting and welding

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