NAME 345 Welding Technology Lecture 10 Gas Welding...

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NAME 345 Welding Technology Lecture 10 Gas Welding (Oxyacetylene Welding) Md. Habibur Rahman Lecturer Department of Naval Architecture & Marine Engineering Bangladesh University of Engineering & Technology Dhaka-1000, Bangladesh

Transcript of NAME 345 Welding Technology Lecture 10 Gas Welding...

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NAME 345Welding Technology

Lecture – 10

Gas Welding(Oxyacetylene Welding)

Md. Habibur Rahman

Lecturer Department of Naval Architecture & Marine Engineering

Bangladesh University of Engineering & TechnologyDhaka-1000, Bangladesh

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‒ Gas welding is a welding process thatmelts and joins metals by heating themwith a flame caused by a reaction of fuelgas and oxygen.

‒ The most commonly used method isOxyacetylene welding, due to its highflame temperature.

‒ The flux may be used to deoxidize andcleanse the weld metal.

‒ The flux melts, solidifies and forms aslag skin on the resultant weld metal.

‒ Gas welding is widely used inmaintenance and repair work because ofthe ease in transporting oxygen and fuelcylinders.

Gas Welding

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‒ Oxyacetylene welding utilizes oxygen and a fuel gas to heat metal untilit is in a molten state and fuse multiple pieces of metal together. It can beused with or without a filler rod.

‒ It is great for brazing dissimilar metals together.

‒ Older technology that can be replaced by GTAW.

‒ OAW is a manual process in which the welder must personally controlthe torch movement and filler rod application.

‒ The term oxyfuel gas welding outfit refers to all the equipment neededto weld.

‒ Cylinders contain oxygen and acetylene gas at extremely high pressure.

Oxyacetylene Welding

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Basic Introduction

▪ The oxyacetylene welding process uses a combination of oxygen and acetylene gas toprovide a high temperature flame.

• Oxyacetylene welding process can be used for joining a variety of metals. Oxygen gas isproduced from commercial liquefaction of air. The liquid air is allowed to boil and whennitrogen and argon escape, pure liquid oxygen is left with. The gas is compressed incylinders at a pressure of 15 MPa.

• Acetylene gas (C2H2) is produced by the reaction of calcium carbide (CaC2)with water.

CaC2 + 2H2O → C2H2 + Ca(OH)2

• Acetylene gas has the tendency to explode if the pressure is increased. So the gas isdissolved in acetone (C3H6O) liquid which acts as a solvent for the gas. One volume ofacetone can absorb about 25 volume of acetylene per atmosphere. The acetylene gasusually compressed at 1.7 MPa.

• The acetylene cylinder will be packed with porous calcium silicate (Ca2SiO4), so that theliquid is distributed in fine form. The cylinders are fitted with fusible safety plugs made ofa low melting alloy (melting point around 97°C) which will allow the gas to escape if thecylinders are exposed to excessive heat.

Oxyacetylene Welding

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Oxyacetylene WeldingTypical Oxyacetylene Welding (OAW) Station

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Oxyacetylene Welding

Torch used in Oxyacetylene Welding

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Oxyacetylene WeldingFlame Characteristics

When acetylene burns with oxygen the reaction can be given in the form

2C2H2 + 5O2 → 2CO2 + 2H2O + heat

Thus one vol. of acetylene combines with 2.5 vol. of oxygen. But in practice, thevolume ratio will be 1:1 from cost point of view.

The combustion takes place in two stages:

1st Stage▪ Oxygen and acetylene (O2 and C2H2) in equal proportion by vol., burn in the

inner white cone. In the cone the oxygen combines with the carbon of theacetylene to form CO, while hydrogen is liberated.

C2H2 + O2 → 2CO + H2 + heat

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2nd Stage▪ On passing into the outer envelope of the flame, two separate reactions

take place as combustion is completed. The carbon monoxide combineswith oxygen from atmosphere and burns to from carbon-di-oxide(CO2). The hydrogen also burns with oxygen from the atmosphere tofrom water vapor (H2O).

2CO + O2 → 2CO2 + heat2H2 + O2 → H2O + heat

▪ The oxygen for these secondary reactions is generally obtained fromthe surrounding atmosphere.

Flame formed by burning a mix of acetylene (C2H2) and oxygen.

Oxyacetylene WeldingFlame Characteristics (Contd.)

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Oxyacetylene WeldingWhy To Produce Flame in Oxy-Acetylene Welding?

In oxyacetylene welding, flame is the most important tool. All the welding

equipment simply serves to maintain and control the flame. The correct type of

flame is essential for the production of satisfactory welds. The flame must be of

the proper size, shape and condition in order to operate with maximum

efficiency.

Types of Flame (or Oxy acetylene Flame settings)

01. Neutral Flame;

02. Carburizing Flame;

03. Oxidizing Flame.

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Oxyacetylene WeldingNeutral Flame

▪ The neutral flame is produced when the ratio of oxygen to acetylene, in the mixtureleaving the torch, is almost exactly one-to-one.

▪ It’s termed “neutral” because it will usually have no chemical effect on the metal beingwelded.

▪ It will not oxidize the weld metal; it will not cause an increase in the carbon content ofthe weld metal.

▪ The neutral flame is commonly used for the welding of:i. Mild Steelii. Stainless steeliii. Cast ironiv. Copperv. Aluminum

▪ In actuality, a neutral flame acts like the inert gases that are used in TIG and MIGwelding to protect the weld from the atmosphere.

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Oxyacetylene WeldingCarburizing (or Reducing) Flame

▪ The Carburizing (or Reducing) Flame, is created when the proportion of acetylene inthe mixture is higher than that required to produce the neutral flame. A Carburizingflame has an approximate temperature of 5500°F (3038°C).

▪ A reducing flame can be recognized by acetylene feather which exists between the innercone and the outer envelope. The outer flame envelope is longer than that of the neutralflame and is usually much brighter in color.

▪ With iron and steel it produces very hard, brittle substance known as iron carbide. Thischemical change makes the metal unfit for many applications in which the weld mayneed to be bent or stretched.

▪ Metals that tend to absorb carbon should not be welded with reducing flame.▪ Strongly carburizing flames are not used in cutting low-carbon steels because the

additional carbon they add causes embrittlement and hardness.▪ These flames are ideal for cutting cast iron because the additional carbon poses no

problems and the flame adds more heat to the metal because of its size.

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Oxyacetylene WeldingOxidizing Flame

▪ The oxidizing flame results from burning a mixture which contains more oxygen thanrequired for a neutral flame. It will oxidize or” burn” some of the metal being welded.

▪ The outer flame envelope is much shorter and tends to fan out. At the end on the otherhand the neutral and carburizing envelopes tend to come to a sharp point.

▪ An oxidizing flame tends to be hotter than the neutral flame. This is because of excessoxygen and which causes the temperature to rise as high as 6300°F.

▪ The Oxidizing Flame is commonly used for the welding of:i. Copper base metalsii. Zinc base metalsiii. A few types of ferrous metals, such as manganese steel and cast iron

▪ The oxidizing flame is rarely used for conventional cutting because it produces excessiveslag and does not leave square-cut edges.

▪ The oxidizing flame gives you fast starts when using high-speed cutting machines and isideal for piercing holes in plate.

▪ Highly oxidizing flames are only used in cutting metal underwater where the onlysource of oxygen for the torch is supplied from the surface.

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Oxyacetylene WeldingComparison between All Three Flames

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Oxyacetylene WeldingBackfires and flashbacks

• Backfires occur when the torch flame burns back into the torch tip and isextinguished with a loud pop.

• They occur either because the torch tip touched the work or because ofinsufficient gas pressure.

• Backfires over time damage the torch.

• It is generally harmless, but can turn into flashback.

• Flashbacks are more serious. When this happens, the popping sound turns toa whistle as the gases burns back through the tip and torch head. If notstopped, the fire can travel through and damage the torch, hoses and explodethe cylinder.

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Oxyacetylene WeldingArrestors

• Flashback arrestors are designed to prevent a flashback from reachingupstream equipment.

• They offer added safety and often include reverse flow check valves in asingle unit.

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Oxyacetylene WeldingBackfireA backfire is a loud noise produced by the explosion of gases at the cutting orwelding tip usually following a minor flashback of the flame, extinguishment, andre-ignition at the tip. Repeated backfire can cause tip to overheat and eventuallycause a sustained flashback.Causes are:• Bringing tip too close to work or touching it;• Foreign particles entering tip and impeding gas flow;• Overheated tip;• Trying to operate with incorrect or too low a gas flow.Backfire remedy• Close the blowpipe control valves (fuel gas first);• Check the nozzle is tight; Check the pressures on regulators;• Re-light the torch using the recommended procedure; If the flame continues to

burn:• Close the oxygen valve at the torch (to prevent internal burning);• Close the acetylene valve at the torch;• Close cylinder valves or gas supply point isolation valves for both oxygen and

acetylene;• Close outlets of adjustable pressure regulators by winding out the pressure-

adjusting screws;• Open both torch valves to vent the pressure in the equipment;• Close torch valves; Check nozzle tightness and pressures on regulators;• Re-light the torch using the recommended procedure

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Oxyacetylene WeldingFlashback and its causesA flashback is a burning back of the flame into the tip, or into or through the torch. It is also called a sustained burning in tip or torch. A flashback can be caused by faulty or misused equipment. If it doesn’t cause fire or hose rupture, then it may produce a hissing or squealing due to burning inside torch or tip (usually at the mixer). Examples of faulty or misused equipment are:

• Failure to purge.• Incorrect pressures.• Distorted or loose tips or adapter seats.• Kinked hoses.• Clogged tip or torch orifices.• Overheated tip or torch

What can you do to prevent flashback?• Before lighting the blowpipe, purge the hoses by opening the gas

supply to each hose for a few seconds. • Fit non-return valves (often called check valves) on the torch, to

prevent back-feeding of gas into the hoses.• Inspect nozzles regularly. Make sure they are not blocked by dirt or

spatter. Replace damaged nozzles.

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Oxyacetylene WeldingAdvantage and disadvantage of gas welding

Advantages Disadvantages

i. Gas welding process finds enormousapplications in the various industriesdue to its versatility.

ii. Unlike arc welding, actual site ofwelding is visible. This providesconsiderable control on the weldingprocess.

iii. Temperature of the process can beeasily controlled by changing thevolumes of input fuel gases.

iv. This is comparatively low-costprocess than most of the arc weldingprocesses.

v. One of the most important advantageof gas welding over arc welding, gaswelding is portable and electricsupply in not needed.

i. Heavy sections can not be joinedwith gas welding.

ii. Flame temperature is lower thanother arc welding processes like TIGwelding, MIG welding etc.

iii. Welding speed is lower due to lowflame temperature.

iv. Flames formed due to burning ofgases are dangerous to human eyesand throat.

v. Combustible gases are needed to becarried in the cylinders which isdangerous.

vi. Gases are expensive. Refractorymaterials can not be welded.

vii. Unlike some arc welding processes, itcan not be automated.

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Required Oxyacetylene Welding Equipment

• Oxygen and Acetylene Cylinders

• Oxygen regulator

• Acetylene regulator

• Oxygen/ Acetylene welding hose

• Hose couplings

• Single-purpose cutting torch or welding torch

• Cylinders and cart for portability

• Flash arrestor and check valves (protective equipment)

• Flint lighter to ignite torch

Oxyacetylene Welding Equipment

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Oxyacetylene Welding Equipment

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Approved Protective Equipment

Installed in OAW system to prevent:

– Backflow of oxygen into fuel-gas supply system.

– Passage of flashback into fuel-gas supply system.

– Excessive back pressure of oxygen in fuel-gas supply system.

Oxyacetylene Welding Equipment

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Oxyacetylene Welding EquipmentTypical torch styles

• A small welding torch, with throttle valveslocated at the front end of the handle.Ideally suited to sheet metal welding. Canbe fitted with cutting

• Attachment in place of the welding headshown. Welding torches of this generaldesign are by far the most widely used.They will handle any oxyacetylene weldingjob, can be fitted with multiflame (Rosebud)heads for heating applications, andaccommodate cutting attachments that willcut steel 6 in. thick.

• A full-size oxygen cutting torch which hasall valves located in its rear body. Anotherstyle of cutting torch, with oxygen valveslocated at the front end of its handle.

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‒ Oxygen is stored within cylinders ofvarious sizes and pressures rangingfrom 2000- 2640 PSI. (Pounds Persquare inch).

‒ Oxygen cylinders are forged fromsolid armor plate steel. No part of thecylinder may be less than 1/4” thick.

‒ Cylinders are then tested to over 3,300PSI using a hydrostatic pressure test.

‒ Cylinders are regularly re-tested usinghydrostatic (NDE) while in service.

‒ Cylinders are regularly chemicallycleaned and annealed to relieve“jobsite” stresses created by handling.

Oxygen Cylinders

Oxyacetylene Welding Equipment

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‒ Acetylene is stored in cylinders speciallydesigned for this purpose only.

‒ Acetylene is extremely unstable in its pureform at pressure above 15 PSI (Pounds perSquare Inch).

‒ Acetone is also present within thecylinder to stabilize the acetylene.

‒ Acetylene cylinders should always bestored in the upright position to preventthe acetone form escaping thus causingthe acetylene to become unstable.

‒ Cylinders are filled with a very poroussubstance “monolithic filler” to helpprevent large pockets of pure acetyleneform forming.

‒ Cylinders have safety (Fuse) plugs in thetop and bottom designed to melt at 212° F(100 °C).

Acetylene Cylinders

Oxyacetylene Welding Equipment

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‒ Never transport cylinderswithout the safety caps inplace.

‒ Never transport with theregulators in place.

‒ Never allow bottles to standfreely. Always chain them to asecure cart or some otherobject that cannot be toppledeasily.

Cylinder Transportation

Oxyacetylene Welding Equipment

Acetylene Gas

‒ Virtually all the acetylene distributed forwelding and cutting use is created byallowing calcium carbide (a man madeproduct) to react with water.

‒ The nice thing about the calcium carbidemethod of producing acetylene is that itcan be done on almost any scale desired.Placed in tightly-sealed cans, calciumcarbide keeps indefinitely. For years,miners’ lamps produced acetylene byadding water, a drop at a time, to lumps ofcarbide.

‒ Before acetylene in cylinders becameavailable in almost every community ofappreciable size produced their own gasfrom calcium carbide.

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‒ Hoses are fabricated from rubber.

‒ Oxygen hoses are green in color andhave right hand thread.

‒ Acetylene hoses are red in colorwith left hand thread.

‒ Left hand threads can be identifiedby a grove in the body of the nutand it may have “ACET” stampedon it.

Regulator Hoses

Oxyacetylene Welding Equipment

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Hose and Connections

Oxyacetylene Welding Equipment

Requires special nonporous hoses.

Hoses are color coded.

Green: oxygen

Red or Black: fuel gas

Connections

Oxygen: right hand

Acetylene: left hand

Hoses should be protected from hot metal and physical damage.

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‒ Reduce high storage cylinder pressureto lower working pressure.

‒ Most regulators have a gauge forcylinder pressure and working pressure.

‒ Regulators are shut off when theadjusting screw is turn out completely.

‒ Regulators maintain a constant torchpressure although cylinder pressuremay vary.

‒ Regulator diaphragms are made ofstainless steel.

Pressure Regulators for Cylinders

Oxyacetylene Welding Equipment

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‒ Reduce high cylinder pressure torelatively low pressure of 0 to 45 psiused for welding.

‒ Maintain pressure without variationduring welding.

‒ Flame remains steady and uniform.

‒ Two pressure gauges mounted onbody of regulator.

‒ One shows pressure in cylinder,other pressure of gas being suppliedto torch.

Pressure Regulators for Cylinders (Contd.)

Oxyacetylene Welding Equipment

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Union for attachment to cylinders

Outlet connection for hose leading to torch

Pressure of cylinder

Pressure of gas being suppliedto the torch

Hand screw which adjusts working pressure

Pressure Regulators for Cylinders (Contd.)

Oxyacetylene Welding Equipment

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‒ Internal mechanism of oxygenregulator.

‒ Oxygen enters regulator at left at2,200 psi. and must leave regulator atdesired pressure for welding – 10 psi.

‒ Force that opens valve provided bytension of spring.

‒ Force that closes valve provided byhigh gas pressure from tank.

‒ When two forces balanced, constantflow of oxygen to torch results.

Regulator Construction

Oxyacetylene Welding

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‒ To make the connections between thetorch and the regulators

‒ Must be strong, nonporous, light, andflexible enough to make torchmovements easy.

‒ They must be made to withstand internalpressures that can reach as high as 100psig.

‒ Welding hose is available in single- anddouble hose lengths.

Hoses

Oxyacetylene Welding

‒ Hose used for light work has a 3/16 or 1/4 inch inside diameter and one or two plies offabric. For heavy-duty welding and cutting operations, use a hose with an inside diameterof 5/16 inch and three to five plies of fabric.

‒ The delivery pressure at the torch varies with the length of the hose. A 20-foot, 3/16-inchhose maybe adequate for a job, but if the same hose was 50 feet long, the pressure dropwould result in insufficient gas flow to the torch.

‒ Longer hoses require larger inside diameters to ensure the correct flow of gas to the torch.‒ The hoses used for fuel gas and oxygen are identical in construction, but they differ in

color. The oxygen hose cover is GREEN, and the fuel-gas hose cover is RED. This colorcoding aids in the prevention of mishaps that could lead to dangerous accidents.

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There are two types of oxyacetylene welding torches:

i. Injector type

‒ Acetylene carried through torch and tip at low pressure bysuction force

• Higher oxygen pressure passing through small orifice ofinjector nozzle

‒ Mixing head and injector integral part of tip

ii. Equal (balanced – pressure) type

‒ Both gases delivered at equal pressures

‒ Mixing head separate replaceable unit in body of torch

Torches

Oxyacetylene Welding

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The acetylene is carried through the torch and tip at low pressure by thesuction force of the higher oxygen pressure passing through the smallorifice of the injector nozzle.

Injector Torch

Oxyacetylene Welding

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Injector Type Mixer

Oxyacetylene Welding

The oxygen passing through the venturi (1), pulls the fuel gas available in chamber (2). The gases are mixed, starting at point (3).

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Oxyacetylene WeldingEqual-pressure (balanced-pressure) torch

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Oxyacetylene WeldingBalanced-pressure Type Mixer

• The two gases are fed in at points: (1) fuel gas and (2) oxygen.• The gases are mixed, beginning at point (3), throughout the mixing

chamber.

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Oxyacetylene WeldingTorch Tips

• Size measured by diameter ofend opening‒ Marked on side of tip

• Most common system: numbersfrom 000-15‒ Larger the number, larger

hole in tip and greatervolume of heat provided

• Tips for injector torches providedwith individual mixers so mixerand tip one unit

• Most made of pure drawn copper

Lighters

• Use friction spark lighter to lightwelding torch

• Safety note: Never use matches‒ Puts welder's hand too close

to torch tip where it may beburned when gases ignite

‒ Gas lighters may explode inyour hand or pocket.

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Oxyacetylene WeldingFiller Rod

• Most gas welding done with filler rod‒ Provides additional metal to form larger weld bead‒ Many diameters, available in bundles of 50 or 100 pounds and boxes up

to 300 pounds net weight‒ 36 inches long

• Steel rods copper coated to prevent rusting

• Some aluminum rods flux coated‒ Both Braze and aluminum 28 inches long

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Oxyacetylene Welding

i. Check regulator valves (make sure both are off – loose).

ii. Open Acetylene tank valve (open slowly – half turn).

iii. Open Oxygen tank valve (open slowly – open all of the way).

iv. Set Acetylene line pressure (slowly turn regulator valveclockwise to 5 psi).

v. Set Oxygen line pressure (slowly turn regulator valve clockwiseto 5 psi – weld; 20 psi – cut).

vi. Turn off Oxygen torch valve (open all the way).

vii. Open torch Acetylene valve (open 1/8 turn).

viii.Light torch (with friction lighter – not another torch, hot metal,or match).

ix. Adjust Acetylene torch valve (adjust to light smoke).

x. Adjust Oxygen torch valve (adjust to neutral flame).

xi. Weld or Cut.

Steps for Turning on the Oxygen and Acetylene Torch

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Oxyacetylene Welding

i. Turn off Acetylene torch valve (allow Oxygen to flow outflame).

ii. Turn off Oxygen torch valve (gentle pressure on valves).

iii. Close Acetylene tank valve (turn to right until valve is seated).

iv. Close Oxygen tank valve.

v. Open Acetylene torch valve (regulator gauge should go to zero).

vi. Close Acetylene torch valve (do not leave open while drainingoxygen).

vii. Open Oxygen torch valve (regulator gauge should go to zero).

viii.Close Oxygen torch valve (do not leave open).

ix. Back out Acetylene regulator valve to loosen.

x. Back out Oxygen regulator valve to loosen.

xi. Hang up hose.

xii. Clean up.

Steps for Turning off the Oxygen and Acetylene Torch

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Oxyacetylene Welding

• In this method, the torch precedes the welding rod. The torch is held atapproximately a 45 degree angle from the vertical away from the direction ofwelding, with the flame directed at the molten puddle. The welding rod isbetween the flame and the molten puddle. This position requires less transversemotion than is used in forehand welding.

• Increased speeds and better control of the puddle are possible with backhandtechnique when metal 1/8 in. (3.2 mm) and thicker is welded, based on thestudy of speeds normally achieved with this technique and on greater ease ofobtaining fusion at the weld root.

OxyFuel Backhand Welding

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Oxyacetylene Welding

• In the OxyFuel Welding forehand method, the welding rod precedes the torch. Thetorch is held at approximately a 45 degree angle from the vertical in the direction ofwelding. The flame is pointed in the direction of welding and directed between therod and the molten puddle. This position permits uniform preheating of the plateedges immediately ahead of the molten puddle. By moving the torch and the rod inopposite semicircular paths, the heat can be carefully balanced to melt the end of therod and the side walls of the plate into a uniformly distributed molten puddle. Therod is dipped into the leading edge of the puddle so that enough filler metal ismelted to produce an even weld joint. The heat which is reflected backwards fromthe rod keeps the metal molten. The metal is distributed evenly to both edges beingwelded by the motion of the tip.

OxyFuel Welding Forehand

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Oxyacetylene Welding

• In general, the forehand oxyfuel welding method is recommended for weldingmaterial up to 1/8 in. (3.2 mm) thick, because it provides better control of thesmall weld puddle, resulting in a smoother weld at both top and bottom. Thepuddle of molten metal is small and easily controlled. A great deal of pipewelding is done using the forehand technique, even in 3/8 in. (9.5 mm) wallthick-nesses.

OxyFuel Welding Forehand (Contd.)

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