WELDING PROCESS

05/06/16 1Hareesha N G, Asst. Prof, DSCE, Bengaluru

• Introduction:
– Definition
– Principles
– Classification
– Application
– Advantages & limitations of welding.
• Arc Welding:
– Principle
– Metal Arc welding (MAW)
– Flux Shielded Metal Arc Welding (FSMAW)
– Inert Gas Welding (TIG & MIG)
– Submerged Arc Welding (SAW)
– Atomic Hydrogen Welding processes. (AHW)
• Gas Welding:
– Principle
– Oxy – Acetylene welding
– Reaction in Gas welding
– Flame characteristics
– Gas torch construction & working
– Forward and backward welding.
05/06/16 Hareesha N G, Asst. Prof, DSCE, Bengaluru 2

INTRODUCTION
• Welding is a process for joining different materials.
• The large bulk of materials that are welded are metals and their alloys, although
the term welding is also applied to the joining of other materials such as thermo
plastics.
• Welding joins different metals/alloys with the help of a number of processes in
which heat is supplied either electrically or by means of a gas torch.
• In order to join two or more pieces of metals together by one of the welding
processes, the most essential requirement is Heat. Pressure may also be
employed.
• Since a slight gap usually exists between the edges of the work pieces, a 'filler
metal’ is used to supply additional material to fill the gap. But, welding can also
be carried out without the use of filler metal.
• The filler metal is melted in the gap, combines with the molten metal of the
work piece and upon solidification forms an integral part of the weld.

• Welding terminology

PRINCIPLE OF WELDING
• An ideal joint between two pieces of metal or plastic can be made by heating the
workpieces to a suitable temperature. In other words, on heating, the materials
soften sufficiently so that the surfaces fuse together.
• The bonding force holds the atoms, ions or molecules together in a solid. This
'bonding on contact' is achieved only when:
– the contaminated surface layers on the workpiece are removed,
– recontamination is avoided, and
– the two surfaces are made smooth, flat and fit each other exactly.
• In highly deformable materials, the above aims can be achieved by rapidly forcing
the two surfaces of the workpiece to come closer together so that plastic
deformation makes their shape conform to each another; at the same time, the
surface layers are broken up, allowing the intimate contact needed to fuse the
materials.
• This was the principle of the first way known to weld metals; by hammering the
pieces together while they are in hot condition.

CLASSIFICATION OF WELDING PROCESSES
• There are about 35 different welding and brazing processes and several soldering
methods in use by industry today.
• There are various ways of classifying the welding and allied processes. For
example, they may be classified on the basis of:
– Source of heat, i.e., flame, arc, etc
– Type of interaction i.e. liquid/liquid (fusion welding) or solid/solid (solid state
welding).
• In general, various welding and allied processes are classified as follows:
1. Gas Welding
 Air Acetylene Welding
 Oxyacetylene Welding
 Oxy hydrogen Welding
 Pressure gas Welding

2. Arc Welding
 Carbon Arc Welding
 Shielded Metal Arc Welding
 Flux Cored Arc Welding
 Submerged Arc Welding
 TIG (or GTAW) Welding
 MIG (or GMAW) Welding
 Plasma Arc Welding
 Electro slag Welding
 Electro gas Welding
 Stud Arc Welding.
3. Resistance Welding
 Spot Welding
 Seam Welding
 Projection Welding
 Resistance Butt Welding
 Flash Butt Welding
 Percussion Welding
 High Frequency Resistance
Welding.
4. Solid State Welding
 Cold Welding
 Diffusion Welding
 Explosive Welding
 Forge Welding
 Friction Welding
 Hot Pressure Welding
 Roll Welding
 Ultrasonic Welding.
5. Thermo-Chemical Welding Processes
 Thermit Welding
 Atomic Hydrogen Welding.
6. Radiant Energy Welding Processes
 Electron Beam Welding
 Laser Beam Welding.

ADVANTAGES OF WELDING
• A good weld is as strong as the base metal.
• General welding equipment is not very costly.
• Portable welding equipments are available.
• Welding permits considerable freedom in design.
• A large number of metals/alloys both similar and dissimilar can be joined by
welding.
• Welding can join workpieces through spots, as continuous pressure tight seams,
end-to-end and in a number of other configurations.
• Welding can be mechanized.
DISADVANTAGES 0F WELDING
• Welding gives out harmful radiations (light), fumes and spatter.
• Welding results in residual stresses and distortion of the work-pieces.
• Edge preparation of the workpieces is generally required before welding them.
• A skilled welder is a must to produce a good welding job.
• Welding heat produces metallurgical changes. The structure of the welded joint is
not same as that of the parent metal.
• A welded joint, for many reasons, needs stress-relief heat-treatment.

PRACTICAL APPLICATIONS OF WELDING
• Welding has been employed in Industry as a tool for:
– Regular fabrication of automobile cars, air-crafts, refrigerators, etc.
– Repair and maintenance work, e.g., joining broken parts, rebuilding worn
out components, etc.
• A few important applications of welding are listed below:
1. Aircraft Construction
• Welded engine mounts.
• Turbine frame for jet engine.
• Rocket motor fuel and oxidizer tanks.
• Ducts, fittings, cowling components, etc.
2. Automobile Construction
• Arc welded car wheels
• Steel rear axle housing.
• Frame side rails.
• Automobile frame, brackets, etc.

3. Bridges
• Section lengths.
• Shop and field assembly of lengths, etc.
4. Buildings
• Column base plates
• Trusses
• formation of structure, etc.
5. Pressure Vessels and Tanks
• Clad and lined steel plates
• Shell construction
• Joining of nozzles to the shell, etc.
6. Storage Tanks
• Oil, gas and water storage tanks.
7. Rail Road Equipment Locomotive
• Under frame
• Air receiver
• Engine
• Front and rear hoods, etc.05/06/16 Hareesha N G, Asst. Prof, DSCE, Bengaluru 11

8. Pipings and Pipelines
• Rolled plate piping
• Open pipe joints,
• Oil gas and gasoline pipe lines, etc.
9. Ships
• Shell frames.
• Deck beams and bulkhead stiffeners.
• Girders to shells
• Bulkhead webs to plating, etc.
10. Trucks and trailers.
11. Machine tool frames, cutting tools and dies.
12. Household and office furniture.
13. Earth moving machinery and cranes.
In addition, arc welding finds following applications in repair and maintenance
work:
14. Repair of broken and damaged components and machinery such as tools,
punches, dies, gears, shears, press and machine tools frames.
15. Hard-facing and rebuilding of worn out or undersized (costly) parts rejected
during inspection.
16. Fabrication of jigs, fixtures, clamps and other work holding devices.

ARC WELDING PROCESS
• Arc welding process is fusion method of welding that utilizes the high intensity
of the arc generated by the flow of current to melt the workpieces.
• A solid continuous joint is formed upon cooling.

PRINCIPLE
• The source of heat for arc welding process is an 'electric arc' generated between
two electrically conducting materials.
• One of the workpiece material called 'electrode' is connected to one pole of the
electric circuit, while the other workpiece which forms the second conducting
material is connected to the other pole of the circuit.
• When the tip of the electrode material is brought in contact with the workpiece
material and momentarily separated by small distance of 2-4 mm, an arc can be
generated.
• The electrical energy is thus converted to heat energy.
• The high heat of the arc melts the edges of the workpieces.
• Coalescence takes place where the molten metal of the one workpiece
combines with the molten metal of the other workpiece.
• When the coalesced liquid solidifies, the two workpieces join together to form a
single component.
• The electrode material can be either a non-consumable material or a
Consumable material.
• The non-consumable electrode made of tungsten, graphite etc., serve only to
strike the arc and is not consumed during the welding process.
• Whereas, the consumable electrode which is made of the same material as that
of the workpiece metal helps to strike the arc and at the same time melt (gets
consumed) and combines with the molten metal of the workpiece to form a
weld.

1. METALLIC ARC WELDING (MAW)
• In metallic arc welding an arc is established between work and the filler
metal electrode.
• The intense heat of the arc forms a molten pool in the metal being welded,
and at the same time melts the tip of the electrode.
• As the arc is maintained, molten filler metal from the electrode tip is
transferred across the arc, where it fuses with the molten base metal.
• Arc may be formed with direct or alternating current.
• Petrol or diesel driven generators are widely used for welding in open, where
a normal electricity supply may not be available.

METALLIC ARC WELDING (MAW) ( continued…….)
• A simple transformer however widely employed for A.C. arc welding.
• The transformer sets are cheaper and simple having no maintenance cost as
there are no moving parts.
• With AC system, the covered or coated electrodes are used, whereas with
D.C. system for cast iron and non-ferrous metals, bare electrodes can be
used.
• In order to strike the arc an open circuit voltage of between 60 to 70 volts is
required.
• For maintaining the short arc 17 to 25 volts are necessary.
• The current required for welding, however, varies from 10 amp. to 500 amp.
depending upon the class of work to be welded.

2. CARBON ARC WELDING
• Here the work is connected to negative and the carbon rod or electrode
connected to the positive of the electric circuit.
• Arc is formed in the gap, filling metal is supplied by fusing a rod or wire into the
arc by allowing the current to jump over it and it produces a porous and brittle
weld because of inclusion of carbon particles in the molten metal.
• The voltage required for striking an arc with carbon electrodes is about 30 volts
(A.C.) and 40 volts (D.C).
• A disadvantage of carbon arc welding is that approximately twice the current is
required to raise the work to welding temperature as compared with a metal
electrode, while a carbon electrode can only be used economically on D.C. supply.

3. FLUX SHIELDED METAL ARC WELDING (MMAW OR SMAW)
a. Definition:
• It is an arc welding process wherein coalescence is produced by heating the
workpiece with an electric arc set up between a flux coated electrode and the
workpiece.
• The flux covering decomposes due to arc heat and performs many functions,
like arc stability, weld metal protection, etc.,
• The electrode itself melts and supplies the necessary filler metal.

b. Principle of the process:
• Heat required for welding is obtained from the arc struck between a coated
electrode and the workpiece.
• The arc temperature and thus the arc heat can be increased or decreased by
employing higher or lower arc currents.
• A high current arc with a smaller arc length produces very intense heat.
• The arc melts the electrode end and the job.
• Material droplets are transferred from the electrode to the job, through the
arc, and are deposited along the joint to be welded.
• The flux coating melts, produces a gaseous shield and slag to prevent
atmospheric contamination of the molten weld metal.
c. Striking the arc:
In manual metal arc welding (MMAW), arc between the electrode and the
workpiece is generally struck either by momentarily touching the electrode
with the workpiece and taking it (electrode) a predetermined distance away
from the workpiece by the wrist motion or by scratching the electrode on the
job in the arc of a circle.
d. Electrode holder:
• It can hold the electrode at various angles and energizes it at the same time.

e. Welding the joint
• Once the arc has been established and the arc length adjusted, the electrode is
inclined to an, angle of approximately 20 degrees with the vertical.
• To achieve comparatively deeper penetration, electrode angle with the vertical
is further reduced.
• The electrode is progressed along the joint at a constant speed, it is lowered, at
the same time, at a rate at which it is melting.
f. Welding Equipment:
– AC or DC welding supply, electrode holder and welding cables.
– Welding electrodes.
• AC transformers and DC generators or rectifiers can be employed for welding
with covered electrodes.
• Both AC and DC power sources produce good quality welds, but depending upon
welding situation one may be preferred over the other.
• The most commonly used power source for AC welding is a transformer.
• A transformer may be operated from the mains on single phase, two phases or
three phases.
• A typical specification for the transformer is as follows:
– Current range up to 600 Amps.
– Open circuit voltage 70 to 100 volts.

Advantages of Shielded Metal Arc Welding (SMAW)
• SMAW is the simplest of all the arc welding processes.
• The equipment can be portable and the cost is fairly low.
• This process finds innumerable applications, because of the availability of a wide
variety of electrodes.
• A big range of metals and their alloys can be welded.
• Welding can be carried out in any position with highest weld quality.
Limitations
• Because of the limited length of each electrode and brittle flux coating on it,
mechanization is difficult.
• In welding long joints (e.g., in pressure vessels), as one electrode finishes, the
weld is to be progressed with the next electrode. Unless properly cared, a defect
(like slag inclusion or insufficient penetration) may occur at the place where
welding is restarted with new electrode.
• The process uses stick electrodes and thus it is slower as compared to MIG
welding.
• Because of flux coated electrodes, the chances of slag entrapment and other
related-defects are more as compared to MIG or TIG welding.

Applications
• Today, almost all the commonly employed metals and their alloys can be welded
by this process.
• Shielded metal arc welding is used both as a fabrication process and for
maintenance and repair jobs.
• The process finds applications in
– Air receiver, tank, boiler and pressure vessel fabrications;
– Shipbuilding;
– Pipes and Penstock joining;
– Building and Bridge construction;
– Automotive and Aircraft industry, etc.

A.C. Welding
1. At higher currents AC gives a smoother
arc.
2. Once established the arc can be easily
maintained and controlled.
3. It is suitable for welding thicker sections.
4. AC is easily available.
5. AC welding power source has no rotating
parts.
6. It does not produce noise.
7. It occupies less space
8. It is less costly to purchase and maintain.
9. It possesses high efficiency (0.8).
10.It consumes less energy per unit weight of
deposited metal.
11.Melting rate of electrode cannot be
controlled in AC as equal heat generates at
electrode and job.
12.An AC welding power source is Transformer
D.C. Welding
1.DC arc is more stable.
2.DC is preferred for welding
certain non-ferrous metals and
alloys.
3.It has lower open circuit voltage
and therefore is safer.
4.ARC heat can be regulated (i.e.,
through DCRP and DCSP)
5.A DC welding equipment is a
self contained unit. It can be
operated in fields where power
supply is not available
6.DC welding power source
is a transformer-rectifier unit
or a DC generator (motor or
engine driven)

TUNGSTEN INERT GAS WELDING (TIG)
• Tungsten inert gas welding or gas tungsten arc welding (GTAW) is a group of
welding process in which the workpieces are joined by the heat obtained from
an electric arc struck between a non-consumable tungsten electrode and the
workpiece in the presence of an inert gas atmosphere.
• A filler metal may be added if required, during the welding process.
• Figure shows the TIG process.

Description
• TIG equipment consists of a welding torch in which a non-consumable
tungsten alloy electrode is held rigidly in the collet.
• The diameter of the electrode varies from 0.5 - 6.4 mm.
• TIG welding makes use of a shielding gas like argon or helium to protect
the welding area from atmospheric gases such as oxygen and nitrogen,
otherwise which may cause fusion defects and porosity in the weld metal.
• The shielding gas flow from the cylinder, through the passage in the
electrode holder and then impinges on the workpiece.
• Pressure regulator and flow meters are used to regulate the pressure and
flow of gas from the cylinder.
• Either AC or DC can be used to supply the required current.

Operation
• The workpieces to be joined are cleaned to remove dirt, grease and other
oxides chemically or mechanically to obtain a sound weld.
• The welding current and inert gas supply are turned ON.
• An arc is struck by touching the tip of the tungsten electrode with the
workpiece and instantaneously the electrode is separated from the
workpiece by a small distance of 1.5 - 3 mm such that the arc still remains
between the electrode and the workpiece.
• The high intensity of the arc melts the workpiece metal forming a small
molten metal pool.
• Filler metal in the form of a rod is added manually to the front end of the
weld pool.
• The deposited filler metal fills and bonds the joint to form a single piece
of metal
• The shielding gas is allowed to impinge on the solidifying weld pool for a
few seconds even after the arc is extinguished (shut off)
• This will avoid atmospheric contamination of the solidifying metal
thereby increasing the strength of the joint.

Advantages
• Suitable for thin metals.
• Clear visibility of the arc provides the operator to have a greater control
over the weld.
• Strong and high quality joints are obtained.
• No flux is used. Hence, no slag formation. This results in clean weld joints.
Disadvantages
• TIG is the most difficult process compared to all the other welding
processes. The welder must maintain short arc length, avoid contact
between electrode and the workpiece and manually feed the filler metal
with one hand while manipulating the torch with the other hand.
• Tungsten material when gets transferred into the molten metal
contaminates the same leading to a hard and brittle joint.
• Skilled operator is required.
• Process is slower.
• Not suitable for thick metals.

METAL INERT GAS (MIG) WELDING
• Metal inert gas welding or gas metal arc welding (GMAW) is a group of arc
welding process in which the workpieces are joined by the heat obtained from
an electric arc struck between a bare (uncoated) consumable electrode and the
workpiece in the presence of an inert gas atmosphere.
• The consumable electrode acts as a filler metal to fill the gap between the two
workpieces.
• Figure shows the MIG welding process.

Description
• The equipment consists of a welding torch in which a bare consumable electrode
in the form of a wire is held and guided by a guide tube.
• The electrode material used in MIG welding is of the same material or nearly the
same chemical composition as that of the base metal.
• Its diameter varies from 0.7 -2.4 mm.
• The electrode is fed continuously at a constant rate through feed rollers driven
by an electric motor.
• MIG makes use of shielding gas to prevent atmospheric contamination of the
molten weld pool.
• Mixture of argon and carbon dioxide in a order of 75% to 25% or 80% to 20% is
commonly used.
• The shielding gas flow from the cylinder, through the passage in the electrode
holder and then impinges on the workpiece.
• AC is rarely used with MIG welding; instead DC is employed and the electrode is
positively charged. This results in faster melting of the electrode which increases
weld penetration and welding speed.

Operation
• The workpieces to be joined are cleaned to remove dust, grease and other oxides
chemically or mechanically to obtain a sound weld. The tip of the electrode is also
cleaned with a wire brush.
• The control switch provided in the welding torch is switched ON to initiate the
electric power, shielding gas and the wire (electrode) feed.
• An arc is struck by touching the tip of the electrode with the workpiece and
instantaneously the electrode is separated from the workpiece by a small distance
of 1.5-3 mm such that the arc still remains between the electrode and the
workpiece.
• The high intensity of the arc melts the workpiece metal forming a small molten
pool.
• At the same time, the tip of the electrode also melts and combines with the
molten metal of the workpieces thereby filling the gap between the two
workpieces.
• The deposited metal upon solidification bonds the joint to form a single piece of
metal.

Advantages
• MIG welding is fast and economical.
• The electrode and inert gas are automatically fed, and this makes the operator
easy and to concentrate on the arc.
• Weld deposition rate is high due to the continuous wire feed
• No flux is used. Hence, no slag formation. This results in clean welds.
• Thin and thick metals can be welded.
• Process can be automated.
Disadvantages
• Equipment is costlier
• Porosity (gas entrapment in weld pool) is the most common quality problem in
this process. However, extensive edge preparation can eliminate this defect.

SUBMERGED ARC WELDING (SAW)
• Submerged arc welding is a group of arc welding process in which the
workpieces are joined by the heat obtained from an electric arc struck between
a bare consumable electrode and workpiece.
• The arc is struck beneath a covering layer of granulated flux.
• Thus, the arc zone and the molten weld pool are protected from atmospheric
contamination by being 'submerged under a blanket of granular flux.
• This gives the name 'submerged arc welding' to the process.
• Figure shows the submerged arc welding process.

Submerged Arc Welding(SAW)

Description
• The equipment consists of a welding head carrying a bare consumable
electrode and a flux tube.
• The fluxtube remains ahead of the electrode, stores the granulated or
powdered flux, and drops the same on the joint to be welded.
• The flux shields and protects the molten weld metal from atmospheric
contamination.
• The electrode which is bare (uncoated) and in the form of wire is fed
continuously through feed rollers.
• It is usually copper plated to prevent rusting and to increase its electrical
conductivity (since it is submerged under flux).
• The diameter of the electrode ranges from 1.6-8 mm and the electrode
material depends on the type of the work piece metal being welded.
• The process makes use of either AC or DC for supplying the required
current.

Operation
• Edge preparation is carried out to obtain a sound weld.
• Flux is deposited at the joint to be welded
• Welding current is witched ON.
• An arc is struck between the electrode and the workpiece under the layer of
flux.
• The flux covers the arc thereby increasing the heat near the weld zone.
• This heat melts the filler metal and the workpiece metal forming a molten weld
pool.
• At the same time, a portion of the flux melts and reacts with the molten weld
pool to form a slag.
• The slag floats on the surface providing thermal insulation to the molten metal
thereby allowing it to cool slowly.
• The welding head is moved along the surface to be welded and the continuously
fed electrode completes the weld.
• The un-melted flux is collected by a suction pipe and reused.
• The layer of slag on the surface of the weld portion is chipped out and the weld
is finished.
• Since the weld pool is covered by flux, solidification of molten metal is slow.
Hence, a backing plate made from copper or steel is used at the bottom of the
joint to support the molten metal until solidification is complete.

Advantages
• High productivity process, due to high heat concentration.
• Weld deposition rate is high due to continuous wire feed. Hence, single pass
welds can be made in thick plates.
• Deep weld penetration.
• Less smoke, as the flux hides the arc. Hence, improved working conditions.
• Can be automated
• Process is best suitable for outdoor works and in areas with relatively high winds.
• There is no chance of spatter of molten metal, as the arc is beneath the flux.
Disadvantages
• The invisible arc and the weld zone make the operator difficult to judge the
progress of welding.
• Use of powdered flux restricts the process to be carried only in flat positions.
• Slow cooling rates may lead to hot cracking defects.
• Need for extensive flux handling.

ATOMIC HYDROGEN WELDING (AHW)
• Atomic hydrogen welding is a thermo-chemical welding process in which the
workpieces are joined by the heat obtained on passing a stream of hydrogen
through an electric arc struck between two tungsten electrodes.
• The arc supplies the energy for a chemical reaction to take place.
• Filler rod may or may not be used during the process.
• Figure shows the arrangement for atomic welding process.

Description
• The equipment consists of a welding torch with two tungsten electrodes
inclined and adjusted to maintain a stable arc.
• Annular nozzles around the tungsten electrodes carry the hydrogen gas
supplied from the gas cylinders.
• AC power source is suitable compared to DC, because equal amount of
heat will be available at both the electrodes.
• A transformer with an open circuit voltage of 300 volts is required to
strike and maintain the arc.
Operation
• The workpieces are cleaned to remove dirt, oxides and other impurities
to obtain a sound weld. Hydrogen gas supply and welding current are
switched ON.
• An arc is struck by bringing the two tungsten electrodes in contact with
each other and, instantaneously separated by a small distance, say 1.5
mm, such that the arc still remains between the two electrodes.
• As the jet of hydrogen gas passes through the electric arc, it dissociates
into atomic hydrogen by absorbing large amounts of heat supplied by the
electric arc.
(endothermic reaction)
• The heat thus absorbed can be released by recombination of the
hydrogen atoms into hydrogen molecule (H2 ).05/06/16 Hareesha N G, Asst. Prof, DSCE, Bengaluru 38

• Recombination takes place as the atomic hydrogen touches the cold
workpiece liberating a large amount of heat.
H + H  H2 + 422kJ (exothermic reaction)
• Note: The hydrogen can be thought of as simply a transport mechanism to
extract energy from the arc, and transfer it to the work.
• A arc is produced due to the heat liberated during the chemical reaction.
A feature of the arc is the speed by which it can deliver heat to the
workpiece surface.
• The welding torch is moved along the surface to be welded with the arc
tip touching the surface.
• The heat of the arc melts and fuses the workpiece and the filler metal to
form a joint.
• The operator can control the heat by varying the distance of the arc
stream between the two electrodes and the distance between the
workpiece.

Advantages
• Intense flame is obtained which can be concentrated at the joint. Hence, less
distortion.
• Welding is faster.
• Workpiece do not form a part of the electric circuit. Hence, problems like striking
the arc and maintaining the arc column are eliminated.
• Separate flux/shielding gas is not required. The hydrogen envelope itself
prevents oxidation of the metal and the tungsten electrode. It also reduces the
risk of nitrogen pick-up.
Disadvantage
• Cost of welding by this process is slightly higher than with the other processes.
• Welding is limited to flat positions only.

GAS WELDING
Definition
• Gas welding is a fusion-welding process.
• It joins metals, using the heat of combustion of an oxygen/air and fuel gas (i.e.
acetylene, hydrogen, propane or butane) mixture.
• The intense heat (flame) thus produced melts and fuses together the edges of the
parts to be welded, generally with the addition of a filler metal.
Principle of gas welding
• When the fuel gas and oxygen are mixed in suitable proportions in a welding
torch and ignited the flame resulting at the tip of the torch is sufficient enough
to melt the edges of the workpiece metals.
• A solid continuous joint is formed upon cooling.
• The two familiar fuel gases used in gas welding are:
– Mixture of oxygen and acetylene gas -called oxy-acetylene welding process.
– Mixture of oxygen and hydrogen gas - called oxy-hydrogen welding process.
• Oxy-acetylene welding is the most versatile and widely used gas welding process
due to its high flame temperature (up to 3500o
C) when compared to that of oxy
hydrogen process (up to 2500o
C)
Note: Oxygen is not a fuel: It is what chemically combines with the fuel gas to
produce the heat for welding. This is called 'oxidation', but the more general and
commonly used term is 'combustion'.05/06/16 Hareesha N G, Asst. Prof, DSCE, Bengaluru 41

OXY ACETYLENE WELDING
Principle of Operation
• When acetylene is mixed with oxygen in correct proportions in the welding torch
and ignited, the flame resulting at the tip of the torch is sufficiently hot to melt
and join the parent metal.
• The oxy-acetylene flame reaches a temperature of about 3200°C and thus can
melt all commercial metals which, during welding, actually flow together to form
a complete bond.
• A filler metal rod is generally added to the molten metal pool to build up the
seam slightly for greater strength.

Description and Operation
• The equipment consists of two large cylinders: one containing oxygen at high
pressure and the other containing acetylene gas.
• Two pressure regulators fitted on the respective cylinders regulates or controls
the pressure of the gas flowing from the cylinders to the welding torch as per the
requirements.
• The welding torch is used to mix both oxygen and acetylene gas in proper
proportions and burn the mixture at its tip.
• A match stick or a spark lighter may be used to ignite the mixture at the torch
tip.
• The resulting flame at the tip has a temperature ranging from 3200°C - 3500°C
and this heat is sufficient enough to melt the workpiece metal.
• Since a slight gap usually exists between the two workpieces, a filler metal is
used to supply the additional material to fill the gap.
• The filler metal must be of the same material or nearly the same chemical
composition as that of the workpiece material.
• The molten metal of the filler metal combines with the molten metal of the
workpiece and upon solidification form a single piece of metal.
• Flux, if required, may be used during the process. It can be directly applied to the
surface of the workpiece or, the heated end of the filler metal may be dipped in05/06/16 Hareesha N G, Asst. Prof, DSCE, Bengaluru 44

Advantages of Gas Welding
• It is probably the most versatile process. It can be applied to a wide variety of
manufacturing and maintenance situations.
• Welder has considerable control over the temperature of the metal in the weld
zone.
• The rate of heating and cooling is relatively slow. In some cases, this is an
advantage.
• Since the sources of heat and of filler metal are separate, the welder has control
over filler-metal deposition rates.
• The equipment is versatile, low cost, and usually portable.
• The cost and maintenance of the gas welding equipment is low when compared
to that of some other welding processes.
Advantages of Gas Welding
• Heavy sections cannot be joined economically.
• Flame temperature is less than the temperature of the arc.
• Fluxes used in certain welding and brazing operations produce fumes that are
irritating to the eyes, nose, throat and lungs.
• Gas flame takes a long time to heat up the metal than an arc.
• More safety problems are associated with the handling and storing of gases.
• Acetylene and oxygen gases are rather expensive.
• Flux shielding in gas welding is not so effective as an inert gas shielding in TIG or
MIG welding.

REACTIONS IN GAS WELDING
• When suitable proportions of oxygen and acetylene are mixed and ignited at the
torch tip, a flame with a temperature of about 3200°C is produced.
• For complete combustion to take place, two volumes of acetylene is combined
with five volumes of oxygen. The reaction is given below:
2C2H2+502->4C02 + 2H20
• Complete combustion takes place in two stages.
1) First stage combustion
• At the beginning of the process, when the gas torch is ignited, equal volumes of
oxygen and acetylene are issued from the torch to burn in the atmosphere.
• The reaction occurs due to which the inner cone is visible at the torch tip.
• For example, consider one volume of each oxygen and acetylene.
C2H2 + 02 -> 2CO + H2 + heat
(1/3 of total heat generation)
• This is an exothermic reaction that produces CO and H2 as products of the first
stage of combustion.

2) Second stage combustion
• The second stage combustion involves the combustion of CO and H2 which are the
products of combustion of first stage.
• Both these products are capable of supporting combustion and hence, utilize 02
from the surrounding atmosphere for combustion.
• The reactions are as follows:
2CO + 02 -> 2C02
and
H2 + 0.5O2->H2O
2/3 of total heat generation
• Carbon monoxide burns and forms carbon dioxide, while hydrogen combines with
oxygen to form water.
• The combustion is therefore complete and carbon dioxide and water (turned to
steam) are the chief products of combustion.

FLAME CHARACTERISTICS
Types of flames
a) Neutral Flame
b) Oxidizing Flame
c) Reducing Flame (carburizing flame)

a) Neutral Flame
• A neutral flame is produced when approximately equal volumes of oxygen and
acetylene are mixed in the welding torch and burnt at the torch tip. (More
accurately the oxygen-to-acetylene ratio is 1.1 to 1).
• The temperature of the neutral flame is of the order of about 3260°C
• The flame has a nicely defined inner cone which is light blue in color. It is
surrounded by an outer flame envelope, produced by the combination of oxygen
in the air and superheated carbon monoxide and hydrogen gases from the inner
cone. This envelope is usually a much darker blue than the inner cone.
• A neutral flame is named so because it affects no chemical change on the molten
metal and, therefore, will not oxidize or carburize the metal.
• The neutral flame is commonly used for the welding of:
– Mild steel Stainless steel
– Cast Iron Copper
– Aluminium

b) Oxidizing Flame (O2 : C2H2 = 1.5 : 1)
• If, after the neutral flame has been established, the supply of oxygen is further
increased, the result will be an oxidizing flame.
• An oxidizing flame can be recognized by the small cone which is shorter, much
bluer in color and more pointed than that of the neutral flame.
• The outer flame envelope is much shorter and tends to fan out (disperse) at the
end.
• An oxidizing flame tends to be hotter than the neutral flame. This is because of
excess oxygen and which causes the temperature to rise as high.
• The excess oxygen, tends to combine with many metals to form hard, brittle, low
strength oxides.
• Moreover, an excess of oxygen causes the weld bead and the surrounding area
to have a scummy or dirty appearance.
• For these reasons, an oxidizing flame is of limited use in welding. It is not used in
the welding of steel.
• A slightly oxidizing flame is helpful when welding most
– Copper-base metals
– Zinc-base metals

c) Reducing Flame
• If the volume of oxygen supplied to the neutral flame is reduced, the resulting
flame will be a carburizing or reducing flame, i.e., rich in acetylene.
• A reducing flame can be recognized by acetylene feather which exists between
the inner cone and the outer envelope.
• The outer flame envelope is longer than that of the neutral flame and is usually
much brighter in color.
• A reducing flame does not completely consume the available carbon; therefore,
its burning temperature is lower and the leftover carbon is forced into the
molten metal. With iron and steel it produces very hard, brittle substance known
as iron carbide.
• This chemical change makes the metal unfit for many applications in which the
weld may need to be bent or stretched.
• Metals that tend to absorb carbon should not be
welded with reducing flame.
• A reducing flame has an approximate temperature
of 3038°C.

d) Carburizing flame
• A reducing flame may be distinguished from carburizing flame by the fact that a
carburizing flame contains more acetylene than a reducing flame.
• A carburizing flame is used in the welding of lead and for carburizing (surface
hardening) purposes.
• A reducing flame, on the other hand, does not carburize the metal, rather it
ensures the absence of the oxidizing condition. It is used for welding with low
alloy steel rods and for welding those metals, (e.g. non-ferrous) that do not tend
to absorb carbon.
• This flame is very well used for welding high carbon steel.
To conclude, for most welding operations, the Neutral Flame
is correct, but the other types of flames are sometimes
needed for special welds, e.g., non-ferrous alloys and high
carbon steels may require a reducing flame, whilst zinc-
bearing alloys may need an oxidizing flame for welding
purposes.

Welding Techniques
• Depending upon the ways in which welding rod and the welding torch may be
used, there are two usual techniques in gas welding, namely:
– Leftward technique or Forehand welding method.
– Rightward technique or Back hand welding method.

Leftward Technique
• The welder holds welding torch in his right hand and filler rod in the left
hand.
• The welding flame is directed away from the finished weld, i.e., towards the
un-welded part of the joint. Filler rod, when used, is directed towards the
welded part of the joint (Fig.).
• The weld is commenced on the right-hand side of the seam, working
towards the left-hand side.
• The blowpipe or welding torch is given small sideways movements, while
the filler rod is moved steadily across the seam.
• The filler rod is added using a backward and forward movement of the rod,
allowing the flame to melt the bottom edges of the plate just ahead of the
weld pool.

Leftward Technique (Continued…..)
• Since the flame is pointed in the direction of the Welding, it preheats the edges
of the joint.
• Good control and a neat appearance are characteristics of the leftward method.
Leftward technique is usually used on relatively thin metals, i.e., having
thicknesses less than 5 mm.
• When workpiece thickness is over 3 mm, it is necessary to bevel the plate edges
to produce a V-joint so that good root fusion may be achieved.
• The included angle of V-joint is 80-90°.
• When welding materials over 6.5 mm thick, it is difficult to obtain even
penetration at the bottom of the V and, therefore, the quality of the weld
decreases as plate thickness increases.
• The leftward technique requires careful manipulation to guard against excessive
melting of the base metal, which results in considerable mixing of base metal
and filler metal.

Rightward Technique
• Here again the welding torch is held in the right hand of the welder and the filler
rod in the left hand.
• Welding begins at the left-hand end of the joint and proceeds towards the right,
hence the name rightward technique.
• The direction of welding is opposite to that when employing the leftward
technique.
• The torch flame in rightward technique is directed towards the completed weld
and the filler rod remains between the flame and the completed weld section
(Fig.).
• Since the flame is constantly directed on the edges of the V ahead of the weld
puddle (Molten metal pool) , no sidewise motion of the welding torch is
necessary. As a result, a narrower V-groove (30° bevel or 60° included angle) can
be utilized than in leftward welding. This provides a greater control and reduced
welding costs.
• During welding, the filler rod may be moved in circles
(within the puddle) or semicircles (back and forth
around the puddle).
• The rightward technique is one used on heavier or
thicker (above 5 mm) base metals, because in this
technique the heat is concentrated into the metal.
Welds with penetrations of approximately 12 mm can
be achieved in a single pass.05/06/16 Hareesha N G, Asst. Prof, DSCE, Bengaluru 56

• Rightward technique has got certain advantages over the leftward one, as
listed below:
– Up to 8.2 mm plate thickness, no bevel is necessary. This saves the
cost of preparation and reduces the consumption of filler rod.
– For welding bigger thicknesses, where beveling of plate edges
becomes necessary, the included angle of V need be only 60°, which
requires less filler metal against 80°V preparation used in leftward
welding technique.
– The welder's view of the weld pool and the sides and bottom of the V
groove is unobstructed. This results in better control and higher
welding speeds.
– The smaller total volume of deposited metal, as compared to leftward
welding, reduces shrinkage and distortion.
– The weld quality is better than that obtained with the leftward
technique.
– Owing to less consumption of the filler metal, the rightward technique
involves lower cost of welding than leftward technique.

Welding Torch or Blow Pipe
• Oxygen and the fuel gas having been reduced in pressure by the gas regulators
are fed through suitable hoses to a welding torch which mixes and controls the
flow of gases to the welding nozzle or tip where the gas mixture is burnt to
produce a flame for carrying out gas welding operation.
• There are two types of welding torches, namely:
– High pressure (or equal pressure) type.
– Low pressure (or injector) type.
• High pressure blow-pipes or torches are used with (dissolved) acetylene stored
in cylinders at a pressure of 8 bar.
• Low pressure blow-pipes are used with acetylene obtained from an acetylene
generator at a pressure of 200 mm head of water (approximately 0.02 bar).

Working of a low pressure blow-pipe
• It is termed as a low pressure blow-pipe because it can be operated at low
acetylene pressure; it is frequently used with acetylene generators.
• As acetylene is of low pressure, it is necessary to use oxygen at a high
pressure (2.5 bar).
• As shown in fig., the oxygen enters the mixing chamber through a passage
located in the centre of the torch. The oxygen passage is surrounded by the
acetylene. The high pressure oxygen passes through a small opening in the
injector nozzle, enters the mixing chamber and pulls (or draws) the
acetylene in after it.
• An advantage of low pressure torch is that small fluctuations in the oxygen
supplied to it will produce a corresponding change in the amount of
acetylene drawn, thereby making the proportions of the two gases
constant while the torch is in operation.

Working of a high pressure blow-pipe
• In this type of blow-pipe, both the oxygen and acetylene are fed to the blow pipe
at equal pressures and the gases are mixed in a mixing chamber prior to being
fed to the nozzle tip.
• The equal pressure or high pressure type of blow-pipe is the one most generally
used because
– It is lighter and simpler.
– It does not need an injector.
– In operation, it is less troublesome since it does not suffer from backfires to
the same extent.
• To change the power of the welding torch, it is only necessary to change the
nozzle tip (size) and increase or decrease the gas pressures appropriately.

WELDING PROCESS

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