Welding

Sivaraman Velmurugan
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Welding

o Welding is a process for joining different
materials.
o 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 thermos plastics.
o 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.
o 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.
2/14/2016 Compiled & Edited by Velmurugan Sivaraman 2
Introduction

o 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.
o 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.
Introduction (Contd.)

Welding Terminology & Definitions
What is a Weld?
o A localised coalescence of metals or non-metals produced either by
heating the materials to the welding temperature, with or without the
application of pressure, or by the application of pressure alone (AWS).
o A permanent union between materials caused by heat, and or pressure
(BS499).
o An Autogenous weld: A weld made with out the use of a filler material
and can only be made by TIG or Oxy-Gas Welding.
What is a Joint?
o The junction of members or the edges of members that are to be joined
or have been joined (AWS).
o A configuration of members (BS499).

Joint Terminology
Five Basic Welded Joints
1. Butt Joint
2. Corner Joint
3. T-Joint
4. Lap Joint
5. Edge Joint

Joint Terminology
Butt Joint
A joint between two members aligned approximately in the same plane.
Different Edge Shapes and Symbols for some Butt-Joint

Joint Terminology
Corner Joint
A joint between two members located at right angles to each other.
Different Edge Shapes and Symbols for some Corner Joint

Joint Terminology
Tee Joint
A joint between two members located approximately at right angles to each other in the
form of a T
Different Edge Shapes and Symbols for some Tee Joint

Joint Terminology
Lap Joint
A joint between two overlapping members
Different Edge Shapes and Symbols for some Lap Joint

Joint Terminology
Edge Joint
A joint between the edges of two or more parallel or nearly parallel members
Different Edge Shapes and Symbols for some Edge Joint

Principle of welding
o 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.
o The bonding force holds the atoms, ions or
molecules together in a solid. This 'bonding
on contact' is achieved only when
o the contaminated surface layers on the
workpiece are removed.
o recontamination is avoided, and
o the two surfaces are made smooth, flat and fit
each other exactly.

o 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.
o This was the principle of the first way
known to weld metals; by hammering the
pieces together while they are in hot
condition.

Arc Welding
Gas tungsten arc welding
Underwater WeldingSpot Welding

Classification of welding process
o There are about 35 different welding
and brazing processes and several
soldering methods in use by industry
today.
o There are various ways of classifying
the welding and allied processes. For
example, they may be classified on the
basis of
o Source of heat, i.e., flame, arc, etc.
o Type of interaction i.e.
liquid/liquid (fusion welding) or
solid/solid (solid state welding).

Classification of welding process (Cont.)
In general, various welding and allied processes are
classified as follows:
1. Gas Welding
o Air Acetylene Welding
o Oxyacetylene Welding
o Oxy hydrogen Welding
o Pressure gas Welding

2. Arc Welding
o Carbon Arc Welding
o Shielded Metal Arc Welding
o Flux Cored Arc Welding
o Submerged Arc Welding
o TIG (or GTAW) Welding
o MIG (or GMAW) Welding
o Plasma Arc Welding
o Electro slag Welding
o Electro gas Welding
o Stud Arc Welding.

3. Resistance Welding
o Spot Welding
o Seam Welding
o Projection Welding
o Resistance Butt Welding
o Flash Butt Welding
o Percussion Welding
o High Frequency Resistance
Welding.

4. Solid State Welding
o Cold Welding
o Diffusion Welding
o Explosive Welding
o Forge Welding
o Friction Welding
o Hot Pressure Welding
o Roll Welding
o Ultrasonic Welding.
5. Thermo‐Chemical Welding Processes
o Thermit Welding
o Atomic Hydrogen Welding.
6. Radiant Energy Welding Processes
o Electron Beam Welding
o Laser Beam Welding.

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

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

Practical application of welding (Cont.)
1. Bridges
o Section lengths.
o Shop and field assembly of lengths, etc.
2. Buildings
o Column base plates
o Trusses
o Formation of structure, etc.
3. Pressure Vessels and Tanks
o Clad and lined steel plates
o Shell construction
o Joining of nozzles to the shell, etc.
4. Storage Tanks
o Oil, gas and water storage tanks.

7. Rail Road Equipment Locomotive
o Under frame
o Air receiver
o Engine
o Fronts and rear hoods, etc.
8. Pipings and Pipelines
o Rolled plate piping
o Open pipe joints,
o Oil gas and gasoline pipe lines, etc.
9. Ships
o Shell frames.
o Deck beams and bulkhead stiffeners.
o Girders to shells
o 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
o 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.
o A solid continuous joint is formed upon cooling.

Principle of ARC welding
o The source of heat for arc welding process is an 'electric arc' generated between two electrically
conducting materials.
o 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.
o 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.
o The electrical energy is thus converted to heat energy.
o The high heat of the arc melts the edges of the workpieces.
o Coalescence takes place where the molten metal of the one workpiece combines with the molten
metal of the other workpiece.
o When the coalesced liquid solidifies, the two workpieces join together to form a single
component.
o The electrode material can be either a non‐consumable material or a Consumable material.
o The non‐consumable electrode made of tungsten, graphite etc., serve only to strike the arc and
is not consumed during the welding process.
o 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)
o In metallic arc welding an arc is established
between work and the filler metal
electrode.
o 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.
o 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.
o Arc may be formed with direct or
alternating current.
o Petrol or diesel driven generators are
widely used for welding in open, where a
normal electricity supply may not be
available.
o A simple transformer however widely
employed for A.C. arc welding.

1. Metallic Arc welding (MAW) (Cont.)
o The transformer sets are cheaper and simple
having no maintenance cost as there are no
moving parts.
o With AC system, the covered or coated electrodes
are used, whereas with D.C. system for cast iron
and on‐ferrous metals, bare electrodes can be
used.
o In order to strike the arc an open circuit voltage
of between 60 to 70 volts is required.
o For maintaining the short arc 17 to 25 volts are
necessary.
o 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 (CAW)
o Here the work is connected to negative and the
carbon rod or electrode connected to the
positive of the electric circuit.
o 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.
o The voltage required for striking an arc with
carbon electrodes is about 30 volts (A.C.) and 40
volts (D.C).
o 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:
o 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.
o The flux covering decomposes due to arc eat and
performs many functions, like arc stability, weld metal
protection, etc.,
o The electrode itself melts and supplies the necessary
filler metal.
b. Principle of the process:
o Heat required for welding is obtained from the arc
struck between a coated electrode and the workpiece.
o The arc temperature and thus the arc heat can be
increased or decreased by employing higher or lower
arc currents.

SMAW) (Cont.)
o A high current arc with a smaller arc length produces
very intense heat.
o The arc melts the electrode end and the job.
o Material droplets are transferred from the electrode to
the job, through the arc, and are deposited along the
joint to be welded.
o The flux coating melts, produces a gaseous shield and
slag to prevent atmospheric contamination of the
molten weld metal.
c. Striking the arc:
o 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.

3. Flux shielded metal arc welding (MMAW or SMAW) (Cont.)
d) Electrode holder:
o It can hold the electrode at various angles and energizes it at the same time.
e) Welding the joint
o 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.
o To achieve comparatively deeper penetration, electrode angle with the vertical is further
reduced.
o 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:
o AC or DC welding supply, electrode holder and welding cables.
o Welding electrodes.

SMAW) (Cont.)
o AC transformers and DC generators or rectifiers can be
employed for welding with covered electrodes.
o Both AC and DC power sources produce good quality welds,
but depending upon welding situation one may be preferred
over the other.
o The most commonly used power source for AC welding is a
transformer.
o A transformer may be operated from the mains on single
phase, two phases or three phases.
o A typical specification for the transformer is as follows:
o Current range up to 600 Amps.
o Open circuit voltage 70 to 100 volts.

Advantages of Shielded Metal Arc Welding (SMAW)
o SMAW is the simplest of all the arc welding processes.
o The equipment can be portable and the cost is fairly low.
o This process finds innumerable applications, because of the availability of a wide variety of electrodes.
o A big range of metals and their alloys can be welded.
o Welding can be carried out in any position with highest weld quality.

Limitations
o Because of the limited length of each electrode and brittle flux coating on it, mechanization is difficult.
o 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.
o The process uses stick electrodes and thus it is slower as compared to MIG welding.
o Because of flux coated electrodes, the chances of slag entrapment and other related‐defects are
more as compared to MIG or TIG welding.
Applications
o Today, almost all the commonly employed metals and their alloys can be welded by this process.
o Shielded metal arc welding is used both as a fabrication process and for maintenance and repair jobs.
o The process finds applications in
o Air receiver, tank, boiler and pressure vessel fabrications;
o Shipbuilding;
o Pipes and Penstock joining;
o Building and Bridge construction;
o Automotive and Aircraft industry, etc.

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

TUNGSTEN INERT GAS WELDING (TIG)
o Tungsten inert gas welding or gas tungsten arc welding (GTAW) is a group of
welding process in which the work pieces 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.
o A filler metal may be added if required, during the welding process.

TUNGSTEN INERT GAS WELDING (TIG) (Contd.)
Description
o TIG equipment consists of a welding torch in which a non‐consumable
tungsten alloy electrode is held rigidly in the collet.
o The diameter of the electrode varies from 0.5 ‐ 6.4 mm.
o 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.
o The shielding gas flow from the cylinder, through the passage in the
electrode holder and then impinges on the workpiece.
o Pressure regulator and flow meters are used to regulate the pressure and
flow of gas from the cylinder.
o Either AC or DC can be used to supply the required current.

Operation
o The workpieces to be joined are cleaned to remove dirt, grease and other
oxides chemically or mechanically to obtain a sound weld.
o The welding current and inert gas supply are turned ON.
o 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.
o The high intensity of the arc melts the workpiece metal forming a small molten
metal pool.
o Filler metal in the form of a rod is added manually to the front end of the weld
pool.
o The deposited filler metal fills and bonds the joint to form a single piece of metal
o The shielding gas is allowed to impinge on the solidifying weld pool for a few
seconds even after the arc is extinguished (shut off)
o This will avoid atmospheric contamination of the solidifying metal thereby
increasing the strength of the joint.

Advantages
o Suitable for thin metals.
o Clear visibility of the arc provides the operator to have a greater control
over the weld.
o Strong and high quality joints are obtained.
o No flux is used. Hence, no slag formation. This results in clean weld joints.
Disadvantages
o 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.
o Tungsten material when gets transferred into the molten metal
contaminates the same leading to a hard and brittle joint.
o Skilled operator is required.
o Process is slower.
o Not suitable for thick metals.

METAL INERT GAS WELDING (MIG)
o 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.
o The consumable electrode acts as a filler metal to fill the gap between
the two workpieces.

Operation
o 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.
o The control switch provided in the welding torch is switched ON to initiate the
electric power, shielding gas and the wire (electrode) feed.
o 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.
o The high intensity of the arc melts the workpiece metal forming a small molten
pool.
o 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.
o The deposited metal upon solidification bonds the joint to form a single piece of
metal.

Advantages
o MIG welding is fast and economical.
o The electrode and inert gas are automatically fed, and this makes the
operator easy and to concentrate on the arc.
o Weld deposition rate is high due to the continuous wire feed
o No flux is used. Hence, no slag formation. This results in clean welds.
o Thin and thick metals can be welded.
o Process can be automated.
Disadvantages
o Equipment is costlier
o 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)
o 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.
o The arc is struck beneath a covering layer of granulated flux.
o Thus, the arc zone and the molten weld pool are protected from
atmospheric contamination by being 'submerged under a blanket of
granular flux.
o This gives the name 'submerged arc welding' to the process.

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

Operation
o Edge preparation is carried out to obtain a sound weld.
o Flux is deposited at the joint to be welded
o Welding current is witched ON.
o An arc is struck between the electrode and the workpiece under the layer of flux.
o The flux covers the arc thereby increasing the heat near the weld zone.
o This heat melts the filler metal and the workpiece metal forming a molten weld pool.
o At the same time, a portion of the flux melts and reacts with the molten weld pool to
form a slag.
o The slag floats on the surface providing thermal insulation to the molten metal thereby
allowing it to cool slowly.
o The welding head is moved along the surface to be welded and the continuously fed
electrode completes the weld.
o The un‐melted flux is collected by a suction pipe and reused.
o The layer of slag on the surface of the weld portion is chipped out and the weld is
finished.
o 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
o High productivity process, due to high heat concentration.
o Weld deposition rate is high due to continuous wire feed. Hence, single pass
welds can be made in thick plates.
o Deep weld penetration.
o Less smoke, as the flux hides the arc. Hence, improved working conditions.
o Can be automated
o Process is best suitable for outdoor works and in areas with relatively high
winds.
o There is no chance of spatter of molten metal, as the arc is beneath the flux.
Disadvantages
o The invisible arc and the weld zone make the operator difficult to judge the
progress of welding.
o Use of powdered flux restricts the process to be carried only in flat positions.
o Slow cooling rates may lead to hot cracking defects.
o Need for extensive flux handling.

ATOMIC HYDROGEN WELDING (AHW)
o 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.
o The arc supplies the energy for a chemical reaction to take place.
o Filler rod may or may not be used during the process.

Description
o The equipment consists of a welding torch with two tungsten electrodes
inclined and adjusted to maintain a stable arc.
o Annular nozzles around the tungsten electrodes carry the hydrogen gas
supplied from the gas cylinders.
o AC power source is suitable compared to DC, because equal amount of
heat will be available at both the electrodes.
o A transformer with an open circuit voltage of 300 volts is required to strike
and maintain the arc.

Operation
o 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.
o 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.
o 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)
o The heat thus absorbed can be released by recombination of the Atoms
into hydrogen molecule (H2).
o Recombination takes place as the atomic hydrogen touches the cold
workpiece liberating a large amount of heat.
H + H = H2 + 422 kJ (exothermic reaction)

Operation
o Note: The hydrogen can be thought of as simply a transport mechanism
to extract energy from the arc, and transfer it to the work.
o A arc is produced due to the heat liberated during the chemical
reaction.
o A feature of the arc is the speed by which it can deliver heat to the
workpiece surface.
o The welding torch is moved along the surface to be welded with the arc
tip touching the surface.
o The heat of the arc melts and fuses the workpiece and the filler metal to
form a joint.
o 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
o Intense flame is obtained which can be concentrated at the joint.
Hence, less distortion.
o Welding is faster.
o Workpiece do not form a part of the electric circuit. Hence, problems like
striking the arc and maintaining the arc column are eliminated.
o 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
o Cost of welding by this process is slightly higher than with the other
processes.
o Welding is limited to flat positions only.

GAS WELDING
Definition
o Gas welding is a fusion‐welding process.
o It joins metals, using the heat of combustion of an oxygen/air and fuel
gas (i.e. acetylene, hydrogen, propane or butane) mixture.
o 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.

GAS WELDING
Principle of gas welding
o 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.
o A solid continuous joint is formed upon cooling.
o The two familiar fuel gases used in gas welding are:
o Mixture of oxygen and acetylene gas ‐called oxy‐acetylene welding
process.
o Mixture of oxygen and hydrogen gas ‐ called oxy‐hydrogen welding
process.
o Oxy‐acetylene welding is the most versatile and widely used gas welding
process due to its high flame temperature (up to 3500°C) when
compared to that of oxy hydrogen process (up to 2500°C).
o 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’.

GAS WELDING

OXY ACETYLENE WELDING
Principle of Operation
o 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.
o 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.
o A filler metal rod is generally added to the molten metal pool to build up
the seam slightly for greater strength.

Description and Operation
o The equipment consists of two large cylinders: one containing oxygen at
high pressure and the other containing acetylene gas.
o 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.
o The welding torch is used to mix both oxygen and acetylene gas in
proper proportions and burn the mixture at its tip.
o A match stick or a spark lighter may be used to ignite the mixture at the
torch tip.
o 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.
o Since a slight gap usually exists between the two workpieces, a filler
metal is used to supply the additional material to fill the gap.

Description and Operation
o The filler metal must be of the same material or nearly the same
chemical composition as that of the workpiece material.
o The molten metal of the filler metal combines with the molten metal of
the workpiece and upon solidification form a single piece of metal.
o 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 in a material and used.

Advantages of Gas Welding
o It is probably the most versatile process. It can be applied to a wide
variety of manufacturing and maintenance situations.
o Welder has considerable control over the temperature of the metal in
the weld zone.
o The rate of heating and cooling is relatively slow. In some cases, this is an
advantage.
o Since the sources of heat and of filler metal are separate, the welder has
control over filler‐metal deposition rates.
o The equipment is versatile, low cost, and usually portable.
o The cost and maintenance of the gas welding equipment is low when
compared to that of some other welding processes.

Disadvantages of Gas Welding
o Heavy sections cannot be joined economically.
o Flame temperature is less than the temperature of the arc.
o Fluxes used in certain welding and brazing operations produce fumes
that are irritating to the eyes, nose, throat and lungs.
o Gas flame takes a long time to heat up the metal than an arc.
o More safety problems are associated with the handling and storing of
gases.
o Acetylene and oxygen gases are rather expensive.
o Flux shielding in gas welding is not so effective as an inert gas shielding in
TIG or MIG welding.

WELDING DEFECTS
Defects affect the quality of weld
o Porous welds
o Poor penetration
o Warping
o Undercut & Underfill
o Distortion
o Cracked welds
o Poor appearance
o Poor fusion
o Brittle welds
o Spatter
o Magnetic blow
o Weld stress

Causes and cures of common welding troubles
Porous welds
why?
o Short arc, with the exception of low hydrogen and stainless.
o Insufficient puddling time.
o Impaired base metal.
o Poor electrodes
What to do?
o Check impurities in base metal.
o Allow sufficient puddling time for gases to escape.
o Use proper current.
o Weave your weld to eliminate.
o Use proper electrodes for job.
o Hold longer arc.

Poor penetration
why?
o Speed too fast..
o Electrodes too large.
o Current too low.
o Faulty preparation.
What to do?
o Use enough current to get desired penetration – weld slowly.
o Calculate electrode penetration properly.
o Select electrode according to welding groove size.
o Leave proper free space at the bottom of weld.

Undercut/Underfill
why?
o Faulty electrode manipulation
o Faulty electrode usage.
o Current too high.
What to do?
o Use uniform weave in butt welding.
o Avoid using an overly large electrode.
o Avoid excessive weaving.
o Use moderate current; weld slowly.
o Hold electrode at a safe distance from
vertical plane in making horizontal fillet welds.

Undercut/Underfill
Examples of various discontinuities in fusion welds
Underfill
Undercut

Distortion
why?
o Uneven heat
o Improper sequence.
o Deposited metal shrinks.
What to do?
o Tack or clamp parts properly.
o Form parts before welding.
o Dispose of rolling or forming strains before welding.
o Distribute welding to prevent uneven heating.
o Examine structure and develop a sequence.

Distortion
Distortion of parts after welding.
a. Butt Joints
b. Fillet Joints
Butt Joints
Fillet Joints

Cracked welds
why?
o Wrong electrode.
o Weld and part sizes unbalanced.
o Faulty welds.
o Rigid joints.
What to do?
o Design structure and welding procedure to eliminate rigid joints.
o Heat parts before welding.
o Avoid weld in string beads.
o Keep ends free to move as long as possible.
o Make sound welds of good fusion.
o Adjust weld size to parts size.
o Allow joints a proper and uniform free space.
o Work with as low an amperage as possible

Cracked welds - Cracks caused by thermal stresses
Types of cracks in welded joints caused by thermal stresses that develop
during solidification and contraction of the weld bead and the welded
structure.
Crater cracks
Various types of cracks in
butt & T-joints

Poor appearance
why?
o Faulty appearance
o Over hang.
o Improper use of electrodes.
o Wrong arc and current voltage.
What to do?
o Use a proper welding technique.
o Avoid over heating.
o Use a uniform weave.
o Avoid over high current. A: Good Weld,
B: Travel Too Fast,
C: Travel Too Slow,
D: Voltage Too Low,
E: Voltage Too High,
F: Amperage Too Low,
G:Amperage Too High.

Poor fusion
why?
o Wrong speed.
o Current improperly adjusted.
o Improper electrode size.
What to do?
o Adjust electrode and ‘V’ size.
o Weave must be sufficient to melt
sides of joints.
o Proper current will allow
deposition and penetration.
o Keep weld metal from curling
away from plates.

Poor fusion - Examples of various discontinuities in fusion welds

Brittle welds
why?
o Wrong electrode.
o Faulty preheating.
o Metal hardened by air.
What to do?
o Preheat at 135 to 260º C if welding on
medium-carbon steel or certain alloy
steel.
o Make multiple-layer welds.
o Anneal after welding.
o Use stainless or low-hydrogen
electrodes for increasing weld
ductility.

Spatter
why?
o Arc blow.
o Current too high.
o Arc too long.
o Faulty electrodes.
What to do?
o Whitewash parts in weld area.
o Adjust current to needs.
o Adjust to proper arc length.
o Lighten arc blow.
o Pick suitable electrodes

Magnetic blow
why?
o Magnetic fields cause the arc to
deviate from its intended course.
What to do?
o Use steel blocks to alter magnetic path
around arc.
o Divide the ground into parts.
o Weld in same direction the arc blows.
o Use a short arc.
o Locate the ground properly on the
work.
o Use a-c welding

Weld stress
why?
o Faulty welds.
o Faulty sequence.
o Rigid joints.
What to do?
o Allow parts to move freely as long as
practical.
o Make as few passes as possible.
o Peen deposits.
o Anneal according to thickness of
weld.
o Move parts slightly in welding to
reduce stresses

WELDING DRAWINGS
The difference between Weld symbol & Welding symbol
Weld symbol indicates the type of weld
Welding symbol is a method of representing the weld on drawings. It
includes supplementary information & consists of the following 8 elements.
1. Reference line
2. Arrow
3. Basic weld symbol
4. Dimension and other data
5. Supplementary symbols
6. Finish symbols
7. Tail
8. Specification, process or other reference

WELDING DRAWINGS

WELDING DRAWINGS
Location significance of
Arrow
Arrow side- for fillet, groove,
flanged weld symbols the
arrow connecting the
welding symbol reference
line to one side of the joint
Other side – the side
opposite the arrow side
Both sides
Symbols for Welds
The weld symbol is used to identify the type of
weld to be made, signify a basic type of weld
joint, or identify the type of joint preparation
needed.

WELDING DRAWINGS
COMPARISON OF A CAST SHAFT SUPPORT WITH A WELDED STEEL SHAFT
SUPPORT

Welding Symbols
o Welding symbols are used on drawings, project specs, and welding
procedure specifications.
o They use a series of symbols to indicate the joint configuration and weld
type, location, size, and length required.
o Welding symbols are part of the language of welding. Welders must be
able to understand this language to ensure their welds meet the design
specs.
Confused Yet?

Welding Symbol - Base
o The base for all welding symbols is the always horizontal reference line
with and arrow at one end.
o The arrow which can come either end of the reference line points to the
location for which the welding symbol applies.
o Information on the reference line is always read from left to right.
o The opposite end of the arrow, or the tail is used for information that aids
in the making of the weld but does not have its owns place on the
symbol. The tail can be omitted when not needed.

Weld Symbol Locations
o The welding symbol distinguishes
between the two sides of a weld
joint by using the arrow and the
space above and below the
reference line.
o Information appearing above the
reference line refers the opposite
side of the joint that the arrow is
pointing to.
o Information appearing below the
reference line refers to the side of
the joint that the arrow is pointing
to.

o Weld symbols appearing on both
sides of the reference line refer to
both sides of the weld joint.

Groove Preparation
o The shape of the groove weld
symbol indicates how the
groove is to be prepared. In the
case of bevel groove, and J-
groove weld symbols only one
of the joint members to be
welded is prepared.
o The arrow indicates which
member is to be prepared by
breaking towards that member.

Combining Weld Symbols
o When more than one type of
weld is to be made on the same
joint, it is necessary to combine
weld symbols.
o The weld symbol is always
placed on top of the groove
weld symbol, just as it would be
on the actual weld.

Size and Dimensions
o Unless defined in a drawing
note, the size data for a fillet
weld is always shown to the left
of the symbol for which it
applies.
o The length of the weld is always
shown to the right of the symbol.
o If the weld is to be an
intermittent weld, the length
(length of each segment) and
the pitch (center to center
spacing) are shown with a dash.
o The length is always shown first
and the pitch second.

Size and Dimensions
o If the welds are to be on both
sides of the welds they can be
back to back or they can be
staggered.
o If the welds are back to back
the symbols are lined up evenly
on both sides of the reference
line.
o If the welds are to be staggered
the symbols above and below
the reference line are offset.

Size and Dimensions
o A groove weld is a weld made in the
groove when one or more of the
members has been beveled.
o There are several items to be
considered when sizing groove welds.
o Groove preparation is the depth to
which the groove penetrates into the
base metal.
o When the groove prep extends all the
way through the joint, no prep size
needs to be shown.
o When the groove prep extends only
part of the way through the joint, the
depth of groove is shown to the left of
the symbol.

Size and Dimensions
o If a root opening is required, the root opening dimension of the groove
weld is shown inside the groove weld symbol.

Size and Dimensions
o The angle for the type of groove is shown above or below the welding
symbol, depending on whether the symbol is above or below the
reference line.

Supplemental Symbols
o Supplemental symbols are used to convey special instructions pertaining
to the welding symbol.
o Weld-all-around is used when a weld is to extend completely around the
weld joint.
o Field weld symbol indicates the weld is to be made on location.
o When the face of the weld must have a finished shape that is not the as-
welded condition a contour symbol is used.
o When complete joint penetration is required a melt through symbol is
used.

Surfacing Weld Symbols
o Surfacing weld symbol always on arrow side
o Applies to single or multiple layers
o Size (thickness) to left of symbol
o Location, direction of welding in tail or a drawing reference

Spot Weld Symbols
o The spot weld symbol may straddle the reference line when there is no
side significance, or be placed on either side if significant.
o Spot size or strength to left of symbol

Back and Backing Welds
o The back and backing weld symbols are identical. The sequence of
welding determines which designation applies.
o Back weld made after groove weld
o Backing weld made before groove weld

Weld Symbols Summary
o Welding symbols are a language expressed in graphic format and are
used to convey precise instructions about how a weld is to be made.
o By learning these symbols and the basic rules for applying them, a
welder can easily understand how to precisely prepare and make the
required welds.
o Understanding these symbols is essential when using WPSs and Drawings.

Thank you very much… … …

Welding

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