Unit 1 GAS AND ARC WELDING PROCESSES-Fundamental

PR 8592 WELDING
TECHNOLOGY UNIT-1
P.VENGALA KUMAR ME.,MBA
TF/MECHANICAL ENGG.,VENGALAKUMAR.ME.,MBA 1

UNIT I GAS AND ARC WELDING PROCESSES
• Fundamental principles of welding
• Types Of Welding – Classification of welding processes
• Gas welding
• Air Acetylene welding,
• Oxyacetylene welding,
• Types of flames
• Carbon arc welding,
• Shielded metal arc welding,
• Submerged arc welding,
• TIG & MIG welding,
• Plasma arc welding and
• Electroslag welding processes
VENGALAKUMAR.ME.,MBA 2

Fundamental principles of welding
• Welding is a FABRICATION or SCULPTURAL
PROCESS that joins materials,
usually METALS or THERMOPLACTICS, by
using high HEAT to melt the parts together and
allowing them to cool causing FUSION. Welding
is distinct from lower temperature metal-joining
techniques such as BRAZING and SOLDERING,
which do not MELT the base metal.

Types Of Welding – Classification of welding processes
• Welding is a process of permanently joining two
or more parts by the application of heat and (or)
pressure. As per American Welding Society 50
different welding processes available today; some
are very popular among the industries such as
SMAW, GMAW, GTAW, SAW, FCAW etc.

welding processes
• welding processes are broadly classified as fusion
welding and solid state welding;
• FUSION WELDING: In Fusion welding
coalescence is done by melting the base metals by
the application of heat. Required heat is generated
by an electric arc formed between the electrode
and the workpiece. Very often filler metal is also
used. Commonly used fusion welding processes
are:

Arc Welding
• Carbon Arc Welding
• SMAW (Shielded Metal Arc Welding or stick Welding)
• GMAW (Gas Metal Arc Welding or MIG welding)
• GTAW (Gas Tungsten Arc Welding or TIG Welding)
• SAW (Submerged Arc Welding)
• FCAW (Flux Cored ARC Welding)
• PAW (Plasma Arc Welding)

Arc welding
• Arc welding is a welding process, in which heat is
generated by an electric arc struck between an electrode and
the work piece.
• Electric arc is luminous electrical discharge between two
electrodes through ionized gas.
Any arc welding method is based on an electric circuit
consisting of the following parts:
• Power supply (AC or DC);
• Welding electrode;
• Work piece;
• Welding leads (electric cables) connecting the electrode and
work piece to the power supply.

Resistance Welding
• Resistance Spot Welding
• Resistance Seam Welding
• Resistance Projection Welding
• Flash Welding
• Oxyfuel welding
• Others’ welding
• Electron beam welding
• Laser beam Welding

SOLID STATE WELDING:
In solid state welding, joining is done by the
application of pressure alone or a combination of
pressure and heat. Normally the base metal is not
reached to the melting point, if heat is applied to
obtain the coalescence and filler metal is not used.
Some of the solid state welding processes are:
• Diffusion welding (DFW)
• Friction welding (FRW)
• Ultrasonic welding (USW)
• Cold Welding
• Forge welding
• Explosion welding (EXW)

Gas Welding
(i) Air Acetylene Welding
• It is a types of welding process in which heat is
produced by combining mixture of acetylene and air. In
this process there is an increase in temperature of
about 2700 degree C. There is formation of weld point
without application of heat or without using filler
metals. In this type of welding only one tank is used
which makes it less expensive and easy to handle.
If we see the working of Bunsen burner that is totally
based on this air acetylene welding.
Applications:
• In case of leads having small area of cross section this
method is used for welding.
• Also sheets of copper having small thickness are also
welded by this process.

B) Oxy-Hydrogen Welding
In this process oxygen and hydrogen are used
for fusing and cutting. The oxy-hydrogen flame
produced in this process is typically of pale blue
color. The temperature of this flame is about 2000
degree C.
The workpiece is firstly heated and the melt
bar fused in to make in the connection or joint. The
time interval is not fixed for the completion of this
process. It will vary from two minutes to five
minutes depending on the size of sheets.

Oxy-Hydrogen Welding
Disadvantages:
• There are more chances of leaking the gas in this
process.
• It is more expensive than other alternative
processes.
Uses:
• Melting of expensive metals.
• Polishing of acrylic glass surfaces.
• In metal manufacturing industries .

OFW) Oxy-Fuel Welding (Gas Welding/Oxy-Acetylene Welding)-
(HISTORICAL)
• In 1903 Edmond and Charles Picard discover this
welding type. This is one of the most important
types of welding process commonly called as
oxy-acetylene or oxy-fuel welding. The unique
feature of this process is that it uses fuel gases
such as hydrogen, natural gas, methane and
oxygen to fuse and cut metal pieces.
• The flame temperature is about 3100 degree
Celsius

OFW) Oxy-Fuel Welding (Gas Welding/Oxy-Acetylene
Welding)
Oxy-fuel welding (OFW) is also known as Gas
welding or Oxy-fuel gas welding. The term ‘Oxy-fuel’ is
used to denote a combination of Oxygen and a Fuel gas,
means it’s a process in which Oxygen and a fuel
(combustible gas) both are required. Most commonly
used fuel gas is Acetylene and thus the name Oxy-
Acetylene welding (OAW) is also used for this process,
when Acetylene is used as a fuel gas. Apart from Oxy-
Acetylene welding, other common variants of Oxyfuel
gas welding are
• Air Acetylene welding (AAW)
• Oxy Hydrogen welding (OHW)
• Pressure gas welding (PGW)

Oxy-Acetylene welding (OAW)
The required heat for welding is generated by a flame
caused by the chemical reaction between oxygen and the fuel gas
(Acetylene). Fuel gas and Oxygen are combined in a mixing
chamber, provided in the welding torch itself. Additional filler
metal can be used with this process. A flux may be used to protect
the molten weld pool. Flux deoxidizes and cleanses the weld metal.
The flux melts, solidifies, and forms a slag on the weld metal. A
typical Oxy-Acetylene welding (OAW) setup contains the
following basic items :
• Oxygen Cylinder
• Acetylene cylinder
• Hose pipes
• Gas torch
• Filler Metal (Optional)
• Flux (Optional)
• Safety valves (Flashback arrestor/Non returning valve or Check
valve)

EQUIPMENT OF GAS WELDING

types of flames
• Three different types of flames can be obtained
depending upon the ratio of Oxygen and
Acetylene, these flames are known as;
• Neutral flame
• Reducing flame
• Oxidizing flame

Neutral Flame
• Neutral Flame: When oxygen and Acetylene
are mixed in equal proportions, Neutral flame
is obtained. This type of flame is characterized
by a short inner cone and a longer outer cone.

Neutral Flame
• In neutral flame combustion takes place in two stages, Primary
combustion takes place at the inner core, when the Oxygen (O2) and
Acetylene (C2H2) meets. The heat produced by this reaction accounts for
two thirds of the total heat generated. Following chemical reaction takes
place at this stage:
2O2 + 2C2H2 – 4CO + 2H2
• Secondary combustion takes place at the outer envelope. In secondary
combustion CO and H2, obtained from primary combustion reacts with
surrounding air (O2) and forms CO2 and H2O. One third of total heat is
generated during this combustion. Chemical formula for this reaction is as
follows:
4CO + 2O2 – 4CO2
2H2 + O2 – 2H20
• CO and H2 present in the Outer envelope consume the additional
oxygen coming from surrounding and hence the molten weld pool remains
protected and Oxidation doesn’t take place. This is why outer envelope is
also called as the protection envelop. Neutral flame is used to weld most of
the metals.

reducingFlame
• Reducing Flame: In Reducing flame, excess
acetylene is used. Due to excess amount, the
combustion of acetylene remains incomplete. This
flame is characterized by a greenish Acetylene
feather between the inert cone and the outer
envelope.
• This excess Acetylene makes this flame reducing
in nature and it’s suitable for welding aluminium
alloys because aluminium oxidizes easily.
• It is also good for welding high-carbon steels
(also called carburizing flame in this case)
because excess oxygen can oxidize carbon and
form CO gas porosity in the weld metal.

OxidizingFlame
• Oxidizing Flame: When excess amount of Oxygen is used then this type of
flame is occurred. Due to the presence of unconsumed oxygen, the flame
becomes oxidizing. This type of flame is characterized by a short white
inner cone. This flame is suitable for welding brass, because copper oxide
covers the weld pool and thus prevents zinc from evaporating from the
weld pool.
• Safety Valves: In Oxyfuel welding, chances of reverse flow of flame or gas
into supply line (or even into the cylinder) is very high which may cause a
flashback, fire, or explosion in any part of the apparatus.
• To prevent such reverse flow of flame a flashback arrestor and
Reverse-flow check valves should be located at the torch inlet. An
additional check valve may be used at the regulator outlet.
• The purpose of a check valve is to help prevent the reverse flow of
gases into the hose, regulator, or cylinder. A flashback arrestor at the torch
inlet offers additional protection to the welder and the hosepipe.

cutting process
It involves two processes:
(i) Fusing or Welding
• This process placed more emphasis on using welding torch
to fuse metals. Tip of this torch is used to fuse metals. The
torch consists of oxygen control unit, Fuel gas control valve
and mixer.
(ii) Cutting or Slicing
• Basically a cutting torch is used to crop the metal. Before
cutting, metal components are heated up to ignition point by
pressing oxygen blast lever present in the torch. Iron oxide
and heat formation takes places after the reaction of metal
with oxygen. The resultant heat is the deciding factor of the
cutting process.

ADVANTAGESOF OXY-FUELGASWELDING:
• Equipment is cheaper than other welding processes,
easy to learn and use
• It’s very portable and can be transported anywhere very
easily
• Since no electricity is required, hence can be used at
locations where power sources are not available.
• The equipment is very versatile and can also be used
for metal cutting, preheating, postheating and surfacing.
• The welder can control the heat input, temperature,
weld bead size and shape very efficiently.
• Very useful for maintenance related work

SPL (HINTS)
Uses:
• 1) For polishing in glass companies.
2) It is used in jewelry designing for the purpose
of water welding,
3) To obtain a bright light in theatres.
Precautions:
• 1) Less than 1/7 the capacity of the cylinder
should be used per hour.
2) Fuel leakage should be avoided.
3) Cylinders should be properly monitored and
maintained.

DISADVANTAGESOF OXY-FUELGAS WELDING:
• Slow welding if compared to other arc welding
processes, hence less productive
• Not good for reactive metals and thick metals
• Large heat affected zone (HAZ)
• Due to the presence of combustible gas
(Acetylene or Hydrogen), lots of Safety
precautions needed because these gases are
highly flammable and can explode if catches
fire.

(SAW) Submerged Arc Welding
• Submerged Arc Welding (SAW) is an arc welding
process in which arc is generated between a bare
electrode and the workpiece.
• The arc and the molten weld pool are
submerged in a blanket of granular fusible flux
on the workpiece.
• Flux covers the arc and prevents fumes, sparks,
spatter, and intense ultra violet radiation from
escaping, which makes this process very different
from other arc welding processes.

Submerged Arc Welding(Cont)
Submerged arc welding is typically used to
weld flat position, however horizontal position
welding can also be achieved by using proper
tooling and fixtures.
This process is highly suitable for automated
operation. It is used extensively in pressure vessel
fabrication, pipe manufacturing, ship and barge
building, railroad car fabrication and the fabrication
of structural members where long welds are
required.

Fundamentals of SAW:
• Initially, the bare electrode (filler wire) is inserted
into a heap of flux that covers the joint to be welded.
Then an arc is initiated and a wire feeding mechanism
begins to feed the electrode (filler wire) in the direction
of the joint at a predefined rate.
• The feeder can be moved manually or the entire
system can be automated. In automated welding, the
work piece moves under a stationary wire feeder or the
welding head is moved over the stationary workpiece.
Additional flux is continuously fed around the
electrodes and gets evenly distributed over the weld
joint.
• The heat generated during welding, melts some of
the flux. These liquid flux floats over the molten metal
and completely shields the molten weld pool from the
atmosphere.

Following factors should be considered before choosing submerged arc welding
process for a particular application:
• Chemical composition and mechanical properties
required of the final weld deposit,
• Frequency or volume of welding to be performed
• Accessibility of the joint and the position in
which the weld is to be made
• Thickness of base metal and alloy to be welded,
• Length of the joint to be welded
• Cost and profitability of the project

The basic components of a SAW processes are:
• Power source
• Welding head
• Flux delivery system (Flux hopper)
• Electrode
• Flux

SAW (CONT):
• Power Source: The power source for a
Submerged arc welding process is of great
significance and should be able to produce high
current at high duty cycle. Both alternating
current power sources as well as direct current
power sources can be used to supply power for
this welding process.
• Welding Head: A typical welding head consists
of the following:
• The gun assembly and contact tip
• Wire feed motor and feed-roll assembly
• Accessories for mounting and positioning the
head

SAW (CONT):
FLUX DELIVERY SYSTEM (FLUX HOPPER):
The flux is stored in a hopper and keeps on feeding sufficient
amount of flux on the joint. A nozzle is usually mounted on the welding
head for the delivery of flux to the weld joint.
Flux: Role of flux is very important in this process, because of the
following three reasons:
• Stability of arc depends on the flux
• Mechanical and chemical properties of the deposited weld metal
can be controlled by the flux
• The control and handling of the flux impacts the soundness of
weld joint
• The flux becomes conductive, flow of current takes place between
the electrode and the work, During molten state. Flux consists of
Silica, Lime, Manganese Oxide, Calcium fluoride and other
compounds

SAW (CONT):
Electrode: Bare solid wire or strip is commonly
used as electrode of submerged arc welding.
However composite metal cored electrodes (similar
to flux cored arc welding electrodes) can also be
used. Following base metals can be welded with
submerged arc welding process:
• Carbon steels
• Low-alloy steels
• Stainless steels
• Nickel-based alloys
• Chromium-molybdenum steels

Advantages of SAW:
• Excellent weld quality and high productivity
• high deposition rates
• controllable penetration (deep or shallow)
• adaptability to automated operation
• Most of the flux can be reused
• Very less fumes

Limitations of SAW:
• Can be used in only flat and horizontal
positions
• Requires post weld slag removal
• Limited to materials that are long and straight
or are rotated pipe
• Residuals from the flux possess a great risk to
health and safety

Shielded Metal Arc Welding (SMAW)
• Shielded Metal Arc Welding (SMAW) also known as
Manual Metal Arc Welding (MMAW), is a manual arc
welding process that uses
a consumable electrode (covered with a flux).
Electrode is connected with one terminal of the power
source and the work is connected with the other
terminal of the power source. Welding cable (or lead) is
used for this purpose. The power source can be used in
AC (alternating current) or DC (Direct current). In AC
the polarity of terminals keeps on changing (almost 100
times in second) but in DC one terminal always acts as
Positive and the other acts as negative.

components of SMAW
• The basic components of SMAW are the
following;
• Power Source
• Electrode Holder
• Electrodes
• Cables/Lead

SMAW (CONT)
• In SMAW, electric arc is used to melt the base
metal, this arc is generated by striking the
electrode with the work piece. Soon after
generation of arc the electrode is withdrawn from
the work piece but it remains in close contact with
the work piece. The gases present between the
gap (between electrode and work piece) gets
ionized and smooth flow of electrons takes place
hence, in spite of the gap between the electrode
and the work piece the circuit remains closed (or
energized) and hence arc doesn’t get
extinguished.

Shielded metal arc welding electrodes are covered with flux
, Purpose of flux is as follows;
• During welding, the flux material decomposes and produces fumes.
These fumes shield the molten weld pool from the atmosphere. In the
absence of those fumes, molten weld pool will be exposed to
atmosphere and may react with oxygen present in the environment
and thus oxidation may take place, which is very dangerous and may
lead to failure.(i.e prevents oxidation of molten weld pool).
• Density of flux material is less than the weld material hence the flux
comes upwards and floats on the weld pool and thus protects the
weld pool. During solidification of molten weld pool, flux also
solidifies and forms a thin layer on the surface of weld called as
slag. This thin layer of slag helps in slow cooling of weld material.
In the absence of this thin layer of slag, rapid cooling of molten
weld pool will take place which is very dangerous because rapid
cooling results in microstructural changes of weld metal (martensite
formation).

• Flux also helps in initiation of arc. Especially
when welding is done in AC (Alternating
Current), polarity keeps on changing at a constant
rate, i.e. it attains a positive value then a negative
value and it happens within a fraction of second.
Polarity changes more than 100 times in a second.
While going from positive to negative it attains a
value ‘0′ and every time when it goes to zero, arc
has to be re-initiated and the flux plays its role. It
happens very fast (i.e. within a fraction of
seconds) hence we can not see this with our eyes.

Advantagesof SMAW:
• This process is suitable for most of the commercially
available metals and alloys.
• The equipment is comparatively inexpensive and portable.
• Equipment is relatively simple.
• This process can be used in all welding positions.
• This welding process is flexible and can be applied to a
variety of joint configurations and positions.
• No need of separate gas shielding.
• Less sensitive to wind and drafts if compared to gas
shielded arc welding processes.
• It can be used at area where access is limited (electrodes an
be bent and even mirror can be used at tight spaces).

Disadvantages of SMAW:
• Low deposition rate than GMAW and FCAW
• More skilled welding operator required than many
other welding processors.
• Not suitable for reactive metals such as Titanium,
Zirconium, Tantalum, and Niobium.
• Not suitable for metals with low melting temperature
such as Lead, Tin and Zinc and their alloys.
• This process cannot be automated.
• More slag due to flux shielded electrode.
• Repeated changing of consumed electrodes with a new
one makes this process quite slow if compared to
GMAW.

Carbon Arc Welding (CAW)
• Carbon Arc Welding (CAW) is a welding process, in
which heat is generated by an electric arc struck
between an carbon electrode and the work piece. The
arc heats and melts the work pieces edges, forming a
joint.
Carbon arc welding is the oldest welding process.
If required, filler rod may be used in Carbon Arc
Welding. End of the rod is held in the arc zone. The
molten rod material is supplied to the weld pool.
Shields (neutral gas, flux) may be used for weld pool
protection depending on type of welded metal.

Carbon Arc Welding(CONT)
• Carbon Arc Welding has been replaced by Tungsten
Inert Gas Arc Welding (TIG, GTAW) in many
applications.
Modification of Carbone Arc Welding is Twin Carbon
Electrode Arc Welding, utilizing arc struck between two
carbon electrodes.
• Work piece is not a part of welding electric circuit in
Twin Carbon Electrode Arc Welding, therefore the
welding torch may be moved from one work piece to
other without extinguishing the arc.

Advantages of Carbon Arc Welding:
• Low cost of equipment and welding operation;
• High level of operator skill is not required;
• The process is easily automated;
• Low distortion of work piece

Disadvantages of Carbon Arc Welding:
• Unstable quality of the weld (porosity);
• Carbon of electrode contaminates weld
material with carbides

Gas Tungsten Arc Welding (GTAW/TIG)
• Gas Tungsten Arc Welding (GTAW) is also
known as Tungsten Inert Gas Welding (TIG
Welding).
• It is an arc welding process that uses a non-
consumable electrode made up of Tungsten (or
alloy of tungsten).
• Arc is generated between the tungsten electrode
and work piece to establish the weld pool.
• Inert gas (Argon or Helium) is used to shield the
molten weld pool during welding. Additional
filler metal may be added in this process.

WORKING PRINCIPLE (GTAW/TIG)
In TIG Welding process, an arc is produced in
between the non consumable tungsten electrode and
work piece which is to be joined.
The arc so produced creates an intense heat which
melts the two metal pieces and fuses them together
to form a strong weld by the use of a filler metal.
The weld so formed exhibits the similar properties
that of the base metal.
A shielding gas environment is used to prevent the
weld from atmospheric contaminations.

TIG WELDING

WORKING PRINCIPLE (GTAW/TIG)
• TIG welding is also called as Gas Tungsten Arc Welding
(GTAW)
• In TIG welding, the electric arc is produced between a non-
consumable electrode and the work piece.
• There is an electrode holder in which the non consumable
tungsten electrode is fixed when the arc is produced.
• By supplying electric power between electrode and work
piece, the inert gas from the cylinder is passed through the
nozzle of the welding head around the electrode.
• The inert gas (Argon, helium, nitrogen and carbon dioxide)
surrounds the arc and it protects the weld from atmospheric
effects and hence, defects free joints are made.

• Filler metal may or may not be used .
• When filler metal is used, it is usually fed, manually
into the weld pool.
• An electrode used in this process is tungsten. It has
high melting point (34300C). Therefore, it will not
be melted during welding.
• Nozzle size, gas flow rate, filler rod size, electrode
diameter and current are chosen depending on
position of the weld and metal thickness.
• Here energy is supplied by constant current welding
power source.
• This power supply produces energy which is
conducted through a column of ionized gas
accompanied by metal vapour is known as plasma.

EQUIPMENTS:
1. TIG Torch or TIG hand piece
2. Power transformer
3. Shielding gas cylinder
4. Pressure regulator and flow meter
5. Work clamp
6. Coolant system(optional).

FUNCTIONS OF TIG TORCH:
• An important of TIG welding equipment is TIG torch.
TIG torch is made up of lead, tungsten holder or collet,
nozzle and back cap.
• Various functions of TIG torch are as follows:
1. It holds the electrode tungsten.
2. It delivers welding current to the tungsten via a
welding power cable.
3. It delivers shielding gas to TIG torch nozzle.
4. TIG torch can be water cooled. Hoses in TIG lead will
supply cooling water to the TIG torch head assembly.
5. TIG torch length will allow a distance from TIG
power source and work piece.

ADVANTAGES:
1. No flux is required.
2. The welding speed is high.
3. It produces high quality weld.
4. No weld clean is necessary.
5. It is more suitable for thin sections.
6. It does not create as much spatter and spark.
7. It is applicable to wide range of materials such
as aluminum, stainless steel, manganese and
copper alloys.
8. It produces welds without contamination.

LIMITATIONS:
1. It generally restricted for flat and horizontal welding.
2. It is slow in operation.
3. Equipment is more sophisticated. So, it is also more
costly.
4. TIG tends to be less forgiving and not as user-friendly as
MIG.
5. It emits brighter UV rays when compared to other
welding processes.
6. Discontinuities in the weld due to contamination of the
tungsten electrode by the molten metal.

APPLICATIONS:
1. Aluminum, magnesium, copper alloys can be
welded easily.
2. Thin parts and sheet metals can be welded easily.
3. Rocket motor chamber fabrication welding can
be done by this process.
4. Expansion bellows and other delicate parts can
be joint.
5. Atomic energy, aircraft, chemical and instrument
industries use this welding process.

Gas Metal Arc Welding (GMAW).
METAL INERT GAS [MIG]
• MIG welding is also called Gas Metal Arc
Welding (GMAW).
• In this arc welding, the electric arc is
produced between a consumable metal wire
electrode and the work piece.
• During welding, the arc and welding zone
are surrounded by an inert gas.
• Argon, helium,CO2,Argon-Oxygen or other
gas mixtures are used as the inert gas.
• The surrounded inert gas protects the weld
from atmosphere.

GMAW/MIG (CONT):
• Consumable electrode wire having chemical
composition similar to the parent material is
continuously fed from a reel to the arc through feed
unit.
• The arc heats and melts both workpiece edges and
electrode wire.
• The fused electrode material is supplied to the
surfaces of the workpiece,fills the weld pool and
forms joint.
• Due to automatic feeding of the filling
wire(electrode),the process is referred to as a semi-
automatic.
• The operator controls only the torch positioning and
speed.

• GMAW/MIG (CONT):
• The welding can be done manually or automatically.
• Either DC generator or AC transformer is used for
welding.
• A constant voltage DC power source is most
commonly with GMAW but constant current systems
as well as AC can be used.
• The current ranges from 100A to 400A depending
upon the diameter of the wire.
• The welding head may be either air or water cooled
depending upon the current being used.
• The process is used for welding thick plates.It is used
for welding aluminium, stainless steel, nickel and
magnesium without weld defects.

MIG WELDING PROCESS

MIG Welding Gases
1. Argon
2. Carbon dioxide
3. Helium.
The types of gas or gas mixtures used is based
on the following factors:
• How deep the weld penetrates the metal
welded
• The characteristics of the welding arc
• The mechanical properties of the weld.

Advantages:
• It is suitable for welding a variety of ferrous
and non ferrous metals.
• No flux is required.
• It is provides greater efficiency.
• It produces high quality weld.
• The process is cheaper.

Disadvantages:
• The process is more expensive than any
other types of Welding.
• It needs a clean joint.
• It needs more maintenance.
• Contact tips may seize due to weld spatter.
• Identifying the problem is difficult due to
more number of parts.

Application:
• It is also used in fabrication and manufacturing
industries.
• Since temperatures are relatively low, it is suitable
for thin sheet section (less than 6mm). Examples:
Motorcar manufacture, shipbuilding, aircraft
engineering, heavy electrical engineering and the
manufacture of tanks, pressure vessels,and pipes.
• The most common application of MIG welding is
automotive repair.
• MIG welding can be used to establish hard facing
or it can even be used to reinforce the surfaces of
a worm out railroad track

Electroslag welding:
Eletroslag welding is a welding process in
which the heat is generated by an electric
current passing between consumable electrode
and work piece through a molten slag covering
the weld surface.
In this process the coalescence is formed by
molten slag and the molten metal pool remains
shielded by molten slag.

Electroslag welding Working:
• In this welding process ,prior to welding the gap
between two work pieces is filled with a welding
flux powder.
• Electro slag welding is initiated by an arc between
electrode and work piece.
• Additional flux is added until the molten slag
reaches the tip of the work piece. The slag reaches
the temperature about 1930°C .
• This temperature insufficient for melting the
consumable electrode and work piece edges the
weld is formed.
• In this welding process, after the molten slag reaches
the tip of the electrode the arc is extinguished.

Electroslag welding WORKING
• To molten contain molten puddle, water cooled
copper shoes or dams are placed on the sides of
the vertical cavity.
• The electrode current passes from the electrode
to the workpiece through the slag pool.
• Eletroslag welding is capable of welding plates
with thicknesses ranging from 50 mm to more
than 900 mm and welding is done.
• The current required is about 600A at 40v to 50
v although higher currents are used for thick
plates.
• The travel speed of the weld is on the range
from 12 mm/min to36 mm/min .Weld quality is
high.

Advantages:
• Heavy thickness metals can economically be
welded.
• Stress formation is low.
• Preparation of joints is easier.
• Slag consumption is low.
• High deposition rate of up to 20 kg/h.
• Distortion is low.

Limitations
• It is difficult to weld cylindrical objects.
• Hot cracking occur.
• Grain size becomes Larger.
• Toughness of the weld is low.
• Only vertical position is possible.
• The cost is high as the equipment as the
equipment is fully automatic and it is of
special design.

Applications:
• It is used mainly to join low carbon steel plates
an sections are very thick
• It is used for welding thick sections of carbon
steel alloys steel and nickel alloys.
• Forgings and castings are welded.
• Heavy plates can be welded.
• It can also be used on structural steel if certain
precautionary observed.

Plasma Arc Welding (PAW)
• Plasma arc welding (PAW) is an arc welding
process in which the heat required for welding is
generated by a constricted arc between a non-
consumable electrode and the workpiece.
• Plasma Arc Welding is essentially an extension of
Gas Tungsten Arc Welding (GTAW). Like GTAW,
a non-consumable electrode is used in Plasma Arc
Welding (PAW). However, a different technique is
used to deliver the heat for welding in Plasma Arc
Welding (PAW).
• The welding torch used in Plasma Arc Welding
(PAW) contains two nozzles an inner nozzle for
orifice gas and an outer nozzle for shielding gas

plasma jet stream
• The inner nozzle contains orifice gas which
surrounds the electrode. The orifice gas is a
neutral gas which gets converted into plasma state
(the fourth state of matter) when an arc is ignited
in the chamber.
• The arc heats the orifice gas to a temperature at
which the electrons presents in the atoms of
orifice gas leave their orbit, due to which, the
orifice gas becomes ionized.
• The ionized gases comes out from the orifice of
the nozzle as a “plasma jet stream”. Plasma is a
good conductor of electricity.

PLASMA ARC WELDING

(PAW) WORKING:
• Plasma emanates from the nozzle of orifice at a
temperature of about 16,700°C (30,000°F),
creating a narrow, constricted arc pattern which
provides excellent directional control and
produces a very favorable depth-to-width weld
profile.
• The outer nozzle contains shielding gas in a
manner similar to Gas Tungsten Arc Welding
(GTAW). The shielding gas covers the area of arc
plasma impingement on the workpiece to avoid
contamination of the weld. Shielding gas may be
same as the orifice gas or it may be different from
orifice gas

Some important terms used in Plasma Arc Welding
• Electrode Setback: The distance between the tip
of the electrode and the face of the constricting
nozzle is known as electrode setback
• Torch Standoff Distance: The distance between
the outer face of the constricting nozzle and the
workpiece is known as the torch standoff distance
• Plenum or Plenum Chamber: The the space
between the inside wall of the constricting nozzle and
the electrode is known as the plenum or plenum
chamber

Equipment
• Plasma arc welding can be performed in manual,
mechanized, or robotic operations. However, for
manual plasma arc welding following items are
used:
• A Power Source
• A Welding Torch
• A Plasma Control Console,
• Gases (Orifice and Shielding gas)
• Torch coolant
• Other accessories such as remote control for
current, gas flow timers and an on-off switch

Power Source:
The power source used for plasma arc
welding is similar to that used for TIG welding
(GTAW). Both GTAW as well as PAW processes
use constant-current power sources and a high
frequency source for arc starting.

Welding Torch:
• A Plasma Arc Welding (PAW) torch has the
following features:
• It holds the electrode and allows current to
pass through the electrode
• Inner nozzle for supply of orifice gas or
plasma gas
• Outer nozzle for supply of shielding gas

Plasma Control :
The crucial control systems of plasma arc
welding are encompassed in a plasma console also
known as plasma control console or console. The
console is generally integrated with the primary
power source, but it can also be available as a
separate stand-alone unit. A typical plasma control
console includes controls for the following;
• Plasma gas flow
• shielding gas flow
• the pilot arc current

Gases (Orifice gas/Shielding gas):
Selection of gases for plasma arc welding
depends on the following criteria
• welding position
• joint configuration
• Base metal

PAW-(CONT)
• Shielding gas is often the same as the orifice gas
for many plasma arc welding applications.
However, some advantages can be observed when
a different gas is used for certain applications.
• Orifice Gas: The orifice gas should be inert with
respect to the electrode in order to avoid rapid
deterioration of the electrode. To enhance the
electrode life 99.99% pure orifice gas must be
used. Flow rates for orifice gases are generally
between 0.1 litre per minute (L/min) to 5
L/min. The most commonly used orifice gases
are;
• Argon
• Argon – Hydrogen Mixture

PAW-(CONT)
• Shielding Gas: Generally inert gases are used as
shielding gas. However, an active gas can also be
used for shielding if it is not considered to
adversely affect weld properties. Following gases
are used for shielding the weld pool;
• Argon
• Argon – Hydrogen mixture
• Argon-helium mixture
• Carbon Dioxide

PAW-(CONT)
• Flow rates for shielding gases are usually in the
range of 5 L/min to 15 L/min for low-current
applications. For high-current welding, flow rates
of 15 L/min to 32 L/min are used.
• Coolant System: Plasma Arc Welding requires a
cooling system. A cooling system should consist
of a coolant reservoir, radiator, pump, flow sensor
and control switches. Corrosion- resistant
materials are used for the construction of the
liquid-contacting surfaces.

PAW-(CONT)
• Electrodes: Like GTAW, tungsten electrodes are used
in Plasma Arc Welding (PAW). Tungsten electrodes
with small additions of thorium, lanthanum, or cerium
can be used for PAW with straight polarity (DCEN).
Pure tungsten and zirconiated electrodes are seldom
used in plasma arc welding, because the electrode tip
geometry cannot be maintained. To learn more about
tungsten electrodes,
• Filler Metal: The filler metal is added externally (if
required). In case of manual welding, filler metal in
form of rods are used. Whereas, filler metal in wire
form is used for mechanized or robotic welding. Filler
metal specification is same as that used in gas Tungsten
Arc Welding (GTAW). To learn more about tungsten
electrodes

ARC MODES:
• Two types of arc modes are used in plasma arc
welding, these are;
• Transferred Arc Mode
• Non-Transferred Arc Mode

TRANSFERRED ARC MODE
• In transferred arc mode, the electrode is
connected to one terminal of the power source
(Generally with Negative polarity) and the
work piece is connected with the other
terminal (Positive terminal).
• Hence the work piece becomes a part of the
electrical circuit (the nozzle remains intact),
and heat is obtained from the anode spot on the
work piece and from the plasma jet.

NON-TRANSFERRED ARC MODE
• In the non-transferred arc mode, the electrode is
connected to one terminal of the power source
(generally with negative polarity) and the nozzle
is connected with the other terminal (positive
terminal). Hence, the arc is established and
maintained between the electrode and the
constricting orifice.
• The workpiece remains out of the electrical arc
circuit. Non-transferred arc mode is suitable for
cutting and joining nonconductive materials.

Advantages of Plasma Arc Welding (PAW):
• The constricted opening yields high heat concentration
in a smaller area
• It gives deeper penetration and produces a sound weld
• Less current input as compared to another welding
process
• The distance between torch and work piece (standoff
distance) does not effects the arc formation
• Can achieve faster travel speeds
• Less Heat affected zone (HAZ) if compared to GTAW
(Gas tungsten arc welding)
• It is more stable and does not gets deflected from the
base metal

Limitations of Plasma Arc Welding (PAW):
• PAW equipment are relatively costly, hence
start-up costs are high
• Safety concern is high due to harmful
radiations
• PAW is very noisy process
• Highly skilled operator is required

Unit 1 GAS AND ARC WELDING PROCESSES-Fundamental

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Unit 1 GAS AND ARC WELDING PROCESSES-Fundamental