Transcript Arc Welding 2

ARC WELDING
WELDING
Welding processes are conveniently divided into
two classes: fusion welding and solid state welding.
 During fusion welding a portion of the base
materials to be joined is melted and mixed, often
with the addition of a filler metal, forming a solid
joint after subsequent solidification.
 As the name implies, during solid state welding
the base material is not melted. Joining occurs
by a process other than solidification, and very
often involves diffusional processes.
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FUSION
WELDING
The fusion welding processes are classified in terms
of the source of heat used to cause localized melting:
electric arc welding, thermal welding and resistance
welding.
 In terms of the absolute number of welds made,
fusion welding is the most common welding
technique.
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PRINCIPLE
2
H=I R
t
ELECTRIC ARC
WELDING
All of the electric arc welding processes rely on
the formation of an arc between an electrode and
the base material to provide heat.
 An electric arc forms when there is an electrical
discharge between two metal objects that are not
in contact with one another - in the case of arc
welding the two objects are the electrode and
base metal work piece.
 The arc ionizes the gas between the electrode
and base metal, thus creating a plasma, which
causes local melting of base metal.
 Depending on the technique, the electrode may or
may not be consumed,
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ELECTRODES
Electrodes can be classified into Non-Consumable
Electrodes and Consumable Electrodes.
 The composition of electrode depends up on the
metal to be welded. For example for welding mild
steel electrode of similar composition is used so
as to get a homogeneous weld joint.
 The size (diameter) of electrode depends up on
the amount of weld metal to be deposited and the
gap between the two plates to be welded.
 Higher currents will be required when bigger
diameter electrodes are used.
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NON-CONSUMABLE ELECTRODES
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Non-consumable electrodes are those electrodes, which do
not consumed during the welding process. Separate filler
metals are necessary to fill the gap between the joints.
Non-consumable electrodes are made up of higher melting
point materials like carbon (melting point -6700°F),
graphite or pure tungsten(melting point - 6150°F).
Non-consumable electrodes are used in Carbon Arc
Welding and TIG Welding.
Using Non-consumable electrodes, good control over the
process is possible.
As compared to carbon electrodes, tungsten electrodes are
more expensive and alloy tungsten electrodes are still more
costlier.
Alloying tungsten increases emissivity, resistance to
contamination, arc stability and contamination. Also
electrode consumption is less.
CONSUMABLE ELECTRODES
Consumable Electrode possess more thermal
efficiency than non-consumable electrode.
 Are consumed during welding operation. May be
made of various metals depending upon the purpose
and chemical composition of the metals to be welded.
 Bare electrodes are used in coil form with out coating
in MIG Welding.
 Metal Arc welding make use of coated electrode.
 Commonly used core wire materials are : mild steel,
low alloy steel, nickel steel etc.
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Consumable Electrodes may be classified into
1. Bare Electrodes : There won’t be any coating of flux in case
of bare electrodes. Arc produced by bare electrode is unstable.
Joint produced by bare electrodes are not strong enough. Also
irregular metal transfer and atmospheric contamination takes
place. Bare electrodes are used when strength is not a primary
concern.
2. Coated Electrodes : Molten metal is exposed to oxygen and
nitrogen in the atmosphere and so undesirable oxides and other
substances decreasing the strength of the weld formed. Coated
electrodes (Flux Coated) are used to prevent the formation of
oxidizes and helps to form slag. Due to Flux coating the molten
metal is not exposed to oxygen and nitrogen in the atmosphere
resulting in strong bond. Coated electrodes produce very good
weld appearances and defect free joints. Commonly used fluxes
are asbestos, mica, silica etc.
Coated Electrodes are again classified into
a) Lightly Coated Electrodes : Thin coating of Flux. eg. Citobest
electrode of Advani Oerlikon.
b) Medium Coated Electrodes: Medium coating of Flux. eg.
Overcord electrode of Advani Oerlikon.
c) Heavily Coated Electrodes : Thick coating of Flux. eg. Citofine
CARBON-ARC WELDING
Carbon-Arc Welding is an arc welding process in
which weld is produced by heating the work-piece
with an arc setup between the carbon electrode and
the work-piece.
 In this method a rod of carbon is used as negative
electrode and work being welded as positive.
 The arc produced between the 2 electrodes heats the
metal to the melting temperature (about 3200° C).
 The reason to use Carbon electrode is that less heat is
generated at the electrode tip than at the work piece
and carbon electrode will fuse with the job.
 In Carbon Arc Welding D.C is used to prevent
electrode disintegration and the amount of carbon
deposit at the weld metal.
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SHIELDED (MANUAL)
METAL ARC
WELDING
The shielded or manual metal arc (SMA) process,
is widely used for the fabrication of pressure vessels,
pipe work and pipeline joints, as well as for the repair
and maintenance of industrial machinery.
 An arc is established between the electrode and the
base metal at the joint line. The arc melts a portion
of the base metal and the electrode to form a
weld pool.
 The molten metal is protected from the
surrounding atmosphere by decomposition of the
electrode coating which forms a gaseous cloud.
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There are two types of Carbon Arc Welding. They
are
1) Single Carbon Electrode Welding
2) Twin Carbon Electrode Welding
DIFFERENCE BETWEEN SINGLE CARBON ELECTRODE WELDING
AND TWIN CARBON ELECTRODE WELDING
Advantages :
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Heat input to the work-piece can be easily controlled.
Work-piece distortion is negligible.
Process can be mechanized.
Suitable for thinner pieces.
Disadvantages :
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Separate filler metal is needed slowing the process.
Chances of carbon deposit.
Applications :
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Welding of Sheet Steel, Copper Alloys, brass , bronze and
aluminum.
On many applications, Carbon Arc Welding has been replaced
by TIG Welding.
SHIELDED (MANUAL)
METAL ARC
WELDING
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Flux Shielded Metal Arc Welding is an arc welding process
in which weld is produced by heating the work-piece with
an arc setup between the flux coated electrode and the
work-piece.
Steel when exposed to air forms oxides and nitrides. These
impurities weaken the weld. To prevent this molten metal
is shielded by enveloping it completely with an inert gas or
flux.
In this method a metal rod is used as negative electrode
and work being welded as positive.
Arc melts the electrode and the job. The flux coating melts,
produces a gaseous shield to prevent the atmospheric
contamination of the molten weld metal.
The arc produced between these two electrodes heats the
metal to the melting temperature (about 2400-2600° C).
Both A.C and D.C can be used.
SHIELDED (MANUAL)
WELDING
METAL ARC
Steps :
1. Before welding work-pieces are suitably prepared
(mating surfaces are cleaned to remove scales).
2. Work-pieces are positioned.
3. Arc is struck.
4. Once arc is obtained, electrode is progressed at a
constant speed along the area to be welded.
Advantages :
 Flux Shielded Metal Arc Welding is the simplest of all the arc
welding process.
 Equipment is portable.
 Big range of metals and alloys can be welded.
 Good weld quality can be obtained.
 Cost is fairly low.
Disadvantages :
 Mechanization is difficult due to the limited length of
electrode.
 Process is slow.
 Metal transfer is not clear.
Applications :
 Used for fabrication work and maintenance work.
 All commonly employed metals and alloys can be welded.
 The process finds applications in
a) Ship building
b) Pipes and penstock joining.
c) Building and bridges construction.
d) Automotive and Aircraft industry.
METAL
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INERT GAS ARC WELDING (MIG)
MIG make use of the high heat produced by the electric arc between the consumable
electrode and material to be welded.
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Gas Metal Arc Welding is a shielded metal arc process.
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The electrode is continuously fed through a gun.
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The current ranges from 100 to 400 A depending upon the diameter of the wire.
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The speed of melting of the wire may be up to 5m/min.
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Usually constant voltage D.C machine is used for MIG Welding.
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Welding Gun is either water cooled or air cooled.
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Welding wire is often bare.
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CO2, argon or argon helium mixtures are often used as shielding gases.
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Shielding is done to prevent contamination of weld.
Advantages :
 Does not require much skill.
 Continuous welding at high speeds can be carried out.
 Deeper penetration is possible.
 Process can be mechanized.
 Thick and thin sections can be welded easily.
 Large metal deposition rates can be obtained.
 No flux is used.
 Faster compared to TIG and Metal Arc Welding.
Disadvantages :
 Welding Equipment is much complex.
 Difficult to weld small corners.
 Slightly complex than TIG.
Applications :
 Used for welding of carbon, silicon and low alloy steels, stainless steels,
aluminum, magnesium, copper, nickel and their alloys, titanium etc.
 Used for manufacture of refrigerator parts.
 Used in industries like aircraft, automobile, pressure vessel and ship
building.
GAS-TUNGSTEN-ARC WELDING(GTAW) OR
TUNGSTEN INERT GAS WELDING (TIG) :
Principle
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Gas Tungsten Arc Welding is a shielded metal arc process.
TIG Make use of the high heat produced by the electric arc between the nonconsumable tungsten electrode and material to be welded.
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Tungsten Electrode is used only to generate an arc.
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Filler metal may be or may not be used.
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Shielding is obtained by an inert gas such as helium or argon or mixture of
two.
Shielding is done to prevent contamination of weld.
Usually A.C machine is used for TIG Welding (for nonferrous alloys) except
for ferrous alloys d.c is used.
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End of the welding gun is water cooled.
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Leftward Welding technique is usually used.
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Base metals welded are carbon and alloy steels, stainless steels, heat
resisting alloys, refractory metals,
aluminum alloys, copper alloys, magnesium alloys, nickel alloys etc.
TUNGSTEN INERT GAS WELDING (TIG)
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TUNGSTEN INERT GAS WELDING (TIG)
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Advantages :
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More different types of metals can be welded such as carbon steel, nickel
steel, aluminum, brass, bronze, titanium.
Unlike metals can be welded to each other like mild steel, stainless steel,
brass to copper.
Heat affected zone is very low.
No flux is used.
Clear visibility of arc.
Smooth welds can be obtained.
Disadvantages :
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Under similar applications MIG is faster than TIG.
Tungsten if transferred can contaminate the same.
Costly.
Applications :
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Welding sheet metals and thinner sections.
Used in precision welding in atomic energy, aircraft and instrument
industries.
ATOMIC HYDROGEN ARC WELDING
Principle :
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In this process arc is struck between the terminals of two tungsten
electrode and the work piece does not form any terminal.
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Heat is generated by striking an arc between the electrodes and work
piece under the shield of hydrogen.
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Electric arc splits molecular hydrogen into atomic hydrogen which is
not stable and has a strong tendency to combine. When it combines in
molecular form generating heat. This combination raises the heat up
to 4200°C.
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Atomic hydrogen features of both arc and flame welding process.
Combined energy of arc and a chemical reaction is utilized for
welding.
ATOMIC HYDROGEN ARC WELDING
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Steps :
1. Hydrogen gas supply and current are switched on.
2. Arc is struck by bringing the two tungsten
electrodes in touch with each other and separating
them by a predetermined distance (say 1.5mm). Arc is
held over the job till a molten pool forms.
Advantages :
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Process is fast.
No flux or separate shielding gas is used. Hydrogen its self
acts as a shielding gas. Welding of thin materials is also
possible.
Uniform welds can be obtained.
Disadvantages :
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Speed is less compared to MAW or MIG.
Cost is more.
Applications :
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Process can be used for welding of most of metals and
alloys like plain carbon steel, alloy steel, aluminum, copper,
nickel and their alloys.
SUBMERGED ARC WELDING (SAW)
( HIDDEN ARC WELDING)
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Submerged Arc Welding is an arc welding process in which coalescence (joint)
is produced by heating the work-piece with an arc setup between a bare metal
electrode and the work-piece.
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In Submerged Arc Welding, the arc is submerged under a layer of Flux and so
the arc is invisible.
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Flux is fed through a Flux Hopper.
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The upper portion of flux is in contact with the atmosphere.
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The Flux may be made of silica, metal oxides or other compounds.
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Bare electrode (Steel stainless steel or copper etc) is fed through the gun.
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Normally d.c is employed for Submerged Welding, but a.c is also used.
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Instead of flux covered electrode, granular flux and a bare electrode is used.
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SAW is an automatic process for the production of high quality butt welds.
SUBMERGED ARC WELDING (SAW)
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Steps :
1. Trigger is pulled and the flux starts depositing
on the joint to be welded.
2. Arc is struck by touching the work-piece with
the electrode of by using a high frequency unit.
3. In all cases arc is struck under the cover of
flux. Flux is a non-conductor of electricity, but
once it melts due to the action of heat it becomes
highly conductive and hence current flow will be
maintained between work-piece and electrode.
4. Electrode at a predetermined speed is fed to
the joint to be welded.
Advantages :
 Often automated, so faster.
 Deep penetration and high quality weld is possible.
 Less distortion.
 Operator can work without safety equipment.
 Wire electrodes are inexpensive.
 No sparks.
 Practically no edge preparation is necessary.
 Smoot welds can be obtained.
Disadvantages :
 Since the operator cannot see the welding being carried out, he cannot judge accurately
the progress of welding.
 Can’t be used for plates less thickness.
 Slag has to be removed continuously.
 Can’t be used for welding cast iron due to high heat input.
 Cast iron, Al alloys, Mg Alloys, Pb and Zn cannot be welded by this process.
Applications :
 Fabrication of pipes, penstocks, pressure vessels, boilers, structural shapes etc.
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Used in automotive, aviation, ship-building and nuclear power industry.
 For welding of metals like mild steel, medium and high tensile low alloy steels.
FLUX-CORED ARC WELDING (FCAW)
Principle :
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Flux-Cored Arc Welding is an arc welding process in which
coalescence (joint) is produced by heating the work-piece
with an arc setup between a continuous tubular
consumable electrode and the work-piece.
Equipment consists of a constant-voltage d.c source, a wire
feeder and a light weight welding gun.
Flux is contained with in the electrode. Additional
shielding may be obtained y an externally supplied
gas(CO2) or gas mixture.
The flux provides the necessary shielding for the pool.
The heat of the arc melts the surface of base metal and the
end of the electrode.
Welding gun is similar to MIG welding gun.
Sometimes additional shielding is provided with a gas.
FCAW is a modification of MIG/CO2 in which a solid wire
is replaced by a flux-cored electrode.
FLUX-CORED ARC WELDING (FCAW)
Advantages :
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Provides high quality weld at lower costs.
Welds variety of steel over a wide thickness range.
Visible arc-easy to weld.
Reduced distortion compared to SMAW.
Disadvantages :
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Used only to weld ferrous metals, primarily steels.
FCAW produces a slag covering which has to be removed.
Equipment is costly. But increased productivity compensates for this.
Applications :
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FCAW is widely used for medium thickness steel fabrication work.
Used for welding in bridges, ship building etc.
Used for welding of
a. Low to medium carbon steels
b. Low alloy high strength steels
c. Cast iron
d. Stainless steels (certain)
e. Quenched and tempered steels.