welding upto gas welding
TRANSCRIPT
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Content
Classification of fabrication(joining) process
Different types of welding Gas welding
Principle, equipment
Different type of flames
Arc welding Manual metal arc welding/Shielded metal arc welding
Inert gas arc welding (GTAW or TIG process, GMAW or MIG)
Submerged arc welding
Plasma arc welding
Resistance welding Spot
Seam
Projection
Some other welding processes
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Some products cannot be manufactured as a single piece.
The desired shape and size of such products can be
obtained by joining two parts of same or different
materials. These parts are manufactured individually andare joined together to obtain the desired product.
For example, aircraft and ship bodies, welded machine
frames, furniture, computers, bridges and the transmissionor electric towers etc., are all fabricated by joining several
different parts.
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Fabrication Processes
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Classification of joining processes:
Temporary Joint
Permanent Joint
A temporary joint can be easily dismantled separating the
original parts without any damage to them
In case it is a permanent joint, an attempt to separate the
parts already joined will result in the damage of the parts.
In a permanent joint, the joint is made such that it has
properties similar to the base metal of the two parts. These
parts cannot be separated into their original shape, size and
surface finish
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Based on the process used for making the joint, the joining
processes can be further classified as:
Mechanical Fastenersbolts-nuts, rivets, screws
Adhesive bonding
Welding, Brazing, Soldering
The choice of a particular joining process depends on
several factors such as application, nature of loads or
stresses, joint design, materials involved and size and shape
of the components
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WELDINGWelding is one of the most extensively used fabrication method.
The joint strength obtained in welding is being equal to or some
times more than that of the parent metal. It is not only used for
making structures, but also for repair work such as the joining
of broken castings.
Welding is a process of metallurgical joining of two pieces of
metals by the application of heat with or without the application
of pressure and addition of filler metal. The joint formed is a
permanent joint.
It is extensively used in the fabrication work in which metalplates, steel sections, castings of ferrous metals are joined
together. It is also used for repairing broken, worn-out or
defective metal parts.
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Modern methods of welding may be classified under two
broad headings.
Plastic or pressure welding process Fusion or non-pressure welding process
In plastic or pressure welding process the pieces of metal
to be joined are heated to a plastic state and then forced
together by external pressure. This procedure is used in
forge welding, resistance welding, spot welding in which
pressure is required.
In the fusion or non-pressure welding, the material at thejoint is heated to a molten state and allowed to solidify.
This includes gas welding, arc welding
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STEPS OF WELDING PROCEDURE
1. Surface Cleaning:
Surfaces of the parts to be welded need to bethoroughly cleaned to remove dust, dirt, oil, grease etc.
2. Edge Preparation:
Preparing a contour at the edges of the pieces to be
joined. It may involve beveling or grooving. This isdone in order to get the fusion or penetration through
the entire thickness of the member.
3. Clamping:Pieces to be welded are clamped suitably so that there
are no undesirable movements during welding.
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4. Initial Weld:
Initial tack welds are done at the opposite corners of the
joint to secure the pieces together. Any cracks at this
stage must be removed as they cause residual stresses.
5. Intermediate and Final Welding:
The weld joint is formed through various weaving
movements (weld beads). During this process, fillermetal and a suitable flux are used. After the
intermediate run of welding, final run is taken.
6. Removal of Excess Material:
Extra material on the weld surface can be removed
using tongs and chipping hammer. The weld is allowed
to cool and then cleaned.
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GAS WELDING
Gas welding is a fusion welding process.
The flame produced by the combustion of gases is
employed to melt the metal. The molten metal is allowed
to flow together thus forming a solid continuous joint
upon cooling.
By burning pure oxygen in combination with othergases, in special torches, a flame up to 33000C can be
attained.
In oxy-fuel gas welding (OFW) the heat is obtained
from the combustion of a fuel gas such as acetylene incombination with oxygen.
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Oxy-acetylene Gas Welding Equipment
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Oxy-acetylene Gas Welding Equipment
1. Welding torch & tip
2. An acetylene cylinder:15.5 bar, red or maroon3. An oxygen cylinder: 125 bar, blue or black
4. Pressure regulator: control flow/pressure of gases
5. Pressure gauge: shows cylinder pressure and gas working
or delivery pressure6. Rubber hoses: black/green hose for oxygen & red/orange
hose for acetylene
7. Safety devices: Goggle with coloured glasses, hand gloves,
helmet, apron, sleeves, shoes etc.
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The oxy-acetylene flame is used to pre heat the parts to be
welded around the joint and also to melt the filler metal.
A jet of oxy acetylene flame issuing from the nozzle of a
burner is played on the junction of the two pieces to be
welded.At the same time a filler rod is held in the zone of jet and its
melt is deposited on the fused junction.
A weld is obtained after the molten metal solidifies. The
coating on the filler rod acts as a flux to keep the joint clean.
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GAS WELDING FLAMES (OXY-ACETYLENE FLAMES)
Neutral F lame
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1. Neutral Flame: (Gas ratio is 1)
A certain amount of oxygen is required for complete combustion of
fuel gases. When the oxygen supply varies, the flame appearance
obtained would also vary.
In neutral flame all the acetylene present is completely burned and
thus all the available heat in the fuel gas is released.
This is the most desirable flame to be used in oxy-acetylene
welding. Due to neutral flame no chemical change and no oxidation
in molten metal. This is desired for most welding operations. Used
for welding steel, stainless steel, cast iron, Cu etc.
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2. Carburizing or Reducing Flame: ( Gas ratio 0.95 to 1)
Excess of acetylene is present, Low temp flame. The excess un-
burnt carbon is absorbed in ferrous metals, making the weld hard and
brittle. An intermediate flame feather exists - reddish in colour. The
length of the flame feather is an indication of the excess acetylene
present.
Carbonizing flame is used for welding high carbon steels and cast
iron, alloy steel.
Since this flame provides a strong reducing atmosphere in the welding
zone, it is useful for those materials which are readily oxidize like
oxygen free copper alloys.
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Advantages of Oxy-acetylene Gas Welding
1.The equipment is inexpensive, simple and is easily
portable.2.Useful for welding light metals such as automobile
bodies and repair works.
3.A large variety of material can be welded.
4.Welds can be produced at reasonable cost.
5.Compared to electric arc welding this provides greater
flexibility with respect to heat impact and cooling rates.
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Disadvantages of Oxy-acetylene Gas Welding
1. It is much slower than electric arc welding and does not
concentrate the heat close to the weld. Thus, the heattreated area is larger, which causes more distortion.
2. If electric arc welding is available gas welding is seldom
used for work over 3.2mm thick.
3. The process is not satisfactory for heavy sections4. Highly skilled operators are required to produce a good
weld.
5. Equipment should be handled carefully as in certain
circumstances acetylene is explosive.
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Applications of Oxy-acetylene Gas Welding:
For joining of thin materials.
For joining materials in whose case excessively high
temperature or rapid heating and cooling of the job
would produce unwanted changes in the metal. For welding both ferrous and non-ferrous metals.
In automotive &aircraft industries, project site works,
workshops etc.
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Other Gas Welding
Oxygen-Hydrogen Welding The oxygen-hydrogen torch can reach temperatures much
higher than the oxy-acetylene torch.
More expensive than oxy-acetylene welding and involves the
flammability risk with hydrogen.Plasma Welding
Hydrogen plasma burns even hotter than hydrogen gas,
permitting the welding of extremely high-melting-point
metals. Very clean procedure that results in very little slag or foreign
matter in the weld.
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