Welding Upto Gas Welding

7/28/2019 Welding Upto Gas Welding

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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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Welding Upto Gas Welding

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