3. WELDING - Diploma Engineering college-New …npkauto.com/oldweb/data/notes/second/4e/amp/WELDING.pdf · Advantages: 1. Suitable for ferrous ... Overlap can be less than that for

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© Rohan Desai- Automobile Dept. – New Polytechnic, Kolhapur. Page 1

3. WELDING

Definition: Welding is a process of joining similar metals by application of

heat with or without application of pressure and addition of filler metal.

Q. What are types of weld joints?

Welded structures are assembled by five basic types of joints: Butt, Lap,

Corner, T and Edge joints.

1. Butt joints are formed by welding the end surfaces or edges of the

members.

2. Lap joints are formed by welding the two surfaces which are

overlapping.

3. T joints are formed by welding the two surfaces which are

approximately right angles to each other.

4. Corner joints are formed by welding the two edges of surfaces which

are right angles to each other.

5. Edge joints are formed by welding the two edges in which a part of

the surface remains parallel to each other.

Q. what are types of welds?

A 'bead' weld is one in which the filler metal is deposited at a joint

where the two surfaces adjoining the joint are in the same plane. A 'bead' is

defined as a single run of weld metal. A 'fillet' weld is one in which the filler

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metal is deposited at the corner of two intersecting surfaces, such as a T or

Lap joint. A 'groove' weld is one in which the filler material is deposited in a

groove formed by edge preparation of one member or of both the members.

A 'plug' or 'slot' weld is one in which a hole is formed through one of the

Pieces to be welded and the filler material is then deposited into this hole

and fused with the mating part.

Q. How welding process is classified?

These may be divided into two groups as follows:

(A) Pressure Processes. In these processes, the parts to be joined are

heated to a plastic state. (Fusion may occur to a limited extent) and forced

together with external pressure to make the joint. Some of the more

common processes in this group are mentioned below:

1. Forge welding

2. Thermit Pressure welding

3. Pressure Gas welding

4. Electric Resistance welding

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(B) Fusion Processes. In these processes, the material at the joint is

heated to the molten state and allowed to solidify to make the joint, without

the application of pressure. Here so joints may be made without the addition

of a filler metal, but, in general, a filler metal must be added to the weld to

fill the space between the parts being welded. The filler metal deposited

should ordinarily be of the same composition as the base metal.

Some of the common welding processes in this group are listed below:

1. Gas welding

2. Electric Arc welding

3. Thermit Fusion welding

Q. What is working principle Gas welding?

Gas welding is done by burning a combustible gas with air or oxygen in a

concentrated flame of high temperature. As with other welding methods, the

purpose of the flame is to heat and melt the parent metal and filler rod of a

joint.

Q. Write short note on Oxy-Acetylene welding.

Oxy-acetylene gas welding is accomplished by melting the edges or surface

to be joined by gas flame and allowing the molten metal to flow together,

thus forming a solid continuous joint upon cooling. With material thicker

than 15 mm, additional metal called filler metal is added to the weld in the

form of welding rod. The composition of the filler rod is usually the same or

nearly the same as that of the part being welded. To remove the impurities

and oxides present on the surfaces of metal to be joined and to obtain a

satisfactory bond a flux is always employed.

Common mixtures of gases are oxygen and acetylene, oxygen and

hydrogen, oxygen and other fuel gas, and air and acetylene. The oxygen-

acetylene mixture is used to a much greater extent than the other and has a

prominent place in the welding industry. The temperature of the oxy-

acetylene flame in its hottest region is about 3,200°C.

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OXY-ACETYLENE FLAME SETTINGS:

In an oxyacetylene flame, O2 and C2H2 are mixed burnt to release heat. The

complete combustion of acetylene in an atmosphere of oxygen represented

by the following reaction:

C2H2 + 2.5O2 = 2CO2 + H2O (vapour) + 1284.57 k J/mol

Q. What are the types of flames used in Gas welding?

Neutral flame:

A neutral flame is obtained when equal amounts of oxygen and acetylene

are mixed and burnt in a torch. The flame is recognized by two sharply

defined zones, the inner white cone flame and the outer blue flame

envelope. Applications: Steel, stainless steel, cast iron and aluminium.

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Carburizing Flame:

A carburising or reducing flame is obtained when acetylene is supplied than

which is theoretically required flame is recognised by three distinct sections:

the inner cone (which is not sharply defined) and outer envelope as for the

neutral flame. The third zone surrounds the inner cone and extends into the

outer enveloping zone. Application: Carbon Steels.

Oxidising Flame:

This flame has an excess of oxygen over that require for a neutral flame. To

obtain an oxidising flame, the flame is first se condition. Then the acetylene

valve is turned down gradually to reduce the amount of acetylene giving an

excess of oxygen. Applications: Brass and Bronze.

Q. Write short note on welding technique.Leftward Welding Technique:

In left ward (also called as forward or forehand) welding technique, the torch

flame progresses from right to left. The method allows preheating of the plate

edges immediately ahead of the molten pool and this is the method more

commonly used.

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Rightward Welding Technique:

In the rightward (or back hand or backward) technique, welding commences

at the left hand side of the plates and proceeds towards the right

Q. Why Rightward Technique is widely used than Leftward

technique?

1. The rightward technique is faster by comparison with the leftward

technique.

2. Plates up to 9 mm thick can be welded with square edge preparation.

3. The technique consumes less gas and filler metal.

4. The mechanical properties of the weld are better.

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Q. Write short note on metal arc welding.

In the metal-arc welding a metal rod is used as one electrode, while the work

being welded is used as another electrode. The temperature produced is

about 2,400°C and 2,600°C on the negative and positive electrode

respectively. During the welding operation, this metal electrode is melted by

the heat of the arc, and is fused with the base metal, thus forming a solid

union after the metal has been cooled. Both ac and dc may be used. A metal-

arc-welding circuit is illustrated.

Q. Write short note on M.I.G. (Gas metal arc welding- GMAW)

Gas-metal-arc welding is a gas shielded; metal arc welding process which

uses the high heat of an electric arc between a continuously fed, consumable

electrode wire and the material to be welded. Metal is transferred through

protected arc column to the work.

In this process, the wire is fed continuously from a reel through a gun to

constant surface which imparts a current upon the wire.

Advantages:

1. Suitable for ferrous as well as nonferrous metals

2. Very high quality of weld

3. Less operator skill is required.

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Applications: Aluminium, Copper, Nickel & their alloys, Variety of

Steels.

Q. Write short note on T.I.G. (Gas tungsten arc welding-GTAW).

This arc-welding process uses the intense heat of an electric arc between a

non-consumable tungsten electrode and the material to be welded.

The shielding is obtained from an inert gas such as helium or argon or a

mixture of the two. The shielding gas displaces the air surrounding the arc

and weld pool. This prevents the contamination of the weld metal by the

oxygen and nitrogen in the air. Filler metal may or may not be used.

Advantages:

1. Produces high quality of welds in nonferrous metals.

2. No weld cleaning is necessary.

3. The arc and weld pool is clearly visible to the welder.

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Applications: Aluminium, Copper, Nickel & their alloys, Variety of

Steels.

Q. What are the types of Resistance welding?

(1) Resistance spot welding, (2) Resistance seam welding, (3) Projection

welding and (4) Resistance butt welding (Upset and Flash butt).

Q. Explain working of Spot welding with applications.

It is used to join overlapping strips, sheets or plates of metals. The pieces are

assembled and squeezed between two electrodes, which must possess high

electrical & thermal conductivity and retain the required strength at high

temperatures. When the current is turned on, the pieces are heated at their

areas of contact to a welding temperature, and with the aid of mechanical

pressure the electrodes are forced against the metal to be welded. The

pressure may be developed by a foot lever or by air pressure or by hydraulic

cylinders.

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Applications: This may be applied to all types of boxes, cans, enclosing

cases, etc.

Q. Explain with sketch Seam welding process.

Seam welding is a method of making a continuous joint between two

overlapping pieces of sheet metal. The normal procedure for making a seam

weld is to place the work between the wheels which serve as conductors for

producing continuous welds. As pressure is applied, the drive is started and

the welding current switched on. Then at the same time, the over-lapping

surfaces of the metal are forced together as fast as they are heated. A

coolant is applied to conserve the electrodes and cool the work rapidly to

speed the operation.

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Advantages:

It can produce gas tight or liquid tight joints.

Overlap can be less than that for spot and projection weld.

Disadvantages:

Welding can be done only along a straight or uniformly curved line.

It is difficult to weld thickness greater than 3 mm.

Applications: Seam welding is used on many types of pressure tight or leak

proof tanks for various purposes, silencer bodies, transformers, refrigerators,

air crafts.

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Q. Explain Projection welding process.Projection welding is a modification of spot welding. The current and pressure

are localised at the weld section by the use of embossed, machined or coined

projections on one or both pieces of the Work. The flattening out of these

projections under pressure results in good welds at all points of contact.

Advantages:

More than one weld at a time hence more output.

Low current density and low pressure increases electrode life.

Disadvantages:

It is limited to combinations of metal thickness and compositions.

For proper welding, all projections must be of same height.

Applications: It is used for joining nuts, bolts and studs to steel plates in car

bodies.

Q. Explain Upset and Flash welding processes.

There are two types of electric resistance butt welding processes: the 'upset'

and the 'flash'.

Upset Welding: In this process, the ends of the two parts (having the same

X-section) to be joined together are clamped in position in the electrodes. The

movable head is moved towards the fixed head until the abutting surfaces of

the work pieces are in light contact. Then the proper current is made to flow

across the interface for a preset time, while the light pressure between the

two parts is maintained. When the interface has been heated to the welding

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temperature (plastic state), the current is switch off and the welding pressure

is increased to form an "upset". In order to obtain uniform heating of the

joint, the two parts to be joined should preferably have the same resistance.

Applications: It is extensively used in the fabrication of tubular sections,

pipes and heavy steel rings. It is also used for joining small ferrous and non-

ferrous strips.

Flash welding: In this process, the parts to be welded approach towards

each other and come into contact with the current switched on. The

procedure of butt welding is as follows: After the parts are properly positioned

and correct current, head speed and time are selected, a cycle start button is

actuated. This makes the movable head to approach the fixed head. As the

abutting surfaces come very near to each other, extremely rapid heating

takes place when surface asperities first make contact. Molten metal is

violently expelled and burns in air with considerable force and "Sparking" or

"arcing", thus giving the process its name "flash butt". In these few seconds,

a very thin layer at the interface is melted. The current is then shut off and

two parts are rapidly pressed together to form a weld.

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Applications: this process is used for joining thin walled tubes, chain links

tools and press manufactured parts.

Q. Give comparison between resistances welding arc welding.

RESISTANCE WELDING ARC WELDING

1) It is a form of a plastic welding. 1) It is a form of fusion welding.

2) External pressure is required. 2) No external pressure is required

hence the equipment is simpler.

3) No filler is required. 3) Suitable filler metal electrodes are

necessary to get proper strength.

4) Supply is AC only. 4) Supply can be AC or DC.

5) Voltage required is very low. 5) The striking voltage is high so

requires voltage control.

Q. What precautions should be taken for welding of Cast iron and

Aluminium?

Cast iron:

Cast iron may be welded either hot or cold.

Hot Welding: The work is preheated to 600 - 650°C and this temperature is

maintained throughout the whole welding process. A considerable volume of

liquid metal is produced at the weld. This retards the cooling of the molten

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metal and thereby prevents the chilling. The hot welding of cast iron can

be done with either an electric arc or a gas flame and with either

complete or do Preheating of work. For welding small parts of intricate

shape, complete preheating is done and is accomplished in metal boxes

heated by charcoal. After completing the weld, the part is covered with

asbestos or coal dust or embedded in hot sand to retard cooling.

Cold welding: The more widespread method is cold welding of cast iron with

an electric arc. No preheating is necessary, because the heat is much more

highly concentrated and localised. The finish weld is covered with sheet

asbestos or embedded in hot sand to retard cooling.

Aluminium:

Aluminium oxidizes easily and the film of aluminium oxide that forms on its

surface has a melting point higher than the parent metal and impedes the

fusion of the edges. So, aluminium and its alloys must be covered with

a flux. Preferred welding methods are: TIG, MIG.

Q. Write short note on Brazing or what are the steps in Brazing?

It is the process to join two metal pieces heated to suitable temperatures by

using a filler metal. Filler metals should have melting point lower than the

base metals. During brazing, the base metals are not melted.

1. The surface to be joined are cleaned and fitted closely together.

2. A flux is applied to all surfaces where the filler metal is to flow.

3. After that joint is heated to the proper brazing temperature. Solid filler

metal may be pre placed on the metal pieces and thus melted as the

metal pieces are heated, or it may be applied to the metal pieces after

the brazing temperature is reached.

Butt joint, seam joint, lap joint, pipe joint and edge joint are obtained from

the brazing process.

Fluxes: Compositions of borates, fluorides, chlorides, borax, and boric acid are

the flux materials. These are in the form of powder, paste or slurry.

Filler metals: Brass, Bronze, Copper, Silicon and Silver alloys. These are in the

form of wire, strips, performs, powder or paste.

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Applications:

Assembly of pipe fittings, Carbide tips to tools, Radiators, heat exchangers,

electrical parts, repair of castings and joining of special materials like

stainless steels.

Q. Give short note on Soldering.

Soft soldering: It is used extensively in sheet-metal work for joining parts

that are not exposed to the action of high temperatures and are not

subjected to excessive loads and forces. Soft soldering is also employed for

joining wires and small parts. The solder, which is mostly composed of lead

and tin, has a melting range of 150 to 350°C. A suitable flux is always used

in soft soldering. Its function is to prevent oxidation of the surfaces to be

soldered or to dissolve oxides that settle on the metal surfaces during the

heating process. Although corrosive, zinc chloride is the most common

soldering flux. Rosin is non-corrosive, but it does not have the cleaning

properties of zinc chloride. A blow torch or soldering iron constitutes the

equipment for heating the base metals and melting the solder and the flux.

Hard soldering: It employs solders which melt at higher temperatures

and are stronger than those used in soft soldering. Silver soldering is a

hard soldering method, and silver alloyed with tin is used as solder. The

temperatures of the various hard solders vary from about 600 to 900°C.

The fluxes are mostly in paste form and are applied to the joint with a

brush before heating. In hard soldering, a blow torch constitutes the

equipment.

Q. What are the steps in hand soldering?

A. Parts to be soldered must be thoroughly cleaned.

B. The flux is applied on the surfaces to be joined.

C. The joint is then heated to the melting temperature of the solder by

soldering iron.

D. Solder is applied which melts and flows into the joint.

Documents

3. WELDING - Diploma Engineering college-New …npkauto.com/oldweb/data/notes/second/4e/amp/WELDING.pdf · Advantages: 1. Suitable for ferrous ... Overlap can be less than that for