Chapter

3
Welding

Aim: To Prepare a Lap Joint by Electric Arc welding.

Materials:M.S. Flat (70 × 50 × 5) mm
Tools & equipments:
A.C. step down transformer/D.C. motor generator, Hand Screen, M.S.
Electrode 3.15 mm, Tong, Chipping hammer, Wire brush, Apron, Hand gloves
etc.
.

Lap Joint

Working Procedure:
1. Marking the flat to given dimensions.
2. Hold the flat in Parallel Jaw Bench vice.
3. Cut the flat with the help of hacksaw Blade.
4. Now take the work piece to the welding table and keep both the work
piece lap position on earthing plate. With the help of welding
equipments tack the both the side ends.
5. Complete the object by one pass bead both sides.
6. Allow the object to cool and the Remove slag and spatter by chipping
hammer and wire brush.
Safety Precautions:
1. Hand Shield should be used to protect the face and eyes.
2. Always wear the apron, hand gloves and shoe.
 3. Apply eye drop (Lucola 10%) after Arc welding.
4. Hot Job never should be catch without tong

Experiment 2

Aim: To prepare a square Butt joint by Electric Arc Welding.

Materials: M.S. Flat (70 × 50 × 5) mm.
Tools & Equipment:
A.C. step down transformer/D.C. motor generator, Hand Screen, M.S. Electrode
3.15 mm, Tong, Chipping hammer, Wire brush, Apron, Hand gloves etc.

Squre Butt Joint

Working Procedure:
1. Marking the Flat to given dimensions with help of Steel Rule, Try Square,
and Scriber.
2. Hold the Flat in Parallel Jaw bench vice.
3. Cut the flat with help of Hacksaw blade.
4. Now take the work piece to the welding table and keep both the work
piece with 1 mm gap on earthing Plate with the help of welding
equipments Tack both side the ends.
5. Complete the welding in down hand position by one pass bead.
6. Allow the object to cool and then remove slag and spatter by chipping
hammer and wire brush.
Safety Precautions:
1. Hand Shield should be used to protect the face and eyes.
2. Always wear the apron, hand gloves and shoe.
3. Apply eye drop (Lucola 10%) after Arc welding.
4. Hot Job never should be hold without tong.

WELDING
 Welding is a materials joining process which produces coalescence of materials
by heating them to suitable temperatures with or without the application of
pressure or by the application of pressure alone, and with or without the use of filler
material.
Types of Welding:
The welding is broadly divided into the following two groups.
1. Pressure Welding (Non–Fusion Welding):
In this type the work pieces are heated to there melting point and then the
work pieces are joined together by applying pressure on them in this case no filler
material are required. The Pressure welding is further classified as follows.
Resistance Welding (Spot, Butt, Seam, Percussion, and Projection), Forge Welding,
and Solid state welding (Friction, Ultrasonic, Explosive, and Stud).
RESISTANCE WELDING :
In this processes a heavy current is passed between two solids in contact
resistance at the contact surface generates heat according to the relationship.
H = I2RT (where H- heat, I- current, R- resistance, T- time)
The heat generated over a limited area in the part of the current softens the
metals locally. A Pressure applied over the solids completes the weld.
FORGE WELDING:
Generally wrought iron can be satisfactory joined by forge welding. In this
processes the parts to be forge are heated to over required melting point then parts
are placed on the anvil end to end and are hammered together.
2. Non–Pressure Welding (Fusion Welding):
In this type the edges of work pieces to be joined and the filler material are
heated to a temperature above the melting point of the metal and then allowed to
solidify. The non–pressure or fusion welding is further classified as follows.
Electric Arc welding, Gas Welding, Metal inert gas (MIG) welding, Tungston inert gas
(TIG) welding, Laser beam welding, Electro Slag welding, Submerged Arc welding
(SMAW), Thermit Welding etc.
Arc Welding:
In Arc welding the heat required for joining the metals is obtained from an
electric Arc. A.C. Stepdown transformers, DC generator and DC Rectifier sets are
used as arc welding machines. These machines supply high electric currents at low
voltages and an electrode used to produce the necessary arc. The electrode serves
as the filler rod and arc melts the surfaces so that the metals to be joined are
actually fused together.
 Arc Welding
Striking an arc
The following are the stages and methods of striking an arc and running a
bead:
(i) Strike and Withdraw: In this method, the arc is started by moving the
end of the electrode on to the work with a slow sweeping motion.
(ii) Rubbing and Withdraw: In this method, the arc is started by keeping the
electrode perpendicular to the work and touching or bouncing lightly on the work.
This method is preferred as it facilitates restarting the momentarily broken arc
quickly.
It is better to make the bead in one continuous sweep. If the welding is stopped for
any reason, chipping the slag from the crater at the end of the bead and restart the
welding.
In addition to the welding machine, certain accessories are needed for carrying out
the welding work.
ARC WELDING EQUIPMENT
(i) Arc welding power source
(a) AC type
(b) DC type
(a) AC Power Source
AC step down transformer: A welding transformer changes high voltage,
low ampere power to low voltage, high ampere welding power.
AC step down transformer is the least expansive, lightest and smallest welding
machine. It’s efficiency is 85% this is an advantage, thereby preventing its over
heating when working under heavy load conditions Instead of being oil–cooled,
some transformers are air–cooled.
 Circuit diagram AC Step-down Transformer
(b) DC power source:
(i) DC Generator: A DC welding generator product direct current in either
straight or Reverse Polarity–depends upon the kind of electrode used or the
material to be welded. A Polarity switch on most machines provides straight or
Reverse Polarity.
Staright Polarity: When the electrode is connected to the(– ve) Pole and object
connected to the (+ ve) pole.It is called as staright Polarity. Heat generated on (–
ve) is 1/3 and (+ ve) is 2/3.
Reverse Polarity: Similarly, when the electrode is connected to the (+ ve) and
object connected to the (– ve) ,this is referred to as Reverse Polarity.

Circuit diagram DC generator

DC Rectifiers:–
 Generally machines are essentially AC step down transformer containing an
electrical device known as rectifier which changes AC to DC. The rectifier may
consist of; metal plates coated with a selenium compound or silicon diodes, each
unit having the special property of allowing the current to flow in one direction only.
WELDING ELECTRODES
An electrode is a piece of wire of a metal or alloy with or without flux covering,
which carries current for welding. The composition of the electrode depends upon
the metal to be welded like to weld mild steel; electrode of similar composition will
be prepared, in order to get a homogeneous welded joint.
Generally two types of electrode are used in welding.
(i) Non–Consumable: Tungsten electrodes and carbon electrodes.
(ii) Consumable: They are low melting point electrodes made up of different
metals and their alloy.
Electrode coating may be light, medium and heavy. The coating may have
(i) Slag forming constituents like silicates of sodium, Potassium, Magnesium,
iron oxide, china clay & etc.
(ii) Deoxidizing constituents like Ferro–silicon and ferro–maganese, refine the
molten metal.
(iii) Allowing constituents like maganese, molybdenum, cobalt, vanadium,
chromium and nickel, etc.
(iv) Arc stability and binding agent ,saw dust,coal dust,iron powder,calcium
carbonate and cellouse etc.
Welding Cables: – The welding current is conducted from the power source
through the electrode holder, the arc, the work piece and back to the welding power
source welding cables or leads are well insulated with rubber, fabric reinforcing
layer.

Welding Electrode
Electrode holder: – Electrode holder is a holding device to hold the electrode
and conducting current to it. The jaws of the holder are made of copper and its
alloys.
 Electrode Holder
Chipping hammer: A chipping hammer used for removing slag formation on
welds. One end of the head is sharpened like a cold chisel and the other, to a blunt,
round point. It is generally made of tool steel.

Chipping Hammer
Wire brush: A wire brush used to clean the surface of base metal of rust, oil,
paint etc. before the welding operation.

Wire Brush
Tong: A tong used to hold the hot object.
 Tong
Hand Screen /Shields: It is a protective device, used in arc welding, for
shielding and protecting the face and eyes of the operator. A hand shield is held in
the hand of the operator and is equipped with a suitable shade glass. Shields are
made of light weight non–reflecting fibre.

Hand Shield/Hand Screen

Protective Clothing:–
(i) Apron: Apron provides protection to the clothes of the operator. It offers
best protection against not spattering particles.

Leather Apron

(ii) Hand gloves: – Hand gloves protect the hands of the operator from
ultraviolet rays and spattering hot metal gloves may be made up of
leather or of a combination of cloth and asbestos.
 Pairs of Hand Gloves
(iii) Shoes: – An operator should wear high topped shoes that go over the
ankle. Ordinary shoes will not prevent small globules of molten metal from
dropping into shoes.
Ground Clamp: It is connected to the end of the ground cable and is clamped
to the work or welding table to complete the electric circuit.

Ground Clamp
Working Table: It is made of steel plate and pipe. It is used for positioning the
parts to be welded properly.
Welding Positions: The welding positions are classified as follows.
(i) Flat position (Down hand Position) : In this position the filler metal is
deposited from the upper side of the joint with the face of the weld.

Flat/Down Hand Position

(ii) Vertical Position: In this position the line of welding is in a vertical plane
and the weld is deposited upon bottom to top or top to bottom vertical
surface.
 Vertical Position
(iii) Horizontal Position: In this position the weld is deposited upon the side
of a horizontal and welding done left to right or right to left.

Horizontal Position
(iv) Overhead Position: In this position the electrode remains below the
work piece.

Overhead Position
Welding Joint: Generally five basic types of joints are used in fusion welding:-
(1) Butt Joint: – The butt joint is obtained by welding the ends or edges of
the two plates which are approximately in the same plane with each
other.
 Butt Joint
(2) Lap Joint: The Lap joint is obtained by over lapping the plates and then
welding the edges of the plates.

Lap Joint
(3) Corner Joint: The corner joint is obtained by joining the edges of two
plates whose surfaces are at an angle of 90º to each other.

Corner Joint
(4) Tee Joint: The Tee Joint is obtained by joining two plates whose surfaces
are at right angle to each other.

Tee Joint
(5) Edge Joint: Edge joint is obtained by joining two parallel plates.

Edge Joint
Welding defects: Some of the common weld defects.
1. Cracks: Cracks in the weld may arise from locked up stresses set up by
non uniform heating and cooling.

Cracks
2. Incomplete Penetration: The Lack of thorough and complete union
between the deposited and Parent metal. This is due to faulty welding
conditions.

Incomplete Penetration
3. Slag inclusions : Slag inclusions may be in the form of slag or any other
foreign material, which does not get a chance to float on the surface of
the solidifying weld metal and thus gets entrapped inside the same
inclusion lower the strength of the joint and make it weaker.
4. Porosity: The formation of blow holes, gas pockets or roughness on the
surface of the weld. This is due to the presence of gases in the metal,
moisture in the flux.

Porosity