Mechanical Workshop

Department of Engineering Science &Management

WELDING
Welding is a process for joining two similar or dissimilar metals by fusion. It joins different
metals/alloys, with or without the application of pressure and with or without the use of filler metal. The fusion
of metal takes place by means of heat. The heat may be generated either from combustion of gases, electric
arc, electric resistance or by chemical reaction. The welding is widely used as a fabrication and repairing
process in industries. Some of the typical applications of welding include the fabrication of ships, pressure
vessels, automobile bodies, off-shore platform, bridges, welded pipes, sealing of nuclear fuel and explosives,
etc. Most of the metals and alloys can be welded by one type of welding process or the other. However, some
are easier to weld than others. To compare this ease in welding term ‘weldability’ is often used. The weldability
may be defined as property of a metal which indicates the ease with which it can be welded with other similar
or dissimilar metals.

Welding Processes:
The number of different welding processes has grown in recent years. These processes differ greatly in the
manner in which heat and pressure (when used) are applied, and in the type of equipment used. Among the
different types of welding processes, two commonly used welding processes are discussed here. One process
is electric arc welding and another is gas welding. The most popular process in arc welding is shielded metal
arc welding (SMAW).
Shielded Metal Arc Welding (SMAW) or Manual Metal Arc Welding (MMAW):
SMAW is an arc welding process that uses a flux covered metal electrode to carry an electrical current.
The current forms an arc that jumps a gap from the end of the electrode to the work. The electric arc creates
enough heat to melt both the electrode and the base material(s). Molten metal from the electrode travels across
the arc to the molten pool of base metal where they mix together. As the arc moves away, the mixture of molten
metals solidifies and becomes one piece. The molten pool of metal is surrounded and protected by a fume
cloud and a covering of slag produced as the coating of the electrode burns or vaporizes. Due to the appearance
of the electrodes, SMAW is commonly known as ‘stick’ welding.

SMAW is one of the oldest and most popular methods of joining metal because of its low cost, flexibility,
portability and versatility. Moderate quality welds can be made at low speed with good uniformity. SMAW is
very flexible in terms of the material thicknesses that can be welded (materials from 1/16” thick to several
inches thick can be welded with the same machine and different settings). It can weld many different types of
metals, including cast iron, steel, nickel & aluminum. Some of the biggest drawbacks to SMAW are (1) that it
produces a lot of smoke & sparks, (2) there is a lot of post-weld cleanup needed if the welded areas are to look
presentable, (3) it is a fairly slow welding process and (4) it requires a lot of operator skill to produce consistent
quality welds.

Schematic diagram showing constituents of SMAW

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Many electric arc welding processes make use of a direct current (DC) rather than alternating current. In any
direct current arc, the specification of polarity can be very important. If the electrode is a cathode and is
negative (DCEN), it refers to as direct current straight polarity, or DCSP. If the welding electrode is an anode
or electrode positive (DCEP), this is referred to as direct current reverse polarity (DCRP). In the normal DCSP
condition, the base metal being bombarded by the electrons is hotter than the electrode and a deep,
“penetrating” weld is produced. In contrast, DCRP produces a wider, shallower weld and the electrode is hotter
than the base metal. The choice of current and polarity depends on the process, the type of electrode, the arc
atmosphere, and the metal being welded.
Arc Welding Equipment:
1) Welding machine with Current regulating hand wheel 2) Welding cable 3) Cable connector
4) Earth clamp 5) Channel for cable protection 6) Electrode Holder 7) Electrode
8) Chipping hammer 9) Welding table 10) Leather apron 11) Hand gloves
12) Hand shield/Welding Helmet 13) Wire brush 14) Tong 15) Job
Apart from the above mentioned equipments, the measuring tools, marking tools and filling tools are also
required.

Shielded metal arc welding equipment

Welding machine with Current regulating hand wheel: Both direct current (DC) and alternating current (AC)
are used for electric arc welding, each having its particular applications. DC welding supply is usually obtained
from generators driven by electric motor or if no electricity is available by internal combustion engines. For
AC welding supply, transformers are predominantly used for almost all Arc-welding where mains electricity
supply is available. They have to step down the usual supply voltage (200-400 volts) to the normal open circuit
welding voltage (50-90 volts). In case of DC welding power is required at 8 to 25 volts and 50 ampere.
Comparison between A.C & D.C Welding:

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Welding cable: Welding cables are required for conduction of current from the power source through the
electrode holder, the arc, the work piece and back to the welding power source. These are insulated copper or
aluminum cables.
Electrode holder: Electrode holder is used for holding the electrode manually and conducting current to it.
These are usually matched to the size of the lead, which in turn matched to the amperage output of the arc
welder. Electrode holders are available in sizes that range from 150 to 500 Amps.

Electrode: An electrode is a piece of wire or a rod of a metal or alloy, with or without coatings. An arc is set
up between electrode and work piece. Welding electrodes are classified into following types
(i) Consumable Electrodes: (a) Bare Electrodes (b) Coated Electrodes
(ii) Non-consumable Electrodes: (a) Carbon or Graphite Electrodes (b) Tungsten Electrodes

Consumable electrode is made of different metals and their alloys. The end of this electrode starts melting
when arc is struck between the electrode and workpiece. Thus consumable electrode itself acts as a filler metal.
Bare electrodes consist of a metal or alloy wire without any flux coating on them. Coated electrodes have flux
coating which starts melting as soon as an electric arc is struck. This coating on melting performs many
functions like prevention of joint from atmospheric contamination, arc stabilizers etc.
The size of electrode is measured and designated by the diameter of the core wire in SWG and length, apart
from the brand and code names; indicating the purpose for which they are most suitable.
Non-consumable electrodes are made up of high melting point materials like carbon, pure tungsten or alloy
tungsten etc. These electrodes do not melt away during welding. But practically, the electrode length goes on
decreasing with the passage of time, because of oxidation and vaporization of the electrode material during
welding. The materials of non-consumable electrodes are usually copper coated carbon or graphite, pure
tungsten, zirconiated tungsten etc.

Parts of a coated electrode

Welding Helmet/Hand Screen:

This is used to protect the eyes and face from the rays of the arc and from spatter or flying particles of hot
metal and supervision of weld bead. It is available either in hand or helmet type. The hand type is convenient
to use wherever the work can be done with one hand. The helmet type though not comfortable to wear, leaves
both hands free for the work.
Chipping hammer:
A chipping hammer is used to remove slag which is formed on welds by striking. 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. Molten
metal dispersed around the welding heads, in the form of small drops, is known as spatter. When a flux coated
electrode is used in welding process, then a layer of flux material is formed over the welding bead which
contains the impurities of weld material. This layer is known as slag. Removing the spatter and slag formed
on and around the welding beads on the metal surface is known as chipping.
Wire brush:
A wire brush is used for cleaning and preparing the work for welding.
Welding table:
It is made of steel plate and pipes. It is used for positioning the parts to be welded properly.

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Earthing Clamp: It is used to connect the metallic table with the earthing cable.

Protective clothing: Operator wears the protective clothing (Apron, Glove) such as apron to keep away the
exposure of direct heat to the body.

Tong: It is used to hold the job for placing the same at the desired location.

Chipping Hammer Wire Brush Welding Helmet Earthing Clamp

Edge preparations:
For welding the edges of joining surfaces of metals are prepared first. Different edge preparations may be
used for welding butt joints, which are given in figure.

Welding joints
Some common welding joints are shown in Figure. Welding joints are of generally of two major kinds
namely lap joint and butt joint. The main types are described as under

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Lap Joint
This joint, made by overlapping the edges of the plate, is not recommended for most work. The single lap
has very little resistance to bending. It can be used satisfactorily for joining two cylinders that fit inside one
another. For obtaining stronger joint than the single-lap joint double-lap joint may be employed, but it has
the disadvantage that it requires twice as much welding.
Single-Vee Butt weld joint:
Butt welds are welds where two pieces of metal are to be joined are in the same plane. It is used for plates up
to 15.8 mm thick. The angle of the vee depends upon the technique being used, the plates being spaced
approximately 3.2 mm.
Double-Vee Butt Weld
It is used for plates over 13 mm thick when the welding can be performed on both sides of the plate. The top
vee angle is either 60° or 80°, while the bottom angle is 80°, depending on the technique being used.
Corner and Tee joints:
These joints are used to join two members located at right angles to each other. In cross section, the corner
joint forms an L‐shape, and the tee joint has the shape of the letter T. Various joint designs of both types
have uses in many types of metal structures.
Edge joint
This type of joint is used to join the edges of two or more members lying in the same plane. In most cases,
one of the members is flanged, view E. While this type of joint has some applications in plate work, it is
more frequently used in sheet metal work.

Welding Positions

Flat or Down-hand Welding Position:

The flat position or down hand position is one in which the welding is performed from the upper side of the
joint and the face of the weld is approximately horizontal.
Horizontal Welding Position:
In horizontal position, the plane of the work piece is vertical and the deposited weld head is horizontal. This
position of welding is most commonly used in welding vessels and reservoirs.
Vertical Welding Position:
In vertical position, the plane of the work-piece is vertical and the weld is deposited upon a vertical surface.
It is difficult to produce satisfactory welds in this position due to the effect of the force of gravity on the
molten metal.
Overhead Welding Position:
The overhead position is probably even more difficult to weld than the vertical position. Here the pull of
gravity against the molten metal is much greater.

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JOB NO: ` BUTT JOINT DATE:

Aim: To make a Butt joint using the given two M.S plates by arc welding.
Material: Mild steel plate of size mm – 2 No’s
Welding Electrodes: M.S electrodes mm X mm
Welding Equipment: Air cooled transformer 26KVA, 3 phase supply, amps up to 400.
Tools and Accessories required:
1. Rough and smooth files. 2. Protractor 3. Arc welding machine (transformer type)
4. Mild steel electrode and electrode holder 5. Ground clamp 6. Tongs
7. Face shield 8. Apron 9. Chipping hammer.

Sequence of operations:
1. Marking 2. Cutting 3. Edge preparation (Removal of rust, scale etc.) by filling
4. Try square leveling 5. Tacking 6. Welding 7. Chipping 8. Cooling
9. Cleaning

Procedure:
1. The given M.S pieces are thoroughly cleaned of rust and scale.
2. One edge of each piece is believed, to an angle of 30 0, leaving nearly ¼ th of the flat thickness, at
one end.
3. The two pieces are positioned on the welding table such that, they are separated slightly for better
penetration of the weld.
4. The electrode is fitted in the electrode holder and the welding current is set to be a proper value.
5. The ground clamp is fastened to the welding table.
6. Wearing the apron and using the face shield, the arc is struck and holding the two pieces together;
first run of the weld is done to fill the root gap.
7. Second run of the weld is done with proper weaving and with uniform movement. During the
process of welding, the electrode is kept at 150 to 250 from vertical and in the direction of welding.
8. The scale formation on the welds is removed by using the chipping hammer.
9. Filling is done to remove any spanner around the weld.
Result:
Thus the desired butt joint of mentioned dimension has been made by Arc welding.
Precaution: (To be filled by student)
Job Drawing:

All dimensions are in mm

Not to true scale

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Welding Defects:

1. Lack of Penetration: It is the failure of the filler metal to penetrate into the joint. It is due to (a) Root
gap too small. (b) Electrode size to big. (c)Travel speed too high. (d) Incorrect use of electrode.
2. Lack of Fusion: Lack of fusion is the failure of the filler metal to fuse with the parent metal. It is duo
to (a) Heat input too low. (b) Weld pool too large and running ahead of the arc. (c) Joint included angle
too small (d) Electrode or torch angle is incorrect. (e) Unfavorable bead positioning.

3. Porosity: It is a group of small holes throughout the weld metal. It is caused by the trapping of gas
during the welding process, due to Moisture, for example from incorrectly stored electrodes or fluxes,
humid shielding gas or leaks in water-cooled welding torches. It is also happened due to Moisture, rust,
grease or paint on the plate edges, insufficient gas shielding and welding onto small gaps filled with air.
4. Slag Inclusion: It is the entrapment of slag or other impurities in the weld. It is caused by slag runs
ahead of the weld, insufficient de-slagging between passes, convex passes which produce slag pockets,
and unfavorable bead sequence.

5. Undercuts: These are grooves or slots along the edges of the weld caused by arc voltage too high,
arc too long, incorrect electrode use or electrode angle, electrode is too large for the plate thickness, travel
speed too high.
6. Cracking: It is the formation of cracks either in the weld metal or in the parent metal. It is due to
unsuitable parent metals used in the weld, bad welding technique, the base material is prone to hardening
(because of a high carbon content or other alloying elements), weld cools down too rapidly, hydrogen in
the weld e.g. because of wet weld edges, wrong or damp electrodes or shielding gases.

7. Distortion: Distortion is due to high cooling rate, small diameter electrode, poor clamping and slow arc
travel speed.

8. Blowholes: These are large holes in the weld caused by (a) Gas being trapped, due to moisture.
(b) Contamination of either the filler or parent metals.

9. Excessive Penetration: It is where the weld metal protrudes through the root of the weld. It is caused by
(a) Incorrect edge preparation (b) Too big a heat concentration (c) Too slow a travel.

10. Overlays: These consist of metal that has flowed on to the parent metal without fusing with it. The defect
is due to (a) Contamination of the surface of the parent metal (b) Insufficient heat.
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