WELDING SHOP
Mechanical Engineering workshop is integration of
the following machine:
·
Machine shop
·
Black smithy shop
·
Foundry shop
·
Welding shop
·
Carpentry shop
·
Filling shop
· Sheet metal shop
Welding shop:
Welding technique used
for joining metallic parts usually through the application of heat. This technique was discovered
during efforts to manipulate iron into useful shapes. The process of
carburization (unite with carbon) of iron to produce hard steel was known at
this time, but the resultant steel was very brittle. The welding
technique—which involved inter layering relatively soft and tough iron with
high-carbon material, followed by hammer forging—produced a strong, tough
blade.
Brief history:
The first real attempt to adopt welding processes on a wide scale was made during World War I. By 1916 the oxyacetylene process was well developed, and the welding techniques employed then are still used. The main improvements since then have been in equipment and safety. Arc welding, using a consumable electrode, was also introduced in this period, but the bare wires initially used produced brittle welds. A solution was found by wrapping the bare wire with asbestos and an entwined (link together) aluminum wire. In the 1940s the tungsten-inert gas process, using a non-consumable tungsten electrode to perform fusion welds, was introduced. In 1948 a new gas-shielded process utilized a wire electrode that was consumed in the weld. More recently, electron-beam welding, laser welding, and several solid-phase processes such as diffusion bonding, friction welding, and ultrasonic joining have been developed.
Basic
principles of welding:
The
basic principle of wending shop is as a coalescence
of metals produced by heating to a suitable temperature with or without the
application of pressure, and with or without the use of a filler material. In fusion welding a heat source
generates sufficient heat to create and maintain a molten pool of metal of the
required size. The heat may be supplied by electricity or by a gas flame. For
example, Electric resistance welding. Welding produces stresses in
materials. These forces are induced by contraction of the weld metal and by
expansion and then contraction of the heat-affected zone. The unheated metal
imposes a restraint on the above, and as contraction predominates, the weld
metal cannot contract freely, and a stress is built up in the joint. This is
generally known as residual stress, and for some critical applications must be
removed by heat treatment
of the whole fabrication.
Types of
welding shop:
There are a wide variety of
welding processes. Several of the most important are discussed below
1)
Forge welding.
The process was first employed to make
small pieces of iron into larger useful pieces by joining them. The parts to be
joined were first shaped, then heated to welding temperature in a forge and
finally hammered or pressed together. The
2) Arc welding
Shielded
metal-arc
welding accounts for the largest total volume of welding today. In this process
an electric arc is struck between the metallic electrode and the work piece.
Tiny globules (small globe) of molten metal are transferred from the metal
electrode to the weld joint. Since arc welding can be done with either
alternating or direct
current,
some welding units accommodate both for wider application. Gas-shielded arc welding, in which the arc is shielded (Protect)
from the air by an inert gas such as argon or helium, has become increasingly
important because it can deposit more material at a higher efficiency and can
be readily automated. Two processes known as spray arc and short-circuiting arc
are utilized. Metal transfer is rapid, and the gas protection ensures a tough
weld deposit. Plasma welding is an
arc process in which hot plasma is the source of heat. It has some similarity
to gas-shielded tungsten-arc welding, the main advantages being greater energy
concentration, improved arc stability, and easier operator control. Better arc
stability means less sensitivity to joint alignment and arc length variation.
In most plasma welding equipment, a secondary arc must first be struck to
create an ionized gas stream and permit the main arc to be started.
3)
Electron-beam welding
.
In
electron-beam welding, the workpiece is bombarded with a dense stream of
high-velocity electrons. The energy of these electrons is converted to heat
upon impact. A beam-focusing device is included, and the workpiece is usually
placed in an evacuated chamber to allow uninterrupted electron travel. Heating
is so intense that the beam almost instantaneously vaporizes a hole through the
joint. Extremely narrow deep-penetration welds can be produced using very high
voltages—up to 150 kilovolts. Workpieces are positioned accurately by an
automatic traverse (pass over) device; for example, a weld in material 13 mm
(0.5 inch) thick would only be 1 mm (0.04 inch) wide. Typical welding speeds
are 125 to 250 cm (50 to 100 inches) per minute.
4) Laser welding
Laser welding is
accomplished when the light energy emitted from a laser source is focused upon
a workpiece to fuse materials together. The limited availability of lasers of
sufficient power for most welding purposes has so far restricted its use in
this area. Another difficulty is that the speed and the thickness that can be
welded are controlled not so much by power but by the thermal conductivity
of the metals and by the avoidance of metal vaporization at the
surface. Particular applications of the process with very thin materials up to
0.5 mm (0.02 inch) have, however, been very successful. The process is useful
in the joining of miniaturized (electronic products)
electrical circuitry.
5) Resistance welding
Spot, seam, and projection welding are resistance welding processes in which the required heat for joining is generated at the interface by the electrical resistance of the joint. Welds are made in a relatively short time (typically 0.2 seconds) using a low-voltage, high-current power source with force applied to the joint through two electrodes, one on each side. Spot welds are made at regular intervals on sheet metal that has an overlap. Joint strength depends on the number and size of the welds. Seam welding is a continuous process wherein the electric current is successively pulsed into the joint to form a series of overlapping spots or a continuous seam. This process is used to weld containers or structures where spot welding is insufficient. A projection weld is formed when one of the parts to be welded in the resistance machine has been dimpled or pressed to form a protuberance that is melted down during the weld cycle.
6) Cold
welding
Cold welding, the joining of materials
without the use of heat, can be accomplished simply by pressing them together.
Surfaces have to be well prepared, and pressure sufficient to produce 35 to 90
percent deformation at the joint is necessary, depending on the material.
Lapped joints in sheets and cold-butt welding of wires constitute the major
applications of this technique. Pressure can be applied by punch presses,
rolling stands, or pneumatic tooling.
7) Ultrasonic
welding
Ultrasonic
joining is achieved by clamping the two pieces to be welded between an anvil
and a vibrating probe or sonotrode. The vibration raises the temperature at the
interface and produces the weld. The main variables are the clamping force,
power input, and welding time. A weld can be made in 0.005 second on thin wires
and up to 1 second with material 1.3 mm (0.05 inch) thick. Spot welds and
continuous seam welds are made with good reliability.
8)
Explosive
welding
Explosive welding takes place when two
plates are impacted together under an explosive force at high velocity. The
lower plate is laid on a firm surface, such as a heavier steel plate. The upper
plate is placed carefully at an angle of approximately 5° to the lower plate
with a sheet of explosive material on top. The charge is detonated from the
hinge of the two plates, and a weld takes place in microseconds by very rapid
plastic deformation of the material at the interface. A completed weld has the
appearance of waves at the joint caused by a jetting action of metal between the plates.
9) Gas welding
Gas welding also commonly
referred as Oxyacetylene welding, involve the combustion of oxygen and
acetylene which produce a relatively hot flame at temperature of 3000 degree. A
rod of filler metal is used which is melted in flame and the molten metal flow
into the join being welded.
Health Effect:
The
primary heath effects associated with welding fumes are:
a. Irritation of
the respiratory system caused by gases
b. Radiation (
arc can generate three types of radiation, Ultra-violet, visible and infra red
radiation)
c. Ultra violet
can cause damage to skin and eyes
d. Visible light
dazzles eyes and impairs vision
e. Infra red
damages skin and eyes
f.
Fire
is an inherent hazard associated with gas welding processes
g. Noise level is
unacceptable
h. Degreasing
(Remove
grease) solvents and coating on the base materials can also decompose e.g.
Halocarbons can break down to release carbonyl chloride, hydrogen chloride and
dichloroacetyl chloride.
Health and
safety considerations:
a. Reduce the
risk of exposure to UV, infrared and visible radiation
b. Protect face
and eyes using a suitable welding shield
c. Protect the
body by wearing suitable clothing
d. Reducing the
risk of fire and explosion
e. Remove
flammable material from welding area
Basic welding equipment:
Here is a
short list of the basic welding
equipment to consider for your shop:
·
Welding
Helmet
·
Welding
Gloves
·
Welding
Blankets
·
Welding
Curtains
·
Welding
Jacket
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