Processes
- Welding
Welding
is the process of fusing two metals together in their molten
state; so can be described on a basic level as melting the
relevant areas of the work and holding them together until
the metal has solidified. Additional metal may be added
to the weld if necessary. The field of welding is very large
and varied; so only the basic principles will be covered
in this section.
There are four main types of welding; Oxy–acetylene,
Arc, T.I.G and M.I.G. |
Oxy-acetlyene Welding is similar in principle to hard
soldering; an oxy–acetylene torch is used to heat up the
work, and usually a filler rod of the same metal is also heated
to its melting point and fed into the heated area, so that it
runs onto and fills the area being welded. The metal solidifies
and forms a bond of similar strength to the parent metal being
welded (unlike hard soldering, which is weaker).
This form of welding requires perhaps the most practice and
skill to achieve an acceptable weld, as there are many critical
areas which need to be carried out or set correctly – the
temperature of the flame, the speed the filler rod is fed into
the weld, the speed of the weld itself and so on.
The Oxy–acetylene torch produces an extremely hot flame,
and care must be taken in selecting the correctly sized nozzle
for the job in hand. There are a variety of different sized
nozzles available, ranging from those which concentrate the
flame and are used for cutting through materials. At the other
end of the scale, there are nozzles that produce a more dispersed
flame, which is used for heating materials. The flame used for
welding must be hot enough to heat the area to be welded and
to melt the filler rod, but must not cut through the work.
Use of the Oxy-acetylene equipment requires a basic amount
of knowledge, so that the tool can be operated safely, correctly
and efficiently.
In use it must be made sure that the pressure of both the oxygen
and Acetylene (now correctly known as Ethyne) are set correctly
(around 2 and 5 bar respectively), and that the valves on the
torch are set correctly to give the desired flame. The acetylene
is always lit first, and then the oxygen added to turn the soot
red flame into the high temperature blue flame required for
welding.
SAFETY
Risk assessments must be carried
out and the appropriate safety guidelines followed.
Correct procedures must be adopted for use and handling of pressurised
gasses.
Oxygen is a very dangerous gas as it has no smell and is difficult
to detect, however in an oxygen rich environment combustion
is easily started and maintained.
The most obvious danger is one of burns.
On modern equipment there are cut off valves situated at the
top of the gas bottles, to prevent the flame flashing back into
the gas supply and causing an explosion. |
Arc Welding
is the oldest example of currently used electric welding procedures,
and perhaps the most straight forward to use and understand.
The principle of arc welding is simple; a relatively large current
is passed through a welding rod (or filler rod) of the appropriate
metal (Mild steel for welding steel etc.) and a circuit is completed
through it when it comes into contact with the earthed metal
being welded. The join between the welding rod and the work
being welded creates an area of high resistance, which produces
enough heat to melt the rod at that point. The rod is drawn
along the work, creating a line of molten metal that solidifies
to create a strong bond between the two metals being welded
together.
There are fewer variables involved in arc welding than in the
Oxy-acetylene process detailed above – the thickness of the
filler rod and the current are perhaps the most crucial variables
in the system; higher currents are used for welding thicker
metals – and require thicker rods.
If the current used is too low for the thickness
of metal being welded the weld will not penetrate sufficiently
into the metals, and hence will be weak. If the current is set
too high, it is likely that the electrode (welding rod) will
splatter (creating a messy weld) or at worst burn through the
work.
As the temperature of the arc can reach as high as 6000oC,
an amount of oxides are produced – these can cause problems
as they form on the metals being welded, creating a barrier
between the work and the filler rod. A layer of oxide both insulates
the metals (meaning the arc cannot be struck) and may affect
the strength and general quality of the weld. To overcome these
problems, the filler rod is coated in flux which, when burned,
produces a shielding gas that protects the metal from the oxygen
in the air. The remains of the flux solidify on the metal in
the form of "slag" – which also protects the weld
against oxidation until it has cooled.
This layer can simply be chipped off with a welding hammer once
the weld has cooled, and sometimes comes off by itself as the
metal contracts.
As with all forms of welding and soldering, it is important
that the areas of the work are cleaned thoroughly of impurities
and oxide before welding takes place; this can be carried out
with a wire brush, or abrasive paper.
M.I.G.(Metal Insert
Gas) Welding is similar in basic principle to arc welding;
using a high current to melt a filler metal onto the work –
the difference lies in the feed of the filler metal and the
method used to combat oxidation. Instead of a filler rod being
used to add metal to the work, a continuous wire feed is run
through a hand held "gun" from a roll inside the welder.
During the wire feed, inert argon gas is expelled from the nozzle
of the gun around the area of the weld, preventing oxidation
of the hot metal.
The feed of the gas and wire are controlled by a trigger mechanism
on the gun, which when depressed initiates the feed.
The speed of the wire feed and current being put through it
can be adjusted and preset on the welder itself. This form of
welding is perhaps easier than arc welding, as (if the machine
is set up correctly) the user can concentrate on the path of
the weld as opposed to the feed of the filler metal, as the
welder controls the feed
T.I.G (Tungsten
Insert Gas) Welding is the most recently developed of the
welding methods discussed here, and shares some similarities
with arc, oxy-acetylene and M.I.G welding.
As with M.I.G welding, a hand held "gun" is used to
control the weld, and an electric current is passed between
the gun and the work via a metal tip on the gun. Unlike M.I.G
welding, where the tip is homogenous to the metal being welded
and acts as a filler rod, T.I.G welding uses a fixed tungsten
bit, which does not come into contact with the work – it is
drawn across the surface at a distance of around 1mm, melting
the metal below as it does so. A filler rod can be fed into
the weld with the user’s free hand as with oxy-acetylene welding.
For more information
on TIG Welding click here
SAFETY
Welding masks are required for
all forms of electrical resistance welding, and to a lesser
extent oxy–acetylene welding as well. Although their primary
function is to protect the eyes from the intense light given
off by the arc, larger types also serve to protect the face
from hot sparks and arc burn. |
Material
Preparation
Across these different types
of welding, there are many techniques that are common to all.
The preparation of the metal to be welded is an example, it
must always be clean and free of oxide.
If thick pieces of metal are to be joined, it is a good idea
to cut a "V" section on one or both of the edges of
the pieces to allow the weld to penetrate sufficiently, and
create a strong joint.
These edge preparation techniques are relevant to all forms
of welding, as well as some types of hard soldering such as
brazing and silver soldering.
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