Welding is a process of joining
similar metals by application of heat with or without the application of
pressure and addition of filler material
Modern methods of welding may
be classified under two broad categories.
a)
Pressure welding
b)
Fusion or non-fusion welding
Pressure welding
In the ‘pressure welding
process’ the pieces of metal to be joined are heated to a plastic state and
then forced together by external pressure exerted by hammering, rolling or
pressing to effect the weld. Forge welding, resistance welding and thermit
welding are of this type.
Fusion welding
In the ‘fusion welding process’
the material at the joint is heated to fusion temperature and a filler metal is
added externally. The weld is then allowed to cool down. Electric arc welding,
electric resistance welding, gas welding and induction welding are included in
this category.
General welding conditions
The following general
conditions are to be observed for getting strong and efficient welded joints.
a) Keep the surfaces to be welded clean and free
from foreign material. This is done by wire brushing, machining or sand
blasting. If the surfaces are not properly cleaned, gas and the slag inclusions
in the weld material will remain, causing weak and brittle joint.
b) Carry out the edge preparation properly to
deposit and fuse weld material into the parent material. This will cause better
penetration of weld material upto the opposite side.
c) Create a non-oxidising atmosphere at the point
where the welding is done.The
tendency to oxidise increases with temperature. At high temperatures, oxidation
of weld metal is likely to have serious weakening effects in the weld. This
influence is counteracted by the use of a flux which removes oxides and permits
a better bond between metals. In electric arc welding flux coated electrodes
are used. In inert gas welding, argon, carbon-dioxide shield is used round the
electrode. In submerged arc welding, the arc is struck under the flux to avoid
contact with the atmosphere and so on.
d) Maintain the required speed of the welding
process, so as to avoid oxidation at high temperature. Some processes are
naturally quicker than others. Select such processes as far as possible.
e) Use the correct quality and size of electrode,
according to its applications.
f) Have a perfect control over the selected
process.e.g. control over the voltage, current and polarity in case of electric
arc welding; control over oxygen and acetylene pressures and nozzle size, etc.
Application of welding processes
A few of them are given below
1) Fabrication of steel structures – frame work of
industrial buildings, bridges, machine frames, towers etc.
2)Welding processes are employed extensively in
the manufacture of automobile bodies, aircrafts, railway coaches etc.
3)
Machine tools, building industry use welding.
4)
In the oil industry, welding is extensively used
at refineries and in pipe line fabrication.
5) The ship building industry is dependent on
welding.
6) Pressure vessels manufacturing industry–boilers and reservoirs and tanks for process equipment in chemical, sugar,
paper and cement industries.
7) General machine repair and maintenance work.
8) The economic and other advantages of welding has
also been felt in the casting industries, since many machine parts that were
formerly cast are now made of steel members, welded together. Such a
construction has the advantage of being lighter and stronger than cast iron.
Types of welds
1.
Bead type
2.
Fillet type
3.
Groove type
4.
Plug type
Fig.
Bead weld
These are used for making edge
joints in thin materials and for building up surfaces. The latter, known as
hard surfacing or hard facing consists of the overlaying by welding of a more
wear-resisting material upon a basic metal blank. This may consist of a high
alloy steel on a mild steel blank or a non-ferrous alloy on a tool steel blank.
Filler weld
These are used for tee joints,
lap joints and corner joints. The size of the fillet weld is measured by the
‘leg length’ of the largest 450 right angle triangle that can be
inscribed within the contour of the cross section of the weld.
Groove welds
The size of such welds is the
depth of the groove, which may be usually taken as the thickness of the plates
to be joined (say ‘t’). Welding from both sides with a double-groove
preparation is to be preferred where accessibility and shop facilities permit,
because less weld metal is required and a saving in time and materials is
effected. The choice of the groove type depends on the type of joint to be
welded and the thickness of the parts.
Fig.
Plug weld
These are used to connect two
parts of welding through a round hole or a slot in either one of the parts.
Plug welds are used for welding stay bolts to thick plates and for
strengthening the support of a fixture, bolted or welded plates.
Types of welded joints
Bead, groove and fillets welds
are used in six basic types of joints, namely
1.
Butt joint
2.
Lap joint
3.
Edge joint
4.
Corner joint
5.
Tee joint
6.
Plug joint
Lap and butt joints are most
commonly used in resistance welding, gas welding and electric arc welding
processes.
In selecting the type of joint
and its weld preparation, the main factors to be considered are
a.
The nature of the load
b.
The cost of the edge preparation and welding
c.
The accessibility for welding
