Welding is a process of joining similar metals by the
application of heat with or without application of
pressure and addition of filler material. The result is
a continuity of homogenous material, of the
composition and characteristics of two parts which
are being joined together.
Welding may be classified under two broad
(1)Plastic welding, (2) Fusion welding
In the plastic or pressure welding, the pieces to be joined
are heated to a plastic state and then forced together by
Ex. Forge welding, resistance welding, thermit welding, gas
In the fusion welding, the material at the joint is heated to
molten state and allowed to solidify. Ex. Gas welding, arc
welding, thermit welding
In the weld, metal solidifies from liquid state and is
therefore essentially a casting. In a single – run weld , long
columnar grains will grow from the sides of the weld and
will meet at the centre of the weld deposit forming a plane
In a multi-run weld, the first run, is as before. The second
run normalizes the first layer causing considerable grain
refinement (giving stronger joint). But the possibility of slag
and gas inclusions will increase.
The contour of weld by forming notches can affect both
fatigue and low temperature properties of a structure,
(welding is a skilled work).
Heat Affected Zone
The effect of welding heat upon the metal immediately
adjacent to the weld is likewise important and dependent on
chemical composition and thermal conductivity of parent
HAZ ranges from over heated area near the weld metal to an
under annealed structure farther away. Pre heating is often
employed when welding C.I and high carbon steel, since it
slows down the cooling rate of parent metal.
Done by burning a combustible gas with air or oxygen in a
concentrated flame of high temperature. The purpose of the
flame is to heat and melt the parent metal and filler rod of a
Equipment is inexpensive
Serves adequately in many job and general repair shops
GAS WELDING TECHNIQUE
According to the position of welding, all welds are
Down hand or flat weld deposited on a horizontal
surface so that the flame is above the weld.
Vertical welds deposited on a vertical surface
(upwards or downwards)
Inclined welds deposited up or down an inclined
Horizontal welds deposited on a vertical surface in a
Overhead welds deposited on a horizontal surface in
any direction so that the weld is above the flame.
This process uses a torch similar to a Bunsen burner
and operates on the Bunsen burner principle. The air
is drawn into the torch required and mixed with the
fuel flame. The gas is then ejected and ignited,
producing an air-fuel flame.
The common fuels used are acetylene, natural gas,
propane and butane. This type of welding has limited
use since the temperature is lower than that attained
by other gas processes.
The air-fuel welding processes are used in lead
welding and many low-melting-temperature metals
and alloys like in brazing and soldering processes.
The oxygen-hydrogen process were once used
extensively to weld low temperature metals like
aluminium, lead and magnesium, but it is not popular
today because more versatile and faster welding
process such as TIG (tungsten inert gas) and MIG
(metal inert gas) have replaced the oxygen-hydrogen
flame. The process is similar to oxygen-acetylene
system, with the only difference being a special
regulator used in metering the hydrogen gas.
Accomplished by melting the edges or surface to be joined by gas
flame and allowing the molten metal to flow together, thus forming
a solid continuous joint upon cooling.
This process is suitable for joining metal sheets and plates having
thickness of 2 to 50 mm.
With material thicker than 15 mm, additional metal called filler
metal is added to the weld in the form of welding rod.
The composition of the filler rod is usually the same as that of the
part being welded. To remove the impurities and oxides present on
the surfaces of the metal to be joined and to obtain a satisfactory
bond a flux is always employed during the welding except mild
steel which has more manganese and silicon that act as deoxidizing
The correct adjustment of the flame is important for
reliable works. A neutral flame is produced when O2 and
acetylene are supplied to torch in equal volumes. The heat
is generated with a pair of chemical reactions which occur
at the inner cone, where the temperature reaches between
3050 to 3450 deg C.
C2H2 + O2 → 2CO + H2 + heat
The secondary combustion process in the outer envelope,
flame temperature is around 2100 deg C near the inner
cone and around 1250 deg C at the end point of the flame.
4CO + 2H2 + 3O2 → 4CO2 + 2H2O + heat
A neutral flame has 2 definite zones
1. Sharp brilliant cone extending short distance from the tip of
torch – develops heat, temp 2100 deg C
2. Outer cone or envelope, faintly luminous bluish colour –
protects the metal from oxidation.
Uses: Welding steel, stainless steel, CI, Cu, Al
Carburizing flame, temp 2700 deg C, there is excess of
acetylene. The flame has 3 zones,
1. Sharp inner cone
2. Intermediate cone of whitish colour
3. Bluish outer cone
Oxidizing flame, temp 3400 deg C, there is excess of oxygen,
the flame has 2 zones
1. Small inner cone with Purple tinge
This flame is necessary for welding brass. In steel this results in
large grain size, high brittleness
Welding torch – tool for mixing oxygen & acetylene
Welding tip – diameter of tip opening depends on the type of metal to
be welded and its thickness.
Pressure regulator – to reduce the cylinder pressure to the required
working pressure. For < 25 mm thickness, reg. pre. is 0.15 to 0.7 kgf /
cm2 and for > 25 mm thickness, reg. pre. is 0.98 kgf / cm2.
Hose & hose fittings
Goggles, Gloves & spark lighter
Gas cylinder – Oxygen gas cylinder, charged with 40 L at 154 kgf / cm2
Acetylene gas – low (0.07 kgf / cm2) & high pressure (1.0 kgf / cm2)
Low pressure acetylene is generated and used in site. To prevent air
blowing back, a back pressure valve must be introduced between the
blow pipe & gas holder.
Every valve has a safety device to provide against dangerous pressure.
In addition to the above welding rods & fluxes are employed.