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L4-Gas Welding.ppt
1. IPE 331: Production Process
Gas welding
Dr. A. B. M. Mainul Bari
Assistant Professor
Dept of IPE, BUET.
2. Gas welding is a fusion welding process. It joins metals, using the heat
of combustion of an oxygen/air and fuel gas mixture.
OFW employs several types of gases such as Acetylene, Hydrogen,
Methylacetylene-Propadiene (MAPP), Propylene, Propane, Butane,
Natural Gas.
Oxy-hydrogen welding was the first gas process to be commercially
developed. The maximum temperature developed by this process is
1980°C.
The most used oxyacetylene welding process uses a combination
of oxygen and acetylene gas to provide a high temperature flame
which has a flame temperature of 3500°C.
Widely used in welding tubes and pipes
Oxyfuel gas is also used in flame cutting torches to cut and separate
metal plates and other parts
Oxyfuel Gas Welding (OFW)
4. There are three distinct types of oxy-acetylene flames, usually termed:
• Neutral
• Carburizing (or “excess acetylene”)
• Oxidizing (or “excess oxygen” )
◦ The type of flame produced depends upon the ratio of oxygen to acetylene
in the gas mixture which leaves the torch tip.
The neutral flame is produced when the ratio of oxygen to acetylene, in the
mixture leaving the torch, is almost exactly one-to-one.
It’s termed ”neutral” because it will usually have no chemical effect on the
metal being welded.
It will not oxidize the weld metal; it will not cause an increase in the carbon
content of the weld metal.
The temperature of the neutral flame is of about 5900ºF.
The flame has a nicely defined inner cone which is light in color and
surrounded by an outer flame envelope produced by combination of
oxygen, in the air, superheated carbon monoxide, and hydrogen gas from
the inner cone. This is much darker blue than the inner cone.
Used for welding-mild steel, cast iron, stainless steel, aluminium, copper.
Flame definition
5. The Carburizing flame is created when the proportion of acetylene in the
mixture is higher than that required to produce the neutral flame.
A reducing flame can be recognized by acetylene feather which exists
between the inner cone and outer envelope. The outer envelope is longer
than the neutral flame and much brighter in color.
A reducing flame does not consume the available carbon, so temperature is
much lower (5500ºF).
With iron and steel it produces very hard brittle substance known as iron
carbide. This chemical change makes the metal unfit for many application
in which the weld may need to be bent or stretched.
It can be used for surface hardening purposes or to ensure the absence of
the oxidizing condition. [Non ferrous alloys and high carbon steel.]
Flame definition
6. The oxidizing flame results from burning a mixture which contains more
oxygen than required for a neutral flame.
An oxidizing flame can be recognized by the small white cone which is
shorter, much bluer in color and more pointed than that of neutral flame.
The outer envelope is much shorter.
An oxidizing flame is burns with a decided loud roar.
The temperature of the oxidizing flame is of about 6300ºF.
At high temperature it will oxidize or ”burn” some of the metal being welded
to form hard brittle, low strength oxides.
An oxidizing flame is of limited use in welding. Used for welding-copper
base metal, zinc base metal, manganese steel and cast iron.
The oxidizing atmosphere in these cases, creates a base metal oxide that
protects the base metal. For example, in welding brass, the zinc has a
tendency to separate and fume away. The formation of a covering zinc
oxide prevents the zinc from dissipating.
Flame definition
7. Fig : Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting
operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing flame. The gas
mixture in (a) is basically equal volumes of oxygen and acetylene.
Type of Flame
8. Oxyacetylene Welding (OAW)
Fusion welding performed by a high temperature flame from
combustion of acetylene and oxygen
Flame is directed by a welding torch
Filler metal is sometimes added
◦ Composition must be similar to base metal
◦ Filler rod often coated with flux to clean surfaces and prevent
oxidation
• Flux are available as powder, paste or liquids.
• No flux for steel.
• Flux is used for cast iron, stainless steel and most non-ferrous
metals other than lead, zinc and some precious metals.
9. The max temperature of the oxy-acetylene flame is 3100-3300 ͦ C and
the center of the heat concentration is just off the extreme tip of the
white cone. Combustion of the gas mixture is recognized as taking
place in two stages:
Primary combustion process
2C2H2 + 2O2 4CO + 2H2 + heat
This reaction dissociates into carbon monoxide and hydrogen.
Secondary combustion process
4CO + 2H2 + 3O2 4CO2 + 2H2O + heat
Combining the above equations
2C2H2 + 5O2 4CO2 + 2H2O + heat
It can be seen that about two-fifth of the oxygen necessary for the
complete combustion of acetylene is got from the cylinder where as
the rest comes from the atmosphere. So, the acetylene- oxygen
flame cannot be used inside of pipes or structure subjected to
oxygen depletion.
Oxyacetylene Welding (OAW)
10. Fig : (a) General view of
and (b) cross-section of
a torch used in
oxyacetylene welding.
The acetylene valve is
opened first; the gas is
lit with a spark lighter or
a pilot light; then the
oxygen valve is opened
and the flame adjusted.
(c) Basic equipment used
in oxyfuel-gas welding.
To ensure correct
connections, all threads
on acetylene fittings are
left-handed, whereas
those for oxygen are
right-handed. Oxygen
regulators are usually
painted green,
acetylene regulators
red.
Torch used in Oxyacetylene
Welding
12. Oxygen
Gaseous Chemical element in air
◦ Necessary for life
Most abundant chemical element in crust of Earth
No color, odor, or taste
Does not burn, but supports combustion
◦ Substances that do not burn in air will in oxygen
13. Acetylene
Most widely used of all fuel gages
◦ Both welding and cutting
Generated as result of chemical reaction that takes place when
calcium carbide comes in contact with water
Tests show oxyacetylene flame temperatures up to approximately
6,300ºF
◦ Very rapid rate of preheating
◦ Burns with smoky flame, gives off carbon, has peculiar odor
14. Figure : Torches: top torch is a welding torch and the bottom is a cutting torch
Torch used in Oxyacetylene
Welding
15. STEPS :
Prepare the edges to be joined and maintain the proper position.
Open the acetylene valve and ignite the gas at tip of the torch.
Then the oxygen valve is opened and the flame adjusted.
Hold the torch at about 45deg to the work piece plane.
Inner flame near the work piece and filler rod at about 30 – 40 deg.
Touch filler rod at the joint and control the movement according to the
flow of the material.
Welding practice
16. Self study
Left ward and right ward techniques of gas welding.
Filler metal and fluxes used for gas welding.
Difference between acetylene and oxygen Gas cylinder.
Difference between acetylene regulator and oxygen regulator.
17. Advantages of OAW
The equipment is low cost, versatile, self-sufficient and usually
portable. Besides welding the oxyacetylene can be used for
welding, brazing, soldering, preheating, post-heating and metal
cutting etc.
It requires little maintenance, and can be used with equal facility in
the field and in the factory.
It can weld most common materials.
The gas flame temperature is lower and easily controllable which is
necessary for delicate work. Therefore oxyacetylene welding is
extensively used for sheet metal fabrication and repairs.
The rate of heating and cooling is relatively low. In some cases this
is an advantage.
Since the source of heat and the filler metal are separate, the
welder has control over filler metal deposition rates. Heat can be
applied preferentially to the base metal and the filler metal.
18. Oxygen and acetylene gases are expensive.
There are safety problems involved in their handling and storing.
The flame takes considerably longer for the metal to heat up. Due
to this, oxyacetylene welding is not suitable for thick sections.
Because the flame is not concentrated and it needs prolonged
heating of the joint which result in a larger heat affected area . This
often leads to increases grain growth, more distortion and in some
cases loss of corrosion resistance.
Flame temperature is less than the arc temperature. Gas flame
takes a long time to heat up the metal than arc.
Refractory metals like Columbium, Tantalum, Molybdenum,
Tungsten and the reacting metals such as Titanium and Zirconium
cannot be welded by this process.
More safety problems are associated with the handling and storing
of gases.
Disadvantages of OAW
19. For joining thin materials.
For joining materials in whose case excessively high temperatures
or rapid heating and cooling of the job would produce unwanted or
harmful changes in the metal.
For joining materials in whose case extremely high temperatures
would cause certain elements in the metal to escape into the
atmosphere.
For joining most ferrous and non-ferrous metals, e.g., carbon
steels, alloy steels, cast iron, aluminium, copper, nickel, magnesium
and its alloys, etc.
In automotive and aircraft industries. In sheet metal fabricating
Applications of OAW
20. Gas cutting
Ferrous metal is heated in to red hot condition by
an oxy-fuel flame where burning can be initiated
and then a stream of pure oxygen is added to the
torch (or the oxygen content of the oxy-fuel mixture
is increased) to oxidize the iron.
The liquid iron and iron oxides are then expelled
from the joint by the kinetic energy of the oxygen
gas stream.
Oxides having lower melting point than the metal,
melt and are blown away by the force of the jet, to
make a cut
Fast and efficient method of cutting steel to a high
degree of accuracy
Torch is different from welding
Cutting torch has preheat orifice and one central
orifice for oxygen jet
PIERCING and GOUGING are two important
operations
Piercing, used to cut a hole at the centre of the
plate or away from the edge of the plate
Gouging, to cut a groove into the steel surface
