GAS WELDING
Sound weld is obtained by selecting proper size of flame, filler material
and method of moving torch
The temperature generated during the process is 33000c
When the metal is fused, oxygen from the atmosphere and the torch
combines with molten metal and forms oxides, results defective weld
Fluxes are added to the welded metal to remove oxides
Common fluxes used are made of sodium, potassium. Lithium and
borax.
Flux can be applied as paste, powder,liquid.solid coating or gas.
GAS WELDING EQUIPMENT...
• 1. Gas Cylinders
•
Pressure
•
Oxygen – 125 kg/cm2
•
Acetylene – 16 kg/cm2
• 2. Regulators
•
Working pressure of oxygen 1 kg/cm2
•
Working pressure of acetylene 0.15 kg/cm2
•
Working pressure varies depends upon the thickness
of the
work pieces welded.
• 3. Pressure Gauges
• 4. Hoses
• 5. Welding torch
• 6. Check valve
• 7. Non return valve
Oxy-Acetylene welding
TYPES OF FLAMES…
• Oxygen is turned on, flame immediately changes into a long white
inner area (Feather) surrounded by a transparent blue envelope is
called Carburizing flame (30000c)
• Addition of little more oxygen give a bright whitish cone surrounded
by the transparent blue envelope is called Neutral flame (It has a
balance of fuel gas and oxygen) (32000c)
• Used for welding steels, aluminium, copper and cast iron
• If more oxygen is added, the cone becomes darker and more
pointed, while the envelope becomes shorter and more fierce is
called Oxidizing flame
• Has the highest temperature about 34000c
• Used for welding brass and brazing operation
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.
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.
GAS CUTTING
• Ferrous metal is heated in to red hot condition and a jet of pure
oxygen is projected onto the surface, which rapidly oxidizes
• 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
GAS CUTTING…
Manual Gas Cutting
What is Gas Welding?
• Gas Welding relies on the
combustion of fuel gases,
usually Oxygen and
Acetylene to heat metals
at the welding point
• Widely used in welding
tubes and pipes
• Commonly referred to as
oxyacetylene welding
Main Components of Gas Welding
• Oxygen and Fuel
Cylinders
• Two-stage Regulators
(one per tank)
• Two Hoses
– Green (Oxygen)
– Red (Fuel)
• Check Valve
• Non-return Valve
• Welding Torch
Dangers of Gas Welding
• Burns
• Eye Damage
– Retinal burning and cataracts
• Heat Exposure
– Torch burns at around 6000° F
Dangers of Gas Welding
• Asphyxiation
– Leaks of poisonous gases
– Confined space welding without proper
ventilation
• Explosions/Fire
• Due to ignition of leaking gases
• Acetylene very unstable when stored above 15 psi
Injury Statistics
• Nearly 365,000 welding-related eye injuries occur
annually
• An estimated cost to employers of $467 million annually
• More than half of injured workers returning to work in
less than two days and 95 percent in less than seven
days
• Deaths occur annually due to explosions, falls while
welding and burns
http://www.ohsonline.com/articles/46296/
Proper Gas Welding Procedures
• Inspect equipment for
wear and leaks
• Replace bad hoses
• Use flint lighter to ignite
flame
• Open cylinder valves very
slowly to keep sudden
high pressures from
exploding the regulators
http://www.cdc.gov/nasd/docs/d001601d001700/d001691/d001691.html
Proper Gas Welding Procedures
• Open and light acetylene
first, then open and
adjust oxygen to a neutral
flame
• When finished, close
valves, bleed the lines,
neatly coil hoses and
replace equipment.
• Have a fire extinguisher
accessible
http://www.cdc.gov/nasd/docs/d001601d001700/d001691/d001691.html
Oxyacetylene Welding (OAW)
The oxyacetylene welding process
uses a combination of oxygen and
acetylene gas to provide a high
temperature flame.
Oxyacetylene Welding (OAW)
• OAW is a manual process in which the
welder must personally control the the
torch movement and filler rod application
• The term oxyfuel gas welding outfit refers
to all the equipment needed to weld.
• Cylinders contain oxygen and acetylene
gas at extremely high pressure.
Typical Oxyacetylene Welding
(OAW) Station
Oxygen Cylinders
• Oxygen is stored within cylinders of
various sizes and pressures ranging from
2000- 2640 PSI. (Pounds Per square inch)
• Oxygen cylinders are forged from solid
armor plate steel. No part of the cylinder
may be less than 1/4” thick.
• Cylinders are then tested to over 3,300
PSI using a (NDE) hydrostatic pressure
test.
Oxygen Cylinders
• Cylinders are
regularly re-tested
using hydrostatic
(NDE) while in service
• Cylinders are
regularly chemically
cleaned and
annealed to relieve
“jobsite” stresses
created by handling .
Cylinder Transportation
• Never transport cylinders without the
safety caps in place
• Never transport with the regulators in
place
• Never allow bottles to stand freely. Always
chain them to a secure cart or some other
object that cannot be toppled easily.
Oxygen Cylinders
• Oxygen cylinders
incorporate a thin metal
“pressure safety disk”
made from stainless
steel and are designed
to rupture prior to the
cylinder becoming
damaged by pressure.
• The cylinder valve
should always be
handled carefully
Pressure Regulators for
Cylinders
• Reduce high storage
cylinder pressure to
lower working
pressure.
• Most regulators have
a gauge for cylinder
pressure and working
pressure.
Pressure Regulators for
Cylinders
• Regulators are shut off
when the adjusting screw
is turn out completely.
• Regulators maintain a
constant torch pressure
although cylinder
pressure may vary
• Regulator diaphragms
are made of stainless
steel
Pressure Regulators Gauges
Using a “Bourdon” movement
• Gas entering the gauge fills
a Bourdon tube
• As pressure in the
semicircular end increases it
causes the free end of the
tube to move outward.
• This movement is
transmitted through to a
curved rack which engages
a pinion gear on the pointer
shaft ultimately showing
pressure.
Regulator Hoses
• Hoses are are fabricated
from rubber
• Oxygen hoses are green in
color and have right hand
thread.
• Acetylene hoses are red in
color with left hand thread.
• Left hand threads can be
identified by a grove in the
body of the nut and it may
have “ACET” stamped on it
Check Valves &
Flashback Arrestors
• Check valves allow gas
flow in one direction only
• Flashback arrestors are
designed to eliminate the
possibility of an explosion
at the cylinder.
• Combination Check/
Flashback Valves can be
placed at the torch or
regulator.
Acetylene Gas
• Virtually all the acetylene distributed for welding and
cutting use is created by allowing calcium carbide (a man
made product) to react with water.
• The nice thing about the calcium carbide method of
producing acetylene is that it can be done on almost any
scale desired. Placed in tightly-sealed cans, calcium
carbide keeps indefinitely. For years, miners’ lamps
produced acetylene by adding water, a drop at a time, to
lumps of carbide.
• Before acetylene in cylinders became available in almost
every community of appreciable size produced their own
Acetylene Cylinders
• Acetylene is stored in cylinders specially
designed for this purpose only.
• Acetylene is extremely unstable in its pure
form at pressure above 15 PSI (Pounds per Square
Inch)
• Acetone is also present within the cylinder to
stabilize the acetylene.
• Acetylene cylinders should always be stored
in the upright position to prevent the acetone
form escaping thus causing the acetylene to
Acetylene Cylinders
• Cylinders are filled with a
very porous substance
“monolithic filler” to help
prevent large pockets of
pure acetylene form
forming
• Cylinders have safety
(Fuse) plugs in the top
and bottom designed to
melt at 212° F (100 °C)
Acetylene Valves
• Acetylene cylinder shut
off valves should only
be opened 1/4 to 1/2
turn
• This will allow the
cylinder to be closed
quickly in case of fire.
• Cylinder valve
wrenches should be
left in place on
Oxygen and Acetylene
Regulator Pressure Settings
• Regulator pressure may vary with different
torch styles and tip sizes.
• PSI (pounds per square inch) is sometimes shown
as PSIG (pounds per square inch -gauge)
• Common gauge settings for cutting
– 1/4” material Oxy 30-35psi Acet 3-9 psi
– 1/2” material Oxy 55-85psi Acet 6-12 psi
– 1” material Oxy 110-160psi Acet 7-15 psi
• Check the torch manufactures data for
optimum pressure settings
Regulator Pressure Settings
• The maximum safe working pressure for
acetylene is 15 PSI !
Typical torch styles
•
A small welding torch, with throttle valves
located at the front end of the handle. Ideally
suited to sheet metal welding. Can be fitted
with cutting
•
attachment in place of the welding head
shown. Welding torches of this general design
are by far the most widely used. They will
handle any oxyacetylene welding job, can be
fitted with multiflame (Rosebud) heads for
heating applications, and accommodate
cutting attachments that will cut steel 6 in.
thick.
•
A full-size oxygen cutting torch which has all
valves located in its rear body. Another style of
cutting torch, with oxygen valves located at
the front end of its handle.
Typical startup procedures
• Verify that equipment visually appears safe IE:
Hose condition, visibility of gauges
• Clean torch orifices with a “tip cleaners” (a small
wire gauge file set used to clean slag and dirt form
the torch tip)
• Crack (or open) cylinder valves slightly allowing
pressure to enter the regulators slowly
• Opening the cylinder valve quickly will “Slam” the
regulator and will cause failure.
Typical startup procedures
• Never stand directly in the path of a regulator
when opening the cylinder
• Check for leaks using by listening for “Hissing”
or by using a soapy “Bubble” solution
• Adjust the regulators to the correct operating
pressure
• Slightly open and close the Oxygen and
Acetylene valves at the torch head to purge any
atmosphere from the system.
Typical startup procedures
• Always use a flint and steel spark lighter to light the
oxygen acetylene flame.
• Never use a butane lighter to light the flame
Flame Settings
• 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.
Pure Acetylene and Carburizing
Flame profiles
Neutral and Oxidizing Flame
Profiles
Flame definition
• The neutral flame (Fig. 4-1) is produced when the ratio of oxygen to
acetylene, in the mixture leaving the torch, is almost exactly one-toone. 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 excess acetylene flame (Fig. 4-2), as its name implies, is
created when the proportion of acetylene in the mixture is higher
than that required to produce the neutral flame. Used on steel, it will
cause an increase in the carbon content of the weld metal.
• The oxidizing flame (Fig. 4-3) results from burning a mixture which
contains more oxygen than required for a neutral flame. It will
oxidize or ”burn” some of the metal being welded.
Oxyfuel Gas Welding
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.
Oxyfuel Gas Welding
• Welding process that uses fuel gas combined with oxygen
to produce flame
• This flame heat melts the metals at the joint
• Acetylene fuel is used in gas welding process
Primary combustion process
C2H2 + O2
2CO + H2 + heat
• This reaction dissociates into carbon monoxide and
hydrogen.
Secondary combustion process
2CO + H2 + 1.5 O2
2CO2 + H2O + heat
Types of flames
• Neutral flame
• Oxidising flame
• Carburising flame
Filler Metals :
•
•
•
•
Additional material to weld the weld zone
Available as rod or wire
They can be used bare or coated with flux
The purpose of the flux is to retard the
Torch Used in Oxyacetylene Welding
Fig : (a) General view of and
(b) cross-section of a
torch used in
oxyacetylene 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.
Welding practice & equipment
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
• 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
Torch used in Oxyacetylene Welding
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 park
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.
Pressure-Gas Welding Process
Fig : Schematic illustration of the pressure-gas welding process.