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IIW Specialist Syllabus
Welding Processes and Equipment
1.2 Oxyfuel Gas Welding
Trainer: Eng. Pedro Vilaa
June 2011
The basis of this presentation was developed
by
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1.2 Oxy Gas Welding 1
OXYFUEL GAS WELDING (OFW) is a
manual process in which the metal
surfaces to be joined are melted
progressively by heat from a gas flame
and are caused to flow together and
solidify without the application of pressure
to the parts
OFW can be applied with or without filler
metal
The most important source of heat for
OFW is the oxyacetylene welding (OAW)
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Oxygen and fuel are stored in
separate cylinders
The gas regulator attached to each
cylinder, whether fuel gas or oxygen,
controls the pressure at which the gas
flows to the welding torch
The mixed gases then pass through
the welding tip and produce the flame
at the exit end of the torch tip
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1.2 Oxy Gas Welding 3
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The equipment is versatile, low-cost, self-sufficient, and usually
portable
It can be used for preheating, postheating, welding, braze welding, and
torch brazing, and it is readily converted into oxygen cutting
The process can be adapted to short production runs, field work,
repairs, and alterations
The oxy-acetylene welding process (OAW) is by far the most important,
and probably the most versatile, of the oxy-fuel gas welding (OFW)
processes but is tending to be superceded by other welding processes,
e.g., the Tungsten Inert Gas (TIG) welding process
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Table: Approximate Maximum Flame Temperatures
deg C
Oxy-acetylene 3,100 to 3,300
Oxy-propane 2,500
Oxy-hydrogen 2,370
Oxy-coal-gas 2,200
Air-acetylene 2,460
Air-coal-gas 1,871
Air-propane 1,750
The gas acetylene (C2H2) is the most important fuel gas employed,
because it has the highest calorific (heat) value. Other hydrocarbon
gases are also used, e.g. Liquid Petroleum Gas (LPG), Propane, etc.
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In practical applications the oxy-acetylene welding process (OAW) is the
only one applied in welding
Higher heat power density
Hotter for all the distances from the center of the flame
More reducing action
Easy to adjust with Oxygen
The oxi-propane flame is more applied in brazing when temperature and
heat power should be lower
Propane and butane are more safe to use fuel gases
The propane, butane and natural gas allow greater autonomy as they canbe easily stored in large volumes.
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Acetylene (C2H2) storing:
When under pressure of 203 kPa and above,
acetylene is unstable, and a slight shock can cause it
to explode, even in the absence of oxygen or air
By dissolving purified and dried acetylene in liquid
acetone, a cylinder such as that shown in Figure can
be used to store about 7.79 m3 of acetylene under a
pressure of 1.7 MPa
The cylinders must be stored in an upright position to
keep the acetone from escaping during use
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1.2 Oxy Gas Welding 8
Manifolded supplies may be used for both acetylene and oxygen
cylinders, although where the oxygen consumption is high it is more
practical to use an oxygen generator/evaporator plant, the oxygen being
delivered and stored in the more compact liquid form.
In each instance the gas is stored away from the workshop in a special
building designed to relevant safety requirements, the gas is piped from
the store to the supply points in the workshop.
The acetylene is conveyed in steel tubes (copper is not used because
of the danger of producing the explosive compound: copper acetylide).
The line is protected by non-return valves and flash-back arresters at
each outlet. The cylinder is also protected by large-capacity flashback
arresters.
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1.2 Oxy Gas Welding 9
Oxygen cylinder(Capacity, 220ft3 at 2 000 lb / in2, Weight 145 lb)
Acetylene cylinder(Capacity, 250ft3 at 250 lb / in2, Weight 215 lb)
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OXYFUEL GAS WELDING (OFW) STATION - Equipment
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1. Welding torch: efficient and light, producing a flame of the right shape and
temperature with controls that are easy to adjust allowing for the quick and easy
changing of nozzle tips, thus affording a wide range of blowpipe power
2. Oxygen and acetylene gas regulators: to reduce the high pressure in a gas
storage cylinder to a lower working pressure, and at the same time maintaining
a steady supply, free from pressure fluctuations3. Supplies of oxygen and acetylene gases, which must be safely stored in
cylinders and suitably piped to the welding areas
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4. Protection equipment, such as suitable colour-tinted goggles,
manufactured to the appropriate standard, e.g. BS 679, and
other protective clothing, such as chrome-Ieather gloves,
aprons, etc
Filter glasses for goggles are invariably green, their main
function being to reduce the glare from the flame cone and
molten metal to a level comfortably acceptable to the eyes ofthe welder, while also protecting the eyes from hot sparks
They are produced in different densities or shades, appropriate
to light or heavy glare. Modern filters are photo chromatic, i.e.
they change shade automatically
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Fire lighters
Protection goggles
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5. Supplies of suitable filler rods in convenient diameters and lengths. These are
usually from 1,2mm to 4,8mm in diameter and 1mm in length
6. Certain other equipment, such as flexible, high-pressure rubber hoses,
coloured red for fuel gas and black for oxygen, with properly designed
connections, threaded left-hand for fuel gas and right-hand for oxygen, to
prevent accidental exchange
Also, safety devices, such as flash-back arresters, or hydraulic backpressure
valves. Equipment such as refractory-surfaced work tables, gas economizers,
etc are desirable but cannot be regarded as absolutely essential
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Types of torches/burners architecture
Features of high-pressure with no injector blowpipe
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Procedure for adding a filler rod into the weld pool
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Fluxes:
A flux prevents the oxidation of molten metal. The flux (material) is
fusible and non metallic During welding, flux chemically reacts with the oxides and a slag is
formed that floats to and covers the top of the molten puddle of metal
and thus helps keep out atmospheric oxygen and other gases
Except for lead, zinc, and some precious metals, OFW of nonferrous
metals, cast irons, and stainless steels generally requires a flux
In welding carbon steel, the gas flame shields the weld adequately,
and no flux is required
Adjustment for correct flame atmosphere is important, but the absence
of flux results in one less variable to control
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The maximum temperature of the oxy-acetylene flame is 3,100 to
3,300 C and the centre of this heat concentration is just off the
extreme tip of the white cone. Combustion is recognised as taking
place in two main stages of combustion
T f Fl i OAW
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Types of Flames in OAW
In oxy-acetylene welding the character of the flame is most important. Certain
technical terms must be learned in this connection
When the acetylene and oxygen are in equal proportions the resultant flame is
said to be neutral; when there is an excess of oxygen the flame is said to
be oxidising; and if more acetylene is present than oxygen the flame is
said to be carburising, orreducing
A reducing flame is on that, because of its need for oxygen will reduce surface
oxides, such as iron oxide. A strictly neutral setting is correct, but the slightest
excess of acetylene may keep scaling (oxidation) to a minimum, particularly
when welding stainless steels
For example, non-ferrous alloys and carbon steels may require a reducing
flame, while zinc-bearing materials may need an oxidising flame
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Three types of flame setting: mixture ratio
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FERROUS AND NONFERROUS METALS THAT CAN BE WELDED BY OAW:
(a) - Match base metal ; (b) - No Flux required
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Acetylene: 2 C2H2+5 O2=4 CO2+2 H2O
Propane: C3
H8
+5 O2
=3 CO2
+4 H2
O
Natural Gas: CH4+2 O2=CO2+2 H2O
Combustion with other fuel gases
Lighting a blowpipe
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Lighting a blowpipe
The correct pressures, as recommended for the appropriate nozzle, should
first be set, initially the fuel gas by opening slightly the blowpipe acetylene
valve and regulating it to the correct pressure by the pressure-regulator screw
This procedure is repeated for the oxygen supply, the oxygen valve then
being closed. The fuel gas is turned on, ignited and adjusted so that the flame
just ceases to form soot but is not blown away from the nozzle tip
The oxygen is now turned on at the blowpipe valve and adjusted until the
acetylene feather just disappears, to obtain a neutral flame setting
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Lighting a blowpipe (cont.)
To extinguish the flame, the fuel gas should be turned off first, followed by the
oxygen. In the event of backfires with either design of torch, the fuel gas
should be turned off first to prevent the internal temperatures from being
destructively high and damaging the blow-pipe body
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Use of grooves to facilitate oxyfuel gas
welding of t> 4.8 mm thick plates