OXY-FUEL WELDING
PRINCIPLE OF WORKING OF OXY-FUEL WELDING
Oxyacetylene welding is a fusion-welding process performed by a high
temperature flame from combustion of acetylene and oxygen.
Oxygen cylinder and acetylene cylinder are filled with gases. Both the cylinders
are attached with pressure gauges, regulators and cylinder valves. The cylinder
containing oxygen is painted black (or green) whereas the acetylene cylinder is
painted maroon. Hose pipes, are provided with each cylinder. These pipes are
connected to welding torch.
A very hot flame is produced by burning of the gases coming through the torch
tip. The flame is directed by a welding torch.
The edges to be welded are heated up to melting. A filler metal is also added to
complete the welding. This molten metal mixture when solidifies on cooling forms
a welded joint.
EQUIPMENTS USED IN OXY-FUEL WELDING
TORCH
Tanks : Oxygen and fuel are stored in tanks. Tanks are color coded. Black for oxygen
tank whereas red/maroon for the fuel tank.
Regulator: Regulator is used to control the flow of gases from high pressure
cylinder. The flow rate is then adjusted by the operator using needle valves on the
torch.
Torch:
• Torch is a device used to mix acetylene and oxygen in the correct proportion and
the mixture flows to the tip of the torch.
• The torch is the part that the welder holds and manipulates to make the weld.
• It has a connection and valve for the fuel gas and a connection and valve for the
oxygen, a handle for the welder to grasp, a mixing chamber (set at an angle)
where the fuel gas and oxygen mix, with a tip (nozzle) where the flame forms.
Different size nozzles are used depending on the thickness of material to be
welded.
Lighter: For starting the flame, the spark should be given by a lighter.
The flame is applied to the base metal and held until a small puddle of molten metal
is formed. The puddle is moved along the path where the weld bead is desired.
TYPES OF FLAMES
CHEMICAL REACTIONS
• During he first stage of reaction (primary combustion), acetylene reacts with
oxygen (from the tank) to form carbon monoxide and water. The first-stage
reaction is seen as the inner cone of the flame (which is bright white).
• During the second stage of reaction (secondary combustion), oxygen from the
surrounding air reacts with the carbon monoxide and hydrogen (reaction
products from the first stage), to form carbon dioxide and water. The secondstage reaction is exhibited by the outer envelope (which is nearly colorless but
with tinges ranging from blue to orange).
• The maximum temperature of the flame is reached at the tip of the inner cone;
the second-stage temperatures are somewhat below those of the inner-cone.
• During welding, the outer envelope spreads out and covers the work surfaces
being joined, thus shielding them from the surrounding atmosphere.
Neutral Flame :
If the ratio is 1:1, all reactions are carried to completion and a neutral flame is produced. The
temperature of the neutral flame is of the order of about 3260°C.
It has a clear, well defined inner cone, indicating that the combustion is complete. The inner
cone is light blue in color. It is surrounded by an outer flame envelope, produced by the
combination of oxygen in the air and superheated carbon monoxide and hydrogen gases from
the inner cone. This envelope is usually a much darker blue than the inner cone. A neutral
flame is named so because it melts metal without changing its properties, and it leaves the
metal clear and clean. This flame is the one most extensively used, having the advantage that
it adds nothing to the metal being joined and takes nothing away – once the metal has fused,
it is chemically the same as before welding.
Applications : The neutral flame is commonly used for the welding of mild steel, stainless
steel, cast Iron, copper, and aluminum.
Oxidizing Flame :
Higher ratio, such as 1.15:1, produces an oxidizing flame, which is hotter than the neutral
flame (about 3600°C) but similar in appearance. Such flames are used only for special
applications. It is considered harmful when welding steel because the excess oxygen reacts
with the carbon in the steel, lowering the carbon in the region around the weld, causes rapid
oxidation resulting in poor weld as well as reduced weld strength.
An oxidizing flame can be recognized by the small cone, which is shorter, much bluer in color
and more pointed than that of the neutral flame. The outer flame envelope is much shorter
and tends to fan out at the end. Such a flame makes a loud roaring sound. The oxidizing flames
are used to cut metals.
Applications - A slightly oxidizing flame is helpful when welding
(i) Copper-base metals (ii) Zinc-base metals and (iii) A few types of ferrous metals such as
manganese steel and cast iron.
Carburizing Flame :
Excess fuel, on the other hand: ratios, such as .9:1, produces a carburizing flame. The excess
fuel decomposes to carbon and hydrogen, and the flame temperature is not as great (about
3050°C). The metal is well protected from oxidation. The addition of carbon atoms from the
excess acetylene fuel will make the metal more brittle (increases hardness).
The carburizing or reducing flame has excess of acetylene and can be recognized by acetylene
feather, which exists between the inner cone and the outer envelope. The outer flame
envelope is longer than that of the neutral flame and is usually much brighter in color.
This flame can be used to advantage in the welding of metals for which the excess oxygen
would cause detrimental oxidation.
However, in some cases this flame is detrimental owing to the fact that it deposits carbon for
example if used on mild steel, weld deposit will be higher in carbon content and therefore
becomes hardened and cracks may result. The addition of carbon atoms from the excess
acetylene fuel will make the metal more brittle (increases hardness).
Applications - Flames of this type are used in welding Monel (a nickel–copper alloy), highcarbon steels, and some alloy steels, and for applying some types of hard-facing material.
Advantages :
• Oxy-acetylene can be used on sites which have no power supply.
• Equipment is cheap as compared to other welding process and it is also portable.
• It can be used far cutting of metals of small thickness. It is specially used for sheet
metal work.
• Oxy-acetylene equipment can also be used to "flame-cut" large pieces of
material.
Disadvantages :
• It takes long time for heating the job as compared to the arc welding.
• Oxy-acetylene weld lines are much rougher in appearance than other kinds of
welds, and require more finishing if neatness is required.
• Gases are expensive.
•
Oxy-acetylene welds have large heat affected zones (areas around the weld line
that have had their mechanical properties adversely affected by the welding
process)