Gas Welding

GAS WELDING PROCESS Gas welding is a fusion welding process. It joins metals, using the heat of

combustion of an oxygen/air and fuel gas (i.e. acetylene, hydrogen,

propane or butane) mixture. The intense heat (flame) thus produced melts

and fuses together the edges of the parts to be welded, generally with the

addition of a filler metal.

When acetylene is mixed with oxygen in correct proportions in the welding

torch and ignited, the flame resulting at the tip of the torch is sufficiently

hot to melt and join the parent metal.

The oxyacetylene flame reaches a temperature of about 3200°C and thus

can melt all commercial metals which, during welding, actually flow

together to form a complete bond.

A filler metal rod is generally added to the molten metal pool to build up

the seam slightly for greater strength. Oxyacetylene welding does not

require the components to be forced together under pressure until the weld

forms and solidifies.

Types of Welding Flames –

Open the acetylene control valve of the welding torch and after the system

has been flushed clean of air the gas is ignited.

At this stage, enough of oxygen is drawn in from the atmosphere to burn

acetylene partially. The acetylene control valve is then adjusted until the

flame ceases to smoke.

The oxygen control valve of the welding torch is then opened in order to

adjust the proportions in which acetylene and oxygen are required to mix

and burn. This results in three distinct types of flames as discussed under.

Types of Flames

1. Neutral Flame (Acetylene oxygen in equal proportions)

2. Oxidising Flame (Excess of oxygen)

3. Reducing Flame (Excess of acetylene)

In oxyacetylene welding, flame is the most important tool. All the welding

equipment simply serves to maintain and control the flame.

The correct type of flame is essential for the production of satisfactory

welds.

The flame must be of the proper size, shape and condition in order to

operate with maximum efficiency.

Neutral Flame

A neutral flame is produced when approximately equal volumes of oxygen

and acetylene are mixed in the welding torch and burnt at the torch tip.

(More accurately the oxygen-to-acetylene ratio is 1.1 to 1). The temperature

of the neutral flame is of the order of about 5900ºF (326ºC)

The flame has a nicely defined inner cone* which is light blue in colour.

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 effects no chemical change in the

molten metal and therefore will not oxidize or carburize the metal.

The neutral flame is commonly used for the welding of:

(i) Mild steel

(ii) Stainless steel

(iii) Cast Iron

(iv) Copper

(v) Aluminium

Oxidising Flame -

If, after the neutral flame has been established, the supply of oxygen is

further increased, the result will be an oxidising flame.

An oxidising flame can be recognized by the small white cone which is

shorter, much bluer in colour and more pointed than that of the neutral

flame.

The outer flame envelope is much shorter and tends to fan out at the end

on the other hand the neutral and carburizing envelopes tend to come to a

sharp point.

An oxidising flame burns with a decided loud roar. An oxidising flame

tends to be hotter than the neutral flame.

This is because of excess oxygen and which causes the temperature to rise

as high as 6300°F.

The high temperature of an oxidizing flame (O2: C2H2 = 1.5: 1) would be an

advantage if it were not for the fact that the excess oxygen, especially at

high temperatures, tends to combine with many metals to form hard,

brittle, low strength oxides.

Moreover, an excess of oxygen causes the weld bead and the surrounding

area to have a scummy or dirty appearance.

For these reasons, an oxidising flame is of limited use in welding.

It is not used in the welding of steel. A slightly oxidising flame is helpful

when welding most

(i) Copper base metals

(ii) Zinc base metals,

Reducing Flame –

If the volume of oxygen supplied to the neutral flame is reduced, the

resulting flame will be a carburising or reducing flame, i.e. rich in

acetylene.

A reducing flame 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 colour.

A reducing flame does not completely, consume the available carbon;

therefore, its burning temperature is lower and the left over carbon is

forced into the molten metal.

With iron and steel it produces very hard, brittle substance known as iron

carbide.

This chemical change makes the metal unfit for many applications in which

the weld may need to be bent or stretched.

Metals that tend to absorb carbon should not be welded with reducing

flame.

A reducing flame has an approximate temperature of 5500°F (3038°C). A

reducing flame may be distinguished from a carburizing flame by the fact

that a carburizing flame contains more acetylene than a reducing flame.

A carburizing flame is used in the welding of lead and for carburizing

(surface hardening) purposes.

Leftward Technique –

The welder holds welding torch in his right hand and filler rod in the left

hand.

The welding flame is directed away from the finished weld, i.e., towards

the unwelded part of the joint.

Filler rod, when used, is directed towards the welded part of the joint.

Fig. Leftward technique

The weld is commenced on the right hand side of the seam, working

towards the left hand side.

The blowpipe or welding torch is given small sideways movements, while

the filler rod is moved steadily across the seam.

The filler rod is added using a backward and forward movement of the

rod, allowing the flame to melt the bottom edges of the plate just ahead of

the weld pool.

Since the flame is pointed in the direction of the welding, it preheats the

edges of the joint.

Good control and a neat appearance are characteristics of the leftward

method.

Leftward technique is usually used on relatively thin metals, i.e., having

thicknesses less than 5 mm.

When work piece thickness is over 3 mm, it is necessary to bevel the plate

edges to produce a V-joint so that good root fusion may be achieved.

The included angle of V-joint is 80-90°.

This large volume weld is uneconomical in terms of time, weld metal

deposited and quantity of gases used and may also over distort the

weldment when welding thick materials.

Data for Leftward Welding –

Plate

thickness

mm

Edge

Preparation

Distance

between

edges,

mm

Filler rod

diameter,

mm

Power of

blowpipe

litre/hr (Gas

consumption)

Rate of

welding

metres/hr

0.8 Square -- 1.6 22 - 44 6 - 7.6

1.6 Square 1.6 1.6 44 - 66 7.6 - 9

2.4 Square 2.4 1.6 66 - 107 6 - 7.6

3.2 Square 3.2 3.2 107 - 153 5.4 - 6

4.0 80°V 3.2 3.2 153 - 214 4.6 - 5.4

5.0 80°V 3.2 3.2 214 - 283 3.6 - 4.6

Rightward Technique –

Here again the welding torch is held in the right hand of the welder and

the filler wire in the left.

Welding begins at the left hand end of the joint and proceeds towards the

right, hence the name rightward technique.

Fig. Rightward technique

The direction of welding is opposite to that when employing the leftward

technique.

The torch flame in rightward technique is directed towards the completed

weld and the filler rod remains between the flame and the completed weld

section, since the flame is constantly directed on the edges of the V ahead

of the weld puddle, no sidewise motion of the welding torch is necessary.

As a result a narrower V -groove (30° bevel or 60° included angle) can be

utilized than in leftward welding.

This provides a greater control and reduced welding costs.

During welding, the filler rod may be moved in circles (within the puddle)

or semicircles (back and forth around the puddle.)

In rightward welding, the weld puddle is less fluid and this result in a

slightly different appearance of the weld surface.

The ripples are heavier and spaced further apart.

The rightward technique is one used on heavier or thicker (above 5 mm)

base metals, because in this technique the heat is concentrated into the

metal.

Welds with penetrations of approximately 12 mm can be achieved in a

single pass.

Rightward technique has got certain. Advantages over the left ward one, as

listed below:

i) Up to 8.2 mm plate thickness no bevel is necessary. This saves the cost of

preparation and reduces the consumption of filler rod.

(ii) For welding bigger thicknesses, where beveling of plate edges becomes

necessary, the included angle of V need be only 60°, which requires less

filler metal against 80°V preparation used in leftward welding technique.

(iii) The welder's view of the weld pool and the sides and bottom of the V

groove is unobstructed. This results in better control and higher welding

speeds.

(iv) The smaller total volume of deposited metal, as compared to leftward

welding, reduces shrinkage and distortion.

(v) The weld quality is better than that obtained with the leftward

technique.

(vi) Owing to less consumption of the filler metal,

Data for Rightward Welding -

Plate

thickness

mm

Edge

Preparation

Distance

between

edges,

mm

Filler rod

diameter,

mm

Power of

blowpipe

litre/hr (Gas

consumption)

Rate of

welding

metres/hr

5.0 Square 2.4 2.4 370 - 520 3.6 - 4.6

6.5 Square 3.2 3.2 520 - 570 3.0 - 3.6

8.2 Square 4.0 4.0 710 - 860 2.1 - 2.4

10.0 60°V 3.2 5.0 1000 - 1300 1.8 - 2.1

13.0 60°V 3.2 6.5 1300 - 1400 1.3 - 1.5

16.2 60°V 3.2 6.5 1600 - 1700 1.1 - 1.3

19.0 60°V (Top) 4.0 6.5 1700 - 2000 0.9 - 1.0

80°V (Bottom)

25.0 60°V (Top)

80°V (Bottom)

4.0 6.5 2000 - 2500 0.6 - 0.7

Gas Welding Equipment -

The basic equipments used to carry out gas welding are:

1. Oxygen gas cylinder.

2. Acetylene gas cylinder.

3. Oxygen pressure regulator.

4. Acetylene pressure regulator.

5. Oxygen gas hose (Blue).

6. Acetylene gas hose (Red).

7. Welding torch or blow pipe with a set of nozzles and gas lighter

8. Trolleys for the transportation of oxygen and acetylene cylinders

9. A set of keys and spanners.

10. Filler rods and fluxes.

11. Protective clothing for the welder (e.g., asbestos apron, gloves, goggles,

etc.)

Manifold System for Gas Welding Manifold System for Gas Welding -

In an industry where considerable gas welding is done and at several

places or where rate of gas consumption is high, it is much advantageous

to use a manifold system.

In a manifold system, instead of having gas cylinders at each place of work,

they are assembled at one centralized position in specially designed racks

and connected by a manifold.

The gas is then distributed by means of a pipeline to different workplaces.

The manifold distributes welding gases at a reduced pressure.

There are separate manifolds for oxygen and acetylene gas supplies.

A manifold system for acetylene.

An oxygen manifold system differs from it in the following ways:

(i) There is no anti flashback device in an oxygen manifold system.

(ii) Oxygen supply line is a copper pipe whereas acetylene supply line is a

steel pipe.

(iii) An oxygen pressure regulator is used in an oxygen manifold system.

Advantages of Using Manifolds -

1. Since gas cylinders have not been scattered throughout the work area,

rather, they have been kept at a central place:

(a) The possibility of accidents is reduced.

(b) More space is available at each workplace.

2. In case of fire, one can easily reach the cylinders.

3. There is more effective use of gases.

4. Cylinders are transported by less distance.

5. There is no replacement of cylinders inside the workshop.

Welding Torch

Oxygen and the fuel gas having been reduced in pressure by the gas

regulators are fed through suitable hoses to a welding torch which mixes

and controls the flow of gases to the welding nozzle or tip where the gas

mixture is burnt to produce a flame for carrying out gas welding operation.

There are two types of welding torches, namely:

(i) High pressure (or equal pressure) type.

(ii) Low pressure (or injector) type.

High pressure blowpipes or torches are used with (dissolved) acetylene

stored in cylinders at a pressure of 8 bar. Low pressure blowpipes are used

with acetylene obtained from an acetylene generator at a pressure of 200

mm head of water (approximately 0.02 bar).

A) Working of a low pressure blowpipe:

It is termed as a low pressure blowpipe because it can be operated at

low acetylene pressures; it is frequently used with acetylene

generators. As acetylene is of low pressure, it is necessary to use

oxygen at a high pressure (2.5 bar).

The oxygen enters the mixing chamber through a passage located in

the centre of the torch.

The oxygen passage is surrounded by the one carrying the acetylene.

The high pressure oxygen passes through a small opening in the

injector nozzle, enters the mixing chamber and pulls (or draws) the

acetylene in after it.

An advantage of low pressure torch is that small fluctuations in the

oxygen supplied to it will produce a corresponding change in the

amount of acetylene drawn, thereby making the proportions of the

two gases constant while the torch is in operation.

B) Working of a high pressure blowpipe:

In this type of blowpipe both the oxygen and acetylene are fed to the

blow pipe at equal pressures and the gases are mixed in a mixing

chamber prior to being fed to the nozzle tip.

The equal pressure or high pressure type of blowpipe is the one most

generally used because

(i) It is lighter and simpler.

(ii) It does not need an injector.

(iii) In operation, it is less troblesome since it does not suffer from

backfires to the same extent.

To change the power of the welding torch, it is only necessary to

change the nozzle tip (size) and increase or decrease the gas

pressures appropriately.

Advantages of Gas Welding -

1. It is probably the most versatile process. It can be applied to a wide

variety of manufacturing and maintenance situations.

2. Welder has considerable control over the temperature of the metal in the

weld zone. When the rate of heat input from the flame is properly

coordinated with the speed of welding, the size, viscosity and surface

tension of the weld puddle can be controlled, permitting the pressure of the

flame to be used to aid in positioning and shaping the weld.

3. The rate of heating and cooling is relatively slow. In some cases, this is

an advantage.

4. Since the sources of heat and of filler metal are separate, the welder has

control over filler metal deposition rates. Heat can be applied preferentially

to the base metal or the filler metal.

5. The equipment is versatile, low cost, self sufficient and usually portable.

Besides gas welding, the equipment can be used for preheating, post

heating, braze welding, torch brazing and it is readily converted to oxygen

cutting.

6. The cost and maintenance of the welding equipment is low when

compared to that of some other welding processes.

Disadvantages of Gas Welding -

1. Heavy sections cannot be joined economically.

2. Flame temperature is less than the temperature of the arc.

3. Fluxes used in certain welding and brazing operations produce fumes

that are irritating to the eyes, nose, throat and lungs.

4. Refractory metals (e.g., tungsten, molybdenum, tantalum, etc.) and

reactive metals (e.g., titanium and zirconium) cannot be gas welded.

5. Gas flame takes a long time to heat up the metal than an arc.

6. Prolonged heating of the joint in gas welding results in a larger heat

affected area. This often leads to increased grain growth, more distortion

and, in some cases, loss of corrosion resistance.

7. More safety problems are associated with the handling and storing of

gases.

8. Acetylene and oxygen gases are rather expensive.

9. Flux shielding in gas welding is not so effective as an inert gas shielding

in TIG or MIG welding.

Applications of Gas Welding -

1. For joining thin materials.

2. 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.

3. For joining materials in whose case extremely high temperatures would

cause certain elements in the metal to escape into the atmosphere.

4. For joining most ferrous and nonferrous metals, e.g., carbon steels, alloy

steels, cast iron, aluminium, copper, nickel, magnesium and its alloys, etc.

5. In automotive and aircraft industries. In sheet metal fabricating plants,

etc.

Welding Filler Metal Rods –

Filter metal is the material that is added to the weld pool to assist in filling

the gap (or groove).

Filler metal forms an integral part of the weld.

Filler metal is usually available in rod form. These rods are called Filler

Rods.

Filler rods have the same or nearly the same chemical composition as the

base metal.

Welding filler rods are available in a variety of compositions (for welding

different materials) and sizes.

Welding Fluxes –

During welding, if the metal is heated/melted in air, oxygen from the air

combines with the metal to form oxides which result in poor quality, low

strength welds or, in some cases, may even make welding impossible.

In order to avoid this difficulty, a flux is employed during welding.

A flux is a material used to prevent, dissolve or facilitate removal of oxides

and other undesirable substances.

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.