Gas Arc welding

Gas welding is the process in which a gas flame is used to raise the temperature of the metal to be joined. The metals are heated up to melting. The metal flows and on cooling it solidifies. A filler metal may be added to the flowing molten metal to fill up cavity made during the end preparation. Many combination of gases are used in Gas welding. But most common of these in oxygen and acetylene.

OXY-ACETYLENE WELDING

The process of oxy-acetylene welding can be used for almost all metals and alloys for engineering purposes. A high temperature flame (3200 C) can be produced by this method. There are two systems of oxygen-acetylene welding.
• High pressure system: In this process the oxygen and acetylene are taken for use from high pressure cylinders.
• Low pressure system: In this system oxygen is taken from high pressure cylinder and the acetylene is produces by the action of calcium carbide and water

PRINCIPLE

A very hot flame is produced by burning of the gases coming through the torch tip. 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.

APPARATUS

Apparatus used for oxy-acetylene (High pressure) welding are:
1. Oxygen cylinder
2. Acetylene cylinder
3. Pressure gauges
4. Valves
5. Hose pipes
6. Torch
7. Welding tip
8. Pressure regulator
9. Lighter
10. Goggles

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 whereas the acetylene cylinder is painted maroon. Hose pipes are provided with each cylinder. These pipes are connected to welding torch.

WELDING PROCESS

To start welding, the acetylene control value is turned first when acetylene comes out of the nozzle, it should be ignited with spark lighter. It will give a yellow-coloured smoke flame. After it oxygen cylinder valve is opened and supply is increased until a best suitable flame is obtained.
Then the flame is focussed on the edges to be welded. Flux and filler metal are also added with the heat of flame, the edges and the filler metal melts and a joint is formed after cooling of the molten metal. The joint may be formed with or without using filler metal.

APPLICATION

All the metals can be welded with proper filler metals. Same equipment may be used for cutting purposes. Oxy-acetylene welding is particularly used for sheet metal work.

ADVANTAGES

1. Equipment is cheap.
2. It can be used for welding all metals.
3. Maintenance of equipment is very less.
4. It is portable process.
5. It can be used for cutting of metals.
6. It is specially used for sheet metal work.

DISADVANTAGES

1. It takes long time for heating the job as compared to the arc welding.
2. Heat affected area is more.
3. Gases are expensive and difficult to store.

TYPES OF GAS FLAMES

Oxidizing flame: 

When the volume if oxygen gas is more than the volume of acetylene mixed into the torch. This flame is used for welding brass and is also used for cutting the metals.

Carburising flame: 

When the volume of acetylene mixed is more than oxygen, carburising flame is formed. This flame is used for welding nickel, monel etc.

Neutral flame: 

It is known as balanced flame. Oxygen and acetylene gases are mixed in equal volumes. Neutral flame is used for normal welding of steel, cast iron etc,

GAS WELDING TECHNIQUES

Leftward welding: 

In this welding the tip of the torch is held at 600-700 to the plates. And the filler rod is inclined at 300-400 in opposite direction. In this method the plate edges are heated immediately after the molten metal. The torch tip and filler rod are moved slowly in the direction towards left.

Rightward welding: 

In right ward welding the torch is kept at 400-500 to the job to be welded. Torch is moved toward right. Right ward welding is done for heavy sections.

Vertical welding: 

This is a method by which metal of any thickness can be welded. In case the thickness of the sheet is lesser then welding from one side will be sufficient but for thicker sheets welding should be done from both the sides. Preparation of edges to be weld is not required. In this case welding starts from bottom and moves till top and the welding blow pipe follows the filler rod. The inclination of the blow pipe is 30-80 degree whereas filler rod is inclined at an angle of 30.

FLUX

The chemicals which de-oxidises the metal surface and provides inert atmosphere around the molten metal are known as fluxes.

Function:
1. To prevent oxides from the hot surfaces.
2. To reduce the viscosity of molten metal.
3. It maintains a steady arc in case of arc welding.

Fluxes are available as liquid, powder, paste and gas. Powder flux is sprinkled on the surfaces to be welded or the filler rod is dipped into the powder. Liquid and paste fluxes are sprayed on the surfaces to be welded. Gas fluxes are used to form inert atmosphere around the joint to be welded.

FILLER

The rod which provides additional metal in completing the welding is known as filler. The composition of filler metal should be the same as that of the metal to be welded.

DETAILS OF GAS WELDING EQUIPMENTS

Oxygen cylinder: 

Cylinder is made up of steel in capacity range 2.25-6.30 m^3. The cylinders are filled with oxygen at about 150Kg/cm^2 at 21 C. A safety valve is also provide on it. The cylinder can be opened or closed by a wheel which operates a valve. A protector cap is provided on the top of the cylinder to safeguard the valve.

Acetylene cylinder: 

Acetylene cylinder as also made up of steel. Gas is filled at a pressure of 18-20Kg/cm^2. The capacity of the cylinder is about 10m^3. Regulator valve and safety valve are also mounted on cylinder. Safety plugs are also provided on the bottom of the cylinder. When filled into the cylinder, the acetylene is dissolved in acetone.

Regulator: 

It is used to control the flow of gases from high pressure cylinder. 

Torch: 

It is a device used to mix acetylene and oxygen in the correct proportion  and the mixture flows to the tip of the torch. There are two types of torches:






a. Low pressure or injector torches: These torches are designed to use acetylene at low pressure. The pressure is kept very low up to 0.7Kg/cm^2. But the oxygen pressure is very high.

b. Medium pressure or equal pressure torches: In this type of torch the acetylene is taken at a pressure equal to 1Kg/cm^2. The oxygen is always supplied at high pressure.

Both type of torches are provided with two needle valves. One regulates the flow of oxygen and the second valve controls the flow of acetylene. A mixing chamber is provide to mix the gases.

Torch tips: 

For different types of jobs, different tips are used. The size of the tip is specified by the outlet hole diameter. More than one hole is also provided in tips. The tip is screwed  or fitted on the front end of the torch.

Goggles: 

Gas flames produce high intensity light and heat rays, which are harmful to naked eyes. To protect the eyes from these rays, goggles are used. Goggles also protect the eyes from flying sparks.

Lighter: 

For starting the flame, the spark should be given by the lighter. Match sticks should not be used, as there is risk of burning hands.

Fire extinguisher: 

Fire extinguishers are used to prevent the fire that may break out by chance. Sand filled buckets and closed cylinders are always kept ready to meet such accidents.

SAFETY MEASURES IN GAS WELDING

1. Always handle the gas cylinder with care.
2. The adjusting screw on the regulator must be fully released before opening a cylinder valve.
3. Never use match stick for lighting a torch.
4. Never lubricate the regulator valve with oil or grease, it may cause explosion.
5. Always use goggles while working.
6. Proper ventilation must be provided in the shop.
7. Acetylene cylinders should be stored in upright position.
8. Do not open acetylene cylinders near spark or fire.
9. Never remove torch tip with pliers.
10. The cylinder should be leak proof.
11. Always use protective caps over the valves.
12. Keep in mind the location of the fir extinguisher.