Gas welding

Apparatus for Gas Welding

Principle of Oxy-acetylene Welding

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.
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 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 valve is turned first. When acetylene comes out of the nozzle, it should be ignited with spark lighter. It will give a yellow-colored smoke flame. After it, oxygen cylinder valve is opened and supply is increased until a most suitable flame is obtained. Then  the flame is focused on the edges to be welded. Flux and filler metal are also added with the heat of flame. The edges and filler metal melt and a joint are formed after cooling of the molten metal. The joint may be formed with or without using filler metal.

Applications

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

Advantages of Oxy-acetylene Welding

The main advantages of oxy-acetylene welding are given  below:

1. Equipment is cheap as compared to other welding process.
2. It can be used for welding of all types of metals.
3. Maintenance of equipment is very less.
4. It is a portable process.
5. It can be used for cutting of metals of small thickness.
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. The heat affected area is more.
3. This is prone to corrosion and brittleness.
4. Gases are expensive and difficult to store.

2.Types of Gas Flames

There are three types of gas  flames:
Oxidizing Flame 2. Carburising Flame  3.Neutral Flame

1. Oxidizing Flame: When the volume of 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.
2. 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.
3. 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.

Fig. 3: Types of Flames

   4.Gas Welding Techniques

There are two types of gas welding  techniques:

1. Left ward welding
2. Right ward welding
1. Left Ward Welding: In this welding the tip of the torch is held at 60 to 70°C to the plates. And the filler rod is inclined at 30 to 40°C 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. The technique is illustrated in the Fig.4.

Fig. 4: Leftward Welding

1. Right Ward Welding: In right ward welding the torch is kept at 40 to 50°C to the job to be welded. Torch is moved towards right as shown in the Fig. 5. Right ward welding is done for heavy sections only.

Fig. 5: Rightward Welding

Flux

The chemicals which deoxidize the metal surface and provide inert atmosphere around the molten metal are known as  fluxes.

The main function of flux is given below:

1. To prevent oxides on 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 & 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 metals to be welded.

Gas Welding Equipment

Details of Gas welding equipment are as  under:

1. Oxygen Cylinder: As shown in Fig 6. Cylinder is made up of steel in capacity range 2.25 to 6.3 m3. The cylinders are filled with oxygen at about 150 kg/cm2 at 21°C. A safety valve is also provided 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 a cylinder to safeguard the valve.

1. Acetylene Cylinder: As shown in Fig. 7. Acetylene cylinders are also made up of steel. Gas is filled at a pressure of 18-20 kg/cm2. The capacity of the cylinder is about 10m3. Regulator valve and safety valve are 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.

1. Regulator: Regulator is used to control the flow of gases from high pressure cylinder.

A simple type of regulator is shown in the Fig. 8.

1. 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. Refer Fig. 9. There are two types of torches:
1. Low pressure or injector torches
2. Medium pressure or equal pressure torches

1. Low Pressure or Injector Torch: These torches are designed to use acetylene at low pressure. The pressure is kept very low up to 0.7 kg/cm2. But the oxygen pressure is very high.
2. Medium Pressure or Equal Pressure Torch: In this type of torch the acetylene is taken at a pressure equal to 1 kg/cm2, the oxygen is always supplied at high pressure. Both types 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 provided to mix the gases.
3. 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. Various types of tips are shown in the Fig.10.

Fig. 10: Torch Tips

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

Fig. 11: Goggles