Oxy acetylene GAS WELDING and Techniques - ITI Online Note

Oxygen acetylene Gas Welding torch or blow pipe are used 

1- Oxygen and acetylene gas pressure is reduced by gas regulator. 

2- These gases are supplied through suitable hose to welding torch. 

3- Gases are mixed in mixing chamber of welding torch and supplied to welding tip or nozzle. 

4- The gas mixture is burnt to produce a flame to carryout welding operation. 

5- There are two types of Gas welding torches, 

      • Low pressure or injector type 

      • High pressure or equal pressure type.

6- Low pressure torches are used with acetylene pressure obtained from acetylene generator at a pressure of 0.02 bar (less than 1 psi). 

7- High pressure torches are used with acetylene stored in cylinder at a pressure of 8 bar.

Low pressure or injector type torch 

1- It is operated by low pressure acetylene at 0.02 bar. 

2- As acetylene is at low pressure, it is necessary to use oxygen at high pressure (approximately 2.8 bar).

3- Oxygen enters into mixing chamber through passage located in the center of torch. 

4- The oxygen passage is surrounded by one carrying acetylene. 

5- The high pressure of oxygen passing through a small opening into injector nozzle enters mixing chamber and pulls acetylene into it. 

Advantages of low pressure torch 

1- Small fluctuations in oxygen will produce a corresponding change in amount of acetylene drawn. 

2- The proportions of two gases are constant while the torch is in operation.

High pressure or equal pressure torch 

1- Oxygen and acetylene are supplied to blow pipe with equal pressures. 

2- The pressure is reduced in two stages instead of one, using two diaphragms and two control valves, so that the pressure reduction ratio is less.

3- Equal pressure or high pressure blow pipe is most commonly used because, 

  • It is lighter and simpler.

  • It does not need an injector.

  • In operation, it is less trouble some, it does not suffer from back fire. 

4- To change the power of welding torch, it is necessary to change the nozzle tip and increase or decrease gas pressure approximately.

Difference between LP and HP blow pipes

Low pressure blow pipe 

1- Injector type blowpipe. 

2- The injector helps to prevent backfiring. 

3- A low pressure blowpipe can be used on high pressure system.

4- Pressure of oxygen is high and acetylene is low.

5- It is more expensive Used in cutting of light sheet metals.

High pressure blow pipe

1- Non injector type blowpipe. 

2- There is no injector to prevent backfiring. 

3- A high pressure blowpipe should not be used on a low pressure system. 

4- Pressure of c oxygen and acetylene are same.

5- It is less expensive .

6- Used in cutting of heavy metals.

Gas welding techniques 

1- Depending upon the ways in which the welding rod and welding torch used it is classified into two techniques. 

  • Leftward welding technique (Forehand technique). 

  • Rightward welding technique (Backhand technique). 

  • Vertical welding technique.

Welding techniques 

1- Depends on the following factors

  • Material being welded.

  • Thickness of the work piece .

  • Shape and size of the work piece

  •Properties anticipated of the joint 

Types of gas welding techniques 

1- Left hand or forward welding 

2- Right hand or backward welding 

3- Vertical welding 

Forward or left hand welding.

1- The weld is commenced at the right hand side of the join and blow pipe is given by steady forward movement, with slight slide ways motion, zig zag along the weld towards the left. 

2- The blow pipe is kept an angle 60-70° to the surface of work, so the flame on stage in front of it. 

3- The filler rod held an angle 30-40° just ahead of flame and progressively fed into it. 

Advantages

1- Vertical joints are welded by this technique. 

Disadvantage

1- This technique is restricted in welding of mild steel plates (up to 5mm thickness), castiron and non ferrous metals. 

Backward or right hand welding 

1- Flame is directed towards the completed part of joint and welding proceeds from left hand right hand.

2- Filler rod is fed into the flame in circular movement. 

3- Horizontal and overhead welding can be done by this technique.

Advantages

 1- Rightward welding is faster by 20-25% from oxygen and 15-25% of less acetylene is need to compare the leftward welding. 

2- The mechanical properties of the weld are better due to annealing effect of flame, which is directed on the completed weld. 

3- The amount of distortion in the work is minimum. 

Disadvantages 

1- Only weld thicker materials. 

2- High content of steels only weld. 

Vertical welding .

1- Welding is commenced at the bottom and proceeds vertically upwards. 

2- Generally two welders are employed for vertical welding, one on either side of the joint.

3- If one operator is employed the angle of the torch varies with the plate thickness. 

4- Does not require edge preparation. 

5- Therefore requires less amount of filler material. 

6- Advantageous for plates more than 6 mm thick.