Oxyacetylene Welding

The highest temperature obtained in oxyacetylene welding is 3,200°C. Acetylene can be used as a gas from a separate cylinder or through reaction of water on calcium carbide. Three different types of flames such as neutral, oxidizing, and carburizing are generated at the tip of welding torch by regulating the amount of acetylene and oxygen with the help of pressure regulators and control valve. These flames are shown in Figure 20.32.

Neutral Flame: Neutral flame is generated at the tip of welding torch with equal volume of oxygen and acetylene mixing in the torch. The two sharply defined zones are inner white cone and outer blue envelope. The maximum temperature occurs at a distance of 3–5 mm from the inner cone.

The reaction at the inner cone is

 

C₂H₂ + O₂ = 2CO + H₂

 

The reactions at outer envelope are

The outer envelope works as a protector and pre-heater of the workpiece. The metals using neutral flame for welding are cast iron, mild steel, stainless steel, copper, aluminium, etc.

Oxidizing Flame: Oxidizing flame is generated with higher proportion of oxygen. The proportion of oxygen and acetylene used is 1.15–1.5. The flame is similar to neutral flame but inner cone is shorter than that of neutral flame; the outer envelope is light blue. In this flame, there is complete combustion of acetylene and it forms carbon dioxide and water vapour. This is oxidizing in nature and used in welding of brass, zinc, bronze, gold, etc.

Figure 20.32 Three Basic Types of Flames Used in Oxyacetylene Gas Welding and Cutting Operations: (a) Neutral Flame, (b) Oxidizing Flame, and (c) Carburizing or Reducing Flame

Reducing Flame or Carburizing Flame: In this flame, acetylene is used in excess amount than the theoretically required. The ratio of oxygen and acetylene used is 0.85–0.95. The three zones in this flame are the inner cone, which is not sharply defined; outer envelope is similar to neutral flame; the third zone surrounding the inner cone extends up to outer envelope. It is whitish colour and shows the excess of acetylene used. This flame is used for welding of low carbon steel, aluminium, non-ferrous metals like monel metal, nickel, etc.

Oxyacetylene Welding Equipments: Following equipments are used in oxyacetylene welding (Figure 20.33):

• Gas cylinders: Two gas cylinders made of steel are used. One is of black colour used for oxygen and other is of maroon or red colour used for acetylene.
• Pressure gages: Each cylinder consists of two pressure gages. One pressure gage shows the pressure of gas inside the cylinder and other shows pressure of gas supplied to blowpipe.
• Pressure regulator: Each cylinder is provided a pressure regulator. The function of the pressure regulator is to control the pressure of gas supply to blowpipe or to maintain the constant pressure of the gas.
• Blowpipe or welding torch: The cross-sectional view of welding torches is shown in Figure 20.33(a) and (b) as high pressure welding torch and low pressure welding torch, respectively.

Figure 20.33 (a) Cross-sectional View of High Pressure Torch, (b) Cross-sectional View of the Low Pressure Torch, and (c) Basic Equipment Used in Oxyacetylene-gas Welding

The high pressure blow pipe consists of two passages: one is for oxygen and other is for acetylene. Both the gases are mixed in a chamber and then driven out through the orifice of the blowpipe nozzle with the desired velocity. These nozzles are usually known as tips and are made interchangeable so that the same blowpipe can be used for different sizes of the tips.

The low-pressure torch works on the principles of injector. The pressure of acetylene used is too low but oxygen is supplied at high pressure from 7 to 50 psi.

Note: Acetylene cannot be filled alone in a cylinder due to dissociation at high pressure so it is mixed with small amount of acetone. For mixing at atmospheric pressure, 25 l of acetylene is mixed with 1 l of acetone.

Advantages

• The equipments used in oxy-fuel welding is less costly and easy maintainable.
• It is portable and can be used anywhere.
• It can be used to join most of the common metals.
• The flame temperature can be easily controlled.
• This can be used for cutting purposes.

Limitations

• Due to lack of concentration of heat, large area of the metal is heated and distortion is likely to occur.
• Oxygen and acetylene gases are expensive.
• Storing and handling of gas cylinders involve greater safety measures.

Flux and shielding provided in oxyacetylene welding is not so effective as in inert gas arc welding.

Welding Rod and Fluxes: Welding rod used in a gas welding has similar composition to work material. The diameter of welding rod depends on the thickness of metal plate. Generally, the diameter of welding rod used is half of the thickness of plate. To increase the fluidity of molten metal and to protect the weld pool from the atmospheric gases, fluxes are used. Various types of fluxes are used to weld the different types of metals but for mild steel, no flux is required. The type of fluxes used, depend on the type of metal to be welded, which are listed below.

• Copper and copper alloys: Mixture of sodium and potassium borates, carbonates, chlorides, sulphates, borax (Na₂B₄O₇·10H₂O), boric acid (H₃BO₃), and di-sodium hydrogen phosphate (Na₂HPO₄) are used for dissolving oxides of copper.
• Ferrous metals– Carbon steel: Dehydrated borax, calcium oxide, dissolved in liquid.– Alloy steel: Boric acid, dehydrated borax, calcium fluoride.
• Aluminium and aluminium alloys: Mixture of alkaline fluorides, chlorides and bisulphate of calcium, sodium, potassium, lithium, and barium.