Mechanical Engineering

Gas Welding Process:

Gas welding is a fusion process in which fusion is obtained by completely melting the joint.  The heat required for heating and melting the parent and filler metal is obtained by the combustion of a fuel gas with oxygen.

Fuel gas used in welding can be acetylene, propane, hydrogen, butane and natural gas. Of these, acetylene is by far the most widely used because the temperatures obtained by combustion of acetylene are much higher than those obtained by combustion of other gases.

Oxy-acetylene flame temperature, for example, is of the order of 3500°C compared to 3000°C for oxy-hydrogen or epoxy butane flame and 2800°C for natural gas flame. The medium of combustion of fuel gases is generally oxygen though sometimes compressed air is also used. Use of air, leads to reduced thermal efficiency, lower welding speed and poorer quality of the welds. The choice of gases for gas welding is thus restricted to choice of correct fuel gas for the desired welding speed and quality of weld.

Oxy-acetylene gas welding process is widely used commercially for welding of ferrous and non-ferrous metals particularly for thin sections up to 6 mm thick. The process is also useful for a large amount of repair work. Typical applications of the process include fabrication of ventilation and air-conditioning ducts and repair of vehicles. The process is also indispensable in early stages of the installation of a new plant. Oxyhydrogen gas welding is used for welding thin sheets of steel and low melting temperature materials.

Oxyfuel gas welding:

Any fuel gas combined with oxygen to produce a flame. This flame is used as the source of heat to melt the metals at the joint.

Fuels are:

• Oxygen,
• Acetylene,
• Hydrogen,
• Methyl acetylene Propadiene.

Ex.: Oxyacetylene Welding

• C₂H₂ + O₂ à 2CO + H₂ + Heat (inner core flame reaction)
• 2CO + H₂ + 1.5O₂ à 2CO₂ + H₂O + heat (Temperature 3300 deg Celsius)

Types of flame:

1. Neutral flame

2. Oxidizing flame

3. Carburizing or Reducing Flame

Apart from their chemical nature, these flames differ in shape and structure as shown. Theoretically all flames consist of three zones: an inner cone, a middle reducing zone known as the acetylene feather and an oxidizing outer zone called the flame envelope or the streamer.

The inner cone is readily distinguishable bright luminous zone. It consists of a mechanical mixture of hot oxygen and dissociated acetylene. The primary combustion starts at the outer boundary of the inner cone and extends into the acetylene feather.

The secondary combustion occurs in the flame envelope using oxygen from atmospheric; air.

Neutral flame:

Acetylene and oxygen ratio is 1: 1

Oxidizing flame:

Greater oxygen supply (it’s used for copper and copper based alloys, steel)

Reducing flame:

Lower oxygen supply (brazing, soldering, flame hardening).

(0₂ : C₂ H₂ = 0.85 to 0.95)

Flame temperature:

Temperature is one of the important characteristics of the flame. Higher the temperature more efficient is the heating and melting of the metal. The temperature of the flame is not constant.

It varies along and across the flame and depends on the composition of the flame. For most of the hydrocarbons which burn with an inner luminous cone the maximum temperature is in the intermediate zone next to the inner cone.

Welding is done using the flame in this zone and accordingly the torch is positioned such that the tip of the inner luminous cone is within 2 to 3 mm of the metal surface to be welded. It is this zone that decides the nature of the flame-reducing, carburizing or oxidizing.

The flame temperature depends on the oxygen to acetylene ratio and increases with increase in this ratio up to a certain point. The limiting values are 1.2 to 1.9 with corresponding temperatures of 3300 to 3500°C. The maximum temperature obtained in a reducing flame is about 2900°C, that in a neutral flame about 3250°C and for an oxidizing flame about 3500°C.

Equipment used:

It consists of a welding torch, which is available in various sizes and shapes, connected by hoses to high pressure gas cylinders and equipped with pressure gases and regulators. Although it can be mechanized, this welding operation is essentially manual and hence slow, and is used typically for fabrication and repair work.

Application of Gas Welding:

1. Fabrication of automotive bodies

2. Repairing work

Advantages of Gas welding:

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

2. The cost and maintenance of the welding equipment is low when compared to other welding process

3. The rate of heating and cooling is relatively slow.

Disadvantages:

1. Fluxes used in certain welding and brazing operations produce flames that are irritating to the eyes, nose, throat and lungs.

2. Heavy sections cannot be joined economically

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

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

5. Refractory metals and reactive metals cannot be gas welded

6. Gas flames takes a long time to heat up the metal than an arc.