Gas Welding & Types of Flames

What is gas welding and different types of flames used in gas welding? It is a type of fusion welding, in which the heat for welding is obtained by the combustion of a fuel gas. The most widely used gas combination for producing a hot flame for welding metals is oxygen and acetylene (C2H2). The approximate flame temperature produced by oxy-acetylene flame is 3200° C.

The basic equipment required to carry out oxy-acetylene gas welding is as follows:

1. Welding torch

It is also known as blow pipe. It is a tool for mixing the oxygen and acetylene in the desired volumes and burning the mixture at the end of a tip, which produces a high temperature flame. The welding torches are commercially available in following two types:

(a) Injector or low-pressure type:

The injector or low-pressure type welding torch operates at an acetylene pressure of less than 7 kN/m2. This low-pressure acetylene is produced at the welding site by the chemical reaction between water and calcium carbide (CaC2). The oxygen is, however, supplied at a pressure ranging from  70 to 280 kN/m2 depending upon the tip size.

(b)  Positive or equal pressure (also known as high pressure) type:

In positive or equal pressure type welding torch, the gases must by delivered to the torch at pressures generally above 7 kN/m2.

2. Welding torch tip

The tips are made of high thermal conductivity material such as copper or copper alloy. The interchangeable tips for the various thicknesses are usually provided with each welding torch.

3. Pressure regulators

There are two gauges on the body of the regulator, one showing the pressure in the cylinder while the other shows pressure being supplied to the torch. The desired pressure at the welding torch for oxygen is between 70 and 280 kN/m2 and for acetylene it is between 7 and 103 kN/m2.·

4. Hose and hose fittings

The standard color for oxygen hose is black and for acetylene hose, it is red.

5. Gas cylinders

The standard color for oxygen cylinder is black and for acetylene cylinder it is maroon.

In addition to the above equipment, welding rods, fluxes, spark lighter, goggles and gloves are required for gas welding.

Types of Flames in Gas Welding

The following three types of flames are used for gas welding.

1. Neutral flame

The neutral flame, as shown in figure below is obtained by supplying equal volumes of oxygen and acetylene. It has the following two sharply defined zones:

• An inner luminous cone (3200° C), and
• An outer cone or envelope of bluish color (1250°C)

The most of the oxy-acetylene welding (e.g., welding of steel, cast iron, copper, aluminum etc.) is done with the neutral flame.

2. Oxidizing flame

The oxidizing flame, as shown in figure below, is obtained when there is an excess of oxygen. It is used for welding brass and bronze.

3. Reducing or carburizing flame

The reducing flame, as shown in figure below, is obtained when there is an excess of acetylene. It is used for welding of molten metal, a certain alloy steels, many of non-ferrous, hard surfacing materials such as satellite.