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Wednesday, 2 November 2011

Gas Welding Process

Gas Welding Process:

In this process, the heat source is combustion of acetylene gas. Chemical reaction of acetylene and oxygen produces a great deal of heat and the oxyacetylene flame burns with temperature exceeding 3250°C, enough to melt most metals and alloys. Two systems are in vogue for oxyacetylene welding:
(i) High pressure system: In this system, both oxygen and acetylene gases are drawn from cylinders in which these gases are stored at high pressure.
(ii) Low pressure system: In this system, oxygen gas is drawn from a cylinder as before, but acetylene gas is produced at site at low pressure. Acetylene gas is produced in a sealed container in which water falls drop by drop on calcium carbide. This acetylene gas is drawn for oxyacetylene welding as per requirement.

High pressure oxyacetylene welding equipment consists of two large steel cylinders. One which is conventionally painted black and is a long thin cylinder contains oxygen filled in at a high pressure of 125–140 kg/sq. cm. The other cylinder which is painted maroon and is shorter but of slightly larger diameter contains acetylene gas dissolved in acetone at a pressure of 16–21 kg/sq. cm. One should handle the D.A. cylinder with great care as acetylene is an inflammable gas and this cylinder should be kept vertical, as far as possible. Both these cylinders are fitted with valves which are normally kept in “closed position”. D.A. means dissolved acetylene gas.
In order to draw gas from the cylinder, each cylinder is fitted with a pressure regulator with two gauges. The function of the pressure regulator is to reduce the pressure of the gas before delivering it. The two gauges indicate the pressure inside the cylinder and the reduced pressure of gas after the pressure regulator stage. The gases are carried from the pressure regulator to the welding torch (also called blow pipe) by means of rubber hose pipes. The pressure regulator and the hose pipe connected to oxygen cylinder are of black colour while those connected to acetylene cylinder are of maroon colour, so that there is no mix up.
A welding torch consists of different passages for oxygen and acetylene gases. Supply of these gases is controlled by pin valves. These two gases are then allowed to mix in a mixing chamber before being drivenout through the orifice of the blow pipe. These orifices are of different sizes and can be screwed on to the blow pipe. The complete assembly of the cylinders, regulator etc. is shown in Fig. Normally the two cylinders are carried in a trolley, which is not shown in Fig. A gas welding operator uses the following safety apparel:
(i) Wears blue coloured goggles to protect his eyes,
(ii) Wears a leather or canvas apron to protect his person,
(iii) Wears leather gloves to protect his hands.
He carries metal welding rods and a supply of flux. He also carries a chipping hammer, a wire brush and a spark lighter. The procedure of lighting a flame is to open the pin valve controlling the flow of acetylene gas in the welding torch and to use spark lighter to burn the gas. The acetylene gas burns with lot of smoke. The oxygen supply valve is then opened and adjusted to get the desired kind of flame.

Three kinds of oxyacetylene flames can be produced with the gas welding equipment. The chemical reaction between acetylene gas and oxygen is represented by the equation
For complete combustion of one volume of acetylene, 2½ volumes of oxygen gas is required. Out of 2½ volumes of oxygen, one volume is drawn from the cylinder and 1½ volume, is supplied by the atmosphere, when the flame burns. When the oxygen is supplied in this proportion, the flame is called neutral flame. If however, oxygen supply is less, the flame is termed reducing flame as it contains some unburnt carbon. If there is excess supply of air (i.e., oxygen), the flame becomes oxidising flame. These three kind of flames can be distinguished from each other by careful observation. These flames are shown in Fig.

A carburising or reducing flame has three distinct zones—inner cone, intermediate feather and outer envelope. When oxygen supply is increased the intermediate feather gradually disappears and only two cones are left the inner cone and the outer envelope. At this point the acetylene and oxygen gases are in chemical balance and the flame is neutral flame. If the supply of oxygen is further increased, the inner cone reduces in length, looses its shape and a sharp hissing sound is produced. The flame has now become oxidising. Such flames have highest flame temperature.
Neutral flame is used for welding of all kinds of steel and cast iron products. Slightly oxidising flame is used for welding brass, bronze and copper products or for welding chromium-Ni and manganese steels. Slightly carburising flame is used in welding of high carbon steel, aluminium and Nickel

Setting of the job: Parts to be welded are cleaned and the joint prepared. Joint preparation depends upon the thickness of work pieces. Thin sheets can be joined by an edge or flange-joint. Sometimes, a lap or fillet joint can be used. A sheet of higher thickness but not exceeding 4.5 mm may be welded with a butt joint without any joint preparation. Different kind of joints commonly used in welding are illustrated.

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