Gas welding Guide, Meaning , Facts, Information and Description
In gas welding, the heat energy and high temperature needed to melt the metal is obtained by the combustion of a fuel gas with oxygen.
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2 Apparatus 3 Setting up the equipment 4 The flame 5 The Blowtorch 6 References 7 See also |
The most commonly used fuel gas is acetylene. Other gases used are liquified petroleum gas (LPG), natural gas, hydrogen and MAPP gas.
Acetylene is obtained from the action of water upon calcium carbide. Calcium carbide in turn is produced by the reaction together in an electric arc furnace of carbon and calcium carbonate. Acetylene can be produced near where the welding is being done in an acetylene generator. More commonly it is produced in a central plant and then shipped to the welding site in special containers. These containers are packed with various porous materials (kapok fibre), then filled about half way with acetone. The acetylene dissolves into the acetone. This method is necessary because acetylene is unstable above 207 kPa (30 lb/in²) and may explode. There is about 1700 kPa (250 lb/in²) of pressure in the tank when full. Acetylene gives a temperature of 3200 to 3500 °C (5800 to 6300 °F) when burned with oygen, which is the highest temperature of any of the gases mentioned here.
Hydrogen has a clean flame and is good for use on aluminum. Can be used at a higher pressure than acetylene and is therefore used for underwater welding.
MAPP gas is a register product of the Dow Chemical Company. It is stabilized methylacetylene- propadiene. It has the storage and shipping characteristics of LPG and has a heat value a little less than acetylene.
Oxygen is most commonly produced at a central plant and then shipped to the welding site in high pressure vessels (commonly called 'tanks') at a pressure of about 14000 kPa (2000 lb/in²). It is also shipped as a liquid in Dewar type vessels (like a large Thermos TM jar) to places that use large amounts of oxygen.
The apparatus used in gas welding consists basically of a torch, two pressure regulators and twin flexible hoses.
The torch is the part that the welder holds and manipulates to effect the weld. It has two valves and two connnections, one each for the fuel gas and the oxygen, a handle for the welder to grasp, a mixing chamber where the fuel gas and oxygen mix, and a tip where the flame issues from.
The regulators are attached to the fuel and to the oxygen sources. The oxygen regulator is attached to the oxygen tank and drops the pressure from about 14000 kPa (2000 lb/in²) to a lower pressure for the torch. This pressure can be adjusted to suit the job at hand by turning a knob on the regulator, and can be set from 0 to about 700 - 1400 kPa (100 - 200 lb/in²). Likewise the fuel regulator is attached to the fuel source and drops the pressure to a level for the torch to use. For acetylene this is 0 to 100 kPa (15 lb/in²).
The flexible hoses connect from the regulators to the torch and carry the fuel gas and the oxygen. The fuel gas connections have left hand threads and the oxygen connectors have right hand threads so that the two cannot be interchanged, so as to help prevent accidents.
The welder wears goggles or a shield with a shaded lens to protect his eyes and wears leather gloves to help protect his hands from burns. He should also wear clothes and shoes appropriate for welding.
Note that the proceedures and equipment used for gas welding are essentially the same as for gas brazing.
When using fuel and oxygen tanks they should be fastened securely to a wall, a post or a portable cart in an upright position. An oxygen tank is especially dangerous for the reason that the oxygen is at a pressure of 14000 kPa (2000 lb/in²) when full and if the tank should fall over and the valve should strike something and be knocked off, the tank will become an unguided and unpredictable missile powered by the compressed oxygen. It is for this reason that an oxygen tank should never be moved around without the valve cap screwed in place.
The acetylene tank should never be laid down while being used as the acetone will start to come out through the valve.
After the oxygen tank is securely fastened, remove the valve cap. With the valve opening pointed away from the welder, the valve is opened slightly for just a moment and then closed. This serves two purposes. For one, it blows out any dirt or dust that may have settled in the valve. This dirt would otherwise end up in the regulator and shorted its life and accuracy. For another, when a tank is filled, the worker has a tendency to tighten the valve securely to make certain it is closed completely. It is better to break it loose now than when the regulator is in place. The oxygen regulator is attached and the nut tighted. Pliers should never be used as the brass nut will soon be damaged by the pliers; a wrench should always be used. Also, there is a tendency of welders to overtighten the nut. If it is not leaking, then it is sufficiently tightened. If a great amount of torque is required to stop it from leaking, or if it will not stop leaking in spite of any amount of tightening, then there is something wrong with the nut or the valve.
The fuel regulator is attached to the fuel tank in the same manner. The nut on the fuel regulator usually has left hand threads.
The flexible hoses are attached from the regulators to the torch. The oxygen hose is usually colored green and the fuel hose red. The fuel hose has left hand threaded connectors at both ends and the oxygen has right hand threaded connectors.
With the valves on the torch closed, and the knobs on the regulators screwed out until loose (0 setting), the valves on the fuel and oxygen tanks are opened. The oxygen valve should be opened slightly and then time given for the high pressure gauge on the regulator to stop rising. Then the valve is opened fully, until it stops turning.
The fuel valve is opened also. An acetylene valve should only be opened one quarter turn. The valve on a LPG tank should be turned out completely as on an oxygen tank.
After this preparation, the regulators are set at the desired pressure. For acetylene, this should never be more than 103 kPa (15 lb/in²). To prevent a large sooty flame at first lighting the torch, both the fuel and the oxygen valves are opened (more fuel than oxygen) and a flame lit with a 'striker' or by some other means. After the flame is adjusted to the proper size, the oxygen valve is opened further and adjusted to give the desired balance of fuel and oxygen. Usually a neutral flame is used. This is a flame where the fuel and oxygen supplied to the torch tip are both completely combined with each other. An oxidizing flame has an excess of oxygen and a reducing flame has an excess of fuel (carbon). An oxidising flame is used for cutting and a reducing flame is used for annealing e.g. to soften steel sheet metal.
An acetylene flame (as is characteristic of most fuel/oxygen flames) has two parts; the inner cone and the outer cone. The inner cone is where the acetylene and the oxygen combine. The tip of this inner cone is the hottest part of the flame. The outer cone is where hydrogen and carbon monoxide from the breakdown of the acetylene and partial combustion of the inner cone combine with the oxygen in the surrounding air and burn. A neutral flame has a well defined inner cone. A reducing flame has a feathery inner cone. An oxidizing flame has a smaller inner cone that is sharply defined and is pale blue. The welder observes this while adjusting the fuel and oxygen valves on the torch to get the correct balance for the job at hand.
The size of the flame can be adjusted to a limited extent by the valves on the torch and by the regulator settings, but in the main it is determined by the size of the orifice in the tip. In fact, the tip should be chosen first and then the regulators set accordingly.
The flame is applied to the base metal and held until a small puddle of molted metal is formed. The puddle is moved along the path where the weld bead is desired. Usually, additional metal is added to the puddle as it is moved along by means of dipping a wire in the molten metal puddle).
A blowtorch is a special type of gas welder. It has a lever on the handle that, when squeezed, supplies an extra flow of oxygen out of a center hole in the mouthpiece. When the steel is heated to its melting point, the extra flow of oxygen will burn (oxidise) the molten iron and blow it away, effectively cutting the steel.
This is an Article on Gas welding. Page Contains Information, Facts Details or Explanation Guide About Gas welding Fuels
Apparatus
Setting up the equipment
The flame
The Blowtorch
References
See also