Welding is given in the Bible and Tuba-Cain was the first smith who forged the Instrument of bronze and iron. About 5000 years back the forge, hammer, anvil bellow and fire heaters were used for forging. It was in the year 1887 that Thomas Fletcher of Harrington invented various blow pipes for burning hydrogen or coal gas with air or oxygen. The bicycle frames were manufactured with the help of these below pipes . This type of below pipe Is used even today where acetylene gas Is used In place of hydrogen or coal gas. In 1892, acetylene gas was first discovered by Thomas Leopold Wilson of North Carolina.
In 1886, two Frenchmen, the Bring brothers formed a company for the liquefaction of alarm from which pure oxygen was obtained. The only acetylene welding blow pipe was Invented by Boucher and Picador. Volta, after whom the term volt Is named, demonstrated In 1800 that the electric arc could be used for melting Iron wire and the process of fusion of metals by electricity was born. (Augural & Tall 1992). Welding Is a process for ]lolling two similar or dissimilar metals by fusion. It joins different metals/alloys, with or without the application of pressure and with or without the use of filler metal.
The fusion of metal takes place by means of heat. The heat may be generated either from combustion of gases, electric arc, electric resistance or by chemical reaction. During some type of welding processes, pressure may also be employed, but this is not an essential requirement for all welding processes. Welding provides a permanent joint but it normally affects the metallurgy of the components. It is therefore usually accompanied by post weld heat treatment for most of the critical components. The welding is widely used as a fabrication and repairing process in industries.
Some of the typical applications of welding include the fabrication of ships, pressure vessels, automobile bodies, off-shore platform, bridges, welded pipes sealing of nuclear fuel and explosives. (Rejoinder Sings 2006) OX-ACETYLENE WELDING In this process, acetylene is mixed with oxygen in correct proportions in the welding torch and ignited. The flame resulting at the tip of the torch is sufficiently hot to melt and Join the parent metal. The ox-acetylene flame reaches a temperature of about 33000C and thus can melt most of the ferrous and non-ferrous metals In common use.
A filler metal rod or welding rod Is generally added to the molten metal pool to build up the seam slightly for greater strength. (Rejoinder Sings 2006) MANUFACTURE OF CALCIUM CARBIDE Calcium carbide is produced by fusing Lime and carbon (coke) at 1900 to 23000C In single or three-phase electric arc furnace. The power of these furnaces varies from 50 to awake. Limestone (C_(a ) [CO) _(3 )) Is first roasted In Lime thus kilns to produce quick lime (C_(a ) O_ ). The quick Lime thus produced Is mixed with ground coke and anthracite In correct proportion to make up the charge.
The charge Is fed Into the hoppers of a carbide furnace. In the furnace the charge Is melted to form molten calcium carbide. It is tapped from the furnace at regular intervals. It is is then carried for cooling and crushing. The crushed carbide is then sent to the sizing department where it is sorted out according to lump size sand packed in roof- iron drums. (Augural and Tail 1992) Calcium carbide to water In this method, the carbide falls into the water from a container above the water tank, the exact quantity is being controlled by means of a valve or other supply devices.
Water to Calcium Carbide In this case, water is added to calcium carbide in quantities of water in accordance tit the amount of acetylene required in the tank. Purification of acetylene gas Generated acetylene chemically contains impurities and for welding purposes, the gas must be pure and therefore the impurities must be removed. The generation of crude acetylene must be slow because the faster the generation, the more heat that is produced and the greater the proportion of impurities.
The impurities include: ammonia hydrides, phosphorous, sulfur, nitrogen water vapor and particles of lime. The passing of the gas through water during generation removes some of the ammonia and sulfated hydrogen but for complete purification the gas must pass wrought proper purification material known as acetylene purifier. The purifier This contains pumice, purification materials and filter wool. In this process, the gas enters the purifiers at the base after passing through a layer at pumice which absorbs water vapor.
The gas then filters through the purification material and then finally through the filter wool which absorbs all the remaining impurities and passes it into service. The filter wool collects all the particles of lime to prevent them being carried forward with the gas. (Sockeye & Megaphone 1996) Test for purity of acetylene gas Acetylene gas after purification is tested for purity by holding silver nitrate test paper (a piece of filter paper soaked in a solution of silver nitrate) in the stream of acetylene gas is for ten (10) seconds.
If the acetylene gas is properly purified there will be no trace of stain on the silver nitrate paper. PRODUCTION OF OXYGEN Oxygen can be produced by; Air liquefaction Electrolysis of water Heating mercuric oxide Heating barium oxide Heating potassium chlorate A substantial quantity of the oxygen used in the industry is extracted from the atmospheric air. The air has the following composition by volume: Nitrogen 8. 03% Oxygen Argon Neon 20. 99% 0. 94% 0. 0018% Carbon -dioxide 0. 03% Helium Krypton Other gases 0. 005% 0. 0001% Rest Liquid-air process is the prime method used in the production of commercial quality oxygen. By this method, air is first washed with caustic soda, then temperature is lowered to -ICC. This low temperature liquefies or vaporizes all of the material within the chamber. The basic principle behind this method is that all gases vaporize at different temperatures. Oxygen being one of the chief constituents of air and having a higher vaporization point than nitrogen and argon vaporizes last.
When the liquid air is allowed to evaporate slowly, nitrogen and argon boil off more rapidity than oxygen until finally almost pure oxygen remains. The gas from the boiling liquid is compressed into steel cylinders specially designed to withstand pressure of keg/ CM at a room temperature of ICC. Oxygen is then ready to be transported for use with oxygen-acetylene welding and cutting equipment. Electrolysis of water By electrolysis of water, hydrogen and oxygen are obtained. An electric current is passed through water containing an electrolyte such as caustic soda to increase the electrical conductivity.
The water will decompose, the oxygen collecting at the positive pole and the hydrogen at the negative pole. The oxygen is distributed to the users as a compressed gas in individual cylinders or by one of several bulk distribution methods. (Augural & Tail 1992) GAS WELDING EQUIPMENTS The basic tools and equipments used for ox-acetylene welding are as follows: Ox-acetylene Equipment. Figure 1. 2 Ox acetylene welding set up (Rejoinder 2006). Cylinders Acetylene and oxygen gas is stored in compressed gas cylinders. These gas cylinders differ widely in capacity, design and color code.
However, in most of the countries, the standard size of these cylinders is 6 to 7 mm and is painted black for oxygen and which is saturated with a chemical solvent acetone. Acetone has the ability to absorb a large volume of acetylene and release it as the pressure falls. If large quantities of acetylene gas are being consumed, it is much cheaper to generate the gas at the place of use with. The help of acetylene gas generators. Acetylene gas is generated by carbide-to-water method. Oxygen gas cylinders are usually equipped with about 40 liters of oxygen at a pressure of about 154 KGB/CM at 21 co.
To provide against dangerously excessive pressure, such as could occur if the cylinders were exposed to fire, every valve has a safety device to release the oxygen before there is any danger of rupturing the cylinders. Fragile discs and fusible plugs are usually provided in the cylinders valves in case it is subjected to danger. (Rejoinder Sings 2006) Gas Pressure Regulators Gas pressure regulators are employed for regulating the supply of acetylene and oxygen gas from cylinders. A pressure regulator is connected between the cylinder and hose leading to welding torch.
The cylinder and hose connections have left- ended threads on the acetylene regulator while these are right handed on the oxygen regulator. A pressure regulator is fitted with two pressure gauges, one for indication of the gas pressure in the cylinder and the other for indication of the reduced pressure at which the gas is going out. (Rejoinder Sings 2006) Welding Torch Figure 1. 3 shows the construction of the welding torch. It is a tool for mixing oxygen and acetylene in correct proportion and burning the mixture at the end of a tip.
Gas flow to the torch is controlled with the help of two needle valves in the handle of the torch. There are two basic types of gas welding torches: (1) Positive pressure (also known as medium or equal pressure), and (2) Low pressure or injector type The positive pressure type welding torch is the more common of the two types of ox- acetylene torches (Rejoinder Sings 2006) Balance type The balance type of blowpipe is used with high-pressure cylinder gases. It has four main parts. The body serves as a handle, and is fitted with the hand valve and the mixing chamber.
There are two hand valves, the oxygen valve and the acetylene valve which open and control the flow of the gases from the hose into the mixing chamber and out of the nozzle. The valves are also used to close the flow of the gases from the hoses. The mixing chamber is where the oxygen and the acetylene are combined in the required proportions before they come out of the nozzle. The mixing chamber can be removed from the body. (Sockeye & Megaphone 1996). Injector type The injector type has the mixing chamber in the nozzle tube.
The oxygen, passing through the injector, draws the acetylene into the mixing chamber and mixes with it. The blowpipe can operate with very low acetylene pressure such as that obtained from the low-pressure acetylene generator. It provides a constant acetylene pressure, whatever the size of nozzle being used or thickness of the metal plate being welded . Let can draw, almost completely, the last bit of gas from the cylinder. Torch Tips It is the portion of the welding apparatus through which the gases pass Just prior to their ignition and burning.
A great variety of interchangeable welding tips differing in size, shape and construction are available commercially. The tip sizes are identified by the diameter of the opening. The diameter of the tip opening used for welding depends upon the type of metal to be welded. Gloves These are required to protect the hands from any injury due to the heat of welding recess Goggles These are fitted with colored lenses and are used to protect the eyes from harmful heat and ultraviolet and infrared rays. Nozzle The nozzle (tip) is where the flame is lit.
Nozzles are of various sizes, according to the diameter of the orifice (hole) at the end: the bigger the orifice, the greater the gas pressure, the higher the gas discharge rate and the grater the intensity of the flame (heat). The nozzles are made of copper, because it is a good conductor of heat. There are two types of nozzle: detachable and integral (in which the nozzle and the nozzle tube are in one piece). There are two inlets. The oxygen inlet has a right-handed thread, which takes the oxygen hose connecting nut.
The acetylene inlet has a left- handed thread, which takes the acetylene hose connecting nut. Figure 1. 4 is an example of a welding nozzle. (Sockeye & Megaphone 1996). Welding nozzle cleaners Welding nozzle cleaners are small round files used for cleaning the orifice of welding nozzle as shown in figurer . 5. They are always supplied in a set of different sizes. When the orifice becomes blocked by soot or particles or molten metal (globules), you need to pass the appropriate size of cleaner into the orifice and then out again as any times as necessary until it is cleaned. (Sockeye & Megaphone 1996) Fig. . 5 Nozzle cleaner (Rejoinder Sings 2006) Welding hoses Welding hoses is of two types as shown in figure 1. 6. The acetylene hose carries the acetylene gas from the regulator to the blowpipe. It is color-coded red, to show that it is carrying a combustible gas. It has left-handed thread connecting fittings at each to fix the regulator and the blowpipe. The oxygen hose carries the oxygen gas from the regulator to the blowpipe. It is color-coded either black or green to indicate non- combustible gas. It is fixed to the regulator and the blowpipe with right-handed hared connecting fittings.
In general, hoses are made from synthetic or natural rubber, which is oil-resistant; it is strengthened by adding a nylon fabric or line braid. Hoses are light, flexible, durable and unaffected by sunlight or the weather. Single or as twin hose. Twin hose has the oxygen and acetylene hoses Joined as one component. (Sockeye & Megaphone 1996) Fig. 1. 6 Ox-acetylene Welding Hoses Hydraulic back pressure valve Acetylene generators or supply must be attached with a hydraulic back pressure valve to prevent the back feeding of gas mixture into supply system to cause explosion.
Blowpipe lighter The safe way to light a blowpipe is by using a spark lighter shown in figure 1. 7. This can conveniently be operated by one hand while the other hand holds the blowpipe. It is quick and fast to use. The spark is caused by a flint fitted to the moving part of the lighter, which runs across a rough striker. Sockeye & Megaphone (1996) Fig. L . 7 Spark Lighters Manifold systems In large workshops, when a single cylinder is not sufficient, several cylinders can be coupled together in a manifold system. The cylinders in a manifold system are connected to the main pipe.
The main pipe then takes the gas to the line regulators ND welding stations. Fig. L . 8 Manifold System (Gibson 1994) LIGHTING THE BLOWPIPE To light the blowpipe, set the regulators to the recommended pressures. When the regulators are set correctly, turn on the acetylene valve on the blowpipe, and light the acetylene at the nozzle with a spark lighter. At first the flame will be yellow and smoky. The soot and smoke being given off are carbon, as combustion is incomplete because of the shortage of oxygen in the flame.
Use the acetylene valve on the blowpipe to increase or decrease the yellow flame until the black smoke reduces and then finally disappears. The acetylene supply is now right for the particular size of nozzle in use. Now turn on the oxygen at the blowpipe valve. As the pressure of the oxygen is increased, the flames gradually take on the appearance of the neutral white plume around the inner cone, this is the carousing flame, which indicates that a little more oxygen is required to obtain the neutral flame. If the inner cone is small and tapered, this indicates that there is too much oxygen.
This flame is called the oxidations flame; the oxygen needs to be reduced slightly at the blowpipe valve in order to obtain the neutral flame. Gibson 1994) Procedure for closing down Turn off the acetylene blowpipe valve first, and then turn off the blowpipe of oxygen valve. There will be a loud popping sound, but this procedure prevents the acetylene from burning back into the nozzle to clog it. Turn off the acetylene and oxygen cylinder valves respectively. Open the acetylene blowpipe vale one-quarter turn to release the trapped pressure in the hose. Close the blowpipe valve again. Loosen the regulator adjusting screw.
Open the oxygen blowpipe valve one-quarter turn to release the trapped pressure in the hose. Fold the hose loosely and secure them safely. Gibson 1994) Gas economizer The gas economizer is a very useful item of equipment designed for use with pipeline distribution of oxygen and acetylene to welding points. It contains two valves which are normally held open by a spring but will be closed by the weight of a blowpipe hung on the control lever arm. Oxygen and acetylene supplies are coupled to the inlet side of the fitting, and the gas hoses to the blowpipe are coupled to the outlet side.
When the blowpipe is lifted off the arm, the gases can be ignited at the pilot light, and the flame can be adjusted to the correct setting in the normal way. Between welding operations, the blowpipe is hung on the arm, shutting off the gases. When the blowpipe is needed again, the flame can be lit from the pilot and welding can be started without loss of time or gas needed to adjust the flame. Without the use of economizers, a lot of gas can be wasted while the flame is being adjusted, and while the blowpipe is laid aside with the flame burning when a Job is being changed or adjusted.
Installing gas economizers can reduce gas consumption by as much as 20% . If a blowpipe is not going to be used for some time, then it should be shut down completely. The economizer is designed for use when there are only short intervals of time between welding operations. (Gibson 1994) Backfire When the flame snaps back inside the blowpipe or nozzle, usually the flame quickly extinguishes itself. This condition is generally caused by insufficient pressure, dirt in the nozzle or some other blockage. (Gibson 1994) Flashback A flashback occurs when the flame moves from the blow torch into the supply system against the flow of the gases.
It is potentially dangerous and its effects can range from soot deposits in the blowpipe and hoses to a fire in the gauge or cylinder often accompanied by squealing or popping noises. It is generally due to incorrect operating practice such as overheating blowpipe and wrong pressures. (Davies 1994). Flashback arrestor The automatic flashback arrestor is made for acetylene, propane, hydrogen and oxygen and is generally connected to the regulator outlet and prevents flame movement into gauges and cylinder, causing regulator damage and even cylinder fires.
Davies (1994) GAS WELDING PROCESSES Gas welding also called an ox-fuel gas welding, derives the heat from the combustion of a fuel gas such as acetylene in combination with oxygen. The process is a fusion welding process wherein Joint is completely melted to obtain the fusion. Operation of gas welding is shown in Figure 1. 1 . The fuel gas generally used is acetylene because of the high temperature generated in the flame. This process is called ox-acetylene welding. The useful fuel gases used for gas welding are; acetylene (C?2 ) H?2 propylene ) H?6 propane(C?3 ) H_8), Hydrogen(H?2 and Natural Gas (C _ Fig. . 10 Gas welding (Gary 2006). Types of welding flames In ox-acetylene welding, flame is the most important means to control; the welding joint and the welding process. The correct type of flame is essential for the production of satisfactory welds. The flame must be of the proper size, shape and condition in order to operate with maximum efficiency. There are three basic types of ox-acetylene flames; Neutral flame A neutral flame is produced by equal amount of acetylene and oxygen. It produces a shiny molten weld pool, with little or no spark.
The characteristics of a neutral flame are a bluish centre envelope (flame), and a white short inner cone the neutral flame is mostly used for general welding and cutting. It is good for welding carbon steels, stainless steel and cast iron. Fig. . 11 Neutral flame (Sockeye & Megaphone 1996) Oxidations flame An excess of oxygen over acetylene produces an oxidize flame. This burns with a hissing sound, producing foam around the molten weld and a large quantity of sparks . Let consists of two flames, an outer envelope (flame), and a short, pointed inner weakens the weld.
Its main uses are for welding brass and bronze and for brazing galvanism iron Fig. 1. 12 Oxidations flame (Sockeye and Megaphone 1996) Carousing flame When the amount of acetylene exceeds the amount of oxygen, a carousing flame is produced. It is sometimes referred to as a reducing flame. It produces a crusty (rough) molten weld pool. The carousing flame is identified by three distinct flames (cone): the outer flame or envelope, the blue-white feather flame and the white inner cone. Because of the excess of acetylene, which contains carbon, the carousing flame tends to add more carbon to the weld, making it brittle . T is therefore rarely used for welding low-carbon steel, but is suitable for welding high carbon steel alloys and for hard facing and aluminum brazing. (Hard facing is the application of a suitable layer of hard material to provide wear resistance) Fig. 1. 3 Carousing flame OX – ACETYLENE WELDING TECHNIQUES Rightward technique (backhand technique) For a right-handed person, welding commences from the left-hand side of the Joint, traveling towards the right-hand side. The flame therefore points towards the weld that has been formed.
The filler rod is positioned between the weld and the flame (blowpipe). This technique is suitable for welding metal with thickness above mm. The advantages of this technique, compared with the leftward technique are: It uses less filler rod. Welding speed is faster, as the filler rod does not obstruct the view of travel. Gas consumption is less as the welding rate is faster. The mechanical properties of the weld are better as the reheating of the weld by the flame has an annealing effect on the weld. There is less risk of oxidations the weld, as the filler rod is kept in the flame most of the time.
Fig. 1. 14 Rightward welding technique To employ this technique, start building the weld from the right-hand side of the joint and move towards the left-hand side. In this situation, the filler rod is in front of the flame’ the flame is pointing in the direction of travel. The position of the blowpipe is between the weld and the filler rod. This technique is used for welding thinner metals, up to mm. The disadvantages of this technique are: The filler rod blocks the view of the Joint edges and has to be withdrawn from time to time.
This in turn slows down the welding rate The end of the filler rod is liable to be oxidized when withdrawn and exposed to the atmosphere. The weld is likely to be oxidized when the oxidized end of the filler rod is fed again into the weld pool. (Sockeye & Megaphone 1996) Fig. 1. 15 Leftward welding technique Filler Rods Gas welding can be done with or without using filler rod. When welding with the filler odd, it should be held at approximately 900 to the welding tip. Filler rods have the same or nearly the same chemical composition as the base metal.
Metallurgical properties of the weld deposit can be controlled by the optimum choice of filler rod. Most of the filler rods for gas welding also contain dioxides to control the oxygen content of weld. Fig. 1. 16 Ox acetylene welding rods Fluxes Fluxes are used in gas welding to remove the oxide film and to maintain a clean surface. These are usually employed for gas welding of aluminum, stainless steel, cast iron, brass and silicon bronze. They are available in the market in the form of dry odder, paste, or thick solutions. Rejoinder Sings 2006)There are four types of welding positions, which are given as: Flat or down hand position Horizontal position Vertical position Overhead position 1 . Flat or Down-hand Welding Position The flat position or down hand position is one in which the welding is performed from the upper side of the Joint and the face of the weld is approximately horizontal. This is the simplest and the most convenient position for welding. Using this technique, excellent welded Joints at a fast speed with minimum risk of fatigue to the welders can be obtained.