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Term Paper on Electric Welding
Term Paper # 1. Introduction to Electric Welding:
Electric welding is a materials joining process that produces coalescence of materials by heating them to the welding temperature with or without the application of pressure or by the application of pressure alone, and with or without the use of filler metal. It is used to make welds. A weld is “a localized coalescence of metals or non-metals produced either by heating materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone and with or without the use of filler metal.”
All metals can be joined by one welding process or another. There is a saying, “If it’s metal, weld it,” and it is certainly true. This should be qualified by stating that all metals commercially employed for structural or strength parts are weldable. Some metals are easy to weld, and others are difficult to weld. The metals that are easily weldable can be welded in thickness from the very thinnest, about the thickness of this paper page, to the thickest or heaviest produced.
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The difficult-to-weld metals require special procedures and techniques that must be developed for specific applications. Some metals may never be welded or joined. For example, mercury is a liquid at room temperature and cannot be welded, while sodium and potassium melt just below the temperature of boiling water and are of no use as a strength member and cannot be welded.
In general, metals that have a low melting point or low strength would not be welded. Some metals are so scarce or expensive that they would not be used where welding would be required. The physical and mechanical properties (such as melting temperature, density, thermal conductivity, tensile strength, ductility etc.), availability, and price all help in determining if a metal will be used in applications where welding is required.
Term Paper # 2. Classification of Electric Welding Processes:
There are many ways to make a weld and there are many different kinds of welds. The welder behind the hood making sparks is using one of the more popular welding processes, known as arc welding. Some welding processes do not cause sparks; in some cases electricity is not used, and in some cases there is not even extra heat. Welding has become complex and technical. It needs considerable knowledge to select the proper welding process for critical work.
Modern methods of welding may be classified under two broad headings—plastic welding and fusion welding. They are also called pressure welding, and non-pressure welding respectively. In the plastic welding or pressure welding, the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure.
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This includes forge welding, resistance welding, thermite welding and gas welding, in which pressure is required. In the fusion welding or non- pressure welding, the material at the joint is heated to a molten state and allowed to solidify. This includes gas welding, arc welding and thermit welding without pressure.
The various electric welding processes are given below:
1. Gas Welding — (i) Oxyacetylene (ii) Air-acetylene and (iii) Oxy-hydrogen.
2. Resistance Welding — (i) Butt (ii) Spot (iii) Projection (iv) Seam and (v) Percussion.
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Some other modern welding techniques used in general engineering are:
1. Arc Welding — (i) Carbon arc (ii) Metal arc (iii) Gas metal arc (iv) Gas tungsten arc (v) Atomic hydrogen arc (vi) Plasma arc (vii) Submerged arc (viii) Flux-cored arc and (ix) Electro-slag.
2. Thermit Welding.
3. Solid State Welding — (i) Friction (ii) Ultrasonic (iii) Diffusion and (iv) Explosive.
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4. Newer Welding — (i) Electron beam and (ii) Laser.
Term Paper # 3. Requirements of Good Weld:
The weld is said to be good weld if it is free from external defects such as irregular width and heights of beads, deviation of the weld from the prescribed dimensions, unfilled craters on the surface of welds, undercuts, slag on the surface of welds, porosity in the outer layer of the welds, visible cracks in and around the weld and internal defects such as poor fusion, hidden porosity in the deposited metal, hidden cracks in the weld and parent metal, poor fusion between layers in multiple layer welding, and entrapment of slag and oxides.
Poor fusion, which is among the most dangerous defects, is lack of through and complete union between the deposited and parent metal, or between individual layers in multiple layer welding. Lack of fusion impairs the strength of welds and makes welded structures unreliable. In order to get sound weld it is essential that absolute fusion of parent metal and electrode metal is obtained.
Porosity is entirely ruled out in weldments where tight seams are critical (pressure vessels, storage tanks, pipelines etc.). Porosity may be brought about by insufficient puddling, as it does not give time for the trapped gas to escape from the molten metal before it solidifies. The gas remaining in the molten metal forms gas pockets. They may be scattered about or link up into strings of pores, giving the welded metal a spongy appearance. Porosity makes welds leaky, pervious to liquids and gases and weak.
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Cracks in welds may arise from locked-up stresses up by non-uniform heating and cooling, excess sulphur or phosphorus in the weld metal, and some other causes. Cracks most often occur in butt welds and may be either longitudinal or transverse.
Apart from visible cracks, there may be hair cracks, usually appearing at the line of fusion between the parent and deposited metal. They may be brought about by lack of fusion, large gas pockets, or slag inclusions where thermal stresses concentrate, giving rise to hair cracks which may develop into large cracks. That is why hair cracks are intolerable.
Term Paper # 4. Preparation for Welding:
There are several factors which must be considered while preparing work for electric welding in order to obtain good results.
For obtaining good results it is desirable that provision may be made for expansion and contraction wherever possible. The strength of the weld will depend on the correct beveling and spacing of parts to be welded. Uniform fusion is directly dependent on the proper beveling and spacing.
The cleaning of the surfaces is another important factor. The cleaning can be done by wire brushing, machining or sand blasting. Impurities if present tend to make the joint weaker as the welded portion is filled with gas and slag inclusions and metal becomes brittle and the cohesion between the metals is, therefore, poor.
Another fundamental of good preparation or joint design is to keep the cross section of the added metal as small as practicable, so as to localise the effective heat in adjacent sections.
Another important point in welding is to overcome the oxidation at high temperatures. The oxides of metals formed at high temperatures encountered in welding have serious weakening effects. This influence is counteracted by using a flux which removes the oxides and permits perfect cohesion of the metals. In arc welding the flux is coated on the electrodes. The flux forms a protective coating of slag over the weld metal and creates a non-oxidising atmosphere.
Term Paper # 5. Requirement of Electrodes for Welding:
An electrode is a piece of wire or rod (of a metal or alloy), with or without flux covering, which carries current for welding. At one end it is gripped in a holder and an arc is set up at the other end.
Electrodes are made of materials having high electrical conductivity as well as thermal conductivity so that heat developed at junction faces between electrodes and work is minimum in the first instance and secondly, whatever heat is produced is conducted away efficiently. In addition to high electrical and thermal conductivity, electrode material should be of very high mechanical strength so that the wear and tear at the electrode tip is minimum.
The materials used for electrodes are- Hard drawn copper (softening temperature 150°C and for welding of aluminium alloys); Cadmium copper (softening temperature 250°C and for joining MS sheets); Chromium copper (softening temperature 500°C and for joining of steel) and Tungsten copper (softening temperature 1,000°C and for welding of steel) and copper alloys.
The diameter of the electrode is approximately given by the expression d = 5√t
Where d is the diameter of tip in mm and t the thickness of sheet in mm.
Either non-consumable or consumable electrodes may be employed in arc welding.
Non-consumable electrodes may be of carbon, graphite or tungsten which do not consume during the welding operation. Consumable electrodes may be made of various metals depending upon their purpose and the chemical composition of the metals to be welded.
In general, the electrodes can be divided into three categories depending upon outer aspects as well as their technological properties.
Bare electrodes are most commonly used in automatic and semi-automatic welding. In using the plain or bare electrodes, as the globules of the metal pass from the electrode to the work, they (globules) are exposed to the oxygen and nitrogen in the surrounding air. This causes the formation of some non-metallic constituents which are trapped in the rapidly solidifying weld metal and thereby reduces the strength and ductility of the metal.
Lightly coated (or washed) electrodes have a coating layer several tenths of a millimeter thick. This coating usually consists of lime mixed with soluble glass which serves as a binder. The primary purpose of the light coating is to increase the arc stability, so they are also called the stabilising coatings. No attempt is made to prevent oxidation and no slag is formed on the weld, nor are the mechanical properties of the weld metal improved. For this reason lightly coated electrodes may only be used for welding non-essential jobs.
Heavy coated electrodes, sometimes referred to as shielded arc electrodes, are used to obtain a weld metal of high quality, comparable with, and even superior to, the parent metal in terms of mechanical properties. Heavy coatings employed are composed of ionising (chalk), deoxidising (aluminium, ferro- manganese etc.), gas generating (starch), slag forming (kaolin), alloying and binding materials.
These electrodes have the following distinct advantages:
(i) The physical and metallurgical properties of the weld can be influenced by adding alloying components to the covering.
(ii) Basic salts of silicon, magnesium and calcium in the covering form slag which floats on the surface of metal and prevents rapid cooling of the weld. Thus weld does not become brittle.
(iii) The weld metal is protected from oxidising action of atmospheric oxygen and nitrifying action of nitrogen of air due to gases formed from the covering material.
(iv) In case of ac supply arc cools at zero current and there is a tendency of deionizing the arc path. Covering gases keep the arc space ionized.
(v) During welding the covering extends beyond the core wire. This directs the arc and concentrates the arc stream, reduces thermal losses and causes increase in temperature of electrode tip.
There may also be powder-cored electrodes which have a good portion of the source of metal for depositing in the joint located as powdered iron mixed in with the flux coating. This makes the electrodes much larger on the outside for the same diameter core wire as standard flux coated electrodes. These electrodes are well suited to down-hand welding and are said to deposit more metal in a given period time than standard flux covered electrodes.
Both bare and coated electrodes, for manual arc welding, are made in the shape of rods up to 12 mm in diameter and 450 mm long. Semi-automatic and automatic welding use electrode wire in coils.
Term Paper # 6. Electric Welding Equipment:
The electric welding sets may be either dc or ac type:
1. DC Welding Sets:
Such sets are of two types namely:
(i) Generator type and
(ii) Rectifier type.
Generator type welding set consists of a differential compound wound dc generator, giving drooping volt-ampere characteristic, driven by any type of prime mover (a squirrel cage induction motor or a petrol or diesel engine). In a differential compound wound dc generator the terminal voltage falls automatically with the increase in load current. The control may be obtained by tapping the series field or by providing a suitable shunt across the series field winding. The open-circuit voltage is adjusted from shunt field.
If supply from existing dc distribution system is to be used for welding then ballast (resistance) is put in series with the equipment and control is obtained by varying this external resistance. This method is also suitable when a number of operators are working on the same supply system. In such cases each operator is provided with separate ballast. The special field of this method is where the service of each arc welding circuit is infrequently in use (say 25% of the time or less). In such cases the loss in the series resistor is less than the no-load loss of a generator type unit.
Another type of dc welding set is a dry type rectifier used in conjunction with a multi-phase, high leakage reactance transformer. Many of these rectifier type welders use selenium rectifiers which are forced air cooled. Rectifier type welders are said to combine some of the desirable arcing characteristics of dc welding, such as easy arc starting, with those of welding transformers, such as reduced no-load losses. DC voltage is controlled by regulating the transformer output in this case.
2. AC Welding Sets:
Single phase or 3-phase step-down transformers which provide low voltage (80-100 voltage on open-circuit) power for welding with some means of output control. One or two taps are provided on the primary side to take care of the voltage variations. The secondary winding of an air-cooled set is generally made of bare wound-on-edge copper strip. The welding transformers may be air or oil-cooled type. The air-cooled sets are lighter in weight and permit the use of insulating materials of the highest thermal classification.
They are also cheaper, less hazardous and easier to maintain. On the other hand, the oil-cooled sets are more compact. Being heavy, they are more suitable for stationary applications in workshops. Synthetic liquid filled sets may be employed in hazardous locations. Some machines are provided with an arc booster that provides a momentary surge of current to give an arc a good start when it is struck.
In transformer type welding machines the current control is achieved by using:
(a) Magnetic shunt or
(b) A choke coil or reactor placed in series with primary or secondary winding or
(c) Tap changing switch in the primary winding.
In the magnetic shunt type, an adjustable gate of iron laminations is placed between the primary and the secondary windings. Part of the flux is diverted through this gate without linking with the secondary winding, thus providing a simple and reliable means of stepless voltage control.
The choke coil type regulator is a variable reactance with a movable iron plunger. The reactor is such that it operates well below the saturation point. Though it improves the arc stability but reduces the power factor.
The tap-changer type regulator is prone to give trouble on account of arcing on the contacts and the moving finger.
The use of series resistance can also be made for current control but efficiency is reduced.
Term Paper # 7. Advantages and Disadvantages of Electric Welding:
Some of the advantages of electric welding are given below:
1. Electric welding is the lowest-cost joining method.
2. It affords lighter weight through better utilization of materials.
3. It joins all commercial metals.
4. It can be used anywhere.
5. It provides design flexibility.
The limitations of electric welding are:
1. Some welding depends on the human factor.
2. It often needs internal inspection.
Most of these limitations can be overcome by means of good controls and supervision.
Welding is also an economical manufacturing method wherever there is a need to join parts permanently. In the high- volume production industries it is common to see welding operations intermixed with bending, machining, forming, and assembly and so on. Welding is an important manufacturing process taking its place with other metal working operations to help bring us good-quality products at economical prices.
Term Paper # 8. Importance of Welding:
Welding is the most efficient way to join metals. It is the only way to join two or more pieces of metal to make them act as one piece. Welding is widely employed to manufacture or repair all products made of metal. Look around, almost everything made of metal is welded; the world’s tallest building, moon rocket engines, nuclear reactors, home appliances, and automobiles barely start the list.
The use of welding is still increasing. If a joint is welded, it is permanent joint. Obviously, if the joint must be disassembled occasionally, it should not be welded. Thus we should change our statement to “welding is the most economical method to permanently join metal parts.” To join two members by bolting or riveting requires holes in the parts to accommodate the bolts or rivets.
These holes reduce the x-sectional area of the members to be joined by up to 10 per cent. The joint may also require the use of one or two gusset plates, thus increasing the weight of material required and the cost. This expense can be eliminated by the use of a weld. The greatest economy of a welded design will be obtained if the x-sectional area of the entire structural member is reduced by the amount of the bolt holes.
This can be done since the entire cross section of a member of a welded design is utilized to carry the load. The amount of material required is reduced as well as its cost. This same design concept applies to joining plates used to build a ship or a container. In view of this material savings, ships and storage tanks are no longer riveted.
Pipes joined by welding offer similar economies. The wall thickness of a pipe should be heavy enough to carry the required load. However, if the pipe is joined by screw threads a heavier wall thickness is used to allow for cutting away a portion of the thickness for the threads. A thinner pipe wall thickness is used for the entire welded pipe system. This reduces the amount of metal required and the cost. The inside surface of the welded joint is smoother. Larger diameter pipes are no longer connected together with screw threads and pipe fittings.
Converting castings to weldments allows the designer to reduce weight by reducing metal thickness. Welding is a design concept which allows freedom and flexibility not possible with cast construction. Heavy plates can be used where strength is required and thin ones can be used where possible. The uniform thickness rule and minimum thickness required for foundry practice are not necessary for weldments. Additionally, high- strength materials can be employed in specific areas, while normal-strength materials are used where required.
Welding is the best way to protect and conserve materials by protecting their surface with special metal overlays. Corrosion and wear of metals account for losses running into billions of dollars annually. Together they are responsible for an untold loss of lives. Waste from both of these destructive forces can be largely reduced by welding.
Special alloys are weld deposited on base metals to provide corrosion resistance surfaces. Hard surfacing overlays can be made by welding to provide special alloys with wear resistant surfaces. A typical application is the resurfacing of a cinder crusher roll with hard weld metal. Weld surfacing is used to reduce the costly abrasive and corrosive wear of machinery.
Thus in short we can say, “Today, the science and art of welding encompasses a wide range of processes and procedures applicable to materials of any thickness and shape—from tiny electronic components to mammoth machines and structures. Welding has opened up prospects for radical improvements in the manufacture of a large variety of machines, devices and structures. It has promoted mechanization and automation in their manufacture. The use of automatic, semi-automatic and mechanized welding equipment has relieved a large number of workers for employment elsewhere in the national economy. Welding is nowadays extensively used in automobile industry, aircraft machine frames, structural work, tanks, machine repair work, shipbuilding, pipeline fabrication in thermal power plants and refineries, fabrication of metal structures.”
Term Paper # 9. Personnel Protection and Safety Rules for Electric Welding:
There are a number of safety and health problems associated with electric welding. When correct precautionary measures are followed, electric welding is a safe occupation. The hazards that are more or less peculiar to welding are electrical shock, arc radiation, air contamination, fire and explosion, compressed gases, welding cleaning and other hazards related to specific processes or occupations.
Combustible materials must not be allowed to collect in or near the welding workplace. Good housekeeping practices should always be employed in the welding shop. Adequate safety devices should be provided, such as fire extinguishers, life- saving and support equipment, first-aid kits, and so on, plus the training of personnel to utilize this equipment properly. Only approved equipment be used, and it must be properly installed and maintained in good working conditions.