Manufacturing Technology PDF

306 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 5. Hose and hose fittings : l Hoses are the rubber and fabric pipes used to connect gas cylinder to blow pipe and are painted black or green for oxygen and red or maroon fo...

306 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 5. Hose and hose fittings : l Hoses are the rubber and fabric pipes used to connect gas cylinder to blow pipe and are painted black or green for oxygen and red or maroon for acetylene. It should be strong, durable, non-porous and light. l Special fittings are used for connecting hoses to equipment. 6. Safety devices : l Goggles fitted with coloured glasses should be used to protect the eyes from harmful heat ultraviolet rays. l Gloves made of leather, canvas and asbestos should be worn to protect hands from any injury. Gloves should be light so that the manipulation of the torch may be done easily. Other requirements include spark-lighter, apron, trolley, wire brush, spindle key, spanner set, filler rods and fluxes and welding tips. Welding rods (Filler materials) for gas welding : The welding wire or rod used as filler material in gas welding should have a chemical compo- t sition similar to that of the base metal. The welding rod diameter, d = + 1 mm (app.), where t is the 2 thickness of the base metal, mm. l Gas welding ‘‘fluxes’’ (composing of borates or boric acid, soda ash and small amount of other compounds e.g., sodium chloride, ammonium sulphate and iron oxide) must melt at a lower temperature than the metals being welded so that surface oxides will be dissolved before the metal melts. 7.7. ELECTRIC ARC WELDING 7.7.1. Introduction Arc welding is the system in which the metal is melted by the heat of an electric arc. It can be done with the following methods : (i) Metallic arc welding. (ii) Carbon arc welding. (iii) Atomic hydrogen welding. (iv) Shielded arc welding. 7.7.2. Advantages and Limitations Following are the advantages and limitations of electric arc welding : Advantages : 1. Portable and relatively inexpensive equipment. 2. Very versatile process. Limitations : 1. Large heat affected zone. 2. Weld quality depends upon operator’s skill in normal operations. 3. Not suitable for thin sections. 7.7.3. Metallic Arc Welding Refer to Fig. 7.14. In metallic arc welding an arc is established between work and the filler metal electrode. The intense heat of the arc forms a molten pool in the metal being welded, and at the same time melts the tip of the electrode. As the arc is maintained, molten filler metal from the electrode tip is transferred across the arc, where it fuses with the molten base metal. Arc may be formed with direct or alternating current. Petrol or diesel driven generators are widely used for welding in open, where a normal electricity supply may not be available. D.C. may also be obtained from electricity mains through the instrumentality of a transformer and rectifier. A simple transformer WELDING AND ALLIED PROCESSES 307 Flux coated electrode Electrode holder Deposited Flame weld metal Lead clamped to the work Crater Leads of generator or transformer Fig. 7.14. Metallic arc welding. is, however widely employed for A.C. arc welding. The transformer sets are cheaper and simple having no maintenance cost as there are no moving parts. l With Arc system, the covered or coated electrodes are used, whereas with D.C. system for cast iron and non-ferrous metals, bare electrodes can be used. l In order to strike the arc an open circuit voltage of between 60 to 70 volts is required. For maintaining the short arc 17 to 25 volts are necessary ; the current required for welding, however, varies from 10 amp. to 500 amp. depending upon the class of work to be welded. l The great disadvantage entailed by D.C. welding is the presence of arc blow (distortion of arc stream from the intended path owing to magnetic forces of a non-uniform magnetic field). With A.C. arc blow is considerably reduced and use of higher currents and large electrodes may be restored to enhance the rate of weld production. Applications : l The field of application of metallic arc welding includes mainly low carbon steel and the high-alloy austenitic stainless steel. l Other steels like low and medium-alloy steels can however be welded by this system but many precautions need be taken to produce ductile joints. 7.7.4. Carbon Arc Welding Refer to Fig. 7.15. Here the work is connected to Carbon negative and the carbon rod or electrode connected to the Filler electrode positive of the electric circuit. Arc is formed in the gap, rod Arc flame filling metal is supplied by fusing a rod or wire into the Pool of arc by allowing the current to jump over it and it produces molten metal a porous and brittle weld because of inclusion of carbon particles in the molten metal. It is therefore used for filling blow holes in the castings which are not subjected to any Fig. 7.15. Carbon arc welding. of the stresses. l The voltage required for striking an arc with carbon electrodes is about 30 volts (A.C.) and 40 volts (D.C.). l A disadvantage of carbon arc welding is that approximately twice the current is required to raise the work to welding temperature as compared with a metal electrode, while a carbon electrode can only be used economically on D.C. supply. 308 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 7.7.5. Atomic Hydrogen Welding Refer to Fig. 7.16. In this system heat is obtained Hydrogen gas from an alternating current arc drawn between two Tungsten tungsten electrodes in an atmosphere of hydrogen. As the electrodes hydrogen gas passes through the arc, the hydrogen molecules are broken up into atoms and they recombine Filler Arc Welding on contact with the cooler base metal generating intense rod flame heat sufficient to melt the surfaces to be welded, together with the filler rod, if used. The envelop of hydrogen gas also shields the molten metal from oxygen and nitrogen and thus prevents weld metal from deterioration. Fig. 7.16. Atomic hydrogen welding. l The welds obtained are homogeneous and smooth in appearance because the hydrogen keeps the molten pool. Advantages : 1. No flux or separate shielding gas is used ; hydrogen itself acts as a shielding gas and avoids weld metal oxidation. 2. Due to high concentration of heat, welding can be carried out at fast rates (specially when filler metal is not needed) and with less distortion of the workpiece. 3. Welding of thin materials is also possible which otherwise may not be successfully carried out by metallic arc welding. 4. The job does not form a part of the electrical circuit. The arc remains between two tung- sten electrodes and can be moved to other places easily without getting extinguished. Limitations : 1. For certain applications, the process becomes uneconomical because of higher operating cost as compared to that of other welding processes. 2. The process cannot be used for depositing large quantities of metals. 3. Welding speed is less as compared to that of metallic arc or MIG welding. Applications : l Atomic hydrogen welding being expensive is used mainly for high grade work on stainless steel and most non-ferrous metals. 7.7.6. Shielded Arc Welding Flux coating In this system molten weld metal is Slag Electrode protected from the action of atmosphere by an coating envelope of chemically reducing or inert gas. Weld Gaseous shield metal As molten steel has an affinity for oxygen Arc stream and nitrogen, it will, if exposed to the Base atmosphere, enter into combination with these metal gases forming oxides and nitrides. Due to this Pool of injurious chemical combination metal becomes molten metal weak, brittle and corrosion resistant. Thus, Fig. 7.17. Shielded arc welding. several methods of shielding have been developed. The simplest (Fig. 7.17) is the use of a flux coating on the electrode which in addition to producing a slag which floats on the top of the molten metal and protects it from atmosphere, has organic constituents which turn away and produce an envelope of inert gas around the arc and the weld. l Welds made with a completely shielded arc are more superior to those deposited by an ordinary arc. WELDING AND ALLIED PROCESSES 309 7.7.7. Arc Blow l Arc blow is the phenomenon of wandering of arc and it occurs in D.C. welding. l When a current flows in any conductor, a magnetic field is formed around the conductor at right angles to the current. Since in the case of D.C. arc welding, there is current through the electrode, workpiece and ground clamp, magnetic field exists around each of these components. The arc thus lacks control as though it were being blown to and by the influence of these complex magnetic fields. This is more common in welding with very high or very low currents, and especially in welding in corners or other confined spaces. Usually arc blow results from the interaction of magnetic fields of the electrode workpiece with that of the arc. The movement of arc blow causes atmospheric gases to be pulled into the arc, resulting in porosity or other defects. The severity of arc blow problem can be reduced by taking the following corrective measures : 1. Change to A.C. welding, if possible (since due to change in the polarity, the effect of magnetic field is nullified). 2. Reduce the current used so that the strength of magnetic field is reduced. 3. Use a short arc length so that filler metal would not be deflected but carried easily to the arc crater. 4. Place more than one ground lead from the base metal (preferably on each from the ends of the base metal plate). 5. The ground cable may be wrapped around the workpieces such that the current flowing in it sets up a magnetic field in a direction which will counteract the arc blow. 7.7.8. Comparison between A.C. and D.C. Arc Welding The Comparison between A.C. and D.C. arc Welding is given below : S. No. Aspects A.C. Welding D.C. Welding 1. Power consumption Low High 2. Arc stability Arc unstable Arc stable 3. Cost Less More 4. Weight Light Heavy 5. Efficiency High Low 6. Operation Noiseless Noisy 7. Suitability Non-ferrous metals cannot Suitable for both ferrous and be joined non-ferrous metals 8. Electrode used Only coated Bare electrodes are also used 9. Welding of thin sections Not preferred Preferred 10. Miscellaneous Work can act as cathode Electrode is always negative while electrode acts as anode and the work is positive. and vice versa. Specifications of A.C. Transformer/D.C. generator : A.C. transformer : Step down, oil cooled = 3 phase, 50 Hz ; Current range = 50 to 400 A ; Open circuit voltage = 50 to 90 V ; Energy consumption = 4 kWh per kg of metal deposit ; Power factor = 0.4 ; Efficiency = 85%. D.C. generator : Motor generator—3 phase, 50 Hz ; Current range = 125 to 600 A ; Open circuit voltage = 30 to 80 V ; Arc voltage = 20 to 40 V ; Energy consumption = 6 to 10 kWh/kg of deposit ; Power factor = 0.4 ; Efficiency = 60%. 310 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Electrodes : The electrodes may be of the following two types : 1. Consumable electrode : (i) Base electrode (ii) Flux coated electrode. 2. Non-consumable electrode : 1. Consumable electrode : (i) Bare electrode : l These electrodes do not prevent oxidation of the weld and hence the joint is weak. They are used for minor repairs where strength of the joint is weak. l Employed in automatic and semi-automatic welding. (ii) Flux-coated electrode : l The flux is provided to serve the following purposes : — To prevent oxidation of the weld bead by creating a gaseous shield around the arc. — To make the formation of the slag easy. — To facilitate the stability of the arc. 2. Non-consumable electrode : l These electrodes are 12 mm in diameter and 450 mm long. l These are not consumed during the welding process. Examples of these electrodes are : Carbon, graphite and tungsten. 7.7.9. Types of Welded Joints The type of joint is determined by the relative positions of the two pieces being joined. The following are the five basic types of commonly used joints : 1. Lap joint 2. Butt joint 3. Corner joint 4. Edge joint 5. T-joint. 1. Lap joint. Refer to Fig. 7.18. Plates Plates Fig. 7.18. Lap joint. Fig. 7.19. Butt joint. l The lap joint is obtained by overlapping the plates and then welding the edges of the plates. l The lap joints may be single traverse, double traverse and parallel lap joints. l These joints are employed on plates having thickness less than 3 mm. 2. Butt joint : l The butt joint is obtained by placing the plates edge to edge as shown in Fig. 7.19. l In this type of joints, if the plate thickness is less than 5 mm, bevelling is not required. When the thickness of the plates ranges between 5 mm to 12.5 mm, the edge is required to WELDING AND ALLIED PROCESSES 311 be bevelled to V or U-groove, while the plates having thickness above 12.5 mm should have a V or U-groove on both sides. l The various types of butt joints are shown in Fig. 7.20. 60° Square butt Single-V—butt Double-V—butt Single-U—butt Double-U—butt Single bevel butt Double bevel butt Single-J—butt Double-J—butt Fig. 7.20. Various types of butt joints. 3. Corner joint. Refer to Fig. 7.21. l A corner joint is obtained by joining the edges of two plates whose surfaces are at an angle of 90° to each other. l In some cases corner joint can be welded, without any filler metal, by melting off the edges of the parent metal. l This joint is used for both light and heavy gauge sheet metal. Fig. 7.21. Corner joint. Fig. 7.22. Edge joint. 4. Edge joint. Refer to Fig. 7.22. l This joint is obtained by joining two parallel plates. l It is economical for plates having thickness less than 6 mm. l It is unsuitable for members subjected to direct tension or bending. 5. T-joint. Refer to Fig. 7.23. l It is obtained by joining two plates whose surfaces are approxi- mately at right angles to each other. l These joints are suitable up to 3 mm thickness. l T-joint is widely used to weld siffeners in aircraft and other thin walled structures. Fig. 7.23. T-joint. 312 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Note : The lap joints, corner joints and T-joints are known as fillet weld joints. The fillet cross- section is approximately triangular. Fig. 7.24 shows the three types of fillet welds. (a) Flush fillet (b) Convex fillet (c) Concave fillet Fig. 7.24 Welding positions : It is easiest to make welds in flat positions, i.e., both the parent metal pieces lying in horizon- tal plane over a flat surface. But, several times it becomes unavoidable to weld the workpieces in some other positions also. The common welding positions are : 1. Flat position 2. Horizontal position 3. Vertical position 4. Overhead position. 1. Flat position. Refer to Fig. 7.25. l In this welding position, the welding is done from the upper side of the joint and the welding material is normally applied in the downward direction. l On account of the downward direction of application of welding material this position is also sometimes called as downward position. Weld beads Electrode arc Fig. 7.25. Flat position. Fig. 7.26. Horizontal position. 2. Horizontal position. Refer to Fig. 7.26. In this case, the weld is deposited upon the side of a horizontal and against a vertical surface. 3. Vertical position. Refer to Fig. 7.27. l In this position, the axis of the weld remains either vertical or at an inclination of less than 45° with the vertical plane. l The welding commences at the bottom and proceeds upwards. l The tip of the torch is kept pointing upwards so that the pressure of the outcoming gas mixture forces the molten metal towards the base metal and prevents it from falling down. Fig. 7.27. Vertical position. WELDING AND ALLIED PROCESSES 313 4. Overhead position. Refer to Fig. 7.28. l In this case, the welding is performed Workpiece from the underside of the joint. The Weld workpieces remain over the head of the welder. Axis of weld l The workpieces as well as axis of the weld all remain in approximately hori- zontal plane. Fig. 7.28. Overhead position. l It is reverse of flat welding. 7.8. THERMIT WELDING Refer to Fig. 7.29. It is the method of uniting iron or steel parts by surrounding the joint with steel at a sufficient high temperature to fuse the adjacent surfaces of the Crucible parts together. Pouring — Here a wax pattern of gate Steel metal box desired size and shape is Slag basin prepared around the joint Thermit Weld or region where the weld Riser is to be affected. Iron — The wax pattern is then plug Heating surrounded by sheet iron gate box and the space be- Mould Thermit Parts being welded tween box and pattern is material collar filled and rammed with Fig. 7.29. Thermit welding. sand. — After cutting, pouring and heating gates and risers a flame is directed into the heating oven due to which the wax pattern melts and drains out, the heating is continued to raise the temperature of the parts to be welded. — The thermit mixture (finely divided aluminium iron oxide) is packed in the crucible of conical shape formed from a sheet-iron casting lined with heat resisting cement and is ignited with magnesium or torch yielding a highly superheated (nearly 3000°C) molten-iron and a slag of aluminium oxide (the reaction is : 8Al + 3 Fe3O4 = 4 Al2O3 + 9Fe + heat). — The molten iron is then run into the mould which fuses with the parts to be welded and forms a thermit collar at the joint. The welds thus obtained are metallurgically very sound and strong. Advantages : 1. Can be used anywhere. 2. Low set-up cost. 3. Not a highly skilled operation. 4. Most suitable for welding of thick sections. Limitations : 1. Only thick sections can be welded. 2. High set-up and cycle time. 314 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Applications : l The process is widely employed in the shipping, steel and railroad industries. l It can also be used for welding non-ferrous parts by selection of a mixture of oxides which on reduction with aluminium will provide an alloy approximating the material to be welded. 7.9. TUNGSTEN INERT-GAS (TIG) WELDING This welding process is also called Gas Tungsten Arc Welding (GTAW) Refer to Fig. 7.30. In this process the heat necessary to Tungsten melt the metal is provided by a very intense electric arc which is electrode struck between a virtually non-consumable tungsten electrode and Power metal workpiece. The electrode does not melt and become a part of source the weld. On joints where filler metal is required, a welding rod Arc Shelding is fed into the weld zone and melted with base metal in the same column gas manner as that used with oxyacetylene welding. The weld zone is shielded from the atmosphere by an inert-gas (a gas which does Puddle not combine chemically with the metal being welded) which is ducted directly to the weld zone where it surrounds the tungsten. Base metal The major inert gases that are used are argon and helium. Fig. 7.30. Tungsten inert-gas TIG process offers the following advantages : (TIG) welding. 1. TIG welds are stronger, more ductile and more corro- sion resistant than welds made with ordinary shield arc welding. 2. Since no granular flux is required, it is possible to use a wide variety of joint designs than in conventional shield arc welding or stick electrode welding. 3. There is little weld metal splatter or weld sparks that damage the surface of the base metal as in traditional shield arc welding. Applications : (i) The TIG process lends itself ably to the fusion welding of aluminium and its alloys, stainless steel, magnesium alloys, nickel base alloys, copper base alloys, carbon steel and low alloy steels. (ii) TIG welding can also be used for the combining of dissimilar metals, hard facing, and the surfacing of metals. 7.10. METAL INERT-GAS (MIG) WELDING This welding process is also called Gas Metal Arc Welding (GMAW). Driving wheels Consumable Refer to Fig. 7.31. The inert-gas consumable electrode electrodes process, or the MIG process is a refinement of the TIG process, however, in this process, the tungsten electrode has been Power replaced with a consumable electrode. The electrode is driven source through the same type of collet that holds a tungsten electrode Arc Shielding by a set of drive wheels. The consumable electrode in MIG process column gas acts as a source for the arc column as well as the supply for the filler material. Puddle MIG welding employs the following three basic processes. Base metal 1. Bare-wire electrode process 2. Magnetic flux process Fig. 7.31. Metal inert-gas welding (MIG). 3. Flux-cored electrode process. WELDING AND ALLIED PROCESSES 315 Advantages : 1. It provides higher deposition rate. 2. It is faster than shielded metal-arc welding due to continuous feeding of filler metal. 3. Welds produced arc of better quality. 4. There is no slag formation. 5. Deeper penetration is possible. 6. The weld metal carries low hydrogen content. 7. More suitable for welding of thin sheets. Limitations : 1. Less adaptable for welding in difficult to reach portions. 2. Equipment used is costlier and less portable. 3. Less suitable for outdoor work because strong wind may blow away the gas shield. Applications : l Practically all commercially available metals can be welded by this method. l It can be used for deep groove welding of plates and castings, just as the submerged arc process can, but it is more advantageous on light gauge metals where high speeds are possible. 7.10.1. Difference between TIG and MIG Welding Processes The difference between TIG and MIG welding processes is given in tabular form below : S. No. Aspects TIG welding MIG welding 1. Name of the process Tungsten inert-gas welding. Metal inert-gas welding. 2. Type of electrode used Non-consumable tungsten electrode. Consumable metallic electrode. 3. Electrode feed Electrode feed not required. Electrode need to be fed at a constant speed from a wire reel. 4. Electrode holder It is called welding torch and has got It is called welding gun or torch. It a cap filled on the back to cover the has facility to continuously feed wire tungsten electrode. It has also got electrodes ; shielding inert-gas, cool- connections for shielding gas, cooling ing water and control table. water and control cable. It may be air- cooled also. 5. Welding current Both A.C. and D.C. can be used. D.C. with reverse polarity is used. 6. Feed metal Filler metal may or may not be used. Filler metal in the form of fire wire is used. 7. Bases metal thickness Metal thickness which can be welded Thickness limited to about 40 mm. is limited to about 5 mm. 8. Welding speed Slow. Fast. 7.11. SUBMERGED ARC WELDING The submerged arc process (which may be done manually or automatically) creates an arc column between a base metallic electrode and the workpiece. — The arc, the end of the electrode, and the molten weld pool are submerged in a finely divided granulated powder that contains appropriate deoxidizers, cleansers and any other fluxing elements. 316 A TEXTBOOK OF MANUFACTURING TECHNOLOGY — The fluxing powder is fed from a hopper that is carried on the welding head. The tube from the hopper spreads the powder in continuous mount in front of the electrode along the line of the weld. — This flux mound is of sufficient depth to submerge completely the arc column so that there is no splatter or smoke, and the weld is shielded from all effects at atmospheric gases. As a result of this unique protection, the weld beads are exceptionally smooth. — The flux adjacent to the arc column melts and floats to the surface of the molten pool ; then it Flux solidifies to form a slag on the top of the welded hopper To power metal. The rest of the flux is simply an insulator supply that can be reclaimed easily. — The slag that is formed by the molten flux solidi- fies and is easy to remove. In fact, in many ap- plications, the slag will crack off by itself as it cools. — The unused flux is removed and placed back into To wire the original hopper for use for the next time. Trigger feed — Granulated flux is a complex, metallic for silicate electrode mechanism that can be used over a wide range of metals. l The process is characterised by high welding currents. The current density in the electrode is Nozzle 5 to 6 times that used in ordinary manual stick electrode arc welding, consequently the melting Fig. 7.32. Apparatus used in manual submerged arc welding. rate of the electrode as well as the speed of welding is much higher than in the manual stick electrode process. Fig. 7.32 shows an apparatus used in manual submerged arc welding. l Welds made by the submerged arc welding process have high strength and ductility with low hydrogen or nitrogen content. Advantages : 1. Higher welding speeds can be employed, effecting saving in welding time. 2. Very high deposition rate. 3. Flux acts as a deoxidiser to purify the weld metal. 4. Shallow grooves can be used for making joints, requiring less consumption of filler metal. In some cases no edge preparation is at all needed. 5. No chance of weld spatter (since the arc is always covered under flux blanket). 6. If required, the flux may contain alloying elements and transfer them to the weld metal. 7. Can be employed with equal success for both indoor and outdoor welding work. 8. Less distortion. 9. Few passes are required due to deep penetration. 10. It is often used in automatic mode. Limitations : 1. This process can be performed only in flat and horizontal welding positions. 2. In order to obtain good weld the base metal has to be cleaned and made free of dirt, grease, oil, rust and scale. 3. Flux may get contaminated and lead to porosity in weld. 4. Normally unsuitable for welding of metal thickness less than 4.8 mm. 5. Removal of slag is an additional follow-up operation. In multiplass beads it has to be done after every pass. WELDING AND ALLIED PROCESSES 317 Applications : l This process is suitable for welding low-alloy, high tensile steels as well as the mild, low- carbon steels. l This process is also capable of joining medium carbon steels, heat resistance steels, and many of high-strength steels. l Also the process is adaptable to nickel, monel and many other non-ferrous metals. l The submerged arc process is also capable of welding fairly thin gauge materials. 7.12. ELECTRO-SLAG AND ELECTRO-GAS WELDING These methods are employed to fuse two sections of thick metal, forming a seam in a single pass. Elimination of the need for making multiple passes and special joint preparations make these methods commonly used welding processes when heavy ferrous metals are to be joined. These proc- esses have reduced costly time in fabrication of large vessels and tanks. There is theoretically no limit to the thickness of the weld bead. 7.12.1. Electro-slag Welding Refer to Fig. 7.33. This process is a vertical and uphill ; two copper shoes, dams, or moulds must be placed on either side of the joint that is to be welded in order to keep the molten metal in the joint area. To power supply — One or more electrodes may be used to weld Wire a joint, depending upon the thick-ness of the guide metal. The electrodes are fed into the weld Electrode joint almost vertically from special wire Molten guides. Electrodes need not be of a special Slag pool deoxidized nature but they may contain a Cooled Liquid metal shoes flux, if it is needed. pool — A mechanism for raising the equipment as Weld bead Solidified the weld is completed and A.C. power source metal Plates being that has approximately 100 amperes output welded and a 100 per cent duty cycle are needed. Fig. 7.33. Electro-slag welding. l Electro-slag welding depends upon the generation of heat that is produced by passing an electric current through molten slag. Applications : Welding of heavy steel forgings, large steel castings, thick steel plates and heavy structural members. 7.12.2. Electro-gas Welding Refer to Fig. 7.34. Electro-gas welding works on the same general principle as electro-slag welding, with the addition of some of the principles of submerged arc welding. Shielding Filler metal Base metal Gas Slag Shielding gas Shielding gas Weld Molten Puddle Weld zone Zone Water cooling Solidified Metal Weld bead Starter tab Fig. 7.34. Electro-gas welding. 318 A TEXTBOOK OF MANUFACTURING TECHNOLOGY l The major difference between electro-slag and electro-gas welding is that an inert gas, such as CO2, is used to shield the weld from oxidation, and there is continuous arc, such as in submerged arc welding, to heat the weld pool. The joints and the use of flux to cleanse the weld are the same as in electro-slag process. The shoes that are used to form the weld, as in electro-slag process, are also used in the electro-gas process to control the weld zone through water cooling. However, the flux, instead of being issued to the weld zone through a hopper mechanism, is incorporated within the electrode itself in the form of cored wires. Applications : Welding of low carbon and medium carbon steels, and with specific precautions for alloy steels and stainless steels as well. — Thickness ranges commonly welded are from 12 mm to 75 mm ; for thinner sections other processes prove more economical and for thickness above 75 mm electro-slag process proves superior. 7.13. ELECTRON-BEAM WELDING Electron-beam welding fusion joins metal by bombarding a specific confined area of the base metal with high velocity electrons. The operation is performed in a vacuum to prevent the reduction of electron velocity. If a vacuum were not used, the electrons would strike the small particles in the atmosphere, reducing their velocity and decreasing their heading ability. l The electron beam welding process allows fusion welds of great depth with a minimum width because the beam can be focused and magnified (Fig. 7.35). The depth of the weld bead can exceed the width of the weld bead by as much as 15 times. l The process joins separate pieces of base metal by fusing of molten metals. The melting is achieved by a concentrated bombardment of a dense stream of electrons, which are acceler- ated at high velocities, sometimes as high as the speed of light. Under most circumstances the entire process is done inside a vacuum chamber. l Most chambers house not only the workpiece but also the cathode, the focusing device and the remainder of the gun, preventing contamination of the weldment and the electron- beam gun itself (Fig. 7.36). To vacuum D.C. supply to filament pump Bevel Cathode (electron emitting heating High voltage filament) D.C. power supply + Positioning diaphragm Electron beam Electromagnetic focusing lens Workpiece Fig. 7.35. Electro-beam welding. Fig. 7.36. Electron-beam gun. Advantages : 1. The greatest advantage of electron-beam welding is that it eliminates contamination of both the weld zone and the weld head because of the vacuum in which the weld is done because of the electrons doing the heating.