A Textbook of Production Technology PDF

Mechanical Working of Metals 267 Extrusion ratio. The extrusion ratio is defined as the ratio of the cross-sectional area of the billet to the cross-sectional area of the product. It reaches about 40 : 1 for hot extrusion...

Mechanical Working of Metals 267 Extrusion ratio. The extrusion ratio is defined as the ratio of the cross-sectional area of the billet to the cross-sectional area of the product. It reaches about 40 : 1 for hot extrusion of steel and may be as high as 400:1 for aluminium. Machines for extrusion. Both mechanical and hydraulic presses can be used for extrusion provided they possess high kinetic energy and long ram guides. Hydraulic presses with either vertical or horizontal plungers are most commonly used. Mechanical presses are used to a lesser extent. The pressure of the working fluid (water or emulsion) may reach 400 atm in hydraulic presses and produce a force of 300 MN. The plunger transmits the pressure to the ram and the pressure plate which force the metal through the die. To reduce the friction between the metal and the container walls and to achieve a more uniform flow of the metal, lubricants are used. Lubricants include a mixture of machine oil and graphite, molten glass (for steel) and other lubricating materials. Extrusion speed is of the order of 0.5 m/s for light alloys and of the order of 4.5 m/s for Cu alloys. 4.4.2. Cold Extrusion. This process is similar to hot extrusion except that the metals worked possess the plasticity necessary for successful forming without heating them. Usually, these metals have a high degree of ductility. Cold extrusion is also done to improve the physical properties of a metal and to produce a finished part (sizing operating). The widely employed cold extrusion methods are : The Hooker or extrusion down method, the impact or extrusion up method and hydrostatic extrusion. Cold extrusion is done mostly on vertical mechanical presses because they are fast and simple. The method is fast, wastes no or little material and gives higher accuracy and tolerances. 1. The Hooker Method. In the Hooker or extrusion down method, the ram/punch has a shoulder and acts as a mandrel, [Fig. 4.45 (a)]. A flat blank of specified diameter and thickness is placed in a suitable die and is forced through the opening of the die with the punch, when the punch starts downward movement. Pressure is exerted by the shoulder of the punch, the metal being forced to flow through the restricted annular space between the punch and the opening in the bottom of the die. If the tube sticks to the punch on its upward stroke, a stripper [see Fig. 4.45 (c)] will strip it from the punch. Small copper tubes and cartridge cases are extruded by this method. In place of a flat solid blank, a hollow slug can also be used. 2. Cold Impact Extrusion. The cold impact extrusion method consists in placing a flat blank of specified diameter with a small hole punched in the centre, in the die cavity and striking it by a punch with a powerful blow. The material gets heated up and becomes plastic and is forced to squirt up around the punch, [Fig. 4.45 (b)]. Thin walled tubes of low flow strength materials (tin, lead, aluminium etc.) are rapidly formed by this method. The end of the tube will correspond to the shape of the die cavity and also of the punch. The outside diameter of the tube takes the shape of the die and the wall thickness is equal to the clearance between the punch and the die. The operation is fully automatic and the production rate is as high as 50 tubes or more per minute. When the punch is on its upward stroke, the tube sticks to it. To effect release, either a stripper [See Fig. 4.45 (c)] or compressed air is directed against the tube, thus stripping it from the punch. Threads may be formed at the end of these collapsible tubes by retractable die portions or by other methods. The tubes are them trimmed, enamelled and printed. These collapsible tubes are used for cosmetics (cream, shaving cream), tooth paste, grease etc. Other product applications include : Cans, fire extinguisher cases, radio shields, food containers, boxes for condensers and cigarette lighter cases (if symmetrical). A variation of the method where a flat solid blank or slug (that is, without a hole) is used in the die cavity (the bottom of the die cavity is closed) is shown in Fig. 4.45 (c). The method is also being employed to make thick walled products such as paint-spray-gun containers and grease-gun containers, by using large presses. 268 A Textbook of Production Technology Impact extruxion speed is 100 to 350 mm/s Punch Extruded Punch Tube Punch Blank Blank Die Die Extruded Tube (a) Hooker Method (b) Impact Extrusion Stripper Punch Seamless Blank/Slug tube Die (c) Container Working Fluid Product B L A N K Die (d ) Hydrostatic Extrusion Fig. 4.45. Cold Extrusion. 3. Hydrostatic Extrusion. In hydrostatic extrusion, [Fig. 4.45 (d)], a high pressure liquid medium is used for the transmission of the force to the billet/blank. The liquid completely envelops the blank and enters die canals. Due to the hydrostatic pressure, the ductility of the material is increased. Even brittle materials like tungsten, cast iron and stainless steel etc. can be extruded. This also permits the extrusion of very long billets or even wire, accompanied by large reductions. There is no container friction and the pressurised fluid also acts as a lubricant and because of this, Mechanical Working of Metals 269 the extruded product has a good surface finish and dimensional accuracy. However, the absence of container friction combined with reduced die friction can increase the tendency to internal crack formation. The pressure transmitting fluids commonly used for hydrostatic extrusion are : Glycerin, Ethylglycol, SAE 30 mineral lubricating oil, castor oil with 10 per cent alcohol and isopentane. The hydrostatic pressure ranges from 1100 to 3150 N/mm2. The main commercial applications of this process are : Cladding of metals, making wires of less ductile materiats and extrusion of nuclear reactor fuel rods. 4.4.3. ADVANTAGES OF EXTRUSION 1. The range of extruded items is very wide. Cross-sectional shapes not possible by rolling can be extruded, such as those with re-entrant sections. 2. No time is lost when changing shapes since the dies may be readily removed and replaced. 3. Dimensional accuracy of extruded parts is generally superior to that of rolled ones. 4. In extrusion, the ductility of the metals is higher as the metal in the container is in composite compression, this advantage being of particular importance in working poorly plastic metals and alloys. 5. Very large reductions are possible as compared to rolling, for which the reduction per pass is generally  2. 6. Automation in extrusion is simpler as items are produced in a single passing. 7. Small parts in large quantities can be made. For example, to produce a simple pump gear, a long gear is extruded and then sliced into a number of individual gears. 8. Extrusions are lighter, more sound and stronger than castings. 9. They do not need draft or flash to trim and needless machining as they are more accurate than forgings. 4.4.4. DRAWBACKS OF EXTRUSIONS 1. Process waste in extrusion is higher than in rolling, where it is only 1 to 3%. 2. Inhomogeneity in structure and properties of an extruded product is greater due to different flows of the axial and the outer layers of blanks. 3. Service life of extrusion tooling is shorter because of high contact stresses and slip rates. 4. Relatively high tooling costs, being made from costly alloy steels. 5. In productivity, extrusion is much inferior to rolling, particularly to its continuous varieties. 6. Costs of extrusion are generally greater as compared to other techniques. 7. Only the shapes with constant cross-section (die outlet cross-section) can be produced. Due to the above factors, the main fields of application of extrusion process are :- (a) working of poorly plastic and non ferrous metals and alloys. (b) manufacture of sections and pipes of complex configuration. (c) medium and small batch production. (d) manufacture of parts of high dimensional accuracy. 4.4.5 Main Process Variable in Extrusion Process Total extrusion pressure is given as, pt   x  p f 0 Where  x = extrusion pressure to deform the material 0 0 (1  B)  [1  R B ] B 270 A Textbook of Production Technology and pf = ram pressure required by container friction. 4i L  D Now 0 = yield strength of work material R = extrusion ratio B =  cot   = Co-efficient of friction  = half Die angle i = Interface shear between billet and container wall. L = Length of billetin container D = Internal diameter of container  Main process variables in Extrusion process are: 1. Strength of billet material 2. Extrusion ratio 3. Co-efficient of friction/lubrication 4. Die-angle 5. Size of billet (Length and diameter) 6. Temperature 7. Speed of extrusion 4.4.6. Some Other Extrusion Processes 1. Side Extrusion. In this process, which is also known as ‘‘Lateral extrusion’’, the material flows at right angle to the direction of ram motion. The method can be : solid side extrusion and hollow side extrusion. In solid side extrusion, a solid body, with a solid protrusion of any profile is extruded. In hollow side extrusion, a workpiece with a hollow protrusion of any profile, is extruded. The tool opening is determined by the split die and the mandrel. The extrusion force required in this Ram process is very high. Due to this, the method is mainly used for non-ferrous Plate metals and highly plastic materials, like lead. Fig. 4.46 shows the principle of side Die holder extrusion process for cable sheathing. 2. Continuous Extrusion. In conventional extrusion process, we start with a finite length of billet and get a finite- length product. However, in continuous Die Container extrusion process, continuous feed stock is converted to continuous product. This is achieved by applying the extrusion Fig. 4.46. Principle of Side Extrusion. pressure on the periphery of the feedstock, instead of on the back, as in conventional extrusion. Again, whereas in conventional extrusion, there is a need to reduce friction between the billet and the container, in continuous extrusion, it is the friction force which provides the driving force for the extrusion. The impetus for the development of continuous extrusion process has been the need for the production of large tonnage of products continuously. Mechanical Working of Metals 271 The most successful technique of continuous extrusion is the ‘‘Conform process’’ illustrated in Fig. 4.47. Continuous feedstock in the form of rod is inserted between the grooved extrusion wheel and the mating die shoe. As the wheel rotates, the rod is carried round the wheel by friction and is finally pressed against a stationary block, known as the abutmen. It gets upset to conform to the container (space between the wheel groove and the mating die shoe). Enough pressure is built up to force the material to extrude through a die opening. Feed particles (Alternative) Shoe Feed Grip slock segment Product Die Abutment Grooved Wheel Product (Alternate) Fig. 4.47. Continuous Extrusion. The process has the followng advantages : (i) The process is truly continuous. (ii) Raw stock, in the form of rod, powder or machined swarf, can be used. (iii) Metallic and non-metallic powders can be intimately mixed and extruded. (iv) Polymeric materials and even fibre reinforced plastics can be extruded. (v) Low capital and operating costs. (vi) If the process is run fast, enough heat is generated due to friction and the product emerges in an annealed condition. No further intermediate heat treatment is needed and the product can be put to use directly. However, the process is limited to the extrusion of non-ferrous metals, mainly copper alloys and aluminium. 3. Helical Extrusion. It is a novel technique for the production of wire or tube. The technique Die more closely resembles machining process. A conical punch is driven slowly into the end of a copper billet, to form a short tube with an annular face. This is Product then steadily deformed by a tool (resembling a negative rake cutting tool) that rotates about the axes of the built and slowly advances in a helical Fuse Compression path. The swarf, so produced, is not wasted but is Charge Billet disk trapped in a small chamber, from which it is forced to Medium escape through an orifice, as a wire. The technique has the following advantages : Fig. 4.48. Explosive Extrusion. (i) The forces are low due to two factors : only a small volume of metal is deformed at a given instant and the yield stress gets reduced, because of severe local heating. 272 A Textbook of Production Technology (ii) Very large reductions in cross-section are possible (upto about 150 times in diameter). 4. Explosive Extrusion : The method, illustrated schematically in Fig. 4.48, has been introduced industrially. When the explosive is detonated by igniting the fuse, pressure shock waves are generated. These are transmitted to the billet with the help of pressure waves transmitting medium (Here Air) and a compression disk. This forces the billet material out of the container through the die. The method belongs to the category of explosive forming methods, HERF methods (See Chapter 9). The rapidity of the process eliminates the drawbacks caused by the cooling of the billet inside a massive container. 4.5. PIPE AND TUBE MANUFACTURING The pipes and tubes can be seamless or with seam. Tubes with seam are manufactured by the various welding techniques. Seamless tubes are made by : extrusion and piercing methods. The extrusion method, ‘‘Tubular extrusion’’ has already been discussed under Art 4.4. Seamless tubes are used in high temperature and pressure conditions/applications as well as for transporting gas and chemical liquids. Extruded tubes are used for gun barrels. Upto 400 mm diameter steel pipes have been made. 4.5.1. Piercing Method of Making Seamless Tubes. Seamless tubes are made in two stages: 1. Manufacture from round blank or billet of a thick walled shell in a piercing mill. 2. Rolling of shell into a pipe of given diameter and wall thickness. A heated round billet with its leading end centre punched, pierced or drilled, is pushed longitudinally in between two large convex shaped (tapered) rolls that revolve in the same direction, their axes being inclined at opposite angles of about 6° from the axis of the billet, Fig. 4.49 (a). The clearance between the rolls is less than the diameter of the billet. As the billet is caught by the rolls and rotated, their inclination causes the billet to be drawn forward into them. The reduced clearance between the rolls forces the billet to deform into an elliptical shape. As the billet tries to rotate under high compressive forces created by the rolls, secondary tensile stresses are set up at 90° to the compressive forces at the centre of the billet. The punched hole at the centre of the billet Mandrel Solid Round Bar (a) Piercing Rolls (b) Rolling Mill (c) Reelers (d )Sizing Rolls Fig. 4.49. Piercing Methods. Mechanical Working of Metals 273 will tear open. The mandrel assists this action. As the billet rotates and feed towards the mandrel, the tearing action is propagated along the length of the billet forming a seamless shell (roughly formed tube). Upon completion of the operation, the mandrel is forced out of the shell. This piercing of the billet to form a rough tube is called ‘‘Mannesmann process’’. The pierced shell is further processed in a ‘‘plug rolling mill’’, (Fig.4.49 (b)), which is a two- high reversing stand with a series of round passes in the rolls. A short mandrel (plug) is held in the centre of the pass by a long bar. The clearance between the mandrel and the pass determines the wall thickness of the rolled tube. This operation elongates the tube and reduces the wall thickness. The outside diameter of the tube is only approximate, however. The tube is still in a rough state. Therefore, another operation, called ‘‘reeling’’ is performed between reelers and over a mandrel as shown in Fig. 4.49 (c). The reeling operation improves the finish of the inside and outside surfaces, eliminates irregularities, scratches and out of roundness, and decreases the differences in wall thickness. Finally, a sizing operation is performed between sizing rolls without use of a mandrel, (Fig. 4.49 (d)) The final operations of reeling and sizing are often conducted on cooled tubes in order to improve their finish and size. Continuous Seamless Tube Mill. A most effective and advanced method for pipe rolling is the manufacture of thick walled shells in a piercing mill and then subsequent rolling in several continuous mills. These mills consist of several stands located one after another, the pipe being reduced in grooves and the direction of reduction changing by 90° in the adjacent stands. Rolling speeds can be adjusted in stands by means of electric motors, this allowing tension rolling and so increasing the reduction per passing and so thinning out pipe wall. A typical continuous mill for manufacturing pipes from carbon and alloy steels in diameters from 30 to 102 mm and wall thickness of 1.75 to 8 mm from blanks 140 mm in diameter, consists of: furnace for rapid heating of blanks/billets, a piercing mill, multi-stand (7 to 9) mill for rolling rough pipes on a long mandrel, a device for extracting the mandrel, an induction furnace for heating rough pipes prior to subsequent rolling, an 11-stand continuous reducing mill in which the pipe is rolled without the use of a mandrel but with tension (when wall thickness is to be reduced) or without tension (when no change in wall thickness is desired) and a 19-stand continuous reducing mill for rolling with high tension in order to sharply reduce the diameter and the wall thickness of the pipe. After rolling, pipes are cut by saws and finished in continuous automated production lines. Extensively used are automatic varieties of mills. An automatic mill is a standard non-reversible two-high stand with grooved rolls. Pipes from 57 to 425 mm in dia. with wall thickness of 3 to 30 mm can be rolled on these mills. 4.5.2. Welded Steel Tubing. Welded pipes are made in two steps : forming a strip into a circular section and then joining strip edges by welding. Welded pipes are much cheaper than seamless one, and their output increases from year to year. Their short comings are : poor strength and corrosion resistance of seams. However, these short comings are minimized by cold rolling. 1. Furnace Butt Welded Pipe. In this method, the skelp with one end trimmed is heated to 1300 to 1350° in a furnace. The hot skelp is drawn out of the furnace by grasping its trimmed end with tongs, over the handles of which a welding bell is slipped, (Fig. 4.50). The tong is pulled by a draw chain. This pulls the skelp through the welding bell which bends and folds the skelp along its longitudinal axis to circular shape, forcing the edges into contact. At the point of contact, the butt edges are welded together due to pressure exerted by the bell. The welded pipe is now transferred to the sizing rolls similar to Fig. 4.49 (d). These rolls give the pipe a fairly correct outside diameter. Next, the pipe undergoes final sizing operation in a series of cross-rolls. This finally corrects the outside diameter of the pipe to the dimension specified and its surface appearance is improved, owing to the fact that the scale is removed in the final sizing operation. After this, the pipe is straightened in straightening rolls. 274 A Textbook of Production Technology Finally, the pipe is washed in water, and its ends trimmed and threaded. This method is suitable for rapid production. 300 pipes of 0.6 m length each may be produced per hour. In a similar manner, pipes may be produced by lap welding instead of butt welding, with the edges overlapping. Tubes from 75 to 100 mm dia. are produced by this method. Flat Skelp Trimmed Skelp Welding Bell Fig. 4.50. Furnace Butt Welding Process. The furnace butt-welded pipes are used Rolls for structural purposes, posts, and for carrying Hot Skelp water, gas and wastes. Butt-welded pipes vary from 3 mm to 75 mm in diameter. Lap welded pipes are used primarily for large sizes from about 50 mm to 400 mm diameter. Continuous Furnace Butt-welding. Conti- nuous furnace butt-welding is used for water and gas pipes of 13.5 to 114 mm in dia. and wall thickness of 2 to 4 mm. A coiled strip is uncoiled in a continuous device, then heated Continuous in a furnace to 1300 to 1350°C. From the Fig. 4.51. Continuous Butt Welding. furnace, the strip (skelp) passes through a series of grooved rolls and is formed into a pipe, Fig. 4.51. Finally the pipes are finished in a reducing mill, then cut to standard lengths by a flying saw. Fig. 4.52. Roll Forming of Pipe. 2. Electric Resistance Welding. This technique enables tubing of higher quality to be obtained than by the furnace welding process. Tubing from 6 to 630 mm in dia and with a wall thickness of 0.5 to 20 mm is produced by this method. The initial material for making the pipe is bright cold- rolled strip in coils, or sheet steel (for large- diameter tubing), which is preliminarily cleaned from scale and rust by pickling or shot blasting. Mechanical Working of Metals 275 The flat strip or sheet is gradually roll formed Electrodes into a tubular shape in a continuous mill of 5 to 12 stands, Fig. 4.52. The formed stock is inserted beneath the electrodes, Fig. 4.53, and the edges are heated by electric current. The heated edges are forced together by side pressure rolls and bottom pressure roll. A welded joint is formed by this action. External and internal flashes are removed by a cutter tool. After welding, pipes are sized or shaped into proper size, in a continuous mill, then cut to standard lengths by saws and transferred to a finishing department. Electric welded pipes are used primarily for pipe line conveying petroleum products and water. Electric resistance welding is the most widely used process in tube production, but other electric welding methods are also employed. For example, SAW is used for thick walled tubes of medium size of carbon and alloy steels and large-size tubes of carbon steels. Atomic-hydrogen welding is used Side for tubes upto 200 mm in diameter with walls from Bottom Pressure Pressure Rolls 2 to 12 mm thick of alloy steels. Argon-arc welding Roll is used for welding thin walled tubing of a diameter upto 450 mm with a wall thickness from 0.6 to 5 mm made of high-alloy austenitic steel, or of non- ferrous metals and alloys. Fig. 4.53. Electric Butt Welded Pipe. 4.6. HOT DRAWING AND CUPPING This is another technique of producing seamless cylinders and tubes. The heated billet or bloom is first pierced on a vertical hydraulic press, (Fig. 4.54 (a)). As the plunger, on its downward stroke, pierces the billet/bloom, confined in a die, the plastic metal is displaced and is moved upward and around the space between the plunger and die to produce a thick walled closed end shell. Upon completion of the downward stroke, the plunger moves upwards and the shell is pushed out of the die with the help of an ejector ram. This hot piercing method is followed by hot drawing. The formed shell is reheated and forced through a series of annular dies (of decreasing dia- meter), built on a push bench, (Fig.4.54(b)), with the help of a hydraulically operated plunger. The cup shaped piece is reduced in diameter and increased in length. To obtain long thin walled cylinders/ tubes, repeated heating and drawing may be necessary. If the final product is to be a tube, then the closed end is cut off and the tube under goes sizing and finishing operations as in piercing method of making seamless tubes. If the final product is to be a cylinder (for storing O2 etc.), the open end is swaged to form a neck, (Fig. 4.55). Swaging involves hammering or forcing a tube or rod into a confining die to reduce its diameter, the die often playing the role of a hammer. Repeated blows cause the metal to flow inward and take the internal form of the die. Seamless cylinders and tubes can also be made by hot drawing or cuppling, (Fig. 4.56). A thick metal blank (usually 8 or 10 mm thick) of circular shape is placed over a cylindrical die. The plunger of the hydraulically operated press forces the heated blank through the die to form a cup shaped product. The thickness of the cup is reduced and its length increased by drawing it through a series of dies having reduced clearance between the die and the punch. The same result can be obtained by drawing on a bench, (Fig. 4.54 (b)). This process is used primarily for forming 276 A Textbook of Production Technology thick walled cylindrical products, for example, oxygen tanks, and artillery shells etc. Piercing Heated Punch Cylinder P U N C H Dies Finished Cylinder Die Ejector Heated Ram Billet (b) Draw Bench (a) Fig. 4.54. Hot Cupping and Drawing. Die 1st Step 2nd Step 3rd Step Finished Gas Cylinder Fig. 4.55. Swaging the Open End of a Cylinder. 4.6.1. Other Methods of Tube Manufacture As discussed before, the manufacture of seamless tubes consists of four stages : (i) Reduction of billets and blooms. (ii) Production of hollows. P (iii) Hot finishing. L U (iv) Cold finishing. N The two methods of making hollows have been G E discussed : Mannesmann process (Art. 4.5.1) and R Hot cupping (Art. 4.6). These two processes do not provide sufficiently large wall reduction and elongation to produce hot-worked tubes. Two Blank methods of hot reduction of both diameter and wall thickness of hollow shells have already been DIE discussed : Plug rolling mill [Fig. 4.49 (b)] and Drawing [Fig. 4.54 (b)]. The other methods employed Cup for the same purpose are : 1. The Assel Elongator. In this technique, three conical driven rolls of special design, all inclined to the tube axis are used. This method has been widely Fig. 4.56. Hot Drawing and Cupping. adopted. Mechanical Working of Metals 277 2. Three-Roll Piercing Machines. The Assel elongator led to the development of three-roll piercing mill, (Fig. 4.57). This rolling mill produces more concentric tubes with smoother inside and outside surface than the old Mannes-Mann mill. 3. Pilgering. A widely used method for hot reduction of both diameter and wall thickness of thick- walled shell is known as the ‘‘Pilger Process’’. A Pilger rolling mill, (Fig. 4.58) has two rolls, with pass grooves of varying profile. Thus, when the rolls rotate, the dimensions of the pass change continuously and, therefore, the form of the pass is variable during one Fig. 4.57. Three-Roll Piercing Mill. revolution of the rolls. At maximum diameter of the Pilger Roll Workpiece Feed Man drel (a) (b) (c) Fig. 4.58. The Pilger Rolling Mill. pass grooves, the rolls form an idle pass (Position a) whose dimensions are larger than the outer diameter of the shell. At this moment, the shell, together with the inserted mandrel, is automatically fed into the gap between the rolls by the amount of feed. Upon further rotation of the rolls, the dimensions of the pass are gradually decreased due to its varying form. The rolls bite the shell and roll the hollow shell on to the central mandrel. The shell is reduced in thickness and diameter, (Position b), this reduction being increased with the decrease in the pass groove diameter. The rolls force the gripped portion of the shell in the direction of rolls rotation, so that the shell and mandrel are made to move backwards, that is, opposite to the direction they were fed by the feeder (Position c). When the rolls have rotated through 360°, they return to the position (a) and the feeder again advances the shell between the rolls. The shell is turned 90° as it is being fed forward, to maintain uniform walls. Hence, in the Pilgering process, the thick hollow shell is reduced both in diameter and thickness and hence elongated in a series of discontinuous steps, two steps forward and one step backward. After rolling the shell, the mandrel is removed from the tube. The next shell is rolled on a new mandrel, while the first one is cooled, lubricated and prepared for further use. Very high reductions (upto about 70%) can be achieved by Pilgering process. After hot finishing, the last stage in the Tube manufacture is cold finishing. This is done by Reelers and Sizing rolls [Fig. 4.49 (c) and (d)] and by Tube drawing (Art. 4.7.2). Another method of Cold-reducing or cold-finishing resem- bles the Pilger hot process, (Fig. 4.59). Two rolls are used with a tapered semi-circular groove in each. The maximum size of the groove is equal to the outside diameter of the ingoing tube and minimum size equal to the outer diameter of the outgoing tube.  Chapter 5 The Welding Process 5.1.GENERAL 5.1.1. Definition. There are many definitions of a welding process. But the most comprehensive definition is given below : Welding is defined as ‘‘a localized coalescence of metals, wherein coalescence is obtained by heating to suitable temperature, with or without the application of pressure and with or without the use of filler metal. The filler metal has a melting point approximately the same as the base metals’’. The welding process is used to metallurgically join together two metal pieces, to produce essentially a single piece of metal. The process results in what is known as a ‘permanent joint’. A good welded joint is as strong as the parent metal. The product is known as ‘Weldment’. 5.1.2. Applications. The welding process finds widespread applica-tions in almost all branches of industry and construction. Welding is extensively employed in the fabrication and erection of steel structures in industrial construction and civil engineering, for example, structural members of bridges and buildings etc. ; vessels of welded-plate construction (steel reservoirs, boilers, pressure vessel tanks and pipelines etc.); and concrete reinforcement. It is the chief means of fastening panels and members together into automobile bodies and also in aviation industry. It has taken the place of castings for a large proportion of machine, jig and fixture bases, bodies and frames. The process is also extensively used for the repair of broken parts, in the building of warn surfaces and in the repair of defective castings. In fact, the future of any metal may depend upon how far it would lend itself to fabrication by welding. 5.1.3. Advantages. The widespread use of welding at the present time is due to its following advantages : 1. Welding results in a good saving of material and reduced labour content of production. 2. Low manufacturing costs. 3. Dependability of the medium, that is, the weldments are safer. 4. It gives the designer great latitude in planning and designing. 5. Welding is also useful as a method for repairing broken, worn or defective metal parts. Due to this, the cost of re-investment can be avoided. 6. Without welding techniques, the light weight methods of fabrication, so vital to the automotive and aircraft industries, would be unthinkable. The welding process has the plus points that it is readily adaptable to streamline structure and the welded joints are very tight. Welded joints are strong, especially under static loading. However, they have poor fatigue resistance due to stress concentration, residual stresses and various weld defects, such as cracks, incomplete fusion, slag inclusions and the like. But, all these drawbacks can be overcome to a large extent. 322 The Welding Process 323 Advantages of Welded Joints over Riveted Joints 1. Economy of material and lighter weight of structure owing to : (a) Better utilization of metal elements (plates, angles) since their working sections are not weakened by the rivet holes, consequently the sections of the welded pieces can be made smaller than the sections of riveted elements, for the same acting forces. (b) Possibility of a wide use of butt-jointed seams, requiring no additional elements, such as cover straps. (c) Lighter weight of the joining elements (rivets weight more than the welds). The weight of welds comprises about 1 to 1.5 per cent of work weight, while the weight of rivets is about 3.5 to 4 per cent. The use of welding instead of riveting, saves on an average 10 to 20 per cent in weight. 2. Greater strength of the joints, due to absence of holes needed for riveting. 3. Less labour is required, since there is no need for marking out and drilling or punching holes. Riveting consumes much more labour and is a much more complicated and less productive job, than welding, which can be often largely automated. 4. Possibility of joining curvilinear parts. 5. Tightness and impermeability of the joint. 6. Noiselessness (riveting is inevitably accompanied by noise). Advantages over Casting Process 1. Lighter weight and saving of material due to : (a) lesser machining allowances and (b) the possibility of utilizing smaller sections, since the wall thickness of cast parts, determined in many cases by the casting process, is as a rule 2 to 3 times greater, and sometimes even more than that of welded parts. The saving of metal in welded machine components, as compared with cast ones, may amount to 40%. 2. Lower Cost. 3. Greater strength. 4. Maximum homogenity. Whether machine parts should be welded, is decided in each particular case, by design and economy considerations. The drawbacks of welding can be : Not all the metals are satisfactori- ly weldable and the weldments are less readily machinable, as compared to castings. 5.1.4. Classification. Welding processes may be classified accord-ing to the source of energy employed for heating the metals and the state of metal at the place being welded. Depending upon the source of heat, the welding processes are classified as :- 1. Chemical (oxygen + combustible fuel gas, e.g., acetylene, propane, butane, natural gas, hydrogen etc.). 2. Chemico – Mechanical (Pressure gas welding, Thermit welding) 3. Electro – chemical (Atomic hydrogen welding) 4. Electro-Mechanical (Electric resistance welding) 5. Electric – Arc welding. 324 A Textbook of Production Technology These may be divided into two groups as follows : (a) Pressure Processes. In these processes, the parts to be joined are heated to a plastic state (fusion may occur to a limited extent) and forced together with external pressure to make the joint. Some of the more common processes in this group are mentioned below : 1. Forge welding 2. Thermit Pressure welding 3. Pressure Gas welding 4. Electric Resistance welding (b) Fusion Processes. In these processes, the material at the joint is heated to the molten state and allowed to solidify to make the joint, without the application of pressure. Here, some joints may be made without the addition of a filler metal, but, in general, a filler metal must be added to the weld to fill the space between the parts being welded. The filler metal deposited should ordinarily be of the same composition as the base metal. Some of the common welding processes in this group are listed below : 1. Gas welding 2. Electric Arc welding 3. Thermit Fusion welding The welding processes can also be classified as : Autogeneous, Homogeneous and Hetrogeneous In ‘Autogeneous’ processes, no filler metal is added to the joint interface, for example, cold and hot pressure welding processes and electric resistance welding. In ‘Homogeneous’ processes, filler metal is added and is of the same type as the parent metal, for example, welding of plain low- C steel with a low- C welding rod and welding of 70 - 30 brass with a 70 - 30 brass welding rod etc. In ‘Hetrogeneous’ processes, a filler metal is used but is of a different type from the parent metal, for example, brazing and soldering processes. Brazing and Soldering are not strictly the welding processes in view of the definition of welding process given above. However, these processes also belong to the family of welding processes. The two most widely used welding methods are : Gas welding and Arc welding. The temperature of an electric arc is much higher than that of a gas flame, so the joint zone melts practically instantaneously in arc welding. Gas welding involves long preheating period which raises the metal adjacent to the joint to a high temperature. This exerts an unfavour- able effect on the crystalline structure, which results in considerable stresses being set up. So, gas welding is unsuitable for relatively large cross-sections due to this trouble and the time involved in preheating. Plates above 20 mm thick are, therefore, best welded by arc welding. 5.1.5. Types of Weld Joints. The relative positions of the two pieces being welded determine the type of joint. Welded structures are assembled by five basic types of joints : Butt, Lap, Corner, T and Edge joints, shown in Fig. 5.1. All other possible joints are variations of these basic joints. To make a weld, the two pieces to be joined are located and clamped or held in the correct position. Butt joints are formed by welding the end surfaces or edges of the members. In Lap joints, the two members being welded should overlap by an amount from 3 to 5 times their thickness. Edge of each piece is welded to the surface of the other. T-joint is used to join two pieces whose surfaces are approximately at right angles to each other. The Welding Process 325 (a) Butt Joint (b) Lap Joint (c) T-Joint Half Open Full open (e) Edge Joint (d) Corner Joint Fig. 5.1. Basic Welded Joints. Corner joints are used to join the edges of two pieces whose surfaces are approximately at right angles to each other. Edge joint is used to join the edges of two pieces in which a part of the surface remains parallel to each other. 5.1.6. Types of Welds. Fig. 5.2 shows (a) Bead Weld the various types of welds used in making a Concave fillet joint. A ‘bead’ weld is one in which the filler metal is deposited at a joint where the two surfaces adjoining the joint are in the same (b) Fillet Weld Convex plane. A ‘bead’ is defined as a single run of Fillet weld metal. A ‘fillet’ weld is one in which the filler metal is deposited at the corner of two intersecting surfaces, such as a T or Lap (c) Groove Weld joint. A ‘groove’ weld is one in which the filler material is deposited in a groove formed by edge preparation of one member or of both the members. A ‘plug’ or ‘slot’ weld is one (d) Spot or seam weld in which a hole is formed through one of the pieces to be welded and the filler material is then deposited into this hole and fused with (e) Plug Weld the mating part. Fig. 5.2. Types of Welds. 5.1.7. Edge Preparation. The pre-paration of the edges of the pieces to be welded depends upon the thickness of metal being welded. Edge preparation is necessary when thickness increases so that heat would be able to penetrate the entire depth. This ensures formation of sound welds. The edge preparation is done by bevelling the edges of the pieces after the rust, grease, oil or paint are completely removed from their surfaces. The various edge preparations are illustrated in Fig. 5.3. There are five basic types of chamfers put on the mating edges prior to welding : Square, V, Bevel, U and J. Butt Joints. The straight square butt joints with no special edge preparation is used when the thickness of the two plates to be welded is small so that heat of welding penetrates the full depth of joint. These joints are suitable for thicknesses from 3 to 8 mm. However, if the plate thickness is more than 4.5 mm, edge preparation is recommended. Single V : For thickness upto 16 mm Double V : For thickness > 16 mm Single and Double U : For thickness greater than 20 mm. 326 A Textbook of Production Technology When comparing V and U-joints it should be noted that U-joints require less electric power and electrode material since the X-section of a U-joint for a given plate thickness is less than that of a V-joint. Another advantage of U-joint lies in the fact that due to the insignificant bevel of the edges, the shrinkage of the molten metal on cooling occurs almost uniformly over the entire section and the plates warp less than in case of V-joints. However, a V-Joint is easy to make than a U- joint. Square Single V Double V Single U Double U Single Bevel Double Bevel Single J Double J BUTT JOINTS Single Fillet Double Fillet LAP JOINTS CORNER JOINTS Straight Single Bevel Double Bevel Single J Double J TEE JOINTS EDGE JOINTS Fig. 5.3. Edge Preparation. Double V and U-joints are by no means inferior to single V-joints when plates of 12 mm and over are welded because the X-sectional areas of double V and U joints is 30-40 percent less than that of single V-joints at the same bevel angle. The short comings of double V and U-joints accrue from poor penetration liable to accur inside the butt and the greater cost of edge preparation especially The Welding Process 327 in the case of U-joints. Other edge preparations for a butt joint are : Single bevel, Double bevel, Single J, Double J. Butt joints are made by Bead or Groove welds. Lap Joints are used to join thin sheets, usually less than 3 mm thick. These joints do not need any special edge preparation. The joint is produced by fillet welds. Corner Joints are used to join sheets upto 5 mm thick. These joints are welded with or without edge preparation, with the help of fillet and/or groove welds. Tee Joints. Only structures subjected to low static loads can be welded without edge preparation. Single bevel joints are employed for critical structures in which the members are from 10 to 20 mm thick and Double bevel designs are used for thicker metals. Single J and Double J joints can also be used for thicker metals. Tee joints are made by using fillet and/or groove welds. Edge Joints. Edge joints are used for metals upto 3 mm thick. The height of flange should be twice the thickness of the sheet. These joints are made by Bead or Groove welds. Fig. 5.4 gives the various definitions concerning welding joints and welds. Fusion Weld Zone Face (a) Flat or Downhand Base Metal Root Face Root Opening (a) V-Joint Groove Weld) (b) Horizontal Toe at ro Toe eld Th W ace F Leg (c) Vertical Fusion Root Root Zone Leg (b) T-Joint (c) Corner Joint (d ) Overhead (Fillet Weld) Fig. 5.4. Definitions. Fig. 5.5. Welding Positions. 5.1.8. Welding positions. The basic welding positions are shown in Fig. 5.5. The flat position is the easiest and quickest for welding. In this position, welding benefits from the force of gravity and maximum deposition rates are obtained resulting in strongest welded joints. Next in ease of welding is the horizontal fillet position in which the force of gravity helps to some extent. However, the major defects that can occur while welding in horizontal position are : undercutting and overlapping of the weld zone. For welding in these positions, the joint should be level or nearly so when possible. Welding in positions other than flat requires the use of manipulative techniques and welding rods/electrodes that result in faster freezing of the molten metal and slag, to counteract the effect of gravity. In vertical position, the welder can deposit the bead in the uphill or downhill direction. Uphill welding is preferred for thick metals because it produces stronger welds. Downhill welding is faster than the uphill welding and is suited for thin metals. 328 A Textbook of Production Technology 5.1.9. Cleaning the Joint and Fluxing. The weld area (joint) must be free of oil, dirt, grease, paint or moisture etc. Firstly, these will interfere with the proper fusion on the metal and result in a weak joint. Secondly, gases may evolve from these (under the action of heat) which may be absorbed by molten metal. For removing oily substances, the cleaning is done by organic solvents like acetone, CCl4 and trichloro ethylene. For removing foreign substances, cleaning is done with a rag soaked in the solvent. Heavier oxide films may be removed by acid pickling, wire brushing or emery. When organic solvents are used for cleaning, these should be completely evaporated from the interfaces before any welding is attempted. Otherwise, highly poisonous gases such as phosgene may form under intense heat of welding. In addition to many gases which may evolve if the joint is unclean, gases such as O2, N2 and H2 are present in the atmosphere. Also, water vapours in air, breakdown under intense heat of welding to form O2 and H2. All these gases may be absorbed by the molten metal. As the metal cools after welding, its solubility for these gases decreases. The gases try to leave the metal as it solidifies. If they are not able to leave, they will form porosities and gas holes in the weld. To avoid this, the joint must be properly cleaned and the molten metal properly shielded from surrounding air. Most oxides present on the metal surface (if the joint is not cleaned properly) or formed during welding (by the reaction between oxygen and metals if the molten metal is not shielded properly) may interfere considerable with the welding. These oxides have higher melting points than the base metals from which they are formed. Due to this, they prevent the proper fusion of the base metals. Therefore, it is necessary to add some material to the welding zone, which is capable of dissolving the oxides. This material is known as ‘Flux’. The flux reacts with the oxides to form what is known as ‘‘Slag’’. The slag has low melting point, is more fluid and is lighter than the molten metal. Due to this, it will float more readily to the surface of the molten metal puddle. The slag will also cover the molten metal puddle and help in preventing the absorption of O2, N2 and other gases from the atmosphere. The slag is chipped off after the weld metal has cooled and solidified. The flux may be applied as a dry powder, paste, thick solution or as an ingredient in the coating of welding rods/electrodes. Fluxes. The flux added to the weld to control the detrimental effects (discussed above) should have the following properties : 1. It should be capable of dissolving oxides and other impurities to clean the metal surface. 2. Its melting point should be lower than that of the base metal and filler metal, so that the surface oxides are dissolved before the base metal melts. 3. Its specific gravity should be lower than that of the base metal and the filler metal, so that the slag floats readily to the surface of the molten metal puddle. 4. It should control the flowability of the molten metal. 5. It should not affect the base metal adversely. The type of flux used will depend upon the type of the metals being welded. The common fluxes used are listed below : (i) For Aluminium and Aluminium Alloys. Mixture of alkaline fluorides, chlorides and bisulphates, for example, mixtures of chlorides of sodium, potassium, Lithium and Barium, fluorides of calcium (Fluorspar, CaF2), sodium and potassium and sodium bisulphate. (ii) Copper and Copper Alloys. Mixtures of sodium and potassium borates, carbonates, chlorides, sulphates; Borax (Na2B4O7.10H2O) Boric acid (H3BO3) and Di-sodium hydrogen phosphate (Na2HPO4) are suitable for dissolving oxides called cuprous oxides. The Welding Process 329 (iii) Ferrous Metals. For welding C.I., Mixtures of Borax, Sodium carbonate and Potassium carbonate ; Sodium carbonate and Sodium bicarbonate ; Borax, Sodium carbonate and Sodium nitrate or Borax alone are suitable fluxes. For Carbon steels : Dehydrated borax and calcium oxide dissolved in liquid are common fluxes. For Alloy steels : Mixture of Boric acid, dehydrated borax and Calcium fluoride. Borax forms compounds with iron oxide and the carbonates cleans and promotes fluidity. 5.2. GAS WELDING Gas welding is ‘‘a group of welding processes wherein coalescence is produced by heating with a gas flame or flames with or without the application of pressure and with or without the use of filler material’’. It may be noted here that out of the various types of gas welding processes, pressure is used only in Pressure Gas Welding (PGW). In all other types of gas welding processes, joint is made without the application of any pressure. The edges or surfaces to be joined are melted (alongwith the filler material when used) by a suitable gas flame and the molten metal is allowed to flow together to form a permanent joint on cooling. The welding heat is obtained from burning a mixture of oxygen and a combustible gas. The gases are mixed in the proper proportion within a welding torch (also called Blowpipe) which provides control of the welding flame. The mixture is burnt at the blowpipe nozzle or tip. The general name of the method is : Oxy Fuel Gas Welding (OFW). The common commercial gases used in gas welding include : acetylene, hydrogen, propane and butane. The most common form of gas welding is oxygen-acetylene (oxy-acetylene) welding, OAW. Acetylene (C2H2) produces higher temperature (in the range of 3200°C) than other gases, (which produce a flame temperature in the range of 2500°C) because it contains more available carbon and releases heat when its components (C and H) dissociate to combine with O2 and burn. The cost of production of acetylene is low and the gases (O2 and C2 H2) can be stored at high pressure in separate steel cylinders. The main drawback of acetylene is that it is dangerous if not handled carefully. 5.2.1. Oxy-Acetylene Welding (OAW). There are two systems of OAW depending upon the manner in which acetylene is supplied for welding : High pressure system and Low pressure system. Acetylene is supplied either from generators or it may be purchased in metal cylinders. O2 is always supplied from metal cylinders. 1. High Pressure System. In this system, both O2 and C2H2 are supplied from high pressure cylinders. Oxygen cylinders are charged to a pressure of 120 atm. gauge. Due to the danger of explosion, pure acetylene can not be compressed to a pressure more than 0.1 Pa above atmosphere. Therefore, acetylene is supplied in cylinders in the form known as ‘‘Dissolved acetylene’’. It is stored in cylinders in which it is dissolved in acetone under a pressure of from 16 to 22 atm gauge. At normal pressure, one litre of acetone dissolves about 25 litres of acetylene. For every additional atmosphere of pressure another 25 volumes of acetylene will be dissolved. As a safety measure, the cylinder of acetylene is filled with a porous filler (usually charcoal) forming a system of capillary vessels. It should not be with drawn from cylinder too rapidly, since some acetone may then be with drawn along with acetylene. The maximum recommended pressure when taking acetylene from a cylinder through a rubber hose is 1 bar (1 × 105). In H.P. system, the pressure of acetylene at the welding torch is from 0.06 to 1.0 bar. 2. Low Pressure System. Here, acetylene is produced at the place of welding by the interaction of calcium carbide and water in an acetylene generator, according to the reaction : 330 A Textbook of Production Technology CaC2 + 2 H 2 O Ca (OH) 2 + C2 H 2 + 127.3 k J per mol. As is clear from above, a great deal of heat is evolved in this reaction. The produced acetylene is supplied to the blowpipe at a low pressure from a gas holder incorporated in the generator. Acetylene is cleaned by passing it through a purifier. To prevent the possibility of an explosion by oxygen or air blowing back and entering the generating plant, a back pressure valve is arranged between the blowpipe and the gas holder. The pressure of acetylene at the torch is upto 0.06 bar For oxygen, the desired pressure at the welding torch is : (i) High Pressure System (Welding and Cutting) = 0.1 to 3.5 bar (ii) Low Pressure System (Welding) = 0.5 to 3.5 bar 5.2.2. Gas Welding Equipment. Equipment required in gas welding includes : cylinders for compressed gases (acetylene generator in place of acetylene cylinder in low pressure system), regulators, blowpipes, Nozzles, Hose and Hose fittings. The assembled basic equipment required for high pressure system is shown in Fig. 5.6. Cylinder Contents Outlet Pressure Gauge Gauge Outlet Pressure Cylinder Contents Gauge Gauge Pressure Regulating Screw Pressure Regulating Screw Valve Valve Welding Torch Acetylene Oxygen Cylinder Painted Cylinder Maroon Painted Black Fig. 5.6. High Pressure Gas Welding Equipment. 1. Cylinders for Compressed Gases. The Oxygen cylinder is painted Black and is made of steel. Acetylene cylinder is painted Maroon and is made of steel. To avoid potentially lethal situations, C2H2 cylinders have L.H., threads and O2 cylinders have R.H. threads. This safety precaution also applies to all valves, connectors and regulators. 2. Pressure Regulators. A pressure regulator or pressure reducing valve, located on the top of both O2 and C2H2 cylinders, serves to reduce the high cylinder pressure of the gas to a suitable The Welding Process 331 working value at the blow pipe and to maintain a constant pressure. The pressure is regulated with the help of a spring loaded diaphragm. Variations of pressure is necessary for different sized nozzles (inside diameter) and the pressure is controlled by a graduated adjusting screw which serves to vary the compression of the spring. 3. Pressure Gauges. Each gas h.p.o2 from cylinder is provided with two pressure Mixing Injector gauges. One gauge indicates the pressure Chamber of the gas inside the cylinder and the other indicates the pressure of the gas supplied to the blow pipe. 4. Blow Pipe. The blow pipe or l.p.c2H2 weld-ing torch serves to mix the gases in (a) proper proportion and to deliver the mixture to the nozzle or tip where it is Nozzle burned. The gases from the cylinders are O2 taken to the blow pipe through reducing valves and with the help of rubber tubes (hoses). On the shank of the blowpipe, two control values (needle type) are Mixing c2 H2 provided, one for controlling the flow of Chamber acetylene and the other of oxygen, entering a chamber called mixing chamber (b) where the two gases are mixed in a Nozzle correct proportion. The control knobs of Connecting Mixing the control valves are usually coloured, thread chamber red for acetylene and blue for oxygen. Control Interchangeable valve nozzle Depending upon the system of OAW, there are two designs of blow pipes: Injector type and High pressure type. A detailed view of the Welding Hand grip Extension Torch (Blow Pipe) is shown in Fig. 5.7 (c). (c) The injector type blow pipe is used Fig. 5.7. Blow Pipes. in the low pressure system. The low pressure of acetylene is not sufficient to force it through the small passage, into the mixing chamber. To overcome this difficulty, high pressure oxygen at a very high velocity is led through an injector nozzle inside the body of the blow pipe. It produces a vacuum in the acetylene channel, drawing acetylene into the mixing chamber, (Fig. 5.7 (a)). From the mixer, the gas mixture flows to the tip. In the high pressure system, (Fig. 5.7 (b)), both the gases are at high pressure. The gases flow on their own to the mixing chamber and no injector is needed. Under no circumstance, must a high pressure torch be used on a low pressure system, because the absence of injector makes it unsafe. Low pressure blow pipe is expensive than the high pressure blow pipe, because the whole head contain- ing both the nozzle (tip) and injector is to be replaced with every nozzle change. 5. Nozzle or Tip. The nozzle is a device screwed to the end of the blow pipe. It is used to permit the flow of oxyacetylene gas mixture from the mixing chamber of blow pipe to the tip of the nozzle to facilitate burning. In order to vary the size of flame (and heat supply) necessary to weld varying thicknesses of metal, a selection of tips is available for the blow pipe. For this, the nozzles are interchangeable so that the correct nozzle is fitted at the end of the blow pipe. Each nozzle is marked showing its gas consumption in litres/hour and a table supplied with the blowpipe shows 332 A Textbook of Production Technology which tip should be used to weld any required thickness of metal. The delivery pressure from the regulator must be varied according to the size of the tip used, and instructions are supplied to obtain the correct conditions. 6. Hose and Hose Fittings. The hose connects the outlet of the pressure reducing valve and the blow pipe. Rubber tubing is necessary for flexibility but it should be of the highest quality, specifically manufactured for this purpose. In accordance with International standards, hose is manufactured to a colour code : Blue for Oxygen and Red for acetylene. Hose fittings are provided at the ends of the hoses for attachment to the blow pipe and the outlet of the pressure reducing valves. To prevent the interchange of fittings, the oxygen hose connection nut has a right handed thread and the acetylene fuel gas fittings have a left handed thread. The other equipment used during gas welding are : 7. Goggles. Welding goggles must be worn to protect the eyes from the heat and light radiated from the flame and molten metal in the weld pool. 8. Welding Gloves. They protect the hands from the heat and metal splashes. 9. Spark Lighter. It is used to conveniently and instantaneously light the blow pipe. 10. Chipping Hammer. It is made of steel and is used to remove metal oxides from the welded bead. 11. Wire-brush. It is used to clean the weld joint before and after welding. Other equipment also includes : safety shields and protective clothing. Welding Inner White Intermediate Torch Tip Cone Flame Flame Feather Outer Blue Flame Envelope (a) Neutral Flame (b) Reducing Flame (c) Oxidising Flame Fig. 5.8. OAW Flame Settings. 5.2.3. Oxy-acetylene Flame Settings. In an oxyacetylene flame, O2 and C2H2 are mixed and burnt to release heat. The complete combustion of acetylene in an atmosphere of oxygen is represented by the following reaction : C2 H 2 + 2.5O2 = 2CO2 + H 2 O (vapour) + 1284.57 k J/mol An oxy-acetylene flame can be set to : neutral, carburising or oxidising condition depending upon the metal to be welded and the type of filler metal to be used. In a neutral flame, O2 and C2H2 are mixed in equal amounts. In carburising or reducing flame, the percentage of C2H2 is more than that of O2 and in the oxidising flame, the percentage of O2 is more than of C2H2. These three types of flames are illustrated in Fig. 5.8. 1. Neutral Flame. A neutral flame is obtained when equal amounts of O2 and C2H2 are mixed and burnt in a torch. The flame is recognised by two sharply defined zones, the inner white cone flame and the outer blue flame envelope, (Fig. 5.8 (a)). The reaction at the inner cone for the neutral flame where equal volumes of cylinder oxygen and acetylene are used is, The Welding Process 333 C2 H 2 + O2  2 CO + H 2 This provides the most concentrated heat with the highest tempera- ture for welding at a distance of 3 to 5 mm from the end of the inner cone. It is also apparent that the environment within the outer envelope consists of carbon monoxide and hydrogen and is relatively inert to materials that oxidise readily. The reactions at the outer envelope are : 2 CO + O2  2 CO2 1 H2 + O  H 2 O (vapour) 2 2 For these reactions, the oxygen is supplied from the surrounding air. During actual welding, the outer envelope spreads over the surface of the work material and serves as a protective shield from the ordinary atmosphere. Also, since the heat developed is not as concentrated, this outer envelope of flame contributes only to preheat the work material for welding. 2. Carburising Flame. A carburising or reducing flame is obtained when an excess of acetylene is supplied than which is theoretically required (O2 : C2H2 = 0.85 to 0.95). A reducing flame is recognised by three distinct sections : the inner cone (which is not sharply defined) and an outer envelope as for the neutral flame. The third zone surrounds the inner cone and extends into the outer enveloping zone. It is whitish in colour and is called ‘‘excess acetylene feather’’. Its length is an indication of the amount of excess acetylene. (Fig. 5.8 (b)). To obtain a neutral flame, first the reducing flame is obtained. Then the supply of oxygen is gradually increased until the intermediate feather disappears. The resulting flame will be a neutral flame. 3. Oxidising Flame. This flame has an excess of oxygen over that required for a neutral flame (O2 : C2H2 = 1.15 to 1.50). To obtain an oxidising flame, the flame is first set to the neutral condition. Then the acetylene valve is turned down gradually to reduce the amount of acetylene, giving an excess of oxygen. The flame resembles the neutral flame except that it acquires a light blue tint and the inner cone is slightly shorter and more pointed than in a neutral flame (Fig. 5.8 (c)). An oxidising flame burns with a harsh sound and the outer envelope is short and narrow. Applications of Three Types of Flames (a) Neutral Flame. In most welding situations, it is theoretically desirable to use a neutral flame, but in practice it is very difficult to discern whether the flame is neutral or oxidising, either a slightly reducing or a slightly Oxidising flame is used. If possible, most welding should be done with a neutral flame, since such a flame has a minimum chemical effect upon most heated metals. The flame is widely used for the welding of steel, stainless steel, cast iron, Copper and Aluminium. (b) Reducing Flame. The acetylene being excess in this flame, the available carbon is not completely consumed. With iron and steel, it will form iron carbide (hard and brittle). Therefore, metals that tend to absorb carbon should not be welded with a reducing flame. This flame is used for materials that oxidise rapidly like steel and aluminium. It is suitable for welding low alloy steels, low c-steels and for welding those metals (for example, non-ferrous) that do not tend to absorb carbon. Such flames are also used for welding Monel metal and Nickel and in hard surfacing with high speed steel and cemented carbides. (c) Oxidising Flame. This flame has limited use because the excess oxygen tends to combine with many metals to form hard, brittle, low strength oxides. Also, excess of oxygen causes the weld 334 A Textbook of Production Technology bead and the surrounding area to have a skummy or dirty appearance. So, this flame is not used for welding steel. It is mainly used when welding materials which are not oxidised readily and which have a high solubility of hydrogen in the molten state and low solubility in the solid state, for example, brasses, bronzes and gold. For these metals the oxidising atmosphere creates a base metal oxide that protects the base metal. For example, in welding brass, Zinc has a tendency to separate and fume away. The formation of a covering copper oxide prevents Zinc from dissipating. Lighting up the Flame. For lighting up the torch for welding, the following steps should be followed : 1. Open both cylinder valves slowly. 2. Adjust the pressure regulators to the required working pressures. 3. Open the acetylene gas valve on the blow pipe and adjust the pressure regulating screw until the gauge reads correctly. 4. Close the acetylene gas valve on the blow pipe. 5. Repeat steps 3 and 4 for oxygen. 6. Turn on the acetylene gas valve on the blowpipe and allow the gas to flush the system. 7. Using a friction spark-lighter, ignite the gas. This will produce an acetylene flame. 8. Adjust the blowpipe valve until the flame just stops smoking and releasing soot. 9. Gradually open the oxygen control valve on blowpipe and adjust it until the required flame is obtained. Closing Down Procedure. When closing down : 1. First turn off the acetylene blowpipe control valve. 2. Then turn off the oxygen blowpipe control valve. 3. Close the cylinder supply valves. 4. Purge each hose in turn by opening the control valves on the blow pipe, first for oxygen and then for acetylene. 5. Release the pressure on the two regulators. 6. Finally check that the blow pipe valves are closed. Power of Blowpipe and Nozzles The power of a blowpipe is denoted in various ways : (i) By the bore diameter of the nozzle. The gas consumption and hence the size of the gas flame will depend upon this dimension. (ii) By the amount of gas flow in litres/hour. Acetylene consump- tion varies from 50 to 2800 litres per hour. Oxygen consumption varies from 50 to 3100 litres/hour. (iii) By a number which represents the thickness of the plate that can be welded with this blowpipe. It is thus clear that the power of the blow pipe determines the size of the flame produced and hence the heat available for welding. The power will be influenced by the velocity and the quantity of the gas mixture leaving the nozzle. Both these factors will depend upon the supply pressure of the gases and the size of the orifice of the nozzle. The velocity of the gas mixture leaving the nozzle orifice ranges from 60 to 200 m/s. The lower velocity flame will burn quietly and is called ‘soft flame’. The higher velocity flame burns with a hissing sound. In general, a flame of average velocity will give best results. 5.2.4. Oxy-Acetylene Welding Techniques. A sound welded joint can be obtained by the proper selection of the torch size, filler material, method of moving the torch along the weld and the angle at which the torch is held, as well as proper regulation of the welding flame. The Welding Process 335 As discussed above, the size of torch required will depend upon the thickness and heat conductivity of the metal being welded. Metals with higher conductivity will require a torch head with a larger gas consumption. The joint is first heated with the torch until the welding pool is formed. A welding rod (if used) should be held in the flame so that its end melts at about the same time as the base metal and the filler metal is added to the pool. In OAW, there are two techniques commonly used, called as : Leftward and Rightward techniques. The choice of either technique will depend upon the metal to be welded, its thickness, any test requirements, and total cost. To compare the two techniques, the blow pipe is held in the right hand and the filler rod in the left hand (with a right handed person). The filler rod is carried to the left of the blowpipe. 1. Leftward Welding Technique. In left ward (also called as forward or forehand) welding technique, the torch flame progresses from right to left, (Fig. 5.9 (a)). The angles of blow pipe and filter rod are shown in the figure. The method allows preheating of the plate edges immediately ahead of the molten pool and this is the method more commonly used. The blowpipe is given a very slight side to side movement and with the filler rod is moved progressively along the joint. With the blowpipe movement, the flame moves away from the just welded portion of the joint, which starts loosing heat and cooling starts soon. Due to this, this technique is restricted to welding of mild steel plates upto 5 mm thick, cast iron and non-ferrous metals. 2. Rightward Welding Technique. In the rightward (or back hand or backward) technique, welding commences at the left hand side of the plates and proceeds towards the right, (Fig. 5.9 (b)). The blow- pipe points in the direction of the completed weld with the inner cone of the flame directed towards the bottom of the joint, concentrating the maximum amount of heat into the plates. Due to this, the weld puddle is kept hot for a longer time and a narrow and deeper weld results. Hence, this technique is principally used for welding thick sections (over 5 mm thick). Travel of Travel of Torch Torch 60° – 70° Torch Weld Rod 40° – 50° Gaseous Envelope 30° – 40° 30° – 40° Weld Base Metal Weld Pool (a) Leftward Technique (b) Right ward Technique Fig. 5.9. OAW Techniques. During welding, the blowpipe moves regularly along the weld seam without any lateral movement. On the other hand, the end of the welding wire describes a series of loops instead of moving steadily. Chief Advantages of Rightward Technique 1. The rightward technique is faster by comparison with the leftward technique. This is because, in the left ward method, the view of the joint edge is interrupted and it is necessary to remove the end of the filler rod to inspect the progress. This action slows down the process. Also, the end of the filler rod becomes oxidised resulting in an unfavourable weld structure. 336 A Textbook of Production Technology 2. Plates upto 9 mm thick can be welded with square edge preparation, whereas with the leftward technique, plates over 3mm thick will have to have edges bevelled. For this reason, this technique is often limited to materials upto 5 mm thick. 3. The technique consumes less gas and filler material in comparison to left ward technique. On larger thicknesses, this technique requires no or little edge bevelling and therefore, less filler metal is required resulting in corresponding savings in gas and time. 4. The mechanical properties of the weld are better due to the annealing effect of the flame which is directed on the completed weld. 5. The included angle of edges is smaller in right ward technique as compared to leftward technique. The heat remains confined to the weld seam and there is less spread of flame. Due to this, the amount of distortion in the work is minimum. Note: (i) Back hand welding is faster by 20 to 25% and from 15 to 25% less acetylene is need in comparison to forehand welding. (ii) Horizontal and overhead welding are usually done by the backhand technique. (iii) Vertical joints are welded by the forehand technique. (iv) The angle at which the torch is inclined to the surface being welded depends upon the thicknessof the metal. Thicker metals require a higher concentration of heat and consequently a largertorch angle. 5.2.5. Filler Material and Fluxes. The welding wire or rod used as filler material in gas welding should have a chemical composition similar to that of the base metal. The diameter of the welding rod is selected to suit the thickness of the base metal. The welding rod diameter d can be approximately determined by the following empirical relation : t d  1, mm 2 where t = thickness of base plate, mm Welding Rods (Filler Materials) for Gas Welding Some typical compositions of welding rods for gas welding are given below : Material to be Welded Welding Rod Chemical Composition 1. L–C steels 0.08%C, 0.36% Mn, 0.13% Cr, 0.013% Ni, 0.20%P 2. Mn–Steels 0.14%C, 0.12% Si, 0.81% Mn, 0.25% Ni 3. Cr-Steel 0.24%C, 0.21%Si, 0.42% Mn, 0.96% Cr, 0.17% Ni, 0.35% S These welding rods are available in different diameters ranging from 0.3 to 12 mm. Flux. Except for lead, zinc and some precious metals, gas welding of non-ferrous metals generally requires fluxing. Fluxes are also needed for cast iron and stainless steel. In welding mild steel, the gas flame adequately shields the weld pool and no flux is needed. For C.I., fluxes are required to increase the fluidity of the fusible iron silicate slag, as well as to aid in the removal of slag. Fluxes compose of borates or boric acid, soda ash and small amounts of other compounds such as sodium chloride, ammonium sulphate and iron oxide. Mixture of equal amounts of boric acid and soda ash, 2% aluminium sulphate and 15% powdered iron makes a satisfactory flux. Gas welding fluxes must melt at a lower temperature than the metals being welded so that surface oxides will be dissolved before the metal melts. The Welding Process 337 5.2.6. Advantages and Disadvantages of OAW Advantages 1. The equipment is low cost, versatile, self-sufficient and usually portable. It requires little maintenance, and can be used with equal facility in the field and in the factory. The oxy-acetylene can be used for welding, brazing, soldering, preheating, postheating and metal cutting etc. 2. It can weld most common materials. 3. The gas flame temperature is lower and easily controllable which is necessary for delicate work. Therefore OAW is extensively used for sheet metal fabrication and repairs. 4. The process is well adapted for short production runs. For the above reasons, gas welding is used in : Automotive and aircraft industry, Sheet metal fabrication plants, and in fabrication of industrial pipes. OAW is best suited for : joining thin sheet metal, thin small tube, small pipe and assemblies with poor fit up and for repairing rough arc welds. Disadvantages 1. Oxygen and acetylene gases are expensive. 2. There are safety problems involved in their handling and storing. 3. The flame takes considerably longer for the metal to heat up. Due to this, OAW is not suitable for thick sections. 4. Because the flame is not concentrated, considerable areas of the metal are heated and distortion is likely to occur. 5. Flux applications and shielding provided by OA flame are not so effective as in inert gas arc welding. Metals unsuited for welding with OAW torch are : refractory metals (Columbium, Tantalum, Molybdenum, Tungsten) and the reacting metals such as Titanium and Zirconium. 5.2.7. Other Fuel Gases. As already noted, the other fuel gases which can be used in gas welding are : propane, butane, natural gas or hydrogen. But because of the lower temperature of the flame (about 2500°C) obtained with these gases, these are used particularly for welding lower melting point alloys such as aluminium, zinc, lead and some precious metals. MAPP Gas. Methyl acetylene propadiene gas is replacing acetylene gas particularly when portability is important, because : 1. It is more dense, thus providing more energy for a given volume. 2. It can be stored safely in ordinary pressure tanks. 5.2.8. Pressure Gas Welding (PGW). In this process, the abutting surfaces of the parts being welded are heated by an Oxy- acetylene flame to a state of fusion or plasticity and then coalescence is produced by the application of pressure and without the use of a filler material. The method is widely employed in butt welding bars, pipes, tubes, railroad rails, tools and rings of low and medium carbon steels and of low and medium alloy steels. Two methods are used commercially for PGW : 1. Closed-joint Method. In closed joint pressure gas welding, clean square surfaces are butted together under moderate pressure. The surfaces are then heated by a water cooled oxyacetylene torch, Fig. 5.10, until the correct temperature is attained. Then an additional upsetting pressure is applied to complete the joint. For low carbon steel, the initial pressure is less than 10 MPa and the final upsetting pressure may be in the range of 28 MPa. An oscillating motion to each side is imparted to the torch to ensure more uniform heating of the abutting surfaces of the parts. 338 A Textbook of Production Technology Clamp Clamp Part Part Pressure Annular Torch (Water Cooled) Fig. 5.10. Pressure Gas Welding. 2. Open-joint Method. In this method, the gas flames play directly upon the square weld joint faces, which have been spaced a short distance apart. When the ends of the joint have reached the fusion temperature, they are brought rapidly in contact under pressure to effect welding under upsetting. Both methods are generally used in partially or fully mechanised set ups. 5.2.9. Oxy-Acetylene Flame Cutting. Pure O2 Oxy gas cutting is based upon the ability of Oxy-Acetylene Mixture certain metals to burn in oxygen with the evolution of a great deal of heat thereby melting the metal and forming oxides. The torch for flame cutting is similar to the welding torch, with two exceptions. First, the welding tip contains only one hole in the centre of the tip through which the mixture END VIEW Oxy-Acetylene of C2H2 and O2 gases flow, whereas the flame Mixture cutting tip contains a centre hole through (a) Welding Tip (b) Cutting Tip which pure oxygen, which does the actual ‘‘cutting’’ flows. There are also several con- Fig. 5.11. Welding and Cutting Tip. centric holes around the centre hole through which mixture of C2H2 and O2 flows and the flame produced by its burning preheats the metal, (Fig. 5.11). Second, the cutting torch has an additional, or third, valve for controlling the flow of pure oxygen. Flame cutting is done both manually and with motor driven heads. As the metal is burnt and eroded away, the torch is moved steadily, (Fig. 5.12), along the path of cut. A uniformly wide slot called a ‘kerf’ is cut by the jet of oxygen. The faster the rate of traverse, the more the bottom lags behind the top of the cut. This is known as ‘drag’ and must be kept small. Thicknesses upto 1.5 m can be cut. This method is suitable for cutting only those metals which have lower ignition/oxidation temperatures than their melting points and the melting point of the formed oxides is lower than that of the metal itself. Also, the oxides must have fair fluidity. The heat conductivity of the metal must be low so as to concentrate the heat. Nearly all flame cutting is done on steel (carbon steels with a carbon content upto 0.7% and low alloy steels). Cast iron can not be cut effectively since its melting point (1200°C) is lower than its ignition temperature (1350°C) Also, the graphite oxidizes more readily than the ferrous matrix and it simply melts the matrix. Aluminium can not be cut because of its high thermal conductivity. Stainless steel can not be cut because of its oxidation resistance. High - alloy chro- mium and chrome-nickel steels and non-ferrous alloys can not be cut since the melting point of their oxides is higher than that of the base metals. The Welding Process 339 When iron powder is added to Direction T T of the gas stream (powder oxyfuel cutting O O cut or powder metal cutting), oxidation of R R C C the metal powder provides the heat to H H melt oxidation-resistant materials. Preheating Underwater Cutting. Flames Techniques have been developed for cutting metal underwater in ship building and repair work, construction and repair requirements associated Kerf Slag + Section of Molten Uncut Metal with offshore exploration, drilling and Metal Drag recovery of oil and natural gas. Fig. 5.12. Oxy-Flame Cutting. A specially designed torch is employed for this purpose. An auxiliary skirt surrounds the main tip of the torch. Compressed air is supplied through the passages in the skirt. The compressed air performs two functions : it expels the water away from the tip area and it provides secondary oxygen and thereby stablises the flame. For depths upto 7.5 m, oxy-acetylene torch is used but for greater depths, oxy-hydrogen torch is employed, because at such depths, C2H2 will have to be used at higher pressure to neutralise the high surrounding pressure created by the depth of water. The use of C2H2 at high pressure is very unsafe. But H2 can be compressed to a higher pressure without any danger. The torch is either ignited in the conventional manner before it is taken underwater or is ignited by an electric spark device after it is submerged. 5.3. ELECTRIC ARC WELDING Electric are welding is the most extensively used method of joining components of metallic parts, the source of heat being an electric arc. An electric arc is a continuous stream of electrons flowing through some sort of medium between two conductors of an electric circuit and accompanied by intense heat generation and radiation. An electric arc for welding is obtained in the following ways : 1. Between a consumable electrode (which also supplies filler metal) and the workpiece. 2. Between a non consumable electrode (carbon, graphite or tungsten etc.) and the workpiece. 3. Between two nonconsumable electrodes. The most common electric arc welding method is the one in which the arc is struck between an electrode and the work. This is called as ‘Direct arc’. The arc struck between two non consumable electrodes adjacent to the parts being welded is called as ‘‘Independent or Indirect arc’’. The metal is heated by the indirect action (by radiation) of the arc. Due to this, the thermal efficiency of the method is poor. Thus it is very rarely used these days. The method is called as ‘‘Double or Twin arc method’’. Atomic hydrogen welding process belongs to this category and is used in a few special applications. To strike an arc, the electrode is brought in contact with the work at the point where the welding is to be started, after connecting the work to the welding circuit. After a light contact, the electrode is immediately withdrawn to a distance of from 2 to 4 mm from the work. Only a comparatively low potential difference is required between the electrode and the work to strike an arc. From 40 to 45 V is usually sufficient for D.C. and from 50 to 60V for A.C. This voltage available at the output terminals of a welding set, before the arc is struck, is known as open circuit voltage (OCV). The voltage falls after the arc is established which is normally less than half the OCV. A stable arc can be maintained between a metal electrode and the work metal with a voltage 340 A Textbook of Production Technology of 15 to 30V while from 30 to 35V is needed to strike an arc between non consumable electrode and the work. The stable arc required for high quality welding can be achieved with an arc length equal to 0.6 to 0.8 of the electrode diameter. The arc length is defined as the distance between the end of the electrode and the surface of the molten metal on the work. When the electrode first makes contact with the job, a large short circuit current flows. When the electrode later is immediately withdrawn, the current continues to flow in the form of spark across the air gap so formed. Due to this, the air gap gets ionized, that is, splits into electrons and positive ions. The lighter electrons flow from cathode to anode and the heavier positive ions flow from anode to cathode. Thus, the air gap becomes conducting and current is able to flow across the gap in the form of an arc. When the lighter, high-velocity electrons strike the anode at great velocity, intense heat is generated at the anode. Heat generated at the cathode is much less, because of the low velocity of impinging positive ions. Thermal and luminous energy is not uniformly evolved in the welding arc. About 43 percent of the total amount of heat is evolved on the anode and about 36 percent on the cathode. The remaining 21 per cent is evolved by the arc. The temperature of an electric arc depends upon the type of electrodes between which it is struck. It is about 3200°C on the cathode and about 3900°C on the anode for carbon electrodes and 2400°C and 2600°C respectively for metal electrodes. The temperature may reach 6000 to 7000°C in the centre of the arc. Welding Lead 150 to (Insulated) 1000 A Electrode Power A Holder Source 20 to D.C. V