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1. UNIT – I II HEATING AND WELDING Explain different types of heating and welding process. C VENKATESH KUMAR

2. UNIT III - HEATING AND WELDING Introduction - advantages of electric heating – modes of heat transfer - methods of electric heating -resistance heating - arc furnaces - induction heating - dielectric heating - electric welding – types -resistance welding - arc welding - power supply for arc welding - radiation welding. C VENKATESH KUMAR

3. Domestic and industrial applications of electric heating. Domestic applications include : Room Heaters Immersion Heaters For Water Heating Hot Plates For Cooking Electric Kettles Electric Irons Pop-corn Plants Electric Ovens For Bakeries Electric toasters etc. Industrial applications of electric heating include: Melting Of Metals heat treatment of metals like annealing, tempering, soldering and brazing etc. Moulding Of Glass Baking of insulators Enamelling of copper wires etc. C VENKATESH KUMAR

4. Advantages of electric heating This system is most clean system of heating. This is free from dirt. This electric heating process does not produce any flue gas. This is much controlled method of heating. Initial and running costs of electric furnaces are much lower than other types of furnaces. Automatic protection schemes for over loading and over current can easily be provided in this system with help of electrical switchgear system. Th e o v e r a l l e f fic i e n cy o f e l ect r i c h e a ting s y s t em i s much more than other systems of heating. There is no upper limit of producing temperature. Electric heating is quite safe because it responds quickly to the controlled signals. C VENKATESH KUMAR

5. Modes / Methods of Heat Transfer Conduction In this mode of heat transfer, one molecule of the body gets heated and transfers some of the heat to the adjacent molecule and so on. There is a temperature gradient between the two ends of the body being heated. Convection In this process, heat is transferred by the flow of hot and cold air. This process is applied in the heating of water by immersion heater or heating of buildings. Radiation It is the transfer of heat from a hot body to a cold body in a straight line without affecting the intervening medium. C VENKATESH KUMAR

6. C VENKATESH KUMAR

7. PARTICULAR CONDUCTION CONVECTION RADIATION Meaning Conduction is a process in which transfer of heat takes place between objects by direct contact. Convection refers to the form of heat transfer in which energy transition occurs within the fluid. Radiation is the mechanism in which heat is transmitted without any physical contact between objects. Represent How heat travels between objects in direct contact. How heat passes through fluids. How heat flows through empty spaces. Cause Due to tem p eratu r e difference. Due to density difference. Occurs from all objects, at temperature greater than K. Differentiate between the methods of heat transfer C VENKATESH KUMAR

8. Occurrence Occurs in solids, through molecular c o l l i s i o ns. Occurs in fluids, by actual flow of matter. Occurs at a distance and does not heat the intervening substance. Transfer of heat Uses heated solid substance. Uses intermedia t e substance. Uses el e ctromag n et i c waves. Speed Slow Slow Fast Law of reflection and refraction Does not follow Does not follow Follow C VENKATESH KUMAR

9. Classification of electrical heating. Power Frequency Method: Resistance heating Direct resistance heating, Indirect Resistance Heating, Arc heating Direct Arc Heating Indirect arc heating. High Frequency Heating: Induction heating and Core type Induction heating Coreless type Induction heating Dielectric Heating C VENKATESH KUMAR

10. Resistance heating . I t is based o n the I 2 R e f f ec t . When cur r e n t is passe d t h r oug h a r es i s t ance eleme n t I 2 R l os s t ak es plac e whi c h produces heat. There are two methods of resistance heating. Direct methods of resistance heating . Indirect Resistance Heating. C VENKATESH KUMAR

11. Direct methods of resistance heating . C VENKATESH KUMAR

12. In this method the material (or charge) to be heated is treated as a resistance and current is passed through it. The charge may be in the form of powder, small solid pieces or liquid. The two electrodes are inserted in the charge and connected to either a.c. or d.c. supply . Two electrodes will be required in the case of d.c. or single-phase a.c. supply but there would be three electrodes in the case of 3-phase supply. When the charge is in the form of small pieces, a powder of high resistivity material is sprinkled over the surface of the charge to avoid direct short circuit. Heat is produced when current passes through it. This method of heating has high efficiency because the heat is produced in the charge itself. C VENKATESH KUMAR

13. Applications of direct heating This method of heating is used in Resistance welding The electrode boiler for heating water t o o ls 3. Salt bath furnace which is used for hardening steel and prevents oxidation C VENKATESH KUMAR

14. Indirect Resistance Heating. C VENKATESH KUMAR

15. In this method of heating , electric current is passed through a resistance element which is placed in an electric furnace. Heat produced is proportional to I 2 R losses in the heating element. The heat so produced is delivered to the charge either by radiation or convection or by a combination of the two. Resistance is placed in a cylinder which is surrounded by the charge placed in the jacket as shown in the Fig. This arrangement provides uniform temperature. Moreover, automatic temperature control can also be provided. C VENKATESH KUMAR

16. Applications of indirect heating This method of heating is used in Room heaters Water heater i.e. immersion heater Ovens like domestic cooking C VENKATESH KUMAR

17. Requirement of good heating element. High-specific resistance so that small length of wire may be required to provide given amount of heat. High-melting point so that it can withstand for high temperature, a small increase in temperature will not destroy the element. Low temperature coefficient of resistance For accurate temperature control, the variation of resistance with the operating temperature should be very low. This can be obtained only if the material has low temperature coefficient of resistance Free from oxidation The formation of oxidized layers will shorten its life. High-mechanical strength Should withstand for mechanical vibrations. Non-corrosive The element should not corrode when exposed to atmosphere or any other chemical fumes. Economical The cost of material should not be so high C VENKATESH KUMAR

18. Materials used for heating element C VENKATESH KUMAR

19. causes for failure of heating elements Formation of Hot Spot Hot spots are the points in the heating element which are formed at higher temperature. Contamination and Corrosion Oil fumes caused by heat treatment of components contaminated with lubricant contaminate the elements and produce dry corrosion. Oxidation of the element and intermittency ofoperation. Vibration Break Excessive vibration may cause the heating element to break, resulting to failure C VENKATESH KUMAR

20. Temperature control methods of resistance furnace Th e t empe r a tu r e of a r es i s t ance fur n a c e c an be changed by controlling the I 2 R or V 2 /R losses . Intermittent Switching. By Changing the Number of Heating Elements Variation in Circuit Configuration. Change of Applied Voltage. Lesser the magnitude of the voltage applied to the load. Bucking-Boosting the Secondary Voltage Autotransformer Control. Series Reactor Voltage. C VENKATESH KUMAR

21. Intermittent Switching. In this case, the furnace voltage is switched ON and OFF intermittently. Hence, by this simple method, the furnace temperature can be limited between two limits. By Changing the Number of Heating Elements. In this case, the number of heating elements is changed without cutting off the supply to the entire furnace. Smaller the number of heating elements, lesser the heat produced . In the case of a 3-phase circuit, equal number of heating elements is switched off from each phase in order to maintain a balanced load condition. C VENKATESH KUMAR

22. (3) Variation in Circuit Configuration. In the case of 3-phase secondary load, the heating elements give less heat when connected in a star than when connected in delta because in the two cases, voltages across the elements is different (Fig. 1). In single-phase circuits, series and parallel grouping of the heating elements causes change in power dissipation resulting in change of furnace temperature. C VENKATESH KUMAR

23. As shown in Fig.1 heat produced is more when all these elements are connected in parallel than when they are connected in series or series-parallel. C VENKATESH KUMAR

24. Change of Applied Voltage. Lesser the magnitude of the voltage applied to the load, lesser the power dissipated and hence, lesser the temperature produced. In the case of a furnace transformer having high voltage primary, the tapping control is kept in the primary winding because the magnitude of the primary current is less. Consider the multi-tap step-down transformer shown in Fig. C VENKATESH KUMAR

25. Autotransformer Control. Fig. shows the use of tapped autotransformer used for decreasing the furnace voltage and, hence, temperature of small electric furnaces. The required voltage can be selected with the help of a voltage selector . C VENKATESH KUMAR

26. Arc Furnace If a sufficiently high voltage is applied across an air-gap, the air becomes ionized and starts conducting in the form of a continuous spark or arc thereby producing intense heat. When electrodes are made of carbon/graphite, the temperature obtained is in the range of 3000°C- 3500°C. The high voltage required for striking the arc can be obtained by using a step-up transformer fed from a variable a.c. supply as shown in C VENKATESH KUMAR

27. Fig. a Fig.b C VENKATESH KUMAR

28. Fig. (a).An arc can also be obtained by using low voltage across two electrodes initially in contact with each other as shown in Fig. (b). The low voltage required for this purpose can be obtained by using a step-down transformer. Initially, the low voltage is applied, when the two electrodes are in contact with each other. Next, when the two electrodes are gradually separated from each other, an arc is established between the two. C VENKATESH KUMAR

29. Direct arc furnace C VENKATESH KUMAR

30. I n this c ase, a r c i s f orme d b e t w e e n the t w o electrodes and the charge in such a way that electric current passes through the body of the charge as shown in Fig. Such furnaces produce very high temperatures. It could be either of conducting-bottom type Fig. (a) or non-conducting bottom type Fig. (b) . As seen from Fig. (a), bottom of the furnace forms part of the electric circuit so that current passes through the body of the charge which offers very low resistance. Hence, it is possible to obtain high temperatures in such furnaces. Moreover, it produces uniform heating of charge without stirring it mechanically. In Fig. (b), no current passes through the body of the furnace. C VENKATESH KUMAR

31. Applications Thes e furnace s i s i n the p r oductio n of s t eel c ompositio n of the fin a l p r od u ct c an be c ause of t h e ea s e with whi c h the be controlled during refining. Most of the furnaces in general use are of non-conducting bottom type due to insulation problem faced in case of conducting bottom. C VENKATESH KUMAR

32. Indirect Arc Furnace C VENKATESH KUMAR

33. Fig. shows a single-phase indirect arc furnace which is cylindrical in shape. The arc is struck by short circuiting the electrodes manually or automatically for a moment and then , withdrawing them apart. The heat from the arc and the hot refractory lining is transferred to the top layer of the charge by radiation. The heat from the hot top layer of the charge is further transferred to other parts of the charge by conduction. C VENKATESH KUMAR

34. Since no current passes through the body of the charge, there is no inherent stirring action due to electro-magnetic forces set up by the current. Hence, such furnaces have to be rocked continuously in order to distribute heat uniformly by exposing different layers of the charge to the heat of the arc. Application : Such furnaces are mainly used for melting nonferrous metals although they can be used in iron foundries where small quantities of iron are required frequently. C VENKATESH KUMAR

35. Induction Heating This heating process makes use of the currents induced by the electro-magnetic action in the charge to be heated. In fact, induction heating is based on the principle of transformer working. The primary winding which is supplied from an a.c. source is magnetically coupled to the charge which acts as a short circuited secondary of single turn. heat produced = V 2 /R. The value of current induced in the charge depends on- (i ) Magnitude Of The Primary Current Turn ratio of the transformer. Co-efficient of magnetic coupling. C VENKATESH KUMAR

36. Types of Induction Heating (a) Core-type Furnaces - which operate just like a two winding transformer. These can be further sub-divided into Direct core-type furnaces Vertical core-type furnaces and Indirect core-type furnaces. (b) Coreless-type Furnaces - I n which an inductively- heated element is made to transfer heat to the charge by radiation . C VENKATESH KUMAR

37. Core type induction furnace. C VENKATESH KUMAR

38. It is shown in Fig.. and is essentially a transformer in which the charge to be heated forms a single-turn short-circuited secondary and is magnetically coupled to the primary by an iron core. The furnace consists of a circular hearth which contains the charge to be melted in the form of an annular ring. When there is no molten metal in the ring, the secondary becomes open-circuited there-by cutting off the secondary current. Hence, to start the furnace, molted metal has to be poured in the annular hearth. Since, magnetic coupling between the primary and secondary is very poor, it results in high leakage and low power factor. In order to nullify the effect of increased leakage reactance, low primary frequency of the order of 10 Hz is used. C VENKATESH KUMAR

39. Advantages of Direct Core Type Induction Furnace Rapid melting. Accurate control of the temperature. Clean heating. The furnace inherently has stirring action of the molten material and this result in uniform end material. C VENKATESH KUMAR

40. Disadvantages Of Direct Core Type Induction Furnace Pinch effect – at high current densities the current flowing through the melt will interact with magnetic field of the core. The furnace cannot be started with a solid material as it may open circuit the The furnace is not suitable for intermittent operation. C VENKATESH KUMAR

41. Applications Used in foundries for melting and refining brass, zinc and other non-ferrous metals Used for heat treatment of metals C VENKATESH KUMAR

42. Coreless type induction furnace. C VENKATESH KUMAR

43. As shown in Fig., the three main parts of the furnace are (i) primary coil (ii) a ceramic crucible containing charge which forms the secondary and (iii) the frame which includes supports and tilting mechanism. The distinctive feature of this furnace is that it contains no heavy iron core with the result that there is no continuous path for the magnetic flux. C VENKATESH KUMAR

44. Th e cru c i b le construction and the c oi l a r e r e l a t i v e l y l i g h t in and c an b e c o n v en i e n tly ti l t e d f or pouring. The charge is put into the crucible and primary winding is connected to a high-frequency a.c. supply. The flux produce by the primary sets up eddy- currents in the charge and heats it up to the melting point. The charge need not be in the molten state at the start as was required by core-type furnaces. The eddy- currents also set up electromotive forces which produce stirring action which is essential for obtaining uniforms quality of metal. Since flux density is low (due to the absence of the magnetic core) high frequency supply has to be used because eddy- current loss W e ∝ B 2 f 2 . C VENKATESH KUMAR

45. Since magnetic coupling between the primary and secondary windings is low, the furnace p.f. lies between 0.1 and 0.3. Hence, static capacitors are invariably used in parallel with the furnace to improve its p.f. C VENKATESH KUMAR

46. Advantages of coreless induction furnaces are as follows : 1. High speed of heating Well suited for intermittent operation High quality of product Low operating cost They produce most uniform quality of product. Thei r ope r a t i o n i s f r ee f r o m smo k e, d i rt, d u s t a nd noises. 7 . T h e y c an b e used f o r all in d u s tr i al app l i c a tions requiring heating and melting. They have low erection and operating costs. Their charging and pouring is simple. C VENKATESH KUMAR

47. Appli c a tions. These are used for steel production These are used for melting of non-ferrous metals like brass , copper, aluminium along with various alloys of these elements The production of carbon from ferrous alloys C VENKATESH KUMAR

48. High frequency eddy current heating. For heating an article by eddy-currents, it is placed inside a high frequency a.c. current-carrying coil. it is the eddy-current loss which is responsible for the production of heat through hysteresis loss also contributes to some extent in the case of non-magnetic materials. The eddy-current loss We ∝ B 2 ƒ 2 . Hence, this loss can be controlled by controlling flux density B and the supply frequency ƒ. This loss is greatest on the surface of the material but decreases as we go deep inside. C VENKATESH KUMAR

49. The depth of the material upto which the eddy-current loss penetrates is given by- C VENKATESH KUMAR

50. Advantages of Eddy-current Heating There is negligible wastage of heat because the heat is produced in the body to be heated. It can take place in vacuum or other special environs where other types of heating are not possible. Heat can be made to penetrate any depth of the body by selecting proper supply frequently. C VENKATESH KUMAR

51. Applications of eddy current heating. Surface Hardening. The bar whose surface is to be hardened by heat treatment is placed within the working coil which is connected to an a.c. supply of high frequency. Annealing. Normally, annealing process takes long time resulting in scaling of the metal which is undesirable. However, in eddy-current heating, time taken is much less so that no scale formation takes place. Soldering. Eddy-current heating is economical for precise high-temperature soldering where silver, copper and their alloys are used as solders. C VENKATESH KUMAR

52. Di-electric heating It is also called high-frequency capacitative heating and is used for heating insulators like wood, plastics and ceramics etc. which cannot be heated easily and uniformly by other methods. The supply frequency required for dielectric heating is between 10-50 MHz and the applied voltage is upto 20 kV. The overall efficiency of dielectric heating is about 50%. C VENKATESH KUMAR

53. When a practical capacitor is connected across an a.c. supply, it draws a current which leads the voltage by an angle φ, which is a little less than 90° or falls short of 90° by an angle δ. It means that there is a certain component of the current which is in phase with the voltage and hence produces some loss called dielectric loss. At the normal supply frequency of 50 Hz, this loss is negligibly small but at higher frequencies of 50 MHz or so, this loss becomes so large that it is sufficient to heat the dielectric in which it takes place. The insulating material to be heated is placed between two conducting plates in order to form a parallel-plate capacitor as shown in Fig C VENKATESH KUMAR

54. Advantages of Dielectric Heating Since heat is generated within the dielectric medium itself, it results in uniform heating. Heating becomes faster with increasing frequency. It is the only method for heating bad conductors of heat. Heating is fastest in this method of heating. 5 . Sinc e n o na k ed f l ame app e ar s in inflammable articles like plastics the p r ocess, and w ooden products etc., can be heated safely. 6.Heating can be stopped immediately as and when desired. C VENKATESH KUMAR

55. Principle of microwave heating . Microwave heating is a multi-physics phenomenon that involves electromagnetic waves and heat transfer; any material that is exposed to electromagnetic radiation will be heated up. The rapidly varying electric and magnetic fields lead to four sources of heating. Any electric field applied to a conductive material will cause current to flow. In addition, a time-varying electric field will cause dipolar molecules, such as water, to oscillate back and forth. A time-varying magnetic field applied to a conductive material will also induce current flow. There can also be hysteresis losses in certain types of magnetic materials. C VENKATESH KUMAR

56. Applications of Microwave Heating Heating Food One obvious example of microwave heating is in a microwave oven. When you place food in a microwave oven and press the "start" button, electromagnetic waves oscillate within the oven at a frequency of 2.45 GHz. These fields interact with the food, leading to heat generation and a rise in temperature. Treating Cancer Another application that leverages the effects of microwave heating is cancer treatment, in particular hyperthermic oncology. This type of cancer therapy involves subjecting tumor tissue to localized heating, without damaging the healthy tissue around it. C VENKATESH KUMAR

57. Definition of Welding It is the process of joining two pieces of metal or non-metal at faces rendered plastic or liquid by the application of heat or pressure or both. Filler material may be used to effect the union. C VENKATESH KUMAR

58. Welding Processes All welding processes fall into two distinct categories : 1. Fusion Welding- It involves melting of the parent metal. Examples are: Carbon arc welding, metal arc welding, electron beam welding, electro slag welding and electro-gas welding which utilize electric energy and Gas welding and thermit welding which utilize chemical energy for the melting purpose. 2. Non-fusion Welding- It does not involve melting of the parent metal. Examples are: Forge welding and gas non-fusion welding which use chemical energy. E xplo s i v e w e l ding , fr i ction w e l din g a n d ult r asonic w e l din g e t c . , w hic h use mechanical energy. Resistance welding which uses electrical energy. Proper selection of the welding process depends on the (a) kind of metals to be joined (b) cost involved (c) nature of products to be fabricated and (d) production techniques adopted C VENKATESH KUMAR

59. Types of electric welding. 1. Resistance welding Seam welding Projection welding Flash welding. spot welding 2. Arc welding Carbon arc welding Metal arc welding Atomic hydrogen arc welding Inert gas metal arc welding Submerged arc welding. C VENKATESH KUMAR

60. Spot Welding C VENKATESH KUMAR

61. C VENKATESH KUMAR

62. The process depends on two factors : Resistance heating of small portions of the two work pieces to plastic state and App l i c a tion o f f o r gin g p r essu r e f or w e l d i n g the two work pieces. Heat produced is H = I 2 Rt/J. The resistance R is made up of (i) resistance of the electrodes and metals themselves (ii) contact resistance between electrodes and work pieces and (iii) contact resistance between the two work pieces. Generally ,contact resistance between the two work pieces is the greatest. C VENKATESH KUMAR

63. As shown in Fig (b), mechanical pressure is applied by the tips of the two electrodes. In fact, these electrodes not only provide the forging pressure but also carry the welding current and concentrate the welding heat on the weld spot directly below them. Fig. (a) shows diagrammatically the basic parts of a modern spot welding. It consists of a step-down transformer which can supply huge currents (upto 5,000 A) for short duration of time. The metals under the pressure zone get heated upto about 950°C and fuse together C VENKATESH KUMAR

64. Application: Spot welding is used for galvanized, tinned and lead coated sheets and mild steel sheet work. This technique is also applied to non- ferrous materials such as brass, aluminium, nickel and bronze etc. C VENKATESH KUMAR

65. Seam Welding C VENKATESH KUMAR

66. The seam welder differs from ordinary spot welder only in respect of its electrodes which are of disc or roller shape as shown in Fig. (a). These copper wheels are power driven and rotate whilst gripping the work. The current is so applied through the wheels that the weld spots either overlap as in Fig. (b) or are made at regular intervals as in Fig. (c). The continuous or overlapped seam weld is also called stitch weld whereas the other is called roll weld. Seam welding is confined to welding of thin materials ranging in thickness from 2 mm to 5 mm. It is also restricted to metals having low harden-ability rating such as hot-rolled grades of low-alloy steels. Stitch welding is commonly used for long water-tight and gas- tight joints. Roll welding is used for simple joints which are not water-tight or gas-tight. Seam welds are usually tested by pillow test. C VENKATESH KUMAR

67. AC arc welding machine (welding transformer). C VENKATESH KUMAR

68. C VENKATESH KUMAR

69. Welding is never done directly from the supply mains. Instead, special welding machines are used which provided currents of various characteristics. Use of such machines is essential for the following reasons : To reduce the high supply voltage to a safer and suitable voltage for welding purposes. To provide high current necessary for arc welding without drawing a corresponding high current from the supply mains. To provide suitable voltage/current relationships necessary for arc welding at minimum. As shown in Fig. it consists of a step-down transformer with a tapped secondary having an adjustable reactor in series with it for obtaining drooping V/I characteristics. The secondary is tapped to give different voltage/ current settings. C VENKATESH KUMAR

70. Advantages of ARC welding Low initial cost Low operation and maintenance cost Low wear No arc blow Disadvantages ARC welding . its polarity cannot be changed it is not suitable for welding of cast iron and non-ferrous metals. C VENKATESH KUMAR

71. Laser( light amplification by stimulated emission of radiation. ) welding C VENKATESH KUMAR

72. It uses an extremely concentrated beam of coherent monochromatic light i.e. light of only one colour (or wavelength). It concentrates tremendous amount of energy on a very small area of the workpiece to produce fusion. It uses solid laser (ruby, saphire), gas laser (CO2) and semiconductor laser. Both the gas laser and solid laser need capacitor storage to store energy for later injection into the flash tube which produces the required laser beam. C VENKATESH KUMAR

73. The gas laser welding equipment consists of capacitor bank for energy storage (ii) a triggering device (iii) a flash tube that is wrapped with wire (iv) lasing material (v) focusing lens and (vi) a worktable that can rotate in the three X, Y and Z directions. When triggered, the capacitor bank supplies electrical energy to the flash tube through the wire. This energy is then converted into short- duration beam of laser light which is pin- pointed on the work-piece as shown in Fig.. Fusion takes place immediately and weld is completed fast. C VENKATESH KUMAR

74. Since duration of laser weld beam is very short (2 ms or so), two basic welding methods have been adopted. In the first method, the work piece is moved so fast that the entire joint is welded in a single burst of the light. The other method uses a number of pulses one after the other to form the weld joint similar to that formed in electric resistance seam welding. C VENKATESH KUMAR

75. Advantages of laser welding It produces high weld quality. it can be easily automated with robotic machinery for large volume production. No electrode is required. No tool wears because it is a non-contact process. The time taken for welding thick section is reduced. It is capable of welding in those areas which is not easily accessible. It has the ability to weld metals with dissimilar physical properties. It can be weld through air and no vacuum is required. It can be focused on small areas for welding. This is because of its narrower beam of high energy. Wide variety of materials can be welded by using laser beam welding. C VENKATESH KUMAR

76. Disadvantages of laser welding. Rapid cooling rate may cause cracking in some metals High capital cost for equipment Optical surfaces of the laser are easily damaged High maintenance costs Energy conversion efficiency is too low, usually below 10%. laser welding machine is expensive. Max welding thickness is 19mm, and it isn’t suitable for production line. Applications of laser welding L a s e r w el di n g is us e d in th e A u t o m ot i ve, air c r a ft an d el e c t ro n i c industries for lighter gauge metals C VENKATESH KUMAR