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1. UNIT – II WELDING Welding : Welding is the process of joining two similar or dissimilar metals by the application of heat with or without application of filler metal and with or without application of pressure. Methods of welding 1. Plastic or Pressure welding 2. Fusion or non pressure welding Plastic welding : In this method of welding the pieces of metal to be joined are heated to plastic state and then forced together by external pressure. The various welding processes classified under this group are : Forge welding, resistance and thermit welding etc. Fusion Welding : Fusion welding is the process of joining two pieces of metal by the application of heat with the addition of filler metal, until they melt and solidify on cooling. This includes Metal arc welding, gas welding etc.

2. CLASSIFICATION OF WELDING :- They may be classified on the basis of sources of heat and type of interaction between two materials. 1. Gas welding: i Oxy acetylene welding ii Oxy hydrogen welding 2. Arc welding: i Carbon arc welding ii Metal arc welding iii TIG welding (GTAW) welding iv MIG (GMAW) welding v Sub-merged arc welding 3. Solid state welding: i Cold pressure welding ii Friction welding iii Diffusion welding iv Forge welding v Ultra sonic welding

3. 4. Resistance welding i Spot welding ii Projection welding iii Flash butt welding iv Seam welding v Percussion welding 5. Special welding Process i . Plasma arc welding ii. Electro slag welding iii. Stud arc welding iv. Explosive welding 6. Thermo-chemical welding processes: i . Thermit welding ii. Atomic hydrogen welding 7. Radiant energy welding processes: i . Electron beam welding ii. Laser beam welding  

4. Welded Joints :   The type of joint is determined by the relative positions of the two pieces which are being joined together. The most common types of joints are : a) butt Joint b) Lap joint c) T- Joint d) corner Joint and e) Edge joint.

5. Lap Joint : Lap joint is used to join two over lapping plates so that the edges of each plate is welded to the surface of the other. Single lap or double lap joints are commonly used. Butt Joints : Butt joints is used to join each of edges of two plates or surfaces located approximately in the same plane with each other. T- Joints : T- Joint is used to join two plates or sections whose surfaces are at right angles to each other. Corner Joint : Corner joint is used to join the edges of two sheets or plates whose surfaces are at approximately 90 to each other. Edge Joint : Edge joint is used to join two parallel plates. This joint is often used in sheet metal work.

6. Square Butt weld or Straight weld : Square butt welds may be used for thickness of from 3 to 5 mm. Before welding, the edges are spaced about 3 mm apart. A square butt weld is a weld in the preparation of which the fusion faces lie approximately at right angles to the surfaces of the components to be joined and are substantially parallel to one another Single V Butt weld :   Single V butt welds are frequently used for metals over 8 mm and upto about 16 mm thick. The edges forming the joint are beveled to form an included angle of 70 to 90 depending upon the welding technique to be used. A single V butt weld is a weld in the preparation of which the edges of both components are beveled so that in the cross section, the fusion faces form a V. Double V butt weld : Double V butt welds are used for metals over 16 mm thick and where welding can be performed on both sides of the plate. A double V butt weld is a weld in the preparation of which the edges of both components are double beveled so that in the cross section, the fusion faces form two opposing V’s.

7. Single U butt joint :   Single U butt welds are frequently used for sheets about 5 mm to 15 mm thickness. A single U butt weld is a weld in the preparation of which the edges of both components are prepared so that in the cross section, the fusion faces form a U. Double U butt weld : The Double U butt welds are used for welding plates over 15 mm thickness. Double U preparation is preferred for very thick sheets due to requirement of lesser filler metals but preparation of U is costlier (difficult). A double U butt weld is a weld in the preparation of which the edges of both components are prepared so that in the cross section, the fusion faces form two opposing U’s. Single J Butt Weld Single J butt welds are used for welding plates about 9 mm to 37 mm thickness. A single J butt weld is a weld in the preparation of which the edges of one component are prepared so that in the cross section, the fusion faces is in the form of a J and the fusion face of the other component is at right angle to the surface of the first component.

8. Double J Butt Weld The Double J butt welds are used for welding thickness plates should be of minimum 18 mm. A double J butt weld is a weld in the preparation of which the edges of one component are prepared so that in the cross section, the fusion faces is in the form of two opposing J’s and the fusion face of the other component is at right angle to the surface of the first component. Single Bevel Butt weld Single bevel butt welds are used for welding plates about 6 mm to 18 mm thickness. A single bevel butt weld is a weld in the preparation of which the edges of one component is beveled and the fusion face of the other component is at right angles to the surface of the firs component.   Double Bevel Butt weld The Double bevel butt welds are used for welding thickness plates should be of minimum 12 mm. A double bevel butt weld is a weld in the preparation of which the edges of one component are double beveled and the fusion face of the other component is at right angles to the surface of the firs component

9. Flanged Joint :  For materials upto about 3 mm the edges may be flanged and fused together without the need of separate filler metal. The gap between plates should be kept an optimum value since penetration will be in sufficient with small gap and with wide gap the molten weld pool will fall through and welding will be difficult to control. Another important point in welding is to overcome the oxidation at high temperature. In gas welding the flux is used from outside in the form of powder where as in arc welding the flux is coated on the electrodes. The flux forms a protective coating of slag over the weld metal and creates a non oxidizing atmosphere.

10. Gas Welding :   Gas welding is a fusion welding process . It is done by melting the edges or surfaces to be joined by the suitable gas flame and then allowing the molten metal to flow together, thus forming an inseparable joint on cooling with or without the addition of filler metal. Normally filler metal is added by inserting it into the molten puddle of the base metals and the puddle then solidifies making the weld bead. The following are the common gas mixtures available for gas welding. i ) Oxy-acetylene ------ 3200 C ii) Oxy- Hydrogen 1900 C iii) Oxy- Methane ----- 2000 C iv) Oxygen – Water 2300 C Among the above gas mixtures oxy acetylene mixture is the one which is widely used, because  1. It has comparatively high flame temperature.  2. It can be conveniently stored in separate steel cylinder.

11. Oxy – Acetylene Gas Welding Equipment :   Oxygen cylinder ; It is used to store the compressed oxygen gas. This cylinder is made of steel and it is in black colour . For safety purposes oxygen cylinders are fitted at a pressure 12500 to 14000 KN/m 2 Acetylene cylinder : It is used to store the Acetylene gas. This cylinder is made of steel and it is in maroon colour . The cylinder is usually fitted to pressure of 1600 to 2100 KN/m 2 . Blow Pipe : It is also known as welding torch. It is used for mixing oxygen and acetylene gases in the desired volume and thus supply the gas mixture to a nozzle connected to its end. Nozzle : It is a device screwed to the end of the blow pipe. It is used to permit the flow of oxygen and acetylene gas mixture from the mixing chamber of the blow pipe to the end called tip of the nozzle to facilitate burning. Pressure Regulator : It is located on the top of the gas cylinder. Its function is to reduce the pressure of the gas inside the cylinder to a pressure suitable for welding. The pressure regulator located on the oxygen cylinder is called oxygen pressure regulator and the one located on the top of the acetylene cylinder is called the acetylene pressure regulator. Pressure Gauges : Each gas cylinder is provided with two pressure gauges, one for indicating the pressure of the gas inside the cylinder and the other for indicating the pressure of the gas supplied to the blow pipe .

12. Hose and Hose fitting : The hose is a rubber tube which permits the flow of gas. It connects the outlet of the pressure regulator and the blow pipe. The hose should be strong, durable, flexible, non porous and light in weight. The hose for the supply of oxygen is green colour and that for the supply of acetylene gas is red colour . Special hose fitting are provided for attachments to the blow pipe and the pressure regulator. Goggles : Goggles fitted with coloured lenses should be provided to protect the eyes from harmful heat and ultra violet and infrared rays. Gloves : It is used to protect the hands from heat and the metal splashes during welding.   Spark lighter : It is used to provide a convenient and instant means for lighting the blow pipe.   Chipping Hammer : It is used to remove the metal oxides and slag deposited if any, from the weld bead. Wire Brush: It function is to clean the surfaces of joints before and after welding.

13. Oxy – Acetylene Welding :   When acetylene gas (C 2 H 2 ), which is produced by the chemical reaction between water and calcium carbide (caC 2 ) is mixed with oxygen in correct propositions in the welding torch and ignited, the acetylene burns to produce a flame which is sufficiently hot to melt and join the base metal. This flame is known as oxy-acetylene. CaC 2 + 2H 2 O C 2 H 2 + Ca (OH) 2

14. Types of flames : The three types of flames obtainable in oxyacetylene welding are : i . Oxidizing flame ( Excess of oxygen) ii. Neutral flame (Acetylene and oxygen in equal proportions) iii. Carburizing flame ( Excess of acetylene) or reducing flame Flame is the most important tool in oxyacetylene welding. To produce satisfactory welds, the correct type of flame i.e., proper size and shape should be chosen. i . Oxidizing flame (Excess of oxygen) : Oxidizing flame is obtained when there is an excess of oxygen than that required for the chemical reaction. The temperature of the oxidizing flame is of the order of about 3482 C. This is the hottest flame that can be produced by any oxygen fuel mixture. The presence of excess oxygen stored at a pressure 13.8 MPa. To 18.2 MPa. Free acetylene is highly explosive is stored at a pressure more than 200 KPa . (O 2 : C 2 H 2 = 1 : 1.15 to 1.5)

15. The oxidizing flame consists of small inner cone which is shorter, pointed and much bluer in colour . The outer blue envelope is also much shorter than the other two flames. The oxidizing flame injects oxygen into the molten metal of the weld puddle causing the metal to oxidize or burn quickly. Also the excess oxygen causes the weld bead to have a dirty appearance. Hence oxidizing flame can’t be used in most applications of oxyacetylene welding. But a slightly oxidizing flame is advantageous in welding copper base metals, zinc base metals and few ferrous metals such as manganese steels and cast irons. It is not used in welding steel. Excess oxygen combines with many metals to form hard, brittle, low strength oxides. 2. Neutral flame (Acetylene and oxygen in equal proportions) : Neutral flame is obtained when approximately equal volumes of oxygen and acetylene are supplied. The temperature of the neutral flame is of the order of about 3260 C. (O 2 : C 2 H 2 = 1 : 1)

16. The neutral flame consists of inner cone which is slightly larger than in case of oxidizing flame. The outer blue envelope of neutral flame is also larger than in case of oxidizing flame. Neutral flame does not cause any chemical change by not oxidizing or carburizing the metal and produce weld bead with clean appearance. The neutral flame is commonly used for welding most of the metals such as mild steels, stainless steels, cast iron, copper and aluminium . 3. Carburizing flame (Excess of acetylene) : Reducing flame is obtained when there is an excess of acetylene than that required for chemical reaction. The temperature of the reducing flame is of the order of about 3038 C. (O 2 : C 2 H 2 = 0.85 : 0.95 ) The reducing flame is distinct in appearance than other two flames by consisting of an inner cone and an intermediate feather. The outer envelope is also larger than the other two flames. The burning temperature of reducing flame is lower since it does not consume all the available carbon. The left over carbon is forced into the metal thereby causing the solidified weld bead to have pitted surface. Metals that tend to absorb carbon should not be welded with reducing flame. However when welding high carbon steel, this excess carbon is an ideal condition ( Aluminium , alloy steel). With iron & steel, it produces a very hard, brittle material called iron carbide. This iron carbide makes the metal unsuitable for many applications that may require bending of welds. Metals that absorb carbon should not be welded with a reducing flame.

17. Oxy-hydrogen flames In this process, hydrogen is used in place of acetylene and the temperature of flame is very low (1980 O C). It is therefore, best suited for welding thin sheets, low melting alloys and for brazing work. An advantage of this process is that no oxides are formed on the surface of the weld if a reducing atmosphere is used. The flame adjustment is very difficult in this welding as there is no distinguishing colour to judge the gas proportions .

18. Arc Welding Equipment :   Arc welding transformer or DC Welding generator: The function of AC welding transformer is to set down the main supply voltage (200 to 400V) to the voltage (60 to 80 V) and current 30 to 500 A, suitable for metal arc welding using AC welding transformer. The function of DC welding generator is to supply suitable voltage for metal arc welding. The machine consists of a generator driven by an AC motor or a gas or oil engine. The most commonly used power source for AC welding is transformer. A transformer may be operated from the mains on single phase, two phase, three phase. Single phase transformer supply low current therefore it is used for welding thinner sections with smaller diameter electrodes. Two phase and three phase supply transformer operates at higher currents and are used for the welding of thick jobs. Transformers are either air cooled or oil cooled depending upon their amperage rating, big transformers are oil cooled. A transformer being the simplest and the cheapest is most common as compared to the other AC welding machines.

19. DC Generator : Advantages : 1. A straight and reverse polarities can be employed. 2. Welding can be carried out in all positions. 3. Nearly all ferrous and non ferrous metals can be welded. 4. Diesel driven generator from self contained units. 5. DC is most universal in application. It can be used in practically all welding operations. An exception is TIG welding of Al & Mg which is normally done with AC. Disadvantages : 1. High Initial Cost. 2. High Maintenance cost. 3. More noise.

20. Electrode : The function of electrode is continuous 1. To act as one of the two conductors to permit continuous flow of electric current. 2. To supply additional metal at the welding during welding process. Cables and cable connectors :   The function of cable and cable connectors is to conduct electrical energy from the welding machine to the end of the electrode and to the work and finally back to the welding machine. The cables connecting the welding machine and the work is called the ground cables. These cables are highly insulated and are sometimes called the leads. Connectors are the devices meant for connecting the ends of the cables to the terminal points of the welding machine. Cable lugs : These are the metal tubes like devices meant for enclosing the end portions of cable to avoid loose contact.

21. Electrode Holder : It is a device used to hold the electrode . Earthing clamp : Its function is to connect the end of the ground cable and work. Chipping Hammer : Its function is to remove the metal oxides and the slag from the weld bead. It is made of steel. Wire Brush : Its function is to clean the surfaces of the plates both before and after welding.   Helmet : Its function is to protect the head of the welder from the sparks during welding. It is made of steel. Safety Goggles : Its function is to protect the eyes of welder from the falling of chips during chipping. Hand Gloves : its function is to protect the chest, belley and clothes of the welder from the fall of sparks during welding. It is made of lather. Welding Shield : Its function is to protect the necked eyes of the welder from the harmful rays of the electric arc. The shield is provided with special dark coloured glass placed in between two clear glasses.

22. S . No. A.C DCSP DCRP 1 Currents higher than those in DCRP can be employed. ( 400 to 500 A for 6 mm dia electrode) Welding Currents upto 100 A can be employed for 6 mm dia electrodes. Currents employed are less than 125 A ( upto 6 mm dia electrodes) to avoid over heating. 2 Equal heat distribution at electrode and job. 1/3 heat is generated at the electrode and 2/3 at the job 2/3 heat is generated at the electrode and 1/3 at the job 3 Normal penetration Deep penetration Least penetration 4 Arc cleaning of the base metal Better arc cleaning action No arc cleaning of the base metal 5 Average arc voltage in Argon atmosphere is 16 V Average arc voltage in Argon atmosphere is 12 V Average arc voltage in Argon atmosphere is 9 V 6 Electrode tip is colder as compared to that in DCRP Electrode runs colder as compared to AC or DCRP Chances of electrode over heating and melting losses.

23. Metal Arc welding

24. Metal Arc welding : Electric arc is used as the main source of heat in arc welding. Electric arc is produced when two conductors i.e. anode and cathode of an electric circuit are brought together and then separated slightly so that an air gap is established such that the current continues to flow through the gaseous medium. This arc produces temperatures of about 6000 O C to 7000 o C. Either from A.C. or D.C source supply electric current is used. With A.C. due to the reversal of the current, the heat generated at each pole is the same and hence changing over the connections to the electrode does not have any effect. But the polarity of D.C has a great effect on electrode performance. With a D.C. source, if the work piece is connected to the positive terminal and the electrode holder is connected to the negative terminal of the welding machine, then the welding set up is said to have straight polarity(DCSP). And if the work piece is connected to the negative terminal and the electrode holder is connected to the positive terminal, then the welding set up is said to have reversed polarity(DCRP). Usually D.C. arc welding machine are D.C. generators which are driven by electric motors where as A.C. welding machines are transformers.

25. In metal arc welding the electric arc is struck between the electrode and the base metal, the base metal melts due to the heat of the arc. The consumable metal electrode will also melts and enters into the molten metal. The depth to which the base metal is melted and deposited is called the penetration of the joint. The amount of penetration depends upon the cross sectional area of the weld bead, the current and the voltage. Penetration increases with increase in current. A crater is formed due to the action of the electric arc. This arc crater should be avoided always otherwise it will become the weak point in the welded structure. Both A.C and D.C. current source may be employed in this method.

26. Advantages 1. It is the simplest of all welding processes. 2. The equipment can be portable and the cost is low. 3. Welding can be carried out in any position with highest weld quality. Disadvantages 1. In welding long joints as one electrode finishes, the weld is to be progressed with the next electrode. 2. Because of flux coated electrode chances of flux entrapment is more. Applications 1. SMAW is used for fabrication, maintenance and repair work. 2. Almost all metals can be welded with this process.

27. Submerged Arc Welding :  The process is called Submerged arc welding because the arc, the end of the electrode and molten weld puddle are hidden or submerged in a finely divided granulated powder. In this process, the arc required for melting the metal at the joint is struck between a continuously fed bare electrode and the workpiece . In Submerged arc welding process instead of flux covered electrode granual flux and bare electrode is used. The flux powder is fed continuously from a hopper ahead of the arc zone. The flux protects the molten pool of metal from all effects of atmospheric gases there by weld bead are exceptionally smooth. The flux adjacent to the arc melts and floats on the surface of the molten pool of metal and then solidifies to form a slag on top of the welded metal. The remaining un melted flux is recovered and reused. Normally, the flux contains silica, metal oxides and other compounds which acts as deoxidizer and cleaner. The bare wire electrode during welding melts and acts as a filler. The electrode wire is continuously fed automatically from reel and the electrode feed rate should match to the speed at which the electrode melts, thus keeping the arc length constant.

28. In submerged arc welding high currents are used. As a results the melting rate of the electrode and the speed of welding is very high. Either A.C or D.C power source can be used but above 1000 a A.C. power source is preferred to reduce arc blow. Submerged arc welding is used for welding medium carbon steels, heat resistant steel, corrosion resistance steels, nickel monel and other non ferrous metals. This welding process produces high quality welds and also eliminates smoke and fumes since the arc column is submerged in flux, adds to its ease of operation and efficiency. This makes submerged arc welding to be used for a wide variety of applications in welding industry.  

29. S. No. Submerged Arc Welding Metal Arc Welding 1 It is an automated process It is the most common type of welding and is normally a manual operation 2 Arc is maintain underneath a mass of flexible, granular flux. Arc is maintained in open 3 Flux is prepared in the form of a coarse powder. Granulated flux is spread over the joint. Flux is supplied from coated electrodes. 4 Thick welding can be done in a single run Several passes are used to weld thick plates 5 Bare wire is fed Coated electrode is used. 6 Deposition rate is high and penetration is deep Deposition rate is low and penetration is less 7 Arc is not visible Arc is visible 8 Weld quality is high Weld quality is not high 9 Limited to flat welding Over head welding is possible 10 Weld spatter is eliminated Weld spatter exists 11 Sparks, smoke and flash are eliminated. Sparks, smoke and flash occurs. 12 High welding speed is possible Welding speed is slow 13 It can’t be used to weld plates less than 5 mm thick. It can be used to weld thin and thick sheets Difference between metal arc welding and submerged arc welding

30. Metal Inert gas welding (MIG) :-   Gas metal arc welding is a gas shielded metal arc welding process which uses the high heat of an electric arc between a continuously fed consumable electrode wire and the material to be welded. Metal is transferred through protected arc column to the work. In this process the wire is fed continuously from a reel through a gun to constant surface which imparts a current upon the wire. A fixed relationship exists between the rate of wire burn off and the welding current so that the welding machine at a given wire feed rate will produce necessary current to maintain the arc. The current ranges from 100 to 400 A depending upon the diameter of the wire, and the speed of melting of the wire may be upto 5 m/min. The welding machine is DC constant voltage, with both straight and reverse polarity available.

31. The welding gun can be either air or water cooled depending upon the current being used. With the higher amperages a water cooled gun is used. The welding wire is generally bare. The wire is usually in diameters of 0.09 to 1.6 mm , however sizes upto 3.2 mm are made. In gas metal arc welding, the welding area is flooded with a gas which will not combine with the metal. The rate of flow of this gas is sufficient to keep oxygen of the air away from the hot metal surface while welding is being done. All ferrous and non ferrous metals can be welded using argon or Helium as protective inert gas. Carbon dioxide is used only for ferrous metals because it is cheap approximately one tenth as much as argon or helium. For welding stainless steel MIG welding is done with either argon-oxygen or helium-argon gas mixture. Titanium requires pure argon gas shielding and copper and nickel alloys use argon helium mixture.

32. Advantages 1. Because of continuously fed electrode, MIG welding process is much faster as compared to TIG welding. 2. The process can be easily automated. 3. It is more economy 4. No flux required. 5. Increased corrosion resistance. Disadvantages 1. The process is slightly more complex as compared to TIG welding. 2. Welding equipment is more complex, more costly and less portable. Application 1. For welding of stainless steel, aluminium , magnesium, nickel and their alloys. 2. For welding of tool steel and dies. 3. Used in industries like aircraft, automobile, pressure vessel, ship building etc.

33. TIG Welding

34. It is also known as Gas tungsten arc welding (GTAW). This process uses a non consumable tungsten electrode which is mounted in special electric holder. This holder is also designed to furnish a flow of inert gas around the electrode and around the arc. In TIG welding, the arc which is required to melt the metal at the joint, is struck between non consumable tungsten electrode and work piece in the presence of protective inert gas. The inert gas protects the molten puddle from atmospheric oxygen and nitrogen. The inert gases that are used are argon, Helium and mixture of argon and Helium. Argon is more widely used than helium because it is heavier gas, producing better shielding at lower flow rate. The shielding gas displaces the air surrounding the arc and weld pool. This prevents the contamination of the weld metal by the oxygen and nitrogen in the air. Filler metal may or may not be used. When a filler metal rod is used , it is usually fed manually into the weld pool. Electrodes used in this process are made of tungsten and tungsten alloys. The tungsten electrode is used only to generate an arc. The arc does not melt the tungsten, which has a melting point over 3300 C. The end of the welding gun where the arc is created is made of high impact ceramic. The torch used in this welding method holds the electrode and directs shielding gas and welding power to the arc. The use of TIG is confined mainly to the welding of relatively thin materials upto about 7 mm. It is especially suited for welding Aluminium and Magnesium based alloys and also used for welding stainless steel, carbon steels and low alloy steels.

35. Advantages 1. No flux is used hence there is no danger of flux entrapment. 2. Process can weld in all positions. 3. TIG welding is very much suitable for high quality welding. Disadvantages 1. Tungsten if it transfers to molten weld pool can contaminate the same. 2. Equipment cost is higher than SMAW welding. Application 1. Welding of aluminium , magnesium, copper, nickel and their alloys 2. Welding of stainless steel. 3. Welding sheet metal of thinner section.

36. Cutting Methods : Gas Cutting 1. Oxy-acetylene cutting 2. Oxygen Lance cutting Arc Cutting 1. Carbon arc cutting 2. Metal arc cutting 3. Arc – oxygen cutting Gas Cutting: Gas flame cutting methods are based on the ability of certain metals mostly iron and steel, to burn in oxygen with the evolution of a large amount of heat. Usually iron and steel oxidizes when exposed to air but this process of oxidation can be enhanced by heating the steel to a red hot condition and then supplying oxygen to the hot spot. By doing this, the metal burns rapidly since the reaction between iron and oxygen is exothermic i.e. produce large amount of heat. This heat keeps the oxide which is formed due to oxidation in molten condition which flows or may be blown off there by exposing more metal to the action of oxygen.

37. Oxyacetylene cutting :   oxy-acetylene cutting employs ordinary gas welding equipment except that the welding torch is replaced by cutting torch. The cutting torch is slightly differs welding torch. Cutting torch is fitted with two valves which regulates the supply of oxygen and acetylene which are mixed together for preheating and the method used for mixing is similar to welding torch. A third valve controls the stream of cutting oxygen. The torch tip consists of four small openings surroundings a large opening at the center. The large opening at the center provides the cutting oxygen stream whereas the small opening around it provide the mixture of oxyacetylene gas. In oxy-acetylene cutting, first the metal is preheated with a preheating flame which is produced by mixing oxygen and acetylene in correct proportions in the cutting torch. After a spot area along the line of cut is heated to red hot, a jet of pure oxygen under high pressure is directed to the heated area through the central opening of the tip of the cutting torch. This cutting oxygen jet penetrates through the steel and causes oxidation reaction between iron and oxygen resulting in the formation of iron oxide.

38. The reaction products of the oxidation reaction are blown off from the reaction zone by the pressure of the oxygen gas thereby exposing more metal to the action of oxygen thus progressively cutting a narrow slit or kerf in the metal along the line of cutting. oxy-acetylene cutting operation may be carried out either manually or in a mechanized way. In general, machine cutting in which the nozzle is traversed mechanically along the cutting line gives a higher quality of cut.

39. Oxy Lance cutting :   Oxy lance cutting is employed to cut heavy thick steel sections which cannot be cut by oxyacetylene cutting. In oxygen lance cutting, the edge of the work piece to be cut is pre heated with a preheating torch. The oxygen is directed to the preheated area using oxygen lance (pipe) at a pressure of 40 to 50 PSI (2.5 to 3.5 Kg/cm 2 ). This causes rapid oxidation of the surface. The oxygen jet blows the reaction products thereby cutting the work piece along the required line of cutting. 

40. Arc Cutting :   In arc cutting the cutting operation is performed by melting the metal with heat supplied by an electric arc which is struck between the electrode and the work piece. Some provision must be made to allow the molten metal to drop by gravity from the cutting area. Carbon Arc cutting :   In carbon arc cutting, the equipment used is similar to that used in carbon arc welding. A graphite electrode is usually preferred and D.C source with straight polarity is used. As usual the arc is struck between graphite electrode and the work piece. The electric arc melts the metal and the molten metal flows away from the cutting area facilitating further cutting of metals. All the metals which can be readily melted can be cut by this method. Advantage being of low cost but the disadvantage is it produces roughest cutting edges and hence this method is employed for cutting scrap metals.

41. Metal arc cutting :   This is similar to carbon arc cutting except that a consumable metal electrode is used. This coating on electrode prevents short circuiting at the sides of the electrode, also stabilizes the arc. As usual here also the arc is struck between the consumable metal electrode and the work piece. This electric arc melts the metal and the molten metal flows away from the cutting arc exposing the metal to be cut. Metal arc cutting is costlier process than the carbon arc cutting since the electrodes are used up during cutting operation. Hence metal arc cutting rarely used. Arc oxygen cutting :   This method is similar to oxy-acetylene cutting except that the preheating is done by an electric arc struck between the consumable tubular electrode and the work piece. The coated tubular electrode possess a small hole at the center through which oxygen is blown to the heated area or cutting area. In this method, the cutting operation involves first striking an electric arc between tubular consumable electrode and the work piece. This electric arc preheats the work piece at the cutting area. Then the oxygen is blown through the center hole of the tubular electrode to the cutting area. This causes rapid oxidation of the metal. This oxidation reaction is an exothermic reaction i.e. it produces large amount of heat. This heat keeps the oxidation reaction products in the molten condition. During cutting these reaction products flows away from the cutting area exposing fresh metal to be cut and thus cutting progress towards the line of cutting. The edges are smoother than carbon arc cutting but not as smooth as those obtained in oxy-acetylene cutting.

42. Weldability of Metals The term weldability has been defined by the American welding society as “ the capacity of metals to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service”. This means that if a particular metal has good weldability it must be welded readily so as to perform satisfactorily in the fabricated structure, and also it must not require expensive or complicated and exacting procedures in order to produce sound joints. There are certain similarities and differences among the various welding processes depending upon the weldability of metals. The weldability of any metal can be changed by physical, chemical, thermal and metallurgical properties i.e. by using a proper welding procedure, shielding atmosphere, fluxing material, filler material and in some cases by proper heat treatment of metal before and after deposition. The following metals have good weldability in the descending order : iron, carbon steel, cast steel, cast iron, low alloy steels and stainless steels.

43. Welding Defects i ) Poor Fusion: It is the lack of thorough and complete union between the deposited and parent metal. This is due to faulty welding conditions or techniques. The main parameters that controls welding is current, if lower than required current, would not sufficiently heat all the faces of the joint to promote poor fusion. ii) Under cut: It is due to excessive current setting excessive arc length, failure in completely filling up crater with the weld metal, improper welding technique. iii) Porosity : It is formation of blow holes, gas pockets or roughness on the surface of the weld. This is due to the presence of gases in the metal, moisture in the flux or rust on the welded edges or filler metal. The main gases that causes porosity are : H 2 , O 2 , N 2 . Porosity if present in large quantity would reduce the strength of the joint. iv) Slag inclusion : It is the presence of non- metallic substances in the metal. These are due to the contamination of the base metal and deposited metal by oxides, non-uniform melting of the electrode coating, insufficient welding heat, rapid solidification and high viscosity of weld metal.

44. v) Crack : Cracks are openings in weld or base metal separated by minute gaps. Cracks in the weld may arise from locked up stresses set up by non-uniform heating and cooling excess sulphur or phosphorus in the weld metal on some other causes. Cracks can be prevented by i ) Avoiding welding of steel containing the high sulphur and phosphorous ii) Spacing the parts to be welded uniformly iii) Designing the joints with provisions for expansion and contraction iv) The weld bead shape slightly convex. vi) Spatter : Metals in the shape of small globules found around weld area at the completion of welding is called spatter. It is due to bubbles of gas entrapped in the molten metal. This is due to: i ) Use of excessive current ii) Use of too Long arc or too high arc voltage. iii) Arc blow making the arc uncontrollable. This defect can be prevented by i ) Reducing excessive current setting ii) Reducing excessive voltage iii) Reducing excessive arc length and arc blow.

45. vii) Weld stresses : The stresses are due to joints which are rigid, improper welding procedure and unequal heating of parts. Weld stresses can be avoided by: i ) Preheating the parts to be welded ii) Annealing the finished weld at 595 to 650 O C for one hour per 25 mm of thickness. iii) Making welds in as fine passes as that practicable. iv) Giving scope for slight movement of parts during welding. viii) Brittleness : It is due to: i ) Use of bare electrode ii) Excessive welding current iii) Single pass deposition of weld metal iv) Welding of high carbon or alloy steels This can be prevented by i ) Use of shielded arc type electrode ii) Reducing the welding current iii) Multiple pass deposition of weld metal iv) Avoiding welding of high carbon or alloy steels

46. ix) Poor penetration : The penetration of weld is the distance from the original surface of the base metal to that point at which fusion stops. Poor penetration is due to: i ) Improper penetration of the joint ii) Use of electrodes having large diameter iii) Too fast welding speed. This can be prevented by i ) Allowing the proper space between the root faces of the pieces to be welded. ii) Use of electrodes having small diameter iii) Reducing the speed of the welding  

47. Advantages, Disadvantages and Applications of Welding Advantages of Welding 1. A good weld is as strong as the base metal. 2. General welding equipment is not very costly. 3. Portable welding equipment are available. 4. Welding permits considerable freedom in design. 5. Welding can be mechanized.  Disadvantages of Welding 1. Welding gives harmful radiations (light), fumes and spatter. 2. Welding results in residual stresses and distortion of the workpiece . 3. Welding heat produces metallurgical changes. 4. Welding joint requires stress-relief heat treatment..  

48. Applications of Welding 1. Aircraft Construction 2. Automobile construction 3. Bridges 4. Buildings 5. Pressure Vessels and Tanks 6. Storage Tanks 7. Pipelines. 8. Ships 9. Household and office furniture 10. Machine tool frames, cutting tools and dies