Pr 8592-wt-unit-1
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This document provides information on various gas and arc welding processes. It describes oxy-acetylene welding, describing the neutral, carburizing and oxidizing flames produced and their characteristics. Arc welding processes discussed include shielded metal arc welding, gas tungsten arc welding and plasma arc welding. Electroslag welding is also summarized, explaining how it uses a molten slag and electric current to join thick metal plates.
Pr 8592-wt-unit-1
- 1. UNIT – 1 GAS AND ARC WELDING PROCESSES
- 2. “Welding is the fabrication process of joining together two pieces of metal so that bonding takes place at their original boundary surfaces”. When two parts to be joined are melted together, heat or pressure or both is applied and with or without added metal for formation of metallic bond.
- 4. Edges of metals to be welded are melted by using a gas flame No pressure is applied during welding except pressure gas welding Flame is produced at the tip of a welding torch Welding heat is obtained by burning a mixture of oxygen and combustible gas Gases are mixed at the required proportion in a welding torch which provides a control for welding flame Commonly used gases are acetylene, hydrogen, propane and butane Flame only melts the metal, so, the additional metal required to the weld is supplied by the filler rod Flux is used during welding to prevent oxidation and remove impurities Metal having 2 mm to 50 mm thick are welded by gas welding 3 types (a) Air-acetylene welding (b) Oxy-acetylene welding (c) Oxy-hydrogen welding
- 5. .
- 6. Most common form of gas welding is OXY- ACETYLENE WELDING
- 7. The combination of oxygen and acetylene produces a flame temperature about 3200oC. The gases Oxygen (O2) and Acetylene can be stored at high pressure in separate cylinders. There are two types of oxy-acetylene system employed 1. High pressure system (Oxygen at 120 bar and acetylene compressed to 1.5 bar) 2. Low pressure system (Acetylene is produced at the place of welding using calcium carbide and water)
- 8. Ca C2 + 2H2O Ca (OH)2 + C2H2 + 127.3 KJ/mol Heat generated in this reaction is very high.
- 9. Similar to Oxy-Acetylene welding. Air is used instead of oxygen. Air taken from the atmosphere is compressed in a compressor Compressed air is mixed with acetylene to the required proportion in the torch. It is used in lead welding and many low melting temperature metals and alloys.
- 10. By varying the ratio of Oxygen and acetylene the following three types of flames can be obtained. 1. Neutral flame 2. Carburising flame 3. Oxidising flame
- 12. Neutral Flame The neutral flame is obtained by supplying equal quantity of oxygen and acetylene. Neutral flame has two zones Sharp bright inner cone Bluish outer cone The reaction of the inner cone for the neutral flame is given by C2H2 + O2 2CO + H2
- 13. Neutral Flame The inner cone develops heat to melt the metal. The maximum temperature neutral flame obtained at the inner cone which is about 3200oC. In this type flame all acetylene gas is completely burned and all available heat in the acetylene is released. It is most desirable flame to be used in oxy- acetylene welding.
- 14. Neutral Flame The reactions of the outer cone are given by 2CO + O2 2CO2 H2 + 1/2O2 H2O The oxygen is supplied from the air. The outer cone protects the molten metal from oxidation reaction because oxygen in the surrounding air is absorbed by gases from the flame.
- 16. Carburising flame: A carburising flame is also called reducing flame. It is obtained by more acetylene than oxygen. This flame has three zones 1. Sharp inner cone 2. White intermediate cone called feather cone. 3. Bluish outer cone
- 17. Carburising flame: The theoretical ratio of mixture O2 : C2H2 = 0.85 to 0.95 Length of the intermediate cone is an indication of excess acetylene in the flame. It is suitable for bracing and flame hardening. The excess carbon causes the steel to become extremely hard and brittle.
- 19. Oxidising flame It is obtained by supplying more oxygen than acetylene. It is similar to neutral flame except the inner white cone is somewhat small. It is giving higher temperature about 3300oC This flame has two zones 1. Smaller inner cone 2. Outer cone
- 20. Oxidising flame The theoretical mixture of oxidizing flame is O2 : C2H2 = 1.15 to 1.5 This flame is harmful for steels because it oxidizes steel. Oxidising flame is desirable for non ferrous alloys such as copper and zinc base alloys.
- 21. .
- 22. In arc welding process heat is developed by an electric arc. An arc is a sustained electric discharge through ionized gas column called plasma between two electrodes. The arc formation is similar for both AC and DC welding.
- 23. The electrodes consists of baked carbon or pure graphite which is inside a copper blanket. The electrodes is not consumed as welding progresses over time. However the electrodes eroded after prolong usage. The average dimensions are 150 mm long, diameter 5mm to 12.5 mm.
- 25. In this welding metals are melted and joined by heating them with an arc between consumable electrode and work piece. The welding electrode consists of core metal wire with an outer coating called flux. The flux assists in creating and stabilizing the arc and provides shielding gas which prevents the reaction of the molten metal with atmosphere. The slag protects the welding from rapid cooling.
- 26. In this process, the work pieces and electrode are melted by the arc and hence, both work pieces become a single piece with out applying external pressure. The temperature of arc is about 5000oC to 6000oC. The depth at which the metal is melted and deposited is called depth of fusion. For better depth of fusion the electrode is kept at 70o inclination to the vertical.
- 27. 1. Bare Electrodes 2. Lightly coated electrodes 3. Heavily coated electrodes.
- 29. It is used where large quantity of flux is required. It is supplied separately instead of using flux coated welding rod. It is also called as hidden arc welding. The arc is completely submerged. The metal electrode is continuously fed from a reel. The flux is supplied in front of the moving head.
- 30. The flux powder is made up of silica, metal oxides and other compounds. Normally DC power is employed using 600A to 1000A and AC power 200A. It is used to weld carbon steels and alloy steels. Plates of 12 mm to 50mm can be welded with one pass.
- 31. Very high quality welds can be produced. Deep penetration can be obtained. Safe for eyes. No edge preparation needed.
- 32. It is not suitable for inclined and vertical. Only specific metals can be welded. Health issues can be caused. Slag removal needed after welding.
- 33. Heavy steel plate fabrication work. Pressure vessel, tanks, Nuclear reactors, Chemical vessels. It is widely used in shipbuilding.
- 35. In this welding the electric arc is produced between an non-consumable tungsten electrode and work piece. The inert gas from the cylinder is passed through the nozzle of the TIG torch. The inert gas (Argon, Helium, Nitrogen and CO2) surrounds the arc and protects the weld from atmospheric effects.
- 36. The tungsten has high melting point (3430oC) and it will not melt during welding. The power supply through the column of ionized gases.
- 37. TIG welding equipment 1. TIG torch 2. Power transformer 3. Shielding gas cylinder 4. Pressure regulator 5. Work clamp 6. Coolant system
- 38. Various functions of TIG torch 1. It holds the tungsten electrode. 2. It delivers welding current to the electrode. 3. It delivers the shielding gas. 4. Water cooling can be provided.
- 39. Advantages 1. It can be used for Aluminum, Stainless steel, Copper alloys. 2. More suitable for thin sections. 3. It does not create much sputter and spark. 4. No flux is required. 5. The welding speed is high.
- 40. Limitations 1. It is generally restricted for flat and horizontal welding. 2. It emits brighter UV rays.
- 41. Applications 1. Aluminum, magnesium, copper alloys can be welded. 2. It can be used in atomic power plants, aircraft industries. 3. Used in rocket motor fabrication.
- 43. Consumable electrode wire having chemical composition similar to the parent material is continuously fed from a reel. The welding zone is surrounded by inert gas such as Argon, Helium, CO2. The welding can be done manually or automatically.
- 44. Either DC generator or AC transformer is used. The current ranges from 100A to 400A. This process is used for welding thick plates. It is used for welding aluminum, stainless steel, nickel and magnesium without defects.
- 45. Advantages It is used for variety of ferrous and non ferrous metals. No flux is required hence no slag to remove. Welding productivity is double as compared to the shielded metal arc welding. High welding speed.
- 46. Limitations It cannot be used for vertical or overhead welding. Process is more expensive. Contact tips may seize due to weld spatter.
- 47. Applications It is suitable for thin sheet sections. Hence it is used for automobile repair works. It can be used to establish hard facing.
- 49. Types of Plasma Arc Welding 1. Non-transferred type 2. Transferred type
- 50. .
- 51. Plasma is high temperature ionized gas. The power is directly connected with the tungsten electrode and copper nozzle. Gas is fed into the nozzle and the arc is generated between tungsten electrode and nozzle. The gas get ionized as it pass through the arc. Due to the restricted shaped of the nozzle orifice ionization is greatly increased. The temperature of about 10000oC can be obtained.
- 53. The restricting orifice is in an inner water- cooled nozzle within which tungsten electrode is centrally placed. Both work is connected to the anode and the tungsten electrode is connected with the cathode. Relatively low plasma gas flow is necessary to prevent turbulence and disturbance of the weld pool.
- 54. It is difficult to initiate the first arc between work piece and electrode. A pilot arc is struck between nozzle and electrode. Thus the pilot arc touches the work piece, main current starts flowing between electrode and work piece. It ignites the transferred arc. The temperature of a constricted plasma arc may be in the order of 8,000oC to 25,000oC
- 55. The flow of cooler gas through these orifices squeezes the circular pattern of the jet in to oval form. Since the tungsten electrode is well inside the nozzle (about 3 mm) tungsten contamination by touch down is avoided .
- 56. The following metals can be welded using PAW 1. Stainless steel 2. Titanium alloys 3. Carbon and low alloy steels 4. Copper alloys 5. Aluminum alloys
- 57. Advantages 1. Penetration is uniform. Deeper penetration and narrower weld is possible. 2. Arc stability is good. 3. High accuracy weld can be obtained. 4. The production rate is high.
- 58. Limitations 1. Huge noise occur during welding. 2. Chances of electric hazards may occur. 3. It is limited to high thickness applications. 4. Frequent orifice replacement is necessary. 5. UV radiations is harmful. 6. The torch is delicate and complex than TIG torch.
- 59. Applications 1. It is used in aerospace applications. 2. It is to weld high melting point metals. 3. It is used for welding titanium plates. 4. It is used for welding nickel alloys. 5. It is used for tube mill applications.
- 61. .
- 62. Electroslag Welding is a highly productive, single pass welding process for thick materials in a vertical or closed to vertical position. Heat is generated by an electric current passing between consumable electrode and work piece through the molten slag covering the welding surface. In this process the coalescence is formed by molten slag and molten metal pool.
- 63. Working Prior to welding the gap between two work pieces is filled with a welding flux powder. The welding is initiated by an arc between starting plate and electrode. Heat generated by the arc melts the flux powder and it forms molten slag. Additional flux is added until the molten slag reaches the tip of the electrode.
- 64. Working After molten slag reaches the tip of the electrode the arc is extinguished. Heat is continuously produced by the electrical resistance of the molten slag. The slag reaches the temperature of about 1930oC. Because the arc is extinguished electroslag welding is not strictly arc welding process. Single of multiple solid as well as flux coated electrodes may be used.
- 65. Working To contain the molten puddle, water cooled copper shoes or dams are placed on the sides of the vertical cavity. As the weld joint solidifies the dams move vertically so as to always remain in contact with the molten puddle. The electric current passes from the electrode to the work piece through the slag pool. This welding is capable of welding plates with thickness ranging from 50mm to more than 900 mm. The current required is about 600A at 40V to 50V. The travel speed is in the range from 12mm/min to 36 mm/min.
- 66. Advantages 1. Heavy thickness metals con continuously be welded. 2. Stress formation is low. 3. Penetration of join is easier. 4. High deposition rate of up to 20 Kg/hr. 5. Distortion is low.
- 67. Limitations 1. It is difficult to weld cylindrical objects. 2. Hot cracking may occur. 3. Grain size becomes larger. 4. Toughness of the weld is low. 5. Only vertical position is possible. 6. The cost of equipment is high.
- 68. Applications 1. It is used to join low carbon steel plates. 2. It is used to weld thick sections. 3. It is used in large structural steel sections such as heavy machinery, bridges, ships, and nuclear reactor vessels. 4. Forgings and castings can be welded.