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L4-Gas Welding.ppt

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L4-Gas Welding.ppt

  1. 1. IPE 331: Production Process Gas welding Dr. A. B. M. Mainul Bari Assistant Professor Dept of IPE, BUET.
  2. 2. Gas welding is a fusion welding process. It joins metals, using the heat of combustion of an oxygen/air and fuel gas mixture.  OFW employs several types of gases such as Acetylene, Hydrogen, Methylacetylene-Propadiene (MAPP), Propylene, Propane, Butane, Natural Gas.  Oxy-hydrogen welding was the first gas process to be commercially developed. The maximum temperature developed by this process is 1980°C.  The most used oxyacetylene welding process uses a combination of oxygen and acetylene gas to provide a high temperature flame which has a flame temperature of 3500°C.  Widely used in welding tubes and pipes  Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts Oxyfuel Gas Welding (OFW)
  3. 3. Temperature distribution
  4. 4.  There are three distinct types of oxy-acetylene flames, usually termed: • Neutral • Carburizing (or “excess acetylene”) • Oxidizing (or “excess oxygen” ) ◦ The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.  The neutral flame is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one.  It’s termed ”neutral” because it will usually have no chemical effect on the metal being welded.  It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal.  The temperature of the neutral flame is of about 5900ºF.  The flame has a nicely defined inner cone which is light in color and surrounded by an outer flame envelope produced by combination of oxygen, in the air, superheated carbon monoxide, and hydrogen gas from the inner cone. This is much darker blue than the inner cone.  Used for welding-mild steel, cast iron, stainless steel, aluminium, copper. Flame definition
  5. 5.  The Carburizing flame is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame.  A reducing flame can be recognized by acetylene feather which exists between the inner cone and outer envelope. The outer envelope is longer than the neutral flame and much brighter in color.  A reducing flame does not consume the available carbon, so temperature is much lower (5500ºF).  With iron and steel it produces very hard brittle substance known as iron carbide. This chemical change makes the metal unfit for many application in which the weld may need to be bent or stretched.  It can be used for surface hardening purposes or to ensure the absence of the oxidizing condition. [Non ferrous alloys and high carbon steel.] Flame definition
  6. 6.  The oxidizing flame results from burning a mixture which contains more oxygen than required for a neutral flame.  An oxidizing flame can be recognized by the small white cone which is shorter, much bluer in color and more pointed than that of neutral flame. The outer envelope is much shorter.  An oxidizing flame is burns with a decided loud roar.  The temperature of the oxidizing flame is of about 6300ºF.  At high temperature it will oxidize or ”burn” some of the metal being welded to form hard brittle, low strength oxides.  An oxidizing flame is of limited use in welding. Used for welding-copper base metal, zinc base metal, manganese steel and cast iron.  The oxidizing atmosphere in these cases, creates a base metal oxide that protects the base metal. For example, in welding brass, the zinc has a tendency to separate and fume away. The formation of a covering zinc oxide prevents the zinc from dissipating. Flame definition
  7. 7. Fig : Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. Type of Flame
  8. 8. Oxyacetylene Welding (OAW) Fusion welding performed by a high temperature flame from combustion of acetylene and oxygen  Flame is directed by a welding torch  Filler metal is sometimes added ◦ Composition must be similar to base metal ◦ Filler rod often coated with flux to clean surfaces and prevent oxidation • Flux are available as powder, paste or liquids. • No flux for steel. • Flux is used for cast iron, stainless steel and most non-ferrous metals other than lead, zinc and some precious metals.
  9. 9. The max temperature of the oxy-acetylene flame is 3100-3300 ͦ C and the center of the heat concentration is just off the extreme tip of the white cone. Combustion of the gas mixture is recognized as taking place in two stages: Primary combustion process 2C2H2 + 2O2 4CO + 2H2 + heat This reaction dissociates into carbon monoxide and hydrogen. Secondary combustion process 4CO + 2H2 + 3O2 4CO2 + 2H2O + heat Combining the above equations 2C2H2 + 5O2 4CO2 + 2H2O + heat It can be seen that about two-fifth of the oxygen necessary for the complete combustion of acetylene is got from the cylinder where as the rest comes from the atmosphere. So, the acetylene- oxygen flame cannot be used inside of pipes or structure subjected to oxygen depletion. Oxyacetylene Welding (OAW)
  10. 10. Fig : (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, acetylene regulators red. Torch used in Oxyacetylene Welding
  11. 11. Various Gas Cylinders Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Thermadyne Industries, Inc.
  12. 12. Oxygen  Gaseous Chemical element in air ◦ Necessary for life  Most abundant chemical element in crust of Earth  No color, odor, or taste  Does not burn, but supports combustion ◦ Substances that do not burn in air will in oxygen
  13. 13. Acetylene  Most widely used of all fuel gages ◦ Both welding and cutting  Generated as result of chemical reaction that takes place when calcium carbide comes in contact with water  Tests show oxyacetylene flame temperatures up to approximately 6,300ºF ◦ Very rapid rate of preheating ◦ Burns with smoky flame, gives off carbon, has peculiar odor
  14. 14. Figure : Torches: top torch is a welding torch and the bottom is a cutting torch Torch used in Oxyacetylene Welding
  15. 15. STEPS :  Prepare the edges to be joined and maintain the proper position.  Open the acetylene valve and ignite the gas at tip of the torch.  Then the oxygen valve is opened and the flame adjusted.  Hold the torch at about 45deg to the work piece plane.  Inner flame near the work piece and filler rod at about 30 – 40 deg.  Touch filler rod at the joint and control the movement according to the flow of the material. Welding practice
  16. 16. Self study  Left ward and right ward techniques of gas welding.  Filler metal and fluxes used for gas welding.  Difference between acetylene and oxygen Gas cylinder.  Difference between acetylene regulator and oxygen regulator.
  17. 17. Advantages of OAW  The equipment is low cost, versatile, self-sufficient and usually portable. Besides welding the oxyacetylene can be used for welding, brazing, soldering, preheating, post-heating and metal cutting etc.  It requires little maintenance, and can be used with equal facility in the field and in the factory.  It can weld most common materials.  The gas flame temperature is lower and easily controllable which is necessary for delicate work. Therefore oxyacetylene welding is extensively used for sheet metal fabrication and repairs.  The rate of heating and cooling is relatively low. In some cases this is an advantage.  Since the source of heat and the filler metal are separate, the welder has control over filler metal deposition rates. Heat can be applied preferentially to the base metal and the filler metal.
  18. 18.  Oxygen and acetylene gases are expensive.  There are safety problems involved in their handling and storing.  The flame takes considerably longer for the metal to heat up. Due to this, oxyacetylene welding is not suitable for thick sections.  Because the flame is not concentrated and it needs prolonged heating of the joint which result in a larger heat affected area . This often leads to increases grain growth, more distortion and in some cases loss of corrosion resistance.  Flame temperature is less than the arc temperature. Gas flame takes a long time to heat up the metal than arc.  Refractory metals like Columbium, Tantalum, Molybdenum, Tungsten and the reacting metals such as Titanium and Zirconium cannot be welded by this process.  More safety problems are associated with the handling and storing of gases. Disadvantages of OAW
  19. 19.  For joining thin materials.  For joining materials in whose case excessively high temperatures or rapid heating and cooling of the job would produce unwanted or harmful changes in the metal.  For joining materials in whose case extremely high temperatures would cause certain elements in the metal to escape into the atmosphere.  For joining most ferrous and non-ferrous metals, e.g., carbon steels, alloy steels, cast iron, aluminium, copper, nickel, magnesium and its alloys, etc.  In automotive and aircraft industries. In sheet metal fabricating Applications of OAW
  20. 20. Gas cutting  Ferrous metal is heated in to red hot condition by an oxy-fuel flame where burning can be initiated and then a stream of pure oxygen is added to the torch (or the oxygen content of the oxy-fuel mixture is increased) to oxidize the iron.  The liquid iron and iron oxides are then expelled from the joint by the kinetic energy of the oxygen gas stream.  Oxides having lower melting point than the metal, melt and are blown away by the force of the jet, to make a cut  Fast and efficient method of cutting steel to a high degree of accuracy  Torch is different from welding  Cutting torch has preheat orifice and one central orifice for oxygen jet  PIERCING and GOUGING are two important operations  Piercing, used to cut a hole at the centre of the plate or away from the edge of the plate  Gouging, to cut a groove into the steel surface
  21. 21. GAS CUTTING Manual Gas Cutting

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