Welding and types


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Welding and types

  1. 1. WELDING
  2. 2. WELDING    Welding is a process of joining similar metals by the application of heat with or without application of pressure and addition of filler material. The result is a continuity of homogenous material, of the composition and characteristics of two parts which are being joined together. Welding may be classified under two broad headings: (1)Plastic welding, (2) Fusion welding
  3. 3.      In the plastic or pressure welding, the pieces to be joined are heated to a plastic state and then forced together by external pressure. Ex. Forge welding, resistance welding, thermit welding, gas welding In the fusion welding, the material at the joint is heated to molten state and allowed to solidify. Ex. Gas welding, arc welding, thermit welding In the weld, metal solidifies from liquid state and is therefore essentially a casting. In a single – run weld , long columnar grains will grow from the sides of the weld and will meet at the centre of the weld deposit forming a plane of weakness. In a multi-run weld, the first run, is as before. The second run normalizes the first layer causing considerable grain refinement (giving stronger joint). But the possibility of slag and gas inclusions will increase.
  4. 4.     The contour of weld by forming notches can affect both fatigue and low temperature properties of a structure, (welding is a skilled work). Heat Affected Zone The effect of welding heat upon the metal immediately adjacent to the weld is likewise important and dependent on chemical composition and thermal conductivity of parent metal. HAZ ranges from over heated area near the weld metal to an under annealed structure farther away. Pre heating is often employed when welding C.I and high carbon steel, since it slows down the cooling rate of parent metal.
  5. 5.  Fusion Welding 1.Oxy-fuel gas welding  2.Arc Welding –  (a) Consumable electrode  (b) Non-consumable electrode  3.Solid state welding – Ultrasonic, Friction, Resistance 
  6. 6. Gas Welding          Done by burning a combustible gas with air or oxygen in a concentrated flame of high temperature. The purpose of the flame is to heat and melt the parent metal and filler rod of a joint. Advantages: Equipment is inexpensive Versatile Serves adequately in many job and general repair shops Types: Oxy-acetylene welding Air-acetylene welding Oxy-hydrogen welding
  7. 7. GAS WELDING TECHNIQUE       According to the position of welding, all welds are classified into: Down hand or flat weld deposited on a horizontal surface so that the flame is above the weld. Vertical welds deposited on a vertical surface (upwards or downwards) Inclined welds deposited up or down an inclined surface Horizontal welds deposited on a vertical surface in a horizontal direction Overhead welds deposited on a horizontal surface in any direction so that the weld is above the flame.
  8. 8. AIR-ACETYLENE WELDING    This process uses a torch similar to a Bunsen burner and operates on the Bunsen burner principle. The air is drawn into the torch required and mixed with the fuel flame. The gas is then ejected and ignited, producing an air-fuel flame. The common fuels used are acetylene, natural gas, propane and butane. This type of welding has limited use since the temperature is lower than that attained by other gas processes. The air-fuel welding processes are used in lead welding and many low-melting-temperature metals and alloys like in brazing and soldering processes.
  9. 9. OXY-HYDROGEN WELDING  The oxygen-hydrogen process were once used extensively to weld low temperature metals like aluminium, lead and magnesium, but it is not popular today because more versatile and faster welding process such as TIG (tungsten inert gas) and MIG (metal inert gas) have replaced the oxygen-hydrogen flame. The process is similar to oxygen-acetylene system, with the only difference being a special regulator used in metering the hydrogen gas.
  10. 10. OXY-ACETYLENE WELDING     Accomplished by melting the edges or surface to be joined by gas flame and allowing the molten metal to flow together, thus forming a solid continuous joint upon cooling. This process is suitable for joining metal sheets and plates having thickness of 2 to 50 mm. With material thicker than 15 mm, additional metal called filler metal is added to the weld in the form of welding rod. The composition of the filler rod is usually the same as that of the part being welded. To remove the impurities and oxides present on the surfaces of the metal to be joined and to obtain a satisfactory bond a flux is always employed during the welding except mild steel which has more manganese and silicon that act as deoxidizing agents.
  11. 11. GAS FLAME     The correct adjustment of the flame is important for reliable works. A neutral flame is produced when O2 and acetylene are supplied to torch in equal volumes. The heat is generated with a pair of chemical reactions which occur at the inner cone, where the temperature reaches between 3050 to 3450 deg C. C2H2 + O2 → 2CO + H2 + heat The secondary combustion process in the outer envelope, flame temperature is around 2100 deg C near the inner cone and around 1250 deg C at the end point of the flame. 4CO + 2H2 + 3O2 → 4CO2 + 2H2O + heat
  12. 12.             A neutral flame has 2 definite zones 1. Sharp brilliant cone extending short distance from the tip of torch – develops heat, temp 2100 deg C 2. Outer cone or envelope, faintly luminous bluish colour – protects the metal from oxidation. Uses: Welding steel, stainless steel, CI, Cu, Al Carburizing flame, temp 2700 deg C, there is excess of acetylene. The flame has 3 zones, 1. Sharp inner cone 2. Intermediate cone of whitish colour 3. Bluish outer cone Oxidizing flame, temp 3400 deg C, there is excess of oxygen, the flame has 2 zones 1. Small inner cone with Purple tinge 2. Envelope This flame is necessary for welding brass. In steel this results in large grain size, high brittleness
  13. 13. WELDING EQUIPMENT           Welding torch – tool for mixing oxygen & acetylene Welding tip – diameter of tip opening depends on the type of metal to be welded and its thickness. Pressure regulator – to reduce the cylinder pressure to the required working pressure. For < 25 mm thickness, reg. pre. is 0.15 to 0.7 kgf / cm2 and for > 25 mm thickness, reg. pre. is 0.98 kgf / cm2. Hose & hose fittings Goggles, Gloves & spark lighter Gas cylinder – Oxygen gas cylinder, charged with 40 L at 154 kgf / cm2 Acetylene gas – low (0.07 kgf / cm2) & high pressure (1.0 kgf / cm2) Low pressure acetylene is generated and used in site. To prevent air blowing back, a back pressure valve must be introduced between the blow pipe & gas holder. Every valve has a safety device to provide against dangerous pressure. In addition to the above welding rods & fluxes are employed.
  14. 14. THANK YOU