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arc welding


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arc welding

  1. 1. ARC WELDING Prof. H. K. Khaira MSME Deptt., MANIT, Bhopal
  2. 2. ELECTRIC ARC WELDING The welding in which the electric arc is produced to give heat for the purpose of joining two surfaces is called electric arc welding. The joining by fusing of two or more pieces of metal together by using the heat produced from an electric arc .
  3. 3. How an arc is formed? • The arc is like a flame of intense heat that is generated as the electrical current passes through a highly resistant air gap.
  5. 5. Arc Welding • It is a fusion welding processes which uses an electric arc to produce the heat required for melting the metal. • The welder creates an electric arc that melts the base metals and filler metal (consumable) together so that they all fuse into one solid piece of metal
  6. 6. Arc Welding • Many things around us are welded … – Pipelines that bring fresh water – Towers that carry electricity to houses – Cars and buses that take people where they need to go • Arc welding continues to be used extensively in the construction of steel structures and in industrial fabrication. • The process is used primarily to weld iron and steels (including stainless steel) but aluminium, nickel and copper alloys can also be welded with this method. • It dominates other welding processes in the maintenance and repair industry, and though flux-cored arc welding is growing in popularity • Is popular because it can be used in the field without complicated equipment and gases
  7. 7. Arc Welding • It is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. • An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. • As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.
  8. 8. Arc Welding • Arc welding is a process that melts and joins metals by heating them with an arc established between a sticklike covered electrode and the metals. • The core wire conducts the electric current to the arc and provides filler metal for the joint. • The electrode holder is essentially a metal clamp with an electrically insulated outside shell for the welder to hold safely. • The heat of the arc melts the core wire and the flux covering at the electrode tip into metal droplets. • Molten metal in the weld pool solidifies into the weld metal while the lighter molten flux floats on the top surface and solidifies as a slag layer.
  9. 9. Arc Welding
  10. 10. Arc Welding • Also known as “stick welding” • Uses an arc between a covered electrode and a workpiece • Shielding is obtained from decomposition of the electrode cover • Pressure is not used • Filler metal is obtained from the electrode
  11. 11. Principle of Arc • A suitable gap is kept between the work and electrode • A high current is passed through the circuit. • The electric energy is converted into heat energy, producing a temperature of 3000°C to 4000°C. • This heat melts the edges to be welded and molten pool is formed. • On solidification the welding joint is obtained
  12. 12. Arc Welding • Process: – Intense heat at the arc melts the tip of the electrode – Tiny drops of metal enter the arc stream and are deposited on the parent metal – As molten metal is deposited, a slag forms over the bead which serves as an insulation against air contaminants during cooling – After a weld ‘pass’ is allowed the cool, the oxide layer is removed by a chipping hammer and then cleaned with a wirebrush before the next pass.
  13. 13. Arc Welding • Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's most popular welding processes.
  14. 14. Basics of Arc Welding • The arc is struck between the electrode and the metal. • It then heats the metal to the melting point. • The electrode is then removed, breaking the arc between the electrode and the metal. This allows the molten metal to “freeze” or solidify.
  15. 15. Arc Welding
  16. 16. Arc Welding
  17. 17. Arc Welding
  18. 18. Basic Steps of Arc Welding • Prepare the base materials: remove paint and rust • Choose the right welding process • Choose the right filler material • Assess and comply with safety requirements • Use proper welding techniques and be sure to protect the molten puddle from contaminants in the air • Inspect the weld
  19. 19. ARC WELDING • An electric arc is generated between an electrode and the parent metal • The electrode carries the electric current to form the arc, produces a gas to control the atmosphere and provides filler metal for the weld bead • Electric current may be AC or DC.
  20. 20. Electric Power for Welding • Current used may be – 1. AC – 2. DC For most purposes, DC is preferred.
  21. 21. AC Arc Welding • instead of 220 V at 50 A, for example, the power supplied by the transformer is around 17–45 V at currents up to 600 A.
  22. 22. DC Arc Welding • D.C. machines are made up to the capacity range of 600 amperes. • 45 to 95 volts • D.C. can be given in two ways: (a) Straight polarity (b) Reverse polarity The polarity will affect the weld size and application
  23. 23. Comparison of A.C. and D.C. arc welding Direct Current (from Generator) 1. Less efficiency 2. Power consumption more 3. Cost of equipment is more 4. Low voltage – safer operation 5. Suitable for both ferrous non ferrous metals 6. Preferred for welding thin sections 7. Positive terminal connected to the work 8. Negative terminal connected to the electrode
  24. 24. Arc Welding Equipments
  25. 25. Metal arc welding
  26. 26. Types of Electrodes 1. Bare electrodes 2. Coated electrodes
  27. 27. Arc Welding • The choice of electrode for SMAW depends on a number of factors, including 1. The weld material 2. Welding position and 3. The desired weld properties.
  28. 28. Welding Electrodes • The composition of the electrode core is generally similar and sometimes identical to that of the base material. • But even though a number of feasible options exist, a slight difference in alloy composition can strongly impact the properties of the resulting weld. This is especially true of alloy steels such as HSLA steels. • Likewise, electrodes of compositions similar to those of the base materials are often used for welding nonferrous materials like aluminium and copper. • However, sometimes it is desirable to use electrodes with core materials significantly different from the base material. For example, stainless steel electrodes are sometimes used to weld two pieces of carbon steel, and are often utilized to weld stainless steel workpieces with carbon steel workpieces.
  29. 29. Coated Electrodes • The electrode is coated in a metal mixture called flux, which gives off gases as it decomposes to prevent 1. Weld contamination 2. Introduces deoxidizers to purify the weld 3. Causes weld-protecting slag to form 4. Improves the arc stability, and 5. Provides alloying elements to improve the weld quality.
  30. 30. Electrode Coating • Electrode coatings can consist of a number of different compounds, including rutile, calcium fluoride, cellulose, and iron powder. • Rutile electrodes, coated with 25%–45% TiO2, are characterized by ease of use and good appearance of the resulting weld. However, they create welds with high hydrogen content, encouraging embrittlement and cracking. • Electrodes containing calcium fluoride (CaF2), sometimes known as basic or low-hydrogen electrodes, are hygroscopic and must be stored in dry conditions. They produce strong welds, but with a coarse and convexshaped joint surface. • Electrodes coated with cellulose, especially when combined with rutile, provide deep weld penetration, but because of their high moisture content, special procedures must be used to prevent excessive risk of cracking. • Finally, iron powder is a common coating additive, as it improves the productivity of the electrode, sometimes as much as doubling the yield.
  31. 31. Functions of electrode (flux) covering • Provides the gaseous shield to protect the molten metal from air. – Cellulose-type electrode (C6H10O5)x , providing gas mixture of H2, CO, H2O and CO2. – Limestone-type electrode (CaCO3) – low in hydrogen and it is used for welding metals that are susceptible to hydrogen cracking such as highstrength steels.
  32. 32. Functions of electrode (flux) covering • Deoxidation - Provide deoxidizers and fluxing agent to deoxidize and cleanse the weld metal. The solid slag also protects the weld metal from oxidation. • Arc stabilization - Provide arc stabilizers which are compounds such as potassium oxalate and lithium carbonate. They readily decompose into ions in an arc, which increase electrical conductivity. • Metal addition - Provide alloying elements (for composition control) and metal powder (increase deposition rate) to the weld pool.
  33. 33. Types of Electrodes Electrodes can be divided into three groups— 1. Fast-fill electrodes, Fast-fill electrodes are designed to melt quickly so that the welding speed can be maximized 2. Fast-freeze electrodes, fast-freeze electrodes supply filler metal that solidifies quickly, making welding in a variety of positions possible by preventing the weld pool from shifting significantly before solidifying. and 3. Intermediate electrodes go by the name "fill-freeze" or "fast-follow" electrodes.
  34. 34. Arc Welding Power Supplies • The current for arc welder can be supplied by line current or by an alternator/generator. – The amount of heat is determined by the current flow (amps) – The ease of starting and harshness of the arc is determined by the electrical potential (volts). • Welding current adjustments can include: – – – – – Amperage Voltage Polarity High frequency current Wave form
  35. 35. Amperage Output • The maximum output of the power supply determines the thickness of metal that can be welded before joint beveling is required. • 185 to 225 amps is a common size. • For an individual weld, the optimum output amperage is determined by – thickness of the metal – type of joint and – type of electrode
  36. 36. Five Common Output Currents For Arc Welding 1. AC (Alternating Current) 2. DC (Direct Current) 3. ACHF (Alternating Current-High Frequency) 4. PC (Pulsed Current) 5. Square wave
  37. 37. Alternating Current • Alternating current: The type of current where the flow of electrons reverses direction (polarity) at regular intervals. • Recommended for general purpose electrodes and flat position.
  38. 38. Alternating Current
  39. 39. Direct Current • Direct current: The type of current where the flow of electrons (polarity) is in one direction. • Controlling the polarity allows the welder to influence the location of the heat. • When the electrode is positive (+) DCRP, it will be slightly hotter than the base metal. • When the base metal is positive (+), DCSP, the base metal will be slightly hotter than the electrode. • DC is required for GMAW • It is frequently used for SMAW
  40. 40. Ampere • Electricity passing through a resistance causes heat. • An air gap is a high resistance • The greater the amperage flowing through the resistance (air gap)  the greater the heat. • The electrode also has resistance. • Excessive amperage for the diameter of the electrode (current density) over heats the electrode. • Insufficient amperage for the diameter of electrode makes the electrode hard to start.
  41. 41. Drooping Characterstics Constant Current Increasing the voltage from 20 to 25 volts (25%) only decreases the amperage from 113 to 120 Amp (5.8%).
  42. 42. Arc Welding Defects The most common quality problems associated with SMAW include • 1. Weld spatter Weld spatter, while not affecting the integrity of the weld, damages its appearance and increases cleaning costs. It can be caused by excessively high current, a long arc, or arc blow, a condition associated with direct current characterized by the electric arc being deflected away from the weld pool by magnetic forces. Arc blow can also cause porosity in the weld, as can joint contamination, high welding speed, and a long welding arc, especially when low-hydrogen electrodes are used. • 2. Porosity Porosity, often not visible without the use of advanced nondestructive testing methods, is a serious concern because it can potentially weaken the weld.
  43. 43. Arc Welding Defects • 3. Poor fusion Another defect affecting the strength of the weld is poor fusion, though it is often easily visible. It is caused by low current, contaminated joint surfaces, or the use of an improper electrode. • 4. Shallow penetration Shallow penetration, another detriment to weld strength, can be addressed by decreasing welding speed, increasing the current or using a smaller electrode. • 5. Cracking. Any of these weld-strength-related defects can make the weld prone to cracking, but other factors are involved as well. High carbon, alloy or sulfur content in the base material can lead to cracking, especially if low-hydrogen electrodes and preheating are not employed. Furthermore, the workpieces should not be excessively restrained, as this introduces residual stresses into the weld and can cause cracking as the weld cools and contracts.[10]
  44. 44. Advantages of arc welding • • • • • • • • • • 1. Simple welding equipment 2. Portable 3. Inexpensive power source 4. Relatively inexpensive equipment 5. Welders use standard domestic current. 6. Process is fast and reliable 7. Short learning curve 8. Equipment can be used for multiple functions 9. Electric arc is about 5,000 oC 10. Used for maintenance, repair, and field construction
  45. 45. Disadvantages • Not clean enough for reactive metals such as aluminium and titanium. • The deposition rate is limited because the electrode covering tends to overheat and fall off. • The electrode length is ~ 35 mm and requires electrode changing lower the overall production rate.
  46. 46. Flux-Cored Arc Welding (FCAW) • Uses an arc between a continuous filler metal electrode and a workpiece • Shielding is provided by a flux contained within the electrode • Additional shielding may be obtained from an externally supplied gas or gas mixture • Commonly used in construction because it is a fast welding process and is easily portable
  47. 47. Arc Welding
  48. 48. Arc Welding