Types of welding

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Welding and it's classification,
Types of welding with their applications, advantages, disadvantages
Arc welding, TIG welding, MIG welding, SAW welding, Thermit welding

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Types of welding

  1. 1. Welding
  2. 2. • Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of Heat alone, and with or without the use of filler material. . Heat may be obtained by chemical reaction, electric arc, electrical resistance, frictional heat, sound and light energy. If no filter metal is used during welding then it is termed as Autogenous Welding Process • Welding is used for making permanent joints. • It is used in the manufacturing of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.
  3. 3.  During ‘Bronze Age' parts were joined by forge welding to produce tools, weapons and ornaments  First application of welding with carbon electrode was developed in 1885 while metal arc welding with bare electrode was patented in 1890.  In the mean time resistance butt welding was invented in USA in the year 1886. Other resistance welding processes such as spot and flash welding with manual application of load were developed around 1905.  With the production of cheap oxygen in 1902, oxy – acetylene welding became feasible in Europe in 1903.  When the coated electrodes were developed in 1907, the manual metal arc welding process become viable for production/fabrication of components in the industries on large scale.
  4. 4. Subsequently other developments are as follows: • Thermit Welding (1903) • Arc Stud Welding (1918) • Seam Welding of Tubes (1922) • Mechanical Flash Welder for Joining Rails (1924) • Extruded Coating for MMAW Electrodes (1926) • Submerged Arc Welding (1935) • Air Arc Gouging (1939) • Inert Gas Tungsten Arc (TIG) Welding (1941) • Iron Powder Electrodes (1944) • Inert Gas Metal Arc (MIG) Welding (1948) • Electro Slag Welding (1951)
  5. 5. • Flux Cored Wire with CO 2 Shielding (1954) • Electron Beam Welding (1954) • Constricted Arc (Plasma) for Cutting (1955) • Friction Welding (1956) • Plasma Arc Welding (1957) • Electro Gas Welding (1957) • Short Circuit Transfer for Low Current, Low Voltage Welding with CO2 Shielding (1957) • Vacuum Diffusion Welding (1959) • Explosive Welding (1960) • Laser Beam Welding (1961) • High Power CO2 Laser Beam Welding (1964)
  6. 6. Advantages of welding • A good weld is as strong as the base metal. • General welding equipment is not very costly. • Portable welding equipment's are available. • A large number of metals/alloys both similar and dissimilar can be joined by welding.
  7. 7. Disadvantages of welding • Welding gives out harmful radiations (light), fumes and spatter. • Welding results in residual stresses and distortion of the work pieces. • Jigs and fixtures are generally required to hold and position the parts to welded. • Edge preparation of the work pieces is generally required before welding them. • A skilled welder is a must to produce a good welding job.
  8. 8. Choice of welding process
  9. 9. GAS WELDING • Gas welding process was introduced in 1903. • Gas welding is a fusion welding process. • It join metals, using the heat of combustion of an oxygen/air and fuel gas (i.e. acetylene, hydrogen, propane or butane) mixture. • Intense heat (flame) thus produces melts and fuses together the edges of the parts to be welded, with addition of a filler metal. • Oxy-acetylene flame temp 3480 degree Celsius
  10. 10. Oxyfuel Gas Welding (OFW) Group of fusion welding operations that burn various fuels mixed with oxygen • OFW employs several types of gases, which is the primary distinction among the members of this group • Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts • Most important OFW process is oxyacetylene welding
  11. 11. Fuels • Propane (LPG) C3H8 • Natural Gas CH4 • Acetylene C2H2 • MAPP (Methylacetylene-propadiene) • Hydrogen
  12. 12. 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
  13. 13. Oxyacetylene Welding
  14. 14. Acetylene (C2H2) • Most popular fuel among OFW group because it is capable of higher temperatures than any other • Up to 3480C (6300F) • Two stage reaction of acetylene and oxygen: • First stage reaction (inner cone of flame) C2H2 + O2  2CO + H2 + heat • Second stage reaction (outer envelope) 2CO + H2 + 1.5O2  2CO2 + H2O + heat
  15. 15. • Maximum temperature reached at tip of inner cone, while outer envelope spreads out and shields work surface from atmosphere • Shown below is neutral flame of oxyacetylene torch indicating temperatures achieved Oxyacetylene Torch
  16. 16. GAS WELDING EQUIPMENT... 1. Gas Cylinders Pressure Oxygen – 125 kg/cm2 Acetylene – 16 kg/cm2 2. Regulators Working pressure of oxygen 1 kg/cm2 Working pressure of acetylene 0.15 kg/cm2 Working pressure varies depends upon the thickness of the work pieces welded. 3. Pressure Gauges 4. Hoses 5. Welding torch 6. Check valve 7. Non return valve
  17. 17. Oxy-Acetylene welding
  18. 18. Typical Portable Oxygen/ Fuel Cutting Rig
  19. 19. Oxygen/ Fuel Hose Green = Oxygen Red = Fuel
  20. 20. Typical Cutting Torch
  21. 21. TYPES OF FLAMES… • Oxygen is turned on, flame immediately changes into a long white inner area (Feather) surrounded by a transparent blue envelope is called Carburizing flame (30000c) • Addition of little more oxygen give a bright whitish cone surrounded by the transparent blue envelope is called Neutral flame (It has a balance of fuel gas and oxygen) (32000c) • Used for welding steels, aluminium, copper and cast iron • If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter and more fierce is called Oxidizing flame • Has the highest temperature about 34000c • Used for welding brass and brazing operation
  22. 22. 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.
  23. 23. 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.
  24. 24. Gas Welding Techniques 1. Fore hand Welding 2. Back hand welding
  25. 25. 1. Fore hand Welding
  26. 26. 2. Back hand welding
  27. 27. Advantages of gas welding • It is probably the most versatile processes. It can be applied to a wide variety of manufacturing and maintenance situations. • Since the sources of heat and of filler metal are separate, the welder has control over filler- metal deposition rates. • The equipment is versatile, low cost, self- sufficient and usually portable. • The cost and maintenance of the welding equipment is low when compared to that of some other welding processes.
  28. 28. Disadvantages • Heavy sections cannot be joined economically. • Flame temp is less than the temp of the arc. • Fluxes used with certain welding and brazing operations produce fumes that are irritating to the eyes, nose, throat and lungs. • Refractory metals (e.g., tungsten, molybdenum, tantalum, etc.) and reactive metals (e.g., titanium and zirconium) can not be gas welded. • More safety problems are associated with the handling and storing of gases.
  29. 29. Arc Welding (AW) • A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work • Electric energy from the arc produces temperatures ~ 10,000 F (5500 C), hot enough to melt any metal • Most AW processes add filler metal to increase volume and strength of weld joint
  30. 30. What is an Electric Arc? • An electric arc is a discharge of electric current across a gap in a circuit • It is sustained by an ionized column of gas (plasma) through which the current flows • To initiate the arc in AW, electrode is brought into contact with work and then quickly separated from it by a short distance
  31. 31. • A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake Arc Welding
  32. 32. Manual Arc Welding and Arc Time • Problems with manual welding: • Weld joint quality • Productivity • Arc Time = (time arc is on) divided by (hours worked) • Also called “arc-on time” • Manual welding arc time = 20% • Machine welding arc time ~ 50%
  33. 33. Two Basic Types of AW Electrodes • Consumable – consumed during welding process • Source of filler metal in arc welding • Nonconsumable – not consumed during welding process • Filler metal must be added separately if it is added
  34. 34. Consumable Electrodes • Forms of consumable electrodes • Welding rods (a.k.a. sticks) are 9 to 18 inches and 3/8 inch or less in diameter and must be changed frequently • Weld wire can be continuously fed from spools with long lengths of wire, avoiding frequent interruptions • In both rod and wire forms, electrode is consumed by the arc and added to weld joint as filler metal
  35. 35. Nonconsumable Electrodes • Made of tungsten which resists melting • Gradually depleted during welding (vaporization is principal mechanism) • Any filler metal must be supplied by a separate wire fed into weld pool
  36. 36. Arc Shielding • At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air • Mechanical properties of joint can be degraded by these reactions • To protect operation, arc must be shielded from surrounding air in AW processes • Arc shielding is accomplished by: • Shielding gases, e.g., argon, helium, CO2 • Flux
  37. 37. Flux A substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal • Provides protective atmosphere for welding • Stabilizes arc • Reduces spattering
  38. 38. Various Flux Application Methods • Pouring granular flux onto welding operation • Stick electrode coated with flux material that melts during welding to cover operation • Tubular electrodes in which flux is contained in the core and released as electrode is consumed
  39. 39. Power Source in Arc Welding • Direct current (DC) vs. Alternating current (AC) • AC machines less expensive to purchase and operate, but generally restricted to ferrous metals • DC equipment can be used on all metals and is generally noted for better arc control
  40. 40. Consumable Electrode AW Processes • Shielded Metal Arc Welding • Gas Metal Arc Welding • Flux-Cored Arc Welding • Electrogas Welding • Submerged Arc Welding
  41. 41. Arc Welding Electrical Terms 1. Electrical Circuit 2. Direct current (DC) 3. Alternating current (AC) 4. Ampere 5. Volt 6. Resistance 7. Ohms Law 8. Constant potential 9. Constant current 10. Voltage drop 11. Open circuit voltage 12. Arc voltage 13. Polarity 14. Watt To understand how an electric arc welder produces the correct heat for arc welding, you must understand the following fourteen (14) electrical terms.
  42. 42. Terms 1 - Electrical Circuit • An electrical circuit is a complete path for electricity. • Establishing an arc completes an electric circuit . Current will not flow through an open circuit.
  43. 43. 2 - Direct Current • Direct current: A type of current where the flow of electrons is in one direction. • In arc welding the direction of flow is called the polarity.
  44. 44. 3 - Alternating Current • Alternating current: The type of current where the flow of electrons reverses direction at regular intervals.
  45. 45. 4 - Ampere • Amperes: the unit of measure for current flow. • One ampere is equal to 6.24150948×1018 electrons passing by a point per second. • Electricity passing through a resistance causes heat. • An air gap is a high resistance Arc welding requires large electrical currents 100-1000A.
  46. 46. 5 - Voltage • Voltage is the measure of electromotive force (Emf). • Emf is measured in units of volts • The voltage at the electrode for MAW determines the ease of starting and the harshness of the arc.
  47. 47. 6 - Resistance • Resistance is the characteristic of a material that impedes the flow of an electrical current. • Measured in units of Ohm’s (  ) • When an electrical current passes through a resistance heat is produced.
  48. 48. 7 - Ohm’s Law • Commonly expressed as: • Voltage is equal to amps x resistance • For arc welding rearranged as: • Amperage is the voltage divided by the resistance. E = IR  I = E R
  49. 49. 8 - Constant Potential A constant potential power supply is designed to produce a relatively constant voltage over a range of amperage changes. Primarily used for GMAW FCAW
  50. 50. Constant Current • In a constant current power supply, the current (amperage) stays relatively constant over a narrow range of voltages. • Primarily used for: SMAW TIG
  51. 51. 10 - Voltage Drop • Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. • Primary cause is resistance. • Excessive voltage drop reduces the heat of the arc.
  52. 52. 11 - Open Circuit Voltage • Open circuit voltage is the potential voltage between the electrode and the work when the arc is not present. • The higher the OCV the easier the arc is to start. • The higher the OCV the steeper the volt – amp curve.
  53. 53. 12 - Arc Voltage Arc voltage is the electrical potential between the electrode and the metal after the arc has started. The arc voltage depends only upon the arc length V = k1 + k2l Volts Where l is the arc length in mm and k1 and k2 are constants, k1 = 10 to 12; and k2 = 2 to 3 The minimum Arc voltage is given by Vmin = (20 + 0.04 l) Volt
  54. 54. 13 - Polarity Polarity (positive & negative) is present in all electrical circuits. Electricity flows from negative to positive Controlling the polarity allows the welder to influence the location of the heat. When the electrode is positive (+) it will be slightly hotter than the base metal. When the base metal is positive (+) the base metal will be slightly hotter than the electrode. What abbreviations are used to indicate the polarity of the electrode? DCEN or DCSP [direct current electrode negative or direct current straight polarity] DCEP or DCRP [direct current electrode positive or direct current reverse polarity]
  55. 55. Arc welding equipment's 1. Droppers: Constant current welding machines Good for manual welding 2. Constant voltage machines Good for automatic welding 58
  56. 56. 14 - Watt Watts are a measure of the amount of electrical energy being consumed. Watts = Volts x Amps The greater the Watts of energy flowing across an air gap the greater the heat produced. Power to drive the operation is the product of the current I passing through the arc and the voltage E across it. This power is converted into heat, but not all of the heat is transferred to the surface of the work. Convection, conduction, radiation, and spatter account for losses that reduce the amount of usable heat
  57. 57. Arc Welding Power Supplies The type of current and the polarity of the welding current are one of the differences between arc welding processes. • SMAW Constant current (CC), AC, DC+ or DC- • GMAW Constant voltage (CV) DC+ • FCAW Constant voltage (CV) DC- • GTAW Constant Current (CC) ), AC, DC+ or DC-
  58. 58. 1: 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. • Welding current depends upon: the thickness of the welded metal, type of joint, welding speed, position of the weld, the thickness and type of the coating on the electrode and its working length. • Welding current, I = k. d, amperes; d is dia. (mm)
  59. 59. 2: Duty cycle • The amount of continuous welding time a power supply can be used is determined by the duty cycle of the power supply. • Duty cycle is based on a 10 minute interval. • Many power supplies have a sloping duty cycle.
  60. 60. 2: Duty cycle The percentage of time in a 5 min period that a welding machine can be used at its rated output without overloading. Time is spent in setting up, metal chipping, Cleaning and inspection. For manual welding a 60% duty cycle is suggested and for automatic welding 100% duty cycle. 63
  61. 61. Atomic hydrogen welding An a.c. arc is formed between two tungsten electrodes along which streams of hydrogen are fed to the welding zone. The molecules of hydrogen are dissociated by the high heat of the arc in the gap between the electrodes. The formation of atomic hydrogen proceeds with the absorption of heat: This atomic hydrogen recombines to form molecular hydrogen outside the arc, particularly on the relatively cold surface of the work being welded, releasing the heat gained previously: 64
  62. 62. Atomic hydrogen welding • Temperature of about 3700 ˚C. • Hydrogen acts as shielding also. • Used for very thin sheets or small diameter wires. • Lower thermal efficiency than Arc welding. • Ceramics may be arc welded. • AC used. 67
  63. 63. THERMIT WELDING It is a process in which a mixture of aluminum powder and a metal oxide called Thermit is ignited to produce the required quantity of molten metal By an exothermic non violent reaction .
  64. 64. Arc Welding processes
  65. 65. MIG
  66. 66. • GMAW stands for Gas Metal Arc Welding • GMAW is commonly referred to as MIG or Metal Inert Gas welding • During the GMAW process, a solid metal wire is fed through a welding gun and becomes the filler material • Instead of a flux, a shielding gas is used to protect the molten puddle from the atmosphere which results in a weld without slag
  67. 67. GMAW Equipment • Power Supply • Wire Feeder • Electrical mechanical device that feed required amount of filler material at a constant rate of speed
  68. 68. GMAW Equipment • Welding filler electrode • Small diameter consumable electrode that is supplied to the welding gun by the roller drive system • Shielding Gas • Gas used to protect the molten metal from atmospheric contamination • 75%Argon (inert gas) & 25% Carbon Dioxide most common gas used for GMAW
  69. 69. GMAW Components • Let’s look a little closer at the GMAW process Travel direction Electrode 1 Arc2 Weld Puddle 3 Shielding Gas4 5 Solidified Weld Metal Generally, drag on thin sheet metal and push on thicker materials
  70. 70. 1 - Electrode • A GMAW electrode is: – A metal wire – Fed through the gun by the wire feeder – Measured by its diameter GMAW electrodes are commonly packaged on spools, reels and coils
  71. 71. 2 - Arc • An electric arc occurs in the gas filled space between the electrode wire and the work piece Electric arcs can generate temperatures up to 10,000°F
  72. 72. 3 - Weld Puddle • As the wire electrode and work piece heat up and melt, they form a pool of molten material called a weld puddle • This is what the welder watches and manipulates while welding .045” ER70S-6 at 400 ipm wire feed speed and 28.5 Volts with a 90% Argon/ 10% CO2 shielding gas
  73. 73. 4 - Shielding Gas • GMAW welding requires a shielding gas to protect the weld puddle • Shielding gas is usually inert gases , CO2 or a mixture of both The gauges on the regulator show gas flow rate and bottle pressure
  74. 74. 5 - Solidified Weld Metal • The welder “lays a bead” of molten metal that quickly solidifies into a weld • The resulting weld is slag free An aluminum weld done with the GMAW process
  75. 75. Advantages of GMAW • High operating factor • Easy to learn • Limited cleanup • Use on many different metals: stainless steel, mild (carbon) steel, aluminum and more • All position • Great for small scale use with 115V and 230V units
  76. 76. Limitations of GMAW • Less portable • GMAW equipment is more expensive than SMAW equipment • External shielding gas can be blown away by winds • High radiated heat • Difficult to use in out of position joints
  77. 77. TIG • Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.
  78. 78. TIG
  79. 79. TIG
  80. 80. Water cooled torch of TIG
  81. 81. TIG Shielding Gases Argon • Good arc starting • Good cleaning action • Good arc stability • Focused arc cone • Lower arc voltages • 10-30 CFH flow rates Helium • Faster travel speeds • Increased penetration • Difficult arc starting • Less cleaning action • Less low amp stability • Flared arc cone • Higher arc voltages • Higher flow rates (2x) • Higher cost than argon
  82. 82. TIG Shielding Gases Argon/Helium Mixtures • Improved travel speeds over pure argon • Improved penetration over pure argon • Cleaning properties closer to pure argon • Improved arc starting over pure helium • Improved arc stability over pure helium • Arc cone shape more focused than pure helium • Arc voltages between pure argon and pure helium • Higher flow rates than pure argon • Costs higher than pure argon
  83. 83. SAW
  84. 84. SAW • The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.
  85. 85. Advantages • High deposition rates (over 100 lb/h (45 kg/h) have been reported). • High operating factors in mechanized applications. • Deep weld penetration. • Sound welds are readily made (with good process design and control). • High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible. • Minimal welding fume or arc light is emitted. • Practically no edge preparation is necessary. • The process is suitable for both indoor and outdoor works. • Low distortion • Welds produced are sound, uniform, ductile, corrosion resistant and have good impact value. • Single pass welds can be made in thick plates with normal equipment. • The arc is always covered under a blanket of flux, thus there is no chance of spatter of weld. • 50% to 90% of the flux is recoverable.
  86. 86. Limitations • Preferred for ferrous (steel or stainless steels) and some nickel-based alloys. • Normally limited to long straight seams or rotated pipes or vessels. • Requires relatively troublesome flux handling systems. • Flux and slag residue can present a health and safety concern. • Requires inter-pass and post weld slag removal.
  87. 87. Also Known AS • Wire Feed • MIG = Metal Inert Gas • Inert Gas= Inactive gas that does not combine chemically with base or filler metal • MAG= Metal Active Gas • Active Gas= Gas will combine chemically with base or filler metal
  88. 88. Advantages • Variety of Metals • All Position Welding • Quality Welds • Little to No Slag • Low Spatter Disadvantages • Cost • Portability • Clean Base Material
  89. 89. GMAW Equipment • Power Supply • Wire Feeder • Electrical mechanical device that feed required amount of filler material at a constant rate of speed
  90. 90. GMAW Equipment • Welding filler electrode • Small diameter consumable electrode that is supplied to the welding gun by the roller drive system • Shielding Gas • Gas used to protect the molten metal from atmospheric contamination • 75%Argon (inert gas) & 25% Carbon Dioxide most common gas used for GMAW
  91. 91. Principles of the GMAW Process
  92. 92. GMAW Process Parameters Steel Material .035” wire Short-Arc Mode Thickness Gas 75%AR- 25%CO2 Amps Wire Speed Volts 1/8” 18-19 140-150 280-300 23-24 3/16” 18-19 160-170 320-340 24-25 1/4” 21-22 180-190 360-380 24-25 5/16” 21-22 200-210 400-420 25-26 3/8” 23-24 220-250 420-520 26-27
  93. 93. TIG • Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas (argon or helium), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.
  94. 94. TIG
  95. 95. TIG
  96. 96. Water cooled torch of TIG
  97. 97. The tungsten arc process is being employed widely for the precision joining of critical components which require controlled heat input. The small intense heat source provided by the tungsten arc is ideally suited to the controlled melting of the material. Since the electrode is not consumed during the process, as with the MIG or MMA welding processes, welding without filler material can be done without the need for continual compromise between the heat input from the arc and the melting of the filler metal.
  98. 98. TIG Shielding Gases Argon • Good arc starting • Good cleaning action • Good arc stability • Focused arc cone • Lower arc voltages • 10-30 CFH flow rates Helium • Faster travel speeds • Increased penetration • Difficult arc starting • Less cleaning action • Less low amp stability • Flared arc cone • Higher arc voltages • Higher flow rates (2x) • Higher cost than argon
  99. 99. TIG Shielding Gases Argon/Helium Mixtures • Improved travel speeds over pure argon • Improved penetration over pure argon • Cleaning properties closer to pure argon • Improved arc starting over pure helium • Improved arc stability over pure helium • Arc cone shape more focused than pure helium • Arc voltages between pure argon and pure helium • Higher flow rates than pure argon • Costs higher than pure argon
  100. 100. SAW
  101. 101. SAW • The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.
  102. 102. Fluxes are fused or agglomerated consisting of MnO, SiO2, CaO, MgO, Al2O3, TiO2, FeO, and CaF2 and sodium/potassium silicate The ratio of contents of all basic oxides to all acidic oxides in some proportion is called basicity index of a flux. CaO, MgO, BaO, CaF2, Na2O, K2O, MnO are basic constituents while SiO2, TiO2, Al2O3 are considered to be acidic constituents.
  103. 103. Electrode wire size, welding voltage, current and speed are four most important welding variables apart from flux. Welding voltage has nominal effect on the electrode wire melting rate but high voltage leads to flatter and wider bead, increased flux consumption and resistance to porosity caused by rust or scale and helps bridge gap when fill up is poor. Lower voltage produces resistance to arc blow but high narrow bead with poor slag removal. Welding voltages employed vary from 22 to 35 V
  104. 104. If the welding speed is increased, power or heat input per unit length of weld is decreased, less welding material is applied per unit length of weld, and consequently less weld reinforcement results and penetration decreases. Travel speed is used primarily to control bead size and penetration. It is interdependent with current. Excessive high travel speed decreases wetting action, increases tendency for undercut, arc blow, porosity and uneven bead shapes while slower travel speed reduces the tendency to porosity and slag inclusion.
  105. 105. Influence of Welding Parameters on Bead Shape.
  106. 106. Advantages • High deposition rates (over 100 lb/h (45 kg/h) have been reported). • High operating factors in mechanized applications. • Deep weld penetration. • Sound welds are readily made (with good process design and control). • High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible. • Minimal welding fume or arc light is emitted. • Practically no edge preparation is necessary. • The process is suitable for both indoor and outdoor works. • Low distortion • Welds produced are sound, uniform, ductile, corrosion resistant and have good impact value. • Single pass welds can be made in thick plates with normal equipment. • The arc is always covered under a blanket of flux, thus there is no chance of spatter of weld. • 50% to 90% of the flux is recoverable.
  107. 107. Limitations • Preferred for ferrous (steel or stainless steels) and some nickel-based alloys. • Normally limited to long straight seams or rotated pipes or vessels. • Requires relatively troublesome flux handling systems. • Flux and slag residue can present a health and safety concern. • Requires inter-pass and post weld slag removal.

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