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Arc Welding Processes

Arc Welding Processes

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Arc Welding Processes

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  1. Arc Welding Processes

  2. Arc Welding Processes • Learning Activities • Read Handbook Pp 1-16, • Look up Keywords • View Slides; • Read Notes, • Listen to • lecture • Do on-line workbook • Do homework • Lesson Objectives • When you finish this lesson you will understand: • The similarities and difference between some of the various arc welding processes • Flux and gas shielding methods • Advantages and disadvantages of the arc welding processes • Need to select between the processes Keywords Welding Flux, Inert Shielding Gas, Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Metal Transfer Mode, Flux Cored Arc Welding FCAW), Submerged Arc Welding (SAW),

  3. Linnert, Welding Metallurgy, AWS, 1994

  4. Arc Welding Processes • Welding processes that employ an electric arc are the most prevalent in industry • Shielded Metal Arc Welding • Gas Metal Arc Welding • Flux Cored Arc Welding • Submerged Arc Welding • Gas Tungsten Arc Welding • These processes are associated with molten metal Electric Arc

  5. Linnert, Welding Metallurgy, AWS, 1994

  6. Protection of the Molten Weld Pool • Molten metal reacts with the atmosphere • Oxides and nitrides are formed • Discontinuities such as porosity • Poor weld metal properties • All arc welding processes employ some means of shielding the molten weld pool from the air

  7. Welding Flux • Three forms • Granular • Electrode wire coating • Electrode core • Fluxes melt to form a protective slag over the weld pool • Other purposes • Contain scavenger elements to purify weld metal • Contain metal powder added to increase deposition rate • Add alloy elements to weld metal • Decompose to form a shielding gas

  8. Shielding Gas • Shielding gas forms a protective atmosphere over the molten weld pool to prevent contamination • Inertshielding gases, argon or helium, keep out oxygen, nitrogen, and other gases • Active gases, such as oxygen and carbon dioxide, are sometimes added to improve variables such as arc stability and spatter reduction Argon Helium Oxygen Carbon Dioxide

  9. Questions? • Turn to the person sitting next to you and discuss (1 min.): • What would happen if there was no flux on the wire to decompose into gas or no inert shielding gas was provided? • What would the weld metal look like?

  10. Shielded Metal Arc Welding SMAW

  11. Shielded Metal Arc Welding (SMAW)

  12. SMAW Electrode Classification Example E7018 • E indicates electrode • 70 indicates 70,000 psi tensile strength • 1 indicates use for welding in all positions • 8 indicates low hydrogen E7018-A1-H8R

  13. ANSI/AWS - 5.1 : Specification for Covered Carbon Steel • ANSI/AWS - 5.5 : Specification for Low Alloy Steel • ANSI/AWS - 5.4 : Specification for Corrosion Resistant Steel AWS Website: http://www.aws.org

  14. Coating Materials -Partial List Slipping Agents to Aid Extrusion Clay Talc Glycerin Binding Agents Sodium Silicate Asbestos Starch Sugar Alloying and Deoxidizing Elements Si, Al, Ti, Mn, Ni, Cr Arc Stabilizers Titania TiO2 Gas-Forming Materials Wood Pulp Limestone CaCO3 Slag-Forming Materials Alumina Al2O3 TiO2 SiO2 Fe3O4

  15. Linnert, Welding Metallurgy AWS, 1994

  16. Linnert, Welding Metallurgy AWS, 1994

  17. Shielded Metal Arc Welding SMAW Advantages • Easily implemented • Inexpensive • Flexible • Not as sensitive to part fit-up variances

  18. Advantages • Equipment relatively easy to use, inexpensive, portable • Filler metal and means for protecting the weld puddle are provided by the covered electrode • Less sensitive to drafts, dirty parts, poor fit-up • Can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, aluminum

  19. Shielded Metal Arc Welding Quality Issues • Discontinuities associated with manual welding process that utilize flux for pool shielding • Slag inclusions • Lack of fusion • Other possible effects on quality are porosity, and hydrogen cracking

  20. Shileded Metal Arc Welding Limitations • Low Deposition Rates • Low Productivity • Operator Dependent

  21. Other Limitations • Heat of welding too high for lead, tin, zinc, and their alloys • Inadequate weld pool shielding for reactive metals such as titanium, zirconium, tantalum, columbium

  22. Questions? • Turn to the person sitting next to you and discuss (1 min.): • Wood (cellulose) and limestone are added to the coating on SMAW Electrodes for gas shielding. What gases might be formed? • How do these gases shield?

  23. Gas Metal Arc Welding Gas Metal Arc Welding

  24. Gas Metal Arc Welding GMAW Modes of Metal Transfer Globular Spray Pulsed Spray Short Circuiting

  25. Gas Metal Arc Welding GMAW Filler Metal Designations ER - 70S - 6 Composition 6 = high silicon Electrode Solid Electrode Rod (can be used with GMAW) Minimum ultimate tensile strength of the weld metal

  26. AWS Specifications for GMAW Wire AWS A5.18 - Carbon Steel Electrodes AWS A5.28 - Low Alloy Steel Electrodes

  27. Gas Metal Arc Welding Shielding Gas • Shielding gas can affect • Weld bead shape • Arc heat, stability, and starting • Surface tension • Drop size • Puddle flow • Spatter Ar He CO2 Ar-He

  28. Gas Metal Arc Welding GMAW Advantages • Deposition rates higher than SMAW • Productivity higher than SMAW with no slag removal and continuous welding • Easily automated

  29. Gas Metal Arc Welding Quality • Spatter • Droplets of electrode material that land outside the weld fusion area and may or may not fuse to the base material • Porosity • Small volumes of entrapped gas in solidifying weld metal

  30. Gas Metal Arc Welding Limitations • Equipment is more expensive and complex than SMAW • Process variants/metal transfer mechanisms make the process more complex and the process window more difficult to control • Restricted access • GMAW gun is larger than SMAW holder

  31. Questions? • Turn to the person sitting next to you and discuss (1 min.): • When comparing processes that have spray and globular metal transfer, which type of transfer mode do you thnk results in more spatter? Why?

  32. Flux-Cored Arc Welding Flux Cored Arc Welding (FCAW)

  33. Linnert, Welding Metallurgy, AWS, 1994

  34. Flux-Cored Arc Welding FCAW Electrode Classification E70 T - 1 Electrode Type Gas, Usability and Performance Minimum UTS 70,000 psi Flux Cored /Tubular Electrode Position American Welding Society Specification AWS A5.20 and AWS A5.29.

  35. Linnert, Welding Metallurgy AWS, 1994

  36. Flux-Cored Arc Welding Advantages • High deposition rates • Deeper penetration than SMAW • High-quality • Less pre-cleaning than GMAW • Slag covering helps with larger out-of-position welds • Self-shielded FCAW is draft tolerant.

  37. Flux-Cored Arc Welding Limitations • Slag must be removed • More smoke and fumes than GMAW and SAW • Spatter • FCAW wire is more expensive • Equipment is more expensive and complex than for SMAW

  38. Questions? • Turn to the person sitting next to you and discuss (1 min.): • What do you suppose would happen if the powder inside the core did not get compacted good?

  39. Submerged Arc Welding Submerged Arc Welding

  40. Submerged Arc Welding SAW Flux / Filler Metal Compositions F7A2-EM12K • F indicates flux • 70-95 ksi UTS, 58 ksi minimum yield strength, 22% elongation • A - as welded; P - postweld heat treated • 2 - minimum impact properties of 20 ft-lbs @ 20°F • E indicates electrode (EC - composite electrode) • M - medium manganese per AWS Specifications • 12 - 0.12% nominal carbon content in electrode • K - produced from a heat of aluminum killed steel

  41. Submerged Arc Welding Advantages • High deposition rates • No arc flash or glare • Minimal smoke and fumes • Flux and wire added separately - extra dimension of control • Easily automated • Joints can be prepared with narrow grooves • Can be used to weld carbon steels, low alloy steels, stainless steels, chromium-molybdenum steels, nickel base alloys

  42. Submerged Arc Welding Limitations • Flux obstructs view of joint during welding • Flux is subject to contamination Þ porosity • Normally not suitable for thin material • Restricted to the flat position for grooves - flat and horizontal for fillets • Slag removal required • Flux handling equipment

  43. Homework Do Homework Assignment 2, “Arc Welding Processes” from the Assignment Page of the WE300 Website. Turn in next Class Period.