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Welding - Processes, Metallurgy and Defects

Welding - Processes, Metallurgy and Defects

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Welding - Processes, Metallurgy and Defects

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  1. Welding - Processes, Metallurgy and Defects Summer 2010 Mikal Balmforth, P.E., CWEng, CWI MIT Research Associate MIT Welding Laboratory

  2. Overview Major welding processes Some welding metallurgy topics Common weld defects Other welding related issues

  3. Arc Welding Processes Shielded metal arc welding (SMAW) Also stick or MMA welding Simple, common process Consumable, flux coated electrode Gas tungsten arc welding (GTAW) Also TIG welding Non-consumable tungsten electrode Inert gas shielding Clean, precision work Filler added externally

  4. Arc Welding Processes Gas metal arc welding (GMAW) Also MIG or MAG welding Consumable wire electrode Gas shielding Often automated Flux cored arc welding (FCAW) Similar to GMAW Hollow electrode wire filled with flux Often no shielding gas

  5. Arc Welding Processes • Others • Submerged arc welding (SAW) • Plasma arc welding (PAW) • Underwater welding

  6. Resistance Welding Processes Spot welding Sheet metal Two copper electrodes Heat from resistance at interface Autos have thousands of spot welds Seam welding Wheel shaped electrodes Projection welding Dimples or projections in sheet or components concentrates heat Upset welding Components forged at elevated temperature

  7. Solid State Welding Processes Friction welding (FRW) Inertia welding (IFW) Friction stir welding (FSW)

  8. Solid State Welding Processes • Diffusion welding (DFW) • Often used in aircraft and aerospace • Ultrasonic welding (USW) • Used for plastics and electronic components • Explosion welding (EXW) • Often dissimilar metals or cladding

  9. High Energy Density Welding Processes Narrow, deep welds are produced Lower heat input and narrower HAZ High speeds are possible Laser beam welding (LBW) Solid state or gas lasers Usually done in air

  10. High Energy Density Welding Processes • Electron beam welding (EBW) • Concentrated stream of high-velocity electrons • Usually done in vacuum • Can be high power

  11. Allied Processes Brazing and soldering (450°C, 840°F) Oxyfuel gas welding Thermal cutting Oxygen, plasma arc, other Gouging

  12. Regions of a Fusion Weld Fusion zone (FZ) – weld metal Fusion boundary (FB) Heat affected zone (HAZ) Unaffected base metal (BM) FB FZ HAZ BM

  13. Fusion Zone Autogenous, homogeneous or heterogeneous Molten to solid

  14. Heat Affected Zone Grain growth, recovery and recrystallization, precipitates, stresses

  15. Welding Metallurgy Issues • How does the heat from welding affect the material and its properties? • Control microstructures and prevent defects • What filler metals, processing conditions, and welding techniques to use • Chemistry • Heat input • Welds often require pre- and/or post-heating

  16. Classification of Weld Defects Fabrication Delayed (hydrogen-induced) Service

  17. Porosity Incomplete fusion Incomplete penetration Undercut Fabrication Defects

  18. Fabrication Defects Cont. Solidification cracking Also called hot or centerline cracking HAZ hot cracking Inclusions – slag or tungsten

  19. Fabrication Defects Cont. Reheat cracking Liquid metal embrittlement (LME)

  20. Fabrication Defects Cont. Lamellar tearing Incorrect weld shape or size Spatter

  21. Delayed (H-induced) Defects Usually initiates in course grained HAZ, but can occur in weld metal Four requirements: Susceptible microstructure Source of hydrogen Low temperature Tensile stress

  22. Service Defects Fatigue failures Cyclic loading conditions Cracks propagate from weld defects Mechanical overload Ductile rupture can initiate at defects

  23. Service Defects • Corrosion • Intergranular corrosion (IGC) • Localized grain boundary attack • Sensitization in austenitic stainless steels • Stress corrosion cracking (SCC) • Tensile stress combined with corrosive media • Transgranular or intergranular crack propagation

  24. Significance of Weld Defects Failure initiation sites Reduced tensile and other mechanical properties Reduced fatigue life Toughness / Fracture mechanics Allowable defect size

  25. Other Welding Related Issues Codes and Standards / Certification WPS, PQR, WPQR AWS, ASME, API, etc. Testing / Inspection Metallography, SEM, mechanical testing NDE Visual, dye penetrant, ultrasonics, x-ray, magnetic particle, eddy current Safety Worker, workplace, fumes, fire prevention Weld repair Stresses and distortion