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8/13/2019 Welding - Processes, Metallurgy and Defects
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Welding -
Processes, Metallurgyand Defects
Summer 2010
Mikal Balmforth, P.E., CWEng, CWI
MIT Research Associate
MIT Welding Laboratory
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Overview
Major welding processes
Some welding metallurgy topics
Common weld defects
Other welding related issues
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Arc Welding Processes
Shielded metal arc welding (SMAW) Also stick or MMA welding
Simple, common process
Consumable, flux coatedelectrode
Gas tungsten arc welding (GTAW)
Also TIG welding Non-consumable tungsten electrode
Inert gas shielding
Clean, precision work
Filler added externally
http://en.wikipedia.org/wiki/Image:GTAW_welding.jpghttp://en.wikipedia.org/wiki/Image:GTAW_welding.jpghttp://en.wikipedia.org/wiki/Image:SMAW.welding.navy.ncs.jpg -
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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 filledwith flux
Often no shielding gas
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Arc Welding Processes
Others Submerged arc welding (SAW)
Plasma arc welding (PAW)
Underwater welding
http://images.google.com/imgres?imgurl=http://www.pagesbydave.com/divepics/images/underwater%2520welding.jpg&imgrefurl=http://weldingbest.blogspot.com/2008_04_01_archive.html&usg=__KAwYHt58qbwP2O3o_JXSmFlxguM=&h=454&w=600&sz=208&hl=en&start=3&um=1&tbnid=66sn_j36VcalaM:&tbnh=102&tbnw=135&prev=/images%3Fq%3Dgmaw%2Bwelding%26um%3D1%26hl%3Den%26rlz%3D1T4GGLJ_en%26sa%3DN -
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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
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Solid State Welding Processes
Friction welding (FRW)
Inertia welding (IFW)
Friction stir welding (FSW)
http://images.google.com/imgres?imgurl=http://www.nrc-cnrc.gc.ca/multimedia/picture/images/html/nrc-iar_technology-lr.jpg&imgrefurl=http://www.nrc-cnrc.gc.ca/multimedia/picture/engineering/nrc-iar_technology_e.html&usg=__IbyS_EVYUc2rmcwXTDIXo6hm8oU=&h=240&w=400&sz=115&hl=en&start=3&um=1&tbnid=3SZnVksr4BgQQM:&tbnh=74&tbnw=124&prev=/images%3Fq%3Dfriction%2Bstir%2Bwelding%26um%3D1%26hl%3Den%26rlz%3D1T4GGLJ_en%26sa%3DXhttp://www.edax.com/technology/EBSD/images/fsw_pic.jpghttp://www.mtiwelding.com/video/Weld_Demo_MR_Broadband.asx -
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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
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High Energy Density WeldingProcesses
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
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High Energy Density WeldingProcesses
Electron beam welding (EBW)
Concentrated stream ofhigh-velocity electrons
Usually done in vacuum Can be high power
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Allied Processes
Brazing and soldering (450C, 840
F)
Oxyfuel gas welding
Thermal cutting
Oxygen, plasma arc, other
Gouging
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Regions of a Fusion Weld
Fusion zone (FZ) weld metal
Fusion boundary (FB)
Heat affected zone (HAZ)
Unaffected base metal (BM)
BM
FB
HAZFZ
http://upload.wikimedia.org/wikipedia/commons/0/0d/Welded_butt_joint_x-section.pnghttp://upload.wikimedia.org/wikipedia/commons/0/0d/Welded_butt_joint_x-section.pnghttp://upload.wikimedia.org/wikipedia/commons/0/0d/Welded_butt_joint_x-section.pnghttp://upload.wikimedia.org/wikipedia/commons/0/0d/Welded_butt_joint_x-section.pnghttp://upload.wikimedia.org/wikipedia/commons/0/0d/Welded_butt_joint_x-section.png -
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Fusion Zone
Autogenous, homogeneous orheterogeneous
Molten to solid
http://www.tms.org/pubs/journals/JOM/0306/fig3.gifhttp://www.tms.org/pubs/journals/JOM/0306/fig4-sm.gif -
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Heat Affected Zone
Grain growth, recovery andrecrystallization, precipitates, stresses
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Welding Metallurgy Issues
How does the heat from welding affect thematerial 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
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Classification of Weld Defects
Fabrication
Delayed (hydrogen-induced)
Service
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Porosity
Incomplete fusion
Incomplete penetration
Undercut
Fabrication Defects
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Fabrication Defects Cont.
Solidification cracking
Also called hot orcenterline cracking
HAZ hot cracking Inclusions slag
or tungsten
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Fabrication Defects Cont.
Reheat cracking
Liquid metalembrittlement (LME)
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Fabrication Defects Cont.
Lamellar tearing
Incorrect weldshape or size
Spatter
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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
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Service Defects
Fatigue failures Cyclic loading conditions
Cracks propagate from weld defects
Mechanical overload
Ductile rupture can initiate at defects
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Service Defects
Corrosion Intergranular corrosion
(IGC) Localized grain boundary
attack
Sensitization in austeniticstainless steels
Stress corrosion cracking(SCC) Tensile stress combined
with corrosive media
Transgranular orintergranular crackpropagation
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Significance of Weld Defects
Failure initiation sites
Reduced tensile and other mechanicalproperties
Reduced fatigue life
Toughness / Fracture mechanics
Allowable defect size
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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, magneticparticle, eddy current
Safety Worker, workplace, fumes, fire prevention
Weld repair
Stresses and distortion