Welding - Processes, Metallurgy and Defects

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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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
http://en.wikipedia.org/wiki/File:GMAW_weld_area.png


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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
http://en.wikipedia.org/wiki/Image:Spot_welder.miller.triddle.jpg


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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
http://en.wikipedia.org/wiki/Image:Welding.jpg


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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
http://www.delftoutlook.tudelft.nl/info/fullimage6ed9.html?ImageID=693


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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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Reheat cracking

Liquid metalembrittlement (LME)


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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

Documents

Welding - Processes, Metallurgy and Defects