Welding - Processes, Metallurgy and Defects Mikal Balmforth, P.E., CWEng, CWI

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

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

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

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

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                                           

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

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

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

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

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

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

Fusion Zone Autogenous, homogeneous or heterogeneous Molten to solid

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

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

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

Fabrication Defects Porosity Incomplete fusion Incomplete penetration Undercut

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

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

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

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

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

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

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

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