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

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. Fabrication Defects Porosity Incomplete fusion Incomplete penetration
Undercut

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 Mechanical overload
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