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Introduction to
Welding Technology
CONSULTANT ENGINEERS - METALLURGY AND WELDING
The WeldNet
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Welding processes
Fusion welding
Involves melting & solidification
Solid phase welding
Explosive bonding
Diffusion welding
Friction welding
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Fusion welding
Most commonly used processes
Heat sourceelectric arc, gas flame, laser
Filler metal
From electrode, rod, wires, powder, fluxes
Independently added filler
No filler (autogenous welding)
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Weld
The AWS definition for a welding processis A materials joining process which producescoalescence of materials by heating them to
suitable temperatures with or without theapplication of pressure or by the application ofpressure alone and with or without the use offiller material".
Filler (if used) has a melting temperaturesimilar to the parts being joined
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Weldability
The capacity of a materialto be welded
under the imposed fabrication
conditionsinto a specific, suitablydesigned structureand to perform
satisfactorily in intended service.
(ANSI / AWS A3.0)
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Factors affecting weldability
Weldability is often considered to be a
material property.
However the effect of other variables should not beignored.
Weldability is also affected by:
Design of a weld
Service conditions
Choice of welding process
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Design
Weld joint design and execution
Thickness, location, access, environment
Restraint
Weldment size, assembly sequence
Service stresses
Safety factor for welds
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Physical properties
Melting and vaporisation temperatures
Electrical and thermal properties
Conductivity, expansion coefficient, thermalcapacity, latent heat
Ionisation potential of electrode
Magnetic susceptibility
Reflectivity
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Solidification of weld metal
Dendritic or cellular growth
Segregation
Depends on composition
Cooling rate
Can lead to solidification cracking
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Dilution
Proportion of weld metal that comes from the
base material
Must be considered for each weld runAffects composition, properties, risk of defects
Greatest effect when filler composition is
different to either or both base metals100% for autogenous welds
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Chemical properties
Affinity of weld metal for O, N and H
Susceptibility to porosity, embrittlement
Presence of a surface film on base metal Oxide films
Paint or metallic surface coating
Fluxing / De-oxidising properties of a slag
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Contaminant gases
Nitrogen and oxygen from air
Hydrogen from
Moisture in air
Moisture in consumables or surface
contaminants
Organic materials (grease, oil, paint etc)
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Gas-metal reactions
Liquid metal may react with air or other gases
Depends on
Liquid metal composition Gas composition
Consequences
Porosity - gas released on solidification
Formation of compounds
Embrittlement
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Metallurgical properties
Strengthening mechanism of base material
Weld versus base material strength
Freezing range Susceptibility to solidification cracking
Susceptibility to detrimental phases forming
during welding
Embrittlement or corrosion
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Service environment
Extreme environments
Corrosive
Low temperature (brittle failure)
High temperature (oxidation, creep, embrittlement) Others (wear, fatigue, nuclear)
The more extreme the environment
The more difficult it is to find suitable materials
The more restricted the welding procedure
becomes to avoid service failure (arc energy)
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Welding variables
Arc energy (heat input)
Preheat and interpass temperature
Filler metal composition
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Arc energy
v
IxEQ 06.0
Q = arc energy in kJ/mmI = welding currentE = arc voltagev = travel speed in mm/min
Low arc energy Small weld pool size Incomplete fusion High cooling rate
Martensite and hydrogen cracking
High arc energy Large weld pool size Low cooling rate Increased solidification
cracking risk Low ductility and strength Precipitation of unwanted particles
(corrosion and ductility)
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Preheat and interpass
Preheat is applied independently
Gas torches
Gas radiant heaters Electric resistance heaters
Interpass temperature
Temperature before next pass is added
Controlled by a cooling time, or air or water cooling
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Raising PH/IP temperature
Slows cooling rate
Reduces HICC in steels
Can increase risk of solidification cracks
Can increase tendency to embrittlement
Improves fusion
Reduces temperature gradient
Minimises distortion and residual stress
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Fusion weld structure
Composite Weldmetal
Unmixed fusedbase metal
HAZ
PartiallyMeltedZone Fusion Line
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Thermal gradients in HAZ
Time
Temperature Fusion lineFusion line + 2mmFusion line + 5 mm
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Thermal HAZ regions
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HAZ Structure
Grain refining
Weld Coarse grain regionDisturbed microstructure
Original base material
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Weld positions and
joints
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Welding positions - plate
Flat 1G Horizontal2G
Vertical3G
Up or Down
Overhead4G
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Welding positions - pipe
Axis vertical2G
Axis horizontal5G
Axis inclined 456G
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Weld joints
Cruciform
LapCorner
Butt Tee
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Weld Types
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Weld types
Butt weld
Between mating members
Best quality High weld preparation cost
Fillet weld
Easy preparation
Asymmetric loads, lower design loads
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Butt welds
Joint types:
Double welded butt
Permanent or temporary backing
Single welded butt
Lower stress concentration
Easier ultrasonic testing or radiography
Expensive preparation
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Butt weld types
Single veecan be single
or double welded
Single bevel Double vee
Backed butt (permanent or temporary)
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Butt weld terms
Root faceRootgap
Fusion face
Included angle
Bevel angle
Cap / Reinforcement
Root run
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J Preparations
Land
Root radiusSingle U preparation
Double U butt
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Fillet welds
Simple to assemble and weld
Stress concentrations at toes and root
Notch at root (fatigue, toughness)Critical dimension is throat thickness
Root gap affects throat thickness
Radiography and ultrasonic testing is oflimited use
Large fillets are uneconomic
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Fillet weld terms
Root
Toe
Weld face
Toe
Throatthickness
Leg length
Gaps shall be taken into account for minimum leg length
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Weld preparation dimensions
Standard preparations
AS/NZS1554, AS/NZS:3992
AWS D1.1, ASME B31.3
Non Standard (Compromise at fabricators risk)
Weld cross sectional area
Cost Ease of welding (risk of defects)
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Weld Defects and
Discontinuities
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Welding discontinuitiesDiscontinuities are essentially defects that fall within the
limitations of the welding standard requirements
Cracks Never a discontinuity !!
Porosity Most common complying weld defect
Incomplete fusion / Inclusions Some allowed by most welding standards
Defective profile Under-weld, over-weld, lack of root bead, burn through, undercut,spatter etc.
Most client specifications limit these types
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Welding defects- Causes Cracks
HACC / HICC, solidification, liquation causes
Porosity Gas entrapment / ejection, poor shielding
Incomplete fusion Sidewall, inter run, root pass, weld toes ( cold lap )
Electrode angle implicated or poor joint profile
Inclusions Slag, oxide, tungsten
Usually operator induced
Defective weld profile / finish Under-weld, over-weld, lack of root bead, burn through, undercut
Usually operator induced
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Some weld defects
Incomplete penetration
Cold lapUndercut
Incomplete sidewall fusion Incomplete root fusion
Slag inclusion
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Solidification cracking Low melting point constituents
Sulphur, Phosphorus, Tin, Lead, Niobium
Undesirable eutectics
Grain boundary segregation Segregation of sulphides etc.
Lowering ductility and raising crack sensitivity
Strains arising during solidification Solidification range
Material types, contamination
Base material dilution, lowering weld strength
Expansion coefficient Differing between base material and weld material
Clad materials
Weld pool shape and size Depth-to-width ratio
Surface concavity
Arc energy
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Solidification cracks
Crater crack
Longitudinal crack Centreline Crack
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Weldability of structural steel
Benchmark against which other materials
are judged
Risk of hydrogen induced cold cracking.Only occurs in ferritic, bainitic or martensitic
steel
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Hydrogen induced cold cracks
HACCHydrogen assisted Presence of hydrogen
Susceptible microstructure
Tensile Stress Temperature
Below ~ 100C
HICCHydrogen induced
Hydrogen embrittlement Susceptible microstructure / stress not always
required
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Susceptible microstructure
Weld metal or HAZ
Martensite or upper bainite
Composition
Hardenability and hardness - carbon equivalent
TTT diagramsCooling rates
Cooling time between 500C and 300C Section thickness
Preheat temperature
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Sources of tensile stress
Residual stress Restraint
Through thickness in thick sections
Applied stress Excessive peening
Lifting
Presetting
Fairing and straightening operations
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Hydrogen From consumables
Moisture absorption
Potential hydrogen test
Basic consumables have lower potential hydrogen
From joint contamination Fabrication practices
Environment
Machinery
Temperature and time dependent
> 150C lower riskdiffusion of hydrogen < 150C to ambient - if susceptible, cracking will occour
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Methods of control
Preheat
Slow down cooling rate between 800C and
500C Remove hydrogen before weld cools
below 150C
Stress relief immediately after weldingLow temp temperature heat treatment (150C
to 250C, known as out-gassing)
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HAZ Cracking
All these approaches are based on studies of the risk ofHAZ cracking.
Weld metal cracking is less understood.
Weld metal cracking is more likely in
Alloy steel weld metals of over 500 MPa yieldstrength
Submerged arc welds (Chevron cracks)
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Lamellar tearing
Pull-out crack (obsolete)
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Lamellar tearing
Separation or cracking along planes
parallel to the principal plane of
deformation. Occurs in rolled sections mainly but can
also occur in extrusions and forgings.
Does not occur in castings Not to be confused with plate lamination.
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Lamellar tearing
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Appearance
Woody looking or stepped crack
Parallel to rolling direction (in rolled
sections) Sometimes associated with HACC / HICC
in the HAZ.
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Factors affecting risk
Material
Through-thickness properties
DesignThrough thickness strains and restraint
Fabricator
Over-welding
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Design approach
Consider corner, tee and cruciform joints a
risk
Thicker members are at risk (morerestrained)
Consider joint details with lower risk
Specify material with adequate throughthickness ductility (testedZ grade)
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Joint details with lower risk
Reduce weld size
Diffuse through thickness strains with joint
design Minimise restraint
Balance weld detail
Avoid welds intersecting in a corner
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Joint detail comparison
Poor details Improved details
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Fabrication practices
Carefully sequence fabrication to minimise
restraint
Choose rolling direction perpendicular toweld axis
Test cold formed materials for tearing
Ultrasonically inspect weld areas forlaminations before fit-up
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Welding practices
Do not over weld
Follow practices that minimise stress and
distortion Buttering can be used to avoid lamellar
tearing but is expensive.
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Residual stress and
distortion
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Residual stress sources
Uneven plastic deformation
Hot or cold forming (rolling, pressing, bending,
shot blasting)Cutting (machining, shearing)
Uneven heating and cooling
Welding, flame cutting, flame straightening Uneven solid phase change
Quenching steelmicrostructure expansion
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0
50
-50
-100
-150
TEMPERATURE IN MIDDLE BAR Deg C
0 100 200 300 400 500 600
100
150
200
-200STR
ESSIN
MIDDLEBA
R
MPa
Heating a restrained barMiddle baris heated to600C andallowed tocool
B
A
C
D
Compression
Tension
EF
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XX XX
Residual stress in a butt weld
ssx
ssy
ssx
0 TensionCompression
XX XX
sy Tension
Compression
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Possible consequences
Distortion
Weld cracking
Brittle failure
Fatigue
Stress corrosion cracking
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Distortion
Angular
Longitudinal Transverse
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Minimising distortion
Avoid over-welding
Use a planned welding sequence
Restrain the weldmentPreset to allow for distortion
Welding techniques
Fast high power techniques, back-stepping,
preheat
Preheatto maximise area of shrinkage
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End of presentation
Questions ??