Introduction to Welding Technology

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Introduction to Welding Technology
The WeldNet

• CONSULTANT ENGINEERS - METALLURGY AND WELDING

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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 source electric 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 process is A
  materials joining process which produces
  coalescence of materials by heating them to
  suitable temperatures with or without the
  application of pressure or by the application of
  pressure alone and with or without the use of
  filler material".
• Filler (if used) has a melting temperature
  similar to the parts being joined

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Weldability

• The capacity of a material to be welded under the
  imposed fabrication conditions into a specific,
  suitably designed structure and 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
  be ignored.
• 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, thermal
  capacity, 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 run
• Affects composition, properties, risk of defects
• Greatest effect when filler composition is
  different to either or both base metals
• 100 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
Q arc energy in kJ/mm I welding current E
arc voltage v 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
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Thermal gradients in HAZ
Fusion line Fusion line 2mm Fusion line 5 mm
Temperature
Time
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Thermal HAZ regions
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HAZ Structure
Weld
Coarse grain region
Disturbed microstructure
Grain refining
Original base material
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Weld positions and joints
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Welding positions - plate
Flat 1G
Horizontal 2G
Vertical 3G Up or Down
Overhead 4G
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Welding positions - pipe
Axis vertical 2G
Axis horizontal 5G
Axis inclined 45 6G
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Weld joints
Butt
Tee
Lap
Corner
Cruciform
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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 vee can be single or double welded
Double vee
Single bevel
Backed butt (permanent or temporary)
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Butt weld terms
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J Preparations
Single U preparation
Root radius
Land
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 of limited
  use
• Large fillets are uneconomic

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Fillet weld terms
Gaps shall be taken into account for minimum leg
length
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Weld preparation dimensions

• Standard preparations
• AS/NZS1554, AS/NZS3992
• 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 andDiscontinuities
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Welding discontinuities

• Discontinuities 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
Undercut
Cold lap
Incomplete penetration
Slag inclusion
Incomplete sidewall fusion
Incomplete root fusion
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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

• HACC Hydrogen assisted
• Presence of hydrogen
• Susceptible microstructure
• Tensile Stress
• Temperature
• Below 100C
• HICC Hydrogen 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 diagrams Cooling 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
• gt 150C lower risk diffusion of hydrogen
• lt 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 welding
• Low 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 of HAZ cracking.
• Weld metal cracking is less understood.
• Weld metal cracking is more likely in
• Alloy steel weld metals of over 500 MPa yield
  strength
• 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
• Design
• Through 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 (more restrained)
• Consider joint details with lower risk
• Specify material with adequate through thickness
  ductility (tested Z 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 to weld
  axis
• Test cold formed materials for tearing
• Ultrasonically inspect weld areas for laminations
  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 steel microstructure expansion

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Heating a restrained bar
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Residual stress in a butt weld
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Possible consequences

• Distortion
• Weld cracking
• Brittle failure
• Fatigue
• Stress corrosion cracking

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Distortion
Angular
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Minimising distortion

• Avoid over-welding
• Use a planned welding sequence
• Restrain the weldment
• Preset to allow for distortion
• Welding techniques
• Fast high power techniques, back-stepping,
  preheat
• Preheat to maximise area of shrinkage

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End of presentation

• Questions ??