SAW FUNDAMENTALS

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SAW FUNDAMENTALS
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Principles of operation
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Principles of operation
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Principles of operation
Factors that determine whether to use SAW

• chemical composition and mechanical properties
  required for the weld deposit
• thickness of base metal to be welded
• joint accessibility
• position in which the weld is to be made
• frequency or volume of welding to be performed

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Principles of operation
SAW methods

• semiautomatic - for relatively small diameter
  electrodes and moderate travel speed
• mechanised - the majority of applications
• automatic - for mass production (e.g. pipes)

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Principles of operation
SAW methods features
Semiautomatic Mechanised Automatic
Starts and maintains the arc
Machine Machine Machine
Feed the electrode
Machine Machine Machine
Control heat input
Person Machine Machine
Provide travel speed
Person Machine Machine
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Principles of operation
SAW methods features
Semiautomatic Mechanised Automatic
Guides the arc along the joint
Person Person Machine
Manipulates welding head to direct the arc
Person Person Machine
Corrects arc to overcome deviations
Person Person Does not
correct!
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Advantages/disadvantages
Advantages of SAW

• high current density ? high deposition rates (up
  to 10 times those for MMA) ? high productivity
• deep penetration ? small welding grooves
• fast travel speed ? less distortions
• deslagging is easy
• uniform bead appearance with good surface finish
  ? good fatigue properties
• can be easily performed mechanised ? high duty
  cycle and low skill level required
• provide consistent quality when performed
  automatic/mechanised ? assure radiographically
  sound welds
• arc is not visible
• little smoke/fumes are developed

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Advantages/disadvantages
Disadvantages of SAW

• limited mainly to flat/horizontal position
• limited to carbon, low alloy, creep resisting and
  stainless steels also nickel alloys
• due to the high heat input, impact strength of
  weld metal/HAZ may be low also high dilution
• slag must be cleared away after welding ? danger
  of slag inclusions
• need flux storage, handling and recirculation
  control
• difficult to apply on-site due to complicated
  equipment
• high capital costs
• weld line must be regular (straight or
  circumferential seams only) with accurate fit-up

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SAW equipment
Power sources can be

• transformers for AC
• transformer-rectifiers for DC

Static characteristic can be

• flat (CV) - most of the power sources
• drooping (CC)

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SAW equipment
Flat (CV) power sources

• most commonly used supplies for SAW
• can be used for both semiautomatic and automatic
  welding
• self-regulating arc ? simple wire feed speed
  control
• wire feed speed controls the current and power
  supply controls the voltage
• applications for DC are limited to 1000A due to
  severe arc blow (also thin wires!)

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SAW equipment
Drooping (CC) power sources

• can be used for both semiautomatic and automatic
  welding
• not self-regulating arc ? must be used with a
  voltage-sensing variable wire feed speed control
• more expensive due to more complex wire feed
  speed control
• arc voltage depends upon wire feed speed whilst
  the power source controls the current
• cannot be used for high-speed welding of thin
  steel

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SAW equipment
Welding heads
Wire reel
Flux recovery system
Slides
Flux hopper
Wire feed motor
Feed roll assembly
Torch assembly
Tracking system
Contact tip
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SAW equipment
Welding heads can be mounted on a
Tractor type carriage
Column/beam carriage
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SAW equipment
Tractor type carriage

• provides travel along straight or gently curved
  joints
• can ride on tracks set up along the joint (with
  grooved wheels) or on the workpiece itself

• can use guide wheels as tracking device

• due to their portability, are used in field
  welding or where the piece cannot be moved

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SAW equipment
Guide wheel bogie
Pilot lamp
Radius from 1500mm
Inside welding from 1150 mm
Simultaneous welding
Guide rail with magnets
Support-roller for fillet weld
Idling-rollers
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SAW equipment
Column/beam carriage

• provides linear travel only
• are capable of linear motion in 3 axes
• because workpiece must be brought to the weld
  station, they are use mostly in the workshop

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SAW equipment
Ancillary equipment

• tilting-rotating positioners bring the area to be
  welded on irregular parts into flat position

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SAW equipment
Ancillary equipment

• roller beads rotate cylindrical parts under the
  weld head

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SAW filler material
Welding wires

• supplied on coils, reels or drums

• random or line winding

drum (approx. 450 kg)
reel (approx. 300 kg)
coil (approx. 25 kg)
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SAW filler material
Welding wires can be used to weld

• carbon steels
• low alloy steels
• creep resisting steels
• stainless steels
• nickel-base alloys
• special alloys for surfacing applications

Welding wires can be

• solid wires
• metal-cored wires

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SAW filler material
Welding wires

• carbon and low alloy wires are copper coated

• stainless steel wires are not coated

• wires must be kept clean and free from oil and
  dust

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SAW filler material
Copper coating functions

• to assure a good electric contact between wire
  and contact tip
• to assure a smooth feed of the wire through the
  guide tube, feed rolls and contact tip (decrease
  contact tube wear)
• to provide protection against corrosion

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SAW filler material
Welding fluxes

• are granular mineral compounds mixed according to
  various formulations
• shield the molten weld pool from the atmosphere
• clean the molten weld pool
• can modify the chemical composition of the weld
  metal
• prevents rapid escape of heat from welding zone
• influence the shape of the weld bead (wetting
  action)
• can be fused, agglomerated or mixed
• must be kept warm and dry to avoid porosity

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SAW filler material
Fused welding fluxes
Components are melted in an electric furnace
Components are dry mixed
Charge is cooled by
Shooting the melt through a stream of water
Product is crushed and screened for size
Pouring melt onto large chill blocks
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SAW filler material
Fused fluxes advantages

• good chemical homogeneity
• easy removal of fines without affecting flux
  composition
• normally not hygroscopic ? easy storage and
  handling
• readily recycled without significant change in
  particle size or composition

Fused fluxes disadvantages

• difficult to add deoxidizers and ferro-alloys
  (due to segregation or extremely high loss)
• high temperatures needed to melt ingredients
  limit the range of flux compositions

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SAW filler material
Agglomerated welding fluxes
Components are powdered and dry mixed
The wet mix is pelletized
Components are bonded
Pellets are broken up and screened for size
Pellets are baked
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SAW filler material
Agglomerated fluxes advantages

• easy addition of deoxidizers and alloying
  elements
• usable with thicker layer of flux when welding
• colour identification

Agglomerated fluxes disadvantages

• tendency to absorb moisture
• possible gas evolution from the molten slag
  leading to porosity
• possible change in flux composition due to
  segregation or removal of fine mesh particles

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SAW filler material
Mixed fluxes - two or more fused or bonded fluxes
are mixed in any ratio necessary to yield the
desired results
Mixed fluxes advantages

• several commercial fluxes may be mixed for highly
  critical or proprietary welding operations

Mixed fluxes disadvantages

• segregation of the combined fluxes during
  shipment, storage and handling
• segregation occurring in the feeding and recovery
  systems during welding
• inconsistency in the combined flux from mix to mix

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SAW filler material

• From the chemical point of view, fluxes can be
  (see BS EN 760)

High basic
Acid (contain acid oxides like SiO2, Al2O3,
TiO2, ZrO2)
Neutral
Basic (contain basic oxides like FeO, MnO, NiO,
CaO)
Welding characteristics (more stable arc,
improved weld appearance, easier slag removal,
higher welding speeds)
Weld metal mechanical properties (YS, KV, UTS),
amount in Mn and Si, melt temperature
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SAW filler material
Welding flux

• can be supplied in bags/pails (approx. 25 kg) or
  bulk bags (approx. 1200 kg)

• handling and stacking requires care
• if flux is too fine it will pack and not feed
  properly ? cannot be recycled indefinitely

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SAW filler material
Wire/flux combination designation acc. BS EN 756
Type of welding flux
Tensile properties
Standard number
EN 756 - S 46 3 AB S2
EN 756
S
46
3
AB
S2
Wire electrode and/or wire/flux combination
Impact properties
Chemical composition of wire electrode