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SAW FUNDAMENTALS Principles of operation Principles of operation 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 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) 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 Person travel speed Machine Machine Principles of operation SAW methods features Semiautomatic Guides the arc along the joint Mechanised Automatic Person Person Machine Manipulates welding Person head to direct the arc Person Machine Person Does not correct! Corrects arc Person to overcome deviations 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 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 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) 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!) 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 SAW equipment Welding heads Wire reel Flux recovery system Slides Flux hopper Wire feed motor Feed roll assembly Torch assembly Tracking system Contact tip Courtesy of ESAB AB SAW equipment Welding heads can be mounted on a: Courtesy of ESAB AB Tractor type carriage Courtesy of ESAB AB Column/beam carriage 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 Courtesy of ESAB AB •due to their portability, are used in field welding or where the piece cannot be moved Courtesy of ESAB AB SAW equipment Pilot lamp Guide wheel bogie Radius from 1500mm Inside welding from 1150 mm Simultaneous welding Guide rail with magnets Support-roller for fillet weld Idling-rollers 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 Courtesy of ESAB AB SAW equipment Ancillary equipment •tilting-rotating positioners bring the area to be welded on irregular parts into flat position Courtesy of ESAB AB Courtesy of ESAB AB SAW equipment Ancillary equipment •roller beads rotate cylindrical parts under the weld head Courtesy of ESAB AB Courtesy of ESAB AB SAW filler material Welding wires •supplied on coils, reels or drums •random or line winding Courtesy of Lincoln Electric coil (approx. 25 kg) Courtesy of Lincoln Electric reel (approx. 300 kg) Courtesy of ESAB AB drum (approx. 450 kg) 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 SAW filler material Welding wires: •carbon and low alloy wires are copper coated •stainless steel wires are not coated Courtesy of Lincoln Electric Courtesy of Lincoln Electric •wires must be kept clean and free from oil and dust 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 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 SAW filler material Fused welding fluxes Components are dry mixed Product is crushed and screened for size Components are melted in an electric furnace Shooting the melt through a stream of water Pouring melt onto large chill blocks Charge is cooled by: 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 SAW filler material Agglomerated welding fluxes Components are powdered and dry mixed Pellets are broken up and screened for size Components are bonded Pellets are baked The wet mix is pelletized 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 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 SAW filler material From the chemical point of view, fluxes can be (see BS EN 760): Acid Neutral (contain acid oxides like SiO2, Al2O3, TiO2, ZrO2) Basic (contain basic oxides like FeO, MnO, NiO, CaO) High basic 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 SAW filler material Welding flux: •can be supplied in bags/pails (approx. 25 kg) or bulk bags (approx. 1200 kg) Courtesy of Lincoln Electric Courtesy of Lincoln Electric Courtesy of Lincoln Electric •handling and stacking requires care •if flux is too fine it will pack and not feed properly cannot be recycled indefinitely 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 Wire electrode and/or wire/flux combination Impact properties Chemical composition of wire electrode