During welding operation, not all of the flux is melted to form slag and a large percentage of this flux can be re-used. It is important to avoid too much mechani- cal abrasion on the flux during flux feed- ing, though, since the flux granules are relatively soft and can easily be reduced to dust. This changes the particle size distribu- tion of the flux, which affects performance characteristics such as wetting, bead appearance and gas shielding. To ensure a constant grain size distribution during recycling it is important to have good grain shape, strength and abrasion resistance to minimise dust formation. A consistent mix- ing ratio of recycled and fresh flux should also be maintained to prevent welding performance being affected. In addition, the flux must be kept dry to avoid moisture- related issues such as gas inclusions, pock marks and hydrogen cracking. The lowest possible level of diffusible hydrogen is essential to reduce the risk of hydrogen-induced cracking. Fluxes with a higher level of diffusible hydrogen nor- mally require higher preheat temperature, especially in the case of base metals with higher Carbon-equivalents, such as flanges and components that use higher strength grades (S420/S460). Robust moisture-proof flux packaging, aluminium composite foil, for example, is key to ensuring low hydro- gen levels without the need to pre-dry the SAW flux. Mechanical properties of the weld metal depend on: the welding wire and its chemi- cal composition; the chemical activity of the flux; the base metal composition and the dilution rate with the base metal; the welding parameters – current type, inter- pass temperatures and wall thickness; and the weld bead thickness and grain refining under single- or multi-pass conditions. Clearly, the SAW wire-flux combination has a major influence on the mechanical Stable, high level mechanical properties
BÖHLER Welding SAW fluxes are designed to deliver good arc stability; nice wetting and bead aspect ratios; good bead appearance; easy slag release; proper grain size distribution; and the required grain shape and strength.
this toughness level, a fluoride-basic flux with a relatively high basicity index is fre- quently used, mostly
properties of the weld metal. Table 1 high- lights how wire-flux combinations have been optimised for different SAW welding requirements. For onshore wind towers, mostly mild steel grades are used with a minimum spec- ified yield strength of 355 MPa. Some parts of the wind tower might be constructed with a higher strength steels such as S420. Charpy V-notch toughness requirements vary from 27 to 50 J at test temperatures of between 0 and -50 °C. In general, there is a preference for us- ing a single flux and wire combination for fabricating all the wind tower sections for onshore projects with different require- ments. This is not only for logistical reasons but also to eradicate errors that could result in inadequate weld metal properties. The combination of the Union S 2 Si wire with UV 408 TT flux has been developed specifi- cally for onshore wind towers to cover this wide application range. For offshore wind towers and founda- tions, the manufacturing requirements are on a higher level. Base metals vary from S355-S460 and Charpy toughness require- ments vary from 27 - 50 J at test tempera- tures of between -40 and -60 °C. However, the industry normally demands wire-flux combinations that provide Charpy-V tough- ness levels greater that 100 J at -60° C. For
with neutral chemical additions with respect to Mn and Si. To ensure the higher strength level, the matching SAW wire has a higher S 3 Si alloy composition. Union S 3 Si wire with UV 418 TT flux is the optimised combination for these off- shore requirements. For large components with high wall thickness that use these high strength steels with higher CE-equivalents, a very low level of diffusible hydrogen is very important to avoid cold cracking is- sues. Fluxes with a higher level of diffusible hydrogen will normally require a higher preheat temperature. The latest improvements of UV 418 TT flux have resulted in a very low level of dif- fusible hydrogen for DCEP and for AC polar- ity for both sinus and square wave shapes. Minimising welding time Tact time – the time between starting units – is a critical number because it decides the total output of a production line. Depend- ing on the situation, the SAW welding sta- tion could be the bottle-neck that governs this tact time. Every subdivided action/ element inside the SAW station should be
SAW wire
Flux
Flux type BI Min yield (MPa) Min tensile (MPa)
CVN toughness M-Run (J)
Application Steel grade 2 run/multi- run Onshore, S355-S420
Classification EN ISO AWS S 42 5 AB S2Si H5 F7A8-EM12K-H4
Union S 2 Si
UV 408 TT AB 1,7
420
500
70 @ -50°C
Union S 3 Si
UV 418 TT FB 2,7
460
530
> 100 @ -60°C Esp. multi- run Offshore, S355-S460 > 100 @ -60°C Esp. multi-run High dep rate > 80 @ -60°C 2 run/multi-run, High dep rate
S 46 6 AB S3Si H5 F7A8-EH12K-H4
Diamondspark S 55 HP Diamondspark S 56 HP
UV 418 TT FB 2,7
460
530
S 46 6 FB T3 H5 F7A8-EC1 S 46 6 FB TZ3 H5 F7A8-ECG
UV 400 AB 1,9
460
530
Table 1: voestalpine Böhler Welding wire and flux combinations optimised for different SAW welding requirements.
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July 2022
AFRICAN FUSION
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