Consumables – Flux

Flux is used in several different forms depending upon the welding process. For the SMAW process, it is extruded as a coating on the electrodes. For the FCAW process, it is added as a powder inside the cored electrode. For the SAW process, it is deposited on the work ahead of the welding arc as a loose granular material. No flux is used for the GTAW and GMAW processes. Flux serves some or all of the following functions:
• Stabilizes the welding arc
• Shields the arc and molten metal from atmospheric oxygen and nitrogen contamination
• Forms a slag blanket over the molten weld metal to help hold it in position and provide further shielding from contamination during cooling
• Provides deoxidizers (Mn and Si) to improve weld metal soundness and alloying elements (e.g., Cr, Mo, or Ni) for low alloy welds

To avoid underbead cracking in pressure vessel steels, SMAW and FCAW electrodes and SAW fluxes should be specified as a low hydrogen type (e.g., EXX18 electrodes) and must be kept clean and dry to prevent hydrogen absorption into the weld metal and heat affected zones. Low hydrogen electrodes and fluxes should impart no more than 10 ml of diffusible hydrogen per 100 gm of typical as-deposited weld metal per the manufacturer’s specifications, as measured by the mercury displacement or gas chromatograph methods per AWS A4.3. Note, however, that the ASME Code does not require the use of low hydrogen electrodes and fluxes.

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