Low temperature vaporisation of Cr from fluoride flux reacted at 1350 ° C with Al -Cr -Fe powder : thermochemical analysis of gas phase reactions and nano-strand formation

Abstract

The submerged arc welding (SAW) process is complex because multi-phase reactions occur simultaneously across a large temperature interval from 2500 °C in the arc cavity, down to the weld pool liquidus temperare at the slag-weld pool interface. This complexity hinders research on specific process metallurgy aspects, such as gas formation from the oxy-fluoride slag. The main objective of this work is to illustrate a low temperature experimental technique as an accurate reaction simulation experiment of SAW flux oxy-fluoride slag behaviour in terms of gas formation and metal powder assimilation reaction mechanisms as observed in the SAW process. The oxy-fluoride slag behaviour was confirmed by identification and analyses of nano-strand formation in the reacted slag formed from the reaction of welding flux with Al-Fe-Cr metal powders at 1350 °C. The observed nano-strand formation agrees with similar observations in SAW post-weld slags used to confirm of oxy-fluoride vaporisation and re-condensation. Element distribution from energy dispersive X-ray spectroscopy (EDS) maps and thermochemical calculations were used to gain insights into the reactions in nano-strand formation. The nano-strands contained chromium patches and spots of less than 1 μm in scale, confirming that added Cr metal powder assimilated via the gas phase. Cr-fluoride formed part of the oxy-fluoride slag and likely formed Cr-fluoride gas. Cr was recovered via re-condensation of Cr vapour formed from the reaction of Cr-fluoride with Al gas. This work presents an accurate low temperature simulation method of gas formation and metal powder assimilation reactions in oxy-fluoride slags as in the SAW process.