Abstract:
Aluminium metal is not typically added to the submerged arc welding (SAW) process
because it is easily oxidised to form unwanted slag in the weld pool. The successful application of
aluminium as a de-oxidiser is illustrated in this study by preventing oxidation of Cr and Co to their
oxides, thereby preventing element loss to the slag. Unconstrained pure metals of Al, Cr, Co and Cu
were applied to investigate the gas formation behaviour of these elements in the SAWarc cavity. Of
interest is the effect of copper in the arc cavity in terms of its possible substitution for aluminium.
The results confirmed that the Al-Cr-Co-Cu alloyed weld metal total oxygen content was lowered to
176 ppm O, in comparison to 499 ppm O in the weld metal formed from welding with the original
flux, which excluded metal powder additions. This lower ppm O value of 176 ppm O confirms
that the added aluminium powder effectively lowered the original flux-induced partial oxygen
pressure in the arc cavity, and at the molten flux–weld pool interface. Carbon steel was alloyed
to 5.3% Co, 5.5% Cr, 5.3% Cu and 4.5% Al at 78% Co yield, 82% Cr yield, 78% Cu yield and 66%
Al yield. Thermochemical equilibrium calculations confirm the partial oxygen pressure-lowering
effect of aluminium when considering the gas–slag–alloy equilibrium. BSE (backscattered electron)
images of the three-dimensional (3D) post-weld slag sample show dome structures which contain
features of vapour formation and re-condensation. SEM-EDX (scanning electron microscope-energy
dispersive X-ray) maps show that the dome surface matrix phase consists of Al-Mg-Ca-Si-Na-K-Ti-Fe-
Mn oxy-fluoride. The spherical 3D structures of 10–40 m in diameter consist of Fe-Mn-Si fluorides
with some Cr, Cu and Co contained in some of the spheres. Cr and Co were observed in distinctive
porous structures of approximately 10 m in size, consisting partly of Cr oxy-fluoride and partly
of Co oxy-fluoride. Nano-sized oxy-fluoride strands and spheres in the dome structures confirm
vaporisation and re-condensation of oxy-fluorides. Cu and Na formed a distinct condensation pattern
on the surface of the Si-Cu-Na-Mn-Fe-Co oxy-fluoride sphere. The results confirm the importance of
including gas phase reactions in the interpretation of SAW process metallurgy.
