Gas Formation of Cobalt and Copper in the Application of Unconstrained Co-Cr-Al-Cu Metal Powders in Submerged Arc Welding: Gas Phase Thermodynamics and 3D Slag SEM Evidence

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 SAW arc 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 µm 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 µm 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.

EERZ (Effective Equilibrium Reaction Zone) Model of Gas-Slag-Metal Reactions in the Application of Unconstrained Al-Ni-Cr-Co-Cu Metal Powders in Submerged Arc Welding: Model and 3D Slag SEM Evidence

“…These studies illustrated that the added aluminium powder controls the weld metal total ppm O in the alloying of the weld metal with titanium-containing unconstrained metal powder formulations [10,11,14] and with chromium-containing unconstrained metal powder formulations [12][13][14][16][17][18]. The importance of controlling the weld metal total ppm O is to ensure high impact toughness of the steel.…”

“…Here, as in the authors' previous similar works the terminology of "unconstrained metal powders" refers to non-alloyed metal powders which are not constrained in tubular wire, such as flux cored and metal-cored wire. In a series of studies by the authors, various combinations of metal powders with Al as a de-oxidiser element were applied in SAW [8][9][10][11][12][13][14][15][16][17][18]. The aluminium powder added as de-oxidiser element lowers the partial oxygen pressure at the interface of the molten flux-weld pool phases.…”

“…The aluminium powder added as de-oxidiser element lowers the partial oxygen pressure at the interface of the molten flux-weld pool phases. In addition, this aluminium de-oxidiser element also lowers the oxygen partial pressure in the arc cavity [8,9,[16][17][18].…”

“…Therefore, the importance of the gas phase in these calculations is clear. The gas-slag-metal equilibrium was also applied to better understand the effect of Al de-oxidiser in SAW [9,[16][17][18]. However, the latter calculations only included the flux and metal powders and did not calculate the weld metal composition.…”

“…The authors conducted several studies on aluminium-assisted alloying of the weld metal in SAW with the elements of Co and Ni included in unconstrained metal powder formulations of Al-Co, Al-Co-Cr, Al-Ni, Al-Ni-Cr, Al-Co-Cr-Cu, and Al-Ni-Cr-Cu [8,9,[15][16][17][18]. Here, the unique Co-Cr-Al-Cu-Ni metal powders combination is applied to the SAW process, with Al as the de-oxidiser element.…”