Insight into the Chemical Behaviour of Chromium in CaF2-SiO2-Al2O3-MgO Flux Applied in Aluminium-Assisted Alloying of Carbon Steel in Submerged Arc Welding

Abstract

Chromium alloying of weld metal is usually accomplished via weld wire. This is done because chromium has a high affinity for oxygen and is therefore not easily transferred across the arc. The formation chromium (VI) is one of the main concerns in welding with chromium containing consumables, especially in open arc processes, and less so in SAW (submerged arc welding). This study investigates the chemical behaviour of chromium in the application of unconstrained metal powders of Al, Cr, Cu and Ti in SAW. The application of aluminium in SAW is used to control the oxygen partial pressure in the process to prevent oxidation of elements of high oxygen affinity, such as chromium. The speciation of chromium was investigated in two-dimensional (2D) and three-dimensional (3D) post-weld slag samples. In the 2D slag samples, the chromium is contained in low concentrations in the oxy-fluoride matrix phase. The 3D samples showed dome structures in which evidence of vapour formation was identified. Chromium presents as distinct chromium spots throughout the oxy-fluoride dome walls, and it is not observed as solute in the oxy-fluoride phase. Chromium presents as chromium-rich Cr-Mn-Mg-O porous particles of less than 20 µm in size, which may agglomerate into larger masses. The calculated thermochemical predominance diagrams show that the Al(l)-Al2O3(s)-AlF(g) equilibrium can maintain the partial oxygen pressure (PO2) at 10−16.5 atmosphere and the partial fluorine pressure (PF2) at 10−11.4 atmosphere at 2500 °C. Under these gas phase conditions in the arc cavity, chromium is present as metallic chromium and may subsequently vaporise as chromium metal and combine with other vaporised elements.