Aluminium-Assisted Alloying of Carbon Steel in Submerged Arc Welding with Al-Cr-Ni Unconstrained Metal Powders: Thermodynamic Interpretation of Gas Reactions

Abstract

Unconstrained metal powders of chromium and nickel, in combination with aluminium, were used in the submerged arc welding (SAW) process to simplify weld metal alloying. Unconstrained metal powders refer to non-alloyed metal powders that are not constrained in tubular wire, such as fluxed-cored and metal-cored wire. Aluminium powder is used to control the oxygen potential at the molten flux–weld pool interface. The results presented here show that the addition of aluminium powder to the weld metal enhances Cr and Ni yields to 89% for Cr and 91% for Ni, compared to lower values reported in pre-alloyed powder application. Alloying of the carbon steel in the base plate and weld wire combination was achieved at 6.0% Cr, 6.2% Ni, and 4.5% Al, with the weld metal oxygen controlled to 162 ppm O. Thermodynamic analysis was applied to investigate the likely gas reactions in the arc cavity emanating from the chemical interaction between Cr, Ni, and Al. The effects of gas-based chemical reactions on the yield of Cr and Ni to the weld pool are discussed and incorporated into our SAW reaction flow diagram. Overall SAW process productivity gains can be accomplished by using unconstrained metal powders to alloy the weld metal because expensive and time consuming steps, such as the manufacturing of alloyed wire and alloyed powder, can now be eliminated.

Modification of Flux Oxygen Behaviour via Co-Cr-Al Unconstrained Metal Powder Additions in Submerged Arc Welding: Gas Phase Thermodynamics and 3D Slag SEM Evidence

“…The weld metal analyses data in Table 3 confirm increased silicon and manganese content in MP7 and MP9 weld metal, as compared to the BC weld metal. This is due to aluminothermic reduction of MnO and SiO 2 from the slag, as identified in our previously reported work on aluminium-assisted alloying of different metal powders in SAW [26][27][28][29][30][31][32][33]. Therefore, the aluminothermic reduction reactions in Equations ( 2) and (3) also occurred in the MP9 weld run.…”

“…Aluminium metal is avoided as a main reactant in SAW because it is easily oxidised in this process. In our novel application of aluminium metal powder in SAW we demonstrate the modification of flux oxygen behaviour to control the weld metal total ppm O, as in our previous works [26][27][28][29][30][31][32][33]. Because alloying element yield is dependent on the flux-induced partial oxygen pressure, the element yield can be improved by de-oxidiser application.…”

“…It was confirmed from our previous similar works on different metal powder combinations with aluminium, that FeO is formed from oxidation of iron by the excess oxygen transferred from the arc cavity [37,38]. This FeO may also be reduced by aluminium powder via reaction (4) to control the oxygen potential at the weld pool-slag interface [26][27][28][29][30][31][32][33]. In the same way, chromium oxide can be reduced by aluminium via Equation ( 5) [28].…”

“…A mass balance was done to quantify the yield of cobalt, chromium, and aluminium from the metal powders added to the weld metal. The mass balance measurements and calculations procedure were described in detail in our previous works [26,[29][30][31][32][33]. The %yield of Co, Cr and Al from metal powder to weld metal can be calculated once the gram mass of Co, Cr and Al added to the weld metal have been calculated from the mass balance.…”

“…Alumina formed as product in the reactions displayed as Equations ( 2) and (3) was easily absorbed into the molten flux because these reactions occurred at the molten fluxweld pool interface [26][27][28][29][30][31][32][33]37]. Calculation of the exothermic reaction heat contributions from Equations ( 2) and ( 3) is possible once the mass Mn and mass Si contributed from these reactions to the weld metal are quantified, using Equation ( 6).…”