Submerged arc welding is a complex metallurgical process with a temperature of nearly 2000 ${}^{\circ }$C (a temperature much higher than that in traditional steelmaking) and different phases, including flux (slag), metal, and plasma. Flux serves vital functions in order to produce the weld metal with desired qualities. It is well known that understandings of the thermodynamic properties regarding flux and slag are essential to aid in flux design and optimization. Actually, the developments of flux design and thermodynamics have been promoting each other. Within this review, the flux design stages have been documented and reviewed in detail from the perspective of thermodynamics. The thermodynamic design principles for fluxes have been evaluated systematically, the limitations of each flux have been elucidated, and the thermodynamic significance of the designed fluxes upon the development of welding thermodynamics has been analyzed. Based on the hypothesis that thermodynamic equilibrium is attained locally considering that the high temperatures and surface-to-volume ratio counteract the short time available for chemical reactions to be completed, both slag–metal and gas–slag–metal equilibrium models have been evaluated, which may provide technical assistance for flux design and matching. Then, recent applications of Calphad (Computer Coupling of Phase Diagrams and Thermochemistry) technology in the fields of flux design and matching have been introduced. The incumbent review demonstrates that thermodynamic consideration is essential to develop new fluxes or upgrade existing ones to meet the growing demands concerning submerged arc welding quality. Furthermore, it is revealed that the thermodynamic approach is capable of facilitating the flux design process geared toward submerged arc welding. Finally, further investigation into welding thermodynamics is proposed to better aid in flux design and matching.

The present study has been undertaken to evaluate the basicity concept of oxygen content estimation subject to submerged arc welded metal. The investigated fluxes cover a wide range of basicity indices from 0.31 to 19.00 at full coverage with acidic, neutral, and basic fluxes. The oxygen content of the submerged arc welded metal (flux oxygen potential) is predicted by employing traditional flux basicity index models and the CALPHAD technique (gas–slag–metal equilibrium model). It is revealed that the gas–slag–metal equilibrium model in this study is capable of remedying the shortcomings of the basicity index models with respect to predicting the oxygen potentials of the fluxes with CaF₂, CaO, SiO₂, MnO, and FeO as major components. This study may provide a vital way to address technical difficulties in flux design. Based on the measured and thermodynamic data, thermodynamic parameters pertinent to flux oxygen potential have been systematically assessed.

Manganese is a vital element in determining the mechanical properties of submerged arc welded metal. To ensure a reliable weld, the equilibrium model has been used for decades to predict and control the manganese content, particularly when MnO-bearing fluxes are applied. However, the equilibrium model only considers chemical interactions within the weld pool zone, leading to significant inaccuracies. To address this limitation, we propose a multi-zone model that accounts for all of the essential reaction zones in the submerged arc process via the Calphad technique. The model’s accuracy is verified by predicting the manganese content, the flux oxygen potential, and the neutral point location for the typical MnO-bearing fluxes covering acidic, neutral, and basic fluxes. The results indicate that the multi-zone model offers superior accuracy compared to the equilibrium model, which neglects significant oxygen improvement and alloy evaporation in the droplet zone. Further analysis of thermodynamic data reveals that the multi-zone model provides a more representative depiction of the variation trends in oxygen and manganese contents during the submerged arc welding process compared to the equilibrium model. Furthermore, this model can be utilized in the optimization of the submerged arc welding process, leading to improved quality and efficiency in heavy engineering industries. This study may provide an improved method for predicting the manganese content in welded C-Mn steel and deepen the understanding of manganese transfer mechanisms during the submerged arc welding process.

Neutral Point indicates the flux formula where no transfer of alloying element between the flux and weld metal occurs. For the submerged arc welding process, Neutral Point is an essential definition for flux design and specification since it helps to identify the flux microalloying ability. The scientific hypothesis that the Neutral Point is only a function of the flux formula is considered as the basis of the Mitra kinetic model. Within this framework, by performing submerged arc welding with CaF₂-SiO₂-Na₂O-MnO agglomerated fluxes under various heat inputs, the moving of Neutral Point has been captured, indicating the scientific hypothesis proposed in Mitra kinetic model may be revised under high heat input welding. Additionally, although some studies have incorporated the consideration of the gas-slag-metal equilibrium, only the effective equilibrium temperature of 2000 or 2100 °C is utilized, which may be insufficient to constrain Mn content in the weld metal. In this study, we have incorporated all possible effective equilibrium temperatures that may be attained in the submerged arc welding process to simulate the transfer behavior of Mn. Then, a novel thermodynamic approach is proposed to detect the moving direction of Neutral Point for Mn from both slag-metal and gas-slag-metal equilibrium considerations, which may pave a vital way for the flux design and the setting of welding parameters. The factors responsible for the deviation between real and predicted data are discussed. The mechanism responsible for the moving of Neutral Point regarding the Mn element is evaluated from the perspective of both slag-metal and gas-slag-metal equilibrium considerations.

From a thermodynamic perspective, the present study has been performed to investigate the effect of SiO₂ level in agglomerated fluxes on the element transfer behavior of essential elements, by applying CaF₂-Na₂O-SiO₂ agglomerated fluxes with varying SiO₂ contents. Element transfer behavior is quantified by the Δ value. The impact of SiO₂ and heat input upon element transfer behavior is interpreted. Additionally, a possible thermodynamic approach to predict high basicity flux O potential and weld metal composition is proposed and evaluated. It is revealed that the consideration of the gas-slag-metal equilibrium is able to place constraints on the transfer behaviors (of O, Si, and Mn) and formation of gases. In submerged arc welding metallurgy, the empirically determined basicity index models proposed by Tuliani have been applied for more than 50 years to predict flux O potential and weld metal oxygen content. However, it is well known by welding practitioners that the flux basicity index model can only predict the changing trend of flux O potential when the flux basicity index is lower than 2.0. The present study has proposed a new thermodynamic method to identify the flux O potential for fluxes with a basicity index higher than 2.0. Additionally, the experimental evidence for the Mitra kinetic model has been provided.