Elucidating the Roles of SiO 2 and MnO upon Decarburization During Submerged Arc Welding: A Thermodynamic Study into EH36 Shipbuilding Steel JIN ZHANG, THERESA COETSEE, HONGBIAO DONG, and CONG WANG By employing CaF 2 -SiO 2 and CaF 2 -SiO 2 -MnO system fluxes, the roles of SiO 2 and MnO in decarburization behaviors during submerged arc welding of EH36 shipbuilding steel have been quantified and evaluated. All possible reactions associated with C transfer and interfaces at which these reactions occur are systematically discussed. It is concluded that the addition of SiO 2 and MnO exerts synergistic effects on the extent of decarburization due to increased partial pressures of O 2 and SiO gases in the plasma, improved O level in the weld pool, and higher activities of the oxides, such as SiO 2 , MnO, and FeO, at the slag–metal interface. The investigation over the macrographic detached slag surfaces shows that the possibility of bubble nucleation is highly influenced by flux formula. The effect of heat input on decarburization is discussed, and the optimal flux compositions expected in the present study are analyzed. https://doi.org/10.1007/s11663-020-01869-x Ó The Minerals, Metals & Materials Society and ASM International 2020 I. INTRODUCTION C is an essential element in the weld metal (WM) for steel grades. It is widely accepted that an increase in C composition enhances steel strength and hardness; however, redundant C may cause a reduction in elongation, toughness, and weldability. [1] C is also one of the most effective soluble elements to influence acicular ferrite (AF) formation, the level of which must be optimized to reach maximum AF fraction in the WM since the formation of AF tends to be suppressed with excessive C content. [2–4] Submerged arc welding (SAW) has been widely used for the joining of thick workpieces due to its inherently high deposition rate and welding efficiency. [5] During SAW, flux is employed to separate the arc and the weld pool from the atmosphere. [6] One salient characteristic of SAW is the significant O uptake from the flux (slag), which will inevitably lead to the decarburization in SAW. [7–12] Therefore, it is essential to understand the decarburization mechanisms in SAW to ensure a sound weld. The behaviors of decarburization in SAW have been discussed previously. Mitra et al. [9] reported that C was oxidized via the gas–metal interfacial reaction between dissolved C and O in the weld pool; they postulated that CO nucleation was feasible and could be responsible for the bubbles under the slag. Indacochea et al., [8] on the other hand, emphasized the importance of the net decarburization reactions between SiO 2 in the slag, gases (SiO and O 2 ) in the plasma, and C in the weld pool; they assumed that CO only tended to nucleate at the plasma–metal interface. Bang et al. [13] performed SAW with different fluxes and found that C loss from the WM generally increased with lower flux basicity. Generally, flux is the major source of O contamina- tion [7,14] and plays an important role in decarburization control as a result of the following reasons: 1. In the presence of the welding arc, most oxide com- ponents in the flux tend to decompose and release gases (such as O 2 and SiO) [5,11,15–17] which may react with C in the weld pool at the gas–metal interface. [8] 2. The level of O dissolved in the weld pool, which largely determines the extent of decarburization, is highly dependent on the oxide decomposition behaviors and slag–metal reactions. [1,6,11] 3. Reactions between C dissolved in the weld pool and oxides in the molten slag may occur. [10] JIN ZHANG is with the School of Metallurgy, Northeastern University, Shenyang 110819, China; THERESA COETSEE is with the Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria, 0002, South Africa; HONGBIAO DONG is with the School of Metallurgy, Northeastern University and also with the Department of Engineering, University of Leicester, Leicester LE1 7RH, UK; CONG WANG is with the School of Metallurgy, Northeastern University and also with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. Contact e-mail: wangc@smm.neu.edu.cn Manuscript submitted March 16, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS B