1. Introduction
The submerged arc welding (SAW) technique is particularly effective in welding for thick steel plates due to its exceptional reliability and high deposition rate [
1]. The arc plasma and weld pool in the SAW process are concealed by a layer flux and slag, rendering them invisible. SAW is a complex metallurgical process with temperatures exceeding 1900 °C, which facilitates chemical reactions among various phases, such as arc plasma, flux/slag, and liquid metal [
2]. These interactions lead to changes in the weld metal (WM) composition, which in turn affect the mechanical properties [
3].
Silicon (Si) is an essential element that significantly impacts the mechanical properties of the weldment, particularly in terms of the strength and toughness. It is widely acknowledged that excessive Si content in the WM should be avoided as it can result in reduced elongation and toughness [
4]. Consequently, it is essential to predict the Si content in submerged arc-welded metal accurately and select appropriate fluxes to achieve the weldment of desired quality [
5].
The temperature achieved in the SAW process is typically above 1900 °C [
6,
7]. Generally, attaining equilibrium is inherently arduous during SAW due to factors such as high temperature gradients, the coexistence of various phases, and the substantial energy transferred from the arc plasma [
8]. Nonetheless, despite these deviations, it remains viable to apply equilibrium principles to place constraints on the chemical mechanisms and interactions within the SAW process [
9,
10]. This is achieved by postulating that local equilibrium can be realized due to the high temperatures and surface-to-volume ratios implicated [
8,
11]. Consequently, although the overall equilibrium may remain elusive in the SAW process, the employment of local equilibrium principles can yield valuable information pertaining to the chemical mechanisms, thereby facilitating the optimization of the SAW process [
12,
13].
Based on the scientific hypotheses mentioned above, equilibrium models have been developed to predict the Si content in the submerged arc-welded metal. Considering the chemical reaction governing the Si transfer at the slag–metal interface, which is shown by Reaction (1), Chai et al. [
4,
9,
10] proposed a slag–metal equilibrium model for estimating the Si content in the submerged arc-welded metal. The novelty of this model lies in its ability to estimate the oxygen (O) content based on the flux basicity index (
BI), which is an empirical index used to estimate the O content of submerged arc-welded metal [
14]. Subsequently, the Si content in the WM ([pct Si]) is estimated from the equilibrium constant of Reaction (1) with the activity of SiO
2 (α
SiO2) in the initial flux, as shown by Equation (2) [
4,
9,
10].
However, the accuracy of the slag–metal equilibrium model in predicting the WM Si content is limited, mainly because the basis for the
BI formula is empirical in nature and does not account for the actual WM O content [
5,
15]. As is well-known, the SAW process enables significant gas formation, which exerts a significant impact on the transfer behavior of Si, especially in terms of Reactions (3) and (4) [
16].
Chai et al. [
16] and Lau et al. [
7] both concluded that the primary source of O in the SAW process is the O
2 generated by the decomposition of oxides inside the arc cavity, such as Reaction (3). Tuliani et al. [
17] suggested that the impact of Reaction (4) on the transfer of Si should also be considered. However, due to the extremely high temperature characteristics of the SAW, it was not possible to perform thermodynamic equilibrium calculations involving gas species under the technological conditions in the early stages of the research [
18]. In recent years, the development of Calphad technology has enabled researchers to perform thermodynamic calculations on metallurgical systems with extremely high temperatures [
19,
20]. The Calphad approach, which stands for “computer coupling of phase diagrams and thermochemistry,” utilizes thermodynamic models to predict the characteristics of material systems with greater accuracy [
19]. Although direct measurement of thermodynamic data is impossible during SAW due to temperatures exceeding 1900 °C, the thermodynamic models used in the Calphad approach have proven reliable in obtaining thermodynamic data above this temperature range [
19,
21]. Based on the gas–slag–metal interface model in SAW, the gas–slag–metal equilibrium model has been updated from the slag–metal equilibrium model [
18]. The gas–slag–metal equilibrium model has demonstrated higher accuracy in predicting Si content compared to the slag–metal equilibrium model [
5].
However, even with the use of thermodynamic equilibrium methods to analyze chemical reactions and estimate the Si content during SAW, there remains a significant margin of error [
5]. The reason for this is that SAW consists of multiple reaction zones rather than solely focusing on the weld pool reaction zone, as emphasized by equilibrium models [
12,
13]. It is well-known that Si is a basic dioxide in the weld pool, and the transfer behavior is largely dictated by the O content in the metal [
4]. Furthermore, the evaporation and redistribution of Si within the SAW process are not considered since the equilibrium models set nominal compositions as the initial compositional input [
1].
To address such issues, the present study has been undertaken to evaluate the transfer behavior of Si in SiO2-bearing fluxes considering all essential chemical reaction zones in SAW. Unlike previous studies that only considered chemical reactions in the weld pool zone, this research will investigate and discuss the elemental behavior of Si in essential reaction zones subject to the SAW process. This method was then compared with the traditional thermodynamic equilibrium models, further enhancing the understanding of the transfer behavior of Si in the SAW process. Then, the shortcomings of the conventional method for quantifying elemental transfer are examined, and recommendations for improvement are put forward.