Thermodynamic Analysis of Rare Earth Oxide Modification of Inclusions in Medium-High Carbon Steel Surfacing Metal

Literature Overview

This 2001 study by Yang Qingxiang, Yao Mei, and Wei Yajuan from Yanshan University and Hebei Building Materials Vocational University, published in the Journal of Chinese Rare Earths, presents a thermodynamic analysis of the modification of inclusions in medium-high carbon steel surfacing metal by rare earth oxides. The research was supported by the State Key Laboratory of Modern Welding Production Technology. The study is significant because inclusion control is a critical quality factor in surfacing alloys, particularly for applications in oil and gas pipelines, pressure vessels, and high-strength structural components where inclusion-induced cracking and fatigue failure are primary concerns.

Core Technical Findings

The thermodynamic analysis demonstrates that rare earth oxides (REO) can be reduced by carbon in the surfacing melt pool to form rare earth elements, which then react with oxygen and sulfur to form rare earth oxides (REO), rare earth sulfides (RES), and rare earth oxy-sulfides (REOS). This sequence of reactions achieves deoxidation, desulfurization, and melt pool purification. Additionally, REO can directly react with sulfur to form rare earth oxy-sulfides, providing an alternative desulfurization pathway.

Thermodynamic Analysis

The thermodynamic feasibility of each reaction is determined by the Gibbs free energy of formation. Rare earth elements have a very high affinity for both oxygen and sulfur, with Gibbs free energies of formation for REO and RES being more negative than those for FeO and FeS. This means that the reactions are thermodynamically favorable at surfacing temperatures (typically 1500–1800 °C). The reduction of REO by carbon is also thermodynamically favorable at these temperatures, as the Gibbs free energy of formation for CO is sufficiently negative to drive the reaction.

The sequence of reactions is important. Carbon first reduces the REO to liberate rare earth atoms, which then preferentially react with oxygen and sulfur in the melt. The resulting REO, RES, and REOS have lower melting points than the original REO and are more easily modified from angular, sharp morphologies to spherical shapes. This morphological modification reduces the stress concentration at inclusion boundaries, improving the fatigue and fracture resistance of the deposited metal.

Engineering Significance for Pipe and Fitting Applications

Inclusion control is particularly critical for surfacing alloys applied to pipe and fitting components that operate under cyclic loading or in corrosive environments. Sulfide inclusions, particularly MnS, are elongated and can act as crack initiation sites during fatigue loading. Oxygen inclusions can form brittle intermetallic compounds that reduce ductility. By modifying inclusions with rare earth oxides, the following improvements are achieved:

• Reduced inclusion size: RE-modified inclusions are finer and more uniformly distributed
• Spherical morphology: Spherical inclusions create less stress concentration than angular or elongated ones
• Lower volume fraction: Effective deoxidation and desulfurization reduce the total inclusion volume
• Improved bonding: Modified inclusions have better interfacial bonding with the matrix, reducing debonding during deformation

For oil and gas pipeline applications, where hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) are major concerns, inclusion control is essential. Sulfide inclusions serve as hydrogen trapping sites, promoting HIC and SSC. By reducing sulfur content through REO addition, the susceptibility to these cracking mechanisms is significantly reduced.

Process Implementation Considerations

The practical implementation of REO modification in surfacing processes requires careful consideration of the following factors:

1. REO addition method: REO can be added to the flux in submerged arc welding, to the flux coating in flux-cored wire, or as a separate powder layer. The addition level is typically 0.1–0.5 wt% of the deposited metal weight.
2. Process parameters: The arc temperature and residence time affect the reduction efficiency of REO by carbon. Higher arc temperatures and longer residence times favor reduction but may also increase RE evaporation losses.
3. Substrate effects: The substrate composition affects the dilution and the final inclusion composition. Low-sulfur substrates (e.g., SS400, A106 Gr. B) are preferred for REO-modified surfacing.
4. Quality verification: Inclusion analysis should be performed using optical microscopy (ASTM E45) or SEM-EDS to verify the inclusion composition and morphology. The inclusion rating should meet the requirements of the applicable specification (e.g., ASTM A240 for stainless steel, API 5L for line pipe).

Key Questions and Reflections

The study provides a thorough thermodynamic analysis but does not include experimental validation of the predicted reactions. While the thermodynamic calculations are sound, the actual kinetics of REO reduction and inclusion modification in a rapidly solidifying surfacing melt pool may differ from equilibrium predictions. The short residence time of the molten pool (typically 1–5 seconds) may limit the extent of reaction, particularly for the reduction of REO by carbon, which may require longer times to reach equilibrium.

Additionally, the study does not address the cost implications of REO addition. Rare earth elements are relatively expensive, and their use in surfacing alloys adds to the material cost. The economic justification for REO modification must be evaluated on a case-by-case basis, considering the cost of inclusion-related failures (cracking, corrosion, fatigue) against the additional material and process costs.

Study Insights and Implications

This study provides a fundamental thermodynamic framework for understanding the role of rare earth oxides in inclusion modification during surfacing. The identification of multiple reaction pathways—reduction by carbon, direct deoxidation, desulfurization, and oxy-desulfurization—offers engineers flexibility in process design. For the pipe and fitting industry, where inclusion-related failures can have catastrophic consequences, REO modification represents a valuable tool for improving deposit quality. The key challenge lies in translating the thermodynamic predictions into practical process parameters that achieve the desired inclusion modification within the constraints of industrial production. Future work should focus on kinetic studies and process optimization to bridge the gap between thermodynamic theory and industrial practice.