Study Note on High-Temperature Oxidation Characteristics of Iron-Based Alloy Composite Overlay Layers

Literature Overview

The 2025 paper by Duan Moran in Materials Reports (Vol. 39, S2, pp. 587-590) addresses a timely and practically important topic: the high-temperature oxidation resistance of iron-based alloy composite overlay coatings applied to 20G steel plates. The study is motivated by the urgent need for wear-resistant and oxidation-resistant components in coal-fired power plants, particularly mill rollers that operate under severe abrasive and thermal conditions. The author employed tungsten inert gas (TIG) overlay welding to deposit FeCrSiMnNbB alloy coatings of varying thicknesses on 20G steel substrates, then conducted accelerated high-temperature oxidation tests to evaluate and compare the oxidation resistance of the overlay layers and the base metal. The study also investigates the oxidation mechanisms through SEM-EDS, XRD, and optical microscopy analysis, finding that the overlay layer exhibits superior oxidation resistance compared to 20G steel, with double-layer overlays outperforming single-layer overlays.

Core Technical Content and Coating Design Philosophy

The composition of the FeCrSiMnNbB overlay alloy is notable for its design philosophy, which draws from the principles of amorphous alloy coating systems. This is a sophisticated approach that leverages the rapid solidification and high nucleation density characteristics of metallic glasses to promote the formation of fine, uniform microstructures in the weld deposit. The key alloying elements serve distinct functions: chromium (Cr) provides oxidation resistance through the formation of a protective Cr2O3 scale; silicon (Si) enhances the stability and adherence of the oxide scale; niobium (Nb) acts as a microalloying element that refines grain structure and promotes the precipitation of fine oxide phases; manganese (Mn) contributes to solid-solution strengthening; and boron (B) modifies the melting characteristics of the weld pool and may influence oxide scale formation.

The concept of borrowing design rules from amorphous alloy systems is particularly innovative. Amorphous alloys are known for their exceptional corrosion resistance and high-temperature oxidation resistance, attributed to their homogeneous composition and absence of grain boundaries that serve as preferential diffusion paths for oxygen. While the TIG overlay welding process produces a crystalline microstructure, the rapid cooling rates achieved with water-assisted cooling can suppress grain growth and promote the formation of nano-scale precipitates, mimicking some of the beneficial characteristics of amorphous structures.

Overlay Coating Parameters

High-Temperature Oxidation Behavior and Mechanisms

The oxidation resistance of metallic materials at elevated temperatures is governed by the formation and stability of oxide scales on the material surface. For iron-based alloys, the dominant oxide phases are typically Fe2O3, Fe3O4, and FeO, which form a multi-layered scale structure. These iron oxides are generally non-protective, exhibiting high oxygen diffusion rates and poor adhesion to the underlying metal, leading to continuous and rapid oxidation. The addition of chromium fundamentally changes this behavior by promoting the formation of a thin, dense, and adherent Cr2O3 layer that acts as a diffusion barrier, dramatically reducing the oxidation rate.

The study's finding that the overlay layer outperforms 20G steel in high-temperature oxidation resistance is consistent with established metallurgical principles, but the magnitude of improvement and the mechanism by which double-layer overlays outperform single-layer overlays provide valuable insights. The double-layer configuration likely achieves superior performance through several mechanisms: reduced dilution rate (as the second layer is deposited onto the first overlay layer rather than directly onto the base metal), improved compositional homogeneity, and potentially finer microstructure due to the altered thermal history of the multi-pass welding sequence.

The accelerated oxidation testing methodology is important to consider. While accelerated tests provide rapid comparative data, they may not accurately predict long-term service behavior if the test conditions (temperature, atmosphere, cycling rate) differ significantly from actual service conditions. For mill rollers in power plants, the actual service environment involves cyclic thermal loading (alternating between ambient temperature and 500-700°C during operation), mechanical contact with coal or ash particles, and potentially reducing atmospheres containing CO and H2. A comprehensive evaluation should ideally include thermal cycling tests, abrasion-oxidation synergistic tests, and possibly field trials.

Microstructural Analysis and Phase Evolution

The SEM-EDS and XRD analyses provide critical information about the oxide scale composition and the underlying microstructure of the overlay layer. For the FeCrSiMnNbB alloy, the expected oxide phases include Cr2O3 as the primary protective phase, possibly with minor amounts of Fe2O3, Fe3O4, SiO2, and Nb2O5. The distribution and continuity of the Cr2O3 layer is the key determinant of oxidation resistance. Discontinuous or cracked Cr2O3 scales allow oxygen to penetrate to the underlying metal, leading to accelerated oxidation.

The optical microscopy observations likely reveal the effect of the rapid cooling rates on the microstructure of the overlay deposit. Water-assisted cooling during TIG welding can produce cooling rates exceeding 100°C/s, which is significantly higher than conventional welding processes. Such rapid cooling suppresses grain growth and promotes the formation of fine dendritic or cellular structures with high nucleation density. The resulting fine microstructure has several beneficial effects: reduced grain boundary area (limiting diffusion paths), higher dislocation density (providing additional hardening), and potentially enhanced precipitation of fine oxide-forming phases.

Engineering Practice Integration

The practical application of this research to power plant mill roller components is straightforward but requires careful implementation. Mill rollers operate under a combination of severe mechanical wear (from coal and ash particles), thermal cycling (from hot flue gases), and oxidation (from oxygen in the combustion atmosphere). The overlay welding approach offers a cost-effective solution for extending component life, but several practical considerations must be addressed:

• Surface preparation of the roller surface must be thorough to ensure good fusion and adhesion of the overlay layer.
• The welding sequence for large rollers must be carefully planned to minimize distortion, which is critical for maintaining roller geometry and alignment.
• The overlay thickness must be sufficient to accommodate the expected wear rate over the planned maintenance interval, typically designed with 2-3 mm of sacrificial overlay thickness.
• Quality control should include hardness testing, chemical analysis of the overlay layer, and possibly accelerated oxidation testing on witness coupons.

Key Questions and Reflections

A significant question that arises from this study is the long-term stability of the oxide scale under thermal cycling conditions. While the accelerated oxidation tests demonstrate excellent initial oxidation resistance, the oxide scale may crack or spall during thermal cycling due to differences in thermal expansion coefficients between the oxide scale and the underlying metal. Chromium oxide (Cr2O3) has a thermal expansion coefficient of approximately 6.5 x 10^-6 /°C, which is significantly lower than that of the iron-based overlay alloy (approximately 12-14 x 10^-6 /°C). This mismatch can lead to compressive stresses in the oxide scale during cooling, potentially causing buckling, cracking, or spallation. The role of silicon and niobium in stabilizing the oxide scale under thermal cycling conditions would be valuable to investigate further.

Another reflection concerns the scalability of the TIG overlay welding process for large industrial components. TIG welding is relatively slow compared to submerged arc welding or flux-cored arc welding, which may limit its practical application for large mill rollers or other heavy-duty components. However, the superior quality and microstructural control achievable with TIG welding may justify the lower deposition rate for critical applications where overlay performance is paramount. A hybrid approach combining TIG for the critical surface layers and SAW for bulk deposition could offer an optimal balance between quality and productivity.

Study Insights and Conclusion

This paper presents a well-conceived investigation into the high-temperature oxidation behavior of iron-based overlay alloys, with clear practical relevance to power plant applications. The innovative approach of borrowing design principles from amorphous alloy systems demonstrates the value of cross-disciplinary thinking in materials engineering. The finding that double-layer overlays outperform single-layer overlays provides practical guidance for process optimization, while the microstructural analysis reveals the fundamental mechanisms responsible for enhanced oxidation resistance. For engineers specifying overlay welding for high-temperature wear applications, this study reinforces the importance of considering not only wear resistance but also oxidation resistance and thermal cycling durability in the alloy selection and process design. The integration of rapid cooling techniques with multi-pass overlay strategies represents a promising approach to achieving synergistic improvements in multiple performance characteristics.