High-Temperature Oxidation Behavior of Iron-Based Composite Overlay Coatings

Literature Overview

This 2025 publication by Duan Moran from Datang International Power Generation Zhangjiakou Branch, published in Materials Reports (Vol. 39, S2, pp. 587-590), investigates the high-temperature oxidation resistance of FeCrSiMnNbB alloy composite overlay coatings deposited on 20G steel via GTAW overlay welding. The research simulates the high-temperature oxidation environment encountered during grinding roller wear in thermal power plants and compares single-layer and dual-layer overlay configurations against the bare 20G substrate.

Technical Approach and Experimental Design

The study addresses a critical operational challenge in coal-fired power plants where grinding rollers in coal mills experience severe wear combined with elevated temperatures from hot flue gas exposure. The FeCrSiMnNbB alloy composition was designed by borrowing design principles from amorphous alloy coating systems, which typically exhibit enhanced oxidation resistance due to their homogeneous, defect-free structure.

Test Condition	Single-Layer Overlay	Dual-Layer Overlay	20G Substrate
Test Temperature	800-1000°C	800-1000°C	800-1000°C
Test Duration	Accelerated cycling	Accelerated cycling	Accelerated cycling
Oxidation Kinetics	Parabolic regime	Parabolic regime	Parabolic with higher rate
Oxide Scale Quality	Dense, adherent	Denser, more protective	Porous, spalling
Relative Oxidation Resistance	Moderate improvement	Best performance	Baseline (poor)

The GTAW overlay process was performed with circulating water cooling assistance, which significantly increased the cooling rate of the molten pool. This rapid solidification suppressed grain growth and promoted the spontaneous formation of nano-sized precipitates within the overlay microstructure. The multi-pass overlay technique in the dual-layer configuration further improved the dilution rate by progressively building up the overlay with each pass, ensuring a more homogeneous and alloy-rich surface composition.

Oxidation Mechanism Analysis

The SEM-EDS and XRD characterization of the oxidation scales revealed distinct differences between the overlay coatings and the substrate. The 20G steel substrate formed thick, porous iron oxide scales (Fe2O3 and Fe3O4) with extensive cracking and spalling, indicating poor protective capability. The single-layer overlay produced a thinner, denser scale with mixed oxide composition including Cr2O3, SiO2, and Nb-containing oxide phases. The dual-layer overlay exhibited the most protective behavior with an extremely thin, continuous, and adherent oxide scale.

The superior oxidation resistance of the dual-layer overlay is attributed to several synergistic mechanisms:

The amorphous-alloy-inspired composition promotes rapid formation of a continuous protective oxide film during early oxidation stages.
The high cooling rate from water-assisted GTAW produces a fine-grained microstructure with abundant nano-precipitates that act as nucleation sites for protective oxide phases.
The multi-pass overlay reduces dilution from the base metal, ensuring higher concentrations of oxidation-resistant alloying elements (Cr, Si, Nb, B) in the final surface layer.
The nano-phase distribution within the overlay provides a larger number of preferential oxidation sites, promoting the formation of a more uniform and protective scale.

Engineering Relevance for Power Plant Applications

For thermal power plant maintenance engineers, this study offers practical guidance for extending the service life of grinding rollers and similar components in coal mills and pulverizing systems. The dual-layer overlay approach provides a significant improvement in oxidation resistance over single-layer coatings, translating to longer service intervals between maintenance shutdowns. The GTAW process with water cooling is particularly suitable for field application where equipment cannot be removed to a workshop for conventional thermal spray or plasma spray coating.

The oxidation kinetics follow a parabolic rate law for both overlay configurations, indicating that the protective oxide scale effectively limits further oxidation through diffusion control. However, engineers should be aware that the protective behavior is temperature-dependent, and the transition from parabolic to linear kinetics may occur at temperatures exceeding 1000°C or under conditions of mechanical stress that disrupt the oxide scale.

Key Reflections and Study Insights

The most innovative aspect of this research is the application of amorphous alloy design principles to crystalline overlay weld deposits. This cross-disciplinary approach demonstrates that composition design strategies from one material system can be effectively transferred to another when the underlying thermodynamic principles are properly understood. The water-cooled GTAW technique is an elegant solution to the challenge of achieving fine microstructures in overlay welding without requiring specialized equipment or post-weld processing. For engineers evaluating overlay solutions for high-temperature wear applications, this study reinforces the importance of considering both wear resistance and oxidation resistance as coupled degradation mechanisms, rather than treating them as independent design criteria. The dual-layer strategy, while more complex to implement, provides substantially better protection and should be considered the preferred approach when component availability is critical and unplanned downtime carries significant economic penalties.