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

Literature Overview

This paper, published in Materials Review (Vol. 39, Suppl. S2, 2025, pp. 587-590) by Duan Moran from Datang International Power Generation Co., Ltd. Zhangjiakou Branch, investigates the high-temperature oxidation resistance of iron-based composite overlay layers deposited on 20G steel plates. The overlay alloy composition FeCrSiMnNbB was applied using tungsten inert gas (TIG) arc overlay welding, and the oxidation behavior was evaluated under simulated high-temperature wear conditions. The study compares single-layer and dual-layer overlay configurations and examines the role of microstructural features in oxidation resistance.

Core Technical Approach

The research addresses a critical need in power generation: the severe wear conditions experienced by coal mill rollers in thermal power plants. These components operate at elevated temperatures (300-600°C) under abrasive and oxidative conditions, leading to rapid material degradation.

Overlay Process Parameters

The use of circulating water-assisted cooling during the overlay welding process is a notable process feature. This technique increases the cooling rate of the deposited metal, which suppresses grain growth and promotes the spontaneous formation of nano-sized precipitates within the overlay microstructure.

Oxidation Test Results

The oxidation behavior was evaluated using accelerated oxidation testing, with weight gain kinetics measured over time:

The dual-layer configuration outperforms the single-layer overlay, which in turn outperforms the unprotected base metal. This improvement is attributed to three key microstructural factors.

Microstructural Analysis and Oxidation Mechanism

Phase Composition

Microstructural Features

The SEM-EDS, XRD, and optical microscopy analyses revealed:

1. Nano-phase formation: The water-assisted cooling produced fine nano-sized precipitates (Cr-rich carbides and intermetallics) within the overlay matrix. These nano-phases provide additional diffusion barriers against oxygen ingress.
2. Grain refinement: The rapid cooling rate suppressed austenite grain growth, resulting in a fine-grained microstructure with increased grain boundary area. While grain boundaries can serve as fast diffusion paths, the overall refinement effect dominates, reducing oxidation rate.
3. Multi-pass dilution improvement: The dual-layer technique effectively reduces dilution from the base metal, ensuring higher concentrations of alloying elements (Cr, Si, Nb, B) in the final overlay surface.

The oxidation mechanism follows a modified parabolic rate law, where the protective scale (primarily Cr₂O₃ with minor SiO₂ and B₂O₃) acts as a diffusion barrier. The quality and continuity of this scale determine the long-term oxidation resistance.

Connection with Engineering Practice

In coal-fired power plants, mill rollers (also called classifier separators or grinding rollers) experience a combined damage mechanism of abrasive wear from coal particles and oxidative degradation at elevated operating temperatures. Traditional high-carbon steel rollers require frequent replacement, leading to significant downtime and maintenance costs.

The overlay approach offers several practical advantages:

• Cost-effectiveness: Overlay welding is significantly less expensive than replacing the entire roller with a corrosion-resistant alloy.
• Repairability: Worn rollers can be re-overlay welded in the field, extending service life without complete replacement.
• Design flexibility: The overlay thickness and composition can be tailored to specific operating conditions (temperature, coal moisture, abrasiveness).

Process Optimization Recommendations

Based on the findings, the following process recommendations are relevant for practical implementation:

Key Questions and Reflections

A significant question raised by this work is the long-term stability of the nano-phase structure under sustained high-temperature exposure. Nano-sized precipitates are thermodynamically metastable and may coarsen (Ostwald ripening) during prolonged service at 500-600°C. The paper does not address this concern, but in practice, the beneficial nano-phase effect may diminish after extended exposure, potentially reducing oxidation resistance over time.

Another reflection concerns the applicability of this overlay technology to other high-temperature wear components in power plants, such as fan blades, air preheater tubes, and boiler tubes. The FeCrSiMnNbB composition, inspired by amorphous alloy design principles, represents an interesting approach to developing cost-effective oxidation-resistant overlays without relying on expensive nickel-based or cobalt-based alloys.

The water-assisted cooling technique is particularly innovative and warrants further investigation. While it effectively refines the microstructure, it may also introduce residual stresses due to thermal gradients. The interaction between these residual stresses and thermal cycling in service could affect overlay adhesion and crack initiation.

Study Insights and Implications

This paper presents a promising approach to improving the oxidation resistance of iron-based overlay alloys through microstructural engineering rather than composition modification. The key insight is that process control—specifically the cooling rate—can be used to create beneficial microstructural features (nano-phases, refined grains) that enhance oxidation resistance without increasing material costs. The dual-layer approach further demonstrates that dilution control is critical for achieving optimal overlay performance. For engineers working on high-temperature wear applications, this study suggests that process optimization should be considered as a primary lever for performance improvement, alongside composition design. The combination of amorphous alloy design principles with conventional overlay welding technology represents a creative approach that bridges fundamental materials science with practical manufacturing.