Zhuojin Pipe Fitting Co., LtdSelf-Protective Paste Boriding Process for Economizer Steel Tubes
Literature Overview
This study by He Jianjun, Luo Baier, Chen Jian, and Ren Yanjie, published in "Ordnance Materials and Engineering Science" (Volume 33, Issue 5, 2010, pages 8–11), investigates the application of self-protective paste boriding technology to economizer tubes in power boiler systems. The research was supported by the Hunan Provincial Natural Science Foundation Key Project (09JJ3097) and the Hunan Provincial Key Discipline Support Project (080703), and conducted at the School of Energy and Power Engineering, Changsha University of Science and Technology.
Technical Background and Process Description
Economizer tubes operate in the hottest zones of boiler systems, where they are subjected to severe oxidation, scaling, and hot corrosion. Conventional protective measures such as alloy coatings or refractory overlays have limitations in terms of durability and cost. Boriding—a surface diffusion treatment that introduces boron into the steel substrate—creates a hard, wear-resistant, and oxidation-resistant surface layer. The self-protective paste method eliminates the need for external furnace atmosphere control, making the process more economical and adaptable to complex geometries.
The boriding process involves applying a paste containing boron source (typically FeB₂ or B₄C) and activator to the tube surface, followed by high-temperature diffusion in air. The self-protective characteristic means the paste itself forms a protective layer during heating, preventing excessive oxidation of the underlying steel.
Process Parameters and Their Effects
The study systematically investigates the influence of key process parameters on boriding quality:
| Parameter | Tested Range | Optimal Value | Effect |
|---|---|---|---|
| Coating thickness | Variable | 6 mm | Thicker coatings provide more boron source but risk cracking; 6 mm balances supply and integrity |
| Boriding temperature | Variable | 900°C | Higher temperatures accelerate diffusion but promote excessive grain growth and brittleness |
| Boriding time | Variable | 5 hours | Longer times deepen the layer but cause tip fragmentation of borides |
At the optimal conditions of 6 mm coating thickness, 900°C temperature, and 5 hours duration, the resulting boride layer exhibits a characteristic finger-like morphology with varying penetration depths. This morphology is typical of boriding in carbon and low-alloy steels, where the diffusion front advances preferentially along grain boundaries and crystallographic planes.
Microstructural Analysis
The finger-like boride structure observed in the study consists primarily of Fe₂B and FeB phases. The Fe₂B phase, which forms at the outer surface, is harder but more brittle, while FeB forms deeper and provides better toughness. The depth variation of the fingers is attributed to the heterogeneous microstructure of the base steel—regions with finer grain size or different crystallographic orientation allow faster boron diffusion.
A notable observation is the tip fragmentation phenomenon that occurs with prolonged boriding times. As the boride layer deepens, the internal stresses generated by the volume expansion during phase transformation cause cracking at the tips of the finger-like structures. This is a critical finding because excessive fragmentation compromises the integrity of the protective layer and may lead to spalling under thermal cycling conditions.
The ease of surface cleaning after boriding is another practical advantage highlighted in the study. The residual paste and oxidized scale can be removed relatively easily, which is important for maintaining the internal cleanliness of economizer tubes where fluid flow efficiency is critical.
Engineering Practice Considerations
For economizer tubes, the boriding treatment must be compatible with the base steel grade. Common economizer tube materials include 12Cr1MoV, 15CrMo, and P91/P92 steels. The boriding process at 900°C is below the tempering temperature for most of these steels, which means the base metal properties remain largely unaffected. However, the thermal cycling during boriding may cause some temper softening in the heat-affected zone immediately beneath the boride layer, which should be evaluated for applications subject to cyclic loading.
The boride layer's resistance to high-temperature oxidation and scaling makes it particularly suitable for economizer tubes exposed to hot flue gas. The hardness of the Fe₂B/FeB layers (typically 1400–2000 HV) provides excellent erosion resistance against fly ash particles, which is a major degradation mechanism in boiler economizers.
From a quality control perspective, the boriding process requires careful monitoring of the following:
- Surface preparation quality (degreasing and roughening to ensure paste adhesion)
- Uniformity of paste application thickness
- Temperature uniformity across the furnace zone
- Post-treatment inspection of boride layer depth and continuity
Key Insights and Reflections
The self-protective paste boriding technique offers a practical surface hardening solution for economizer tubes that balances performance, cost, and process simplicity. The optimal parameters identified (6 mm, 900°C, 5 h) provide a clear process window for industrial implementation. The finger-like morphology, while appearing irregular, actually provides good stress relief through its branching structure, which can accommodate thermal expansion differences between the boride layer and the base steel.
One area requiring further investigation is the long-term durability of the boride layer under actual boiler operating conditions. Thermal cycling, mechanical vibration, and chemical attack by combustion products may all contribute to layer degradation. The tip fragmentation observed at extended boriding times suggests that the process should not be pushed beyond the optimal window in an attempt to increase layer depth.
The study's focus on air-environment boriding is significant because it eliminates the need for inert atmosphere furnaces, substantially reducing equipment costs. However, the air environment also introduces oxygen into the diffusion process, which may affect the phase composition and porosity of the boride layer. Future work should address the mechanical properties (hardness, fracture toughness, adhesion strength) and corrosion resistance of the borided surface under simulated service conditions.
Summary
The self-protective paste boriding process provides an effective surface treatment for economizer steel tubes, producing a hard, oxidation-resistant boride layer with a characteristic finger-like morphology. The optimal process parameters of 6 mm coating thickness, 900°C temperature, and 5 hours duration yield a well-formed boride layer that is easily cleaned and resistant to erosion and high-temperature oxidation. The tip fragmentation phenomenon observed at extended times serves as a clear indicator of process limits, and engineers should adhere to the established parameter window to ensure consistent quality. This technology represents a cost-effective enhancement of economizer tube life in power generation applications.