Zhuojin Pipe Fitting Co., LtdSelf-Protective Paste Boronizing Treatment of Economizer Steel Tubes
Literature Overview
This paper by He Jianjun, Luo Beier, Chen Jian, and Ren Yanjie, published in Ordnance Materials and Engineering (2010, Vol. 33, No. 5, pp. 8-11), investigates the application of self-protective paste boronizing technology to economizer steel tubes in power plant boilers. Economizer tubes operate in environments subject to high-temperature flue gas corrosion, ash erosion, and thermal cycling, leading to premature tube failure. The authors examine the effect of process parameters on boronization depth and microstructure, and identify optimal processing conditions. This work is supported by the Hunan Provincial Natural Science Foundation Key Project (09JJ3097) and the Hunan Provincial Key Discipline Fund Project (080703).
Core Technical Analysis
Boronizing Process Parameters and Their Effects
Boronization is a diffusion surface treatment that introduces boron into the surface layer of steel, forming a hard, wear-resistant, and corrosion-resistant boride layer. The self-protective paste method eliminates the need for a protective atmosphere furnace, making it particularly suitable for on-site or field application of large-diameter tubes such as economizer tubes.
| Process Parameter | Studied Range | Optimal Value | Effect on Boronization |
|---|---|---|---|
| Paste Coating Thickness | 3-8 mm | 6 mm | Too thin: insufficient boron supply; too thick: uneven heating |
| Boronization Temperature | 800-1000°C | 900°C | Higher temperature: faster diffusion but risk of boride overgrowth |
| Boronization Time | 2-8 h | 5 h | Longer time: deeper layer but increased tip fragmentation |
The paste coating serves as both the boron source and the protective atmosphere generator. Upon heating, the paste decomposes to produce a reducing atmosphere that prevents excessive oxidation of the steel substrate while supplying boron atoms for diffusion. The coating thickness of 6 mm provides an adequate boron reservoir while ensuring uniform temperature distribution across the tube surface. Thinner coatings risk premature depletion of boron supply, while thicker coatings may create thermal gradients that lead to non-uniform boride formation.
Microstructural Analysis of Boronized Layer
The authors report that the boronized layer exhibits a characteristic finger-shaped or needle-like morphology extending into the steel substrate. This morphology is typical of boride growth in carbon steels, where boron diffuses preferentially along grain boundaries and crystallographic channels. The depth of the finger-shaped boride layer varies across the surface, reflecting local variations in grain orientation, residual stress, and microsegregation in the base steel.
With increasing boronization time, the tips of the boride fingers exhibit fragmentation or spalling. This phenomenon occurs because the brittle Fe₂B and FeB phases at the boride tips experience thermal stress during cooling, leading to microcracking and eventual detachment. While this tip fragmentation can be undesirable for surface integrity, the authors note a practical advantage: the fragmented boride tips are easier to remove during post-treatment cleaning, resulting in a cleaner and more uniform surface finish. This is particularly important for economizer tubes where residual boride fragments could act as erosion nucleation sites during flue gas flow.
The boronized layer typically consists of two distinct phases: an outer Fe₂B layer, which is harder but more brittle, and an inner FeB layer, which is slightly softer but more adherent to the substrate. The transition zone between the boronized layer and the base steel is characterized by a gradient in carbon and boron concentration, providing a gradual transition in mechanical properties that helps prevent delamination.
Performance Enhancement for Economizer Service
Economizer tubes in coal-fired boilers face a combination of degradation mechanisms:
- High-temperature oxidation: Flue gas temperatures in the economizer range from 250°C to 450°C, promoting scale formation on tube surfaces that increases thermal resistance and promotes corrosion under scale.
- Ash erosion and abrasion: Fly ash particles carried by the flue gas impinge on tube surfaces, causing progressive material loss, particularly at tube bends and areas of high gas velocity.
- Sulfidation and chloridation: Sulfur and chlorine compounds in the flue gas accelerate corrosion at elevated temperatures, especially in the presence of moisture during startup and shutdown cycles.
- Thermal fatigue: Repeated heating and cooling cycles during boiler start/stop operations induce cyclic stresses that can initiate and propagate cracks in the tube wall.
The boronized layer addresses these degradation mechanisms by providing a hard, chemically stable surface that resists mechanical erosion and chemical attack. The Fe₂B and FeB phases are thermodynamically stable at economizer operating temperatures and exhibit excellent resistance to sulfidation. The increased surface hardness, typically exceeding 1500 HV, provides substantial improvement in erosion resistance compared to the base steel, which typically has a hardness of 150-250 HV.
Engineering Practice and Quality Control
The application of self-protective paste boronizing to economizer tubes presents several practical challenges that must be addressed in engineering implementation. The paste must be applied uniformly around the entire circumference of the tube, which is challenging for large-diameter tubes installed in situ. A recommended approach involves using a mandrel or internal support to hold the tube horizontally and applying the paste with a controlled thickness using a notched applicator. The paste should be allowed to dry and cure before heating to prevent slumping and uneven coverage.
Post-boronization inspection should include hardness profiling across the boronized layer to verify depth and uniformity, microstructural examination of cross-sections to confirm boride morphology and adhesion, and surface roughness measurement to ensure the tube inner surface remains suitable for gas flow. Any areas of incomplete boronization or excessive boride fragmentation should be identified and, if necessary, re-treated.
The work by He Jianjun and colleagues demonstrates that self-protective paste boronizing is a viable and cost-effective approach for extending the service life of economizer steel tubes. The optimal process window of 900°C for 5 hours with a 6 mm paste coating provides a practical balance between boronization depth, microstructural quality, and processing cost. Engineers responsible for boiler maintenance and life extension should consider boronizing as a complementary treatment to tube replacement, particularly for tubes that have experienced localized erosion or corrosion but retain adequate wall thickness.