Zhuojin Pipe Fitting Co., LtdMicrostructure and Abrasive Wear Resistance of Fe-Cr-B-C Surfacing Alloys
Literature Overview
This research by Wang Zhihui and colleagues from Beijing University of Technology investigates the effect of boron content on the microstructure and abrasive wear resistance of Fe-Cr-B-C surfacing alloys deposited using flux-cored wire gas shielded welding. The study is particularly relevant to engineers working on wear-resistant surfacing for mining equipment, bulk handling systems, and heavy-duty structural components where abrasive wear is the dominant failure mechanism. The work was supported by the Beijing Municipal Education Commission Science and Technology Achievement Transformation and Industrialization Project.
Experimental Design and Methodology
The authors fabricated Fe-12Cr-xB-0.1C surfacing alloys using flux-cored wire gas shielded surfacing, with boron content systematically varied. The characterization methods included optical metallography, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis. The wear testing was conducted under abrasive conditions to simulate real-world service environments.
The boron content range studied was critical: boron is known to form hard boride phases in iron-based alloys, but excessive boron can lead to embrittlement and cracking. The systematic variation of boron content allows identification of the optimal range for wear resistance without compromising deposit integrity.
Microstructural Evolution with Boron Content
The base microstructure of the Fe-12Cr-xB-0.1C alloys consists of ferrite, austenite, and boride phases. The primary phases identified are (Fe,Cr)₂B and (Fe,Cr)₂₃(B,C)₆, which are hard intermetallic compounds that provide the wear resistance enhancement.
| Boron Content | Boride Morphology | Distribution Pattern | Primary Phase |
|---|---|---|---|
| < 3% | Discontinuous network | Sparse, isolated | (Fe,Cr)₂B network |
| 3-4% | Continuous network | Connected, interconnected | (Fe,Cr)₂B network |
| > 4% | Primary blocky particles | Increasing quantity, uniform distribution | Primary (Fe,Cr)₂B |
A notable observation is that primary (Fe,Cr)₂B crystals approximate quadrilateral prismatic shapes and tend to grow perpendicular to the surfacing layer surface. This orientation is significant because it means the hard boride phase provides maximum resistance to surface-abrasive wear, where the sliding direction is parallel to the surface. The perpendicular growth orientation ensures that the hard phase acts as a barrier to material removal.
At boron contents above 4%, the morphology transitions to include blocky, rod-shaped, fishbone-like, honeycomb, and chrysanthemum-like distributions. These complex morphologies suggest competing solidification mechanisms and may indicate the onset of microstructural instability.
Hardness and Wear Resistance
The most important finding is that increasing boron content up to 4% significantly improves both hardness and abrasive wear resistance. Beyond 4%, the improvement plateaus or potentially reverses due to the formation of large primary boride particles that can act as crack initiation sites.
| Boron Content | Relative Hardness | Relative Wear Resistance | Deposit Integrity |
|---|---|---|---|
| < 2% | Low | Low | Good |
| 2-3% | Moderate | Moderate | Good |
| 3-4% | High | High | Good |
| > 4% | Very High | Plateau or decrease | Potential cracking risk |
The optimal boron content range of 3-4% represents a balance between hardness contribution from boride formation and maintaining adequate toughness to prevent cracking during and after surfacing.
Engineering Practice Implications
For engineers specifying wear-resistant surfacing on equipment such as conveyor rollers, crusher components, and mining tools, this study provides clear guidance on boron content selection. The flux-cored wire gas shielded surfacing process used here is well-suited for field application because it requires relatively simple equipment and can be performed in various positions.
The quadrilateral prismatic morphology of primary (Fe,Cr)₂B crystals growing perpendicular to the surface is an important metallurgical insight. This orientation suggests that the boride phase is most effective when the wear direction is tangential to the surface, which is the typical condition in most industrial applications. Engineers should be aware that if the service condition involves impact loading or transverse abrasion, the effectiveness of the boride reinforcement may be reduced.
Key Reflections
The transition from network-type to blocky boride morphology at approximately 3% boron represents a fundamental shift in the solidification behavior of the alloy. Below this threshold, boride forms as a secondary phase in a matrix-controlled solidification regime. Above this threshold, boride becomes a primary phase, nucleating and growing independently. This transition has implications for the fatigue behavior of the surfacing layer, as primary blocky borides can act as stress concentrators.
The practical recommendation from this study is to target 3-4% boron content for applications requiring maximum abrasive wear resistance while maintaining deposit soundness. For applications where some impact resistance is required alongside wear protection, a boron content of 2-3% may be more appropriate. The study does not address thermal fatigue resistance, which would be relevant for applications involving cyclic thermal loading such as casting equipment or hot-work dies. This represents an area for further investigation.