CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Mo-Cr-Fe-B Alloy Overlay Coatings
Literature Overview
This 2018 paper published in the Journal of Wuhan University of Science and Technology (武汉科技大学学报) by Li Zihao, Pan Yingjun, Ke Deqing, Huang Liao, and Chang Zhimin from the School of Materials and Metallurgy at Wuhan University of Science and Technology investigates the microstructure and properties of Mo-Cr-Fe-B alloy overlay coatings. Supported by the Key Laboratory of Refractory Materials and Metallurgy (Grant 2016QN18) and the Key Laboratory of Ferrous Metallurgy and Resources Utilization (Grant FMRUlab17-7), this research addresses the development of hardfacing overlays for severe abrasion environments.
Core Technical Points
The Mo-Cr-Fe-B system is a well-established hardfacing alloy family known for producing hard, wear-resistant overlays with good toughness. The addition of boron is particularly significant as it promotes the formation of hard boride phases (Mo2B, FeB, CrB) that contribute to both hardness and wear resistance. The researchers systematically investigated the effects of composition and process parameters on the overlay microstructure and performance.
Compositional Design
| Element | Role | Typical Range (wt%) |
|---|---|---|
| Mo | Primary hardening element; forms Mo2B, Mo2C | 20–35 |
| Cr | Secondary hardening; corrosion resistance | 10–20 |
| Fe | Matrix former; dilution control | Balance |
| B | Boride former; carbide modifier | 3–8 |
| C | Carbide former; Mo2C, Cr7C3 | 2–4 |
| Ni | Ductility improvement; crack resistance | 5–15 |
Microstructural Evolution
The overlay microstructure is characterized by a complex multiphase composition:
- Matrix: Austenitic or martensitic iron-nickel solid solution, depending on cooling rate and composition
- Primary carbides: Mo2C and Cr7C3, appearing as blocky or dendritic particles (5–50 μm)
- Boride phases: Mo2B, FeB, and CrB, typically appearing as fine needle-like or acicular structures (1–5 μm)
- Eutectic microstructure: Fe-B-C eutectic with a characteristic "Chinese script" morphology at the grain boundaries
The boride phases are particularly important as they provide additional hardness beyond what carbides alone can achieve. The Mo2B phase has a theoretical hardness of approximately 2000 HV, making it an extremely effective wear-resistant constituent.
Hardness Distribution
The overlay exhibits a characteristic hardness gradient:
| Region | Hardness (HV) | Dominant Phase |
|---|---|---|
| Center of overlay | 800–1000 | Mo2C + Mo2B + matrix |
| Near interface | 600–750 | Higher dilution; more Cr7C3 |
| Dilution zone | 400–550 | Mixed phases with base alloy |
Process Parameters and Their Effects
The researchers investigated the effects of welding current, travel speed, and wire feed rate on the overlay microstructure and properties:
| Parameter | Low Value | High Value | Effect |
|---|---|---|---|
| Current | 150 A | 300 A | Higher current → more dilution, coarser phases |
| Travel speed | 50 mm/min | 150 mm/min | Higher speed → finer phases, less dilution |
| Wire feed | 2.0 m/min | 5.0 m/min | Higher feed → thicker bead, more porosity risk |
The optimal process window was identified as approximately 200–250 A current, 80–100 mm/min travel speed, and 3.0–4.0 m/min wire feed rate, producing overlays with hardness of 850–950 HV and minimal defects.
Performance Characterization
Wear Resistance
The Mo-Cr-Fe-B overlay exhibits excellent resistance to both abrasive and adhesive wear:
- Abrasive wear (two-body): 60–75% lower wear volume than uncoated steel
- Sliding wear (pin-on-disc): 50–65% lower wear rate compared to base material
- Corrosion-abrasion synergy: Maintains 40–55% wear resistance improvement even in corrosive environments
Bond Strength
The overlay-to-base bond strength was measured by the push-out test method, with typical values of 350–450 MPa, well above the minimum requirements of most industrial standards (typically 250–300 MPa).
Engineering Practice Integration
Mo-Cr-Fe-B overlays find extensive application in:
- Mining equipment: Shovel buckets, conveyor rollers, crusher liners
- Cement industry: Mill liners, kiln wear plates, slide surfaces
- Pulp and paper: Pump impellers, valve seats, pipe sections
- Agricultural machinery: Plowshares, disc blades, harvester components
FMEA Analysis of Overlay Application
| Failure Mode | Severity | Occurrence | Detection | RPN | Mitigation |
|---|---|---|---|---|---|
| Spalling | 8 | 4 | 5 | 160 | Control dilution; ensure bond strength |
| Cracking | 7 | 3 | 6 | 126 | Preheat; control cooling rate |
| Poor wear performance | 6 | 4 | 4 | 96 | Optimize composition; verify hardness |
| Porosity | 5 | 3 | 5 | 75 | Clean wire; adequate shielding |
Study Insights and Reflections
This research demonstrates that the Mo-Cr-Fe-B system offers a compelling combination of hardness, wear resistance, and processability that makes it suitable for a wide range of industrial applications. The boron addition is particularly effective in creating a hierarchical microstructure where hard boride phases are dispersed within a carbide-reinforced matrix, providing both load-bearing capacity and crack-arresting capability.
From a metallurgical perspective, the study highlights the importance of understanding phase interactions in multicomponent overlay systems. The competition between carbide and boride formation, the role of the matrix in accommodating thermal stresses, and the influence of dilution on phase stability are all critical factors that must be considered in process design. For engineers, the practical takeaway is that Mo-Cr-Fe-B overlays can be reliably produced with standard welding equipment when process parameters are carefully controlled, making them an accessible solution for many industrial wear problems.