CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Molybdenum on Microstructure and Wear Resistance of High Hardness Open Arc Overlay Alloy Study
Literature Overview
This study examines the influence of molybdenum (Mo) addition on the microstructural evolution and abrasive wear resistance of high-hardness open arc overlay alloys, specifically those deposited by gas shielded arc welding (GMAW) or submerged arc welding (SAW) processes. The investigation focuses on the Fe-Cr-Mo-C system, where Mo content is varied from 0 to 15 wt% to establish composition-property relationships. The research provides critical guidance for alloy design in applications requiring extreme hardness and wear resistance, such as ball mill liners, grinding rolls, and high-pressure pump components.
Microstructural Analysis Across Mo Content Range
Phase Composition and Carbide Morphology
| Mo Content (wt%) | Primary Carbides | Matrix Composition | Hardness (HV0.3) | Carbide Volume Fraction (%) |
|---|---|---|---|---|
| 0 | M7C3, M23C6 | Fe-Cr solid solution | 580 | 35 |
| 3 | M7C3, Mo2C | Fe-Cr-Mo solid solution | 640 | 38 |
| 6 | M7C3, Mo2C, M6C | Fe-Cr-Mo solid solution | 700 | 42 |
| 9 | Mo2C, M6C, M7C3 | Mo-rich solid solution | 760 | 48 |
| 12 | Mo2C, M6C | Mo-rich solid solution | 810 | 52 |
| 15 | Mo2C, M6C, MC | Mo-Cr solid solution | 840 | 55 |
The microstructural evolution follows a clear trend with increasing Mo content. At low Mo levels (0-3 wt%), the dominant carbide is M7C3 with a lamellar morphology along dendrite boundaries. As Mo increases to 6-9 wt%, Mo2C hexagonal carbides begin to appear as discrete particles within the dendrite cores, while M7C3 remains at the boundaries. Above 9 wt% Mo, the carbide network transitions to a predominantly Mo2C and M6C mixture, with the continuous film morphology gradually giving way to a more dispersed particulate distribution.
The matrix composition shifts significantly with Mo addition. At 0 wt% Mo, the matrix is a Cr-enriched Fe solid solution. At 15 wt% Mo, the matrix becomes a Mo-Cr solid solution with reduced Fe content due to preferential carbide formation of Fe and Mo. This compositional shift has profound implications for both the solid solution strengthening and the thermal stability of the coating.
Wear Resistance Mechanisms and Quantitative Analysis
The abrasive wear resistance improves monotonically with Mo content, but the rate of improvement diminishes at higher Mo levels. This is explained by three competing mechanisms:
- Carbide hardening effect: Mo2C (2900 HV) and M6C (2600 HV) are harder than M7C3 (1800 HV), providing superior resistance to micro-cutting.
- Carbide dispersion refinement: Higher Mo content promotes finer carbide particles with more uniform spacing, reducing the distance between load-bearing particles.
- Matrix strengthening: Mo in solid solution provides significant lattice distortion strengthening, contributing 30-50 HV per wt% Mo at the matrix level.
Abrasive Wear Test Results
| Mo Content (wt%) | Dry Sliding Wear Rate (mg/N·m) | Three-Body Abrasion Rate (mg/N·m) | Impact Abrasion Rate (mg/N·m) |
|---|---|---|---|
| 0 | 1.25 | 1.85 | 2.10 |
| 3 | 0.95 | 1.45 | 1.75 |
| 6 | 0.72 | 1.10 | 1.40 |
| 9 | 0.58 | 0.88 | 1.15 |
| 12 | 0.48 | 0.75 | 0.98 |
| 15 | 0.42 | 0.68 | 0.88 |
The impact abrasion results are particularly informative. While all Mo additions improve impact abrasion resistance, the improvement from 9 to 15 wt% Mo is only 24%, compared to the 45% improvement from 0 to 9 wt% Mo. This diminishing return is attributed to the increasing brittleness of the overlay at high Mo content, which promotes spallation under impact loading despite the higher hardness.
Process Considerations for High-Mo Open Arc Overlay
Welding Parameter Optimization
The open arc welding process presents unique challenges when depositing high-Mo alloys due to the tendency for Mo-rich carbides to form large, coarse particles during the relatively slow cooling rates associated with SAW and GMAW processes.
| Process Parameter | Low Mo (0-6 wt%) | High Mo (9-15 wt%) |
|---|---|---|
| Arc current | 300-400 A | 250-350 A |
| Travel speed | 200-300 mm/min | 250-400 mm/min |
| Wire/powder feed rate | 8-12 kg/h | 6-10 kg/h |
| Preheat temperature | 100-150°C | 150-200°C |
| Interpass temperature | 200-250°C | 200-250°C |
| Shielding gas | Ar + 5% CO2 | Pure Ar |
Reduced heat input is essential for high-Mo alloys to prevent excessive carbide coarsening. The lower current and higher travel speed combination produces thinner, faster-cooling weld passes that inhibit the growth of Mo2C and M6C particles beyond 20 micrometers. The use of pure argon shielding (instead of CO2 mixtures) is critical to prevent carbon pickup, which would form excessive M7C3 at the expense of Mo-rich carbides.
Engineering Practice and Application Guidance
For ball mill liner applications, the optimal Mo content is 9-12 wt%, which provides a hardness of 760-810 HV0.3 with adequate impact resistance for the high-energy impact environment. The coating thickness should be 25-40 mm, deposited in 4-6 passes with interpass temperature control to prevent excessive grain growth.
For high-pressure pump wear rings operating in cavitation and erosion environments, 6-9 wt% Mo is recommended, as the slightly lower hardness (700-760 HV0.3) provides better resistance to cavitation pitting through improved fracture toughness. The erosion resistance is further enhanced by a smooth, dense surface finish achieved through post-weld machining.
Key Reflections and Study Insights
The most significant finding of this study is the identification of an optimal Mo content window of 9-12 wt% for high-hardness open arc overlay alloys. Below 9 wt%, the hardness is insufficient for severe abrasion conditions, while above 12 wt%, the brittleness-induced spallation under impact loading negates the hardness advantage. This finding has direct implications for alloy selection in mining and mineral processing applications where both abrasion and impact are present.
A secondary but important insight is the process-alloy interaction. The same Mo content can produce markedly different microstructures depending on the welding process and parameters used. A 9 wt% Mo alloy deposited by SAW with high heat input may exhibit coarse, brittle carbides, while the same alloy deposited by PTA with controlled heat input produces fine, uniformly dispersed carbides with superior wear resistance. This reinforces the principle that alloy composition and process parameters must be optimized simultaneously, not independently.
The study also highlights the importance of considering the full spectrum of wear mechanisms when evaluating alloy performance. A coating that excels in dry sliding abrasion may perform poorly in impact abrasion or three-body abrasion due to brittleness. The engineering selection must be based on the dominant wear mechanism in the specific service environment, not on a single hardness value.
This research provides a solid foundation for the rational design of high-Mo overlay alloys for severe wear applications. The key engineering takeaway is that the pursuit of maximum hardness must be balanced against toughness and spallation resistance, and that the optimal Mo content is application-specific rather than universal. Future development efforts should focus on multi-component alloy systems that combine Mo with other strengthening elements (W, V, Ti) to achieve synergistic property enhancement without the brittleness penalty associated with high single-element Mo content.