Zhuojin Pipe Fitting Co., LtdEffect of Molybdenum on Microstructure and Wear Resistance of High-Hardness Open-Arc Surfacing Alloys
Literature Overview
The research by Gong Jianxun, Liu Jiangqing, and Li Yi from Xiangtan University (2016, Metal Heat Treatment, Vol. 41, No. 5) systematically investigates the influence of molybdenum content on the microstructure and tribological performance of high-hardness iron-based surfacing alloys. Funded by the Hunan Provincial Natural Science Foundation Xiangtan Joint Fund (Project No. 2015JJ5031), this study provides valuable insights into the role of molybdenum in optimizing overlay performance.
Material System and Experimental Parameters
The researchers developed a series of flux-cored wire compositions with controlled Mo variation while maintaining a consistent base composition. The alloys were deposited using self-shielded flux-cored wire open-arc welding and characterized comprehensively.
| Parameter | Specification |
|---|---|
| Base composition | 11–13% Cr, 3.5–3.8% C, 2.1–2.3% Nb, 0.6–0.7% B |
| Mo variation range | 0–4.0 wt% |
| Achieved hardness | Up to 65 HRC |
| Surfacing method | Flux-cored wire self-shielded open-arc |
| Wear test | Wet sand abrasion |
| Characterization | OM, XRD, SEM |
Microstructural Evolution with Mo Addition
The study reveals a complex but systematic evolution of microstructure with increasing molybdenum content:
- Overall grain refinement: Mo addition promotes microstructural refinement across all phases, likely through solid solution effects on nucleation kinetics and possible pinning of grain boundaries.
- Matrix transformation: The base matrix evolves from predominantly brittle transformed ledeburite (Ld') to a composite structure of austenite plus lath martensite with excellent strength-toughness combination.
- Hard phase modification: M₂₃(C,B)₆ phase becomes more ductile with Mo addition, while NbC particles formed in-situ provide additional wear resistance.
- Ld' suppression: A critical finding is the significant inhibition of brittle transformed ledeburite formation with appropriate Mo addition.
| Mo Content (wt%) | Matrix Structure | Hard Phase Character | Ld' Content | Hardness |
|---|---|---|---|---|
| 0 | Predominantly Ld' | Coarse M₂₃(C,B)₆ | High | High but brittle |
| 1.0–2.0 | Austenite + lath martensite | Refined M₂₃(C,B)₆ + NbC | Low | 60–65 HRC |
| 3.0–4.0 | Refined austenite + martensite | Fine NbC, reduced spacing | Very low | High |
Wear Mechanism Analysis
The wet sand abrasion testing reveals an important transition in wear mechanism:
- Low Mo content: Wear dominated by micro-cutting of brittle Ld' phase, resulting in high material removal rates
- Optimal Mo content (approximately 1.5–2.5%): Reduced carbide spacing and decreased NbC particle detachment probability significantly improve wear resistance through a balanced mechanism of matrix plastic deformation and hard phase resistance
- Excessive Mo content (>3%): The wear mechanism transitions to micro-cutting by abrasive particles, as the refined microstructure provides less resistance to individual abrasive particle penetration
This non-monotonic relationship between Mo content and wear resistance is a critical finding for practical alloy design.
Engineering Practice Implications
For pipeline and equipment applications requiring high-hardness wear-resistant overlays:
- Optimal Mo range: 1.5–2.5% provides the best balance of hardness, toughness, and wear resistance
- Quality control consideration: The narrow optimal window requires precise control of consumable chemistry
- Application suitability: The austenite + lath martensite matrix provides good impact resistance, making these alloys suitable for components subject to both abrasion and impact loading
Specific applications include:
- Slurry pump liners and impellers in mining and mineral processing
- Abrasion-resistant overlays on pipeline elbows in sand-laden service
- Hardfacing of drag line bucket teeth and conveyor wear plates
- Protection of heat exchanger tubes in erosive service
Study Insights and Critical Reflection
The demonstration that molybdenum can simultaneously refine the microstructure, suppress brittle Ld' formation, and improve the toughness of M₂₃(C,B)₆ phase represents a sophisticated understanding of multi-element interactions in high-carbon iron-based alloys. The identification of a clear optimal Mo range with distinct wear mechanism transitions on either side provides actionable guidance for consumable development.
From a quality assurance perspective, this research highlights the importance of wear testing in qualifying surfacing alloys. Hardness alone is insufficient to predict wear performance—the microstructural composition and phase relationships are equally critical. The transition from abrasive resistance to micro-cutting at high Mo content serves as a cautionary example of why comprehensive tribological characterization is essential.
The work also demonstrates the value of systematic single-variable studies in understanding multi-element alloy systems. By varying only Mo content while maintaining all other parameters constant, the researchers isolated the specific effects of this element, providing clear guidance for alloy optimization.