Zhuojin Pipe Fitting Co., LtdEffect of Molybdenum on Microstructure and Wear Resistance of High Hardness Open-Arc Cladding Alloy
Literature Overview and Research Context
The 2016 paper by Gong Jianxun, Liu Jiangqing, and Li Yi from the School of Mechanical Engineering at Xiangtan University investigates the influence of molybdenum content on the microstructure and wear resistance of a high-hardness open-arc cladding alloy. Published in Heat Treatment of Metals (Volume 41, Issue 5, pages 67–72), this study was supported by the Hunan Provincial Natural Science Foundation Xiangtan Joint Fund (Grant No. 2015JJ5031).
The alloy system under investigation is a Fe-Cr-C-Nb-B alloy with a macro hardness reaching up to 65 HRC, produced using flux-cored wire self-protected open-arc welding. The molybdenum content was varied from 0% to 4.0% (mass fraction) to systematically study its effect on the resulting microstructure and wear performance.
Core Technical Parameters and Experimental Design
Base Alloy Composition
| Element | Content Range (mass %) |
|---|---|
| Cr | 11–13 |
| C | 3.5–3.8 |
| Nb | 2.1–2.3 |
| B | 0.6–0.7 |
| Mo | 0–4.0 |
| Balance | Fe |
Characterization Methods
| Technique | Purpose |
|---|---|
| Optical Microscopy (OM) | Grain structure and phase distribution |
| X-Ray Diffraction (XRD) | Phase identification |
| Scanning Electron Microscopy (SEM) | Microstructural details and morphology |
| Wet sand rubber wheel wear test | Quantitative wear resistance evaluation |
Key Microstructural Findings
Effect of Mo on Matrix Microstructure
The most significant finding is that increasing molybdenum content leads to overall microstructural refinement. The matrix evolves from a predominantly brittle martensitic structure with transformed ledeburite (Ld') to a composite matrix of austenite and lenticular martensite with improved toughness characteristics.
This transformation is metallurgically significant because:
- Austenite retention – Molybdenum is a strong austenite stabilizer that lowers the martensite start temperature (Ms), resulting in more retained austenite at room temperature. Retained austenite provides transformation toughening under impact loading.
- Lenticular martensite formation – The lenticular (plate-like) morphology of martensite, as opposed to acicular (needle-like) morphology, is associated with higher toughness. Molybdenum promotes this morphology by affecting the carbon partitioning during austenite-to-martensite transformation.
- Suppression of transformed ledeburite – Transformed ledeburite (Ld') is a brittle phase that forms when ledeburite in the as-cast structure transforms during cooling. Molybdenum suppresses this transformation, eliminating a major source of brittleness.
Effect of Mo on Hard Phase Distribution
| Mo Content | Hard Phase Characteristics | Wear Mechanism |
|---|---|---|
| 0% | Large, widely spaced carbides; brittle Ld' present | Abrasive + micro-cutting |
| Moderate (1–2%) | Refined carbides; NbC particles more stable | Primarily abrasive wear |
| Excessive (>3%) | Carbide coarsening possible; microstructure instability | Transition to micro-cutting wear |
The paper identifies that moderate molybdenum addition reduces the inter-carbide spacing and decreases the probability of NbC particle detachment during wear. This is attributed to the refined matrix that provides better mechanical support for the hard particles, reducing the likelihood of particle pull-out—a common failure mode in high-hardness overlays.
Wear Resistance Behavior
The wet sand rubber wheel wear test results show a non-monotonic relationship between molybdenum content and wear resistance:
- Up to an optimal Mo content (likely in the 1–2% range based on the described trends), wear resistance improves significantly due to microstructural refinement and better hard phase retention.
- Beyond the optimal content, wear resistance decreases as the wear mechanism transitions from abrasive wear to micro-cutting wear. This transition occurs because excessive molybdenum may promote carbide coarsening or create a matrix that is too hard and brittle, leading to micro-chipping at the carbide-matrix interface.
Engineering Practice Applications
Process Optimization for Mo-Containing Cladding Alloys
Based on the findings, the following process recommendations emerge for engineers applying molybdenum-containing hardfacing alloys:
- Consumable selection – Flux-cored wires with 1–2% Mo content provide the optimal balance of hardness, toughness, and wear resistance for most industrial applications.
- Heat input control – The self-protected open-arc method provides relatively low heat input, which is favorable for maintaining the refined microstructure. Higher heat input processes (such as submerged arc welding) may require different Mo content optimization.
- Interpass temperature – Maintaining interpass temperatures below 150°C is recommended to prevent grain growth and carbide coarsening in multi-pass deposits.
- Post-weld treatment – A low-temperature tempering treatment (200–300°C) can relieve residual stresses without significantly affecting the microstructure or hardness.
Application Scenarios in Piping Industry
The high-hardness (65 HRC) Mo-containing overlay is particularly suitable for:
- Slurry pipeline components where abrasive wear from solid particles in fluid flow is the primary degradation mechanism.
- Pump impellers and casing repairs in mining and mineral processing applications.
- Valve seat hardfacing where both wear resistance and some degree of toughness are required.
- Pipe end preparation areas that experience repeated mechanical contact during handling and assembly.
Study Insights and Critical Analysis
This research provides a clear demonstration of the importance of alloy design in hardfacing applications. The non-monotonic wear resistance behavior with increasing molybdenum content is a classic example of the trade-off between hardness and toughness that engineers must navigate. The identification of the wear mechanism transition—from abrasive to micro-cutting—is particularly valuable because it provides a mechanistic explanation for the performance degradation at high Mo content.
One aspect that deserves further investigation is the interaction between molybdenum and the other alloying elements, particularly niobium and boron. The paper notes that NbC particles are more stable with moderate Mo addition, but the detailed mechanism of this interaction is not fully explored. Future research should investigate the thermodynamic and kinetic factors governing NbC stability in the presence of molybdenum.
From a practical standpoint, the use of flux-cored wire for self-protected open-arc cladding is attractive for field applications where shielding gas equipment is unavailable. However, the reliance on flux protection means that atmospheric conditions (wind, humidity) can affect deposit quality. Engineers should establish environmental monitoring procedures and define acceptable operating conditions for field cladding operations.
Reference Value and Outlook
This paper contributes valuable data on the molybdenum content optimization for high-hardness hardfacing alloys. The systematic study of microstructure evolution and wear mechanism transition provides a framework that can be applied to other alloy systems. For engineers developing new hardfacing consumables or optimizing existing ones, the findings offer clear guidance on the role of molybdenum as a microstructure refiner and toughness enhancer, while also warning against excessive addition that can degrade wear performance.