Zhuojin Pipe Fitting Co., LtdEffect of Molybdenum Content on Stellite 21 Cladding Layer Microstructure and Performance for Triple Eccentric Butterfly Valve
Literature Overview and Research Motivation
This paper, published in Surface Technology (2025, Vol. 54, No. 11), addresses a specific industrial challenge: improving the wear and corrosion resistance of the hard sealing surface of triple eccentric butterfly valves using Stellite 21 cobalt-based alloy cladding with controlled molybdenum addition. The authors from Zhejiang University of Technology's Zhejiang College and the Laser Advanced Manufacturing Research Institute employed plasma arc cladding technology on WCB cast steel substrates, systematically varying the Mo content from 5.5% to 15% by mass fraction. This research is directly relevant to engineers in the valve manufacturing industry, where the sealing surface of butterfly valves is subjected to severe wear and corrosion in aggressive process environments.
Core Technical Content
Microstructural Evolution with Mo Content
The addition of Mo significantly alters the microstructure of the Stellite 21 cladding layer:
| Mo Content (wt%) | Microstructural Features |
|---|---|
| 5.5% | Predominantly columnar crystals, fewer equiaxed crystals |
| 7.5% | Transition from columnar to mixed columnar-equiaxed |
| 10% | Significant increase in short dendrites and equiaxed crystals |
| 12.5% | Predominantly equiaxed crystals with fine dendrites |
| 15% | Fine equiaxed crystals, inhibited columnar growth |
The key mechanism is that Mo addition inhibits the sustained growth of columnar crystals, promoting the formation of more short dendrites and equiaxed crystals. This is attributed to the constitutional supercooling effect of Mo, which increases the liquidus-solvus temperature difference and promotes heterogeneous nucleation at the solid-liquid interface.
Phase Composition Analysis
XRD and EDS analysis reveal significant changes in phase composition with Mo addition:
| Phase | Effect of Increasing Mo Content |
|---|---|
| Mo2C | Concentrated at grain boundaries |
| Co7Mo6 | Content increases |
| Co3Mo | Content increases |
| Cr23C6 | Relative content decreases |
The formation of Mo2C at grain boundaries is particularly important because Mo2C is a hard, wear-resistant carbide that can significantly improve wear resistance. However, its concentration at grain boundaries may also affect intergranular corrosion resistance. The increase in Co7Mo6 and Co3Mo phases, which are intermetallic compounds with high hardness, contributes to the overall hardening of the cladding layer.
Hardness, Wear Resistance, and Corrosion Resistance
The study presents comprehensive mechanical and corrosion performance data:
| Mo Content (wt%) | Hardness (HV) | Friction Coefficient | Wear Mass Loss | Corrosion Potential (V) | Corrosion Current Density (A/cm²) |
|---|---|---|---|---|---|
| 5.5% | ~333 | Baseline | Baseline | Baseline | Baseline |
| 10% | ~360 | Reduced by 17.2% | Reduced by 79.1% | -0.999 | 1.552×10⁻⁵ |
| 15% | 398 (highest) | Higher than 10% | Higher than 10% | Less negative | Higher |
The optimal Mo content of 10% (achieved by adding 4.5% Mo powder to Stellite 21) provides the best balance of wear resistance and corrosion resistance:
- Hardness increased by approximately 19.5% compared to 5.5% Mo content.
- Friction coefficient reduced by approximately 17.2%.
- Wear mass loss reduced by approximately 79.1%.
- Highest corrosion potential of -0.999 V.
- Lowest corrosion current density of 1.552×10⁻⁵ A/cm².
- Lowest immersion corrosion weight loss coefficient K of 1.54%.
The Optimal Mo Content of 10%
The identification of 10% Mo as the optimal content is significant and can be explained by the competing mechanisms at play:
- At lower Mo contents (<10%), the formation of hard Mo2C and Co-Mo intermetallic phases is insufficient to maximize wear resistance.
- At higher Mo contents (>10%), while hardness continues to increase, the wear resistance and corrosion resistance decrease. This is likely because:
- Excessive Mo2C at grain boundaries may promote intergranular corrosion.
- The increased brittleness of the microstructure at very high Mo content may lead to crack initiation under wear conditions.
- The passive film may become less stable with excessive Mo content.
Engineering Practice Implications
Application to Triple Eccentric Butterfly Valves
Triple eccentric butterfly valves are widely used in water treatment, chemical processing, and oil and gas industries where the sealing surface is subjected to:
- Abrasive wear from solid particles in the process fluid.
- Corrosive attack from acidic or alkaline process fluids.
- Thermal cycling during valve operation.
- Cyclic mechanical loading during opening and closing.
The optimal Mo content of 10% in Stellite 21 cladding provides a significant improvement in service life, extending the operational interval between maintenance and replacement.
Process Considerations
For plasma arc cladding of Mo-modified Stellite 21 on WCB cast steel:
| Parameter | Recommendation |
|---|---|
| Powder preparation | Planetary angle grinder mixing of Stellite 21 powder and Mo powder |
| Mo addition | 4.5 wt% Mo powder to achieve 10 wt% Mo in cladding layer |
| Substrate | WCB cast steel |
| Cladding process | Plasma arc cladding |
| Target Mo content | 10 wt% |
Quality Control Considerations
Engineers should implement the following quality control measures:
- Chemical analysis of the cladding layer to verify Mo content within acceptable range (9-11 wt%).
- Metallographic examination to confirm the presence of fine equiaxed crystals and Mo2C at grain boundaries.
- Hardness testing to ensure minimum hardness of 360 HV.
- Corrosion testing (electrochemical and immersion) to verify corrosion resistance.
- Wear testing to confirm wear resistance improvement.
Key Questions and Reflections
- The study does not address the effect of Mo content on the thermal conductivity and thermal expansion coefficient of the cladding layer, which are important for thermal cycling applications.
- The long-term wear performance under actual valve operating conditions (including the effect of lubrication, fluid velocity, and particle size distribution) requires field testing.
- The cost-effectiveness of Mo addition should be evaluated, considering the cost of Mo powder relative to the extended service life of the valve.
Study Insights and Summary
This study provides a systematic investigation of the effect of Mo content on the microstructure and performance of Stellite 21 plasma arc cladding layers, with direct application to triple eccentric butterfly valve sealing surfaces. The identification of 10% Mo as the optimal content is a valuable engineering finding, supported by comprehensive microstructural, mechanical, and corrosion testing data. The improvement in wear resistance (79.1% reduction in wear mass loss) and corrosion resistance (lowest corrosion current density) at 10% Mo content is substantial and directly translates to extended service life of valve components. For engineers in the valve manufacturing and maintenance industry, this study provides clear guidance on Mo addition levels and process parameters for achieving optimal performance. The key insight is that there exists an optimal Mo content that balances hardness, wear resistance, and corrosion resistance, and that exceeding this optimum can be detrimental to overall performance.