Zhuojin Pipe Fitting Co., LtdEffect of Cr3C2 on Microstructure and Wear Resistance of Plasma Surfaced Cobalt-Based Alloy
Literature Overview
This paper by Gao Huhe, Ma Shujin, Ding Tingting, and Hou Qingyu, published in Heat Treatment in 2015 (Vol. 30, No. 5, pp. 23-28), investigates the influence of Cr3C2 particle addition on the microstructure and wear resistance of plasma-transferred arc (PTA) surfaced cobalt-based alloy coatings. The research was supported by the Anhui Provincial Natural Science Foundation (Grant No. 1508085ME95). The study is particularly relevant to the surface engineering of components subjected to severe abrasive wear in high-temperature environments.
Baseline Microstructure of PTA Surfaced Cobalt-Based Alloy
Without Cr3C2 addition, the PTA surfaced cobalt-based alloy layer exhibits a hypoeutectic microstructure consisting primarily of primary (Co) solid solution and (α(Co) + M7C3) eutectic. The M7C3 carbides form a network along the interdendritic regions, which is a characteristic feature of cobalt-based alloys with carbon content above the eutectic composition.
The hypoeutectic structure, while providing reasonable wear resistance, has limitations in terms of hardness and wear life under severe abrasive conditions. The network-type M7C3, while hard, can act as stress concentration sites and crack initiation points under impact or cyclic loading.
Effect of Cr3C2 Addition on Microstructure
The addition of 10-20% Cr3C2 fundamentally transforms the microstructure from hypoeutectic to hypereutectic. This transformation is metallurgically significant because it changes the primary phase from (Co) solid solution to Cr3C2 particles, which have a much higher hardness and stability.
| Cr3C2 Addition | Microstructure Type | Primary Phase | M7C3 Morphology |
|---|---|---|---|
| 0% | Hypoeutectic | (Co) solid solution | Network |
| 10-20% | Hypereutectic | Cr3C2 particles | Unmelted blocky + spherical |
The Cr3C2 particles appear in two distinct morphologies: unmelted blocky particles that survived the melting process, and spherical particles that precipitated during solidification. The blocky particles provide excellent resistance to abrasive wear due to their high hardness (approximately 15-18 GPa), while the spherical particles contribute to the overall hardness and refine the microstructure.
Wear Mechanism Analysis
The wear mechanism of the PTA surfaced cobalt-based alloy layers was identified through abrasive wear testing. The baseline coating (without Cr3C2) exhibited two primary wear mechanisms:
- Brittle spalling: The network-type M7C3 carbides act as crack initiation sites, and cracks propagate through the coating, causing large flakes to detach from the surface.
- Ploughing: The relatively soft (Co) solid solution matrix is deformed and displaced by the abrasive particles, leading to material removal through cutting and gouging.
With 10-20% Cr3C2 addition, the wear mechanism shifts predominantly to mild ploughing. This is a significant improvement because mild ploughing involves minimal material removal and results in much lower wear rates. The hard Cr3C2 particles resist abrasion by ploughing, and the refined microstructure reduces the severity of the ploughing action.
Wear Resistance Enhancement Mechanism
The improvement in wear resistance with Cr3C2 addition can be attributed to several synergistic mechanisms:
- Hardness enhancement: The Cr3C2 particles have a hardness significantly higher than the (Co) matrix, providing resistance to abrasive penetration.
- Microstructure refinement: The presence of Cr3C2 particles breaks up the interdendritic network of M7C3, resulting in a finer and more uniform microstructure.
- Crack suppression: The blocky Cr3C2 particles interrupt crack propagation paths, reducing the likelihood of brittle spalling.
- Load sharing: The hard particles bear a significant portion of the applied load, reducing the stress on the softer matrix.
Process Parameters and Dilution Control
PTA surfacing of cobalt-based alloys requires careful control of process parameters to minimize dilution with the base metal and ensure proper melting of the Cr3C2 particles. Key parameters include:
- Arc current: Must be sufficient to melt the Cr3C2 particles but not so high as to cause excessive dilution.
- Travel speed: Affects the cooling rate and consequently the microstructure.
- Powder feed rate: Must be balanced with the arc current to maintain a stable melt pool.
- Standoff distance: Influences the powder delivery efficiency and heat input distribution.
The dilution ratio between the surfacing layer and the base metal is a critical factor that affects the final composition and properties of the coating. Excessive dilution can alter the phase composition and reduce the effectiveness of the Cr3C2 addition.
Engineering Applications
Cobalt-based surfacing alloys are widely used in applications requiring combined high-temperature strength, wear resistance, and corrosion resistance. The addition of Cr3C2 makes these coatings particularly suitable for:
- Turbine blades and hot sections in gas turbines
- Injection molds in polymer processing
- Mining and quarrying equipment
- Cement kiln components
- Slag handling equipment in steel mills
The improved wear resistance with mild ploughing mechanism is especially valuable in applications where the component is subjected to continuous sliding contact with abrasive particles at elevated temperatures.
Reflections and Technical Insights
The transformation from hypoeutectic to hypereutectic microstructure with Cr3C2 addition is a well-known metallurgical phenomenon, but its specific manifestation in PTA surfaced cobalt-based alloys deserves careful consideration. The PTA process, with its rapid cooling rates, tends to preserve the as-deposited microstructure more faithfully than conventional welding processes. This means that the Cr3C2 particles added to the powder feed are more likely to survive the melting process in their original morphology, contributing to the final coating properties.
The observation that the wear mechanism shifts from brittle spalling to mild ploughing is particularly significant from a reliability standpoint. Brittle spalling is a catastrophic failure mode that can lead to sudden and unpredictable coating failure, while mild ploughing is a gradual wear mechanism that can be monitored and predicted. This makes Cr3C2-enhanced coatings more suitable for safety-critical applications where predictable wear behavior is essential.
The optimal Cr3C2 addition range of 10-20% represents a balance between wear resistance enhancement and potential brittleness concerns. Beyond 20%, the increasing volume fraction of hard Cr3C2 particles may compromise the toughness and crack resistance of the coating. The selection of the specific Cr3C2 content within this range should be guided by the expected service conditions, including load magnitude, sliding speed, and temperature.
Summary
This research demonstrates that the addition of 10-20% Cr3C2 to PTA surfaced cobalt-based alloy coatings fundamentally transforms the microstructure from hypoeutectic to hypereutectic and significantly improves wear resistance by shifting the wear mechanism from brittle spalling and severe ploughing to mild ploughing. The findings provide valuable guidance for the design of high-performance surfacing coatings for severe abrasive wear applications, particularly in high-temperature environments where cobalt-based alloys are traditionally used.