CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Friction Wear Properties of Nickel-Based Alloy Plasma Cladding Layer
Literature Overview
This study provides a comprehensive examination of the microstructure, hardness, and friction wear behavior of nickel-based alloy overlay layers produced by plasma transferred arc (PTA) cladding. The nickel-based alloys examined include Inconel 625, Stellite 6, and Haynes 230, which are widely used in high-temperature, corrosive, and wear-critical applications. The study systematically correlates the microstructure with the tribological performance under various sliding conditions.
Core Technical Content
Nickel-based alloy cladding layers are valued for their excellent combination of high-temperature strength, corrosion resistance, and wear resistance. The PTA cladding process is particularly suitable for these alloys because it provides a dilution-free or low-dilution overlay with a fully dense, crack-free microstructure.
Microstructural Characteristics
The microstructures of the three nickel-based alloy cladding layers are characterized as follows:
Inconel 625 (Ni-22Cr-9Mo-3Nb-1Ti):
- Dendritic microstructure with interdendritic precipitation of gamma-prime (Ni3Nb) and gamma-double-prime (Ni3Ti) phases.
- Grain size of 50 to 150 μm, with fine grain refinement near the fusion boundary.
- Hardness of 300 to 340 HV in the as-deposited condition.
- The gamma-prime and gamma-double-prime phases provide precipitation strengthening, which is particularly effective at elevated temperatures.
Stellite 6 (Co-Cr-W type, often classified as cobalt-based but sometimes grouped with nickel-based in broader classifications):
- Austenitic matrix with extensive MC and M23C6 carbide networks.
- The carbide volume fraction is approximately 30 to 40 percent.
- Hardness of 380 to 420 HV in the as-deposited condition.
- The carbide network provides excellent resistance to abrasive and erosive wear.
Haynes 230 (Ni-20Cr-2.5Mo-1.5W-0.5Ta):
- Dendritic microstructure with Laves phase (Ni2Mo) and gamma-prime precipitates.
- Grain size of 60 to 180 μm.
- Hardness of 320 to 360 HV in the as-deposited condition.
- The Laves phase provides excellent high-temperature strength and creep resistance.
Friction and Wear Behavior
The tribological tests were conducted using a pin-on-disk tribometer under the following conditions:
- Normal load: 20, 50, and 100 N
- Sliding distance: 1000 to 5000 m
- Counterface: GCr15 bearing steel disk
- Environment: Ambient air and argon atmosphere
| Alloy | Load (N) | Friction Coefficient | Specific Wear Rate (×10⁻⁶ mm³/N·m) | Dominant Wear Mechanism |
|---|---|---|---|---|
| Inconel 625 | 20 | 0.45 | 1.2 | Adhesive |
| Inconel 625 | 50 | 0.52 | 2.8 | Mixed (adhesive + abrasive) |
| Inconel 625 | 100 | 0.61 | 5.5 | Severe adhesive |
| Stellite 6 | 20 | 0.38 | 0.5 | Abrasive |
| Stellite 6 | 50 | 0.42 | 1.1 | Abrasive |
| Stellite 6 | 100 | 0.48 | 2.3 | Mixed (abrasive + adhesive) |
| Haynes 230 | 20 | 0.42 | 0.9 | Adhesive |
| Haynes 230 | 50 | 0.50 | 2.1 | Mixed |
| Haynes 230 | 100 | 0.58 | 4.2 | Severe adhesive |
The friction coefficients of all three alloys increase with increasing load, which is consistent with the transition from mild to severe wear regimes. The Stellite 6 alloy exhibits the lowest friction coefficient and the lowest specific wear rate across all load conditions, which is attributed to the hard carbide network that resists material removal.
Effect of Temperature on Wear Behavior
The study also examines the effect of elevated temperature on the wear behavior of the nickel-based alloy cladding layers:
- At 200°C, the wear rate of Inconel 625 increases by approximately 20 percent due to the reduction in yield strength and the increased tendency for adhesive wear.
- At 400°C, the wear rate of Inconel 625 increases by approximately 50 percent, but the Stellite 6 alloy shows only a 15 percent increase due to the thermal stability of the carbide network.
- At 600°C, Inconel 625 exhibits severe wear with a wear rate increase of over 100 percent, while Stellite 6 shows a 30 percent increase. Haynes 230 shows intermediate behavior with a 40 percent increase.
These results demonstrate that the selection of the nickel-based alloy for high-temperature wear applications should be based on the service temperature, with Stellite 6 being the preferred choice for temperatures above 400°C.
Defect Analysis and Countermeasures
PTA cladding of nickel-based alloys can produce several characteristic defects:
- Lack of fusion: Incomplete melting of the previous pass or the substrate, resulting in a weak bond between the overlay and the substrate or between successive passes. Countermeasures include increasing the plasma current, reducing the travel speed, and ensuring proper powder feed rate.
- Porosity: Gas porosity from hydrogen absorption in the molten pool, or lack-of-fusion porosity from incomplete melting. Countermeasures include using dry shielding gas, preheating the substrate, and optimizing the process parameters to ensure full penetration.
- Cracking: Hot cracking due to solidification cracking in the dilution zone, or cold cracking due to hydrogen embrittlement in the heat-affected zone. Countermeasures include controlling the dilution ratio, using appropriate filler materials, and applying post-weld heat treatment to relieve residual stresses.
Engineering Practice Applications
Nickel-based alloy PTA cladding is widely used in the following industrial applications:
- Gas turbine blade repair and coating, where the overlay must resist high-temperature oxidation and thermal fatigue.
- High-pressure valve seat cladding, where the overlay must resist erosion from high-velocity fluid flow containing solid particles.
- Chemical reactor lining, where the overlay must resist corrosion from aggressive chemical media.
- Nuclear reactor components, where the overlay must resist radiation damage and corrosion in high-temperature water environments.
A typical specification for a gas turbine blade PTA cladding might call for a minimum overlay thickness of 0.5 mm, a maximum dilution of 10 percent, and a minimum hardness of 300 HV. The Inconel 625 alloy is commonly specified for this application due to its excellent high-temperature strength and corrosion resistance.
Study Insights and Recommendations
The most important insight from this study is that the wear behavior of nickel-based alloy cladding layers is strongly dependent on the load, temperature, and environment. The selection of the appropriate alloy and process parameters must be based on a thorough understanding of the service conditions and the dominant wear mechanism.
For low-load, low-temperature applications, Inconel 625 provides an excellent combination of wear resistance, corrosion resistance, and high-temperature strength. For high-load, high-temperature applications, Stellite 6 is the preferred choice due to its superior abrasive wear resistance and thermal stability. Haynes 230 offers a good compromise between the two, with excellent high-temperature strength and moderate wear resistance.
The study also emphasizes the importance of the dilution ratio in determining the properties of the cladding layer. A high dilution ratio can significantly reduce the hardness and wear resistance of the overlay by introducing softer substrate material into the overlay microstructure. Therefore, process parameters must be carefully optimized to minimize dilution while ensuring complete bonding.
In conclusion, this literature provides a comprehensive framework for understanding the microstructure and wear behavior of nickel-based alloy PTA cladding layers. The systematic examination of the effects of alloy composition, load, temperature, and environment on the tribological performance offers engineers a practical basis for material selection and process optimization in high-performance cladding applications.