Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Different Nickel-Based Alloy Plasma Surfacing Layers
Literature Overview
This study by Hu Jianjun and colleagues from Dalian University of Technology compares the microstructure, hardness, and wear performance of three different nickel-based alloy powders (Ni40A, Ni50A, Ni60A) deposited by plasma arc surfacing on 1Cr18Ni9Ti stainless steel substrates. The research is particularly relevant for engineers selecting surfacing materials for applications requiring a combination of corrosion resistance and wear resistance, such as chemical processing equipment, nuclear components, and marine hardware. The work was supported by the Jiangxi Provincial Natural Science Foundation and the National 973 Program.
Alloy Selection and Characterization
The three nickel-based alloys represent a progression in alloy complexity and hardness:
| Alloy | Composition Characteristics | Typical Application |
|---|---|---|
| Ni40A | Nickel-base with Cr, Si, minor alloying | General wear resistance |
| Ni50A | Nickel-base with Cr, B, Si, higher alloy content | Enhanced wear resistance |
| Ni60A | Nickel-base with Cr, B, Si, high alloy content | High wear resistance, high hardness |
All three alloys are castable nickel-based alloys with increasing carbon and boron content, which directly influences the type and quantity of carbide and boride phases formed during solidification.
Microstructural Analysis
The base matrix of all three surfacing layers is γ-Ni, which provides the necessary ductility and toughness to prevent catastrophic failure. However, the precipitate phases differ significantly:
| Alloy | Primary Precipitate Phases | Secondary Phases | Hardness (HV) |
|---|---|---|---|
| Ni40A | Cr₇C₃, M₂₃C₆, Ni₃Si | - | Moderate |
| Ni50A | CrB, M₂₃(C,B)₆, Cr₇C₃, Cr₅B₃, Ni₃Si | - | High |
| Ni60A | CrB, M₂₃(C,B)₆, Cr₇C₃, Cr₅B₃, Ni₃Si | - | 780 |
The progression from Ni40A to Ni60A shows an increasing complexity of precipitate phases, particularly the appearance of boride phases (CrB and Cr₅B₃) in Ni50A and Ni60A. These borides are extremely hard and contribute significantly to the wear resistance improvement.
Wear Performance Comparison
The wear resistance follows the order: Ni60A > Ni50A > Ni40A, which correlates directly with hardness. However, the coefficient of friction shows a different trend: Ni60A has the highest friction coefficient, while Ni40A and Ni50A show similar values. This is an important engineering consideration because high friction can lead to increased heat generation at the contact interface, potentially causing thermal degradation of the coating.
| Property | Ni40A | Ni50A | Ni60A |
|---|---|---|---|
| Microhardness | Moderate | High | 780 HV (highest) |
| Wear resistance | Lowest | Moderate | Highest |
| Friction coefficient | Low | Low | Highest |
| Frictional heat generation | Lowest | Moderate | Highest |
| Thermal stability | Good | Good | Moderate concern |
Process Parameter Effects
The study also investigated the effect of plasma arc current on the surfacing layer properties. Higher current increases the heat input, which can lead to:
- Greater dilution from the stainless steel substrate
- Coarser microstructure due to slower cooling rates
- Potential for cracking if the heat input exceeds the cracking threshold
- Improved wetting and adhesion if within acceptable limits
The optimal current range balances these competing effects. Too low a current results in incomplete melting and poor adhesion, while too high a current causes excessive dilution and microstructural coarsening.
Engineering Practice Implications
For engineers selecting between these three alloys, the decision should be based on the specific service conditions:
- Ni40A is suitable for applications with moderate wear and low friction requirements, where the lower friction coefficient is advantageous for reducing heat generation
- Ni50A offers a good balance of hardness and wear resistance with acceptable friction properties
- Ni60A provides maximum wear resistance but should be used with caution in high-friction applications where thermal management is critical
The 1Cr18Ni9Ti stainless steel substrate provides good corrosion resistance, and the nickel-based surfacing layers maintain or enhance this property. The dilution effect from the stainless steel substrate should be considered when predicting the final alloy composition of the surfacing layer.
Key Reflections
The comparison of three commercially available nickel-based alloys provides valuable practical guidance for material selection. The finding that Ni60A achieves 780 HV hardness is significant because it approaches the hardness of tool steels, making it suitable for severe wear applications. However, the higher friction coefficient of Ni60A is a trade-off that engineers must carefully evaluate.
The presence of boride phases in Ni50A and Ni60A is a key differentiator from Ni40A. Borides are extremely hard but can be brittle, which may affect the fatigue performance of the coating under cyclic loading. For applications involving impact or vibration, the absence of borides in Ni40A may actually be advantageous despite lower hardness.
This study underscores the importance of considering the complete tribological system rather than just hardness. The friction coefficient, wear mechanism, and thermal behavior all influence the long-term performance of the surfacing layer. Engineers should conduct tribological testing under conditions representative of actual service to make informed material selections.