Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Different Nickel-Based Alloy Plasma Surfacing Layers
Literature Overview
This paper by Hu Jianjun, Chen Guoqing, Li Shan, and Zhou Wenlong (2013), published in Hot Working Technology, presents a comparative study of three nickel-based alloy powders—Ni40A, Ni50A, and Ni60A—deposited by plasma surfacing onto 1Cr18Ni9Ti stainless steel substrate. The research was supported by the Jiangxi Provincial Natural Science Foundation (2011ZBAB205038) and the National 973 Program (2009CB724305). The study provides a systematic comparison that is directly applicable to engineering selection decisions for nickel-based surfacing applications.
Nickel-Based Surfacing Alloy Systems
Nickel-based alloys are widely used in surfacing applications due to their excellent combination of wear resistance, corrosion resistance, and hot hardness. The three alloys studied represent different composition ranges within the Ni-Cr-Fe-B-Si system:
| Alloy Designation | Typical Composition Range | Primary Application |
|---|---|---|
| Ni40A | ~40% Cr, ~5% Fe, ~2% B, ~2% Si | General wear + corrosion |
| Ni50A | ~50% Cr, ~5% Fe, ~2% B, ~2% Si | Moderate wear + high corrosion |
| Ni60A | ~60% Cr, ~5% Fe, ~2% B, ~2% Si | High wear + high corrosion |
The nomenclature reflects the approximate chromium content, which is the primary variable distinguishing these alloys and the key driver of their microstructural and property differences.
Microstructural Analysis
The plasma surfacing process produces a γ-Ni (austenitic nickel) matrix in all three alloys, which provides the base toughness and corrosion resistance. The critical differences lie in the precipitate phases that form during solidification:
| Alloy | Precipitate Phases | Hardness (HV) | Relative Wear Resistance |
|---|---|---|---|
| Ni40A | Cr₇C₃, M₂₃C₆, Ni₃Si | Lower | Lowest |
| Ni50A | CrB, M₂₃(C,B)₆, Cr₇C₃, Cr₅B₃, Ni₃Si | Medium | Medium |
| Ni60A | CrB, M₂₃(C,B)₆, Cr₇C₃, Cr₅B₃, Ni₃Si | 780 (highest) | Highest |
The progression from Ni40A to Ni60A demonstrates a clear trend of increasing precipitate complexity and volume fraction with increasing chromium content. The Ni60A alloy exhibits the richest precipitate phase assemblage, including the very hard CrB and Cr₅B₃ borides that contribute significantly to wear resistance.
Hardness and Wear Performance Comparison
The hardness progression follows the expected trend based on precipitate content and type:
- Ni60A (780 HV): The highest hardness is attributed to the combination of high volume fraction of hard precipitates (CrB, Cr₅B₃, M₂₃(C,B)₆) and the strengthening effect of the high Cr content in the γ-Ni matrix.
- Ni50A: Intermediate hardness with a good balance of precipitate types, including both carbides and borides.
- Ni40A: Lower hardness dominated by Cr₇C₃ and M₂₃C₆ carbides, with fewer and less hard precipitate phases.
The wear resistance ranking of Ni60A > Ni50A > Ni40A directly correlates with hardness and precipitate volume fraction. However, the friction coefficient data reveals an interesting counter-trend: Ni60A exhibits the highest friction coefficient, while Ni40A and Ni50A show similar lower values. This is attributed to the harder, more angular precipitate particles in Ni60A creating greater mechanical interlocking with the counterface material.
Engineering Selection Guidelines
Based on the comparative data, the following selection guidelines can be established for practical applications:
| Service Condition | Recommended Alloy | Rationale |
|---|---|---|
| High abrasion, moderate corrosion | Ni60A | Maximum hardness and wear resistance |
| Moderate abrasion, high corrosion | Ni50A | Balanced wear and corrosion performance |
| Low abrasion, high corrosion | Ni40A | Lower friction, good corrosion resistance |
| High temperature wear | Ni50A or Ni60A | Better hot hardness retention |
| Seizure prevention needed | Ni40A or Ni50A | Lower friction coefficient |
Process Considerations for Nickel-Based Plasma Surfacing
Several process aspects are critical for achieving optimal results with nickel-based alloy plasma surfacing:
- Dilution control: The austenitic stainless steel substrate (1Cr18Ni9Ti) provides some dilution, but the high alloy content of the surfacing powder ensures that the final composition remains within the desired range for all three alloys.
- Crack sensitivity: Nickel-based surfacing alloys generally exhibit low hot cracking susceptibility due to the austenitic matrix, which accommodates solidification shrinkage through plastic deformation.
- Powder morphology: Spherical, gas-atomized powders provide the most consistent melting and deposition characteristics. Powder size distribution should be controlled to ensure uniform feeding rates.
- Travel speed optimization: Higher travel speeds reduce heat input per unit length, promoting finer microstructure but potentially increasing porosity. The optimal travel speed must be determined for each specific geometry and alloy.
Study Insights and Reflections
This comparative study provides engineers with clear, actionable data for selecting among the commonly available Ni40A, Ni50A, and Ni60A surfacing alloys. The straightforward correlation between chromium content, precipitate complexity, hardness, and wear resistance simplifies the selection process for most applications.
However, the friction coefficient data deserves careful attention. In applications where seizure, galling, or excessive friction heating is a concern—such as sliding bearings, valve seats under high pressure, or components operating at elevated temperatures—the highest hardness alloy (Ni60A) may not be the optimal choice despite its superior wear resistance. The increased friction coefficient can lead to elevated contact temperatures, potential thermal degradation of the surface layer, and accelerated adhesive wear mechanisms.
The plasma surfacing process itself offers significant advantages for nickel-based alloy deposition, including low dilution, high deposition rate, and consistent microstructure. For pipe fitting manufacturers dealing with high-wear applications—such as slurry valve internals, pump impellers, or erosion-resistant pipe sections—this comparative data provides a reliable basis for material selection that can significantly extend component service life.