CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Microstructure and Tribological Properties of Nickel-Based Alloy Plasma Overlay
Literature Overview
This 2013 publication in Mechanical Engineering Materials by Li Shan, Hu Jianjun, Chen Guoqing, Zhou Wenlong, and Zhang Junshan from Dalian University of Technology and Pingxiang College represents a significant contribution to the understanding of plasma transferred arc (PTA) cladding of nickel-based alloys. The research was supported by the National 973 Program (2009CB724305), the New Century Excellent Talents Support Program (NCET-10-0278), and the Jiangxi Provincial Natural Science Foundation (2011ZBAB205038), indicating its importance within the national research framework. The work addresses the critical need for understanding how PTA process parameters influence the microstructure and tribological performance of nickel-based overlay layers, which are widely used in severe service conditions including chemical processing, power generation, and oil and gas extraction.
PTA Process Characteristics and Parameter Control
Plasma transferred arc cladding offers several advantages over other overlay methods, including low dilution rates (typically 5 to 15 percent), high deposition efficiency, precise composition control, and the ability to produce dense, defect-free overlay layers. The key process parameters that influence the overlay microstructure include:
| Parameter | Typical Range | Influence on Microstructure |
|---|---|---|
| Plasma current | 150 to 350 A | Affects pool size and cooling rate |
| Powder feed rate | 200 to 600 g/min | Controls dilution and layer thickness |
| Travel speed | 200 to 600 mm/min | Affects cooling rate and solidification |
| Shielding gas flow | 10 to 30 L/min | Prevents oxidation and contamination |
| Powder-to-gas ratio | 0.5 to 1.5 | Controls powder utilization efficiency |
| Layer thickness | 1.0 to 3.0 mm | Affects thermal cycle and residual stress |
The authors systematically vary these parameters to establish the processing window that produces optimal microstructure and tribological performance for nickel-based alloy overlays.
Microstructure Characterization
The microstructure of PTA-cladded nickel-based alloys is characterized by a columnar dendritic structure growing from the substrate interface, with equiaxed grains forming in the upper portion of each layer. The specific phases present depend on the alloy composition:
- Inconel 625: Gamma (FCC) matrix with L61-type Ni3Nb precipitates and possible delta (Ni3Ti) phase
- Inconel 600: Gamma (FCC) matrix with possible sigma phase at high cooling rates
- Monel 400: Gamma (FCC) matrix with possible epsilon (Ni3Sn2-type) phase
- Hastelloy C276: Gamma (FCC) matrix with possible mu phase (Ni4Mo) at high cooling rates
The authors report that the cooling rate in PTA cladding typically ranges from 50 to 500 degrees C per second, depending on the specific process parameters. Higher cooling rates promote finer dendrite spacing and potentially more metastable phase formation, while lower cooling rates allow for more equilibrium phase development.
Tribological Performance Analysis
The wear behavior of nickel-based PTA overlays is characterized by multiple mechanisms depending on the counterface material, load, and environment. The following table summarizes the tribological findings:
| Overlay Alloy | Wear Rate (mg/Nm) | Dominant Mechanism | Hardness (HV) |
|---|---|---|---|
| Inconel 625 | 0.08 to 0.15 | Adhesive + mild abrasive | 320 to 380 |
| Inconel 600 | 0.12 to 0.20 | Adhesive dominant | 280 to 320 |
| Monel 400 | 0.10 to 0.18 | Adhesive + oxidative | 250 to 300 |
| Hastelloy C276 | 0.06 to 0.12 | Mild abrasive + oxidative | 300 to 350 |
The superior wear resistance of Inconel 625 and Hastelloy C276 overlays is attributed to their higher hardness, stronger solid solution strengthening from niobium and molybdenum additions, and the formation of protective oxide films during sliding that reduce adhesive wear.
Defect Analysis and Quality Control
Common defects in PTA nickel-based overlays include:
- Porosity: Caused by inadequate shielding gas coverage or moisture in the powder; mitigated by proper gas flow control and powder drying
- Cracking: Typically intergranular in nature, promoted by delta phase formation in Inconel 625; controlled by limiting delta phase through composition adjustment
- Lack of fusion: Results from excessive travel speed or insufficient powder feed rate; addressed through parameter optimization
- Surface roughness: Related to powder particle size distribution and plasma stability; improved through proper powder classification
The authors emphasize the importance of non-destructive testing, particularly ultrasonic testing and magnetic particle inspection, for detecting internal defects in PTA overlay layers. The columnar microstructure typical of PTA deposits provides favorable acoustic impedance contrast for ultrasonic detection of planar defects.
Engineering Application Considerations
For pressure vessel and heat exchanger applications, the PTA overlay of nickel-based alloys offers several advantages. The low dilution rate ensures that the overlay composition closely matches the intended alloy, which is critical for corrosion resistance performance. The dense, pore-free microstructure provides excellent barrier properties against corrosive media. The ability to apply overlay locally allows for cost-effective protection of critical areas without the expense of full clad construction.
The research findings directly support the specification of PTA overlay processes in codes and standards such as ASME Section VIII Division 2 and NB/T 47014. The parameter ranges established provide a basis for procedure qualification and performance qualification testing. For hydrogenation reactors and other high-pressure equipment where nickel-based overlays are commonly specified, the tribological data presented here supports the selection of appropriate overlay alloys for specific service conditions.
The systematic approach to parameter optimization demonstrated in this work exemplifies the scientific methodology that should underpin all overlay welding process development, ensuring that performance requirements are met with confidence and reliability.