Effect of Welding Parameters on Microstructure and Microhardness of Ni60 Alloy Plasma Cladding Layer

Literature Overview and Research Context

Ni60 alloy, a castable nickel-iron-chromium alloy containing approximately 57-63% Ni, 25-31% Cr, and 1-2% Fe with minor additions of Mo, B, and Si, is widely employed in weld overlay applications for its exceptional resistance to cavitation erosion, dry sliding wear, and chemical corrosion in aggressive environments. The present literature examines the influence of plasma transferred arc (PTA) welding parameters on the microstructure, phase composition, and microhardness of Ni60 cladding layers deposited on carbon steel substrates.

PTA cladding is preferred over other welding overlay methods for Ni60 applications due to its ability to produce narrow, dilution-free deposits with fine microstructures. The plasma arc provides a highly concentrated heat source with temperatures exceeding 15,000°C, enabling rapid melting and solidification of the powder feedstock. However, the process is sensitive to parameter variations, and improper settings can lead to excessive dilution, porosity, or undesirable phase transformations that compromise the functional properties of the cladding layer.

Process Parameters and Their Influence on Dilution

The study systematically varies plasma current, travel speed, powder feed rate, and powder gas flow rate to determine their individual and combined effects on cladding quality. The following table presents the parameter ranges investigated and their primary effects:

Dilution is the single most critical quality metric for Ni60 cladding, as it directly determines the chromium and nickel content of the final deposit and, consequently, its corrosion and wear resistance. The study demonstrates that dilution can be maintained below 15% when the powder-to-arc ratio is between 0.6 and 0.8, plasma current is 80-100 A, and travel speed is 180-220 mm/min. At these settings, the Ni60 deposit retains its characteristic composition with chromium content above 25% and nickel above 55%.

Microstructural Characteristics and Phase Analysis

The microstructure of Ni60 cladding layers deposited under optimal PTA conditions consists of a dendritic solidification pattern with an austenite matrix and dispersed carbide particles. The primary carbide phase is M7C3, which forms as discrete particles at dendrite tips and interdendritic regions. The size and distribution of these carbides are strongly influenced by the cooling rate, which is in turn governed by the welding parameters.

At lower travel speeds (100-150 mm/min), the cooling rate is reduced, resulting in coarser dendrite spacing (50-80 μm) and larger carbide particles (5-15 μm). This microstructure provides adequate wear resistance but may exhibit reduced toughness. At higher travel speeds (250-300 mm/min), the increased cooling rate produces fine dendrite structures (15-30 μm spacing) with smaller, more uniformly distributed carbides (1-5 μm), offering superior wear resistance and improved crack resistance.

The following table summarizes the microstructural evolution across different parameter combinations:

An important observation from the study is that the transition from M7C3 to M23C6 carbide morphology occurs when dilution exceeds 20%, indicating excessive carbon steel substrate melting. The M23C6 phase is larger, more continuous, and less beneficial for wear resistance, marking a clear quality boundary for the process.

Microhardness Distribution and Wear Performance

The microhardness profile across the cladding cross-section reveals a characteristic gradient from the substrate interface to the cladding surface. At the substrate interface, where dilution is highest, hardness values of 350-450 HV are typical. Moving toward the cladding surface, hardness increases to 550-720 HV in the fully melted zone. The maximum hardness is achieved at the surface layer, where the cooling rate is highest and the microstructure is finest.

The hardness variation within a single deposit is typically ±15% of the average value, which is acceptable for most engineering applications. However, significant hardness variation between successive cladding passes (exceeding ±25%) indicates parameter instability and requires process correction. The study recommends implementing real-time monitoring of plasma current and powder feed rate to maintain consistency across multi-pass cladding operations.

For wear testing, specimens cladded under optimal parameters exhibit a wear rate of 0.003-0.008 mm³/N·m in dry sliding conditions against a 100Cr6 steel counterpart, representing a 15-30 times improvement over the uncladded carbon steel substrate. The wear mechanism transitions from adhesive wear at the substrate interface to abrasive wear in the cladding zone, with the fine M7C3 carbides acting as effective abrasion-resistant particles.

Engineering Application Considerations

For pressure vessel and heat exchanger applications involving Ni60 cladding, the following engineering considerations are critical:

1. Substrate preparation: The base material must be preheated to 150-200°C to reduce thermal stress and minimize the risk of substrate cracking, particularly for low-alloy steels with carbon equivalent exceeding 0.4%.
2. Multi-pass strategy: For cladding thicknesses exceeding 2 mm, a multi-pass approach is recommended with each pass diluting the previous one. The first pass should use slightly higher current to ensure good bonding, while subsequent passes should reduce current to minimize additional dilution.
3. Post-cladding heat treatment: An annealing treatment at 800-850°C for 1-2 hours followed by air cooling can relieve residual stresses without significantly altering the microstructure. This treatment is particularly important for thick-walled components where residual stress could lead to delayed cracking.
4. Quality verification: Each cladding job should include dilution analysis (optical emission spectroscopy or XRF), microhardness profiling, and metallographic examination of a cross-section to verify carbide morphology and absence of porosity.

Key Reflections and Study Conclusions

The literature provides valuable guidance for PTA cladding of Ni60 alloy, but several practical limitations deserve emphasis. First, the parameter windows identified are specific to the powder grade and substrate composition used in the study; different Ni60 formulations (e.g., with varying B and Si content) may require adjusted parameters. Second, the study does not extensively address the long-term corrosion performance of the cladding layer, which is critical for applications in chemical processing and marine environments.

The most important practical takeaway is that dilution control is the cornerstone of quality Ni60 PTA cladding. Engineers must invest in process development and qualification that rigorously establishes and monitors dilution levels, rather than relying solely on visual inspection of the deposit surface. The transition from M7C3 to M23C6 carbides serves as a reliable microstructural indicator of excessive dilution and should be included in quality acceptance criteria.

For engineers transitioning from conventional welding overlay methods to PTA cladding, the learning curve is steep but the rewards in terms of deposit quality and functional performance are substantial. Systematic parameter studies, metallographic verification, and hardness profiling should be integral components of any PTA cladding qualification program.