Effect of Welding Current on Microstructure and Properties of Ni60-Cr3C2 Plasma Cladding Layer

Literature Overview and Research Background

Ni60 (Stellite 6 equivalent) is a nickel-based alloy containing chromium, cobalt, and iron, widely employed for its outstanding wear resistance, corrosion resistance, and high-temperature strength. When combined with Cr3C2 ceramic particles, the resulting composite cladding layer exhibits enhanced tribological performance due to the synergistic effect of the tough nickel matrix and hard ceramic reinforcement. Plasma transferred arc (PTA) cladding is the predominant method for depositing such composite layers, as it provides high deposition rates, low dilution, and good metallurgical bonding with the substrate.

This study investigates how welding current — which directly controls arc power density, melt pool geometry, and cooling rate — affects the microstructure, hardness, wear resistance, and dilution rate of Ni60-Cr3C2 plasma cladding layers deposited on carbon steel substrates.

Core Technical Points and Process Parameters

Arc Power and Melt Pool Characteristics

Welding current in PTA cladding determines the arc power, which governs the melt pool depth, width, and cooling rate. At lower currents (below 100 A), the arc power is insufficient to fully melt the powder feedstock, leading to incomplete melting of Cr3C2 particles and poor metallurgical bonding. At higher currents (above 200 A), excessive heat input causes significant substrate melting, increasing dilution and altering the alloy composition of the cladding layer.

The study examines a current range of 100-220 A with the following process parameters:

Microstructural Evolution with Current

At 100-120 A, the cladding layer exhibits a dendritic structure with interdendritic regions enriched in Cr3C2 particles. The dendrites are relatively fine, and the matrix shows a solid solution of Cr, Co, and Fe in Ni. The hardness distribution is heterogeneous, with Cr3C2 particles reaching 1800-2200 HV and the matrix at 350-400 HV.

At 160-180 A, the dendrite spacing increases due to lower cooling rates, and some Cr3C2 particles begin to partially dissolve at the dendrite boundaries. The matrix hardness remains relatively stable, but the overall composite hardness begins to decline slightly due to the reduced volume fraction of intact ceramic particles.

At 200-220 A, significant substrate dilution occurs (exceeding 30%), and the cladding layer composition shifts toward a Ni-Fe-Cr austenitic structure. Cr3C2 particles are extensively dissolved, and the composite character is largely lost. The layer hardness drops to 300-350 HV, approaching that of the substrate.

Mechanical and Tribological Properties

The wear rate data clearly demonstrates the inverse relationship between welding current and tribological performance. The optimal current range of 100-140 A provides the best combination of low dilution, high hardness, and superior wear resistance.

Defect Analysis and Countermeasures

Common Defects at Different Current Levels

Crack Mitigation Strategies

Cracking in Ni60-Cr3C2 cladding layers is primarily hot cracking (solidification cracking) due to the wide solidification range of the alloy and the restraint from the substrate. At higher currents, the increased thermal gradient and the formation of a liquid film at dendrite boundaries promote crack initiation. The following countermeasures are recommended:

1. Preheat the substrate to 150-200 °C to reduce thermal gradient
2. Use multiple thin layers instead of a single thick deposit
3. Add small amounts of sulfur or selenium to narrow the solidification range
4. Apply controlled post-weld stress relief annealing at 700-750 °C

Engineering Practice Implications

For industrial applications such as mining equipment, cement mill liners, and chemical pump impellers, this study provides clear guidance on process parameter selection:

• The optimal welding current for Ni60-Cr3C2 PTA cladding on carbon steel is 100-140 A
• Dilution must be kept below 20% to maintain composite hardness above 600 HV
• Multi-layer cladding with alternating current levels can be employed to achieve desired thickness while maintaining low dilution in the critical surface layers
• Powder feed rate must be synchronized with arc power to ensure complete melting without excess spatter

The study also highlights the importance of substrate preparation. Surface roughness, contamination, and oxide layers significantly affect the metallurgical bond between the substrate and the first cladding layer. Mechanical grinding to a surface finish of Ra 6.3 μm or better is recommended before cladding.

Key Questions and Reflections

A critical question arising from this study is the long-term stability of Cr3C2 particles under service conditions involving elevated temperatures and cyclic loading. While the as-deposited microstructure shows excellent particle integrity at optimal currents, prolonged exposure to temperatures above 600 °C may cause particle coarsening and matrix softening. Additionally, the thermal fatigue behavior of the cladding-substrate interface under repeated thermal cycling warrants further investigation.

Another reflection is the economic aspect of cladding thickness. While multiple thin layers provide superior properties, they increase production time and cost. Engineers must balance performance requirements against manufacturing economics, particularly for large-area cladding applications.

Summary and Study Insights

This literature establishes that welding current is the dominant parameter governing the microstructure and performance of Ni60-Cr3C2 plasma cladding layers. The optimal current range of 100-140 A yields a dilution rate below 20%, a composite hardness exceeding 600 HV, and a wear rate reduced by more than 50% compared to the uncladded substrate. For engineers designing cladding processes for wear-critical components, the key insight is that current must be selected not merely to achieve adequate deposition but to preserve the integrity of ceramic reinforcement particles within the nickel matrix. The interplay between arc power, cooling rate, and particle dissolution is a delicate balance that demands careful process control and thorough understanding of the underlying metallurgical mechanisms.