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

Literature Overview

Published in Materials for Mechanical Engineering (2025) by Zhang Yu, Tang Biao, Ma Teng, and Bi Lige from Jiamusi University, and Zhou Heng from University of Science and Technology Beijing, this study investigates the influence of welding current on the microstructure and properties of Ni60/Cr₃C₂ composite plasma cladding layers. Funded by the Heilongjiang Provincial Department of Education Basic Scientific Research Business Fee (2023-KYYWF-0561), the work addresses a practical engineering problem: optimizing the plasma transferred arc cladding process for composite coatings that combine the matrix properties of Ni60 cast iron with the extreme hardness of chromium carbide particles.

Core Technical Content

Ni60 (ASTM A540 Type I) is a nickel-iron alloy with approximately 57-65% Ni, 2.5-3.5% C, 1.5-3% Cr, and 1-2% Mo. It is widely used for hardfacing applications due to its excellent combination of wear resistance, corrosion resistance, and ability to withstand thermal cycling without cracking. The addition of Cr₃C₂ particles (hardness approximately 2000-2400 HV) creates a composite coating where the hard carbide particles provide wear resistance while the ductile Ni60 matrix prevents catastrophic brittle fracture.

The plasma transferred arc cladding process parameters studied include welding current (the primary variable), arc voltage, travel speed, powder feeding rate, and shielding gas flow. The welding current directly controls the heat input, which affects:

1. Dilution rate between the coating and substrate
2. Cooling rate and solidification microstructure
3. Carbide particle dissolution and redistribution
4. Residual stress state and cracking susceptibility
5. Coating thickness and deposition efficiency

Microstructural Response to Current Variation

At low welding currents, the heat input is insufficient to fully melt the Cr₃C₂ particles, resulting in a composite microstructure where intact carbide particles are embedded in a partially melted Ni60 matrix. The dilution rate is low, preserving the Ni-rich composition of the coating. However, the lower current may also result in poor bonding with the substrate due to insufficient melting of the substrate surface.

At moderate currents, partial dissolution of Cr₃C₂ occurs, releasing chromium and carbon into the melt pool. This leads to the formation of additional carbide phases (Cr₇C₃, Cr₂₃C₆) in addition to the undissolved Cr₃C₂ particles, creating a more complex and potentially more wear-resistant microstructure. The dilution rate increases moderately, slightly reducing the Ni content but not compromising the overall coating performance.

At high currents, extensive carbide dissolution occurs, and the coating composition approaches that of the base material due to high dilution. The microstructure becomes coarser, and carbide networks may form at grain boundaries, which can act as crack initiation sites. Cracking susceptibility increases significantly at high currents due to the combination of high thermal stress and reduced matrix ductility.

Mechanical Performance Analysis

The mechanical properties of the Ni60/Cr₃C₂ composite cladding layer exhibit a non-monotonic relationship with welding current:

• Hardness: Increases with current up to an optimal point (approximately 200-250 A) due to carbide redistribution and matrix solid solution strengthening, then decreases at higher currents due to dilution and coarse microstructure.
• Wear resistance: Follows a similar trend to hardness, with optimal performance at moderate currents where the composite structure is most effective.
• Bond strength: Improves with increasing current up to a point, as better substrate melting enhances metallurgical bonding, but excessive current may cause substrate damage.
• Impact resistance: Decreases with increasing current as the matrix becomes more brittle due to higher dilution and carbide network formation.

Process Optimization Strategy

Based on the study findings, the following process optimization recommendations emerge:

1. Current selection: For most applications, a welding current of 200-250 A provides the best balance of hardness, wear resistance, and bonding quality.
2. Current-to-speed ratio: Maintaining a consistent heat input per unit length (current/speed ratio) ensures uniform microstructure across the coating.
3. Multi-pass strategy: Using lower current for the first pass (to establish bonding) and moderate current for subsequent passes (to build thickness) optimizes both bonding and microstructure.
4. Powder composition: The Cr₃C₂ particle size (typically 45-150 μm) and volume fraction (10-30%) should be matched to the selected current level.
5. Substrate preparation: Preheating to 200-300°C and proper surface preparation (grinding to remove oxide and scale) are essential for achieving good bonding at all current levels.

Engineering Practice Integration

The Ni60/Cr₃C₂ composite cladding system finds application in:

• Mining equipment: Excavator buckets, crusher jaws, and conveyor components
• Cement industry: Kiln rollers, mill liners, and wear plates
• Power generation: Coal handling equipment, ash handling systems
• Oil and gas: Downhole tools, drilling equipment, and valve components

The selection of welding current should be based on the specific service requirements: lower currents (150-200 A) for applications where impact resistance and toughness are critical, moderate currents (200-250 A) for general wear resistance applications, and higher currents (250-300 A) only where maximum hardness is required and the coating is not subjected to impact loading.

Study Insights and Reflections

This research highlights the critical role of welding current as the primary control parameter in plasma cladding of composite coatings. The non-linear relationship between current and coating properties underscores the importance of process optimization rather than simply maximizing deposition rate or hardness. The engineering challenge lies in finding the current level that produces the optimal composite microstructure—sufficient carbide dissolution to create additional hard phases, but not so much that the reinforcing particles are completely consumed.

A key insight is that the dilution rate, which is directly controlled by welding current, has a more profound effect on coating performance than the current itself. Engineers should focus on controlling dilution through current selection, powder feeding rate adjustment, and multi-pass strategies to achieve consistent coating quality.

Summary

The systematic study of welding current effects on Ni60/Cr₃C₂ plasma cladding layers demonstrates that moderate current levels (200-250 A) produce the optimal balance of hardness, wear resistance, and bonding quality. The underlying mechanism is the controlled partial dissolution of Cr₃C₂ particles, which creates a synergistic composite microstructure with both undissolved hard particles and newly formed carbide phases. Engineers implementing this cladding system should prioritize dilution control through current management and adopt multi-pass strategies to achieve both strong bonding and optimal microstructure. The practical value of this work lies in providing clear process guidelines for producing high-performance composite coatings in industrial hardfacing applications.