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

Literature Overview

This 2025 study by researchers from Jiamusi University and University of Science and Technology Beijing investigates the systematic influence of plasma transferred arc (PTA) welding current on the microstructure, hardness, and wear resistance of Ni60/Cr₃C₂ composite overlay layers. The research is supported by the Heilongjiang Provincial Department of Education Basic Scientific Research Business Fee Project (2023-KYYWF-0561) and addresses a critical parameter optimization challenge in industrial plasma cladding applications. The Ni60 alloy combined with Cr₃C₂ cermet particles represents one of the most widely used wear-resistant overlay systems in mining, cement, and power generation industries.

Core Technical Points

Plasma Transferred Arc Overlay Process Parameters

The study examines the effect of welding current as the primary variable while maintaining other parameters within typical industrial ranges:

Parameter	Range Studied	Optimal Value
Welding current (I)	150–400 A	250–300 A
Arc voltage (U)	25–35 V	28–30 V
Travel speed (V)	300–600 mm/min	400–450 mm/min
Gas flow rate (Ar)	15–25 L/min	20 L/min
Powder feed rate	0.8–1.5 kg/h	1.0–1.2 kg/h
Layer thickness	0.8–1.5 mm	1.0–1.2 mm
Number of layers	2–4	3

Microstructural Evolution with Current Variation

The welding current directly controls the heat input per unit length (Q = UI/V), which governs the solidification conditions and subsequent microstructure of the overlay deposit. The study reveals distinct microstructural transitions across the current range:

Low current range (150–200 A):

Heat input: 7.5–10.5 kJ/mm
Solidification rate: relatively slow
Primary austenite dendrites with Cr₃C₂ particles distributed at interdendritic regions
Higher proportion of soft austenitic matrix (approximately 60–70%)
Hardness: 420–480 HV
Dilution with base metal: 12–18%

Medium current range (250–300 A):

Heat input: 17.5–22.5 kJ/mm
Optimal solidification conditions
Fine cellular-austenite structure with uniformly distributed Cr₃C₂ particles
Balanced matrix/carbide ratio (approximately 50/50)
Hardness: 620–720 HV
Dilution with base metal: 8–12%
Cracking tendency: minimal

High current range (350–400 A):

Heat input: 30.6–38.6 kJ/mm
Excessive heat input causing overheating
Coarse columnar dendrites with large Cr₃C₂ particle agglomeration
Increased delta ferrite content at grain boundaries
Hardness: 550–600 HV (decreased due to coarsening)
Dilution with base metal: 15–22%
Cracking tendency: moderate to high due to thermal stress

Cr3C2 Particle Distribution and Matrix Interaction

The Cr₃C₂ particles (typically 15–45 μm in size) interact with the Ni60 matrix through several mechanisms:

Heterogeneous nucleation: Cr₃C₂ particles serve as nucleation sites for austenite dendrites, promoting finer grain structure at medium current levels.
Matrix hardening: The presence of Cr₃C₂ particles increases the chromium content in the surrounding matrix, promoting the formation of secondary carbides (M₇C₃, M₂₃C₆) that contribute to overall hardness.
Particle pull-out resistance: The bonding strength between Cr₃C₂ particles and the Ni60 matrix determines the wear mechanism. At optimal current levels, the interface bonding is strong enough to prevent premature particle pull-out during sliding wear.

Wear Performance Correlation

The relationship between welding current and wear resistance follows a non-linear trend:

Current (A)	Hardness (HV)	Wear Rate (mm³/N·m)	Wear Mechanism
150	445	4.2×10⁻⁶	Adhesive + Abrasive
200	475	3.5×10⁻⁶	Abrasive dominant
250	645	1.8×10⁻⁶	Micro-ploughing
300	710	1.2×10⁻⁶	Micro-ploughing (optimal)
350	620	2.1×10⁻⁶	Abrasive + Particle pull-out
400	565	3.0×10⁻⁶	Particle pull-out + Abrasive

Process Analysis and Standards Compliance

Heat Input Management

The welding current is the primary determinant of heat input in PTA overlay welding. According to ASME IX and NB/T 47014 qualification requirements, the qualified procedure must control heat input within specified limits to ensure acceptable weld metal properties. For Ni60-based overlay applications:

Minimum heat input: 7 kJ/mm (to ensure complete melting and bonding)
Maximum heat input: 25 kJ/mm (to prevent excessive dilution and cracking)
Optimal heat input window: 15–22 kJ/mm

Dilution Control

Dilution from the base metal is a critical quality parameter that directly affects overlay layer composition and properties. The study demonstrates that medium current levels (250–300 A) provide the best dilution control at 8–12%, which maintains the Ni60 matrix composition within acceptable limits while ensuring adequate Cr₃C₂ particle incorporation. Excessive dilution beyond 15% significantly reduces the chromium equivalent and nickel content, compromising both wear resistance and corrosion resistance.

Defect Analysis

Defect Type	Current Range	Root Cause	Countermeasure
Lack of fusion	< 180 A	Insufficient heat input	Increase current to > 200 A
Excessive dilution	> 350 A	Excessive heat input	Reduce current to < 300 A
Cracking	> 350 A	Thermal stress + high carbon	Reduce current, add preheat
Crater cracking	All ranges	Crater solidification	Use arc crater fill
Porosity	< 150 A	Poor gas coverage	Increase gas flow rate
Particle segregation	> 350 A	Excessive turbulence	Reduce current, optimize feed rate

Engineering Practice Applications

Industrial Application Scenarios

The optimized PTA overlay parameters identified in this study are directly applicable to several industrial scenarios:

Mining equipment: Wear plates for excavator buckets, crusher liners, and conveyor rollers operating under severe abrasive conditions with sand, gravel, and rock materials.
Cement industry: Kiln liners, preheater cyclone components, and mill liners exposed to high-temperature abrasive slurry containing limestone particles.
Power generation: Boiler tube overlays, fan blades, and pump impellers experiencing erosive wear from fly ash and flue gas.
Marine applications: Propeller shafts, rudder stocks, and pump components exposed to abrasive seawater containing sand and shell particles.

Procedure Qualification Considerations

For production implementation of Ni60/Cr₃C₂ PTA overlay, the following qualification requirements must be met per NB/T 47014 and ASME IX:

Welder qualification test coupon: minimum 3 layers with total thickness of 4 mm
Mechanical testing: hardness profile across full thickness showing uniform 600–720 HV
Metallographic examination: no cracks, lack of fusion, or excessive porosity (> 1% area fraction)
Wear testing: ASTM G99 pin-on-disk test with wear rate below 2.0×10⁻⁶ mm³/N·m
Dilution analysis: chemical composition at 0.5 mm depth confirming Ni > 55%, Cr > 25%

Quality Control Implementation

A comprehensive quality control program for PTA overlay production should include:

Incoming inspection: Verify Ni60 powder composition and Cr₃C₂ particle size distribution (15–45 μm, D50 = 28 μm)
Process monitoring: Real-time recording of current, voltage, and travel speed with automated alarms for parameter deviations exceeding ±10%
In-process inspection: Visual examination of each layer for surface quality, followed by magnetic particle testing for crack detection
Final inspection: Full thickness hardness survey, cross-sectional metallographic examination, and ultrasonic testing for subsurface defects
Documentation: Complete weld log including all process parameters, material heat numbers, and inspection results

Key Questions and Reflections

The study raises several important questions regarding the scalability of optimized parameters from laboratory conditions to industrial production environments. Laboratory trials typically employ single-pass welding on flat coupons under controlled conditions, whereas industrial applications involve multi-pass welding on complex geometries with variable joint configurations. The transition from laboratory to production requires additional parameter optimization for factors such as:

Joint geometry effects: Convex surfaces, internal corners, and edge locations create local variations in heat input and dilution that may shift the optimal current window.
Layer-to-layer interaction: Subsequent layers are deposited on partially cooled previous layers, creating different thermal conditions than the initial layer on the base metal. The optimal current for each layer may differ progressively.
Base metal variability: Different base materials (carbon steel, low-alloy steel, stainless steel) have different thermal conductivities and melting points, affecting dilution behavior and requiring individual parameter optimization.

A particularly interesting observation is the non-monotonic relationship between current and hardness. The peak hardness at 300 A followed by a decrease at higher currents suggests an optimal balance between solidification refinement and thermal coarsening. This behavior is consistent with the classic solidification theory where moderate cooling rates produce the finest microstructure. However, the exact mechanism of hardness decrease at high currents involves both matrix coarsening and potential Cr₃C₂ particle coarsening through dissolution and re-precipitation, which warrants further investigation through in-situ observation techniques.

The study also highlights an important consideration regarding the role of Cr₃C₂ particle size and distribution. The wear performance is not solely determined by matrix hardness but also by the particle-matrix interface bonding strength. At high current levels, the increased thermal energy may partially dissolve Cr₃C₂ particle edges, creating a diffusion bonding zone that improves interface strength but reduces particle volume fraction. This complex interaction between particle dissolution, matrix modification, and wear mechanism transition represents an important area for future research.

Study Insights and Reference Value

This 2025 study provides valuable parameter optimization data for Ni60/Cr₃C₂ plasma overlay welding that can be directly applied to industrial production settings. The systematic investigation of welding current effects establishes a clear relationship between process parameters, microstructure, and wear performance that serves as a reference for procedure development and qualification. The identified optimal current range of 250–300 A for single-pass overlay on carbon steel substrates provides a practical starting point for production welding operations.

The research contributes to the broader understanding of composite overlay metallurgy by demonstrating how process parameters influence particle distribution, matrix structure, and overall wear mechanism. The findings support the development of more sophisticated multi-parameter optimization approaches that consider the simultaneous effects of current, voltage, travel speed, and powder feed rate on overlay quality. Future work should extend this investigation to include multi-pass welding scenarios, different base materials, and long-term wear testing under actual industrial service conditions to establish comprehensive performance databases for production decision-making.