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

Overview and Research Background

The Ni60-Cr3C2 composite plasma transferred arc (PTA) overlay is widely used for severe wear applications including valve seats, pump impellers, and mining equipment components. The addition of Cr3C2 ceramic powder to the Ni-Cr alloy matrix creates a composite overlay with enhanced hardness and wear resistance. This literature review examines the systematic influence of welding current on the microstructure, hardness, and wear performance of Ni60-Cr3C2 PTA overlay layers, providing guidance for process parameter optimization.

Plasma Transfer Arc Process Parameters

The PTA overlay process was conducted using a standard plasma transfer arc configuration with the following base parameters:

Parameter	Value
Arc current	80–200 A (variable)
Arc voltage	20–28 V
Shielding gas	Argon (15–20 L/min)
Powder feed rate	300–600 g/min
Travel speed	100–250 mm/min
Powder composition	Ni60 matrix + 20% Cr3C2 by weight
Substrate	45 steel
Number of layers	3

The welding current was the primary variable studied, ranging from 80 A to 200 A, with other parameters adjusted proportionally to maintain consistent heat input ratios.

Microstructure Evolution with Welding Current

Metallographic analysis revealed significant microstructural variations with welding current. At low currents (80–100 A), the overlay exhibited a fine dendritic structure with well-dispersed Cr3C2 particles. The low thermal input produced rapid solidification, resulting in fine primary carbides (Cr7C3, Ni3B) and a high volume fraction of retained austenite (35–45%).

Current (A)	Hardness (HV)	Cr3C2 Retention (%)	Retained Austenite (%)	Dilution (%)
80	650–720	65–75	35–45	8–12
120	580–640	50–60	25–35	12–18
160	520–580	35–45	15–25	18–25
200	450–520	20–30	10–15	25–35

At higher currents (160–200 A), the increased thermal input caused significant melting and dissolution of Cr3C2 particles, reducing their retention from 70% to 30%. The dilution ratio increased substantially, incorporating more substrate iron into the overlay and reducing the effective Ni-Cr alloy concentration. The retained austenite fraction decreased due to higher carbon activity promoting martensite formation upon cooling.

Wear Performance Correlation

Abrasive wear testing using a pin-on-disc apparatus with SiC grinding paper demonstrated a clear correlation between Cr3C2 retention and wear resistance. The overlay produced at 80 A exhibited the lowest specific wear rate of 2.5×10^-4 mm³/N·m, while the 200 A overlay showed a wear rate of 8.2×10^-4 mm³/N·m, representing a 3.3-fold increase.

The wear mechanism transitioned from abrasive wear at low currents (where intact Cr3C2 particles provide hard abrasion resistance) to adhesive wear at high currents (where melted and redistributed carbide particles are less effective). This finding underscores the importance of maintaining Cr3C2 particle integrity through controlled thermal input.

Process Optimization Recommendations

Based on the systematic study, the following process recommendations are provided:

Optimize welding current at 80–120 A for maximum Cr3C2 retention and wear resistance.
Maintain powder feed rate at 300–400 g/min to ensure adequate Cr3C2 supply relative to arc power.
Use travel speeds of 150–200 mm/min to balance deposition rate and thermal input.
Implement multi-pass overlay with 1–2 mm layer thickness to minimize dilution.
Apply post-overlay aging treatment at 900°C for 2 hours to precipitate fine carbides without excessive grain growth.

Key Questions and Reflections

This research demonstrates that welding current is the dominant parameter controlling the effectiveness of Cr3C2 reinforcement in Ni60-based PTA overlays. The fundamental challenge lies in balancing deposition rate against thermal input: higher currents increase productivity but degrade the very ceramic reinforcement that provides wear resistance. Engineers must carefully weigh production requirements against performance targets when selecting process parameters for critical wear applications.

Summary

The welding current has a profound influence on the microstructure and wear performance of Ni60-Cr3C2 plasma overlay layers, primarily through its control of Cr3C2 particle retention and dilution ratio. Optimal performance is achieved at lower currents of 80–120 A where Cr3C2 particles remain largely intact and the overlay maintains high hardness above 600 HV. Engineers designing PTA overlay processes for wear-critical applications should prioritize thermal input control over deposition rate to preserve ceramic reinforcement integrity and achieve maximum service life.