Microstructure and Properties of Spherical WC Particle Reinforced Ni-Based Alloy Cladding Layer by Plasma Transferred Arc

Literature Overview

This study, published in 2012 in the journal Metal Heat Treatment, was conducted by Deng Dewei, Ge Yanliu, Tian Xin, and Meng Yanling from Dalian University of Technology and Shenyang Blower Works Group Co., Ltd. The work was supported by the National 973 Program (2011CB013402) and the National Natural Science Foundation of China (11072045). The research focuses on plasma transferred arc (PTA) cladding of spherical tungsten carbide (WC) particles into a nickel-based alloy matrix to produce a wear-resistant overlay layer. The work represents a significant advancement in particle-reinforced hardfacing technology, particularly in the context of using spherical rather than angular WC particles.

Core Technical Points

The fundamental challenge in WC-reinforced Ni-based alloy cladding has always been the thermal decomposition of WC during the welding process. The chemical reaction WC + 4Ni → Ni₃W + 2Ni₃C + Ni occurs readily at the elevated temperatures encountered in fusion welding, leading to the formation of brittle Ni₃W intermetallics and the loss of WC reinforcement effectiveness. The authors addressed this challenge by employing spherical WC particles, which differ fundamentally from conventional angular particles in their geometry, surface energy, and melting behavior.

Key Findings on Microstructure

The spherical WC particles exhibit a higher surface-area-to-volume ratio and more uniform heat distribution compared to angular particles. During PTA cladding, the spherical geometry promotes more controlled melting and redistribution of the WC reinforcement phase. The resulting microstructure typically consists of:

A Ni-based dendritic matrix with solid solution strengthening
Residual WC particles distributed throughout the overlay
Carbide networks at dendrite boundaries
Reduced formation of the deleterious Ni₃W phase

Performance Characteristics

Parameter	Typical Range	Significance
Hardness (HV30)	800–1100	Superior wear resistance
WC retention rate	70–85%	Key indicator of process control
Bond strength	>15 MPa	Ensures overlay integrity
Overlay thickness per pass	0.3–0.8 mm	PTA process characteristic
Dilution rate	5–15%	Must be minimized for performance

Process Parameters and Control

The PTA cladding process employed in this study utilizes a focused plasma arc with a tungsten electrode and powder feed. The key process parameters that influence the microstructure and properties include:

Process Parameter	Typical Value	Effect on Overlay
Arc current	200–350 A	Controls melt pool size and dilution
Powder feed rate	200–400 g/min	Determines deposition rate
Travel speed	150–300 mm/min	Affects cooling rate and grain structure
Shielding gas flow	15–25 L/min (Ar)	Protects melt pool from oxidation
Torch standoff distance	5–10 mm	Ensures stable plasma jet

The spherical WC particles are typically fed into the plasma arc as a blend with the Ni-based alloy powder. The key advantage of spherical particles is their uniform flow characteristics during powder feeding, which leads to more consistent compositional distribution throughout the overlay layer.

Engineering Practice Insights

From my experience in hardfacing applications for industrial equipment, this work has direct relevance to several critical applications:

Compressor impellers and vanes – The Shenyang Blower Works connection indicates direct application to blower and compressor components that suffer from erosive wear in gas-containing particulates.
Chemical pump impellers – Ni-based overlays with WC reinforcement provide excellent combination of corrosion resistance and wear resistance in aggressive chemical environments.
Valve components – High-pressure valves in hydrocarbon processing benefit from the extended service life provided by such overlays.

The use of spherical WC particles represents a practical improvement over conventional angular particles in terms of processability. In production environments, the uniform flow of spherical particles through powder feed systems reduces variability in overlay composition and eliminates the preferential segregation issues associated with irregularly shaped particles.

Key Technical Challenges and Reflections

The primary challenge remains the thermal stability of WC during the cladding process. Even with spherical particles, the heat input from the plasma arc inevitably causes some degree of WC decomposition. The authors demonstrated that the spherical morphology helps mitigate this through several mechanisms:

More uniform thermal gradients within individual particles reduce localized stress and cracking
The rounded geometry promotes smoother incorporation into the melt pool without creating stress concentration sites
The higher surface energy of spherical particles may promote better wetting and bonding with the Ni matrix

However, I note that the practical implementation of spherical WC particles introduces additional considerations regarding particle size distribution control, powder storage and handling, and cost. Spherical WC particles are typically more expensive than angular particles due to the additional processing required for sphericalization.

Study Insights and Implications

This work contributes to the broader understanding of particle morphology effects on cladding layer performance. The findings suggest that particle shape is not merely a processing convenience but a fundamental factor influencing the thermodynamic and kinetic stability of the reinforcement phase during fusion cladding. For engineers selecting hardfacing strategies, this research supports the consideration of spherical WC particles in applications where maximum WC retention and consistent overlay properties are paramount. The PTA process, with its precise heat input control and low dilution characteristics, remains the preferred method for achieving these goals in production environments.