Spherical WC-Reinforced Iron-Based Composite Plasma Cladding Layer: Microstructure and Tribological Performance

Literature Overview

This 2018 publication from Shanghai Maritime University, published in the Journal of Tribology (Chinese and English editions), investigates the microstructure and tribological properties of spherical tungsten carbide (WC) reinforced iron-based composite plasma transferred arc (PTA) cladding layers. The research team, led by Fan Li, Chen Haiyan, Liu Shanshan, Dong Yaohua, Dong Lihua, and Yin Yansheng, was supported by the Ocean Public Welfare Industry Research Special Fund Project (201405013-3), the National Natural Science Foundation of China (51609133), and Shanghai Maritime University Research Fund Project (20130448). The marine application context is particularly significant, as ship propellers, thrusters, and underwater equipment face severe erosion-corrosion environments.

Core Technical Content

Plasma Transferred Arc Cladding Process Parameters

PTA cladding offers superior control over dilution, microstructure, and surface quality compared to conventional welding-based cladding methods. The key process parameters for spherical WC reinforced iron-based composite cladding are:

Parameter	Typical Range	Effect on Performance
Arc current	200-350 A	Controls melt pool size and dilution
Arc voltage	22-28 V	Affects penetration depth
Travel speed	100-200 mm/min	Controls heat input and dilution
Shielding gas flow	10-20 L/min	Prevents oxidation of WC particles
Powder feed rate	0.5-1.5 kg/h	Controls overlay thickness per pass
Powder nozzle height	8-12 mm	Affects powder trajectory and melting
Preheat temperature	100-200 °C	Reduces cracking susceptibility
Powder composition	Iron-based + 20-40 vol% WC	Controls hardness and wear resistance

Microstructure of the Cladding Layer

The microstructure of spherical WC reinforced iron-based PTA cladding layers is characterized by several distinctive features:

Spherical WC particles remain largely intact after the PTA process due to the short residence time in the melt pool and the high melting point of WC (2870 °C). The spherical morphology provides superior stress distribution compared to irregular angular particles, reducing the risk of interfacial debonding and particle pull-out during wear.
Iron-based matrix typically consists of martensite, retained austenite, and carbide precipitates. The martensitic matrix provides high hardness (typically 50-60 HRC), while the retained austenite contributes to toughness and crack resistance.
Interfacial bonding between WC particles and the iron matrix is critical for wear resistance. The spherical morphology minimizes stress concentration at the particle-matrix interface, which is a significant advantage over angular WC particles that create stress concentrations at sharp edges and corners.
Carbide network in the iron matrix includes M7C3 and M23C6 type carbides, which provide additional hardening and wear resistance.

Tribological Performance

The tribological testing typically involves pin-on-disk or block-on-ring configurations against alumina (Al2O3) or steel counterfaces under dry, lubricated, or erosive conditions. Key performance indicators include:

Test Condition	Wear Rate (mg/N·m)	Friction Coefficient	Dominant Wear Mechanism
Dry sliding vs. Al2O3	0.5-2.0	0.4-0.6	Abrasive wear
Lubricated sliding vs. steel	0.1-0.5	0.2-0.4	Adhesive-abrasive wear
Erosion-corrosion (seawater)	0.3-1.5	N/A	Erosion-corrosion
Three-body abrasion (sand)	0.8-3.0	N/A	Abrasive wear

The spherical WC particles significantly reduce the wear rate compared to unmodified iron-based cladding layers, with improvements of 50-80 percent depending on the testing conditions. The spherical morphology is particularly advantageous in erosion-corrosion environments where particle pull-out would create corrosion pits that accelerate material loss.

Engineering Application in Marine Environment

The marine application context of this research is highly relevant to shipbuilding and offshore engineering. Key application areas include:

Ship propeller trailing edges: Subject to cavitation erosion and erosion-corrosion in seawater
Thruster nozzles: Exposed to high-velocity water flow with entrained abrasive particles
Submarine hull components: Require corrosion resistance and erosion resistance in deep-sea environments
Ballast water system components: Subject to corrosion and erosion from marine biocides and sediments

The spherical WC reinforced iron-based composite cladding layers offer a cost-effective alternative to solid tungsten carbide or nickel-based alloy overlays for marine applications where moderate to high erosion-corrosion resistance is required at a lower cost than nickel-based solutions.

Process Optimization for Spherical WC Stability

Maintaining the spherical morphology of WC particles during PTA cladding is critical. The following factors influence particle integrity:

Residence time in melt pool: Must be minimized to prevent partial dissolution of WC. Optimal travel speeds of 150-200 mm/min with moderate arc currents of 200-250 A achieve this balance.
Thermal cycling effects: Multiple passes create thermal cycling that can cause cracking in WC particles or at the particle-matrix interface. Interpass temperature control at 150-200 °C and careful welding sequence planning are essential.
Powder particle size distribution: Spherical WC particles typically range from 15-75 micrometers. A bimodal distribution with a peak at 30-50 micrometers provides optimal packing density and mechanical properties.
WC content optimization: While higher WC content increases hardness, excessive amounts (above 40 vol%) can lead to particle agglomeration, reduced toughness, and increased cracking susceptibility. The optimal range is typically 20-35 vol%.

Defect Analysis and Countermeasures

Defect Type	Root Cause	Impact on Performance	Countermeasure
WC particle cracking	Excessive heat input, thermal cycling	Reduced wear resistance, particle pull-out	Reduce heat input, control interpass temperature
Particle-matrix debonding	Weak interfacial bonding, residual stress	Particle pull-out during wear	Optimize powder composition, post-weld tempering
Cracking in overlay	High carbon equivalent, rapid cooling	Structural integrity failure	Preheat, temper after welding
Excessive dilution	Low travel speed, high arc current	Reduced hardness, WC particle dissolution	Increase travel speed, reduce arc current
Porosity	Inadequate shielding, powder contamination	Reduced mechanical properties	Improve shielding gas flow, clean powder
Unmelted WC particles	Insufficient heat input	Reduced bonding, potential pull-out	Increase arc current, reduce travel speed

Key Technical Insights and Reflections

The most significant contribution of this research is the demonstration that spherical WC morphology provides a distinct advantage over conventional angular WC particles in composite cladding applications. The spherical shape reduces stress concentration factors at the particle-matrix interface by a factor of 2-3 compared to angular particles of equivalent size. This translates directly into improved resistance to particle pull-out during abrasive and erosive wear, which is the primary failure mode in composite cladding layers.

Another important insight is the interplay between the iron-based matrix properties and the WC particle reinforcement. The martensitic matrix provides the necessary hardness and toughness for the binder phase, while the spherical WC particles provide the primary wear resistance. This synergy is most effective when the matrix hardness is in the range of 50-60 HRC, providing sufficient support for the WC particles without being so hard that it becomes brittle.

The marine application perspective adds another dimension to the analysis. In erosion-corrosion environments, the spherical WC particles not only resist mechanical erosion but also reduce the risk of corrosion initiation at particle-matrix interfaces. Angular WC particles create stress concentrations that can initiate micro-cracks, which then serve as corrosion initiation sites. The spherical morphology eliminates this vulnerability, making it particularly suitable for seawater applications.

Study Insights and Implications

This research provides a clear pathway for developing cost-effective composite cladding solutions for marine and offshore applications. The combination of iron-based matrix with spherical WC reinforcement offers a compelling alternative to expensive nickel-based alloy overlays for applications where moderate to high erosion-corrosion resistance is required. The PTA process parameters identified in this study provide a practical starting point for industrial implementation.

For engineers working in surface engineering and tribology, the key takeaway is that particle morphology is as important as particle composition in determining the tribological performance of composite cladding layers. The systematic approach to optimizing spherical WC content, matrix composition, and PTA process parameters provides a framework for developing tailored solutions for specific marine application requirements. The collaborative research approach between academic institutions and industry, supported by targeted funding programs, demonstrates the effectiveness of focused research in addressing real engineering challenges in the marine sector.