Slurry Wear Performance of Tungsten Carbide Composite Wear-Resistant Cladding Layers

Literature Overview

This study by Du Xueming, Shi Yuxiang, and Li Ainong from Wuhan University of Technology and Wuhan University, published in the Journal of Wuhan University of Technology (Transportation Science and Engineering) in 2002, investigates the abrasive wear performance of tungsten carbide (WC) composite cladding layers under slurry abrasion conditions. The research was funded under the "Ninth Five-Year" National Key Science and Technology Program (95-02-07-08-01). The work is particularly relevant to engineers in the mining, mineral processing, pulp and paper, and wastewater treatment industries, where slurry erosion is a dominant failure mode for equipment components.

Core Technical Viewpoints

The study addresses a critical practical problem: the wear behavior of WC-based composite overlay layers in slurry conditions differs fundamentally from that in dry solid abrasion. In slurry wear, the presence of water or other liquids changes the wear mechanism from primarily mechanical ploughing to a combination of mechanical erosion, hydrodynamic impacts, and chemical interactions. Understanding these mechanisms is essential for selecting appropriate overlay compositions and process parameters for slurry service.

Slurry Wear Mechanisms

Slurry abrasion involves three primary mechanisms that operate simultaneously:

Mechanical erosion: Solid particles in the slurry impact and plough the overlay surface, removing material through cutting and micro-fracture. The severity depends on particle hardness, size, concentration, and impact velocity.
Hydrodynamic impact: The liquid medium transmits impact energy from particles to the surface, and the fluid dynamics of the slurry flow create additional erosive forces.
Chemical and electrochemical interactions: The liquid medium can dissolve or corrode the binder phase of the composite overlay, weakening the bond between WC particles and the matrix.

The composite overlay layer studied typically consists of WC particles (5–50 μm) dispersed in a nickel-based or cobalt-based binder matrix, deposited by hardfacing processes such as FCAW, oxy-fuel, or thermal spray. The WC particles provide the primary wear resistance through their extreme hardness (HV 2400–2800), while the binder matrix provides toughness and adhesion.

Wear Test Methodology and Results

The slurry wear tests typically employ a pin-on-disc or slurry pot test configuration, where the overlay specimen is exposed to a circulating slurry containing abrasive particles (typically silica, alumina, or natural sand) suspended in water. The test parameters include:

Parameter	Typical Range	Effect on Wear Rate
Particle size	25–250 μm	Larger particles increase wear rate
Particle concentration	5–50 wt%	Higher concentration increases wear rate
Particle hardness	SiO₂ (HV 1000–1200) to Al₂O₃ (HV 1500–1800)	Harder particles increase wear rate
Slurry velocity	1–10 m/s	Higher velocity increases wear rate
Test temperature	20–80°C	Higher temperature may increase chemical wear
Slurry pH	4–10	Extreme pH increases chemical degradation

The study demonstrates that WC composite overlay layers exhibit significantly lower wear rates than conventional hardfacing alloys (such as Cr-C-Mo martensitic or Ni-Cr-C composite) under slurry conditions. The wear resistance improvement factor is typically 3–8 times compared to standard hardfacing alloys, depending on the slurry severity.

Microstructural Factors Influencing Slurry Wear

The wear performance of the WC composite overlay is governed by several microstructural factors:

WC particle size and distribution: Uniform distribution of WC particles is critical. Agglomeration of WC particles creates weak zones where particles can be pulled out during abrasion. Optimal WC particle sizes for slurry wear are in the range of 10–30 μm, balancing hardness contribution against the risk of particle pullout.
Binder matrix composition: Ni-based binders (such as Ni-Cr-B-Si) generally provide better corrosion resistance in aqueous environments than Co-based binders, making them more suitable for slurry service. The binder hardness should be in the range of HV 400–600 to provide adequate support for the WC particles without being so soft as to be rapidly eroded.
Bond line quality: The interface between the overlay layer and the base metal is a critical weak point. Poor bond quality leads to overlay spallation, which dramatically reduces the effective wear life. The dilution at the bond line should be controlled to less than 30% to maintain the overlay's wear properties.
Cracking in the overlay: Microcracking in the WC composite overlay, particularly at the WC-binder interfaces, provides paths for slurry ingress and accelerates wear. Process parameters must be optimized to minimize cracking while maintaining adequate hardness.

Engineering Applications and Case Studies

The findings of this study have direct applications in several industrial sectors:

Mining and mineral processing: Slurry pumps, sand pumps, and hydrocyclones are subject to severe slurry abrasion. WC composite overlay on impeller surfaces, wear rings, and cyclone liners can extend service life by 5–10 times compared to uncoated carbon steel.
Pulp and paper industry: Pulp digesters, screen plates, and pulp pumps experience continuous slurry abrasion from wood fibers and chemicals. WC overlay on these components reduces maintenance frequency and unplanned downtime.
Wastewater treatment: Sludge pumps and grit removal equipment in wastewater treatment plants benefit from WC overlay on impeller blades and wear surfaces.
Hydropower: Penstock gates, turbine runner surfaces, and sediment-laden water passages experience slurry erosion. WC overlay extends the maintenance interval for these critical components.

Defect Analysis and Countermeasures

Defect Type	Cause	Countermeasure
WC particle pullout	Insufficient binder hardness, poor particle-matrix bonding	Optimize binder composition; ensure adequate wetting
Overlay spallation	Poor bond line quality, high residual stress	Control preheat temperature; optimize interpass temperature
Microcracking	Thermal stress from high cooling rate, brittle matrix	Reduce cooling rate; add ductilizing elements
Porosity	Gas entrapment during solidification	Improve shielding; optimize process parameters
Excessive dilution	High heat input, thin overlay layers	Reduce current; increase wire feed rate; use multiple thin passes

The defect prevention strategy follows a systematic FMEA approach: identify potential failure modes, assess severity and occurrence, and implement design and process controls to reduce risk. For slurry wear applications, the most critical defects are overlay spallation and WC particle pullout, as these directly compromise the wear protection function.

Study Insights and Implications

The research by Du and colleagues provides valuable guidance for engineers selecting and specifying WC composite overlay layers for slurry service. The key insight is that slurry wear is a multi-mechanism phenomenon, and the overlay design must address all contributing mechanisms simultaneously. A WC composite overlay that performs excellently in dry abrasion tests may fail prematurely in slurry service if the binder matrix is susceptible to corrosion or if the bond line is poorly characterized.

For engineering practice, the study reinforces the importance of matching the overlay composition to the specific slurry environment. A Ni-based WC composite is generally preferred for neutral to slightly acidic slurries, while a Co-based WC composite may be more appropriate for high-temperature or oxidizing environments. The overlay thickness should be specified with sufficient margin to accommodate the expected wear rate over the design service life, typically 3–5 mm for severe slurry service and 1.5–3 mm for moderate service.

The study also highlights the importance of process control in producing high-quality WC composite overlays. The dilution rate, cooling rate, and interpass temperature must be carefully controlled to produce a crack-free, pore-free overlay with uniform WC particle distribution. This is particularly challenging for FCAW overlay, where the high heat input tends to promote WC dissolution and coarsening. For critical applications, plasma transferred arc (PTA) cladding or laser cladding may be preferred, as these processes offer better control over dilution and microstructure.