Preparation and Wear Resistance Analysis of Cobalt-Based Tungsten Carbide Overlay Cladding

Literature Overview and Technical Context

This paper investigates the preparation methodology and tribological performance of cobalt-based tungsten carbide (Co-WC) overlay cladding layers, which are widely employed in applications demanding extreme wear resistance under abrasive and erosive conditions. Cobalt-based hardfacing alloys, such as Stellite 6 and Stellite 21, combined with WC particles, represent a well-established solution in the surface engineering field for components subjected to severe mechanical wear, including mining equipment, cement mill liners, valve seats, and pressure vessel internals in slurry service.

The research addresses the critical challenge of optimizing the microstructure of Co-WC overlay layers to maximize the synergistic effect between the tough cobalt-based matrix and the extremely hard WC particles (Vickers hardness approximately 2400 HV). The key technical challenge lies in achieving complete wetting and bonding of the WC particles by the molten cobalt matrix while avoiding excessive dissolution of the carbide particles, which would reduce hardness and wear resistance.

Overlay Preparation Methodology and Process Parameters

The study examines multiple deposition methods for Co-WC overlay cladding, with particular emphasis on submerged arc welding (SAW) overlay and gas metal arc welding (GMAW) overlay using wire-powder composite consumables. The following table summarizes the process parameters and their effects:

Process Parameter	SAW Overlay	GMAW Overlay	Effect on Microstructure
Welding Current	350–550 A	150–250 A	Higher current increases WC dissolution
Travel Speed	0.15–0.30 m/min	0.10–0.25 m/min	Lower speed promotes WC wetting
Preheat Temperature	150–250 °C	100–200 °C	Reduces cracking susceptibility
Interpass Temperature	150–250 °C	100–200 °C	Controls cooling rate and phase formation
Wire-Powder Ratio	60:40 to 70:30	50:50 to 60:40	Higher wire ratio increases matrix toughness
WC Particle Size	25–75 μm	25–53 μm	Smaller particles improve distribution uniformity
Shielding Flux	Low-hydrogen basic	Argon/CO₂ mixture	Controls atmosphere and inclusion content

The composite wire-powder consumable design is critical to the process. The cobalt-based wire serves as the matrix material, while the WC powder is fed separately through a powder feeder positioned to deliver particles into the arc zone. The powder delivery rate and position relative to the arc must be precisely controlled to ensure uniform incorporation of WC particles throughout the overlay thickness.

Microstructural Characteristics and Phase Analysis

Metallographic examination of the Co-WC overlay layers reveals a microstructure consisting of a dendritic Co-Cr solid solution matrix with dispersed WC particles and secondary carbide phases. The primary phases identified include:

FCC Co-Cr solid solution matrix (γ-Co)
Undissolved WC particles (hexagonal structure)
Cr₇C₃ and Co₃W₃C carbide phases formed by WC dissolution
M₆C type carbides (M = Co, Cr, W) in interdendritic regions
M₂₃C₆ type carbides at grain boundaries

The degree of WC dissolution is strongly influenced by the thermal cycle during welding. Higher heat inputs and longer residence times at elevated temperatures promote greater WC dissolution, leading to the formation of hard secondary carbides but potentially reducing the overall hardness if excessive dissolution occurs. The optimal balance is achieved when approximately 20–40% of the WC particles remain undissolved, providing a hard phase reinforcement while maintaining adequate matrix toughness.

The overlay-substrate interface shows a diffusion zone of 100–300 μm where cobalt and chromium atoms diffuse into the substrate and iron atoms diffuse into the overlay. This interdiffusion zone is critical for bond strength and crack resistance. The research demonstrates that controlled preheating and interpass temperature management can minimize the formation of brittle martensitic phases in the dilution zone, which is particularly important when overlaying cobalt-based alloys on carbon steel substrates.

Wear Resistance Performance and Mechanism Analysis

The wear resistance of Co-WC overlay layers is evaluated through pin-on-disk abrasion testing, dry sand rubber wheel abrasion testing, and slurry erosion testing. The following table presents typical performance data:

Test Method	Co-WC Overlay	Hardened Steel (50 HRC)	Uncladded Steel	Relative Wear Rate
Pin-on-disk (SiC paper)	0.08–0.12 mm³/N·m	0.35–0.50 mm³/N·m	0.80–1.20 mm³/N·m	1.0–1.5×
Rubber wheel (dry sand)	0.15–0.25 mm³/1000 m	0.80–1.20 mm³/1000 m	2.00–3.00 mm³/1000 m	1.0–1.8×
Slurry erosion (SiC)	0.02–0.05 mg/cm²	0.15–0.30 mg/cm²	0.50–1.00 mg/cm²	1.0–2.0×

The wear resistance mechanism in Co-WC overlays is multifaceted. The hard WC particles (2400 HV) and secondary carbides (1500–2000 HV) provide resistance to abrasive wear through micro-ploughing and micro-cutting resistance. The tough cobalt matrix (200–300 HV) provides resistance to crack propagation and delamination, absorbing energy during impact and preventing catastrophic failure. The synergistic effect between the hard and tough phases results in wear rates 5–10 times lower than conventional hardened steels under severe abrasive conditions.

The erosion resistance performance is particularly important for pressure vessel applications involving slurry service. The cobalt matrix's ability to deform plastically without cracking, combined with the hard carbide particles that resist cutting by abrasive particles, provides excellent protection against erosive wear. The research demonstrates that overlay layers with 30–50 vol% WC content exhibit optimal erosion resistance, as this composition provides sufficient hard phase reinforcement while maintaining adequate matrix ductility.

Engineering Practice Applications

In pressure vessel fabrication, Co-WC overlay cladding is commonly specified for components exposed to abrasive slurry service, including cyclone internals, filter housings, slurry pumps, and heat exchanger tubes in mining and mineral processing applications. The overlay is typically applied to carbon steel or low-alloy steel substrates to provide a wear-resistant surface while maintaining the structural integrity and cost-effectiveness of the base material.

For hydrogenation reactor applications, where catalyst particles can cause erosive wear on vessel internals, Co-WC overlay provides an effective solution. The overlay thickness is typically specified at 2–5 mm, applied using SAW overlay with composite wire-powder consumables. The qualification of Co-WC overlay welds under NB/T 47014 requires demonstration of adequate bond strength (minimum 250 MPa shear strength), acceptable hardness profile (minimum 50 HRC for the overlay layer), and satisfactory macrographic examination showing no cracks or incomplete fusion at the overlay-substrate interface.

Common Defects and Countermeasures

The following table summarizes common defects encountered in Co-WC overlay fabrication and their countermeasures:

Defect Type	Cause	Countermeasure
Cracking	High carbon equivalent, excessive cooling rate	Increase preheat, control interpass temperature, use low-carbon filler
Porosity	Inadequate shielding, moisture in flux	Improve shielding coverage, dry flux thoroughly
Incomplete Fusion	Excessive travel speed, insufficient current	Increase current, reduce travel speed, improve surface preparation
Excessive WC Dissolution	High heat input, prolonged high-temperature exposure	Reduce current, increase travel speed, use smaller WC particles
Delamination	Poor substrate preparation, hydrogen contamination	Thorough surface cleaning, control hydrogen content in shielding gas

Study Insights and Conclusions

This research provides comprehensive technical guidance for the engineering application of Co-WC overlay cladding in wear-critical applications. The fundamental insight is that optimal wear performance requires careful balancing of the hard phase content and matrix toughness, which is achieved through precise control of process parameters and consumable composition. For engineers specifying Co-WC overlay in pressure vessel fabrication, the key considerations include the selection of appropriate wire-powder ratio, control of thermal cycles to minimize WC dissolution while ensuring complete wetting, and rigorous quality assurance through hardness profiling, macrographic examination, and bond strength testing. The technology remains a mature and reliable solution for abrasive wear protection, with ongoing development in consumable design and process control continuing to improve performance and reduce manufacturing costs.