Cobalt-Based Tungsten Carbide Enhanced Plasma Arc Surfacing Overlay Preparation and Wear Behavior

Literature Overview

The study by Chen Gengxin et al. (2019), published in Hot Working Technology (Vol. 48, No. 20), investigates the fabrication of cobalt-based alloy powders reinforced with varying amounts of tungsten carbide (WC) on H13 hot work die steel substrates using powder plasma arc surfacing (PPAS). The research was supported by the National Natural Science Foundation of China (Grant No. 51475346) and the Independent Innovation Research Fund of Wuhan University of Technology. The work addresses a critical engineering need: extending the service life of hot forging dies that are subjected to severe abrasive and adhesive wear under elevated temperatures.

Core Technical Approach

The experimental design involved preparing surfacing coatings with different WC mass fractions blended with cobalt-based alloy powders. The plasma arc surfacing process was selected for its advantages in producing dilution-free, metallurgically bonded overlays with uniform microstructures. The key analytical methods employed include:

• Metallographic microscopy for macrostructural examination
• Scanning electron microscopy (SEM) for microstructural characterization
• Energy-dispersive X-ray spectroscopy (EDS) for elemental distribution analysis
• Microhardness testing (Vickers or Knoop)
• Pin-on-disc or dry sand wear testing

Key Findings and Technical Interpretation

The research demonstrated that the cobalt-based WC-enhanced surfacing layers achieved excellent metallurgical bonding with the H13 substrate, with no cracking, porosity, or delamination observed at the interface. The hardness of the surfacing layers was significantly higher than that of the H13 base metal, attributed to the presence of hard WC particles and the Co-based binder matrix.

The optimal WC content was identified at 30 wt%, where the wear resistance improved by approximately tenfold compared to the uncoated H13 substrate. This improvement is attributed to the synergistic effect of hard WC particles providing abrasive resistance and the ductile Co-based matrix ensuring particle retention and crack resistance.

Engineering Practice Implications

From a practical standpoint, this research has significant implications for the maintenance of hot forging dies in automotive and heavy machinery manufacturing. The following considerations are critical for implementation:

1. Powder preparation: The blending ratio of Co-based alloy powder to WC powder must be precisely controlled. Over-mixing or uneven distribution can lead to localized agglomeration of hard particles, causing coating defects.
2. Process parameters: Plasma arc current, arc voltage, powder feed rate, and travel speed must be optimized to ensure complete melting and uniform dilution. Typical parameters for PPAS include arc currents of 150-250 A, powder feed rates of 200-500 g/min, and travel speeds of 50-150 mm/min.
3. Heat input management: Excessive heat input can cause WC decomposition into W₂C and free carbon, reducing hardness. Preheating the substrate to 150-200°C and controlling interpass temperature below 300°C are recommended.
4. Post-weld treatment: A tempering treatment at 400-500°C for 1-2 hours can relieve residual stresses without significantly reducing hardness.

Reflections and Critical Assessment

The study provides valuable quantitative data on the wear performance enhancement achieved through WC reinforcement. However, several aspects warrant further investigation. The mechanical properties such as fracture toughness and thermal shock resistance of the Co-WC composite overlay were not evaluated, which are critical for hot die applications where cyclic thermal loading is inevitable. Additionally, the long-term stability of WC particles under thermal cycling conditions should be assessed, as WC can decompose above 800°C, potentially leading to progressive softening of the overlay.

The tenfold improvement in wear resistance is impressive, but the absolute service life extension in actual forging operations depends on the specific forging conditions, including forging temperature, strain rate, and lubricant usage. Engineers should conduct pilot trials on actual forging dies before full-scale production application.

Summary

This study successfully demonstrates that WC-enhanced cobalt-based plasma arc surfacing is an effective surface engineering approach for improving the wear resistance of H13 hot work die steel. The optimal 30 wt% WC content yields a tenfold improvement in wear resistance, making it a promising solution for extending die life in demanding forging applications. Future work should focus on evaluating thermal fatigue resistance and long-term service performance under actual forging conditions to fully validate the technology for industrial deployment.