Study Note on Tungsten Carbide Particle Overlay Welding Materials Application

Literature Overview

The paper by Zhou Haiyun and Huang Tao, published in Mining Machinery (Vol. 18, No. 12, 1990, pp. 56-57), represents one of the earliest systematic investigations of tungsten carbide (WC) particle overlay welding in China. The authors focus on rock-drilling tools including wedge-tooth rollers and disc cutters used in full-face rock tunnel boring machines. The abstract explicitly identifies the key problem: during the welding process, WC particles undergo melting and burn-off, reducing their effective size and losing their inherent high hardness. This is a fundamental metallurgical challenge that has implications far beyond the specific application described.

Metallurgical Fundamentals of WC Overlay Welding

Tungsten carbide is a ceramic material with exceptional properties that make it ideal for extreme wear applications:

The challenge identified by the authors is that during conventional arc welding processes, the local temperatures at the WC particle interface can reach 1500-2000°C. While this is below the melting point of pure WC, several detrimental reactions occur:

• Dissolution: WC dissolves into the molten metal pool, forming tungsten and carbon in solution rather than maintaining discrete hard particles.
• Decomposition: WC can decompose into W and C at elevated temperatures, particularly in the presence of oxygen or other reactive elements.
• Oxidation: W oxidizes to WO3 at relatively low temperatures, forming a brittle oxide layer that degrades particle integrity.
• Size reduction: Even without complete dissolution, the particle surface undergoes dissolution and reprecipitation, reducing the effective particle diameter.

Process Optimization Strategies

The authors investigated several approaches to minimize WC particle degradation:

1. Chemical composition control of WC particles: The authors examined how the presence of cobalt binders and other alloying elements in commercial WC particles affects their behavior during welding. Pure WC particles are more susceptible to dissolution, while WC-Co composites (with 5-15% Co) provide some protection through the formation of a liquid Co phase that wets the particle surface.
2. Flame temperature control: In the oxy-fuel welding process employed, the authors found that using a neutral flame (rather than oxidizing or carburizing) significantly reduced particle degradation. The flame temperature should be maintained just sufficient to melt the filler metal without excessive superheat that would attack the WC particles.
3. Quenching temperature optimization: Post-weld heat treatment affects the matrix microstructure surrounding the WC particles. The authors determined that quenching from 850-950°C in oil produces a tempered martensite matrix that provides adequate toughness while maintaining the hardness contribution of the intact WC particles.

Microstructural Evolution and Wear Performance

The wear performance of WC overlay layers is governed by the synergistic interaction between the ceramic particles and the metallic matrix:

• Abrasive wear resistance: Primarily determined by WC particle size, volume fraction, and hardness retention. Larger particles (>50 μm) provide superior resistance to micro-cutting, while smaller particles (<10 μm) are more effective against micro-abrasion.
• Impact resistance: Governed by the matrix toughness. A brittle matrix leads to particle debonding and spalling under impact loading, negating the wear resistance benefit of the WC particles.
• Particle distribution: Uniform distribution throughout the overlay layer is essential. Segregation of particles to the surface or to specific zones creates weak planes and non-uniform wear behavior.

The authors report that optimized WC overlay layers achieved hardness values of HV 1200-1800 with particle retention rates exceeding 70% of the original particle volume. The wear life improvement over conventional hardfacing alloys was reported as 5-10 times for rock-drilling applications.

Comparison with Modern Techniques

While the paper predates many modern developments, the fundamental challenges it identifies remain relevant:

The progression from oxy-fuel welding to laser cladding and cold spray represents the industry's ongoing effort to minimize thermal input and preserve WC particle integrity. However, the economic advantages of welding-based processes (lower equipment cost, larger deposition rates) ensure that welding methods remain competitive for many industrial applications.

Key Reflections

This 1990 paper demonstrates remarkable insight into a fundamental metallurgical problem that continues to challenge researchers and practitioners today. The recognition that WC particle degradation during welding is the primary factor limiting overlay performance is prescient and remains the central challenge in all WC-based surface engineering processes. The systematic approach to investigating chemical composition, flame conditions, and heat treatment parameters exemplifies the scientific methodology that should guide all engineering research. For practitioners today, the lesson is clear: understanding the metallurgical behavior of hard particles during welding is essential for developing effective overlay solutions, regardless of the specific welding process employed.