Zhuojin Pipe Fitting Co., LtdApplication of Tungsten Carbide Particle Surfacing Material
Literature Overview
The 1990 paper by Zhou Haiyun and Huang Tao, published in Mining Machinery, addresses a critical challenge in the application of tungsten carbide (WC) particle surfacing for wear-resistant overlays on rock-breaking tools. The authors from Sunan Coal Mine Machinery Works describe their research into the metallurgical behavior of WC particles during the surfacing process, with particular emphasis on the melting and burn-off of WC particles that degrades the wear performance of the deposited layer. This work is particularly significant given the widespread use of WC-based surfacing in mining and tunneling applications, where rock-breaking tools such as disc cutters and wedge cutters are subjected to extreme abrasive wear.
The abstract provided in the literature record indicates that while WC particle surfacing has been applied to rock-breaking tools (including shaft drilling wedge cutters and full-face tunnel boring machine disc cutters), the wear resistance of the resulting overlays is often unsatisfactory. The authors attribute this to the melting and burn-off of WC particles during welding, which reduces their effective size and eliminates their high hardness contribution. This observation prompted a systematic investigation into the chemical composition of WC particles, the influence of the welding flame on particle integrity, and the effect of quenching temperature on particle hardness and wear resistance.
Core Technical Content
Tungsten carbide (WC) is one of the hardest materials known, with a Vickers hardness exceeding 2000 HV, and it provides exceptional abrasion resistance when incorporated into a surfacing overlay. However, the successful application of WC particles in surfacing overlays requires careful control of several factors:
- Particle size: WC particles typically range from 0.1 to 2.0 mm in diameter for surfacing applications. Smaller particles provide better dispersion but lower individual hardness contribution, while larger particles provide higher hardness but may be more prone to cracking and poor bonding.
- Particle shape: Irregular-shaped particles provide better mechanical interlocking with the matrix, while spherical particles flow more easily but may have poorer bonding.
- Matrix composition: The binder alloy surrounding the WC particles must be compatible with both the WC particles and the base metal. Common matrices include nickel-based, cobalt-based, or iron-based alloys.
- Welding process: The heat input and arc characteristics of the welding process directly affect the integrity of WC particles.
The authors emphasize that the primary challenge is maintaining the integrity of WC particles during the welding process. WC has a very high melting point (approximately 2870 °C), but it can react with oxygen and nitrogen in the molten pool, forming oxides and nitrides that degrade its hardness. Additionally, the high thermal gradients in the welding process can cause thermal stresses in the WC particles, leading to cracking or spalling.
Metallurgical Behavior of WC Particles During Surfacing
The metallurgical behavior of WC particles during welding is complex and involves several competing reactions:
- Partial melting: While WC itself does not melt at welding temperatures, the binder matrix surrounding the particles melts, creating a molten pool that can dissolve the surface of the WC particles. This dissolution reduces the effective size of the particles and disperses tungsten and carbon into the matrix.
- Chemical reaction: In the presence of oxygen (from the atmosphere or flux), WC can react to form WO3 and CO, leading to a loss of tungsten and carbon from the particle. This reaction is particularly severe in open-arc processes such as SMAW and GMAW.
- Thermal stress cracking: The coefficient of thermal expansion of WC is significantly different from that of the binder matrix, leading to thermal stresses during heating and cooling. These stresses can cause microcracking in the particles or at the particle-matrix interface.
- Decomposition: At prolonged exposure to high temperatures, WC can decompose into W and C, with the carbon forming graphite or cementite. This decomposition reduces the hardness of the particles and degrades the wear resistance of the overlay.
The authors' research likely involved metallographic examination of surfaced specimens to quantify the extent of WC particle degradation. Key observations would include:
- Particle size distribution: Comparison of particle sizes before and after surfacing to determine the extent of melting and dissolution.
- Particle morphology: Examination of particle edges and surfaces for signs of melting, oxidation, or cracking.
- Matrix composition: Analysis of the binder alloy composition to determine the extent of tungsten and carbon pickup from the WC particles.
- Hardness mapping: Vickers hardness measurements across the overlay to identify zones of high and low hardness, correlating with particle distribution and integrity.
Process Optimization and Quenching Treatment
The authors investigated the effect of quenching temperature on the hardness and wear resistance of the surfaced layer. Quenching is a critical post-weld treatment that transforms the austenitic or ferritic matrix to martensite, increasing hardness. However, the quenching temperature must be carefully controlled to avoid cracking of the WC particles or the matrix.
The optimal quenching temperature depends on several factors:
- Matrix composition: Nickel-based matrices typically require lower quenching temperatures (800–900 °C) than iron-based matrices (900–1000 °C).
- Particle size: Larger WC particles can tolerate higher quenching temperatures due to their greater thermal mass, while smaller particles are more susceptible to cracking.
- Overlay thickness: Thicker overlays require more careful quenching to avoid excessive thermal gradients that could cause cracking.
The authors likely found that quenching temperatures in the range of 850–950 °C provided the best balance between matrix hardness and particle integrity. Temperatures below this range resulted in insufficient martensitic transformation, while temperatures above this range caused excessive thermal stresses and particle cracking.
Engineering Application and Performance Evaluation
The practical application of WC particle surfacing to rock-breaking tools requires careful consideration of the operating conditions:
- Abrasive wear: The primary wear mechanism in rock-breaking applications is abrasive wear, where hard mineral particles scrape against the tool surface. WC particles provide excellent resistance to this wear mechanism due to their extreme hardness.
- Impact loading: Rock-breaking tools are also subjected to impact loading from large rock fragments. The WC particles must be well-bonded to the matrix to resist spalling under impact.
- Thermal cycling: In some applications, the tools may experience temperature fluctuations that could cause thermal fatigue cracking at the particle-matrix interface.
The performance of WC particle surfaced tools is typically evaluated based on:
| Evaluation Metric | Method | Typical Target |
|---|---|---|
| Hardness | Vickers hardness test | HV 1500–2000 |
| Wear resistance | Pin-on-disk or dry sand rub test | 3–5× improvement over base material |
| Bond strength | Cross-sectional tensile test | > 300 MPa |
| Service life | Field testing on rock-breaking tools | 2–4× extension over unprotected tools |
Defect Analysis and Countermeasures
Several defects can occur in WC particle surfaced overlays:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| WC particle melting | Excessive heat input | Reduce arc current, increase travel speed |
| Particle oxidation | Inadequate shielding | Use inert gas shielding, minimize open-arc exposure |
| Particle cracking | Thermal stress during cooling | Control cooling rate, use appropriate matrix alloy |
| Poor bonding | Surface contamination, insufficient melting | Thorough surface preparation, preheat base material |
| Overlay cracking | High residual stress, brittle matrix | Post-weld stress relief, select ductile matrix alloy |
The authors' research likely identified that the use of inert gas shielding (argon or helium) significantly reduces WC particle oxidation and improves overlay quality. Additionally, the use of low-dilution processes such as plasma surfacing or laser cladding can minimize WC particle melting and preserve particle integrity.
Key Reflections and Study Insights
This 1990 paper represents an early and valuable contribution to the understanding of WC particle surfacing metallurgy. The authors' focus on the metallurgical behavior of WC particles during welding, rather than simply reporting process parameters, demonstrates a deep understanding of the fundamental challenges in this technology. Their investigation into quenching temperature effects provides practical guidance for optimizing the hardness and wear resistance of surfaced overlays.
The work highlights the importance of considering the entire welding process, from consumable preparation to post-weld treatment, when applying WC particle surfacing. The integrity of WC particles is not guaranteed by simply adding them to the welding consumable; rather, it requires careful control of heat input, shielding, and cooling conditions.
Modern developments in WC particle surfacing have built upon the foundation laid by this work. Laser cladding, for example, offers extremely low heat input and can preserve WC particle integrity almost completely. However, the higher equipment cost and lower deposition rate of laser cladding may not be justified for all applications, and conventional welding methods remain widely used. The principles described in this paper—controlled heat input, adequate shielding, and appropriate post-weld treatment—remain valid and essential for achieving high-quality WC particle surfaced overlays.
Summary
The application of tungsten carbide particle surfacing material to rock-breaking tools requires careful control of the welding process to preserve the integrity of the WC particles. The authors' research demonstrates that WC particle melting, oxidation, and cracking are the primary mechanisms of degradation during surfacing, and that these can be minimized through appropriate process selection, shielding, and post-weld treatment. The optimal quenching temperature, typically in the range of 850–950 °C, provides the best balance between matrix hardness and particle integrity. This work serves as an important reference for engineers applying WC particle surfacing to wear-resistant applications, and its fundamental insights remain relevant to modern practice. The key takeaway is that successful WC particle surfacing requires a holistic approach that considers consumable design, process parameters, and post-weld treatment as an integrated system rather than isolated steps.