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22 March 2019 | Hardfacing

What you need to know about hardfacing with tungsten carbides

Hardfacing deposits made from tungsten carbides are composite materials consisting of a metal alloy matrix reinforced with tungsten carbide particles. These deposits can contain up to 85% carbide, with matrices made of steel, nickel alloys (Ni-B-Si), copper alloys (Cu-Zn-Ni), or cobalt alloys (Co-Cr-W or Mo).

Steel matrices provide significant hardening but are more prone to cracking due to partial dissolution of the carbides during exposure to the molten bath. In contrast, non-ferrous alloy matrices are less likely to crack and offer better wetting capabilities for the carbides, along with superior corrosion resistance.

Copper, nickel, and cobalt alloys melt at lower temperatures than steel-based alloys: Cu-Zn-Ni (750-950°C / 1400-1750°F), Ni-B-Si (1050°C / 2100°F), and Co-Cr-W or Mo (1285-1435°C / 2340-2615°F). Choosing these matrix types reduces particle dissolution and minimizes distortion of the base metal. Additionally, the Ni-Si-B alloy offers advantages due to its self-fluxing properties and high resistance to chemical attacks.

Tungsten carbide hardfacing provides superior abrasion resistance compared to chromium carbides, resulting in improved performance and longevity. This makes them particularly valuable for parts and equipment subjected to severe abrasion, such as drilling equipment, tunnel boring machines, augers in brickworks, mixer blades, foundry scrapers, scraping blades, dredging equipment, and pump impellers.

Properties of Tungsten Particles

Tungsten particles are characterized by:

  • High density: 15 g/cm³ (twice that of steel).
  • Oxidation resistance: up to 500°C (1000°F).
  • Stability in acidic and basic environments.

Tungsten particles often exist as eutectic mixtures (polycrystalline) of WC/W₂C. WC particles (HV 2000-3000) are harder than W₂C particles (HV 1200-2300). Sometimes they appear in monocrystalline form, which is slightly less hard but richer in carbon and more stable.

Unlike chromium carbides, which form through the reaction of carbon and chromium during the solidification of the molten bath, tungsten carbides are already present as carbides within tubular electrodes (SMAW), in the flux of flux-cored wires (FCAW), or incorporated into the matrices of oxy-fuel welding (OFW) rods.

During welding, tungsten particles blend into the molten bath of the filler metal and bond with the base metal during solidification. The abrasive wear resistance is also dependent on the preservation of the carbides in the molten bath.

Factors Influencing Wear Resistance

Several factors influence the wear resistance and integrity of tungsten carbide hardfacing deposits:

  1. Ductility of the matrix: This allows for multi-pass deposits with fewer cracks. Copper-based matrices are the most ductile, followed by nickel, cobalt, and finally iron-based matrices.
  2. Amount of carbides: The hardness of the deposit increases with the number of layers deposited.
  3. Particle morphology: Spherical carbides have a lower tendency to dissolve compared to angular carbides, improving wear resistance.
  4. Welding energy and interaction time: These must be optimized to reduce particle dissolution. Abrasion tests according to ASTM G65 have shown that deposits from OFW provide the best resistance, followed by TIG, SMAW, and FCAW.
  5. Matrix reactions: Nickel, cobalt, and copper matrices do not react with tungsten carbides, unlike steel matrices, where dissolution can occur, especially with WC/W₂C and angular particles. Dissolution in steel matrices leads to hardening through quenching as carbon (C) and tungsten content increases, transforming the matrix into a tool steel type.

Due to their high density, segregation occurs, with larger particles settling at the bottom of the deposit while smaller particles may remain near the surface. Having particles of different sizes and limiting solidification time helps reduce segregation.

To discover the best tungsten carbide hardfacing solution tailored to your specific needs, don’t hesitate to contact us. Our experts are available to advise you and optimize the performance of your abrasion-resistant equipment.