Grain Size (F.S.S.S.) — the most important variable
Grain size is typically expressed as Fisher Sub-Sieve Size (F.S.S.S.) in micrometres, measured by air permeability. It controls the final hardness and toughness of the sintered carbide in the opposite directions: finer powder produces harder, more wear-resistant carbide (higher hardness, lower toughness); coarser powder gives tougher carbide that resists impact and shock loads but at lower hardness.
As a practical guide: F.S.S.S. 0.6–1.5 µm (sub-micron/ultra-fine) is used for precision micro-grain grades targeting hardness ≥ 1,800 HV — used in PCB micro-drills, dental burs, and fine-blanking dies. F.S.S.S. 2–4 µm (fine) suits general-purpose cutting inserts and wood-working tooling with cobalt binders of 6–10%. F.S.S.S. 4–8 µm (medium) is used for road milling picks, mining drill button bits, and wear plates at 10–15% Co. F.S.S.S. 8–15 µm (coarse) goes into rock drill bits, oil-field PDC cutter substrates, and heavy wear components at 15–20% Co where impact resistance matters more than hardness.
Carbon Content — bonded vs. free
WC powder specification sheets list total carbon and sometimes separate bonded (combined) carbon from free carbon. What matters in practice:
Total carbon should be close to the stoichiometric WC value of 6.128% by weight. Most WC powder specifications require total carbon ≥ 6.11% (bonded) to ensure complete WC phase formation during sintering. Free carbon (graphite particles present in the powder) should be as low as possible — typically max. 0.03%. Free carbon causes porosity (A-type pores) in the sintered carbide, reducing transverse rupture strength. On the other side, if total carbon is too low, eta-phase (W₃Co₃C) forms during sintering — a brittle phase that makes the carbide crack.
When comparing WC powder from different sources, always request the carbon analysis table showing both total carbon and free carbon per lot, not just a nominal value.
Apparent Density
Apparent density (g/cc or g/cm³, measured by Hall flowmeter) affects how the powder fills the die cavity during pressing and its flowability on automated press lines. Typical ranges by grain size: sub-micron grades 1.5–2.5 g/cc; fine grades 2.4–4.0 g/cc; medium/coarse grades 4.0–6.0 g/cc.
High apparent density within a grade usually indicates good particle morphology — rounded, well-crystallised particles rather than irregular or agglomerated fragments from poor milling or insufficient reduction. For automated multi-cavity press systems, consistency of apparent density lot-to-lot is as important as the absolute value — variation in apparent density causes weight variation in pressed parts, leading to dimension scatter after sintering.
Oxygen Content and Other Impurities
Oxygen in WC powder (measured as total O, typically max. 0.07–0.15% depending on grain size) comes from surface oxide layers on WC particles. During sintering, CO gas evolved from oxide reduction can cause porosity. Fine grades have higher surface area and thus more surface oxide, so acceptable oxygen limits are typically higher for sub-micron grades and lower for coarser grades.
Key metallic impurities to watch: Fe (iron), Ni (nickel), Mo (molybdenum), Ti. Iron is particularly harmful — it dissolves in the cobalt binder during liquid-phase sintering and reduces transverse rupture strength at low ppm levels. Request a full ICP impurity analysis, not just W+C+O. Maximum Fe is typically 50–100 ppm for tooling grades.
Matching grade to application: a quick reference
Precision cutting inserts (ISO P/M/K grades, low Co): fine WC 2–4 µm F.S.S.S., total C ≥ 6.11%, free C max. 0.02%, O max. 0.10%.
Mining and rock drilling (high Co, impact-loaded): coarse WC 6–12 µm F.S.S.S., total C ≥ 6.10%, free C max. 0.03%, O max. 0.15%.
Wear parts, pump components, valve seats: medium WC 4–8 µm, total C ≥ 6.11%, free C max. 0.03%.
Sprayed carbide coatings (HVOF/plasma): spherical or irregular WC-Co composite powder — different specification than conventional WC; F.S.S.S. is less relevant than particle size distribution (PSD) by laser diffraction.
When requesting a quote, always specify F.S.S.S. target and tolerance, total carbon minimum, free carbon maximum, and the impurity elements you test on incoming inspection. This gives the supplier the information they need to match a lot from their production, or to manufacture to your specification.