Tungsten carbide powder: what it is, how it is sized and where it goes

Tungsten carbide powder is tungsten metal powder reacted with carbon. ITIA, the tungsten industry association, says the metal powder is mixed with carbon black and held at 1,300 to 2,200 °C in hydrogen. Stoichiometric WC holds 6.13% carbon by weight, and a 2023 paper calls WC a line compound that barely strays from that ratio.

The tungsten gets there by a chain: most ore concentrate is processed into ammonium paratungstate (APT), the main tungsten raw material traded (USGS), then tungsten oxide, which hydrogen reduces to metal powder (ITIA). Most tungsten powder made goes on to become WC powder.

Cemented carbide is the solid material of inserts (small replaceable cutting tips) and end mills (rotating milling cutters): hard carbide grains, mainly WC, held in a tough metal binder such as cobalt, nickel or iron (ITIA).

Sources: ITIA: tungsten intermediates; Materials Science-Poland 41(1), 2023; USGS Open-File Report 2005-1028; ITIA Newsletter, cemented carbides.

WC powder is made coarse or fine on purpose. ITIA says its grain size follows the size of the tungsten powder it starts from and the carburising temperature: the hotter the furnace, the coarser the carbide. The tungsten powder itself can be made at any average size from 0.1 to 100 µm (thousandths of a millimetre) by changing the reduction conditions.

Beyond size, ITIA lists grain size, size distribution, apparent and tap density, and a carbon content matched to the grade (the recipe of grain size and binder) being made, and asks for "the complete absence of coarser grains in fine grained powders".

One standard lab figure is the Fisher Number. ASTM B330 uses air permeability to derive an average equivalent spherical diameter over 0.2 to 75 µm. Its maths assumes smooth, solid spheres of one size, so ASTM calls the result an average particle size or Fisher Number: not absolute, but useful on a relative basis for process control.

ISO 4499-2:2008 notes that words such as fine or submicron have no commonly agreed size ranges, and recommends these classes for WC grain size measured by the mean-linear-intercept method, not by Fisher Number:

Sources: ITIA: tungsten intermediates; ASTM B330-20 abstract; ISO 4499-2:2008 preview.

WC powder becomes a solid tool by powder metallurgy: milled with cobalt, pressed and sintered. NTP says the powders are milled in heptane or a similar solvent with paraffin or polyethylene glycol added, then dried and pressed to rough shape. Commercial cobalt grades generally carry 3% to 16% cobalt (NTP).

Sintering, the heat step that turns pressed powder into solid carbide, runs at 1,350 to 1,550 °C (NTP). ITIA puts the lateral shrinkage between 17 and 24% and residual porosity commonly below 0.02 vol%. The full chain is on how tungsten carbide is made.

Sources: NTP RoC background document, hard metals; ITIA Newsletter, cemented carbides.

Outside cemented carbide, WC powder becomes a wear surface on steel.

Thermal spray coatings

In high-velocity oxy-fuel (HVOF) spraying, powder is fed into a supersonic flame and the softened particles strike the part to build a dense, well-adhered coating (SERDP/ESTCP). In a 2021 study the commercial HVOF carbide powders were agglomerated and sintered, sized -45 +15 µm (between 15 and 45 µm), with a median of about 30 µm. The US Navy chose a WC-17Co HVOF coating as its alternative to electroplated hard chrome (NASA-archived report).

Hardfacing

Hardfacing is welding a hard, wear-resistant alloy onto a surface that wears (1943 Government Printing Office manual). On drill bits and chisels, tungsten hardfacing is sprayed, welded or infiltrated onto the steel (a 2021 recycling review), and a 2023 paper names WC-Co as its most common strengthening phase.

Diamond-tool and drill-bit matrices

USGS says coarse macrocrystalline WC goes into hardfacing and diamond tool matrices. PDC (polycrystalline diamond compact) drill bits stand on a body of coarse WC or cast carbide infiltrated with a copper alloy (the same 2021 review).

Sources: SERDP/ESTCP WP-200023; Materials (Basel) 2021, 15(1):40; NASA NTRS 20130009937; GPO welding manual, 1943; Int. J. Refractory Metals and Hard Materials 98 (2021); Materials Science-Poland 41(1), 2023; USGS Open-File Report 2005-1028.

Cast carbide and macrocrystalline WC are both hardfacing materials, but different materials. Cast (fused) carbide is a melted and quenched eutectic of WC and W₂C; macrocrystalline WC is WC alone, made straight from ore concentrate.

Cast carbide comes crushed or spherical. Crushing leaves many cracks in the grains, which considerably lowers their strength, and irregular particles flow poorly (2010 paper). Spherical cast carbide is made by spraying; that paper reports its microhardness as HV 26,000-33,000 MPa (about HV 2,650-3,370 in kgf/mm²) and calls round particles the best choice for hardfacing powders in most cases, because they flow best through powder feeders.

In the 2023 hardfacing paper, fine carbides dissolve more readily than coarse ones. The 2021 recycling review adds that less pure tungsten scrap can be made into cast eutectic carbide.

Sources: ITIA: tungsten intermediates; Paton Welding Journal 2010, no. 4; USGS Open-File Report 2005-1028; Materials Science-Poland 41(1), 2023; Int. J. Refractory Metals and Hard Materials 98 (2021).

In carbide powder and dust, the cobalt is the bigger concern. OSHA's 1987 letter reports NIOSH's finding that in mixtures over 2% cobalt, the cobalt adds more to the potential hazard than the tungsten carbide does. The Tungsten Consortium's industry classification under EU CLP lists plain WC as not classified (ITIA), but NTP lists cobalt-tungsten carbide powders and hard metals as reasonably anticipated to be a human carcinogen.

Powder processing is generally tied to the highest airborne exposures in hard-metal making, NTP says, with several studies reporting cobalt near or above 5,000 µg/m³ (5 mg/m³). For tungsten carbide over 2% cobalt, OSHA's chemical data table lists these time-weighted average limits:

OSHA names enclosure, local exhaust ventilation, general ventilation and personal protective equipment as controls, and ITIA adds respirators for powder handling. If tungsten dust gets on your clothes, ATSDR's advice is to shower and change before you head home. Fire is a separate risk: OSHA says cobalt dust can form explosive mixtures in air, and the industry table lists fine tungsten metal powder of 0.6-0.9 µm as a flammable solid, unlike WC.

This summarises what agencies publish; it is not medical advice. More is on is tungsten carbide toxic.

Sources: OSHA letter of interpretation, 1987-02-11; ITIA: health and environment; NTP Report on Carcinogens profile; OSHA chemical data: tungsten carbide; OSHA: cobalt dust; ATSDR ToxFAQs: tungsten.

Powder, grinding sludge and sweepings are carbide in a form we buy. USGS and ITIA both count grinding sludge and loose powder as soft scrap, and ITIA says most soft scrap is recycled by the chemical conversion route.

Keep it sealed: powder in a lidded container, sludge in a leak-proof one. If coolant or oil is in it, USPS makes the mailer identify its flashpoint and other hazards, and a package holding liquid must be marked and carry orientation arrows, so name the coolant in the form. USPS takes no mailpiece over 70 pounds.

Tell us in the quote form what it is, wet or dry, about how much it weighs, and add photos. Ship from anywhere in the US and we pay the shipping, or we pick up in every city in California, Arizona and Nevada; we pay cash where state law allows. More on selling tungsten carbide.

Sources: USGS Open-File Report 2005-1028; ITIA Recycling of Tungsten Products, part 2; USPS Publication 52, section 323; USPS DMM 601; USPS DMM 201.