Is tungsten carbide conductive? Electricity, EDM and heat

Yes. The US National Toxicology Program calls tungsten carbide a good electron conductor, and a 2024 EDM paper on a 6% cobalt grade describes hard metal as a conductive ceramic.

Cemented carbide is tungsten carbide (WC) grains held together by a metal binder. Cobalt is the most common binder, and commercial grades generally carry 3% to 16% cobalt, per NTP. A grade is a set mix of carbide and binder. ITIA names the grain size of the WC and the cobalt content as the two main settings that tune a grade's hardness and toughness.

Cobalt-rich spots conduct better than the rest of the part. In die-sinking EDM of a 6% cobalt grade, the 2024 study found that areas with more cobalt melted more, because of their higher electrical conductivity.

For scale against a metal, ITIA lists tungsten metal at a resistivity of 5.28 microhm-cm at 298 K (about 25 C), and its conductivity at 0 C at about 28% of silver's, silver being the best conductor of all. None of the sources behind this handbook gives a matching resistivity for sintered tungsten carbide, so no number is printed here.

Magnetism is a separate property: cobalt, the usual binder metal, is ferromagnetic, meaning attracted to a magnet (USGS).

Sources: NTP Report on Carcinogens profile; NTP background document on hard metals; OSTI 1488769, cobalt as binder; Materials 2024, EDM of WC-6Co; ITIA, tungsten properties; USGS Circular 1196-M, cobalt.

Because carbide conducts, it can be shaped by electrical discharge machining (EDM), which removes conductive material by melting and vaporizing it with a stream of sparks. The tool never touches the part. The condition, in the 2024 paper's words, is that the material has sufficient electrical conductivity.

Two forms show up in the carbide studies cited here. In die-sinking EDM a shaped electrode is sunk into the part, as in the 2024 study of a 6% cobalt grade. In wire EDM a thin wire cuts a profile through it, as in the 2003 study of WC-Co below. A 2024 review lists cemented carbide beside electrically conductive ceramics and quenched steel as hard materials that EDM machines precisely.

Sources: Micromachines 2024, EDM review; Materials 2024, EDM of WC-6Co; OSTI 20903702, wire EDM of WC-Co (2003).

EDM leaves a thin damaged layer on the cut surface, and more discharge energy makes it thicker. The sources describe four kinds of damage:

• Recast or white layer. Material melted by the spark re-solidifies on the surface. In electrical discharge turning of WC-Co at 5 rpm it measured 32.8 microns after roughing and 9.4 microns after finishing, about three and a half times thinner.
• A softer skin. Nanoindentation of wire-EDMed WC-Co in 2003 found the recast layer softer and less stiff than both the bulk and the heat-affected zone below it.
• Cracks and pores. The same study traced irregular indents to porosity, soft matrix material, separated grain boundaries and thermal cracks from the EDM process.
• Cobalt loss. A Rocky Flats report (RFP-4498) says early EDM machines were notorious for eroding the cobalt binder first, which cracked the carbide. It calls that largely solved, because nearly all machines of its day had carbide settings that avoid cobalt depletion.

The same report adds two warnings. An EDM recast layer sits in tension, where milling and turning layers are usually in compression, and that tension contributes to the shorter fatigue life of EDMed parts in general. And even with the right settings, "carbides are fundamentally brittle materials that are susceptible to cracking."

Sources: Micromachines 2024, EDM review; OSTI 20903702, wire EDM of WC-Co (2003); Rocky Flats RFP-4498, EDM surfaces.

Carbide carries heat about as well as alloy steel at the low end of its range and more than twice as well at the high end. The Fraunhofer and tungsten figures are at room temperature; the 4140 and 6% cobalt papers state no temperature for theirs.

So the 6% cobalt grade carries about 2.4 times as much heat as 4140, and the 40 W/m·K grades carry slightly less. Thermal conductivity fell as binder content rose and as the WC grain got finer in the Fraunhofer work, and a 2023 model of WC-Co-Ni likewise found it rising with grain size. So a fine-grain, high-cobalt grade moves heat more slowly than a coarse, low-cobalt one.

Heat changes it too. Between room temperature and 1000 C, the high-conductivity grades lost about 30% of their conductivity, while the low ones gained 6% to 16%.

High-speed steel (HSS), a tool steel that ITIA says carries 1.5% to 20% tungsten, has no thermal conductivity figure in these sources, so the table leaves it out. University lecture notes sum HSS up as high in toughness but low in hot hardness, and list high thermal conductivity and low thermal expansion among carbide's traits. The full comparison is on carbide vs HSS.

Sources: Materials 2020, Fraunhofer IKTS carbide grades; Materials 2024, EDM of WC-6Co; ITIA, tungsten properties; Materials 2023, WC-Co-Ni conductivity model; KSU lecture, cutting tool materials; ITIA, Tungsten in Steel; ITIA, Tungsten in Steel (2008).

In calculated tests, a carbide that conducts heat better runs a cooler edge. In the Fraunhofer study's calculated temperature fields for grooving 4140, raising the carbide's conductivity from 48 to 72 W/m·K lowered the peak edge temperature by about 7%, at every edge rounding studied.

The hottest point was always the cutting edge itself, with temperature falling away from the contact zone. For a 5% cobalt grade with a 30 micron edge rounding, the calculated peak rose from 671 C to 715 C as cutting speed went from 120 to 180 m/min. A 2021 paper puts the highest tool/chip interface temperature at 600 to 1000 C, hot enough for carbide to oxidize in dry, high-speed cutting.

Those numbers sit above the roughly 550 C to which ITIA says hardened HSS keeps most of its hardness; a 2021 review cites 600 to 650 C as the limit for safe use of HSS tools. Carbide keeps high hardness over a wide range of temperatures and draws heat out of the edge, which suits an insert (a replaceable carbide cutting tip) or a solid carbide end mill working in that heat. The temperatures at which carbide itself softens and oxidizes are on melting point and heat.

Sources: Materials 2020, Fraunhofer IKTS carbide grades; Frontiers in Materials 2021, carbide oxidation; ITIA, Tungsten in Steel; Materials 2021, HSS review; KSU lecture, cutting tool materials.

Carbide expands about half as much as steel when heated, and brazing locks that mismatch into the joint. A 2019 brazing study gives 11.6 millionths per C for its steel and 5.7 millionths per C for its cemented carbide, grade not stated.

That study brazed the two with BAg-24 silver-copper filler at 750 C. As a rough sum, if each figure held over the whole cool-down to a 20 C room, a 20 mm braze line would shrink by about 0.17 mm on the steel side and 0.08 mm on the carbide side. The joint has to absorb roughly 0.09 mm of difference.

The authors note that this difference may leave tensile residual stress on the steel side, because the carbide shrinks less. A 2000 study of brazed carbide-steel tools says brazing leaves residual stress from that mismatch as the joint cools, and that such stresses can cause early failure by cracking of the cutting edge under steady or repeated loads.

Brazed tips also matter in scrap. ITIA says carbide brazed onto or cast into steel is the main reason those products are recycled at a lower rate, and the solder has to be removed before the carbide is processed. We buy carbide in any form, brazed tools and EDM offcuts included, so send photos and a rough weight or read how we buy on sell tungsten carbide.

Sources: Materials 2019, steel-to-carbide brazing; Materials Science Forum 2000, brazed tools; ITIA, recycling of tungsten.